Special Issue
Polymer materials and composites
Polimerni materiali in kompoziti
Guest Editor:
Miran Mozeti~

VSEBINA – CONTENTS
Predgovor/Foreword
M. Mozeti~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
IZVIRNI ZNANSTVENI ^LANKI – ORIGINAL SCIENTIFIC ARTICLES
Microwave-assisted non-aqueous synthesis of ZnO nanoparticles
Sinteza nanodelcev ZnO v nevodnem mediju pod vplivom mikrovalov
G. Ambro`i~, Z. Crnjak Orel, M. @igon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
Removal of a thin hydrogenated carbon film by oxygen plasma treatment
Odstranjevanje tanke plasti hidrogeniranega ogljika s kisikovo plazmo
U. Cvelbar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
Low temperature destruction of bacteria Bacillus stearothermophilus by weakly ionized oxygen plasma
Nizkotemperaturno uni~evanje bakterij Bacillus stearothermophilus s {ibko ionizirano kisikovo plazmo
M. Mozeti~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
Surface characterization of polymers by XPS and SIMS techniques
Analiza povr{ine polimerov z metodama XPS in SIMS
J. Kova~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
Modification of PET-polymer surface by nitrogen plasma
Modifikacija povr{ine PET-polimera z du{ikovo plazmo
R. Zaplotnik, M. Kolar, A. Doli{ka, K. Stana-Kleinschek. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
Functionalization of AFM tips for use in force spectroscopy between polymers and model surfaces
Funkcionalizacija AFM-konic za uporabo v spektroskopiji sil med polimeri in modelnimi povr{inami
T. Maver, K. Stana - Kleinschek, Z. Per{in, U. Maver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
A novel approach for qualitative determination of residual tin based catalyst in poly(lactic acid) by X-ray photoelectron
spetroscopy
Nov na~in kvalitativne dolo~itve vsebnosti katalizatorja kositra v polilakti~ni kislini z rentgensko fotoelektronsko spektroskopijo
V. Sedlaøík, A. Vesel, P. Kucharczyk, P. Urbánek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
Hydrophobization of polymer polystyrene in fluorine plasma
Hidrofobizacija polimera polistiren s fluorovo plazmo
A. Vesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
Plasma treatment of biomedical materials
Plazemska obdelava biomedicinskih materialov
I. Junkar, U. Cvelbar, M. Lehocky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
Radiofrequency induced plasma in large-scale plasma reactor
Radiofrekven~no inducirana plazma v reaktorju velikih dimenzij
R. Zaplotnik, A. Vesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
Modification of surface morphology of graphite by oxygen plasma treatment
Sprememba morfologije grafita med obdelavo s kisikovo plazmo
K. Eler{i~, I. Junkar, M. Modic, R. Zaplotnik, A. Vesel, U. Cvelbar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
Properties of particleboards made by using an adhesive with added liquefied wood
Lastnosti ivernih plo{~, izdelanih z uporabo lepila z dodanim uteko~injenim lesom
N. ^uk, M. Kunaver, S. Medved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
Poly(styrene-CO-divinylbenzene-CO-2-ethylhexyl)acrylate membranes with interconnected macroporous structure
Poli(stiren-KO-divinilbenzen-KO-2-etilheksil)akrilatne membrane s povezano porozno strukturo
U. Sev{ek, S. Seifried, ^. Stropnik, I. Pulko, P. Krajnc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
Modification of non-woven cellulose for medical applications using non-equlibrium gassious plasma
Modifikacija celuloznih kopren, uporabnih v medicinske namene, z neravnovesno plinsko plazmo
K. Stana - Kleinschek, Z. Per{in, T. Maver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253
ISSN 1580-2949
UDK 669+666+678+53 MTAEC9, 45(3)167–292(2011)
MATER.
TEHNOL.
LETNIK
VOLUME 45
[TEV.
NO. 3
STR.
P. 167–292
LJUBLJANA
SLOVENIJA
MAY–JUNE
2011
Use of AFM force spectroscopy for assessment of polymer response to conditions similar to the wound, during healing
Uporaba AFM-spektroskopije sil za spremljanje odziva polimernih molekul na v rani podobna okolja med celjenjem
U. Maver, T. Maver, A. @nidar{i~, Z. Per{in, M. Gaber{~ek, K. Stana-Kleinschek. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
Biodegradable polymers from renewable resources: effect of proteinic impurityon polycondensation products of
2-hydroxypropanoic acid
Biorazgradljivi polimeri iz obnovljivih virov: vpliv proteinskih ne~istot na produkte polikondenzacije 2-hidroksipropanojske kisline
I. Poljan{ek, P. Kucharczyk,V. Sedlaøík, V. Ka{párková, A. [alaková, J. Drbohlav . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265
Submikrometrski in nano ZnO kot polnilo v PMMA-materialih
Sub micrometer and nano ZnO as filler in PMMA materials
A. An`lovar, Z. Crnjak Orel, M. @igon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269
Tuning of poly(ethylene terephtalate) (PET) surface properties by oxygen plasma treatment
Prilagoditev lastnosti povr{ine polietilen tereftalata (PET) z obelavo v kisikovi plazmi
A. Doli{ka, M. Kolar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275
Probability of recombination and oxidation of O atoms on a-C:H surface
Verjetnost za rekombinacijo in oksidacijo za atome kisika na povr{ini a-C:H
A. Drenik, K. Eler{i~, M. Modic, P. Panjan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
Hydrothermal growth of Zn5(OH)6(CO3)2 and its thermal transformation into porous ZnO film used for dye-sensitized solar
cells
Hidrotermalna rast Zn5(OH)6(CO3)2 s termi~no transformacijo v porozno plast ZnO, uporabljeno za elektrokemijske son~ne celice
M. Bitenc, Z. Crnjak Orel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
19. KONFERENCA O MATERIALIH IN TEHNOLOGIJAH, 21. – 23. november 2011, Portoro`, Slovenija
19th CONFERENCE ON MATERIALS AND TECHNOLOGY, November 21–23, 2011, Portoro`, Slovenia . . . . . . . . . . . . . . . . . . . . . 293
MISLI GOSTUJO^EGA UREDNIKA
FOREWORD
Posebno izdajo na{e znanstvene revije namenjamo
polimerom in materialom, ki vsebujejo polimere.
Tovrstne materiale sre~ujemo povsod okoli nas, od
nekaterih naravnih organskih materialov do visoko-
tehnolo{kih bio~ipov. Hitra rast uporabe polimernih
materialov je vedno nov izziv za razvoj naprednih
izdelkov in tehnologij za modifikacijo povr{inskih
lastnosti, ki v vedno ve~ji meri temeljijo na sodobnih
neravnovesnih postopkih. Ta izdaja na{e revije obsega
nekatere vidike polimerov in polimernih kompozitov od
nanokompozitov do hidrogeniranega ogljika v sodobnih
fuzijskih reaktorjih. Tematski sklopi, ki so predstavljeni
v tej izdaji, spadajo med raziskovalne aktivnosti Centra
odli~nosti "Polimerni materiali in tehnologije", ki je bil
ustanovljen leta 2010 z namenom zdru`evanja raziskav
na tem podro~ju in uporabnikov tovrstnih materialov.
Gostujo~i urednik:
prof. dr. Miran Mozeti~,
vodja projekta
Center odli~nosti PoliMaT
This special issue is devoted to polymers and
polymer-containing materials. They are found every-
where around us in various forms ranging from some
natural organic materials to sophisticated biochips.
Rapidly growing applications dictate development of
novel materials with improved characteristics as well as
innovative techniques for surface modification – many
are based on application of advanced non-equilibrium
processing. The present issue covers some scientific
aspects of polymers and polymer composites, from
nanocomposites to hydrogenated carbon in fusion
reactors. The topics presented in the issue are essentially
research activities performed within the Centre of
excellence "Polymer materials and technologies"
established in 2010 in order to unite Slovenian research
centers and users of such materials.
Guest Editor
Prof. Dr. Miran Mozeti~
Project manager
Centre of excellence PoliMaT

G. AMBRO@I^ et al.: MICROWAVE-ASSISTED NON-AQUEOUS SYNTHESIS OF ZnO NANOPARTICLES
MICROWAVE-ASSISTED NON-AQUEOUS SYNTHESIS
OF ZnO NANOPARTICLES
SINTEZA NANODELCEV ZnO V NEVODNEM MEDIJU POD
VPLIVOM MIKROVALOV
Gabriela Ambro`i~, Zorica Crnjak Orel, Majda @igon
Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, SI-1000 Ljubljana, Slovenia.
gabriela.ambrozic@ki.si
Prejem rokopisa – received: 2011-02-14; sprejem za objavo – accepted for publication: 2011-04-12
In this work, the microwave-assisted non-aqueous synthesis of ZnO nanoparticles is reviewed. In comparison with the
"classical" (conductive) heating, the microwave method provides for a better control over experimental parameters and,
therefore, opens exciting opportunities for a better understanding of the influence of the reaction conditions on the reaction and
growth mechanisms of ZnO particles. As the morphology and size determine the physicochemical behavior of ZnO, a variety of
polymer-ZnO nanomaterials with tunable optoelectrical and mechanical properties can be in-situ or ex-situ prepared by the
microwave-assisted synthesis.
In the article, a special attention is devoted to investigations performed in our laboratory on the systems of zinc acetylacetonate
as a precursor and 1-butanol as media as well as oxygen supplier. Contrary to analogous syntheses performed under reflux
conditions, we showed that under microwave conditions the precursor’s initial amount exerts a tremendous effect on the
morphology and size of ZnO particles.
Keywords: ZnO nanoparticles, microwave chemistry, non-aqueous synthesis, crystal growth
V delu podajamo pregled sinteze nanodelcev ZnO v nevodnem mediju pod vplivom mikrovalov. V primerjavi s segrevanjem s
konvekcijo mikrovalovno segrevanje omogo~a bolj{i nadzor nad eksperimentalnimi parametri in tako ponuja zanimive
prilo`nosti za bolj{e razumevanje vpliva reakcijskih pogojev na mehanizem in rast delcev ZnO. Ker pa velikost in oblika delcev
dolo~ata fizikalno-kemijske lastnosti ZnO-nanomateriala, bo mikrovalovna sinteza v prihodnosti omogo~ila in-situ ali ex-situ
pripravo {irokega spektra nanokompozitov ZnO/polimer z nastavljivimi optoelektronskimi in mehanskim lastnostmi.
V ~lanku je posebna pozornost namenjena raziskavam, ki smo jih opravili v na{em laboratoriju na sistemu cinkovega
acetilacetonata kot prekurzorja in 1-butanola kot medija in donorja kisika. V nasprotju s prej{njimi {tudijami na analognem
sistemu pod pogoji refluksa smo pokazali, da pod mikrovalovnimi vplivi za~etna koli~ina prekurzorja klju~no vpliva na
morfologijo in velikost delcev ZnO.
Klju~ne besede: ZnO-nanodelci, mikrovalovna kemija, brezvodna sinteza, rast kristalov
1 INTRODUCTION
The unique and fascinating properties of nanostruc-
tured materials have triggered tremendous motivation
among scientists to explore and understand their for-
mation and growth processes as well as their subsequent
implementation in the steady and fast growing research
on the preparation of new polymer composite nano-
materials, which can exhibit new functionalities based on
nanoparticle’s optical and electrical properties.1 Due to
the fact that morphologies and sizes of inorganic
particles determine the properties and applications of the
polymer nanocomposites, the possibility to tune the
physical and chemical properties of nanoscale materials
through varying the crystal size and shape is a major
driving force in nanoparticle research.
In general, three types of processes are applied for
the synthesis of metal oxide particles in solution: the
"classical" synthesis under reflux conditions, the auto-
clave synthesis and, the most recent microwave-assisted
synthesis.2
In the oil bath synthesis, where the walls of the
reactor are heated by convection or conduction, the core
of the sample needs longer time to achieve the target
temperature and this may result in inhomogeneous
temperature profiles within the reaction flask.
One possibility to overcome this problem is the use
of microwave or dielectric heating, which is becoming an
increasingly popular synthetic method for organic3, 4 as
well as inorganic materials.5 Microwave process enables
the rapid and homogeneous heating of the reaction
mixture to the desired temperature without heating the
entire furnace or oil bath, which saves time and energy.
The microwave heating is based on two mechanism
of the conversion of the electromagnetic radiation into
heat energy, namely dipolar rotation and ionic conduc-
tion (Figure 1). The dipolar rotation (Figure 1a) occurs
when polar molecules having an electrical dipole
moment align with the oscillating electromagnetic field.
In this process, energy is lost as heat due to the mole-
cular friction and dielectric loss.
The ionic conduction heating mechanism (Figure 1b)
results from the dissolved (dissociated) charged particles
(ions). In dependence from the charge, the ions oscillate
back and forth under the influence of the electric com-
ponent of microwave radiation. In comparison with the
Materiali in tehnologije / Materials and technology 45 (2011) 3, 173–177 173
UDK 66.017:620.1/.2 ISSN 1580-2949
Pregledni ~lanek/Review article MTAEC9, 45(3)173(2011)
former mechanism, the absorption of microwave
irradiation by this mechanism is more efficient and the
energy transfer is faster.
Due to the fact that different compounds convert
microwave radiation to heat to different extents, i.e., they
have different microwave absorbing properties, micro-
wave radiation allows a selective heating of compounds
in the reaction mixture.
Therefore, the general advantages of microwave-
mediated synthesis over conventional heating are
(Figure 2): a) reaction rate acceleration as a con-
sequence of high heating rates, b) versatility of applied
reaction conditions, i.e. mild conditions (low-power/
low-temperature synthesis) or autoclave conditions (high
temperatures and pressures), c) higher chemical yields
with less by-products, d) different reaction selectivity
due to different microwave absorbing properties, e)
better reproducibility (excellent control over reaction
conditions), f) easy handling, which allows fast and easy
optimization of the experimental parameters.
However, there are few disadvantages of microwave
over the conventional heating. The monitoring of the
reaction course or, in the case of inorganic species, the
time-dependent monitoring of the particles growth can
not be performed. Furthermore, the microwave synthesis
reactor is expensive in comparison with glassware
equipment used for conventional (oil-bath) synthesis
and, up-to-now, the microwave-mediated scale-up
production has not yet reached the production quantities
of the "classical" reactors.
Nevertheless, the microwave synthesis promise to
offer many new benefits in the field of inorganic
nanoparticle precipitations, and, particularly in gaining
an insight into the dynamics of the formation of inorga-
nic nanoparticles under defined experimental conditions.
Given that microwave-mediated synthesis of inorganic
materials offer even better control over reaction para-
meters that conventional approach, methodological
investigations are indispensable to explore the full
potential of this synthetic method.
The purpose of this article is first a brief review of
the state-of-the art of the ZnO microwave synthesis using
organic solvents, particularly alcohols, and second, to
report on our own results of the non-aqueous micro-
wave-mediated synthesis of ZnO nanoparticles from zinc
acetylacetonate hydrate (Zn(acac)2xH2O) in 1-butanol.
2 MICROWAVE HEATING IN THE SYNTHESIS
OF ZNO NANOPARTICLES
As a wide band gap II–VI semiconductor (3.37 eV),
zinc oxide (ZnO) is promising material for numerous
applications, such as gas sensors, transparent electrodes,
pH sensors, biosensors, acoustic wave devices, and UV
photodiodes.6 Its extraordinary properties makes it one of
the most intensively studied materials and, as so, there is
a rapid growth in number of recent publications con-
cerning the synthesis performed under microwave
conditions (Figure 3).
The investigations on the solution-based microwave-
assisted growth of ZnO were mainly performed in the
water systems.7–10
In nonaqueous routes, the organic reaction mecha-
nisms play a dominant role in ZnO formation. The
organic species (i.e., the solvent and the organic ligands
in the precursor) undergo chemical reactions that are
responsible for supplying the monomers for nucleation
G. AMBRO@I^ et al.: MICROWAVE-ASSISTED NON-AQUEOUS SYNTHESIS OF ZnO NANOPARTICLES
174 Materiali in tehnologije / Materials and technology 45 (2011) 3, 173–177
Figure 1: Microwave-mediated heating process: a) dipolar rotation
and b) ionic conduction
Slika 1: Proces segrevanja pod vplivom mikrovalov: a) dipolarna
rotacija in b) ionska prevodnost
Figure 3: Number of publication per year on the solution-phase
(aqueous and non-aqueous) microwave-assisted synthesis of ZnO
particles (source ISI Web of Knowledge).
Slika 3: [tevilo objav na leto o sintezi ZnO-delcev v vodnem ali
nevodnem mediju pod vplivom mikrovalov (vir ISI Web of Know-
ledge)
Figure 2: Pros (+) and cons (-) of the microwave-assisted synthesis.
Slika 2: Prednosti (+) in pomanjkljivosti (–) sintez pod vplivom mi-
krovalov
and growth of ZnO nanoparticles. Therefore, in these
systems, a high sensitivity of both the organic reactions
and the crystal growth towards microwave irradiation can
be expected. Because of the fact that species differ in
their microwave absorption behavior, a selective heating
in a homogeneous or heterogeneous system might lead to
diverse morphology and size of the ZnO particles.
The wide parameter window for controlling the
microwave-assisted synthesis of ZnO has been investi-
gated in numerous of alcohol reaction systems. Hu et
al.11 reported on the rapid microwave-polyol process that
leads to the formation to monodisperse spherical ZnO
clusters, which average size can be tuned by the amount
of the precursor (zinc acetate). The striking effects of the
microwave heating parameters and the solvent compo-
sition on morphology hierarchical ZnO nanostructures
were demonstrated in ethylene glycol-water systems.12,13
Straw-bundle-like, wide chrysanthemum-like and oat-
arista-like morphologies and microspheres were pre-
pared on the basis of the applied cycling mode, which
determine the growth of the particles in the nucleation
stage, whereas T-like, X-like and cross-like linked
hexagonal prisms were observed by changing the volume
ratio of ethylene glycol to water.
Bilecka et al 14,15 studied in details the kinetic and
thermodynamic aspects of microwave-assisted synthesis
of ZnO nanoparticles from zinc acetate and benzyl
alcohol. It was concluded that microwave radiation
drastically accelerated the particle formation by
enhancing the dissolution of the precursor in alcohol as
well as increasing the rate constants of the esterification
reaction and the crystal growth.
3 SYNTHESIS OF ZnO BY MICROWAVE
HEATING USING ZINC ACETYLACETONATE
The syntheses were carried out in a Milestone MEGA
1200 laboratory microwave oven. The precursor (2.3, 4.6
and 6.9) mmol was initially dispersed in 17 mL of
1-butanol and heated in 2 min to 120 °C. Samples were
irradiated for 30 min with 250 W pulsed microwave
irradiation power. After irradiation, the samples were
cooled to room temperature, centrifuged and vacuum
dried at 40 °C.
The mechanism of the zinc acetylacetonate con-
version into ZnO16 relies on the nucleophilic attack of the
alcohol to the carbonyl group of the acetylacetonate
ligand and subsequent hydrolytic formation of the
reactive Zn-OH intermediates, which polycondensate
forming repeating -Zn-O-Zn- bonds. (Figure 4).
For the microwave-assisted synthesis, besides the
alcohol reactivity, the crucial property of the alcohol as a
medium is its ability to convert effectively the electro-
magnetic energy into heat energy. Based on the experi-
mental data17, 1-butanol is efficient microwave absorbing
solvent so under the microwave conditions the trans-
formation of zinc acetylacetonate into ZnO occurs in an
excellent yield (up to 85 %).
FTIR spectra (Perkin Elmer’s Spectrum One spec-
trometer) and XRD diffractograms (Bruker AXS D4
Endeavor diffractometer with Cu K radiation and a
Sol-X energy-dispersive detector) of the precipitates
obtained from 2.3 mmol and 6.9 mmol of initial amount
of zinc acetylacetonate are shown in Figure 5. Both of
the FTIR spectra (Figures 5a and 5b) consist of bands
G. AMBRO@I^ et al.: MICROWAVE-ASSISTED NON-AQUEOUS SYNTHESIS OF ZnO NANOPARTICLES
Materiali in tehnologije / Materials and technology 45 (2011) 3, 173–177 175
Figure 5: FTIR spectra and XRD diffractograms of the as-prepared
ZnO particles obtained from 2.3 (curves a and c) and 6.9 mmol
(curves b and d) of initial amount of zinc acetylacetonate in 17 mL of
1-butanol. The samples were exposed for 30 min with 250 W pulsed
microwave irradiation at 120 °C.
Slika 5: FTIR-spektra in XRD-difraktograma ZnO-delcev, priprav-
ljenih iz 2.3 mmol (krivulji a in c) in 6.9 mmol (krivulji b and d) cink
acetilacetonata v 17 mL 1-butanola. Vzorci so bili 30 min izpostav-
ljeni pulznemu mikrovalovnemu sevanju z mo~jo 250 W pri 120 °C.
Figure 4: Reaction mechanism of ZnO formation from zinc acetyl-
acetonate and 1-butanol.16
Slika 4: Mehanizem nastanka ZnO iz cinkovega acetilacetonata in
1-butanola 16
representing Zn-O stretching vibration at approximately
450 cm–1 and diffuse signals at cca. 3400 cm–1 attributed
to water molecules adsorbed on the ZnO surface. The
FTIR spectrum of as-prepared precipitate obtained from
higher amount of zinc acetylacetonate (Figure 5b)
shows also numerous signals of unreacted precursor.
Correspondingly, besides the intensive zincite reflec-
tions, the XRD pattern in Figure 5d shows additional
low intensity signals in the low-angle region representing
the crystalline phase of the precursor.
Figure 6 shows FE-SEM images (FE-SEM SUPRA
35VP (Carl Zeiss) equipped with an energy-dispersive
spectrometer (EDXS, Inca 400, Oxford Instruments)) of
as-prepared ZnO nanoparticles synthesized from
different amounts of zinc acetylacetonate (2.3, 4.6 and
6.9) mmol in 17 mL of 1-butanol. It was observed that
the initial quantity of the precursor exerts a tremendous
effect on the morphology of the particles, similarly to the
reported ethylene-glycol-water systems.12,13 Contrary to
the mentioned microwave-assisted ZnO preparation, in
the syntheses performed under reflux conditions by
using analogous concentration of the precursor in
1-butanol, the rod-shaped ZnO particles were formed
from all of the investigated solution meaning that in this
case the product shape is concentration-independent.16
The particles in Figure 6a are approximately 10 nm
to 60 nm in size, while the complex ZnO architectures in
Figure 6b consist of a few-nanometers-large ZnO
G. AMBRO@I^ et al.: MICROWAVE-ASSISTED NON-AQUEOUS SYNTHESIS OF ZnO NANOPARTICLES
176 Materiali in tehnologije / Materials and technology 45 (2011) 3, 173–177
Figure 7: FE-SEM images of as-prepared ZnO nanoparticles syn-
thesized from 2.3 mmol of zinc acetylacetonate and 1.16 mmol of
1-hexadecyl-3-methyl imidazole chloride monohydrate in 17 ml of
1-butanol under a) 250 W pulsed microwave irradiation microwave
conditions (30 min, 120 °C) and b) reflux conditions after 3 h.
Slika 7: FE-SEM-mikrografije ZnO-nanodelcev, pripravljenih iz 2.3
mmol cink acetilacetonata in 1.16 mmol 1-heksadecil-3-metil imida-
zol klorid monohidrata v 17 mL 1-butanola. Vzorci so bili 30 min
izpostavljeni pulznemu mikrovalovnemu sevanju z mo~jo 250 W pri
120 °C.
Figure 6: FE-SEM images of as-prepared ZnO particles synthesized
from a) 2.3, b) 4.6 and c) 6.9 mmol of zinc acetylacetonate in 17 ml of
1-butanol. The samples were exposed for 30 min with 250 W pulsed
microwave irradiation at 120 °C.
Slika 6: FE-SEM-mikrografije ZnO-delcev pripravljenih iz a) 2.3
mmol, b), 4.6 mmol in c) 6.9 mmol cink acetilacetonata v 17 mL
1-butanola. Vzorci so bili 30 min izpostavljeni pulznemu mikro-
valovnemu sevanju z mo~jo 250 W pri 120 °C.
crystallites, which form approximately 100-nm-long
nanorods growing on one side of the agglomerated,
densely packed, quasi-spherical ZnO particles. In
accordance with FTIR and XRD data (Figures 5b and
5d), in the case of 6.9 mmol of precursor, the micrograph
in Figure 6c shows that after 30 min of microwave
exposure approximately 100 nm long ZnO nanorods
formed close to the surface of the undissolved pyramidal
precursor. The similar phenomenon was observed during
the monitoring of the ZnO growth from the oversaturated
solution of zinc acetylacetonate in 1-butanol under the
reflux conditions, where, due to the local supersaturation
of the reactive monomer species, the initial formation of
ZnO occurs around the solid precursor.16
Ionic liquids are particularly interesting species in
microwave chemistry not only as they can act as
template to control the particle shape and assembly
behaviour, but also their ionic properties can drastically
enhance the microwave absorbance of the system. For
that mean, we have performed the microwave assisted
synthesis of ZnO from the 1-butanol solution of zinc
acetylacetonate in the presence of 1-hexadecyl-3-methyl
imidazole chloride monohydrate. Contrary to the reac-
tions carried out under "classical" reflux conditions
(Figure 7a), under microwave radiation the product
precipitates in form of round-shape architectures, which
are formed by the aggregation of ZnO nanocrystals
(Figure 7b). The similar effect of ionic liquid on the
aggregation of primary ZnO nanoparticles was already
reported in the microwave irradiated system of tetra-
butylammonium hydroxide hydrate and zinc acetate
dehydrate.18 The formation of thermodinamically favo-
rable spherical shape was attributed to the electrostatic
interactions of polar charges in small ZnO particles,
which uncontrolled growth is prevented by the absorp-
tion of charged species of the ionic liquid. This shows
that microwaves can be used to influence and control the
oriented attachment process, so new exciting opportu-
nities for the assembly of ZnO nanoparticles could be
opened.
4 CONCLUSIONS
The application of microwave radiation for the
preparation of metal oxides under aqueous and
non-aqueous conditions, especially relevant for the
preparations of polymer/ZnO nanocomposites, has been
shown to be a versatile approach to the design on novel
nanoparticles’ morphologies. Particularly the fast
reaction rates (short reaction times), better product yields
and the possibility to automatically combine different
experimental parameters makes microwave-assisted
synthesis suitable for the studies of the influences of the
reaction conditions on the morphology and sizes of ZnO
particles, which determine its properties and appli-
cations.
The different examples of the microwave-assisted
synthesis of ZnO described in this article show that the
general rules of the influence of the reaction parameters
on the mechanism of the particle growth in microwave
processes are not yet fully understand. This opens new
research challenges in designing the nanoparticles with
defined sizes, morphologies and complex architectures
by using different experimental conditions.
ACKNOWLEDGEMENT
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Tech-
nology of the Republic of Slovenia through the contract
No. 3211-10-000057 (Center of Excellence for Polymer
Materials and Technologies).
5 REFERENCES
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(2008), 3148
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(2008), 886
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and Interface Science, 346 (2010), 317
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2002, Chapter 2
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Current Opinion in Solid State & Materials Science, 14 (2010), 75
G. AMBRO@I^ et al.: MICROWAVE-ASSISTED NON-AQUEOUS SYNTHESIS OF ZnO NANOPARTICLES
Materiali in tehnologije / Materials and technology 45 (2011) 3, 173–177 177

U. CVELBAR: REMOVAL OF A THIN HYDROGENATED CARBON FILM BY OXYGEN PLASMA TREATMENT
REMOVAL OF A THIN HYDROGENATED CARBON
FILM BY OXYGEN PLASMA TREATMENT
ODSTRANJEVANJE TANKE PLASTI HIDROGENIRANEGA
OGLJIKA S KISIKOVO PLAZMO
Uro{ Cvelbar
Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, 1000 Ljubljana, Slovenia
uros.cvelbar@ijs.si
Prejem rokopisa – received: 2011-02-15; sprejem za objavo – accepted for publication: 2011-03-09
Capability of low pressure oxygen as a medium for removal of hydrogenated carbon thin films is demonstrated. The film was
deposited onto polished iron discs by CVD method. Discs were mounted into a discharge chamber which was evacuated to the
ultimate pressure of about 5 Pa. Methane of commercial purity was leaked into the chamber during continuous pumping so the
pressure of 100 Pa was established. Weakly ionized plasma was then created in the chamber by an inductively coupled RF
generator operating at the frequency of 27.12 MHz and the power of about 200 W. The methane molecules dissociated in the
plasma forming CHx radicals that adsorbed on the sample surface causing formation of a thin film of hydrogenated carbon. AES
depth profiling showed that a 3 nm thick film was formed after 200 s of plasma treatment. Samples were then exposed to oxygen
plasma in the same chamber, and characterized by AES as well as water drop contact angle. After 12 s of oxygen plasma
treatment the samples were visually free of carbon and the AES depth profiling proved it. The contact angle of a water drop
decreased from initial 80° to about 10° indicating a rapid transformation of the surface properties from hydrophobic to
hydrophilic character. The experiments allowed for estimation of the cleaning efficiency which was about 0.25 nm/s.
Keywords: hydrogenated carbon, gaseous plasma, methane, oxygen, Auger electron spectroscopy, hydrophilic
Prispevek opisuje mo`nosti, ki jih daje kisikova plazma pri odstranjevanju tankih plasti hidrogeniranega ogljika. Plasti so bile
pripravljene na `eleznih poliranih vzorcih z nanosom iz parne faze. Vzorci so bili postavljeni v razelektritveno posodo
vakuumskega sistema. Sistem je bil najprej iz~rpan do kon~nega tlaka okoli 5 Pa, potem pa smo ob stalnem ~rpanju dovajali
metan pri tlaku 100 Pa. V metanu smo ustvarili plinsko plazmo z razelektritvijo, ki smo jo vzbujali z radiofrekven~nim
generatorjem mo~i okoli 200 W in frekvence 27,12 MHz. Molekule metana v plazmi disociirajo, nastali radikali CHx pa se
lahko ve`ejo na povr{ino vzorca in po~asi tvorijo tanko plast hidrogeniranega ogljika. Profilna analiza s spektroskopijo
Augerjevih elektronov (AES) je pokazala, da je po 200 s obdelave nastala plast debeline okoli 3 nm. Vzorci so bili potem
izpostavljeni delovanju kisikove plazme v istem rektorju. Po izpostavitvi so bili analizirani z AES, izmerili pa smo tudi
kontaktne kote vodnih kapljic v odvisnosti od ~asa izpostavitve kisikovi plazmi. Globinski profili AES so pokazali izredno
veliko stopnjo ~istosti vzorcev po okoli 12-sekundni obdelavi. Kontaktni kot vodne kapljice je bil sprva 80°, kar ka`e na
hidrofobnost plasti hidrogeniranega ogljika, `e po 1 s obdelave s kisikovo plazmo pa se je zmanj{al na okoli 10°, kar ka`e na
hitro funkcionalizacijo s polarnimi skupinami. Rezultati raziskav so omogo~ili izra~un hitrosti jedkanja, ki je za navedene
vzorce okoli 0,25 nm/s.
Klju~ne besede: hidrogenirani ogljik, plinska plazma, metan, kisik, spektroskopija Augerjevih elektronov, hidrofilizacija
1 INTRODUCTION
Energy supply is a major global consideration of
future generations. Currently, the majority of energy is
supplied by burning fossil fuels. The problem of this
energy source is not only that it will be exhausted sooner
or later, but the burning also causes release of huge
quantities of greenhouse gases, such as CO2, which
contributes to the heating of Earth’s atmosphere. More-
over, as recent history shows, the availability of energy
sources is often subject to local political situation. The
future energy sources should therefore be both friendly
to the environment and independent from local political
situation. While many methods for energy production
have been invented, none of them is effective enough to
be a suitable replacement of carbon fuels as the
consumption of energy is expected to grow continuously
due to the expected growth of population as well as
higher consumption per capita in third world countries.
The most natural solution of the problem is usage of
energy arising from our Sun. The Sun is a rather young
star and is expected to supply energy for the next billion
years. The temperature on the Sun surface is close to
6 000 K so it radiates energy as a black body covering a
broad range from IR to UV part of the spectrum. The
majority of energy reaching the Earth is in the visible
range, i. e. photons of the energy of few eV. The Earth
receives the energy flux of close to 1000 W/m2. Taking
into account the Earth diameter which is about 12 700
km one can calculate the power reaching the earth
P = ¼  d2 E0 = 4 × 10
17 W (1)
Here d is the Earth’s diameter and E0 the flux of
energy from the Sun at the Earth distance (close to 1000
W/m2). The current needs of all population are
approximately 3 × 1010 W so the energy coming from the
Sun is 10 million times the needs of the population. The
humans therefore use only a negligible fraction of
available energy.
Materiali in tehnologije / Materials and technology 45 (2011) 3, 179–183 179
UDK 533.5:546.26:542.428.8 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)179(2011)
The problem arises from the fact that the energy in
the form of visible light photons does not cover all of
humanity’s energy needs. Beside visible light, we need
other forms of energy, such as electricity. Not sur-
prisingly, efforts have been made worldwide to develop
methods for conversion energy from visible light photons
to electricity. Solar panels have been developed decades
ago and are currently being used, but only a small
fraction of the globally consumed energy comes from
this source. There are several reasons that prevent
general adoption of solar cells including geographic
(climate) reasons, high investment costs, rather poor
efficiency, and aging effects. Also, it seems that
contamination of the panel surface by dust is far from a
minor problem, especially in the deserts where other
conditions are favorable. Although recent results in
development of new generation of solar cells with
quantum dots or nanowires 1–5 are promising, their
application is still questionable.
The original source of energy from our Sun are not
photons of visible light. Sun makes energy from nuclear
fusion (reactions between hydrogen isotopes). Huge
amount of energy is released by fusion. At the reaction
D + T  He + n (2)
an amount of energy equal to almost 18 MeV is
released. The majority of energy (more than 14 MeV) is
taken by the neutron in terms of its kinetic energy, and
the rest is taken by the He nucleus, also in terms of the
kinetic energy. This reaction is really energetic: 1 kg of
fuel (hydrogen isotopes) produces about 100 · 106 kW h
of energy. A kilogram of coal, on the other hand, gives
only about 5 kW h – 8 orders of magnitude less energy.
Nuclear fusion is possible only at extremely high
temperatures, because a substantial amount of energy
(0.4 MeV) has to be invested in the pair of nuclei in
order to overcome the electrostatic energy barrier. Such
high temperature is not available at the sun surface, but
only in the Sun core. The temperature in the Sun centre
is estimated to 15 · 106 K. This is obviously enough for
fusion reactions heating our Sun. Extremely fast
neutrons and helium nuclei formed at fusion reactions
collide with other nuclei and are thermalized (achieve
Maxwell – Boltzmann kinetic energy distribution
function) so they cannot reach the Sun surface. Strong
convection allows for transfer of energy from the Sun
core toward the surface. Adiabatic cooling occurs on the
way so the Sun crust is not as hot as the core. Finally, the
Sun surface emits photons according to the Stefan –
Boltzmann law and what we get at the Earth surface is a
flux of mostly visible light photons at the value close to
1000 W/m2.
Since the early 1940s, scientists began experimenting
with nuclear fusion on Earth, as the energetic neutrons
produced in the reaction are more promising particles for
energy production than the photons of visible light.
Recent results are encouraging so the international
community is making many efforts to bring the idea of
having a small Sun here on Earth into fruition.
Extremely high temperatures that should be achieved in
human–made fusion devices bring many technical
problems; a major one is deposition of hydrogenated
carbon layers in experimental fusion devices. They are
formed when protective graphite coatings, needed due to
extremely high temperatures, are eroded by hydrogen
atoms. Several research groups worldwide are therefore
involved in development of methods for removal of the
deposits.
An original approach for removal of hydrogenated
carbon deposits is application of thermodynamically
non-equilibrium gases, especially oxygen. Such a state
of oxygen is obtained by passing molecular gas through
a gaseous discharge 6–11. Oxygen molecules in the dis-
charge are partly ionized, dissociated and excited, and
the resultant particles are chemically very reactive 11–16.
A unique property of such particles is that they are very
selective – they do interact with certain polymers even at
room temperature, but the interaction with graphite is
poor 17–23. Exposure of fusion reactors to such oxygen
particles would therefore lead to removal of hydro-
genated carbon deposits while leaving the massive
graphite blocks of protective coatings in fusion reactors
rather intact.
Although it is well known that oxygen particles
interact chemically with hydrocarbons including
polymers 24–30, very little work has been performed on
quantification of results for hydrogenated carbon
deposits prepared by plasma deposition. The present
paper reports recent results on this phenomenon.
2 EXPERIMENTAL
Experiments were performed in a plasma reactor
explained to details elsewhere 30–34. The reactor is made
from glass in order to prevent heterogeneous surface
recombination of neutral oxygen atoms so a rather high
dissociation fraction is obtained already at a rather low
power. The dissociation fraction of 10 % is typical 35–39.
The original experimental setup was modified for current
experiments. Two different gases were leaked into the
system – methane and oxygen. Methane was used to
deposit a thin film of hydrogenated carbon, and oxygen
for its removal.
Samples were iron discs with a diameter of 10 mm
and a thickness of 0.5 mm. The discs were polished and
cleaned by chemical methods. The appearance of hydro-
genated film during treatment with methane plasma was
observed by a naked eye, but some quantification was
performed by Auger Electron Spectroscopy (AES) depth
profiling. A classical AES device with a rather poor
lateral resolution was applied. The primary electron
beam has the diameter of about 40 μm. The samples
were sputtered by argon ions in order to measure the
depth profiles. The ions had the kinetic energy of 3 keV
and were rastered over the surface area of about 5 mm x
U. CVELBAR: REMOVAL OF A THIN HYDROGENATED CARBON FILM BY OXYGEN PLASMA TREATMENT
180 Materiali in tehnologije / Materials and technology 45 (2011) 3, 179–183
5 mm. The sputtering rate for hydrogenated carbon de-
posits was estimated previously using standard samples
and the value was about 2 nm/min.
Water drop contact angle measurements were used in
order to determine the surface conditions. A home-made
device comprising a CCD camera and a computer with
appropriate software was used to measure contact angles
of a drop of distilled water with the volume of 3 μL.
3 RESULTS
The discharge chamber was fist cleaned by a brief
exposure to oxygen plasma, then evacuated to ultimate
pressure of about 5 Pa. Methane was leaked continuously
during pumping so a stable working pressure of 100 Pa
was obtained. Samples were exposed to methane plasma
for 200 s and removed from the discharge system. A
typical AES depth profile of a sample is presented in
Figure 1. Samples were then mounted back into the
system individually. Oxygen plasma was used to remove
the hydrogenated carbon deposits. Samples were visually
clean after about 10 s of treatment with oxygen plasma.
A depth profile of a sample treated for 12 s is presented
in Figure 2. Many samples were exposed to plasma for
different periods. These samples were characterized by
the water drop contact angle method. The contact angle
versus plasma treatment time is presented in Figure 3.
4 DISCUSSION
The depth profile of a sample presented in Figure 1
indicates formation of a thin carbon film. Unfortunately,
the AES technique does not recognize hydrogen but
since methane was used to deposit the film it is expected
that it contains much hydrogen. Namely, methane
molecules entering the discharge are not only partially
ionized but also well-dissociated. The dissociation
products are neutral H atoms and CHx radicals. Since our
plasma is not powerful, full dissociation is unlikely to
occur. The CHx radicals stick on the surfaces forming a
thin film of hydrogenated carbon. The appearance of the
film is easily observed with a naked eye because it is
black. The thickness of the layer cannot be determined
by the eye but AES depth profile gives a good approxi-
mation. The thickness is calculated from the Figure 1 by
taking into account approximate sputtering yield which
was determined previously with standard samples. In our
case, the thickness is estimated to about 3 nm. The
deposition rate is pretty low, what can be explained
either by low deposition rate or by spontaneous removal
of formed layer by neutral hydrogen atoms and hydrogen
ions. Namely, these particles cause slow erosion of
carbon in fusion reactors 40–45 and it was already shown
that hydrogen plasma was suitable for removal of hydro-
genated carbon prepared by other techniques 46–48.
It is interesting that the oxygen concentration at the
interface between iron and carbon in Figure 1 is pretty
small. According to the literature, iron tends to form a
thin native oxide film spontaneously 49–51. The absence of
U. CVELBAR: REMOVAL OF A THIN HYDROGENATED CARBON FILM BY OXYGEN PLASMA TREATMENT
Materiali in tehnologije / Materials and technology 45 (2011) 3, 179–183 181
Figure 3: Contact angle of a water drop on iron samples with
hydrogenated carbon deposits versus oxygen plasma treatment time.
Slika 3: Kontaktni kot vodne kapljice na jeklenih vzorcih s plastjo
hidrogeniranega ogljika v odvisnosti od ~asa obdelave s kisikovo
plazmo
Figure 1: AES depth profile of an iron sample after exposure to
methane plasma for 200 s
Slika 1: AES profilni diagram jeklenega vzorca po izpostavi metanovi
plazmi za 200 s
Figure 2: AES depth profile of an iron sample after exposure to
methane plasma for 200s and subsequently to oxygen plasma for 12 s
Slika 2: AES profilni diagram jeklenega vzorca po izpostavi metanovi
plazmi za 200 s in nato kisikovi plazmi za 12 s
rather large oxygen concentration is thus explained by
effects of methane plasma. As already mentioned, lots of
H atoms are created in methane plasma and these atoms
cause reduction of metal oxide thin films 52–55. The first
effect of iron exposure to methane plasma is thus
reduction of the native oxide film.
Once the film is reduced the deposition of hydro-
genated carbon is observed. This film is removed rather
efficiently by oxygen plasma treatment as demonstrated
in Figure 2. This sample is almost free form carbon (a
small concentration is observed only at the surface and
this is probably due to contamination on the way from
the plasma lab to the surface characterization lab. The
oxide film is now present on the surface as expected
since iron is quickly oxidized after exposure to air.
Figure 3 represents results of the water drop contact
angle measurements. Metals have large surface energy so
the contact angle on pure metal should be very low.
Samples with hydrogenated carbon deposits, however,
exhibit a high contact angle, which indicates hydro-
phobic character of the deposits. The result is not sur-
prising – all oxygen-free polymers are hydrophobic 55–59.
The contact angle quickly drops at exposure to oxygen
plasma as shown in Figure 3. The huge decrease of the
contact angle after 1s of plasma treatment cannot be
attributed to removal of the film, since the sample is still
black. The effect is rather explained by surface function-
alization of the hydrogenated carbon film. Numerous
authors have shown that even a brief exposure of
polymer to oxygen plasma causes an appearance of
oxygen-rich functional groups on the surface of a
polymer 60–64. These functional groups are extremely
polar and the surface functionalized by them is usually
hydrophilic. The contact angle shown in Figure 3 keeps
decreasing with increasing oxygen plasma treatment
time and finally stabilizes at the value of approximately
10°, typical of very clean metals.
Knowing the thickness of the original hydrogenated
carbon films, z, and length of oxygen plasma pro-
cessing needed for complete removal of the film, t,
allows for an estimation of the removal rate. The removal
rate is
z/t = 3 nm/12 s = 0.25 nm/s (3)
This value is approximate since the removal is not all
homogeneous. Also, the estimation of the film thickness
is not very accurate, and finally, it is worth mentioning
that this value holds for room temperature. It has been
shown that removal rates of amorphous carbon deposits
increase with temperature 23, so we expect that this remo-
val rate would also be higher at elevated temperature.
5 CONCLUSION
The experimental results presented in this paper
clearly show that weakly ionized oxygen plasma created
in inductively coupled radiofrequency discharge is a
suitable medium for removal of hydrogenated carbon
thin films. The films that were deposited by CVD using
methane as precursor were effectively removed in about
10 s which makes this technique suitable for cleaning of
large areas. The removal rate was estimated to about 0.25
nm/s. This value may be somewhat too small for
application in fusion reactors, but it should be stressed
that modern fusion devices operate at elevated
temperature of few 100 °C where the removal rate is
expected to be much higher.
ACKNOWLEDGEMENT
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Tech-
nology of the Republic of Slovenia through the contract
No. 3211-10-000057 (Center of Excellence Polymer
Materials and Technologies).
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U. CVELBAR: REMOVAL OF A THIN HYDROGENATED CARBON FILM BY OXYGEN PLASMA TREATMENT
Materiali in tehnologije / Materials and technology 45 (2011) 3, 179–183 183

M. MOZETI^: LOW TEMPERATURE DESTRUCTION OF BACTERIA BACILLUS STEAROTHERMOPHILUS ...
LOW TEMPERATURE DESTRUCTION OF BACTERIA
BACILLUS STEAROTHERMOPHILUS BY WEAKLY
IONIZED OXYGEN PLASMA
NIZKOTEMPERATURNO UNI^EVANJE BAKTERIJ BACILLUS
STEAROTHERMOPHILUS S [IBKO IONIZIRANO KISIKOVO
PLAZMO
Miran Mozeti~
Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, 1000 Ljubljana, Slovenia
miran.mozetic@guest.arnes.si
Prejem rokopisa – received: 2011-02-03; sprejem za objavo – accepted for publication: 2011-03-10
Weakly ionized plasma created in oxygen at low pressure was applied in order to destruct bacteria Bacillus stearothermophilus.
Plasma was created in an inductively coupled radiofrequency discharge. The kinetic temperature of neutral gas was kept close to
room temperature in order to prevent substantial thermal damage. Plasma parameters depended on the pressure of oxygen in the
reactor. The density of charged particles was decreasing with increasing pressure while the density of neutral oxygen atoms
exhibited an opposite behavior. Plate count technique as well as scanning electron microscopy was used to monitor the bacterial
destruction. Experiments performed at different pressures indicate that the neutral O atoms are major reactants causing etching
of organic material, while ultra violet radiation from plasma is responsible for rapid deactivation in the first few seconds of
plasma treatment.
Keywords: sterilization, oxygen plasma, bacteria, Bacillus stearothermophilus, plate count technique, scanning electron
microscopy
Nizkotla~no kisikovo plazmo smo uporabili za uni~evanje bakterij Bacillus stearothermophilus. Plazmo smo vzbujali v
induktivno sklopljeni visokofrekven~ni razelektritvi. Kineti~na temperatura nevtralnega plina je bila blizu sobni, s ~imer smo se
izognili ne`elenim termi~nim efektom. Plazemski parametri so bili odvisni od tlaka kisika v reaktorju. Gostota nabitih delcev je
z nara{~ajo~im tlakom padala, medtem ko je gostota nevtralnih kisikovih atomov nara{~ala. Za dolo~anje {tevila pre`ivelih
bakterij smo uporabili inkubacijo na krvnem agarju s {tetjem kolonij, za dolo~anje po{kodb pa vrsti~no elektronsko
mikroskopijo. Raziskave, opravljene pri razli~nih tlakih, so pokazale, da je jedkanje organskega materiala, ki sestavlja bakterije,
predvsem posledica interakcije nevtralnih kisikovih atomov. Hiter padec {tevila pre`ivelih bakterij v prvih nekaj sekundah
plazemske obdelave pa smo pripisali vplivu ultravijoli~nega sevanja.
Klju~ne besede: sterilizacija, kisikova plazma, bakterije, Bacillus stearothermophilus, krvni agar, vrsti~na elektronska
mikroskopija
1 INTRODUCTION
Polymer materials applied in biology and medicine
should be sterile. This is especially the case for body
implants where the requirements for sterility are sever.
Several techniques have been developed to assure ste-
rility of materials and devices. They include poisonous
chemical reagents, a variety of energetic beams and heat
treatments. All these techniques have advantages and
drawbacks. In practice, heat treatment using both dry and
wet atmosphere is particularly popular since the equip-
ment is rather inexpensive and easy to use. Samples are
mounted into an appropriate chamber and heated to
elevated temperatures. Dry heating usually requires
higher temperatures and/or longer treatment times
comparing to wet treatment. Water vapour is an excellent
medium for rapid heat transfer from a boiler to the
samples. The device is called an autoclave and is widely
used in common practice.
A small disadvantage of heat treatment is a simple
fact that many materials do not stand prolonged heating.
This is especially the case for many polymers since
structural modifications may occur. Many body implants
are made from or contain polymer components so the
problem is pretty severe. In such cases autoclaving is not
the right sterilization method and other methods should
be applied. The most straight-forward method is appli-
cation of poisonous liquids or gases. There are a handful
of such reagents such as ethylene- oxide, fluorine and
chlorine. No bacteria or spores can stand prolonged
treatment with such poisonous compounds. A small pro-
blem arises from the simple fact that such reagents are
also poisonous for humans. Such sterilization should be
performed using strict protocols, and accidents are not
uncommon. It is especially difficult to remove remains of
reagents as well as death bacteria that may remain toxic.
Energetic beams are increasingly popular in sterili-
zation of delicate materials. Energetic electrons, ions,
-rays and hard UV radiation can be applied. The proce-
dure is effective, but unsuitable for sterilization of
objects with complex shape since the energetic beams do
not always reach gaps and other small features. The
sterilization of implants is therefore not a trivial task and
many researchers worldwide attempt to develop more
Materiali in tehnologije / Materials and technology 45 (2011) 3, 185–190 185
UDK 678:621.785:542.428.8 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)185(2011)
reliable methods. One of them is application of cold
weakly ionized gaseous plasma.1–16 The present paper
addresses plasma sterilization of materials contaminated
with heat resistive bacteria Bacillus stearothermophilus.
2 EXPERIMENTAL
Destruction of bacteria Bacillus stearothermophilus
was performed in a small plasma reactor which has been
described to details previously.1–11 Plasma has been
well-characterized with a Langmuir probe and a catalytic
probe.17–23 Plasma parameters have been measured in a
wide range of pressures from 10 Pa to 200 Pa.28 The
density of charged particles and the density of neutral
oxygen atoms at the pressures at which sterilization was
performed are summarized in Table 1.
Table 1: Two most important plasma parameters (ion and neutral atom
densities) at three selected pressures in the vacuum chamber
Tabela 1: Dva najpomembnej{a plazemska parametra (gostota ionov
in nevtralnih kisikovih atomov) pri treh izbranih tlakih v vakuumski
posodi
Pressure Charged particledensity
Neutral O atom
density
30 Pa 2.5 · 1016 m–3 1.9 · 1021 m–3
75 Pa 8 · 1015 m–3 3.2 · 1021 m–3
150 Pa 4 · 1015 m–3 4.6 · 1021 m–3
The samples of Bacillus stearothermophilus were
prepared according to the standard procedure. The
bacteria were first incubated and then rinsed thoroughly
in order to remove traces of nutrition in the incubating
suspension. The initial concentration of bacteria in
suspension was determined by a standard plate count
technique and was about 8 × 108 cells per ml. A drop of
water containing bacteria with the volume of 100 μL was
deposited onto carriers. We used standard well-polished
silicon substrates. The substrates were pre-cleaned by
chemical methods and exposed to oxygen plasma for few
seconds in order to remove any possible traces of
impurities that may prevent an even distribution of the
water drop. The substrates were dried at ambient
temperature and after water evaporation a rather uniform
film of bacteria was created. The number of bacteria per
carrier was about 8 × 106 cells (vegetative form only).
Carriers with bacteria were exposed to plasma for
various times and at three different pressures of (30, 75
and 150) Pa. The corresponding plasma parameters are
presented in Table 1. After each treatment the bacteria
were washes by distilled water, diluted and applied onto
agar plates. They were incubated for about 24 h so that
visible colonies appeared on the agar plate. The number
of visible colonies was measures and, taking into
account the dilution ratio, the number of survived
bacteria on the substrates was calculated.
Selected samples were mounted into a scanning
electron microscope to determine any visible changes of
bacteria. We used a high resolution scanning electron
microscope (SEM) with a field emission source of
electrons Carl Zeiss Supra 35. The kinetic energy of
primary electron beam was adjusted to1 keV. No coating
was applied onto the samples prior to SEM analyses.
3 RESULTS
Results of systematic measurements of the survival
rate are summarized in Figure 1 and Figure 2. Figure 1
represents results at the oxygen pressure of 75 Pa while
Figure 2 is for two different pressures: 30 Pa and 150
Pa. A pretty sharp drop of the live bacteria is observed
after plasma treatment for a short time and the concen-
M. MOZETI^: LOW TEMPERATURE DESTRUCTION OF BACTERIA BACILLUS STEAROTHERMOPHILUS ...
186 Materiali in tehnologije / Materials and technology 45 (2011) 3, 185–190
Figure 2: Survival curves of Bacillus stearothermophilus by PCT. The
graph represents CFUs of Bacillus stearothermophilus vs. treatment
time by oxygen plasma at pressures of 30 Pa and 150 Pa.
Slika 2: [tevilo bakterij Bacillus stearothermophilus, ki smo jih
ugotovili z metodo nanosa na plo{~e krvnega agarja. Slika predstavlja
pre{teto {tevilo enot, ki sestavljajo kolonije, v odvisnosti od ~asa
plazemske obdelave pri tlakih 30 Pa in 150 Pa.
Figure 1: Survival curve of Bacillus stearothermophilus by PCT. The
graph represents CFUs of Bacillus stearothermophilus versus oxygen
plasma treatment time at 75 Pa.
Slika 1: [tevilo bakterij Bacillus stearothermophilus, ki smo jih
ugotovili z metodo nanosa na plo{~e krvnega agarja. Slika predstavlja
pre{teto {tevilo enot, ki sestavljajo kolonije, v odvisnosti od ~asa
plazemske obdelave pri tlaku 75 Pa.
tration was decreasing monotonously with increasing
oxygen plasma treatment time.
Selected samples were imaged by scanning electron
microscopy. Figure 3 represents a typical image of an
untreated sample. The SEM image of a sample exposed
to plasma for 3 s is presented in Figure 4. No visible
changes are detected as compared to the untreated
sample. Figure 5 represents SEM image of a sample
exposed to plasma for 10 s. Here, some modifications of
the bacteria are observed, but the original shape is more
or less preserved. The sample treated for 55 s is
presented in Figure 6. The bacteria are now visibly
damaged.
4 DISCUSSION
The results presented in Figures 1 and 2 reveal
interesting features. The initial number of survived
bacteria drops significantly even for the shortest
treatment time. Further on, a plateau is observed in the
survival curves, and for prolonged treatment the number
of survived bacteria is very small, but still measurable.
The appearance of the plateau can be explained by
different destruction mechanisms. Oxygen plasma is a
rich source of particles that may be harmful to bacteria.
The particles include excited oxygen molecules, atoms in
the ground state, metastable excited atoms, positively
charged ions, negatively charged ions and photons
emitted at relaxation of excited states by electrical dipole
radiation.24–27 All these particles may react with bacteria.
It has been shown by numerous authors that organic
material is slowly etched during treatment with oxygen
plasma.29–33 The etching has been attributed to chemical
interaction between neutral oxygen atoms that abound in
oxygen plasma and organic materials. The etching rate
depends on plasma parameters as well as on the substrate
temperature. At low temperature, the rate is often of the
order of 10–4.29–31 This value increases as the temperature
is increasing.
The sharp decrease of the number of survived
bacteria observed in the first few seconds of plasma
treatment (Figures 1 and 2) can be hardly attributed to
etching by neutral oxygen atoms. The sample tempe-
rature remains close to room temperature for this short
period of plasma treatment so the reaction probability
remains low. Also, the SEM image after 3 s of plasma
treatment does not reveal any visible modification of the
bacterial cell wall (Figure 4). The sharp drop in the
number of survived bacteria in first few seconds of
treatment by oxygen plasma should be therefore
attributed to other mechanisms.
M. MOZETI^: LOW TEMPERATURE DESTRUCTION OF BACTERIA BACILLUS STEAROTHERMOPHILUS ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 185–190 187
Figure 5: A SEM image of bacteria treated by oxygen plasma for 10 s
Slika 5: Posnetek bakterij, ki so bile obdelane s kisikovo plazmo 10 s
Figure 3: A SEM image of untreated bacteria
Slika 3: Posnetek neobdelanih bakterij
Figure 6: A SEM image of bacteria treated by oxygen plasma for 55 s
Slika 6: Posnetek bakterij, ki so bile obdelane s kisikovo plazmo 55 s
Figure 4: A SEM image of bacteria treated by oxygen plasma for 3 s
Slika 4: Posnetek bakterij, ki so bile obdelane s kisikovo plazmo 3 s
Excited oxygen atoms are definitely more aggressive
against organic materials, but they cause similar damage
as neutral oxygen atoms in the ground state. The same
applies for metastable molecules that are expected to be
present in oxygen plasma in a pretty high concentration.
Negatively charged oxygen ions are unlikely to reach the
surface of the samples as a thin sheath is always formed
between the unperturbed plasma and the sample surface.
A potential drop within this sheath repels negatively
charged particles so that the flux onto the surface is
negligible. Positively charged molecules and ions, on the
other hand, are accelerated across the sheath and
bombard the surface with the kinetic energy obtained
within the sheath. The voltage drop within the sheath
depends on the external biasing and could be pretty large
in capacitively coupled plasma. In our case, however, we
use inductively coupled plasma and the samples are not
connected to an electrode. The samples are just kept at
the floating potential. The difference between the plasma
and floating potentials depends on electron temperature.
In our case, this temperature is only a few 10 000 K, so
the potential difference is of the order of 10 V. Assuming
a collision-less sheath, the ions gain energy of the order
of 10 eV within the sheath. The penetration depth of
such ions in a solid material is only of the order of a nm,
so they are unlikely to cause any damage to vital parts of
bacteria. The rapid drop in the number of survived
bacteria observed in Figure 1 and 2 therefore cannot be
explained by the action of ions.
The only particles left are photons. Plasma is a rich
source of radiation in the range from infra-red to
ultraviolet light. The radiation is best observed by optical
emission spectroscopy.35–41 The transitions between
excited and ground states as well as between higher and
lower excited states may occur in oxygen molecules and
atoms as well as impurities that are always presented in
gaseous plasma. Neutral oxygen molecules are poor light
emitters since the transition by electrical dipole radiation
is prohibited by laws of quantum mechanics. Charged
molecules are scarce in our plasma (see Table 1) so the
emission of light quanta is easily neglected. Atoms, on
the other hand, abound in our plasma as shown in Table
1. Some atoms are excited to highly excited states that
radiate at different wavelengths. The photons have
energies either in the far UV (approximately 10 eV) or in
visible range (mostly at about 2 eV). The visible light in
the red part of spectrum is obviously not lethal to
bacteria so they can be excluded from discussion on
killing mechanisms. Hard UV radiation, on the other
hand, is lethal for bacteria, but the concentration of
atoms in highly excited states that produce them is pretty
low (unfortunately very little work has been performed
on quantitative determination of the radiation from this
source). The rapid deactivation of bacteria as observed in
Figures 1 and 2 can be therefore only partly attributed
to radiation from oxygen particles.
Plasma often contains impurities. The main impurity
in simple vacuum systems is water vapour. Since the
ultimate pressure in our system is few pascals, the
concentration of water in the chamber filled with oxygen
is few percent. Water molecules are quickly dissociated
to OH and H radicals. Both are excellent sources of UV
radiation. The Lyman series of H atoms is all in UV
range, while OH molecules have rich transitions in the
near UV region.42 The UV photons destroy the DNK of
bacteria and are therefore likely to contribute signifi-
cantly to deactivation of bacteria. The quick drop in the
survival curves in first few seconds of plasma treatment
(Figures 1 and 2) is therefore likely to be explained by
absorption of UV radiation. UV radiation does not cause
a visible damage to organic materials so this mechanism
explains the preserved shape of bacteria observed in
Figure 4.
Once many bacteria are damaged by UV radiation,
the bacteria in agglomerates are only preserved. While it
was difficult to observe such agglomerates they are
definitely present. As long as bacteria are sheltered in
agglomerates their deactivation by UV radiation is not
likely to occur. They have to wait for other mechanisms
to become effective. Such mechanism is chemical
etching predominantly by neutral oxygen atoms. Figure
5 represents a SEM image of bacteria after 10 s. Visible
changes in morphology can be observed. The effect is
even much more pronounced after 55 s of plasma
treatment. The corresponding SEM image is presented in
Figure 6. In fact, bacteria presented in Figure 6 are
really badly damaged. Only ashes are left as remains of
bacteria. Figure 6 also reveals small but perfectly visible
spheres that are left after ashing process induced by
oxygen plasma treatment. The origin of the spheres is
unknown and any discussion on this phenomenon is
beyond the scope of this paper.
There is a discrepancy between results presented in
Figures 1 and 6. Namely, Figure 6 reveals only ashes on
the substrates while the results presented in Figure 1
clearly show that not all bacteria are damaged lethal. A
possible explanation of this discrepancy may be
connected to appearance of agglomerates. Since the
number of SEM images taken at these experiments was
limited, we might have missed such agglomerates.
Namely, the bacteria presented in Figure 6 are definitely
damaged so badly that they cannot be revitalized.
It is worth discussing the effect of pressure on the
bacteria deactivation. Comparison of results presented in
Figure 2 indicates that the deactivation is much faster at
high pressure. At the treatment time of 120 s, for
instance, the number of survived bacteria at 30 Pa is
about 80 000, while at 150 Pa it is only little more than
10 000 – almost an order of magnitude smaller. This
pretty big difference may be explained taking into
account different plasma parameters at different
pressures as shown in Table 1. The density of charged
particles at 30 Pa is almost an order of magnitude larger
M. MOZETI^: LOW TEMPERATURE DESTRUCTION OF BACTERIA BACILLUS STEAROTHERMOPHILUS ...
188 Materiali in tehnologije / Materials and technology 45 (2011) 3, 185–190
than at 150 Pa. If bacteria were deactivated by charged
particles one would expect higher deactivation at lower
pressure. The results show right the opposite effect. The
effect is explained by interaction of neutral oxygen
atoms with bacteria. The density of oxygen atoms at 150
Pa is more than double of the value measured at 30 Pa.
This result, in combination with the results presented in
Figure 2, indicate that neutral oxygen atoms play a
dominant role in deactivation of bacteria in the second
phase, i.e. after destruction by UV radiation. Any
quantification of the destruction probability versus the
density of atoms is probably not justified for many
reasons including the experimental error (both in
determination of the O density and in determination of
the number of survived bacteria), the unknown rate of
agglomeration, and the fact that the destruction rate is
not linear with the flux of neutral oxygen atoms onto the
surface of bacteria. The temperature effects may not be
negligible, too. While any substantial heating can be
excluded for first few seconds of plasma treatment the
thermal effects may not be negligible after prolonged
treatment. Namely, although the neutral gas kinetic
temperature is close to room temperature, exothermic
reactions take place on the substrate surface due to
interaction of various plasma particles with the solid
material. The reactions include heterogeneous surface
recombination of neutral oxygen atoms, neutralization of
charged particles, weak bombardment with positive ions
and de-excitation of metastable atoms and molecules.
The reaction rates are, unfortunately, not known so any
estimation of the bacterial temperature versus the plasma
treatment time is beyond the scope of the paper.
Unfortunately, the experimental system in present confi-
guration does not allow for measuring the temperature.
Finally it is interesting that complete sterilization was
not obtained even after 5 minutes of plasma treatment.
This effect is difficult to explain by agglomerates since
Figure 6 shows that even after 55 s the bacteria in
monolayer are ashed. The facts observed in Figures 1
and 2 may be explained with un-careful handling of
bacteria. Namely, the plate count technique applied for
determination of the number of bacteria is definitely
sensitive to contamination. If foreign bacteria are
presented in the experimental room they would add to
the measured values obtained by this technique. Plate
count technique unfortunately does not distinguish
between bacteria used for the experiments and foreign
bacteria that might have been present in the experimental
room.
5 CONCLUSION
A method for sterilization of substrates contaminated
with bacteria Bacillus stearothermophilus was presented.
The method is based on application of weakly ionized
oxygen plasma. Particles created in oxygen plasma
interact with bacteria causing their degradation. The
degradation was studied using the plate count technique.
Rather rapid degradation was observed. The degradation
mechanisms were discussed to some details. The combi-
nation of the results obtained by plate count technique
and those obtained by scanning electron microscopy
indicate that both destruction of bacterial DNA by
absorption of UV radiation and chemical etching are
responsible for deactivation or destruction of bacteria. In
the first few seconds of plasma treatment the most likely
mechanism is deactivation of bacterial DNK, and the
chemical etching prevails thereafter. In any case it has
been shown that plasma treatment is a promising
technique for sterilization of delicate biomedical
materials.
ACKNOWLEDGEMENT
The author acknowledges the financial support from
the Ministry of Higher Education, Science and
Technology of the Republic of Slovenia through the
contract No. 3211-10-000057 (Center of Excellence
Polymer Materials and Technologies).
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M. MOZETI^: LOW TEMPERATURE DESTRUCTION OF BACTERIA BACILLUS STEAROTHERMOPHILUS ...
190 Materiali in tehnologije / Materials and technology 45 (2011) 3, 185–190
J. KOVA^: SURFACE CHARACTERIZATION OF POLYMERS BY XPS AND SIMS TECHNIQUES
SURFACE CHARACTERIZATION OF POLYMERS BY
XPS AND SIMS TECHNIQUES
ANALIZA POVR[INE POLIMEROV Z METODAMA XPS IN SIMS
Janez Kova~
Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, 1000 Ljubljana, Slovenia.
janez.kovac@ijs.si
Prejem rokopisa – received: 2011-02-14; sprejem za objavo – accepted for publication: 2011-03-10
Polymers are very often subjects of different surface treatments. To understand and control the basic processes during surfaces
modifications, analyses with advanced analytical techniques are required. We present two of such state of the art techniques,
X-ray photoelectron spectroscopy – XPS and secondary ion mass spectroscopy – SIMS, often applied for characterization of
polymers surfaces. In this work we give an overview of these techniques and their advantages and limitations related to polymer
characterization. The combined use of these techniques is demonstrated in the application example, in which the cotton fibers
were characterized before and after deposition of fluorine-based thin coating.
Keywords: X-ray photoelectron spectroscopy, XPS, TOF-SIMS, polymers, cotton
Polimerni materiali so pogosto predmet povr{inskih obdelav. Za razumevanje in kontrolo osnovnih procesov, ki potekajo med
povr{inskimi modifikacijami, je potrebna uporaba naprednih analitskih tehnik. Predstavljamo dve tak{ni moderni tehniki:
rentgensko fotoelektronsko spektroskopijo – XPS in masno spektroskopijo sekundarnih ionov – SIMS, ki se pogosto uporabljata
pri karakterizaciji povr{in polimernih materialov. V delu predstavljamo osnove teh tehnik ter njihove prednosti in omejitve,
povezane s preiskavo polimerov. Kombinirana uporaba obeh analitskih tehnik je predstavljena na primeru preiskave bomba`nih
vlaken pred nanosom tanke plasti na osnovi fluorovih spojin in po njem.
Klju~ne besede: rentgenska fotoelektronska spektroskopija, XPS, TOF-SIMS, polimeri, bomba`
1 INTRODUCTION
Polymers are widely used materials in everyday life.
Their inertness, low specific weight, low production cost,
variability of mechanical properties and other unique
properties, make them even more attractive for future
applications. Among modifications of polymers the
polymer surface treatments play an important role since
many surface related properties are crucial for techno-
logical applications of these materials. These appli-
cations are relevant for improvement of wettability,
adhesion, surface barrier properties, biocompatibility,
thin film deposition … The reason for surface modifi-
cations is often the intrinsic property of many polymers,
i.e. low surface energy, which should be changed in
order to improve polymers compatibility with other
materials and processes. Another reason for modification
can be to introduce new functional properties at the
polymer surface. The technological processes for
polymer surface modifications are physical (corona,
plasma, UV, laser treatments …) and chemical (wet
treatment, surface grafting …) modifications 1. The
effect of such surface modification may be followed by
different more or less sophisticated analytical tools.
Surface analytical techniques with high surface sensi-
tivity have been used to understand basic phenomena
during surface modifications. Among them are X-ray
photoelectron spectroscopy – XPS and secondary ion
mass spectroscopy – SIMS, which were very often
applied for advanced surface characterization of organic
and inorganic solid surfaces 2–5. Both of them have very
high surface sensitivity combined with high elemental
and molecular sensitivity. There are many other methods
to analyze polymer surfaces like optical, electron and
scanning probe microscopy, vibrational spectroscopies
(FTIR, Raman spectroscopy), methods for surface
energy measurements like contact angle measurements
and others 1.
The aim of this work is to present the main features
of XPS and SIMS techniques as two state of the art
analytical techniques for surface polymer characteri-
zation. Both of them have very high surface, elemental
and molecular sensitivity. The main features and
limitations of these two techniques are described in rela-
tion to polymer surface characterization. An application
of combined use of the XPS and SIMS technique is
given for cotton fabric.
2 X-RAY PHOTOELECTRON SPECTROSCOPY –
XPS
XPS analyses give information on the chemical
composition and chemical bonds of solid surface 2–4,6.
During the XPS analysis, a sample is illuminated with
the monochromatic X-ray light in an XPS spectrometer
and the energy of emitted photoelectrons from the
sample surface is analyzed. In the photoelectron
spectrum, which represents the distribution of emitted
photoelectrons as a function of their binding energy,
Materiali in tehnologije / Materials and technology 45 (2011) 3, 191–197 191
UDK 678:542.428 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)191(2011)
peaks can be observed which are typical of elements
present on the sample surface. The analysis area can be
from some microns to mm in diameter and the signal
during the XPS analysis originates from the surface layer
up to 6 nm in thickness. During the analysis, two types
of XPS spectra are usually recorded. Firstly, a spectrum
through a wide energy range is acquired, in which the
peaks of present elements are identified. Their concen-
tration is calculated by dividing the peak intensities with
the relative sensitivity factors provided by the XPS
spectrometer manufacturer 6. The attained results are
normalized to 100 %. The relative error at the calculation
of surface composition is approximately 20 % of
reported value, while the elemental sensitivity is about
0.5 %. The XPS method does not enable the analysis of
hydrogen and helium. In addition to wide energy range
spectra, high-energy resolution spectra of characteristic
peaks of the elements like C 1s, O 1s and others (N 1s, S
2p, Cl 2p, F 1s, Si 2p …) are recorded through a narrow
energy range. From the shape and binding energy of the
peaks within these XPS spectra, the chemical bonding of
surface elements can be identified with the help of data
from the literature. For example in the case of organic
materials one can identified from the carbon C 1s
spectrum (energy range 282–297 eV) the bonding of
carbon atoms like C-C/C-H, C-O, C=O, C-N, O-C-O,
O=C-O, C-F and others. In addition to this other spectral
features like shake-up peak in C 1s spectrum at 292 eV
helps to identify aromatic pendant groups. During the
analysis, the polymer samples are usually charging
electrically, thus, a low-energy electron gun-neutralizer
should be used. Prior to the spectra processing, spectra
should be shifted, so that within the spectrum of carbon
C 1s, the peak typical of the chemical bonds C-C/C-H is
at the binding energy of 285.0 eV. The analysis took
place in an ultra-high vacuum, which was during the
analysis approximately 10–7 Pa or less. For precise XPS
analysis of chemical bonding of surface atoms a special
X-ray source should be used. In such a source X-rays are
emitted from the Al-anode with energy of 1486.6 eV and
they are additionally monochromatized by special
monochromator in order to reduce their energy spread to
about 0.25 eV. The total recommended energy resolution
during polymer analysis is 0.6 eV.
In order to get depth distribution of elements beneath
the surface, ion bombardment with argon ions with
energy of few keV was traditionally applied what results
in a control removal of surface material. By subsequent
XPS analyses one can in this way obtain XPS depth
profiles presenting the concentration of elements as a
function of sputtering time what can be converted into
depth. In this way also polymers and thin organic films
can be analyzed. Unfortunately due to local damage of
chemical bonds in soft material like polymers and
preferential sputtering effects very limited information
on subsurface chemical bonds can be obtained by depth
profiling. In last years large progress was achieved
towards reducing surface damage introducing ion
bombardment by cluster ion beams like C60+.
Table 1 gives some characteristic features of the XPS
technique. XPS technique was often applied for
characterization of surface modifications of polymeric
materials by other and also by our research group 7–24.
3 SECONDARY ION MASS SPECTROSCOPY –
SIMS
SIMS is an analytical technique for compositional
analysis of the solid surfaces and thin films 2,5. Due to its
ability to identify the molecular structure i.e. type of
molecules at the surface, SIMS technique was
extensively applied in last decades just for polymer
analyses 12,15,24. During the SIMS analysis the surface is
bombarded with focused high energy (1–30 keV) ion
beam what results in the ejection or sputtering of the
species from the solid surface. Most of the emitted
particles are neutral but small fraction of them is charged
ions which are called secondary ions. They are measured
J. KOVA^: SURFACE CHARACTERIZATION OF POLYMERS BY XPS AND SIMS TECHNIQUES
192 Materiali in tehnologije / Materials and technology 45 (2011) 3, 191–197
Table 1: Comparison of main features of two surface analytical techniques XPS and SIMS
Tabela 1: Primerjava glavnih zna~ilnosti dveh povr{insko ob~utljivih tehnik: XPS in SIMS
Feature XPS SIMS
Main information Elemental composition and type of
chemical bonds of surface atoms
Type of atoms, molecules and pendant
groups at surface
Surface sensitivity From 3–6 nm for polymers 1–2 uppermost monolayers (1 nm)
Sensitivity for elements All, except H and He All
Need for ultra-high vacuum ambient Yes Yes
Spatial resolution 1–100 μm Less than 1 μm
In-depth information for thin films Yes Yes
faautoImaging Limited Yes with high resolution and sensitivity
3D analysis No Yes with high resolution and sensitivity
Quantification Yes Limited
Typical acquisition time for high
resolution spectrum
1–3 min 5–15 min
Required expertise for data
interpretation
High High
with a mass analyzer to determine the elemental com-
position and molecular structure of the surface. SIMS is
a very surface sensitive technique because the emitted
particles originate from the 1–2 top-most monolayers
(1 nm). During the SIMS analysis the surface atoms are
removed from the surface therefore the SIMS is locally
destructive. In order to obtain chemical information of
the original surface, the primary ion dose must be low
enough (<1013 cm–2) to prevent a surface damage. This
regime is so-called "static SIMS" mode and it is widely
used for the characterization of polymer surfaces. If the
primary ion dose is large enough it can be used to erode
the surface layer in a controlled manner. In this way
information on the in-depth distribution of elements can
be obtained similar as it was described above for XPS
depth profiling. This, so called "dynamic SIMS" mode is
widely applied for depth-profiling of thin films, layer
structures and dopant concentration. Layers of up to few
microns thick can be depth-profiled using SIMS tech-
nique.
The sensitivity of the SIMS technique depends on the
yield of secondary ion sputtering which is a function of
the specimen’s material (matrix effect), and the type,
energy and incidence angle of the primary beam of ions.
This is a reason for limited possibility for precise
quantification of SIMS results in terms of surface
composition. On the other hand the sensitivity of the
SIMS technique is very high and is in the range of 10–6.
This technique allows the routine measurement of many
trace elements at very low concentration, even in the
ppb-range. The proper choice of primary ions is
important for optimum sensitivity 25. For different
materials different types of ion guns should be used to
provide an optimum detection sensitivity for secondary
ions. In general inorganic materials do not exhibit very
complex structure with high mass units. Therefore for
inorganic materials the SIMS spectra acquired in the low
mass region (up to 100 u) can provide enough infor-
mation for successful analysis. Opposite is valid for
organic materials containing often ingredients with very
high molecular mass. In this case it is important to get as
much information as possible from the SIMS spectra in
the high mass region (above 200 u) where small mass
differences between similar molecular structures can be
distinguished. For this purpose ion beams with high yield
of surface sputtering of higher mass secondary ions is
required. In last decade liquid metal ion sources (LMIG)
for enhanced excitations in organic SIMS analyses ware
developed. They are based mainly on gold and bismuth
clusters of atoms, like Au3+, Bi3+, Bi5+. Similar characte-
ristics are valid also for the ion beam of the C60+ clusters.
Such ions sources provide enhanced increase for
sputtering of high mass secondary ions and are very
suitable for the analysis of organic and bio-organic
systems. In the case of inorganic materials oxygen ions
are usually used for analysis of electropositive elements
or those with low ionization potentials, whereas electro-
positive ions like cesium atoms are better for sputtering
negative ions from electronegative elements.
From the construction point of view there are three
basic types of ion guns. In one, ions of gaseous elements
are usually generated by electron ionization, for instance
noble gases (Ar+, Xe+), oxygen (O–), or even ionized
molecules such as SF5+ or C60+. This type of ion gun is
easy to operate and generates high current ion beams. A
second source type, the surface ionization source, is one
that generates Cs+ primary ions. A third source type, the
liquid metal ion source (LMIG), operates with metals or
metallic alloys, which are liquid at room temperature.
While a gallium source is able to operate with elemental
gallium, recently developed sources for gold, indium and
bismuth use cluster ions. The LMIG source provides a
high focused ion beams (<60 nm) for high-resolution
work with moderate intensity and is additionally able to
generate short pulsed ion beams. It is therefore
commonly used in static TOF-SIMS instruments. Highly
focused ion beam allows chemical or molecular imaging
of the surface, an analytical approach in which focused
ion beam is rastered over the surface and mass spectra
are collected at every pixel point. Retrospectively, for
any peak in the mass spectrum, an image can be
generated.
The mass analyzer may be a quadrupole mass
analyzer, magnetic sector mass analyzers or time-of-
flight (TOF) analyzer. They differ with respect to
transmission, mass resolution, mass detection (parallel or
sequential) and sensitivity. Among them the TOF-SIMS
analyzers provide the highest sensitivity and mass
resolution, a much greater mass range and they are
preferred for static SIMS analyses of polymers. In SIMS
measurements the spectra of positive and negative ions
are measured. The TOF-SIMS spectra obtained from
polymer surfaces can be divided into three regions. In the
first low mass range (up to 300 u) ions consisting of end
groups, fractions of repeat units, or side chains are
detected (e.g. CxHy+/–). In general the detection of these
low mass fragments is sufficient for an identification of
inorganic materials. In the second range (300–2000 u),
typically ions consisting of multiple repeat units with
and without the loss of functional groups are detected,
what is important for polymer analyses. In the third high
mass range (up to 10 000 u) intact oligomer ions can be
observed. These quasimolecular ions are typically
formed by an attachment (M + H) or loss of hydrogen
(M – H) and are important for recognition of surface
species. Recently developed polyatomic or cluster ions
primary sources such as gold (Aun+), bismuth (Bin+) and
C60+ (buckminster fullerens) overcome the problem of
insufficient yields of these higher mass species observed
under atomic primary bombardment 26. The use of cluster
ions has also another advantage. It decreases also the
residual chemical damage on the surface comparing to
monoatomic ions due to larger size, shallower implan-
tation depth and higher sputtering yields. For these
J. KOVA^: SURFACE CHARACTERIZATION OF POLYMERS BY XPS AND SIMS TECHNIQUES
Materiali in tehnologije / Materials and technology 45 (2011) 3, 191–197 193
reasons cluster ions are also very suitable for molecular
depth profiling of most organic and biological samples,
where in-depth molecular distribution is measured.
At this point we can compare also the acquisition
time between SIMS and XPS techniques. In the
TOF-SIMS instruments due to the time-of-life type of
measurements whole mass spectrum is acquired in
parallel at very high sensitivity. This is a reason that the
typical acquisition of SIMS spectrum at one point takes
about 1–3 min. It depends also on the required mass
region and mass resolution. This time is shorter than
typical time needed for acquisition of high-energy
resolution spectra of different elements in the XPS
analysis. In the XPS analyses such acquisition time is
typical from 5 min to 15 min.
In comparison to other surface analytical techniques
the SIMS offers several advantages, namely the ability to
identify all elements, including H and He and the ability
to identify elements present in very low concentration
levels, such as dopants in semiconductors. Some of the
SIMS main features are summarized in Table 1.
4 APPLICATION OF XPS AND SIMS
TECHNIQUES FOR COTTON FIBERS
CHARACTERIZATION
In the following an example is given, which shows
the combined application of XPS and SIMS techniques
for surface characterization of cotton fabrics treated with
fluoroalkylsilane (FAS). The cotton is an example of the
natural polymer and it was often characterised by XPS
and SIMS techniques 11,12,15,16,18,20,23. The thin fluoro-
alkylsilane treatment was applied in order to modify the
wettability of the cotton surface towards the hydrophobic
surface. Surface analyses were performed on the
untreated and treated cotton fabrics. The XPS and SIMS
techniques were used to follow the surface composition,
chemical bonds of surface atoms, molecular structures
and spatial distribution of coated material.
The XPS analysis was performed in the XPS
spectrometer produced by Physical Electronics Inc.,
model TFA XPS. The Al monochromatized source of
X-ray light with the power of 200 W was used. The
energy resolution was approximately 0.7 eV and the
analysis area was about 0.4 mm in diameter. Figure 1
shows an XPS survey spectra obtained on the cotton
fabrics and the fabrics after treatment. In the spectrum of
untreated sample the peaks of C 1s and O 1s are present
indicating the pure cotton surface. The surface com-
position deduced from this spectrum is given in Table 2.
There is the mole fraction 61.6 % of C and 38.4 % of
oxygen on the surface. This composition corresponds
well to expected cotton nominal composition. As
mentioned above XPS method can in addition to surface
composition yield also information on bonding of
surface atoms. Figure 2a shows high-energy resolution
carbon C 1s spectrum obtained on the untreated sample.
The spectrum is composed from more peaks, which were
identified by de-convolution procedure. After C 1s
spectrum de-convolution into different components, C1,
C2, C3, we assigned them to different chemical bonds of
carbon atoms. In this case a peak C1 at binding energy of
285.0 eV is assigned to C-C and C-H bonds, a peak C2
at 286.4 eV is assigned to C-O, a peak C3 at 287.8 eV is
assigned to C=O and O-C-O bonds. The origin of peaks
C2 and C3 is related to backbone of the cellulose
molecules from the cotton matrix. The C1 component
related to C-C/C-H bonds probably originates from some
non-cellulosic components as waxes or pectine or from
additives or surface contamination. Namely the C-C/C-H
bonds are not present in cotton backbone structure. The
relative concentration of C1, C2, C3 components is given
in Table 2.
J. KOVA^: SURFACE CHARACTERIZATION OF POLYMERS BY XPS AND SIMS TECHNIQUES
194 Materiali in tehnologije / Materials and technology 45 (2011) 3, 191–197
Table 2: Surface composition and relative concentration of different bonds of carbon atoms obtained by the XPS analysis of the cotton fabrics
before and after treatment with fluoroalkylsilane
Tabela 2: Sestava povr{ine in relativne koncentracije kemijskih vezi ogljikovih atomov, dobljene z XPS-analizo bomba`ne tkanine pred obdelavo
s fluoroalkilsilanom in po njej
Sample x(C)/% x(O)/% x(F)/% x(Si)/%
C1 (%),
C-C/C-H
C2 (%),
C-O
C3 (%),
C=O,
O-C-O
C4 (%),
O=C-O
C5 (%),
CF2
C6 (%),
CF3
Non-treated cotton
fabrics 61.6 38.4 0 0 38.0 43.4 18.6 0 0 0
Treated fabrics 37.7 9.6 48.6 4.2 15.9 18.6 3.1 5.0 48.4 9.0
Figure 1: XPS survey spectra obtained on the surface of the non-
treated cotton fabrics (lower) and on fabrics treated with fluoro-
alkylsilane (upper).
Slika 1: Pregledni XPS-spektri, dobljeni na povr{ini neobdelane
bomba`ne tkanine (spodaj), in na tkanini, obdelani s fluoroalkilsi-
lanom (zgoraj)
After the treatment of the cotton fabrics with
fluoroalkylsilane, the upper XPS survey spectrum
present in Figure 1 was obtained. In addition to C 1s and
O 1s the peaks of F 1s, Si 2p and Si 2s appeared
indicating successful deposition of functional coating.
The new surface composition is also given in Table 2.
The major element on the surface is fluorine (48.6 %)
originating from the deposited coating. The deposition of
functional coating can be recognized also in different
chemical bonding of the surface atoms with respect to
untreated cotton. Carbon C 1s spectrum from the coated
cotton surface is shown in Figure 2b. New peaks are
present in this spectrum representing new types of
C-atoms bonding. The two peaks are at binding energies
of 291.7 eV (C5) and 293.9 (C6) eV. These energies are
characteristic for CF2 and CF3 bonding which shows the
fluorinated-carbon compound on the surface. Also the
ratio between the C1 and C2 peaks changed after coating
deposition (Table 2). Applying the XPS method we
concluded on the surface composition and chemical
J. KOVA^: SURFACE CHARACTERIZATION OF POLYMERS BY XPS AND SIMS TECHNIQUES
Materiali in tehnologije / Materials and technology 45 (2011) 3, 191–197 195
Figure 3: TOF-SIMS spectra of positive polarity obtained on the cotton fibers treated with fluoroalkylsilane
Slika 3: TOF-SIMS-spektri pozitivnih ionov, dobljeni na bomba`nih vlaknih, obdelanih s fluoroalkilsilanom
Figure 4: TOF-SIMS images obtained on the cotton fibers treated
with fluoroalkylsilane. Images were taken using raster size of (80 ×
80) μm. Images on left side were obtained summarizing the signals of
SixOy clusters (left top), fluorocarbon (left middle) and aliphatic
hydrocarbons (left down). An image on the right side is a composite
image obtained from three images on the left.
Slika 4: TOF-SIMS-slike, dobljene na bomba`nih vlaknih, obdelanih
z fluoroalkilsilanom. Slike so bile posnete po podro~ju (80 × 80) μm.
Slike na levi strani so bile dobljene iz signalov, zna~ilnih za SixOy
spojine (levo zgoraj), floroogljikovih spojin (sredina levo) in alifatskih
ogljikovodikovih spojin (levo spodaj). Slika na desni strani je dobljena
kot vsota slik na levi strani.
Figure 2: XPS high-energy resolution spectra C 1s obtained on the
non-treated cotton fibrics (a) and fabrics treated with fluoroalkylsilane
(b). The spectra are doconvoluted in the C1–C6 peaks presenting
different C-atom bonds.
Slika 2: Energijsko visoko lo~ljivi spektri XPS C 1s, dobljeni na
povr{ini neobdelane bomba`ne tkanine (a), in na tkanini, obdelani s
fluoroalkilsilanom (b). Spektri so bili razstavljeni na vrhove C1–C6, ki
pomenijo razli~ne vezi ogljikovih atomov.
bonding of elements on non-treated cotton fabrics and on
the fabrics after coating deposition.
Due to the lack of spatial resolution of the XPS
method there is an open question how the deposited
coating is distributed over the surface. Is the coating
homogenously distributed or there are some non-treated
regions. To analyse the homogeneity of the applied
coating we carried out the SIMS analysis on the treated
sample. The SIMS instrument with TOF mass analyzer
was used. The primary ion beam of Bi-ions of energy 25
keV was produced by LMIG source. In the first step the
spectroscopic analysis was performed at one point. A
typical SIMS mass spectrum in the range between 5 u
and 790 u obtained with positive ions from the treated
fabrics is presented in Figure 3. The spectrum consists
of great number of peaks at different masses. The peaks
assignment is more complicated comparing to XPS
method. Anyhow there are known groups of peaks at
characteristic masses belonging to expected chemical
molecules and their fragments. In the case of coated
cotton the SIMS spectrum shows the main peaks at
masses m/u (12, 31, 69, 131, 240), which originate from
the fluorocarbon compound due to fluoroalkylsilane
treatment, the peaks at (77, 116, 129) u originating from
aromatic hydrocarbons and peaks at (347, 407, 488, 548)
u originating from the SiOx clusters. Beside mentioned
peaks which were used to identify the molecular
structure of the surface, other peaks exist in the SIMS
spectrum, as for example at 18 u (H3O+), 28 (Si), 29
(CHO+), 41 (C3H5+), 55 (C4H7) … After SIMS point
analysis the SIMS images were recorded analyzing the
region of (256 × 256) pixels and acquiring the whole
mass spectra at each pixel. The characteristic peaks of
fluorocarbon compound, aromatic hydrocarbons and a
peak from the SiOx clusters were used to construct the
images, which help to follow the distribution of
deposited coating. These SIMS images are presented in
Figure 4 as three separated images (on the left side) and
as a composite image. These results show that different
species are distributed in different manner. From the
composite image it can be recognized that there are
regions were the SiOx-clusters and fluorocarbon com-
pounds from the fluoroalkylsilane film are more concen-
trated. In this way we conclude that some inhomo-
genities from the coating deposition process which can
be further improved by modifying process parameters.
5 CONCLUSIONS
In this work the main features of XPS and SIMS
analytical techniques were presented and discussed. Both
of them are relevant for surface characterization of
polymer materials. These two techniques have very high
surface sensitivity. The main features of the XPS method
are the ability to obtain quantitative information on
chemical composition and on types of chemical bonds in
particular of carbon atoms, which are the most abundant
in organic materials. On the other hand the main features
of the SIMS techniques are the ability to obtain the
chemical information of the type of atoms, clusters and
molecules at the surface with very high lateral resolution.
The combination of both methods gives the very
complete information on polymer surface chemistry. The
combined use of the XPS and SIMS techniques was
demonstrated on the analyses of cotton fibers coated
with fluorine-containing film. XPS data confirm the
formation of film through the presence of fluorine and
C-F bonds on the cotton surface. The spatially resolved
SIMS images revealed partially nonuniform distribution
of coated surface species related to the reactivity of
deposited material.
ACKNOWLEDGEMENT
The author acknowledges the financial support from
the Ministry of Higher Education, Science and Tech-
nology of the Republic of Slovenia through the contract
No. 3211-10-000057 (Center of Excellence Polymer
Materials and Technologies). The author thanks to dr. B.
Simon~i~ and B. Tom{i~ for sample preparation and to
ION-TOF company from Münstrer, Germany, for help at
SIMS measurements.
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Materiali in tehnologije / Materials and technology 45 (2011) 3, 191–197 197

R. ZAPLOTNIK et al.: MODIFICATION OF PET-POLYMER SURFACE BY NITROGEN PLASMA
MODIFICATION OF PET-POLYMER SURFACE BY
NITROGEN PLASMA
MODIFIKACIJA POVR[INE PET-POLIMERA Z DU[IKOVO
PLAZMO
Rok Zaplotnik1, Metod Kolar2, Ale{ Doli{ka2, Karin Stana-Kleinschek2
1Jo`ef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
2Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, 1000 Ljubljana, Slovenia
rok.zaplotnik@ijs.si
Prejem rokopisa – received: 2011-02-09; sprejem za objavo – accepted for publication: 2011-03-05
Nizkotla~no, {ibko ionizirano du{ikovo plazmo smo uporabili za povr{insko modifikacijo polimera polietilen tereftalat z
du{ikovimi funkcionalnimi skupinami. Plazmo smo vzbujali v brezelektrodni visokofrekven~ni plinski razelektritvi z
generatorjem, ki deluje pri koristni mo~i okoli 200 W in osnovni frekvenci 27,12 MHz. Du{ikove molekule, ki vstopijo v
plinsko razelektritev, se vzbudijo v visoka vzbujena stanj, delno disociirajo in {ibko ionizirajo. Transformacija plina v stanje
plazme omogo~a nastanek kemi~no reaktivnih delcev z veliko potencialno energijo, medtem ko ostane njihova kineti~na
energija blizu vrednosti, ki je zna~ilna za plinske molekule pri sobni temperaturi. Velika kemijska reaktivnost du{ikove plazme
omogo~a hitro funkcionalizacijo povr{ine PET-polimera z du{ikom bogatimi funkcionalnimi skupinami. Ta pojav smo
spremljali z rentgensko fotoelektronsko spektroskopijo. Polimerni vzorci so postali hitro nasi~eni z du{ikom, iz ~esar je mogo~e
sklepati, da je funkcionalizacija omejena na zelo tanko plast prav ob povr{ini vzorcev.
Klju~ne besede: poly(ethylene terephthalate), du{ikova plazma, modifikacija povr{ine, funkcionalne skupine, rentgenska
fotoelektronska spektroskopija
Low pressure weakly nitrogen plasma was applied for incorporation of nitrogen-containing functional groups onto
poly(ethylene terephthalate) – PET polymer. Nitrogen plasma was created in an electrode-less radiofrequency discharge at the
nominal power of 200 W and the frequency of 27.12 MHz. Nitrogen molecules entered the discharge region were highly
excited, partially dissociated and weakly ionized. Transformation into the state of plasma allowed for creation of chemically
reactive particles with a high potential energy while the kinetic energy remained close to the value typical for room temperature.
The chemical reactivity allowed for rapid functionalization with nitrogen-rich functional groups. The appearance of these
groups was monitored by X-ray photoelectron spectroscopy – XPS. The polymer surface was quickly saturated with nitrogen
indicating that the modification was limited to an extremely thin surface film.
Keywords: poly(ethylene terephthalate), nitrogen plasma, surface modification, functional groups, X-ray photoelectron
spectroscopy
1 INTRODUCTION
Polymer materials are widely used not only in
industry but also in biomedical applications. A polymer
which is of particular interest in biomedical applications
is poly(ethylene terephthalate) – PET. The structural
formula is presented in Figure 1. PET is supplied in
variety of forms from bulk pieces through thin foils to
fabrics. Perforated foils are often used as the first layer
of plasters, while artificial blood vessels are made from
knitted material. The PET material has excellent che-
mical and mechanical properties and expresses
reasonably good bio-compatibility. The latter property,
however, is not always adequate since un-specific
interaction with body fluids is often observed. It is
therefore desirous to modify the surface properties of
PET before specific applications. The modification can
be performed by a variety of wet chemical treatments,
but such procedure often does not yield optimal results.
A popular technology for modification of polymer
surface properties is treatment by gaseous plasma.1–14
The surface properties of PET polymer can be modified
using oxygen plasma treatment (to make it more hydro-
philic),15–21 or fluorine plasma (to obtain excellent
hydrophobicity).22–23 Such treatments have been applied,
but it seems that in some cases they do not allow for
optimal modification of surface properties. This is
especially the case when optimal interaction with
proteins is required. In such cases other types of plasma
should be used. In the present paper, modification of
PET by nitrogen plasma is addressed.
2 NITROGEN PLASMA
Plasmas of diatomic gases are often used in order to
modify surface properties of polymer and other
materials. It has been shown that oxygen molecules
dissociate in plasma forming highly reactive atoms.24–27
Similar effects were observed in hydrogen plasma.28–35
Nitrogen, however, behaves much different in an
Materiali in tehnologije / Materials and technology 45 (2011) 3, 199–203 199
UDK 678:535.33 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)199(2011)
Figure 1: Structural formulae of poly(ethylene terephthalate)
Slika 1: Strukturna formula polietilen tereftalata
electrical discharge. This is due to a particular property
of excited states as well as collision phenomena. In
oxygen plasma, for instance, the probability for
super-elastic collisions between vibrationally excited
molecules and neutral atoms is huge, while in nitrogen it
is negligible. This particular phenomenon allows for
extremely high vibrational temperature of molecules in
nitrogen plasma.
Appearance of highly vibrationally excited molecules
is not the only particularity of nitrogen plasma. Nitrogen
molecules also have a variety of electronically excited
states and most are metastable. Plasma, created in
nitrogen, is therefore a rich source of excited nitrogen
molecules that are chemically pretty reactive. Some
common excited states are presented in Figure 2.
3 EXPERIMENTAL
Samples of PET foils were treated in nitrogen
plasma. The reactor has been described to details
elsewhere.15,36,37 Due to the completeness of the paper let
us just summarize the most important features of the
experimental system. The system comprises a discharge
chamber, a vacuum pump, gas feeding system, radio-
frequency generator and an optical spectrometer. The
discharge chamber is a borosilicate glass tube (Schott
8250) with the length of 60 cm and the outer diameter of
4 cm. It is connected to kovar rings on both sides, and
the rings are welded to standard FK 40 flanges. Rubber
gaskets are used for vacuum seals. The discharge
chamber is pumped on one side with a two stage oil
rotary pump with the nominal pumping speed of 28 m3/h.
The pressure is measured with an absolute vacuum
gauge. Nitrogen is leaked continuously on the other side
of the discharge tube through a manually adjustable
needle valve. The valve is connected to a nitrogen flask
via a copper tube. The vacuum system is often exposed
to ambient atmosphere and never baked so the ultimate
pressure is several Pa. The residual atmosphere contains
mostly water vapour. Plasma is created in the discharge
chamber by radiofrequency generator with the nominal
power of 700 W and the fundamental frequency of 27.12
MHz. A coil of 14 turns is wound up the discharge
chamber and connected directly to the generator. Since
no matching network is applied between the generator
and the coil, the matching is rather poor so it was estima-
ted that the forward power was only about 200 W 38–42.
Both inductive and capacitive components are present in
the discharge. The glowing plasma fills the entire volume
in the discharge tube at low pressure, but at high pressure
it is limited to a small volume inside the coil. At the
pressure of 75 Pa the plasma is at its optimal condition
so the samples are usually treated at this pressure. An
optical spectrometer of rather poor resolution is used to
monitor the plasma radiation from about 300 nm to 1000
nm. The best sensitivity of the spectrometer is between
400 nm and 800 nm where the borosilicate glass is pretty
transparent.
Commercially available PET foils were cut to small
pieces and exposed to plasma for (3, 10 and 30) s. No
cleaning or other pretreatments were performed prior to
exposure to plasma, but handling was careful so any
presence of foreign material is unlike. Samples were
R. ZAPLOTNIK et al.: MODIFICATION OF PET-POLYMER SURFACE BY NITROGEN PLASMA
200 Materiali in tehnologije / Materials and technology 45 (2011) 3, 199–203
Figure 4: XPS survey spectrum of an untreated sample
Slika 4: Pregledni XPS-spekter neobdelanega vzorca
Figure 2: Some excited states of nitrogen molecule. The width of the
states is due to vibrationally excitation
Slika 2: Nekatera vzbujena stanja du{ikove molekule. [irok razpon po
potencialni energiji je posledica vibracijskih vzbujenih stanj
Figure 3: A typical optical spectrum of nitrogen plasma
Slika 3: Zna~ilen opti~ni spekter du{ikove plazme
analyzed by X-ray photoelectron spectroscopy (XPS).
We used our spectrometer AvaSpec 3648 from Avantes.
The surface of the plasma treated PET samples was
analyzed with an XPS instrument TFA XPS Physical
Electronics. The base pressure in the XPS analysis
chamber was about 6 × 10–8 Pa. The samples were
excited with X-rays over a 400-μm spot area with a
monochromatic Al K1,2 radiation at 1486.6 eV. The
photoelectrons were detected with a hemispherical
analyzer positioned at an angle of 45° with respect to the
normal to the sample surface. Survey-scan spectra were
made at a pass energy of 187.85 eV a 0.4 eV energy step,
while for C1s individual high-resolution spectra were
taken at a pass energy of 23.5 eV and a 0.1 eV energy
step. Since the samples are insulators, we used an addi-
tional electron gun to allow for surface neutralization
during the measurements. The spectra were fitted using
MultiPak v7.3.1 software from Physical Electronics,
which was supplied with the spectrometer. The curves
were fitted with symmetrical Gauss-Lorentz functions. A
Shirley-type background subtraction was used. Both the
relative peak positions and the relative peak widths
(FWHM) were fixed in the curve fitting process.
3 RESULTS
Optical spectra of nitrogen plasma were measured
during treatment of the samples. The spectra did not
change noticeably during the treatment indication pretty
poor interaction of plasma with the samples. A typical
spectrum is shown in Figure 3.
Samples were characterized by XPS before and after
plasma treatment. A typical survey spectrum for
untreated sample is presented in Figure 4, while Figures
5, 6 and 7 represent spectra measured for samples
exposed to nitrogen plasma for (3, 10 and 30) s,
respectively. The survey spectra allow for quantification
of the composition in the surface layer from which
photo-electros can reach the detector. The elemental
composition in atomic percent is given in boxed text
placed in each spectrum presented in Figures 3 – 7.
4 DISCUSSION
The treatment of PET samples by nitrogen plasma
caused interesting modification of the surface layer that
is worth discussing. First, and most significant effect, is
appearance of nitrogen. The spectrum of untreated
sample, presented in Figure 4, is free from nitrogen.
Only carbon and oxygen are presented and the concen-
tration (79 % C and 21 % O) is close to the theoretical
value for bulk poly(ethylene terephthalate). Even a 3 s
long exposure to nitrogen plasma causes a pretty large
concentration of nitrogen atoms at about 12 % (Figure
5). The appearance of nitrogen is a consequence of inte-
raction between reactive particles generated in plasma
and surface atoms. Namely, nitrogen molecules in the
thermally equilibrium gas at room temperature do not
interact chemically with PET. A question arises, which
particles are responsible for breaking of surface chemical
bonds and formation of functional groups containing
R. ZAPLOTNIK et al.: MODIFICATION OF PET-POLYMER SURFACE BY NITROGEN PLASMA
Materiali in tehnologije / Materials and technology 45 (2011) 3, 199–203 201
Figure 7: XPS spectrum of the sample treated with nitrogen plasma
for 30 s
Slika 7: XPS-spekter vzorca, ki je bil obdelan z du{ikovo plazmo 30 s
Figure 5: XPS spectrum of the sample treated with nitrogen plasma
for 3 s
Slika 5: XPS-spekter vzorca, ki je bil obdelan z du{ikovo plazmo 3 s
Figure 6: XPS spectrum of the sample treated with nitrogen plasma
for 10 s
Slika 6: XPS-spekter vzorca, ki je bil obdelan z du{ikovo plazmo10 s
nitrogen. The list of candidates is pretty long. Figure 2
reveals the candidates. The most reactive particles are
usually those with the highest potential energy –
positively charged molecules. The density of ions in
plasma, however, is pretty low (the order of magnitude is
solely 1015 m–3) so they cannot cause such rapid func-
tionalization. The list of neutral particles with potential
energy abound 10 eV, which are chemically very
reactive, too, include a variety of molecular excited
states as shown in Figure 2. Finally, there are also
neutral atoms in the ground as well as excited states. In
plasma of some other two-atom molecules such as
hydrogen, the concentration of excited molecules is
pretty low because the radiative life time of those states
is short, and their contribution to chemical reactivity is
easily neglected. Nitrogen molecules, on the other hand,
have most excited states metastable meaning that the
radiative decay due to electrical dipole interactions is not
allowed. The concentration of such highly excited
molecules is thus pretty large, definitely larger than the
concentration of ions. Namely, in plasma with a rather
low electron temperature the positive ions are created by
step-like process and not directly by electron impact
ionization of a molecule in the ground state. Since a
variety of excited states is available, the interaction of
nitrogen plasma with polymer materials is far from being
well understood. In addition, nitrogen plasma is also rich
in high vibrationally excited molecules. Figure 4 reveals
some. Unlike oxygen plasma, where the vibrational
population is usually limited to few lowest excited states,
molecules nitrogen plasma are found in energetic excited
states with potential energy up to about 10 eV. Such
molecules are chemically reactive, too, so they can also
contribute to functionalization of polymer materials.
Figures 5, 6 and 7 reveal that the concentration of
nitrogen does not depend on treatment time, as long as it
is at least 3 s. This effect is explained by saturation of the
surface layer nitrogen functional groups. As mentioned
earlier, the molecules and atoms capable of chemical
interaction with a polymer abound in nitrogen plasma so
saturation is obtained quickly.
More surprising is enrichment of the surface film
with oxygen. This effect cannot be explained by poor
purity of nitrogen, since the original gas in the flask
contains over 99.99% of nitrogen and since the tube
connecting the flask with the needle valve is made from
copper that does not represent a significant source of
oxygen. Also, the vacuum system is hermetically tight as
no nitrogen is detected in the residual atmosphere. The
enrichment is rather explained by interaction of water
molecules with the surface film. As already mentioned,
the ultimate pressure is several Pa and the residual
atmosphere contains practically only water vapor. The
dissociation energy of water molecules is below the
excitation energy of nitrogen metastables, so a collision
with a nitrogen molecule found in a metastable excited
state is likely to cause dissociation of water molecule to
OH and O radicals. These radicals are chemically very
reactive and interact with organic materials causing
formation of oxygen functional groups. In fact, a
competition between oxygen and nitrogen probably
appears: both functional groups can be created on a
polymer surface and the exact functionalization depends
on the fluxes of nitrogen and oxygen reactive particles
onto the polymer surface. A study of the reaction pro-
babilities is definitely beyond the scope of current paper
but should be addressed in order to understand the
functionalization of polymers with nitrogen rich func-
tional groups in industrial environments. Finally, it is
worth mentioning that many polymers absorb substantial
amounts of water which is slowly released under vacuum
conditions so avoiding of oxygen radicals is a hard task
in real environment.
5 CONCLUSION
The functionalization of polymer surface with
nitrogen was addressed. XPS results clearly showed that
simple nitrogen plasma was a suitable medium for
modification of FET surface with nitrogen groups. The
exact mechanisms that lead to functionalization are,
however, far from being well understood. This is
predominantly due to the fact that numerous highly
excited molecules are found in nitrogen plasma. Optical
emission spectroscopy revealed that not only electro-
nically excited, but also vibrationally excited molecules
abound in our plasma. The enrichment of the surface
layer with oxygen observed at current experiments
reveals that functionalization with nitrogen functional
groups only may be a pretty difficult task in industrial
environments. The oxygen containing radicals are
formed in nitrogen plasma by dissociation of water
molecules. The concentration of water in a vacuum
system can be minimized by intensive and/or prolonged
pumping, using high vacuum compatible systems and
drying of the polymer before entering the system, but all
these procedures are difficult to apply in real industrial
environments.
ACKNOWLEDGEMENT
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Tech-
nology of the Republic of Slovenia through the contract
No. 3211-10-000057 (Center of Excellence Polymer
Materials and Technologies).
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Marcandalli, G. Poletti, Appl. Surf. Sci., 211 (2003) 1–4, 386–397
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42 (2004) 20, 3727–3839
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Atmani, F. Poncin-Epaillard, Surf. Coat. Technol., 201 (2006) 3–4,
868–879
24 M. Mozetic, A. Vesel, U. Cvelbar, A. Ricard, Plasma Chem. Plasma
Process., 26 (2006) 2, 103–117
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1–2, 17–22
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1088–1093
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(2010) 7, 969–974
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Balat-Pichelin, Vacuum, 84 (2010) 1, 90–93
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45 (1994) 10/11, 1095–1097
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N. Krstulovic, M. Klanjsek-Gunde, N. Hauptman, J. Phys. D: Appl.
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R. ZAPLOTNIK et al.: MODIFICATION OF PET-POLYMER SURFACE BY NITROGEN PLASMA
Materiali in tehnologije / Materials and technology 45 (2011) 3, 199–203 203

T. MAVER et al.: FUNCTIONALIZATION OF AFM TIPS FOR USE IN FORCE SPECTROSCOPY ...
FUNCTIONALIZATION OF AFM TIPS FOR USE IN
FORCE SPECTROSCOPY BETWEEN POLYMERS AND
MODEL SURFACES
FUNKCIONALIZACIJA AFM-KONIC ZA UPORABO V
SPEKTROSKOPIJI SIL MED POLIMERI IN MODELNIMI
POVR[INAMI
Tina Maver1,2, Karin Stana - Kleinschek1,2, Zdenka Per{in1,2, Uro{ Maver3,*
1University of Maribor, Faculty of Mechanical Engineering, Laboratory for Characterisation and Processing of Polymers, Smetanova 17,
SI-2000 Maribor, Slovenia
2Centre of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, SI-1000 Ljubljana, Slovenia
3National Institute of Chemistry, Laboratory for materials electrochemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia
uros.maver@ki.si
Prejem rokopisa – received: 2011-02-12; sprejem za objavo – accepted for publication: 2011-03-03
The following work presents the use of two different methods for the attachment of different functional groups onto the AFM tip
surface. Such functionalized tips then allow for further binding of molecules with different origins and natures, thus allowing for
use when measuring forces, and the extent of interactions appearing between two model surfaces and in real systems. Force
spectroscopy, in combination with chemical force microscopy (CFM), as used in this study, exhibits great potential for chemical
sensing in the field of polymer sciences. In modern wound treatment, it is very important to know the type and ranges of
interactions between different polymer materials, which are mostly crucial components of the dressings. Precise measurement of
these interactions would help to choose those materials that fit together without the use of additional chemical modifications on
their surfaces. Such modifications are often the cause of unpredictable complications during the course of wound healing. This
same method could also be used for interaction evaluation between chosen polymer materials with biological macromolecules,
which appear within the wound during the healing process. Such in vitro testing could be of great help when optimal wound
dressing materials need to be chosen in order to alleviate a patient´s suffering after application. Scanning electron and atomic
force microscopies were used in order to prove the effectiveness and applicability of the used functionalization procedures.
Keywords: atomic force microscopy, chemical force microscopy, force spectroscopy, functionalization of AFM tips
V prispevku predstavljamo uporabo dveh razli~nih metod za pripravo AFM-konic z razli~nimi funkcionalnimi skupinami.
Tak{na funkcionalizacija je primerna za nadaljnje pripenjanje razli~nih molekul, kar je osnova za uporabo pri dolo~anju mo~i in
obsega interakcij med modelnimi povr{inami in v realnih sistemih. Spektroskopija sil v kombinaciji z mikroskopijo na kemijsko
silo, ki je bila uporabljena v tem delu, ima velik potencial na podro~ju kemijskega zaznavanja v polimerni znanosti. Pri pripravi
ve~plastnih polimernih materialov, ki se uporabljajo pri moderni obravnavi kroni~nih ran, nam natan~no dolo~anje interakcij
lahko pomaga pri njihovi izbiri. Na podlagi takih meritev bi lahko izbrali materiale, ki med seboj najbolje interagirajo, in ne bi
bilo treba vpeljati nobene kemijske modifikacije, ki bi lahko negativno vplivala na potek celjenja ran. Prav tako bi bilo mogo~e
metodo uporabiti za dolo~anje interakcij izbranih polimernih materialov z biolo{kimi molekulami v rani, s ~imer bi in vitro
dolo~ili optimalen material za plast obli`a, ki je v stiku z rano. V delu smo uspe{nost in uporabnost funkcionalizacije preverjali
z elektronsko mikroskopijo in mikroskopijo na atomsko silo.
Klju~ne besede: mikroskop na atomsko silo, mikroskop na kemijsko silo, spektroskopija sil, funkcionalizacija konic
1 INTRODUCTION
Polymers have found their way into all fields of
science and industry over the last decades. Their
potential applications range from binders in batteries1 to
composite materials in drug delivery.2 Whilst the range
of possible combinations between different monomers is
endless, polymers found or based on natural polymers
have recently become the subject of thorough research,
once again.3,4 Synthetic changes to their native structure
make them even more propelling; especially cellulose
derivatives exhibit a lot of potential for satisfying most
industrial needs.3
Although AFM has been used for more than three
decades; we are still experiencing some breakthroughs
for its use in several areas of science and industry. This
AFM method is particularly useful for studying those
modifications obtained on surfaces of polymer materials
treated with reactive non-equilibrium gaseous plasma.5–18
Namely, it has been shown that even a mild plasma
treatment results in important changes in surface
morphology. Such changes may eventually lead to a
nano–textured surface and thus to an extremely high
hydrophilicity for otherwise moderately hydrophobic
polymer materials.19–32
From its origins in 1986,33 this technique has evolved
from a shaky method to the ultimate tool on the
nanoscale.34 The first years of AFM use were dedicated
to pushing the resolution boundary, to some unimagi-
nable extent,35 during the last decade research has been
dedicated more to force measurements,36 the identifi-
Materiali in tehnologije / Materials and technology 45 (2011) 3, 205–211 205
UDK 678:535.33 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)205(2011)
cation and characterization of processes at the molecular
level37and mainly by the use of force spectroscopy and
chemical force microscopy (CFM).
The latter is, at the moment, the most propelling
within the field of polymer examination on the nano-
scale.38,39 CFM provides the possibility of gaining
knowledge about those interactions appearing between
polymer molecules or polymers, and different surfaces.40
This additional information allows for the prediction of
final material characteristics based on the examined
polymers, even before their finalization. Quantitative
assessment of the involved forces and their extent makes
it easier to choose the correct polymers for achieving
desired interactions between those materials used in
several different interest fields (adhesion, adsorption,
repelling etc.). Multilayer polymeric materials are, at the
moment, first choice materials for the preparation of
modern wound dressings. When sticking together layers
of different polymeric origins, their interaction gains
importance regarding the behavior of the final product.
Methods enabling us to monitor this would greatly
improve any success when choosing the right material
and consequentially, avoid any chemical modification.
Wound healing is a precisely regulated process, which
can be interrupted or even worsened by even small inter-
ventions, thus making derivatives of previously proven
biocompatible polymers dangerous and sometimes even
toxic. The use of CFM for this aspect could decrease the
need for chemical modification and thus improve the
success of those polymers used for wound dressings.
Successful research using AFM is impossible without
the proper tools, so choosing right tips is crucial in this
regard. Many commercial tips are available at the
moment, but only some exhibit characteristics that allow
for easy and repeatable functionalization. Whilst the
functionalization of tips may seem quite simple during
the first iteration, it quickly becomes clear, that a lot of
chemical skills are needed to bind the right species to the
right place.41 As a careful researcher always desires to
know as much as possible about each preparation step, a
lot of statistical evaluation is needed in order to prove
and evaluate the success of any attachment42. During the
course of our research, we found that silicon-based tips
are the most promising in regard to ease of chemical
modification. Their biggest advantage proved to be the
commercial availability of several different silica precur-
sors, which are highly suitable for decorating the AFM
tips with the desired functional groups. Although the
literature shows some direction towards the attachment
of NH2 groups on the surface,43 we attempted to widen
the field of possible attachments by introducing other
functional groups, such as the SH groups or certain
hydrophobic ones like CH3. Gold-coated tips were also
used additionally in order to prove their applicability for
functionalization purposes.
The purpose of our study was to evaluate, test, and
alter available techniques for AFM tip functionalization,
thus enabling its later use for the binding of test
molecules. Different types of silica precursors were used
to prepare those tips with the desired functional groups,
which are known to allow further functionalization at
mild conditions. These are often necessary when soft
molecules, such as polymers or biological macromole-
cules, are the test subjects.
2 EXPERIMENTAL
2.1 Preparation of functionalized AFM tips
2.1.1 Silicon-based tips
Table 1 shows the chemical formulae and short
names for the used silica precursors in this study. A
known procedure was used for the attachment of NH2
groups to the surface, which is described elsewhere.43
Depending on the characteristics of the used precursors,
the initial procedure was modified to suit the attachment
of desired functional groups. The used parameters are
written along with their chemical structures and names,
in Table 1.
2.1.1.1 Attachment of NH2 groups
Firstly APTES (30 μL) triethylamine-TEA (10 μL)
was poured into separate Eppendorf tubes of 1 ml. These
tubes were then placed inside an exiccator, which was
thoroughly cleaned with acetone and ethanol, respec-
tively and blown through using argon gas. Fresh tips
(NP-S10 contact-mode tips, Veeco) were placed beside
the two tubes on a flat surface and then the exiccator was
again mildly blown through with argon gas and sealed.
The precursors and tips were left inside the exiccator for
two hours. Figure 1 shows the scheme of the procedure.
2.1.1.2 Attachment of SH and CH3 groups
This procedure was similar to the previous one but
with slight changes regarding the used volumes and
times of tip exposure to the reagents in the exiccator.
Molecular weight and relative density were the only
parameters taken into account when adjusting the used
volume because the structural differences between the
three used silica precursors appear on the same fragment
of the molecule (whilst all other parts remained the
T. MAVER et al.: FUNCTIONALIZATION OF AFM TIPS FOR USE IN FORCE SPECTROSCOPY ...
206 Materiali in tehnologije / Materials and technology 45 (2011) 3, 205–211
Figure 1: Preparation scheme for the amino functionalized AFM tips
Slika 1: Shema priprave AFM-konic s amino skupinami
same). Vapour pressure had, in our case, the biggest
impact on the relative rate of functional group attach-
ment to the surface and was therefore used to adjust the
time of tips exposure in order to achieve the required
coverage. The used volumes and times for the attach-
ment of SH and CH3 groups were 32 μL, 25 μL and 4 h,
4 h, respectively.
2.1.2 Gold-coated tips
Tips without coating on the reflective side were
chosen for this purpose, in order to avoid any possible
loss of resolution. Functionalization of the tips reflective
side could cause laser refraction, resulting in signal
decrease. Fresh gold-coated tips (ATEC-CONTAu-10,
Nanosensors) were dipped into a mixture of MPTES (20
μL) and EtOH (2 mL). They remained in the solution for
1 h in order to achieve the desired surface coverage.44
Immediately after removal from the mixture, tips were
washed over a few stages, by combined dipping into
water (millie Q) and chloroform (CF), respectively and
then dried using high grade nitrogen (5.0.). Figure2
shows a schematic depiction of this procedure.
2.2 Attachment of polymers to the tip surface
Two approaches were used to show that the
functionalized tips are able to bind additional species to
their decorated surface. The first enabled us to adsorb
two different cellulose derivatives (amylose – AM and
carboxymethyl cellulose – CMC), and the second was
used to covalently bind one of them (CMC) to the tip for
further use. Additionally silicon spheres were bound to
the surface of the functionalized tips (as shown in Figure
2), which is proven to be very useful for examining the
elasticity of films, biological structures, such as cells and
macromolecules.45
2.2.1 Adsorption
Solutions with 10 mmol concentrations were used for
both polymers in order to achieve adsorption on the tips.
Pure tips, out of the box, were gently dipped into the
T. MAVER et al.: FUNCTIONALIZATION OF AFM TIPS FOR USE IN FORCE SPECTROSCOPY ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 205–211 207
Table 1: Used silica precursors and their physic–chemical characteristics
Tabela 1: Uporabljeni silikatni prekurzorji in njihove fizikalno-kemijske lastnosti
SILICA PRECURSOR STRUCTURAL FORMULA
Name MPTES [3-mercaptopropyltriethoxysilane]
/(g/cm3) 0.98
Mr/(g/mol) 238.42
Ttal./°C /
Tvrel./°C 210
Vapour pressure p/hPa 6.66 (at 20 °C)
Name MTES [methyl–triethoxysilane]
/(g/cm3) 0.895
Mr/(g/mol) 178.30
Ttal./°C –40
Tvrel. /°C 142
Vapour pressure p/hPa 6,40 (at 20 °C)
Name APTES [(3-aminopropyl)triethoxysilane]
/(g/cm3) 0.949
Mr 221.37
Ttal./ °C /
Tvrel. /°C 122–123
Vapour pressure p/hPa 13,30 (at 20 °C)
Figure 2: Preparation scheme for gold-coated AFM tips
Slika 2: Shema funkcionalizacije z zlatom oblo`enih AFM-konic
solution using freshly cleaned tweezers (they were
sonicated in acetone for 30 min and dried using clean
nitrogen), and left there for 2 h. Afterwards, tips were
removed from the solution and carefully washed 6 times
using two different solvents (water and chloroform –
CF), alternately.
2.2.2 Covalent binding
The amino decorated tips, prepared as mentioned
previously, were used for the covalent attachment. CMC
was attached to amino-functionalized tips, via amide
bond, using N,N’-carbonyldiimidazole (N-CDI) (CDI
method). In this way, the carboxyl group of CMC was
activated, which was then prone to binding to the amino
species of the tips, under mild conditions. The reaction
took place in a small beaker containing the solution of
the activated CMC in water. Amino-functionalized tips
were placed inside the beaker, and left there for 24 h.
Tips were washed and dried in the same way as pre-
viously mentioned, within the section for the adsorption
of polymers.
2.2.3 Attachment of silicon balls
The same procedure, as shown in Figure 3 was used
for the attachment of silicon balls. First the gold-coated
tips were functionalized using MPTES, as mentioned in
the section Gold-coated tips. These were then gently
dipped in an extremely diluted water suspension of
silicon balls with a pH below 4, which was adjusted
using citric acid, and left there for 2 h to allow for attach-
ment of the balls. Silicon balls were obtained during the
same preparation, procedure as stated elsewhere.46
All the used solvents and other chemicals (unless
stated otherwise) were of analytical grade (high purity),
and bought from Sigma-Aldrich.
2.3 Methods
2.3.1 Scanning electron microscopy
Dried AFM tips for scanning electron microscopy
were pressed on a double-sided adhesive carbon tape
(SPI Supplies, USA). Tips were imaged using field
emission scanning electron microscopy (FE-SEM, Supra
35 VP, Carl Zeiss, Germany) operated at 1 keV.
2.3.2 AFM
Force spectroscopy measurements were performed
using an Agilent 5500 AFM. All measurements were
done in liquid medium (milli–Q water) to avoid capillary
forces. 100 curves were measured during each time scale
at different points on the sample´s surface, and the
outliners eliminated. Some of the measurements were
repeated and the measurements included in the final
results in order to achieve better statistics and prove the
measurements were performed correctly. In all cases, a
model atomically flat silicon surface was used to avoid
any discrepancies due to specific surface characteristics
or various roughness at different spots where force
spectroscopy was performed.
3 RESULTS
3.1 SEM imaging of the functionalized tips
Scanning electron microscopy showed that almost all
functionalization attempts were successful. No clear
differences could be seen between the images of those
functionalized tips with different functional groups.
Images also showed that the functionalization was
mostly confined to the actual end of the tip and not to the
surrounding chip. Figure 3 shows representative images
of the functionalized tips.
3.2 AFM measurements
These measurements were performed to show the
applicability of the proposed functionalization proce-
dures in further investigations regarding interacting poly-
mers species to be used in materials preparation or drug
delivery. All the measured force curves were analyzed
and those maximal forces that the functionalized tips
experienced were compared for chosen tips. Addition-
T. MAVER et al.: FUNCTIONALIZATION OF AFM TIPS FOR USE IN FORCE SPECTROSCOPY ...
208 Materiali in tehnologije / Materials and technology 45 (2011) 3, 205–211
Figure 3: SEM micrographs of functionalized AFM tips: a) decorated
with amino groups, b) decorated with thiol groups, c) with adsorbed
amylose and d) with adsorbed carboxymethyl cellulose
Slika 3: Slike funkcionaliziranih AFM-konic, pridobljene z uporabo
vrsti~ne elektronske mikroskopije: a) s pripetimi amino skupinami, b)
s pripetimi tiolnimi skupinami, c) z adsorbirano amilozo in d) z
adsorbirano karboksimetil celulozo
Table 2: Results of force spectroscopy measurements
Tabela 2: Rezultati spektroskopije sil
Interaction type Average force
[nN]
Average
distance [nm]
Amylose (AM)
Adsorption on pure tips 50 150
Adsorption on hydrophobic
tips 80 200
Carboxymethyl cellulose (CMC)
Adsorption on pure tips 27 90
Covalent bond with amino
functionalized tips 103 300
ally, those distances were analyzed, at which interaction
with the surface disappeared. Table 2 shows the repre-
sentative data as measured and evaluated.
3.2.1 Force curve explanation
A typical force curve is shown in Figure 4. The
explanation is as follows: when approaching the surface
the cantilever is in an equilibrium position (A) and the
curve is flat. As the tip approaches the surface (B), the
cantilever is pushed up to the surface – being deflected
upwards, which is seen as a sharp increase in the
measured force. At (C) the approach half-cycle ends and
the retract half–cycle begins. Once the tip starts
retracting, the deflection starts to decrease and passes its
equillibirum position at (D). As we start moving away
from the surface the tip snaps in due to interaction with
the surface, and the cantilever is deflected downwards
(E). Once the tip-sample interactions are terminated due
to increased distance, the tip snaps out, and returns to its
equilibrium position.
Figure 5 shows representative force curves, and their
comparison when appropriate. It can be seen that the
covalently bound CMC exhibits far greater forces and
ranges when compared to the adsorbed species.
Comparison of the response for the two used polymer
species also reveals that the additional carboxyl groups
in CMC contribute to a greater force and range when
compared to AM. The final comparison shows the
differences between the measured forces and ranges for
the AM bound to two differently functionalized tips,
namely the slightly more hydrophobic ones (additional
CH3 groups on the surface) and the fresh tips without any
functionalization. As the more hydrophobic nature of
AM suggests, the interaction between AM and the
slightly more hydrophobic tips (functionalized with
additional CH3 groups) is greater, which is also reflected
in the increased force involved in the interaction.
4 DISCUSSION
Chemical force microscopy can be a very powerful
tool when used correctly. The most crucial part of
achieving its purpose is the functionalization of the tips,
thus enabling progressive sensing. Its use in polymer
sciences is no exception. It seems that by employing
some alterations to the known functionalization proce-
dures, we are now able to attach different functional
groups to the tip surface, thus providing numerous
possibilities for the further bonding of a wide variety of
different species. All used procedures proved to be
successful for mainly decorating mostly the edge of tips,
leaving the surroundings almost as clean as before the
functionalization. In this way, there is no response by the
AFM feedback system and none of the resolution is lost.
SEM images confirm the latter by showing that the tip’s
surroundings remain almost unchanged, whilst the
surface of the tip exhibits some new features when
compared to the non-functionalized ones (Figure 4). The
introduction of two different polymers to the tip’s
surface proves to be almost equally successful, as no
increase in "chemical waste" can be examined on the
surface surrounding the tip. Attached molecules seem to
concentrate right at the end of the tip’s pyramid, which is
T. MAVER et al.: FUNCTIONALIZATION OF AFM TIPS FOR USE IN FORCE SPECTROSCOPY ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 205–211 209
Figure 5: Representative force curves after tip functionalization. a)
comparison between three different procedures for CMC attachment
to the tip surface and b) comparison of force curves as measured using
two types of amylose attachment. It can be clearly observed that the
type of functionalization has a huge impact on any interaction between
the tip and the model surface.
Slika 5: Vzor~ni primeri spektroskopije sil. a) primerjava treh raz-
li~nih na~inov vezave CMC na povr{ino AFM-konice in b) primerjava
meritev sil med dvema na~inoma pripetja amiloze. Krivulje jasno
nakazujejo, da tip funkcionalizacije drasti~no vpliva na obseg inte-
rakcij med konico in modelno povr{ino.
Figure 4: Example of a simple force curve. The upper is the approach
curve, and the bottom the retract curve. More details regarding this
characteristic feature can be found in the text
Slika 4: Primer krivulje pri spektroskopiji sil. Zgornja predstavlja
krivuljo med pribli`evanjem, medtem ko je spodnja krivulja oddalje-
vanja. Podrobnosti v zvezi s potekom meritve so zapisane v besedilu.
therefore ready to interact with the model surface
(Figures 3c and d).
AFM measurements revealed that, when using
different bonding natures to bind the polymers to the tip,
these changes alter the forces and distances at which they
disappear. In the case of CMC, it can be seen that the
forces involved are a couple of scales bigger when they
are bound to the tip via a covalent bond, as compared to
the preparation procedure, in which they are only
adsorbed. Such knowledge could be of extreme
importance when preparing novel polymer based
materials; as such forces contribute to the materials final
characteristics, such as adhesion properties, sensitivity to
mechanical stress and others.
These measurements also reveal that the used
preparation procedure is applicable for chemical sensing
purposes. We have successfully showed that a different
amount of force is exhibited when using different bond
types and polymers. At the same time, we have
discovered that these forces disappear at different
distances when the tips are pulled away from the model’s
surface. This clear difference between measurements
proves that our preparation procedure is applicable for
chemical sensing, and can be further used during the
characterization process of polymers and polymeric
materials. The used method could be of great benefit to
the public health because polymers are the most used
materials in the preparation of wound dressings.
5 CONCLUSIONS
We have successfully prepared different function-
alized AFM tips, which are not only capable of binding
additional chemical species to their surfaces, but also
allow for their use during chemical sensing. With further
improvement and transfer to those systems found in real
materials, these techniques could make a significant
contribution to the characterization process of polymers
in order to maximize the effectiveness of the preparation
process, especially during the preparation of wound
dressings.
Acknowledgement
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Techno-
logy of the Republic of Slovenia through the contract
No. 3211-10-000057 (Center of Excellence Polymer
Materials and Technologies).
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Materiali in tehnologije / Materials and technology 45 (2011) 3, 205–211 211

V. SEDLAØÍK et al.: A NOVEL APPROACH FOR QUALITATIVE DETERMINATION ...
A NOVEL APPROACH FOR QUALITATIVE
DETERMINATION OF RESIDUAL TIN BASED
CATALYST IN POLY(LACTIC ACID) BY X-RAY
PHOTOELECTRON SPETROSCOPY
NOV NA^IN KVALITATIVNE DOLO^ITVE VSEBNOSTI
KATALIZATORJA KOSITRA V POLILAKTI^NI KISLINI
Z RENTGENSKO FOTOELEKTRONSKO SPEKTROSKOPIJO
Vladimír Sedlaøík1,3*, Alenka Vesel2, Pavel Kucharczyk3, Pavel Urbánek3
1Jo`ef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
2Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, 1000 Ljubljana, Slovenia
3Centre of Polymer Systems, Polymer Centre, Tomas Bata University in Zlín, nám. T. G. Masaryka 5555, 760 01 Zlín, Czech Republic
sedlarik@ft.utb.cz
Prejem rokopisa – received: 2011-02-15; sprejem za objavo – accepted for publication: 2011-03-04
This work is focused on description of qualitative determination of residual tin based catalysts in poly(lactic acid) by using
X-ray photoelectron spectroscopy and fluorimetry as standard technique. Three types of poly(lactic acid) were prepared by
direct melt polycondensation of lactic acid by using methanesulfonic acid (w = 0.5 %) or various amount of stannous
2-ethylhexanoate (w = (0.5 and 2) %). The results from both methods show that the amount of observed tin in the samples
correlates with the concentration of the stannous 2-ethylhexanoate used for polycondensation. X-ray photoelectron spectroscopy
was proven to be suitable for detection of tin residuals in polymer matrix.
Keywords: poly(lactic acid), catalyst residuals, tin, X-ray photoelectron spectroscopy, fluorimetry
Delo je osredinjeno na opis kvalitativne dolo~itve katalizatorjev na osnovi kositra v polilakti~ni kislini z uporabo rentgenske
fotoelektronske spektroskopije in fluorimetrije. Tri vrste polilakti~ne kisline smo pripravili z neposredno polikondenzacijo
lakti~ne kisline z uporabo metansulfonske kisline (w = 0,5 %) ali razli~ne koli~ine kositrovega 2-etilheksanoata (w = (0,5 in
2)%). Rezultati obeh metod ka`ejo, da je vsebnost kositra v vzorcih povezana s koncentracijo kositrovega 2-etilheksanoata, ki se
uporablja za polikondenzacijo. Rentgenska fotoelektronska spektroskopija se je izkazala kot primerna metoda za odkrivanje
majhnih vsebnosti kositra v polimerni matrici
Klju~ne besede: polilakti~na kislina, katalizator, kositer, rentgenska fotoelektronska spektroskopija, fluorimetrija
1 INTRODUCTION
Polymers are widely used for a variety of appli-
cations, which include packaging, adhesives, engineering
materials, composites, and electronic as well as medical
devices. The lastly mentioned application of polymers,
production of medical devices, has gained an enormous
interest in the last decades.1–6
Bioresorbable polymer based items are one of the
most developing areas in the field of medical device
production, due to necessity to introduce a material with
favourable chemical, biological and mechanical proper-
ties, which would also bring a comfort with respect to
traditional materials (metals or ceramics). An elimina-
tion of the second surgery after healing process could be
mentioned as an example of using of an implant made of
bioresorbable polymers.7,8
Poly(lactic acid) (PLA) is well know biodegradable
polyester,9–13 which has been already used in biomedical
applications such as implant for bone fixation and tissue
engineering scaffold. The synthesis techniques and its
modification have been described elsewhere. However,
one can find that mostly organic tin based catalysts are
used for PLA preparations. It has been observed that the
uptake of tin by human beings can cause acute effects
(eye irritation, headache, stomach-ache, urination prob-
lems etc.) as well as long-term effects (depressions, liver
and brain damage, chromosomal changes etc.).14 In
addition, Cam et al. has reported negative influence of
residual metallic compounds (including tin) in the PLA
matrix on a thermal stability of the polymer during its
thermoplastic processing.3 These facts must be consi-
dered during PLA based medical devices evaluation.
Tin can be analytically determined by several
techniques including atomic absorption spectrometry,
inductively coupled plasma atomic emission spectro-
metry, energy-dispersive X-ray fluorescence.15 Never-
theless, these techniques are reported for inorganic
materials. On the other hand, fluorimetry has been
introduced for organic samples. A disadvantage of this
method is a complex procedure of sample preparation.
This works is dedicated to introduction of a novel
approach to tin determination in PLA matrix by using
X-ray photoelectron spectroscopy, which has been
already used for analysis of polymer surfaces. This
approach offers fast and reliable and experimentally
Materiali in tehnologije / Materials and technology 45 (2011) 3, 213–216 213
UDK 546.811:535.33 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)213(2011)
undemanding way of Tin determination in polymer
matrix. This method was correlated with flourimetric
measurements.
2 EXPERIMENTAL
2.1 Polymer samples preparation
Three types of PLA were used for investigation in
this study. The detailed characteristics of the prepared
samples are shown in Table 1. Generally, PLA samples
were prepared by direct melt polycondensation of lactic
acid. A typical procedure was as follows: 50 mL lactic
acid solution (80 %, LachnerNeratovice, Czech Repub-
lic) was added into a double necked flask (250 mL)
equipped with a Teflon stirrer. The flask was then placed
in an oil bath heated by magnetic stirrer with heating and
connected to a laboratory apparatus for distillation under
reduced pressure. The dehydration step followed at 160
°C, reduced pressure 15 kPa for 4 h. After that, the
reactor was disconnected from the vacuum pump and the
relevant amount of the catalyst (Stannous 2-ethyl-
hexanoate (Sn(Oct)2) or methanesulfonic acid (MSA)
both Sigma Aldrich, Steinheim, Germany) was added
dropwise under continuous stirring. The flask with
dehydrated mixture was connected back to the source of
vacuum (100 Pa) and the reaction continued for 24 h at
the temperature 160 °C. The resulting product was
allowed to cool down at room temperature and then
dissolved in acetone. The polymer solution was
precipitated in a mixture of chilled methanol/distilled
water 1 : 1 (v/v). The obtained product was filtrated,
washed with methanol and dried at 45 °C for 48 h. The
average values of molecular weight were determined by
gel permeation chromatography. Detailed procedure is
shown in one of our previous works.9
Table 1: Characteristics of the PLA samples
Tabela 1: Karakteristike vzorcev PLA
Sample
designation Catalyst
Concen-
tration of
catalyst
(w/%)*
Mw/
(g mol–1) Mw/Mn
PLA1 MSA 0.5 17 200 1.60
PLA2 Sn(Oct)2 0.5 47 000 2.08
PLA3 Sn(Oct)2 2 67 700 2.06
*related to monomer
2.2 Characterization techniques
2.2.1 X-ray photoelectron spectroscopy (XPS)
Samples in powder form were analyzed with an XPS
instrument TFA XPS Physical Electronics. The base
pressure in the chamber was about 6 × 10-8 Pa. The
samples were excited with X-rays over a 400-μm spot
area with a monochromatic Al K?1,2 radiation at 1486.6
eV. The photoelectrons were detected with a hemisphe-
rical analyzer positioned at an angle of 45° with respect
to the normal to the sample surface. Survey-scan spectra
were made at a pass energy of 187.85 eV and 0.4 eV
energy step. An electron gun was used for surface
neutralization. The concentration of elements was deter-
mined by using MultiPak v7.3.1 software from Physical
Electronics, which was supplied with the spectrometer.
On each sample two survey spectra were recorder on two
different spots on the surface. The sensitivity of the
method is 0.5 %.
2.2.2 Fluorimetric analysis
Fluorimeter FSL 920 Edinburg was used for
fluorescence spectra and intensity measurements. The
device was equipped with a xenon lamp. A quartz cell
was used for the measurement (10 mm). A uniform
resolution of 1 nm was kept in all cases. The procedure
of samples treatment before the analyte determination is
described in detail by Mazooriet al.15
3 RESULTS AND DISCUSSION
As can be noticed in Table 1, the expected tin con-
centration in the investigated samples has following
trend: PLA1< PLA2 < PLA3. Moreover, it can be
expected that tin concentration is equal zero in PLA1.
A method which can be used for detecting small
amounts of tin in PLA samples is XPS (X-ray photo-
electron spectroscopy).16–28 The technique is particularly
useful for characterization of polymer materials whose
surface properties have been modified using non-equili-
brium plasma treatments29–45Figure 1 shows XPS survey
spectra of the PLA samples. The measured spectrum
consists of characteristic peaks which correspond to
electronic energy levels of the investigated material.
Because each element has a unique set of binding
energies, XPS can be used to identify and determine the
concentration of the elements at the surface. XPS survey
spectra shown in Figure 1 represent characteristic peaks
due to the presence of oxygen (O1s), carbon (C1s) and
V. SEDLAØÍK et al.: A NOVEL APPROACH FOR QUALITATIVE DETERMINATION ...
214 Materiali in tehnologije / Materials and technology 45 (2011) 3, 213–216
Figure 1: XPS survey spectra of PLA samples with tin additives
Slika 1: Pregledni spekter XPS povr{ine vzorca PLA z dodatkom
kositra
tin (Sn3d) elements in the examined samples. Carbon
and oxygen are constituents of PLA polymer while a
small peak of tin found in the samples PLA2 and PLA3
clearly shows the presence of the small amounts of
catalyst.
Surface composition of the samples as determined
from the XPS survey spectra (Figure 1) is shown in
Table 2. Surface composition was determined from the
area of the measured peaks which was correlated by
taking into account special sensitivity factors, which are
due to different ionization cross section. It can be noticed
that no tin evidence is observed in the case of PLA1
where only carbon and oxygen was detected. On the
other hand PLA2 proves a trace of tin presence in the
polymer matrix. Interestingly, almost four times higher
values were found for PLA3. It means that this
observation directly correspond to mass of the added
catalyst during PLA preparation (Table 1).
Table 2: Surface composition of the samples (in mole fractions, x/%)
as determined from the XPS survey spectra (average ± standard
deviation)
Tabela 2: Povr{inska sestava vzorcev (v molskih dele`ih, x/%) dolo-
~ena iz preglednih spektrov XPS
Sample designation x(C)/% x(O)/% x(Sn)/%
PLA1 62.7 ± 0.8 37.4 ± 0.8 0.00 ± 0.00
PLA2 61.0 ± 0.1 38.8 ± 0.2 0.23 ± 0.05
PLA3 61.1 ± 1.4 37.8 ± 1.8 1.09 ± 0.36
The emission spectra of PLA samples show noti-
ceable peak at 357 nm in all cases (Figure 2). The
maximal intensities of the observed peaks for PLA
samples are presented in Figure 2a. It can be noticed
that PLA1 emits at the given wavelength as well as tin
containing samples PLA2 and PLA3. However,
differences between the observed detector responses for
PLA2 and PLA3 after subtraction of the signal (Figure
2b) of PLA1 is related with initial concentration of the
catalyst used for PLA polycondensation (Table 1) by
means of the intensities ratio PLA3/PLA2.
The results presented above reveal that both XPS and
flourimetric method show similar observations in
qualitative detection of tin based compounds in PLA
matrix. The former method, however, consider only
surface composition of a sample. That can be a limitation
of the method. Practically, it brings high requirements
for samples homogeneity. On the other hand, undisputed
advantage of XPS technique is low demand for sample
mass, which is important in the small scale productions
such as medical grade PLA synthesis.
4 CONCLUSIONS
This study introduces a novel way of residual tin
based catalyst determination in poly(lactic acid) by using
X-ray photoelectron spectroscopy (XPS). In addition,
XPS observations were correlated with fluorimetry. The
results from both methods correspond with the
concentration of tin based catalyst used for polymer
preparation. XPS techniques can be considered as
suitable for qualitative or even semi-quantitative analysis
of tin based compounds in polymer matrixes providing
achieving of sufficient homogeneity of the studied
material.
Acknowledgements
This research was financially supported by the
Ministry of Education, Youth and Sports of the Czech
Republic (project No. 2B08071) and Operational
Program Research and Development for Innovations
co-funded by the European Regional Development Fund
(ERDF, project No. CZ.1.05/2.1.00/03.0111). The
authors acknowledge the financial support from the
Ministry of Higher Education, Science and Technology
of the Republic of Slovenia through the contract No.
3211-10-000057 (Center of Excellence for Polymer
Materials and Technologies). Furthermore, this work was
also co-supported Slovenian Research Agency (ARRS)
and Science and Education Foundation of the Republic
of Slovenia within the "Ad futura" programme and by
the Ministry of Higher Education, Science and
Technology of the Republic of Slovenia through the
contract No. 3211-10-000057 (Center of Excellence
Polymer Materials and Technologies).
V. SEDLAØÍK et al.: A NOVEL APPROACH FOR QUALITATIVE DETERMINATION ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 213–216 215
Figure 2: Fluorescence emission of PLA samples, values of maximal
intensities observed for PLA samples at 257 nm (a) and intensities of
polycondensates catalyzed by stannous 2-ethylhexanoate after
subtraction (b)
Slika 2: Fluorescen~na emisija vzorcev PLA: vrednosti maksimalne
intenzitete, dolo~ene na vzorcu PLA pri 257 nm (a) in intenziteta
polikondenzatov kataliziranih s kositrovim 2-etilheksanoatom (b)
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A. VESEL: HYDROPHOBIZATION OF POLYMER POLYSTYRENE IN FLUORINE PLASMA
HYDROPHOBIZATION OF POLYMER POLYSTYRENE IN
FLUORINE PLASMA
HIDROFOBIZACIJA POLIMERA POLISTIREN S FLUOROVO
PLAZMO
Alenka Vesel
Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, 1000 Ljubljana, Slovenia
alenka.vesel@ijs.si
Prejem rokopisa – received: 2011-02-10; sprejem za objavo – accepted for publication: 2011-03-05
Modification of surface properties of polymers by plasma fluorination was studied by X–ray Photoelectron Spectroscopy (XPS).
Samples of polystyrene (PS) were exposed to weakly ionized reactive gaseous plasma created in tetrafluormethane (CF4) gas.
Reactive plasma particles rapidly interacted with the samples forming a surface film rich with CFx functional groups. The
appearance of these groups was monitored by high-resolution XPS, while the resultant modification of surface properties was
monitored by water drop contact angle technique. Survey XPS spectra showed a substantial concentration of over mole fraction
50 % of fluorine, while high resolution C1s spectra revealed the appearance of different functional groups. The CF2 group was
predominant as approximately a third of carbon atoms was found in this group. The contact angle measurement revealed
simultaneous modification of the surface free energy from moderately hydrophobic original material to highly hydrophobic
state, as the water contact angle increased from original 85° to 111°. Plasma fluorination is therefore a suitable method for
improving of surface properties of polymers for biomedical applications.
Keywords: CF4 plasma, fluorination, PS polymer, polystyrene, surface functionalization, hydrophobization, XPS
Z metodo rentgenske fotoelektronske spektroskopije (XPS) smo preiskovali spremembe v povr{inski sestavi polimerov pri
plazemski fluorinaciji. Vzorce polistirena (PS) smo izpostavili {ibko ionizirani reaktivni plinski plazmi, ustvarjeni v
fluorometanu (CF4). Reaktivni delci iz plazme so izredno hitro reagirali s povr{ino vzorca, zaradi ~esar je nastala tanka
povr{inska plast, bogata s CFx-skupinami, kar je povzro~ilo spremembo omo~ljivosti vzorca. Nastanek teh skupin smo dokazali
z metodo XPS, medtem ko smo spremembo omo~ljivosti dolo~ali s kontaktnim kotom vodne kapljice. Iz preglednih XPS
spektrov je razvidna znatna koncentracija fluora na povr{ini vzorca z molskim dele`em ve~jim od 50 %. Iz visoko lo~ljivih
spektrov ogljika smo dolo~ili razli~ne fluorove funkcionalne skupine, med katerimi je najve~ji dele`, skoraj eno tretjino,
zavzemala skupina CF2. Z merjenjem kontaktnega kota smo ugotovili porast hidrofobnosti povr{ine, saj je kontaktni kot narasel
iz za~etnih 85° na 111°. Rezultati so pokazali, da je plazemska fluorinacija primerna metoda za izbolj{anje povr{inskih lastnosti
polimerov za biomedicinske aplikacije, kjer `elimo imeti ~im bolj inertno povr{ino.
Klju~ne besede: CF4-plazma, fluorinacija, PS-polimer, polistiren, povr{inska funkcionalizacija, hidrofobizacija, XPS
1 INTRODUCTION
Polymer materials are nowadays widely used in
industry and medicine. The type of polymer suitable for
particular application is chosen according to chemical,
mechanical and thermal properties, as well as the pro-
duction cost. The surface properties of selected polymers
are often not optimal for particular application. In such
cases they should be modified prior to application.
Improved hydrophilicity is usually obtained by surface
functionalization with polar functional groups, and better
hydrophobicity is assured by incorporation of highly
non-polar groups. Numerous techniques have been
invented for modification of surface properties but it
seems that nowadays one technique predominates. The
technique is mild treatment by non-equilibrium gaseous
plasma1–11. Namely, this technique is extremely fast,
ecologically benign and rather inexpensive. Increased
hydrophilicity is obtained by treatment with oxygen (and
sometimes nitrogen) plasma. Numerous authors reported
excellent results for several types of polymers including
polyethylene terephthalate12–14, polyethersulphone15–16,
polyphenylene sulphide17, polystyrene18,19, polyvinyl-
chloride20–22, polyethylene naphthalate23, polymethyl
methacrylate24, cellulose25–29 etc. The opposite effect, i.e.
increased hydrophobisation, is achieved by treatment
with gases rich in fluorine such as CF4 or C2F6.30–34
Hydrophobisation of polymers is required in cases
where adhesion of liquids should be minimized. A
classical example is medicine. The wounds are protected
by plasters. Both for the medical reasons as well as
patient comfort it is important that the plaster does not
stick to the wounded place too strongly. Although the
major part of the plaster made from cellulose to soak the
liquids35–39 the uppermost layer is made from a hydro-
phobic polymer, typically in the form of perforated
membrane, which allows for a good transport of liquids
and simultaneously for poor adhesion onto the wounded
place. Typical examples of currently applied membrane
include PET, PES and PP. In some cases the surface
properties of these materials assure a good functionality,
but in many cases they fail and the plaster stick on to a
wound. Obviously, a material with better properties
should be applied or, alternatively, the surface of such
materials should be made more hydrophobic.
Materiali in tehnologije / Materials and technology 45 (2011) 3, 217–220 217
UDK 678:542.428.8:546.16 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)217(2011)
As mentioned earlier, a suitable method for hydro-
phobization of polymer materials is application of
gaseous plasma. In the present paper we address the
hydrophobization of polystyrene by treatment with mild
plasma created in tetraflour methane.
2 EXPERIMENTAL
2.1 Plasma treatment of polymer
Experiments were performed with a polystyrene (PS)
foil from DuPont. The samples were treated in the
experimental system which was pumped with a two-
stage oil rotary pump with a pumping speed of 4.4 · 10–3
m3 s–1. The discharge chamber was a Pyrex cylinder with
a length of 0.6 m and an inner diameter of 0.036 m. The
plasma was created with an inductively coupled RF
generator, operating at a frequency of 27.12 MHz and an
output power of about 200 W. Commercially available
CF4 gas was leaked into the discharge chamber. The
pressure was measured by an absolute vacuum gauge. At
our experiments, the pressure was fixed at 75 Pa. The
samples of PS foil were treated in CF4 plasma for 3 s and
10 s.
Since it is known that polymer surface which was
treated in plasma is not stable with time, we have
performed also ageing studies. After plasma treatment
one of the samples which was treated for 3 s was stored
in a dry plastic box and left ageing for 4 d.
2.2 X-ray photoelectron spectroscopy (XPS) characte-
rization
The surface of the plasma treated PET samples was
analyzed with an XPS instrument TFA XPS Physical
Electronics. The base pressure in the XPS analysis
chamber was about 6 × 10–8 Pa. The samples were
excited with X-rays over a 400–μm spot area with a
monochromatic Al Kα1,2 radiation at 1486.6 eV. The
photoelectrons were detected with a hemispherical
analyzer positioned at an angle of 45° with respect to the
normal to the sample surface. Survey-scan spectra were
made at a pass energy of 187.85 eV a 0.4 eV energy step,
while for C1s individual high-resolution spectra were
taken at a pass energy of 23.5 eV and a 0.1 eV energy
step. Since the samples are insulators, we used an addi-
tional electron gun to allow for surface neutralization
during the measurements. The spectra were fitted using
MultiPak v7.3.1 software from Physical Electronics,
which was supplied with the spectrometer. The curves
were fitted with symmetrical Gauss-Lorentz functions. A
Shirley-type background subtraction was used. Both the
relative peak positions and the relative peak widths
(FWHM) were fixed in the curve fitting process.
2.3 Water-contact angle measurements
Wettability was examined immediately after the
plasma treatment by measuring the water contact angle
with a demineralised water droplet of a volume of 3 μL.
A home made apparatus equipped with a CCD camera
and a PC computer was used for taking high resolution
pictures of a water drop on the sample surface. Each
determination was obtained by averaging results of 5
measurements. The relative humidity (45 %) and tempe-
rature (25 °C) were monitored continuously and were
found not to vary significantly during the contact angle
measurements. The contact angles were measured by our
own software which enables fitting of the water drop on
the surface in order to allow a relatively precise deter-
mination of the contact angle.
3 RESULTS AND DISCUSSION
Immediately after the plasma treatment the surface
wettability of the samples was measured by water drop.
The measurements showed a decrease of surface wettabi-
lity since the contact angle has increased from 85° for
untreated PS sample to about 110° for plasma treated
sample. Such contact angle is often found on polymers
treated in CF4 plasma and was found also for PET
polymer. A well pronounced increase of the contact
angle is a good sign for surface hydrophobization which
is due to formation of unpolar fluorine functional groups
at the surface. This was proved by XPS measurements.
Figure 1 shows the XPS survey spectrum of un-
treated and treated PS sample. Since PS is hydrocarbon
polymer we can observe only one peak due to carbon on
untreated sample (hydrogen can not be detected by
A. VESEL: HYDROPHOBIZATION OF POLYMER POLYSTYRENE IN FLUORINE PLASMA
218 Materiali in tehnologije / Materials and technology 45 (2011) 3, 217–220
Figure 1: XPS survey spectra of untreated and plasma-treated PS
showing attachment of fluorine to the surface
Slika 1: Pregledni XPS-spekter neobdelanega in plazemsko obdela-
nega vzorca PS
Table 1: Surface composition of the plasma treated PS samples (atom
fractions, x/%)
Tabela 1: Povr{inska sestava plazemsko obdelanih vzorcev PS (atom-
ski dele`i, x/%)
Sample C F O F/C O/C
Untreated PS 100
Treated in CF4 plasma for 3 s 43.0 56.5 0.5 1.31 0.012
Treated in CF4 plasma for 10 s 43.4 56.1 0.5 1.29 0.012
Treated 3 s and aged 1 d 43.0 56.3 0.7 1.31 0.016
Treated 3 s and aged 4 d 42.6 56.0 1.4 1.31 0.033
XPS). After plasma treatment a new huge peak is
observed which corresponds to fluorine. Corresponding
surface composition is shown in Table 1 and Figure 2.
The concentration of fluorine at the surface is rather high
more than 50 atomic %. The surface is well saturated
with fluorine atoms since there is no difference between
3 s and 10 s of treatment. Small amount of oxygen which
is also detected on plasma treated surface is due to the
presence of water vapour in the vacuum system.
Incorporation of fluorine to the polymer surface caused
formation of different functional groups, which can be
deduced from high resolution spectrum of carbon. In
Figure 3 is shown a comparison of carbon peaks before
and after plasma treatment. Untreated sample has only
one big peak due to C-C (from aliphatic chain) and
–C=C bonds (from phenyl ring). Another small peak can
be observed at high binding energies at about 291 eV
which is a satellite peak that is typical for aromatic
polymers. A huge difference is observed in the shape of
the carbon peak of the treated samples indicating
formation of different functional groups at the surface.
Functional groups can be determined by decomposition
of the measured peak into subpeaks which are shown in
Figure 4. Figure 4 shows that incorporation of fluorine
resulted in formation of different functional groups at the
surface which can be attributed to CF3, CF2, CF and CHF
groups30. The concentration of these groups is presented
in Table 2. The majority of fluorine is found in CF2
groups.
Here we should also mention, the fluorine-containing
polymers may be sensitive to x–ray degradation during
recording of spectra. In our case we did not observe any
degradation (modification of the shape of carbon spectra)
with time, since exposure times were low.
The ageing of the samples was studied as well. The
surface composition was measured for samples aged one
day and four days. As shown in Table 1 it was not
possible to observe any trend of ageing during first four
days since the concentration of fluorine is stable.
Normally the plasma treated samples are ageing very
fast18, especially the first day because we can observe
remarkable changes in water contact angle on samples
treated in oxygen plasma. Here the contact angle was
stable within these four days. An explanation for this is
the following: polymers treated in oxygen plasma which
is used to make surface hydrophilic have high surface
energy, which is energetically unfavourable. Therefore
such surface tends to decrease an excess of surface
energy which leads to surface rearrangement and ageing.
A. VESEL: HYDROPHOBIZATION OF POLYMER POLYSTYRENE IN FLUORINE PLASMA
Materiali in tehnologije / Materials and technology 45 (2011) 3, 217–220 219
Figure 4: Example of curve fitting of C1s peak of a PS sample treated
for 3 s showing formation of new peaks due to formation of different
carbon-fluorine functional groups, which are presented in Table 2
Slika 4: Primer spektra ogljika C1s vzorca PS obdelanega 3 s, v
katerem smo z metodo prilagajanja krivulj dolo~ili podvrhove, ki
pripadajo razli~nim fluorovim funkcionalnim skupinam, nastalih med
obdelavo (glej tudi tabelo 2)
Table 2: Chemical binding of carbon atoms of plasma treated PS
samples.
Tabela 2: Vezava atomov ogljika na plazemsko obdelanih vzorcih PS
Peak assignment C1 C2 C3 C4 C5 C6
Binding energy
(eV) 284.8 286.5 287.8 289.2 291.2 293.2
Chemical binding C-C CH2-CHF CHF CF CF2 CF3
Untreated 100 %
Treated 3 s 23.8 9.2 6.2 19.8 33.0 8.0
Treated 10 s 22.7 9.0 5.6 21.9 33.4 7.4
Treated 3 s and
aged 1 d 22.9 7.6 5.7 21.6 34.3 8.0
Treated 3 s and
aged 4 d 24.8 7.6 6.1 20.5 32.8 8.1
Figure 2: Comparison of surface composition of the PS samples (see
also Table 1)
Slika 2: Primerjava povr{inske sestave vzorcev PS (glej tudi tabelo 1)
Figure 3: Comparison of carbon C1s spectra of untreated and
plasma-treated PS samples
Slika 3: Primerjava spektrov ogljika C1s neobdelanega in plazemsko
obdelanega vzorca PS
During hydrophobization with CF4 plasma treatment the
surface energy is lowered. Surfaces having low surface
energy are more stable than the one with high surface
energy.
During treatment of samples in plasma we may either
have substitution of fluorine atoms to the surface or
deposition of CxFy radicals. Therefore the formation of
fluorine functional groups needs further discussion. CF4
molecules are dissociated in plasma into CF3, CF2, CF
molecules and F atoms. In our case we have only
substitution of fluorine atoms to the surface, because
deposition is normally observed when using gas with
higher carbon content and bigger molecular mass (e.g.
C2F6, C3F8). It was observed for PET (polyethylene
terephthalate) polymer which was treated at the same
conditions, that the mass of the sample was little lower
after the treatment. This was due to etching and
definitely not due to deposition.
4 CONCLUSION
The results presented in this paper clearly indicate
that treatment of PS with CF4 plasma improves the
hydrophobic character of this polymer. Even a brief
exposure allowed for functionalization of the surface
film with fluorine rich functional groups. High resolution
XPS C1s spectra revealed formation of different fluorine
groups including CHF, CF, CF2, CF3. The majority of
carbon atoms are bounded in the CF2 group as in
polytetraflourethylene (PTFE), but the XPS results also
revealed about 8 percent of fluorine richest CF3 groups.
All this groups allow for a stable hydrophobization of the
PS polymer, since the concentration did not decrease
even after prolonged ageing. The technology is therefore
suitable for application in medicine for improving of the
medical plaster quality.
ACKNOWLEDGEMENT
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Tech-
nology of the Republic of Slovenia through the contract
No. 3211-10-000057 (Center of Excellence Polymer
Materials and Technologies).
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A. VESEL: HYDROPHOBIZATION OF POLYMER POLYSTYRENE IN FLUORINE PLASMA
220 Materiali in tehnologije / Materials and technology 45 (2011) 3, 217–220
I. JUNKAR et al.: PLASMA TREATMENT OF BIOMEDICAL MATERIALS
PLASMA TREATMENT OF BIOMEDICAL MATERIALS
PLAZEMSKA OBDELAVA BIOMEDICINSKIH MATERIALOV
Ita Junkar1, Uro{ Cvelbar2, Marian Lehocky3
1Odsek za tehnologijo povr{in in optoelektroniko, Institut "Jo`ef Stefan", Jamova cesta 39, 1000 Ljubljana, Slovenija
2Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, 1000 Ljubljana, Slovenia.
3Medical Materials Research Centre, Technology Park, Tomas Bata University, Nad Ovcirnou 3685, Zlin, Czech Republic
ita.junkar@ijs.si
Prejem rokopisa – received: 2011-02-15; sprejem za objavo – accepted for publication: 201103-06
Surface plasma treatment techniques for modification of biomedical polymeric materials are presented. The emphasis is on the
use of non-equilibrium radiofrequency (RF) oxygen and nitrogen plasma. By variation of discharge parameters (power,
discharge frequency, type of gas) and plasma parameters (density of neutrals and ions, kinetic energy of electrons, gas
temperature) it is possible to produce polymer surfaces with different surface properties. Already after short plasma treatment
time the surface of polymeric material becomes hydrophilic. Formation of nitrogen and oxygen functional groups is observed
immediately after plasma treatment. By optimisation of plasma treatment time the number of newly formed functional groups
can be increased. Plasma treatment also produces morphological changes of the surface; nanohills of different shapes and height
can be formed on PET surface depending on the treatment time and type of gas. Evidently the change in surface morphology
affects the change in surface roughness, which increases with longer plasma treatment time. Plasma treatment influences also on
the biological response, as all plasma treated surfaces exhibit improved proliferation of fibroblast and endothelia cells. The
number of adherent platelets practically does not change after nitrogen plasma treatment, however much lower number of
adherent platelets is observed on oxygen plasma treated surfaces.
Key words: plasma treatment, biocompatibility, polymer, vascular grafts, endothelia cells, platelets
Predstavljene so tehnike plazemske obdelave povr{in, s katerimi lahko modificiramo povr{ine biomedicinskih polimernih
materialov. Poudarek je na uporabi neravnovesne radiofrekven~ne (RF) plazme du{ika in kisika. S spreminjanjem
razelektritvenih (mo~, razelektritvena frekvenca, vrsta plina) in plazemskih parametrov (gostota nevtralnih delcev, ionov,
kineti~ne energije elektronov, temperature plina) je mogo~e pripraviti povr{ine polimerov z razli~nimi lastnostmi. @e po kratkih
~asih izpostavitve polimernih povr{in du{ikovi ali kisikov plazmi le-ta postane hidrofilna. Takoj po obdelavi je na povr{ini
mogo~e opaziti novonastale du{ikove oziroma kisikove funkcionalne skupine. Z optimizacijo ~asa izpostavitve plazmi je
mogo~e koncentracijo le-teh {e nekoliko pove~ati. Plazemska obdelava vpliva tudi na spremembe v morfolo{kih lastnostih
povr{ine, tako je mogo~e na plazemsko obdelanih povr{inah opaziti nanostrukture, katerih oblika in velikost je odvisna od ~asa
izpostavitve plazmi, kot tudi od vrste plina uporabljenega za plazmo. Morfolo{ke spremembe vplivajo tudi na spremembe v
hrapavosti povr{ine, ki se pove~a s ~asom plazemske obdelave. Modifikacija povr{ine vpliva na biolo{ki odziv, saj se po
plazemski obdelavi proliferacija endotelijskih in fibroblastnih celic na povr{ini pove~a. [tevilo adheriranih trombocitov na
povr{inah, obdelanih z du{ikovo plazmo, se bistveno ne spremeni, medtem ko se njihovo {tevilo bistveno zmanj{a na povr{inah,
obdelanih s kisikovo plazmo.
Klju~ne besede: plazemska obdelava, biokompatibilnost, polimer, umetne `ile, endotelijske celice, trombociti
1 INTRODUCTION
Surface properties of biomaterials play a major role
in determining biocompatibility; they have a significant
influence on biological response and also determine the
long term performance in vivo. The main goal in
designing biomaterials is therefore to ensure that they
exhibit appropriate surface properties as well as desired
physical and mechanical characteristics, which would
enable them to function properly in the biological envi-
ronment. It is hard to satisfy all of these characteristics;
this is why surface treatment techniques are commonly
employed in order to improve surface properties. It is
still a highly challenging task to modify surface
properties in order to produce hemocompatible surfaces,
and many controversial results are reported in the
literature.
Biological response to biomaterials is very complex
and still not fully understood. As the surface of the
biomaterial is responsible for initiating the primary
interaction with body fluids it is of vital importance to
ensure that the surface is suitably conditioned to ensure
an appropriate biological response (biocompatibility). It
was thought for many years that the surface of the
biomaterial should be inert. However, nowadays it has
been found that the contact of biomaterials with blood
enables integration with the body, prevents infections,
inflammatory reactions, blood coagulation and other
correlated reactions. It is of primary importance that the
surfaces of hemocompatible materials exhibit anti-
thrombogenic properties, as this prevents thrombosis.
Thrombosis is initiated with the adsorption of blood
plasma proteins on the surface of the biomaterial and is
strongly influenced by its physical and chemical
properties.
Surface properties of implants are usually described
with wettability, chemistry, surface charge and texture
(roughness). These factors all influence the sequence of
protein adsorption and subsequent platelet adhesion/
thrombus formation. Although the mechanism of
occlusion and dysfunction of artificial prostheses is
Materiali in tehnologije / Materials and technology 45 (2011) 3, 221–226 221
UDK 533.9:678 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)221(2011)
multifactorial, all the studies performed suggest that
fibrinogen and platelet deposition play a predominant
role.1,2 It also seems that the outermost atomic layer of
the surface of an alloplastic implant is a decisive factor
for determining biocompatibility.3 One possible method
to alter surface characteristics, such as wettability, che-
mistry, charge and morphology to improve biocompa-
tibility of implant devices4 is by treatment of the surface
with different gaseous plasma, like glow discharge
created in different gases and by variation of discharge
parameters (discharge power, pressure, etc.),5 which in
turn influence plasma parameters (density of atoms,
energy of plasma particles, etc.). Plasma modification
has been used recently to enhance biocompatibility of
implant devices made from stainless steel, titanium and
various polymers.6–14 The unique advantage of plasma
modification of implant devices is that the surface can be
modified without altering the bulk properties of the
material.15 It is thus possible to obtain desired mecha-
nical and physical properties of implant material and at
the same time also improve its surface properties to
accomplish biocompatibility.
2 SURFACE MODIFICATION BY PLASMA
TREATMENT
Combined surface treatments incorporating photons,
ions and electrons and some other excited particles, are
found in gas-electric discharges, often denominated
plasmas. Ionised gas is usually called plasma when it is
electrically neutral (i.e., electron density is balanced by
that of positive ions) and contains a significant number
of electrically charged particles, which is sufficient to
affect its electrical properties and behaviour.16 Therefore
plasma is composed of highly excited atomic, molecular,
ionic, and other native radical species. It is typically
obtained when gases are excited into energetic states by
radio-frequency (RF), microwave, or electrons from a
hot filament discharge. To produce plasma, electron
separation from atoms or molecules in gas state, or
ionization is required. When an atom or a molecule gains
enough energy from an outside excitation source or via
interaction (collisions) with one another, ionization
occurs.17
Plasmas are divided into thermodynamically balanced
and unbalanced. Plasma characteristics are dependent
upon the electrical discharge type, the type of gas or
gaseous mixture, and the pressure. Thermodynamically
balanced plasmas are characterized by very high
temperatures of heavy particles (often about 10 000 K).
These types of plasmas are not suitable for the treatment
of polymeric materials, as the gas temperature is so high
as to cause their thermal degradation. While in thermo-
dynamically unbalanced plasma the gas temperature is
significantly lower, as they are composed of low tempe-
rature heavy particles (charged and neutral molecular
and atomic species) and very high temperature electrons
(often about 50 000K). This type of plasmas are are
suited for the treatment of delicate polymeric pro-
ducts18–32 such as PET, PDMS or PTFE, which are used
for biomedical applications.
Clark and Hutton33 showed that with hydrogen
plasma they can rapidly defluorinate fluoropolymers to a
depth of 2 nm. On the other hand it was reported that
plasma treatment with oxygen increased endothelia cell
attachment on vascular grafts made of PTFE.34 Compa-
rison of plasma treatment with oxygen, nitrogen and gas
mixture of nitrogen and oxygen, where conducted by M.
Chen et al., where it has been shown that the mixture of
gases uniquely modifies PTFE surfaces and reduces
levels of inflammatory cells.5 Surfaces having incor-
porated nitrogen were more effective than those of
oxygen containing functional groups in promoting cell
adhesion.35 Though, appropriate surface modification is
not only a function of working gas, but also of other
discharge parameters, such as pressure, type of gas,
power etc. Chevallier et. al. showed that nitrogen plasma
treatment of PTFE at low-power (10 W) experimental
conditions exhibits more alkene and less amino groups
formed on the surface than a higher-power plasma
treatment (20 W). Consequently, surface chemistry could
be modulated through appropriate selection of discharge
parameters.36
In our investigation surface properties of polymeric
biomedical implants have been tailored by RF oxygen
and nitrogen plasma treatment. The PET polymeric
implants were treated in the experimental system shown
in Figure 1. The plasma was created with an inductively
coupled RF generator, operating at a frequency of 27.12
MHz and an output power of about 200 W. The plasma
parameters were measured with a double Langmuir
probe and a catalytic probe36–44. In our experiments, the
pressure was fixed at 75 Pa, as at this pressure the
highest degree of dissociation of molecules, as measured
by the catalytic probes, was obtained. At these discharge
parameters, plasma with an ion density of about 2 · 1015
I. JUNKAR et al.: PLASMA TREATMENT OF BIOMEDICAL MATERIALS
222 Materiali in tehnologije / Materials and technology 45 (2011) 3, 221–226
Figure 1: The RF plasma reactor chamber with the sample in position
Slika 1: Cev RF plazemskega reaktorja z vzorcem
m–3, an electron temperature of 4 eV, and neutral atoms
density of about 4 · 1021 m–3 for oxygen plasma and
about 1 · 1021 m–3 for nitrogen plasma was obtained.
After plasma treatment surface chemistry, wettability and
topography was altered.
Change in chemical composition was determined
from (x-ray photolelectron spectroscopy) XPS. It has
been shown that newly formed oxygen, or nitrogen
functional groups are formed on the surface, depending
on the type of gas used for modification. Already after
short exposure to oxygen plasma the increase in oxygen
concentration from initial mole fraction 21 % to 39 %
was observed. With prolonged exposure to oxygen
plasma the concentration slowly increased, and at 90 s it
reached 44 %. On nitrogen plasma treated samples
nitrogen concentration increased from 0 % to 12 % after
3 s of treatment time, and after 90 s of treatment it
reached about 14 %. During nitrogen plasma treatment a
small increase in oxygen concentration was also ob-
served.
Wettability was examined immediately after the
plasma treatment by measuring the water contact angle
with a demineralised water droplet of a volume of 3 μL.
The relative humidity (45 %) and temperature (25 °C)
were monitored continuously and were found not to vary
significantly during the contact angle measurements.
Contact angle measurements show a decrease in contact
angles after oxygen and nitrogen plasma treatment
Figure 2, corresponding to a higher hydrophilicity of the
polymer surface. During the water contact angle
measurements room temperature was 21 °C and The
oxygen plasma treated samples exhibit lower values of
contact angles, and thus demonstrating that this treat-
ment provides a higher hydrophilic character. Even after
short exposure times the surfaces show an increased
hydrophilicity, regardless of the type of gas used. The
treatment with nitrogen plasma, however, seems to be
less efficient in reaching high hydrophilicity. However
the high hydrophilycity of oxygen plasma treated sur-
faces could be attributed to degradation products, which
are formed on the surface after longer treatment times
and could cause a lower contact angle, due to surface
roughening.
The morphology of untreated and plasma treated
surfaces was analyzed by atomic force microscopy
(AFM) and scanning electron microscopy (SEM). In
both cases change in surface morphology was observed.
In Figure 3 phase AFM images are shown. In Figure 3 a
surface of untreated sample is shown, while in Figure 3
b and c change in surface morphology after treatment in
nitrogen and oxygen plasma can be observed, respec-
tively. Untreated sample has smooth surface, without any
particular features on the surface, while treated samples
exhibit small nanostructures on its surface. The diffe-
I. JUNKAR et al.: PLASMA TREATMENT OF BIOMEDICAL MATERIALS
Materiali in tehnologije / Materials and technology 45 (2011) 3, 221–226 223
Figure 3: Phase AFM images of PET polymer; a) untreated, b) treated
for 90 s in nitrogen plasma and c) treated for 90 s in oxygen plasma
Slika 3: Fazne AFM-slike PET polimera; a) neobdelanega, b) obde-
lanega 90 s v du{ikovi plazmi in c) obdelanega 90 s v kisikovi plazmi
Figure 2: Water contact angle measured on the PET polymer as a
function of treatment time and plasma gas, () oxygen plasma
treatment, () nitrogen plasma treatment
Slika 2: Kontaktni kot vodne kapljice, izmerjen na povr{ini PET
polimera, v odvisnosti od ~asa obdelave in vrste plina, () obdelava v
kisikovi plazmi, () obdelava v du{ikovi plazmi
rence between the samples treated with oxygen (Figure
3 a) and nitrogen (Figure 3 b) plasma is noticeable: the
samples treated by oxygen plasma have structures which
are higher and further apart, than those treated with
nitrogen plasma. This could be attributed to highly
oxidative nature of oxygen plasma. However the height
of these nanostructures is not only dependent on the type
of gas employed for treatment, but also on the treatment
time. By longer treatment time the height of these nano-
structures can be increased, especially for the case of
oxygen plasma treatment. Due to growth of nanostruc-
tures, surface roughness is increased on these surfaces as
well.
3 BIOLOGICAL RESPONSE
To achieve a desired biological response, the attach-
ment of cells to the surface of biomaterials is of primary
importance. When a biomaterial is exposed to a living
organism many extremely complex reactions may occur
at the cell-biomaterial surface. These reactions include
coagulation, healing, inflammation, mutagenicity, and
carcinogenity and play an important role in the
successful implementation of the implemented material
or device45-48. Surface parameters, such as surface
chemistry, wettability, surface topography and surface
roughness influence protein adsorption, and the adsorbed
protein layer further dictates subsequent cellular
reactions. Thus, by carefully tailoring surface properties
by plasma treatment one could engineer the surface for a
specific protein adsorption which would lead to a desired
cellular response.
Gas plasma treatment is one of the strategies for
enhancing surface properties by enriching the surface
with new functional groups known to enhance cell
proliferation – such as oxygen or nitrogen.49,50 One of the
strategies to improve biocompatibility/hemocompatibi-
lity of the surface is to introduce new functional groups,
such as hydroxyl (-OH), amine (-NHx), methyl (-CH3)
sulphate (-SO4) or carboxylic (-COOH).51–54 This is
either employed to tailor the biological response (im-
prove cell proliferation, reduce platelet adhesion etc.) or
to enable immobilisation of biomolecules (enzymes,
proteins etc.). The effects of functional groups on
hemocomaptibilility have been extensively studied, but
again results are not always consistent. The study by
Wilson et. al. has shown that treatment of polymer
(polyethyetherurethane- PEU) surface with RF ammonia
and nitrogen plasma (incorporation of nitrogen groups)
significantly reduces contact activation54. However, no
changes in thrombogenicity, as compared to the un-
treated surface, were observed after oxygen and argon
plasma (incorporation of oxygen groups). Similar results
were obtained for RF plasma treatment of polydimethyl-
siloxane (PDMS) by Williams.55
On the other hand surface wettability is also believed
to be one of the important parameters which affect
biological response to a biomaterial. It is established that
protein adsorption is the first event that takes place on
the surface of a biomaterial with biological fluids,52–54
and that the biological response is controlled by the
nature and confirmation of the proteins adsorbed to the
surface. Thus, wettability is believed to play an im-
portant role in the amount and conformational changes
of adsorbed proteins59 platelet adhesion/activation, blood
coagulation60 and adhesion of cells.61,62
Generally hydrophobic surfaces are considered to be
more protein-adsorbent than hydrophilic surfaces, due to
strong hydrophobic interactions occurring at these
surfaces.63–65
Nevertheless surface morphology should also be
taken in account when talking about biological response
to biomaterials. Surface morphology is important in
protein adsorption and subsequent cell response. Reidel
and colleagues showed that adsorption of albumin dra-
matically increased due to presence of nanoislands.66
While Vertegel et. al. showed that the adsorption of lyso-
zyme to silica nanoparticles decreased with decreasing
nanoparticle size67. Surface topography plays an import-
ant role in providing three-dimensionality of cells68. For
instance the topography of the collagen fibres, with
repeated 66 nm binding, has shown to affect cell shape.69
It has been shown that RF oxygen and nitrogen
plasma treatment improve proliferation of fibroblast as
well as endothelia cells. Figure 4 shows the measured
absorbance, which is directly proportional to viability of
endothelia cells, cultured on different samples. These
results show that proliferation of endothelia cells is
improved on all plasma treated surfaces, which is in
accordance with the results published in the literature.5, 70
Improved proliferation of cells can be attributed to newly
formed functional groups (oxygen and nitrogen) intro-
duced after short plasma treatment time (3 s), as well as
to higher hydrophilicity of the surface, surface morpho-
logy etc. It seems that longer treatment time (longer than
30 s) by oxygen plasma is more affective in promoting
endothelia cell attachment than nitrogen plasma.
I. JUNKAR et al.: PLASMA TREATMENT OF BIOMEDICAL MATERIALS
224 Materiali in tehnologije / Materials and technology 45 (2011) 3, 221–226
Figure 4: Viability of endothelia cells (HUVEC) cultured on surfaces
treated by oxygen and nitrogen plasma for different treatment times
Slika 4: Viabilnost endotelijskih celic (HUVEC) na povr{inah, obde-
lanih s kisikovo in du{ikovo plazmo pri razli~nih ~asih obdelave
Endothelia cell seeding is a common approach to
improving hemocompatibility of vascular grafts, as
endothelia cells are thought to be an ideal hemocompa-
tible surface. However adhesion of platelets is not
desired for hemocompatible surfaces. Thus lower or
practically no adhesion of platelets would be desired.
Interestingly our study showed significant differences in
adhesion of platelets to oxygen and nitrogen plasma
treated surfaces. Observable differences in the number of
adherent platelets and their shape can be seen in Figure
5. The number of adherent platelets decreased dramati-
cally on oxygen plasma treated surfaces; as can be seen
from Figure 5 c after only 3 s of oxygen plasma treat-
ment, a lower number of platelets was observed. Those
that did adhere seemed to be in a more round form,
which is thought to be attributed to low platelet activity.
On the contrary there were many aggregated platelets on
untreated (Figure 5 a) and nitrogen plasma treated sur-
faces (Figure 5 b). Fibrin formation was also observed
on these surfaces, especially on the untreated surface.
The platelets on the untreated polymer surface are
mostly in well spread form and start to aggregate.
4 CONCLUSIONS
It has been shown that plasma treatment techniques
enable surface modification of biomedical materials and
thus enable desired biological response of the surface.
Therefore many biomedical materials have been treated
by plasma in order to improve their surface properties to
accomplish biocompatibility. By fine tuning the dis-
charge and plasma parameters the surface can be appro-
priately modified.
Our study showed that by oxygen and nitrogen
plasma treatment surface chemistry, wettability and
morphology can be altered. Furthermore plasma
treatment enables improved proliferation of fibroblast
and endothelia cells and influences on adhesion pro-
perties of platelets. Interestingly adhesion of platelets
was noticeably reduced on oxygen plasma treated surfa-
ces, while adhesion on nitrogen plasma treated surfaces
was similar to the untreated ones. It has been shown that
oxygen plasma treatment is a promising way to improve
hemocompatible properties of PET surface, as surfaces
modified in this manner exhibit improved proliferation
of endothelia cells and reduced platelet adhesion.
ACKNOWLEDGEMENT
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Tech-
nology of the Republic of Slovenia through the contract
No. 3211-10-000057 (Center of Excellence Polymer
Materials and Technologies).
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R. ZAPLOTNIK, A. VESEL: RADIOFREQUENCY INDUCED PLASMA IN LARGE-SCALE PLASMA REACTOR
RADIOFREQUENCY INDUCED PLASMA IN
LARGE-SCALE PLASMA REACTOR
RADIOFREKVEN^NO INDUCIRANA PLAZMA V REAKTORJU
VELIKIH DIMENZIJ
Rok Zaplotnik1, Alenka Vesel2
1Jo`ef Stefan Institute, Department F4, Jamova 39, 1000 Ljubljana, Slovenia
2Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, 1000 Ljubljana, Slovenia
rokzaplotnik@yahoo.com
Prejem rokopisa – received: 2011-02-05; sprejem za objavo – accepted for publication: 2011-03-05
In this contribution we describe the characteristics of an electrical discharge in rarified gas created by dual-excitation coil. By
using two parallel and overlapping excitation coils we can achieve a better transfer of the electromagnetic power from the
radiofrequency (RF) generator to gaseous plasma. Dual-excitation coil is connected via a matching unit and a high frequency
cable to the RF generator. Such special assembly (coupling of plasma with RF generator) has several advantages in comparison
with normally used set-up. These advantages are the following: power transfer to plasma is increased, plasma is more
homogeneous, the voltage necessary for generating plasma is lower, and many side-effects which are due to capacitive coupling
of the plasma system are reduced.
Keywords: radiofrequency discharge, gaseous plasma, large plasma reactor, matching network
V prispevku opisujemo meritve karakteristik plinske razelektritve, inducirane z dvojno vzbujevalno tuljavo. Z uporabo dveh
vzporedno vezanih prekrivajo~ih se in zamaknjenih vzbujevalnih tuljav dose`emo bolj{i prenos elektromagnetne mo~i od
radiofrekven~nega generatorja v plinsko plazmo. Tuljavi sta vezani zaporedno v sklopu: tuljava-uskladitveni ~len-visoko-
frekven~ni kabel-generator. Tovrstna vezava tuljav ima pred doslej opisanimi sklopi ve~ prednosti, med drugim naslednje:
izkoristek mo~i generatorja se pove~a, plazma v razelektritveni posodi je bolj homogena, napetost na generatorju, ki je potrebna
za generiranje plazme, je manj{a, obenem pa so bistveno zmanj{ani stranski pojavi, ki so posledica kapacitivnih sklopitev v
plazemskem sistemu.
Klju~ne besede: radiofrekven~na razelektritev, plinska plazma, sklopitveni ~len, veliki plazemski reaktorji
1 INTRODUCTION
In last few decades, plasma has become an important
medium for treatment of materials in many modern
technologies like surface cleaning,1–4 sterilization,5–6 sur-
face functionalization,7–16 selective etching17–20 and
surface (nano)modification.21–25 Plasma can be divided to
thermodynamically thermal and nonthermal plasma.
Thermal plasma, where the gas particles are in thermo-
dynamic equilibrium, is used for plasma cutting and
welding, for synthesis of ceramics, for the degradation of
hazardous chemical waste, for the plasma spraying
etc.26–28 A request for ecologically suitable technologies
has led to the development of new types of technologies
like thermodynamically nonequilibrium plasma for
processing of materials. Examples of application are:
vacuum deposition of thin films, laser industry, micro-
electronics, macroelectronics (e.g. plasma displays),
silicon micromachining (e.g. production of silicon
pressure sensors) etc.29 Numerous examples have been
used in the automotive, optical and military industry and
in biomedicine.
For processing of materials we can use different
types of thermodynamic nonequilibrium plasmas, which
are excited in different gases. Plasma can be created by
passing the gas through an electric field. Such gas is
partially ionized, which means that it contains not just
neutral particles but also free electrons and ions. Free
electrons are accelerated in the electric field and make
collision with the atoms or molecules. At collisions
atoms and molecules are excited from the ground state
into different excited states.
Electrical discharges are further divided according to
the frequency of the electric field needed for excitation
of plasma to: DC discharge, corona discharge 50–450
kHz, radiofrequency (RF) discharge 5–100 MHz, micro-
wave (MW) discharge 2.45 GHz and ECR (electron
cyclotron resonance) discharge 2.45 GHz with magnetic
field.30–31
Radiofrequency (RF) plasma usually works at two
different frequencies 27.12 MHz or 13.56 MHz. RF
plasma can be divided into capacitive and inductive
coupled plasma, depending on the method used for
creation of electric field. In the case of capacitive
coupling the electric field is established between two
electrodes (i.e. a capacitor) while for inductive coupling
we need an excitation coil or a spiral.30–31
Inductively coupled plasma is not perfectly inductive
but we can also have a contribution of capacitive
component. Therefore, inductively coupled plasma can
operate in two different operating modes: E- and
H-mode.32–38 At low RF powers the inductively coupled
Materiali in tehnologije / Materials and technology 45 (2011) 3, 227–231 227
UDK 533.9:621.039.5 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)227(2011)
plasma is characterized by weak light emission intensity,
low electrons density and relatively high electron tempe-
rature. This mode is called E-mode and here capacitive
coupling prevails. With increasing RF power we can
observe a transition to H-mode which is characterized by
sudden increase of light intensity and electron density,
while electron temperature is slightly reduced. In this
mode, the inductive component prevails and plasma is
concentrated in a small volume inside the excitation coil
or in the vicinity of the excitation coil.
H-mode is fairly easy to create in a small plasma
reactor,39–49 but on the other hand it can be a big problem
in large-scale plasma reactors. In large plasma reactor it
is very difficult to create uniform inductive plasma in
H-mode. In this paper we present characteristics of large
plasma reactor, where plasma is excited with special
dual-excitation coil, which is connected to RF generator
via a special matching unit. Electrical characteristics and
light emission was measured and compared to plasma
created by normal excitation coil.
2 EXPERIMENTAL
2.1 Construction of plasma reactor
Schematic of the plasma system is shown in Figure
1. Plasma was created in a large quartz discharge tube
with a length of 200 cm and a diameter of 20 cm. The
discharge tube was pumped by rotary pump with a
nominal pumping speed of 15 m3 h–1. The pressure was
measured with an absolute vacuum gauge. The base
pressure in the system was about 1 Pa. Oxygen was
leaked into the discharge chamber through a precise leak
valve and was set to two different pressures: 10 Pa and
40 Pa.
For plasma excitation special dual-excitation coil was
constructed. This coil was connected to the RF generator
via a special matching unit (Figure 2). The RF generator
is working at a frequency of 27.12 MHz and a maximum
nominal power of 8 kW. Matching unit consist of two
high-frequency, high voltage, vacuum variable capaci-
tors, whose capacitance can be adjusted with the servo
motors that are controlled by the RF generator. This
matching unit is used to match the output impedance of
the power supply and the impedance of the plasma sys-
tem (load) to maximize the power transfer to plasma.31
2.2 Electrical characterization
Voltage on the excitation coils was measured with
high voltage probe Tektronix P6015a connected to
oscilloscope Tektronix TDS3024B. We measured root
mean square-RMS voltage. Forwarded power measure-
ment of the radiofrequency generator was taken from the
generator display.
2.3 Plasma characterization
Optical emission spectroscopy (OES) was applied as
a method for measuring the difference in light emission
intensity of normal and dual-excitation coil. Spectra
were measured in the range from 200 nm to 1100 nm by
an optical spectrometer (Avantes AvaSpec-3648-USB2).
Spectral resolution was about 1 nm. Integration time was
100–200 ms.
3 RESULTS AND DISCUSSION
The matching unit is attached to a dual excitation
coil, which consists of two parallel overlapping exci-
R. ZAPLOTNIK, A. VESEL: RADIOFREQUENCY INDUCED PLASMA IN LARGE-SCALE PLASMA REACTOR
228 Materiali in tehnologije / Materials and technology 45 (2011) 3, 227–231
Figure 2: Excitation coil and matching network of plasma system: 8 –
RF generator (8 kW), 9 – high frequency cable, 10 – matching unit,
11, 12 – normal or dual-excitation coil, 13 – high-voltage probe, 14 –
oscilloscope
Slika 2: Vzbujevalni in sklopitveni ~len vakuumskega sistema: 8 – RF
generator, 9 – visokofrekven~ni kabel, 10 – uskladitveni ~len, 11, 12 –
navadna ali dvojna vzbujevalna tuljava, 13 – visokonapetostna sonda,
14 – osciloskop
Figure 1: Schematic of the vacuum system: 1 – vacuum pump, 2 –
valve, 3 – air-inlet valve, 4 – quartz discharge tube, 5 – absolute
vacuum gauge, 6 – leak valve, 7 – gas
Slika 1: Shema vakuumskega sistema: 1 – vakuumska ~rpalka, 2 –
ventil, 3 – ventil za vpust zraka, 4 – razelektritvena kremenova cev, 5
– merilnik absolutnega tlaka, 6 – precizni dozirni ventil in 7 – plinska
jeklenka
tation coils. Excitation coils, which are of the same
diameter, are overlapping in such a way that they have
the same axis and are fixed together only at the
beginning and the end, where they are linked to the
matching unit. First excitation coils is shifted along the
axis in comparison to the second excitation coil in such a
way that turns of the second excitation coil are wound in
the middle of the turns of the first excitation coil (Figure
2). Furthermore, both excitation coils are wound in the
same direction, so that turns of the first excitation coils
run parallel with the turns of the second excitation coil.
Dual-excitation coil is made of 25 mm wide copper band
with a thickness of 0.4 mm. The first excitation coil has
five turns, while the second one has four turns. The
entire length of the dual-excitation coil is 800 mm.
Comparison of measurements made in oxygen plas-
ma generated by a normal excitation coil and measure-
ments made with a dual-excitation coil is shown in
Figures 3–6. Here we should also note that a length of
the normal excitation coil is the same as a length of
dual-excitation coil and has five turns. Figure 3 shows
the measured voltage of normal excitation coil and
dual-excitation coil as a function of power of RF gene-
rator. From Figure 3 we can conclude that in the case of
dual-excitation coil lower voltage is required for the
same power transfer than in the case of normal excitation
coil. By using dual-excitation coil the applied voltage is
reduced in comparison with the normal excitation coil
and this is from the technological point of view very
useful.
The emission intensity of oxygen plasma measured in
the middle of the excitation coil as a function of voltage
or RF power shows that the plasma generated in a
dual-excitation coil is much more intense than plasma,
generated in the normal excitation coil (Figures 4 and
5). Figure 4 presents the results of measurements of the
intensity of oxygen emission lines 777 nm and 845 nm
(transition O(3p5P  3s5S) and O(3p3P  3s3S))50–52 in a
R. ZAPLOTNIK, A. VESEL: RADIOFREQUENCY INDUCED PLASMA IN LARGE-SCALE PLASMA REACTOR
Materiali in tehnologije / Materials and technology 45 (2011) 3, 227–231 229
Figure 6: Light emission intensity of oxygen plasma measured along
normal excitation coil and dual-excitation coil
Slika 6: Meritve intenzitete izsevane svetlobe vzdol` navadne vzbu-
jevalne tuljave in dvojne vzbujevalne tuljave
Figure 4: Light intensity measured in the centre of normal excitation
coil and dual-excitation coil versus the applied voltage to the coil. The
pressure was 10 Pa.
Slika 4: Meritve intenzitete izsevane svetlobe na sredini navadne
vzbujevalne tuljave in dvojne vzbujevalne tuljave v odvisnosti od
napetosti na vzbujevalni tuljavi pri tlaku 10 Pa
Figure 5: Light intensity measured in the centre of normal excitation
coil and dual-excitation coil versus the applied voltage to the coil. The
pressure was 40 Pa.
Slika 5: Meritve intenzitete izsevane svetlobe na sredini navadne
vzbujevalne tuljave in dvojne vzbujevalne tuljave v odvisnosti od
napetosti na vzbujevalni tuljavi pri tlaku 40 Pa
Figure 3: Voltage measured on normal excitation coil and dual-exci-
tation coil versus RF power
Slika 3: Meritve napetosti na navadni vzbujevalni tuljavi in dvojni
vzbujevalni tuljavi v odvisnosti od mo~i generatorja
plasma at a pressure of 10 Pa. The integration time was
200 ms. We can see that the intensity of light in plasma
generated in a normal excitation coil, is about three times
lower than in the case of dual-excitation coil. In Figure 5
are presented the same results for the pressure of 40 Pa.
In this case the spectrometer integration time was 100
ms. The difference is even more evident than at a
pressure of 10 Pa. The light emission intensity in a
dual-excitation coil is at the same applied voltage up to
4-times bigger than in the case of normal excitation coil.
With OES method we have measured also the diffe-
rence in homogeneity of the plasma in H-mode (Figure
6) for both coils (normal and dual). This was done by
measuring the plasma emission in the range between 200
nm and 1100 nm at eight different points along the coil.
Figure 6 shows the normalized intensity of oxygen 777
nm and 845 nm emission lines as a function of position
of spectrometer along the excitation coil. First position
(point at x = 100 mm in Figure 6) of the spectrometer
was at the beginning of the excitation coil. The graph
shows that the intensity of oxygen lines in the case of
dual-excitation coil is at four measured points higher
than a half of the maximum intensity, while in the case
of normal excitation coil the intensity is higher at only
two points. Furthermore, the width of the measured
curve at half of the maximum intensity is for a factor of
two wider for the case of a dual-excitation coil in compa-
rison to the normal one. It can be concluded that the
length of plasma column in H-mode is larger when we
used dual-excitation coil, while in the case of normal
excitation coil it is shorter.
At the end we should also mention, that instead of
dual-excitation coil we can also use triple-excitation coil
(or even multiple coils). The only limitation is that turns
of individual excitation coil may not overlap turns of
another excitation coil. So this means that another
excitation coil must be shifted along the discharge tube
in such a way that turns of the second (third …) coil run
between turns of the first coil. From practical point of
view, if we have dense coil windings around the dis-
charge tube, we have limit space for further wrapping of
additional coils between the turns of the first coil. So this
means that too many coils are not useful.
4 CONCLUSION
In this paper we present characteristics of a large
plasma reactor, where plasma is excited with a special
dual-excitation coil, which is connected to RF generator
via a matching unit. By this special set-up plasma is run
in inductive mode without capacitive coupling. We have
shown that in such configuration power transfer to
plasma is optimized and plasma is homogenous.
Measurements of voltage on the excitation coil show that
in the case of dual-excitation coil lower voltage is
required for the same transfer of power than in the case
of normal excitation coil. Comparison of normal and
dual-excitation coils which run at the same voltage
shows that plasma is much more intense if it is generated
in dual-excitation coil.
ACKNOWLEDGEMENT
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Techno-
logy of the Republic of Slovenia through the contract
No. 3211-10-000057 (Center of Excellence Polymer
Materials and Technologies) and Slovenian Technology
agency – TIA for a grant for young researcher. Operation
also part financed by the European Union, European
Social Fund.
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R. ZAPLOTNIK, A. VESEL: RADIOFREQUENCY INDUCED PLASMA IN LARGE-SCALE PLASMA REACTOR
Materiali in tehnologije / Materials and technology 45 (2011) 3, 227–231 231

K. ELER[I^ et al.: MODIFICATION OF SURFACE MORPHOLOGY OF GRAPHITE BY OXYGEN PLASMA TREATMENT
MODIFICATION OF SURFACE MORPHOLOGY OF
GRAPHITE BY OXYGEN PLASMA TREATMENT
SPREMEMBA MORFOLOGIJE GRAFITA MED OBDELAVO S
KISIKOVO PLAZMO
Kristina Eler{i~1, Ita Junkar1, Martina Modic1, Rok Zaplotnik1, Alenka Vesel2,
Uro{ Cvelbar2
1Institut "Jo`ef Stefan", Jamova cesta 39, 1000 Ljubljana, Slovenia
2Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, 1000 Ljubljana, Slovenia
kristina.elersic@ijs.si
Prejem rokopisa – received: 2011-02-13; sprejem za objavo – accepted for publication: 2011-030
Samples of highly oriented pyrolitic graphite (HOPG) were exposed to fully dissociated oxygen plasma created by a
radiofrequency discharge in pure oxygen. The discharge was powered with a radiofrequency generator operating at the standard
industrial frequency of 13.56 MHz and the output power of 1000 W. Samples of pyrolitic graphite were in pieces with
dimensions of 10 mm × 10 mm × (1.7) mm in high and exposed to plasma for different periods up to 90 s. The morphology of
originally perfectly flat samples was monitored with a high resolution scanning electron microscope (SEM). Oxygen plasma
caused non-homogeneous etching of samples. The first visible effect of plasma treatment was an appearance of rather spherical
features of sub-micrometer dimensions as well as randomly oriented channel - like structures. Increasing the treatment time
caused transformation of the spherical features into cones. The size of cones increased until about 40 s of plasma treatment.
Large cones of few micrometers in size were observed at spots with surface dust particles. Prolonged treatment caused
destruction of the cones and formation of sub-micrometer large holes. The effects were explained by interaction of highly
excited oxygen Particles with surface carbon atoms.
Keywords: graphite, oxygen plasma, scanning electron microscopy, morphology, nanocones
Vzorce visoko orientiranega pirolitskega grafita smo izpostavili delovanju popolnoma disociirane kisikove plazme, ki smo jo
vzbujali z radiofrekven~nim generatorjem. Generator deluje pri industrijski frekvenci 13,56 MHz in izhodni mo~i 1000 W.
Grafitne vzorce, narezane na kose z merami 10 mm × 10 mm × (1,7) mm v vi{ino in jih obdelovali s plazmo v razli~nem ~asu
do 90 s. Morfologijo prvotno povsem ravnih vzorcev smo opazovali z vrsti~nim elektronskim mikroskopom visoke lo~ljivosti.
Opazili smo, da plazemska obdelava vodi k nehomogenemu jedkanju. Sprva se na povr{ini grafita tvorijo kroglasti skupki
dimenzije pod 1 μm, ki kaj kmalu prerastejo v sto`ce. Velikost sto`cev nara{~a s ~asom plazemske obdelave do okoli 40 s, z
nadaljnjo obdelavo pa postopoma izginjajo. Po dolgem ~asu obdelave sto`ci povsem izginejo, namesto njih pa nastanejo okrogle
vdolbine. Pri kraj{ih ~asih obdelave smo opazili tudi nepravilno orientirane kanale. Na mestih, kjer so se nahajali pra{ni drobci,
smo opazili tudi ve~je sto`ce dimenzije ve~ mikrometrov. Opa`ene pojave smo razlo`ili z interakcijo visoko vzbujenih kisikovih
delcev s povr{inskimi atomi ogljika.
Klju~ne besede: grafit, kisikova plazma, vrsti~ni elektronski mikroskop, morfologija, nanodelci
1 INTRODUCTION
Treatment of materials by oxygen plasma is a popular
technique for modification of surface morphology. The
development of high resolution scanning electron micro-
scopes allow for monitoring the surface features at
almost nanometer scale. It has been found that oxygen
plasma treatment causes oxidation of metals.1–6 The
oxidation is never uniform, but interesting nanofeatures
appear on the surface of samples exposed to plasma with
right parameters.3,7,8 Interaction of aggressive plasma
with polymer materials usually results in burning of
samples. Mild oxygen plasma treatment, however, causes
both functionalization of polymers with polar functional
groups 9–11 and modification of surface morphology.
Quite often it happens that extremely rough surface is
obtained with well-defined and dense nanocones and
similar structures on the surface of polymer exposed to
highly reactive but cold oxygen plasma.12,13 Quite inte-
resting as well, plasma treatment also causes modifica-
tion of organic cells and tissues and represents an
interesting method for modification of bacterial cell wall
during plasma sterilization of delicate materials.14–16
Plasma treatment of different materials therefore results
in modification of surface properties of practically all
materials.17
Different discharges were applied for generation of
oxygen plasma but it seems that the majority of authors
like high frequency electrode-less discharges such as
radiofrequency 18–21 and microwave one.22–24 The proper-
ties of plasma created in such discharges vary enor-
mously and depend on the size of the discharge chamber,
the type of materials facing plasma, the discharge power,
frequency and coupling, the pressure and flow of oxygen
through a vacuum system, the concentration of impuri-
ties in the discharge vessel, and (usually neglected but
often very important) the properties of samples treated
by plasma. The major reactants in such plasma are
neutral oxygen atoms in the ground state,25–29 although
other excited particles may play a certain role in
modification of materials as well. The density of oxygen
Materiali in tehnologije / Materials and technology 45 (2011) 3, 233–239 233
UDK 533.9:661.666:542.428.8 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)233(2011)
atoms depends on many discharge parameters including
the discharge power, but in highly concentrated plasma
the dissociation fraction is practically 100 % as long as
the materials facing plasma exhibit a low coefficient for
heterogeneous surface recombination. Although graphite
samples have been exposed to oxygen plasma by several
authors,30–34 the evolution of surface morphology during
plasma treatment is still far from being well understood.
The current paper presents recent results obtained in our
laboratories using extremely aggressive oxygen plasma
for treatment of highly oriented pyrolitic graphite
samples.
2 EXPERIMENTAL
Commercially available graphite samples were used.
Samples were supplied by NT-MDT,HOPG marked as
ZYH, with piece thickness of 1,7 mm. Samples have
special properties as they are prepared layer by layer.
After exposure to plasma we can renew upper working
layer with simply removing it with scotch tape. Graphite
molecular layers have a spalling angle in relation to the
surface layer, at which the layers are easily removed and
the sample can be exposed to plasma modification more
then once.
Samples were exposed to extremely reactive oxygen
plasma created in a discharge chamber of a vacuum
system. The system was pumped with a two stage rotary
pump with a pumping speed of 80 m3 h–1 and commer-
cially available oxygen was leaked into the chamber
during continuous pumping. The pressure of oxygen
inside the chamber was 75 Pa. Plasma was generated
with a radiofrequency generator operating at the standard
industrial frequency of 13.56 MHz and the output power
of 1000 W. The plasma volume was pretty small (less
than 1 L) so plasma was pretty energetic. Any attempt to
measure plasma parameters by an electrical or a catalytic
probe resulted in a rapid melting of the probe so we were
unable to obtain a meaningful result. Only optical
emission spectroscopy was used for estimation of plasma
parameters. The measurements showed that oxygen
molecules are almost fully dissociated. Taking into
account the measured pressure in the system before
turning on the discharge it was possible to calculate the
density of oxygen atoms using the standard vacuum
equation
p = nM k T (1)
where p is the measured pressure, n the density of
molecules, k Boltzmann constant and T the absolute gas
temperature before turning on the discharge. Assuming
100 % dissociation as suggested by optical emission
spectroscopy measurements, the density of O atoms in
plasma is twice the density of molecules before turning
on the discharge, i.e.
nA = 2 p / (k T) (2)
Taking into account numerical values the density of
oxygen atoms in our plasma is 3.7 × 1022 m–3.
The morphology of samples was monitored by
scanning electron microscopy. We used the high-reso-
lution scanning electron microscopy FEG-SEM7600F
from JEOL. Images were taken by angle of 45° and
magnitudes of 1 000-times and 10 000-times at low
accelerating voltage 2 kV. Lower secondary electron
images (LEI) are presented in this paper.
3 RESULTS
Samples of pyrolitic graphite were cut to pieces with
dimensions of (10 × 10) mm with thickness of working
layer (1.7 ± 0.2) mm and exposed to plasma for different
periods up to 90 s. After the plasma treatment they were
mounted in the electron microscope and imaged at
different magnifications. In this paper we present only
results obtained at two different magnifications: 1 000-
times and 10 000-times. The surface of an untreated
sample was almost perfectly flat so we do not show a
corresponding SEM image. Interesting modification of
surface morphology starts after about 15 s of plasma
treatment. Figure 1 represents a couple of SEM images
of the samples exposed to oxygen plasma for 18 s. The
original image is obtained at magnification of 1 000-
times, while the insert was obtained 10 000-times. The
same applies for all other Figures. The SEM image pre-
K. ELER[I^ et al.: MODIFICATION OF SURFACE MORPHOLOGY OF GRAPHITE BY OXYGEN PLASMA TREATMENT
234 Materiali in tehnologije / Materials and technology 45 (2011) 3, 233–239
Figure 1: SEM image of a sample exposed to plasma for 18 s; a) at
magnification of 1 000-times, b) at magnification of 10 000 times
Slika 1: Povr{ina vzorca po obdelavi s kisikovo plazmo za 18 s; a) pri
pove~avi 1 000-krat, b) pri pove~avi 10 000-krat
K. ELER[I^ et al.: MODIFICATION OF SURFACE MORPHOLOGY OF GRAPHITE BY OXYGEN PLASMA TREATMENT
Materiali in tehnologije / Materials and technology 45 (2011) 3, 233–239 235
Figure 5: SEM image of a sample exposed to plasma for 35 s; a) at
magnification of 1 000-times, b) at magnification of 10 000-times
Slika 5: Povr{ina vzorca po obdelavi s kisikovo plazmo za 35 s; a) pri
pove~avi 1 000-krat , b) pri pove~avi 10 000-krat
Figure 3: SEM image of a sample exposed to plasma for 25 s; a) at
magnification of 1 000-times, b) at magnification of 10 000-times
Slika 3: Povr{ina vzorca po obdelavi s kisikovo plazmo za 25 s; a) pri
pove~avi 1 000-krat, b) pri pove~avi 10 000-krat
Figure 4: SEM image of a sample exposed to plasma for 31 s: a) at
magnification of 1 000-times, b) at magnification of 10 000-times
Slika 4: Povr{ina vzorca po obdelavi s kisikovo plazmo za 31 s; a) pri
pove~avi 1 000-krat, b) pri pove~avi 10 000-krat
Figure 2: SEM image of a sample exposed to plasma for 20 s; a) at
magnification of 1 000-times, b) at magnification of 10 000-times
Slika 2: Povr{ina vzorca po obdelavi s kisikovo plazmo za 20 s; a) pri
pove~avi 1 000-krat, b) pri pove~avi 10 000-krat
sented in Figure 1 shows interesting surface morpho-
logy. Randomly oriented channels are observed already
at 1 000-times, but the 10 000-times magnification
reveals spherical features on the surface of (as mentioned
above) originally flat surface. The features are trans-
formed quickly to cone – like structures as shown in
Figure 2 which is the image after 20s of plasma treat-
ment. The features become somewhat bigger since they
may be visible even at the small magnification. Figure 3
presents a couple of images obtained after 25 s of plasma
treatment. Numerous small cones with randomly distri-
buted large cones are observed in Figure 4 which
represents the images obtained after 31 s. The features
remain rather unchanged for 35 s (Figure 5). After 52 s
of plasma treatment, however, they almost vanished as
demonstrated in Figure 6. Also, small craters become
visible. Further increase of treatment time causes an
important increase of the crater diameters and disappear-
ance of all surface cones. Figure 7 represents the SEM
images after 90 s. It is interesting that small cones are
observed at the bottom of craters as revealed from
Figure 7.
4 DISCUSSION
The SEM images presented in Figures 1–7 are attrac-
tive if not all that un-expected and definitely deserve a
discussion. As mentioned in Introduction, exposure of
almost any material to oxygen plasma causes increased
roughness. In the case of materials that form rather stable
oxides, exposure to oxygen plasma leads to formation of
a thin oxide film. The oxide film created at exposure of a
metal to non-equilibrium oxygen plasma, however, may
have different morphology than an oxide film obtained
by thermal oxidation performed close to thermal equili-
brium conditions. For instance, bundles of nanowires
with an extremely favorite aspect ratio may grow on the
surface of some metals. The first report on this
phenomenon appeared in 1995,35 but until recently, no
solid hypothesis that would explain the phenomenon
appeared. Much work has been performed on oxidation
of iron and an appropriate hypothesis appeared recent-
ly.36 The hypothesis may explain interaction of oxygen
plasma with metals, but is inappropriate for the case of
carbon containing materials. Namely, carbon does not
form stable oxides but the interaction between reactive
oxygen particles results in formation of CO radicals and
perhaps also CO2 molecules that desorb from the surface
even at room temperature. Exposure of carbon contain-
ing materials to oxygen plasma therefore results in
etching. Surprisingly enough, etching is far from being
homogeneous. Numerous authors showed that exposure
of many polymers to oxygen plasma yields to formation
of cone – like structures. While no internationally
accepted theory appeared, an appropriate hypothesis was
launched recently by Junkar et al.12 The authors
proposed a hypothesis that the extremely non-homo-
K. ELER[I^ et al.: MODIFICATION OF SURFACE MORPHOLOGY OF GRAPHITE BY OXYGEN PLASMA TREATMENT
236 Materiali in tehnologije / Materials and technology 45 (2011) 3, 233–239
Figure 7: SEM image of a sample exposed to plasma for 90 s; a) at
magnification of 1 000-times, b) at magnification of 10 000-times
Slika 7: Povr{ina vzorca po obdelavi s kisikovo plazmo za 90 s; a) pri
pove~avi 1 000-krat, b) pri pove~avi 10 000-krat
Figure 6: SEM image of a sample exposed to plasma for 52 s; a) at
magnification of 1 000-times, b) at magnification of 10 000-times
Slika 6: Povr{ina vzorca po obdelavi s kisikovo plazmo za 52 s; a) pri
pove~avi 1 000-krat, b) pri pove~avi 10 000-krat
geneous etching was due to non-homogeneity of the
sample. Namely, polymers are often semi-crystalline and
a hypothesis was that the crystalline part of polymer is
etched at lower rate than the amorphous component.
Unfortunately, the authors of that paper presented no
solid confirmation in terms of the size of polymer
crystallites. In any case, the hypothesis of Junkar is not
applicable to the case of highly oriented graphite since it
is almost mono-crystalline. The results observed in
Figures 1–7 therefore cannot be explained by inhomo-
geneity of the original material.
One possible explanation that seems obvious when
monitoring Figure 2 is an existence of surface dust
particles. Namely, the insert in Figure 2 reveals a dust
particle above the large cone. The dust particle prevents
reactive oxygen particles that abound in our plasma
reaching the surface of graphite. The result is a side
erosion of material. Such effects are known to exist in
natural formations on a meter-scale practically in all
cases where the uppermost layer of material is eroded at
lower rate than the material beneath. This hypothesis
therefore explains formation of large cones such as the
one observed in Figure 2. The hypothesis, however,
cannot explain formation of numerous small features
observed in Figures 1–5.
The lack of appropriate hypothesis may lead to a
speculation that an extremely thin, by SEM not visible
film of impurities is presented on the surface of the
graphite. The film may shrink to small islets upon
plasma treatment either due to electrostatic forces
(assuming the impurities are not conductive), or due to
thermodynamic forces (heating of materials often leads
to formation of droplets, or both. The speculation may be
justified by careful examination of cone peaks, for
instance the largest cone in the insert of Figure 3. It
seems that this cone as well as many others is peaked
with a foreign material. The EDX analysis, however, did
not reveal any impurities so the speculation is question-
able. Still, appearance of quite a few cones scattered
randomly on the surface of the sample best observed in
Figure 4 keeps this speculation appropriate.
Scientists familiar with materials etching by ion
beams would quickly find an explanation for appearance
of the cones. Namely, it is well known that etching of
organic as well as some other materials by ion beams
(for instance in Auger electron depth profiling) leads to
formation of nanostructured surface. The explanation,
however, fails in our case since the samples are not
exposed to ions with any kinetic energy worth speaking
about. Namely, the samples were kept at floating poten-
tial during plasma treatment. The floating potential
depends on the electron temperature in plasma as well as
on the mass of ions. As mentioned earlier, the electron
temperature has not been measured but it can be
estimated to few electron volts. The ion mass is known
since we have only two ions: O2+ and O+. The concen-
tration of other gases in the system is negligible. The
voltage across the sheath between unperturbed plasma
and the sample can be estimated to about 10 V so the
ions gain kinetic energy of only 10 eV. This energy is
definitely too low for any decent discussion of ballistic
effects caused on the surface of samples due to exposure
to oxygen ions.
The formation of the features observed in Figures
1–5 therefore remains an unresolved problem. Instead of
presenting any other speculation let us point to another
observation – this one is rather unexpected. Figure 6
reveals a disappearance of the nano-cones after pro-
longed plasma treatment. The Figure clearly indicates
that only largest cones are visible if not preserved as
those shown in Figures 2–5. Obviously, the mechanism
that is responsible for formation of cones becomes less
important than a destruction mechanism. Also, well
defined holes appear on the surface of the graphite
sample. While the destruction may be explained by ther-
mal effects (very high temperature of features stretching
away from the surface) the explanation of the hole
formation is a difficult task. The hole size increases with
further treatment as shown in Figure 7. This Figure
represents SEM images obtained after plasma treatment
for 90 s. The size and depth of the holes vary but it is
clear that the size of holes increases with increasing
plasma treatment time. Any speculation on the observed
fact that at least some holes contain small features at the
bottom is beyond the scope of this paper.
The fact that molecules in our oxygen plasma are
fully dissociate allows for estimation of the oxygen atom
flux onto the surface and speculations of the effects they
may cause on the graphite surface. The density of atoms
in plasma was calculated using equation (2). The flux of
atoms onto the surface is calculated using the standard
equation
j = ¼ n v (3)
where v is the average thermal velocity of oxygen
atoms. This value is not known since the gas kinetic
temperature could not be measured. Still, the order of
magnitude is 1000 K so the velocity in equation (3) is of
the order of 1 000 m/s. The resultant flux is then of the
order of 1025 m–2 s–1. This is a really huge flux. Taking
into account the recombination probability of the order
of 10–2 38 the sample is heated due to heterogeneous
surface recombination at the rate of
P = ½ j  W A (4)
Here,  is the probability for surface recombination,
W the dissociation energy of and oxygen molecule and A
the total surface of the sample. Taking into account
numerical values, the power is of the order of 1 W. The
power is pretty low and definitely does not suggest
extensive heating of the graphite sample. Obviously, if
thermal effects are important for disappearance of cones
on the surface of graphite samples after prolonged
plasma treatment, they should be due to other effects.
Such effects may include neutralization of charged
K. ELER[I^ et al.: MODIFICATION OF SURFACE MORPHOLOGY OF GRAPHITE BY OXYGEN PLASMA TREATMENT
Materiali in tehnologije / Materials and technology 45 (2011) 3, 233–239 237
particles and relaxation of metastables. Unfortunately,
any attempt for a decent characterization of plasma
failed so any discussion of the heating mechanisms is
beyond the scope of the paper.
5 CONCLUSION
Exposure of highly oriented pyrolitic graphite
samples to aggressive oxygen plasma revealed formation
of nanocones on the surface. The appearance of nano-
cones was explained by non-homogeneous etching of
material upon exposure to reactive oxygen particles
created in plasma. The nanocones were of similar size of
few 100 nm apart from few that were several micro-
meters large. The formation of such large cones was
explained by presence of dust particles on the sample
surface. The dust particles prevented reactive oxygen
atoms from reaching the surface so side erosion
prevailed. At prolonged treatment of samples the cones
disappeared and were slowly but graduate replaced by
holes whose dimensions increased with increasing
plasma treatment time. Several speculations about the
observed phenomena were presented and discussed but
the results did not allow for presenting a decent
hypothesis. The results, however, are interesting and
represent just another example of un-usual modification
of a material upon exposure to non-equilibrium oxygen
plasma.
ACKNOWLEDGEMENT
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and
Technology of the Republic of Slovenia through the
contract No. 3211-10-000057 (Center of Excellence
Polymer Materials and Technologies).
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Materiali in tehnologije / Materials and technology 45 (2011) 3, 233–239 239

N. ^UK et al.: PROPERTIES OF PARTICLEBOARDS MADE BY USING AN ADHESIVE ...
PROPERTIES OF PARTICLEBOARDS MADE BY USING
AN ADHESIVE WITH ADDED LIQUEFIED WOOD
LASTNOSTI IVERNIH PLO[^, IZDELANIH Z UPORABO LEPILA
Z DODANIM UTEKO^INJENIM LESOM
Nata{a ^uk1,2, Matja` Kunaver1,2, Sergej Medved3
1National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia
2Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki Park 24, SI-1000 Ljubljana, Slovenia
3Biotechnical Faculty, Department of Wood Science and Technology, Ro`na dolina, Cesta VIII/34, 1000 Ljubljana, Slovenia
natasa.cuk@ki.si
Prejem rokopisa – received: 2011-02-10; sprejem za objavo – accepted for publication: 2011-03-28
In this study, three-layer particleboards were produced using liquefied wood in an adhesive mixture. The influence of two
different formaldehyde resins: melamine-formaldehyde and melamine-urea-formaldehyde resin and two various catalysts:
ammonium sulfate and ammonium formate on the particleboard properties was investigated. There were also two pressing
parameters examined: temperature and time. The following physical and mechanical properties of particleboards were
measured: board thickness, density, moisture content, bending strength and modulus of elasticity, internal bonding strength,
surface soundness, thickness swelling and formaldehyde content. The results showed that properties of produced particleboards
were better when melamine-formaldehyde resin and ammonium formate as a catalyst were used in combination with liquefied
wood in the adhesive mixture. Also, mechanical properties were improved as the press time and press temperature increased.
The optimal mechanical properties of particleboards made with the utilization of the liquefied wood in the adhesive mixture
were achieved at 3 min press time and 180 °C press temperature using melamine-formaldehyde resin and 3 % of ammonium
formate as a catalyst. A very important characteristic is the low formaldehyde content and that is extremely important in the
provision of better quality of life. With the liquefaction of wood and the application of liquefied wood in the particleboard
production wood biomass exploitation could be significantly increased. We are able to produce new materials from renewable
resources that can be a great alternative to the raw materials, originating from crude oil.
Keywords: liquefied wood, renewable resources, particleboards, mechanical properties
Izdelali smo trislojne iverne plo{~e, kjer smo za pripravo lepila uporabili uteko~injen les. Ugotavljali smo, kako v kombinaciji z
uteko~injenim lesom na lastnosti ivernih plo{~ vplivata dve razli~ni formaldehidni smoli: melamin-formaldehidna in
melamin-urea-formaldehidna smola, ter dva razli~na katalizatorja: amonijev sulfat in amonijev formiat. Ugotavljali smo tudi
vpliv dveh parametrov stiskanja, in sicer temperature stiskanja in ~asa stiskanja. Fizikalne in mehanske lastnosti, ki smo jih
ivernim plo{~am dolo~ili, so bile: debelina plo{~e, gostota, vsebnost vlage, upogibna trdnost in modul elasti~nosti, razslojna
trdnost, ~vrstost povr{ine, debelinski nabrek in vsebnost prostega formaldehida. Rezultati so pokazali, da smo izbolj{ali lastnosti
ivernih plo{~, ko smo v kombinaciji z uteko~injenim lesom uporabili melamin-formaldehidno smolo in amonijev formiat kot
katalizator. Ugotovili smo tudi, da so se lastnosti izdelanih ivernih plo{~ s ~asom in temperaturo stiskanja izbolj{ale. Optimalne
lastnosti ivernih plo{~, pri katerih smo lepilni me{anici dodali uteko~injen les, smo dosegli pri ~asu stiskanja 3 min in
temperaturi stiskanja 180 °C, pri uporabi melamin-formaldehidne smole in 3 % amonijevega formiata kot katalizatorja. Zelo
pomembna pri izdelanih ivernih plo{~ah je tudi majhna vsebnost prostega formaldehida, kar mo~no vpliva na izbolj{anje
kvalitete bivanja. Z uteko~injanjem lesa in njegovo uporabo pri izdelavi ivernih plo{~ pove~amo izrabo lesne biomase. Iz
obnovljivih virov tako lahko sintetiziramo nove materiale, ki so odli~na alternativa materialom, ki jih sicer pridobivamo iz
surove nafte.
Klju~ne besede: uteko~injen les, obnovljivi viri, iverne plo{~e, mehanske lastnosti
1 INTRODUCTION
Wood is recyclable, renewable, biodegradable and
one of the most abundant natural polymers. It can be
used for many different purposes and converted to many
useful industrial chemicals. Nowadays, it can also be
liquefied using different polyhydric alcohols and acid
catalysts. Celullose, hemicelullose and lignin are the
main wood components and during liquefaction these
components are decomposed – depolymerized. The final
product has lots of free hydroxyl groups and can
therefore be used to prepare an adhesive for gluing the
wood and wood composites. Particleboards for which we
use 10–15 % of adhesive (based on the weight of
particles) represent almost 60 % of the wood composites
production.
To produce particleboards wood is broken down into
particles of various size and glued together. Wood par-
ticles are bonded using synthetic adhesives and pressed
into boards at high temperature and high pressure. The
pressing operation provides increased density and
strength.1
The properties of particleboards are engineered in a
manner to meet many service requirements. Particle-
boards can be made in large panel sizes, with a full range
of thicknesses. With the introduction of continuous
presses, endless boards emerge and are subsequently cut
into any length desired.2 There are many factors affecting
the properties of the particleboards and the most
important are:
• species of wood, fiber structure, density, hardness,
compressibility;
Materiali in tehnologije / Materials and technology 45 (2011) 3, 241–245 241
UDK 678:674.07:620.17 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)241(2011)
• form and size of raw wood;
• share of bark;
• non-wood lignocellulose materials;
• type and size of particles;
• method of particle drying;
• particle screening and separating, particle size distri-
bution;
• type and amount of binding agents;
• method of mat formation, structure of particleboard;
• moistening of particles prior to pressing, final moi-
sture content of board, conditioning;
• curing conditions;
• thickness of board;
• sand content of particleboard;
• surface quality;
• primings, coats of varnish or lacquer;
• laminating, veneering, overlaying.3
Over the years many different lignocellulosic mate-
rials beside wood were used in particleboard production:
coconut chips,4 paper sludge,5 waste tea leaves,6 castor
stalks,7 wheat straw,8 flax shiv,9 kenaf stalks,10 needle
litter,11 waste grass clippings,12 bagasse,13 saline creeping
wild rye,14 peanut hull,15 cotton carpel,16 vine prunings,17
kiwi prunings,18 waste tissue paper and corn peel,19
almond shell20 and others.
Next to the commonly used urea-formaldehyde resin
for particleboard manufacturing, there were also other
adhesives applied: melamine-modified urea-formalde-
hyde resin,21 phenol-formaldehyde resin and melamine-
urea-phenol-formaldehyde resin,22 PTP resin,23 soy-pro-
tein adhesive,8, 24 cement,25 rice bran adhesive,26 EMDI
isocyanate resin,4 MDI resin.8
Lee and Liu prepared resol resin based on liquefied
bark and used it for particleboard production.27 Particle-
board made from liquefied Taiwan acacia bark, using
H2SO4 as a liquefaction catalyst, had the best properties
in bending strength, internal bonding strength and
thickness swelling. Liquefied wood was also used in
preparation of phenol-formaldehyde resin,28 polyure-
thane resin,29 isocyanate adhesive30 and epoxy resin.31
Since we have already proved in our previous
research that liquefied wood can be used in the adhesive
mixture for particleboard production,32 the aim of this
research was to evaluate the influence of some factors on
the properties of particleboards made from liquefied
wood. We investigated the impact of two different
formaldehyde resins and two various catalysts used in
combination with liquefied wood in the adhesive mixture
on the properties of particleboards. We also examined
the influence of two pressing conditions: temperature
and time. Other parameters such as liquified wood
loading, resin content and press pressure were held
constant.
2 EXPERIMENTAL
2.1 Wood liquefaction
Sawdust of spruce wood (Picea abies (L.) Karst.)
was liquefied using the mixture of glycerol and
diethylene glycol (mass ratio 80/20,) as a liquefying
reagent and p-toluenesulphonic acid as a catalyst. The
mass ratio of wood sawdust to glycols was 1 : 3 and the
amount of p-toluenesulphonic acid was 3 % based on the
mass of glycols.
The mixture of glycols and acid was charged into
three neck glass reactor equipped with mechanical
stirrer, thermometer, condenser and external heating.
When the mixture in the reactor reached temperature 160
°C wood sawdust was added. After all the wood sawdust
was added the temperature was elevated to 180 °C and
maintained for 90 min to carry out the liquefaction
reaction. After the liquefaction the liquefied product was
cooled down.
2.2 Adhesive preparation
The adhesive was prepared with the addition of
liquefied wood (LW) to the melamine-formaldehyde
(MF) or the melamine-urea-formaldehyde (MUF) resin.
Both resins, MF and MUF, were received from Melamin
Ko~evje where they also measured the properties. The
properties of MF and MUF resin are presented in Table
1. The liquefied wood loading was 30 % based on the
mass of adhesive needed (dry content based), other 70 %
was either MF or MUF resin. The mixture was
thoroughly blended, then water and catalyst were added.
The catalyst was only added into the adhesive mixture
for the core layer. Catalysts used were 20 % solution of
ammonium sulfate ((NH4)2SO4) and 10 % solution of
ammonium formate (NH4HCO2). The catalyst content
was 3 % based on the weight of the adhesive mixture,
except when the influence of catalyst on the properties of
particleboards was investigated. The whole resin mixture
was blended for a few minutes before use.
Table 1: Characteristics of MF and MUF resin
Preglednica 1: Karakteristike MF in MUF smole
Properties MF MUF
Solid content (58 ± 1) % (63 ± 2) %
Viscosity* 18–22 s 80–200 s
Free formaldehyde content below 0.2 % max. 0.5 %
pH value 9–10 9.2–9.5
Stability (at 20 °C) 20 d 2 months
*viscosity was determined according to the standard DIN EN ISO
2431,  = 4 mm, 20 °C
2.3 Particleboard production
Three-layer particleboards of 500 mm by 500 mm in
size, 16 mm of thickness and 0.600 g/cm3 of target
density were produced in laboratory conditions. The
N. ^UK et al.: PROPERTIES OF PARTICLEBOARDS MADE BY USING AN ADHESIVE ...
242 Materiali in tehnologije / Materials and technology 45 (2011) 3, 241–245
resin content was 10.50 % in the surface layer and 6.50
% in the core layer. The resin content was calculated
based on the dry weight of wood particles.
Wood particles mixture composed from 75 % of
spruce wood (Picea abies (L.) Karst.) and 25 % of beech
wood (Fagus sylvatica L.) was first dried at 70 °C to
obtain the moisture content approximately 4 %, then the
resin was added. After blending for several minutes the
glued wood particles were hand-formed into a mat using
wood mold. For the determination of resin and catalyst
type impact on the particleboard properties, the wood
mat was hot-pressed for 3 min at temperature 180 °C.
Later the press times were (2, 2.5, 3 and 3.5) min at the
press temperature 180 °C and the press temperatures
were 140 °C, 160 °C, 180 °C and 200 °C at the press
time of 3 min. Before testing, all particleboards were
conditioned for 4–5 d at temperature 20 °C and relative
humidity 65 %.
2.4 Particleboard testing
The physical and mechanical properties of produced
particleboards were tested according to the European
standards: the board thickness EN 324, the density EN
323, the moisture content EN 322, the bending strength
and the modulus of elasticity EN 310, the internal
bonding strength EN 319, the surface soundness EN 311,
the thickness swelling EN 317 and the formaldehyde
content EN 120.
3 RESULTS AND DISCUSSION
The results of the physical and mechanical properties
of laboratory made particleboards bonded with LW
combined with the MF and the MUF resin are presented
in Table 2. The results indicate that mechanical proper-
ties of particleboards were up to 25 % better when MF
resin was used. The reason may be that the resin con-
taining more melamin is more reactive. Also, the cyclic
structure of the melamine imparts greater stability to the
resulting linkages. The three amine groups assure a
three-dimensional, cross-linked molecular structure
when fully cured.33 Consequently, mechanical properties
of particleboards using LW and MF resin were better in
comparison to the particleboards where LW and mela-
mine enhanced UF (MUF) resin was used. Nevertheless,
in both cases, the properties satisfied the standard
requirements. There was no major difference in thick-
ness swelling with regard to the resin type used.
From previous research we have already found out
that the addition of LW in the adhesive mixture reduces
the content of formaldehyde in the boards.32 In this
research we have again obtained low formaldehyde
content. We observed that formaldehyde content was
lower when MF resin was used. Lower formaldehyde
content at use of MF resin, compared to formaldehyde
content at use of MUF is related to the nature of reaction
between formaldehyde and melamine or formaldehyde
and melamine-urea, as it was reported by Eom et al.34
Such boards can easily be used in dry conditions for
general and interior purposes.
Table 2: Properties of particleboards produced by using an adhesive
mixture made of LW combined with MF or MUF resin
Preglednica 2: Lastnosti ivernih plo{~, izdelanih iz uteko~injenega
lesa v kombinaciji z MF in MUF smolo v lepilni me{anici
Particleboard properties LW-MF LW-MUF
Board thickness (mm) 15.71 16.18
Density (g/cm3) 0.627 0.610
Moisture content (%) 7.98 7.70
Bending strength (N/mm2) 13.42 11.06
Modulus of elasticity (N/mm2) 2322 1823
Internal bonding strength (N/mm2) 0.47 0.45
Surface soundness (N/mm2) 1.28 1.18
Thickness swelling (%) 19.51 19.97
Formaldehyde content
(mg/100 g of dry board) 1.40 2.55
Table 3: Properties of particleboards produced by using an adhesive
with added LW and ammonium formate or ammonium sulfate as a
catalyst
Preglednica 3: Lastnosti ivernih plo{~, izdelanih iz uteko~injenega
lesa, dodanega lepilni me{anici pri uporabi amonijevega formiata in
amonijevega sulfata kot katalizatorja
Particleboard
properties NH4HCO2 (NH4)2SO4
Catalyst content 1 % 2 % 3 % 4 % 1 % 2 % 3 % 4 %
Board thickness
(mm) 15.58 15.70 15.71 15.69 16.04 15.83 15.53 15.71
Density (g/cm3) 0.658 0.643 0.627 0.660 0.637 0.642 0.659 0.651
Moisture content
(%) 7.80 7.55 7.98 8.27 8.23 7.49 7.18 7.08
Bending strength
(N/mm2) 12.37 12.71 13.42 12.70 10.48 11.96 12.98 11.71
Modulus of
elasticity (N/mm2) 2286 2327 2322 2240 2091 2193 2404 2321
Internal bonding
strength (N/mm2) 0.40 0.54 0.47 0.47 0.34 0.40 0.46 0.39
Surface soundness
(N/mm2) 1.22 1.22 1.28 1.18 1.01 1.09 1.14 1.24
Thickness swelling
(%) 20.82 18.77 19.51 18.20 21.62 20.74 21.33 22.00
Formaldehyde
content (mg/100 g
of dry board)
2.51 2.06 1.40 1.22 3.44 2.85 2.71 2.25
We also investigated the impact of type and amount
of catalyst on the particleboard properties. We used two
different ammonium salts: ammonium formate
(NH4HCO2) and ammonium sulfate ((NH4)2SO4). The
results of particleboard testing are shown in Table 3. It
can be seen that the properties of particleboards were
better when ammonium formate was used. We also
varied the quantity of the catalyst from 1 % to 4 %. The
properties of particleboards were the best when the 3 %
loading of each catalyst was applied. When comparing
the efficiency of both catalysts and the influence of the
amount of the catalyst the difference in thickness
swelling is not very distinctive. Nevertheless, there are
some differences in formaldehyde content. With higher
amount of catalyst the formaldehyde content was lower
N. ^UK et al.: PROPERTIES OF PARTICLEBOARDS MADE BY USING AN ADHESIVE ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 241–245 243
for the both catalysts. Furthermore, the formaldehyde
content was higher when ammonium sulfate was used.
Further on, we evaluated the influence of pressing
conditions, i.e. time and temperature, on the properties of
particleboards made by using an adhesive with added
liquefied wood. The results of testing particleboards
pressed at different times are presented in Table 4. The
prolongation of the press time resulted in an improve-
ment of mechanical properties. The best properties were
achieved at press time of 3 min. The time of pressing can
be shorten to 2.5 min and consequently, the energy could
be saved significantly. Namely, the reduction of press
time means greater level of production, i.e. increased
output, and consequently, the reduction of the unit
production costs. Of course, press time reduction must
be carefully watched because of the impact on the board
quality. The press time had no larger effect on the
thickness swelling and the formaldehyde content.
Table 4: Properties of particleboards produced by using an adhesive
with added LW pressed at different times
Preglednica 4: Lastnosti ivernih plo{~, izdelanih iz uteko~injenega
lesa, dodanega lepilni me{anici pri razli~nih ~asih stiskanja
Particleboard properties 2 min 2,5 min 3 min 3,5 min
Board thickness (mm) 16.35 16.15 16.12 15.85
Density (g/cm3) 0.614 0.595 0.596 0.624
Moisture content (%) 6.18 5.97 5.43 5.53
Bending strength (N/mm2) 8.53 9.53 10.62 9.65
Modulus of elasticity
(N/mm2) 1872 1806 2106 1904
Internal bonding strength
(N/mm2) 0.37 0.52 0.46 0.48
Surface soundness (N/mm2) 0.95 0.94 1.03 0.93
Thickness swelling (%) 19.58 19.59 19.75 17.82
Formaldehyde content
(mg/100 g of dry board) 3.49 2.94 2.83 3.02
Table 5: Properties of particleboards produced by using an adhesive
with added LW pressed at different temperatures
Preglednica 5: Lastnosti ivernih plo{~, izdelanih iz uteko~injenega
lesa, dodanega lepilni me{anici pri razli~nih temperaturah stiskanja
Particleboard properties 140 °C 160 °C 180 °C 200 °C
Board thickness (mm) 17.51 16.47 16.12 16.27
Density (g/cm3) 0.592 0.629 0.596 0.628
Moisture content (%) 6.03 5.63 5.43 4.77
Bending strength (N/mm2) 8.10 9.34 10.62 10.99
Modulus of elasticity
(N/mm2) 1655 1974 2106 2037
Internal bonding strength
(N/mm2) 0.26 0.44 0.46 0.43
Surface soundness (N/mm2) 0.94 0.99 1.03 0.96
Thickness swelling (%) 19.86 20.56 19.75 20.44
Formaldehyde content
(mg/100 g of dry board) 2.74 2.76 2.83 2.71
The results of testing of particleboards which were
pressed at various temperatures are presented in Table 5.
When the press temperature was elevated, the properties
of particleboards were improved. The optimal properties
were obtained at press temperature of 180 °C, while the
temperature of 140 °C was too low to achive required
strength and the temperature of 200 °C did not provide
better quality of the board than of the 180 °C. The press
temperature can be lowered to 160 °C and the properties
would still satisfy the European standards requirements.
The press temperature did not have much effect on both
the thickness swelling and the formaldehyde content,
what was also observed when evaluating the influence of
the press time.
4 CONCLUSIONS
The three-layer particleboards were successfully
produced using adhesive with the addition of 30 % of
liquefied wood. The physical and mechanical properties
of produced particleboards met the European standard
requirements and were better when using melamine-
formaldehyde resin and 3 % of ammonium formate. The
optimal properties of particleboards were achieved at the
press time of 3 min and the press temperature 180°C. By
shortening the press time to 2.5 min or by decreasing the
press temperature to 160°C reduction of the energy costs
could be achieved. Nevertheless, the additional research
will be needed to confirm these findings. Particleboards
made with the adhesive with added liquefied wood have
low formaldehyde content, and that is of special import-
ance when using particleboards for interior applications.
With the liquefaction of wood and application of
liquefied wood in particleboard production the wood
biomass explotation can be increased. That would enable
efficient lignocellulosic waste recycling, increase
renewable resources usage and reduce the crude oil
consumption.
ACKNOWLEDGEMENTS
The authors wish to thank for the support to Ministry
of Higher Education, Science and Technology of the
Republic of Slovenia within the contract No.
3211-10-000057 (Center of Excellence for Polymer
Materials and Technologies), GG Postojna for their
financial and material support, Melamin Ko~evje for
their material support and Janez Renko for his technical
help.
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Materiali in tehnologije / Materials and technology 45 (2011) 3, 241–245 245

U. SEV[EK et al.: POLY(STYRENE-CO-DIVINYLBENZENE-CO-2-ETHYLHEXYL)ACRYLATE MEMBRANES ...
POLY(STYRENE-CO-DIVINYLBENZENE-CO-2-ETHYLH
EXYL)ACRYLATE MEMBRANES WITH
INTERCONNECTED MACROPOROUS STRUCTURE
POLI(STIREN-KO-DIVINILBENZEN-KO-2-ETILHEKSIL)AKRILATNE
MEMBRANE S POVEZANO POROZNO STRUKTURO
Ur{ka Sev{ek1, Silvo Seifried1, ^rtomir Stropnik1, Irena Pulko2,3, Peter Krajnc1,3
1University of Maribor, Faculty of Chemistry and Chemical Engineering, Smetanova 17, Maribor, Slovenia
2Polymer Technology College, Pod gradom 4, Slovenj Gradec, Slovenia
3Centre of Excellence PoliMaT, Tehnolo{ki park 24, Ljubljana, Slovenia
peter.krajnc@uni-mb.si
Prejem rokopisa – received: 2011-02-15; sprejem za objavo – accepted for publication: 2011-03-28
A combination of doctor blading and emulsion templating was used to prepare macroporous poly(styrene-co-
divinylbenzene-co-2-ethylhexylacrylate) and poly(styrene-co-divinylbenzene) membranes with an interconnected porous
structure. Water in oil high internal phase emulsions including monomers in the oil phase were cast onto a glass plate and
polymerised at elevated temperature. After purification porous polyHIPE membranes were obtained. The volume ratio of
aqueous phase (75 % or 85 %) and the molar ratio of divinylbenzene (2 % or 4 %) were varied, while the addition of
chlorobenzene to the oil phase influenced the viscosity of the emulsions. A comonomer, 2-ethylhexylacrylate substantially
improved the flexibility of the membranes. All yielding membranes were characterized by measuring their cast thicknesses and
flow densities for deionised water. Scanning electron microscopy was used to study the morphological features of the
membranes.
Keywords: membranes, porous polymers, polyHIPE, emulsions
Za pripravo makroporoznih poli(stiren-ko-divinilbenzen-ko-2-etilheksil)akrilatnih in poli(stiren-ko-divinil)benzenskih membran
z odprto porozno strukturo smo uporabili kombinacijo nanosa z no`em in emulzij. Emulzije tipa voda v olju z visokim dele`em
notranje faze, z monomeri v oljni fazi smo nanesli na stekleno plo{~o in polimerizirali pri povi{ani temperaturi. Po ~i{~enju smo
pridobili porozne poliHIPE-membrane. Variirali smo volumski dele` vodne faze (75 % ali 85 %) in molski dele` divinilbenzena
(2 % ali 4 %), medtem ko je dodajanje klorobenzena v oljno fazo vplivalo na viskoznost emulzije. Z dodatkom komonomera,
2-etilheksil akrilata so se elasti~ne lastnosti membran bistveno izbolj{ale. Dobljene membrane smo okarakterizirali z merjenjem
njihove kon~ne debeline in merjenjem njihove preto~nosti za deionizirano vodo. Elektronsko vrsti~no mikroskopijo smo
uporabili za {tudij morfolo{kih lastnosti membran.
Klju~ne besede: membrane, porozni polimeri, poliHIPE, emulzije
1 INTRODUCTION
Macroporous polymers with interconnected porous
structure can be prepared by polymerising the conti-
nuous phase of a high internal phase emulsion. The most
common definition of a high internal phase emulsion
describes it as an emulsion where the volume fraction of
the internal phase is very high, normally higher than 74
% (figure representing the maximum occupiable space
by uniform spheres).1 Polymers of this type are referred
to as polyHIPEs.2 Typically, hierarchically porous
structure is obtained, with larger pores (referred to as
cavities or voids) as the result of internal phase droplets
and smaller connecting pores as a result of polymer
shrinkage during the polymerisation. The most studied
polyHIPE system is the styrene/divinylbenzene class
material. Hydrophilic polyHIPE materials can also be
prepared from HIPE emulsions.3,4 Cooper et. al de-
scribed the preparation of a number of polyHIPE mate-
rials using supercritical CO2 in water (C/W) HIPE
emulsions.5 Other polyHIPE materials that have been
investigated include those prepared from urea and
formaldehyde.6 Monolithic polyHIPE materials from
glycidyl methacrylate have also been described.7,8
PolyHIPE material is suitable for a variety of applica-
tions, such as solid phase organic synthesis, separation of
biomacromolecules, biocatalyst supports, water purifica-
tion, sensors material, filtration, etc.9 Due to relative
simplicity of high internal phase emulsion preparation
(although the issue of stability is frequently problematic)
and its liquid form, a wide range of shapes and sizes of
these porous materials can be prepared. It has been found
that the nature of the mould substrate against which the
polyHIPE material is prepared has a profound influence
on its surface morphology and the degree of adhesion to
the mould.10 The influence of the substrate on the
morphology of polyHIPE material is significant but not
yet completely understood.10,11 Problems with HIPE
stability were encountered when using PVC as a mould
substrate, which could be due to leaching of plasticizer.
Even samples that did form tended to adhere to the
substrate. Polypropylene did not result in adhesion,
however, the surfaces polymerised against the substrate
were largely of a closed-cell structure. This is probably
Materiali in tehnologije / Materials and technology 45 (2011) 3, 247–251 247
UDK 678:542.428.8 ISSN 1580-2949
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due to the presence of a surface film of monomer, caused
by localised HIPE collapse at the HIPE-substrate
interface. Different applications of polyHIPE materials
require different materials properties, such as physical,
mechanical, thermal, etc. Therefore good control over
properties is desirable. One of these properties is
elasticity. In order to increase flexibility, hydrophobic
monomers, that produce low Tg polymers, such as
2-ethylhexylacrylate (EHA) and n-butyl acrylate (BA)
are normally used.12 In the case of using 2-ethylhexyl-
acrylate, the lowering of Tg is the result of integration of
the flexible 2-ethylhexylacrylate units into the polymer
structure. The hydrophobicity of the plasticizing mono-
mer ensures that emulsion stability is not compromised.
Another very important characteristic of porous poly-
HIPE materials is their pore size. The average pore size
can vary from about 1 μm to over 100 μm. It was noticed
that increasing the DVB content in a styrene/DVB
polyHIPE, from 0 % to 100 % DVB, caused a significant
decrease in average void diameter from 15 μm to 5 μm.13
Most polyHIPE materials are prepared in a mono-
lithic form following the shape of the mould. Thin films
can be advantageous for numerous applications and in
the case of polyHIPE membranes flexible mechanical
properties are especially important. Furthermore, surface
morphology is crucial for the performance of polyHIPE
membranes.
In this paper, the preparation of styrene/divinyl-
benzene/ethylhexyl acrylate based polyHIPE membranes
and the influence of monomer mixture, film thickness
and added porogenic solvent on the morphology, is
described.
2 EXPERIMENTAL
2.1 Materials and methods
Styrene (Merck), divinylbenzene (DVB, 80 %, tech.;
Aldrich), 2-ethylhexylacrylate (EHA; Aldrich, 98 %)
were purified by passing through basic alumina (Al2O3;
Aldrich) to remove the inhibitors. Sorbitan monooleate
(Span 80; Aldrich), chlorobenzene (CB; Aldrich), cal-
cium chloride hexahydrate (CaCl2 × 6H2O; Merck), po-
tassium persulphate (KPS; Fluka) were used as received.
Thicknesses of membranes were measured with
MINIMER HD1 (Seltron), which is used for measure-
ment of nonmagnetic coatings with accuracy 1 μm.
Fluidity was measured with an ultrafiltration test cell
AMICON, model 8400. Scanning electron micrographs
were recorded on a Philips FEI XL30 ESEM.
2.2 Preparation of high internal phase emulsions and
membranes
In a typical experiment, styrene, divinylbenzene and
Span 80, 2-ethylhexylacrylate or chlorobenzene were
placed in a 250 mL 3-neck round bottomed flask, and the
mixture was stirred at 300 r/min with a glass stirring rod
fitted with a D-shaped polytetrafluoroethylene paddle,
connected to an overhead stirrer motor. The aqueous
phase was then added dropwise to the organic solution
with continuous stirring. Once all the aqueous solution
had been added, stirring was continued for further 30
min at 300 r/min to yield HIPEs. Both phases were
degassed prior to use.
To prepare membranes, a doctor blade (slit 500 μm or
600 μm) was used to spread the HIPE emulsion onto a
polished glass substrate ((300 × 300 × 15) mm). The
spread of the emulsion was carefully covered with
another glass plate ((300 × 280 × 3) mm) and heated to
60 °C for 24 h. After polymerisation, membranes were
carefully retrieved from glass substrate, washed with
deionised water and ethanol (both for 24 h) and then
dried.
3 RESULTS AND DISCUSSION
Various methods can be used for the preparation of
thin films of emulsions. With regards to polyHIPE
membrane applications, the preparation method should
be efficient and should offer the control of film
thickness. Casting the emulsion onto a substrate using a
doctor blade has already been demonstrated as an
appropriate preparation procedure.14 Doctor blades with
different slits can be used to obtain films with various
thicknesses.
Precursor emulsions with volume ratio of internal
phase to continuous phase 7.5 to 2.5 and 8.5 to 1.5 were
stabilised by sorbitan monooleate (Span 80, hydrophili-
city lipophilicity balance number 4.3). Span 80 is a
standard surfactant used for numerous water in oil high
internal phase emulsions where hydrophobic monomers
are used in the continuous phase and the amount of
surfactant necessary for obtaining open porous structure
is around mass fraction 20 % with regards to the conti-
nuous phase.15 Emulsions A1, A2, A3 and A4 (Table 1)
were prepared in that way, using mole fractions 2 % or 4
% of divinylbenzene in the continuous phase for cross-
linking the styrenic chains. In our previous work, we
have shown that higher degree of crosslinking reduces
the flexibility of polymerised films to a degree that
wrapping of membranes around a centimeter tube was
not possible.12 The casting of emulsions was done using
a doctor blade on a polished glass substrate and it was
found that a second glass plate was needed for retaining
the emulsion stability until polymerisation. Generally,
when preparing membranes from solution using doctor
blading and subsequent polymer precipitation16,17, the
thickness of the resulting polymer membrane was close
to the slit opening of the doctor blade used. In our work
this was not the case and the resulting membranes were
substantially thinner than the slit of the blade (Table 2).
The explanation for this phenomenon lies probably in the
viscous nature of the high internal phase emulsions.
Since the shrinkage of polyHIPE with regards to precur-
U. SEV[EK et al.: POLY(STYRENE-CO-DIVINYLBENZENE-CO-2-ETHYLHEXYL)ACRYLATE MEMBRANES ...
248 Materiali in tehnologije / Materials and technology 45 (2011) 3, 247–251
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Materiali in tehnologije / Materials and technology 45 (2011) 3, 247–251 249
Table 1: Preparation data for polyHIPE membranes
Tabela 1: Podatki za pripravo poliHIPE-membran
Membrane PVa/% ×DVBb/ % ×EHAc/ % (porogen)/%
V (AP)d/ V (OP)e/ V (STY)/ V (DVB) V (EHA)/ V (CB)/
mL mL mL mL mL mL
A1 75 2 0 0 12.1 4.87 3.91 0.13 0 0
A2 85 2 0 0 22.86 4.87 3.91 0.13 0 0
A3 75 4 0 0 12.18 4.81 3.81 0.25 0 0
A4 85 4 0 0 23 4.81 3.81 0.25 0 0
B1 75 2 10 0 13.08 5.16 3.51 0.13 0.73 0
B2 85 2 10 0 24.71 5.16 3.51 0.13 0.73 0
B3 75 4 10 0 13.16 5.19 3.41 0.25 0.73 0
B4 75 2 20 0 14.06 5.54 3.11 0.13 1.45 0
B5 85 2 20 0 26.56 5.54 3.11 0.13 1.45 0
B6 75 4 20 0 14.14 5.57 3.01 0.25 1.45 0
B7 75 2 30 0 15.04 5.93 2.71 0.13 2.18 0
B8 75 4 30 0 15.11 5.96 2.61 0.25 2.18 0
B9 85 2 30 0 28.41 5.93 2.71 0.13 2.18 0
C1 75 2 0 5 12.1 4.98 3.91 0.13 0 0.2
C2 75 4 0 15 12.18 5.42 3.81 0.25 0 0.61
C3 85 2 0 40 22.86 6.39 3.91 0.13 0 1.61
C4 85 4 0 50 23 6.84 3.81 0.25 0 2.03
a pore volume (volume fraction of internal phase), b % mol of DVB, c % mol of 2-ethylhexylacrylate, d aqueous phase, eoil phase
Table 2: Thicknesses of membranes
Tabela 2: Debeline membran
Sample A1 A2 A3 A4 B1 B2 B3 B4 B5 B6 B7 B8 B9 C1 C2 C3 C4
Da/μm 500 500 500 500 600 600 600 600 600 600 600 600 600 500 500 500 500
d b/μm 231.5 253.6 174.7 286 380 397 344 305 407 403 223 251 115 78.3 162.2 174.4 340
a slit opening on doctor blade, b average thickness of membrane
Figure 1: SEM images of the membranes A1 (a), B4 (b), B7 (c) and C1 (d)
Slika 1: SEM-posnetki membran A1 (a), B4 (b), B7 (c) in C1 (d)
sor emulsion is usually less than 10% (established from
our work on monolithic polyHIPEs), we believe that the
lower thicknesses of the resulting membranes are due to
the thickness of the spread itself rather than subsequent
shrinkage. However, a slight influence of emulsion
composition on the final membrane thickness was also
observed. Membranes A1 and A2 (same monomer com-
position, A1 with 75 % internal phase, A2 with 85 %
internal phase) show a trend of increasing thickness with
increasing pore volume. The same is true for membranes
A3 and A4 and for the membranes including ethylhexyl
acrylate (B1-B9). The reason again might be in different
viscosity of emulsions with different phase volume
ratios. Further experiments will however be needed to
clarify this trend.
With regards to producing membranes with increased
elasticity in order to meet the criteria of flexible films, a
plasticizing comonomer, ethylhexyl acrylate (membranes
B1-B9) was added to the oil phase of the emulsions.
Visually, the emulsion stability was not compromised,
using the same surfactant type and amount. The same
can be concluded from SEM images (Figure 1, b and c).
Together with a lower amount of crosslinking, the
inclusion of EHA resulted in elastic membranes that
could be easily wrapped around a one centimeter diame-
ter tube. However, permeability results have shown that a
high degree of EHA content increases back pressure of
the membranes so a compromise in mechanical and
permeable characteristics needs to be reached with a
specific application in mind.
Scanning electron microscopy inspection of cross
sections of membranes (Figure 1) revealed open porous
structure in all cases. Larger pores, in diameter ranging
from 18 μm (B4) to 32 μm (A1), are connected by a
series of interconnecting smaller pores. The size of the
bigger pores reflects the emulsion preparation and
kinetic stability since the creation of large pores is the
consequence of non monomer containing droplet phase
and the size of the droplets guides the pore size. As we
can see from Table 3, membrane B7 with the highest
share of 2-ethylhexylacrylate has the least opened
structure. We believe the reason for this is the stabilizing
effect of this hydrophobic monomer on the emulsion.
FTIR spectroscopy of membranes (typical examples
shown in Figure 2) confirm the inclusion of 2-ethyl-
hexylacrylate in the polymer matrix in the case of B
samples. The signal at 1729 cm–1 represents the carbonyl
group of acrylate ester and this signal is not present in
the spectra of samples A and C which do not include
2-ethylhexylacrylate.
When comparing polyHIPE membranes to polyHIPE
monoliths, more attention in the case of membranes has
to be devoted to the surface since there is no possibility
of removing a layer prior to application. The importance
of casting substrate for open surface of polyHIPE
membranes has already been demonstrated.8 Images of
membrane surfaces, in most cases, show the open
U. SEV[EK et al.: POLY(STYRENE-CO-DIVINYLBENZENE-CO-2-ETHYLHEXYL)ACRYLATE MEMBRANES ...
250 Materiali in tehnologije / Materials and technology 45 (2011) 3, 247–251
Figure 2: FTIR spectra of membranes A1 (no 2-ethylhexylacrylate)
and B1 (10% 2-ethylhexylacrylate)
Slika 2: FTIR-spektra membran A1 (brez 2-etilheksil akrilata) in B1
(10 % 2-etilheksil akrilata)
Table 3: Pore size data
Tabela 3: Velikosti por
A1 B4 B7 C1
D a/μm 31.6 18.3 21.1 22.0
d b/μm 9.6 7.0 5.2 7.7
(d/D) 0.30 0.38 0.24 0.34
a average pore diameter, b average interconnecting pore diameter; both
determined from SEM images
Table 4: Permeability of the membranes for deionised water
Tabela 4: Prepustnosti membran za deionizirano vodo
sample p/bar J(kg/m2 h) sample p/bar
J
(kg/m2 h)
A1
0.5 36.9
B4
0.5 2.9
1 54.7 1 2.9
2 64.1 2 5.7
A2
0.5 25.25
B6
0.5 0.8
1 30.00 1 1.0
2 48.87 2 1.9
A4
0.5 87.1
B7
0.5 4.75
1 66.8 1 6.79
2 95.7 2 7.74
B1
0.5 32.6
B8
0.5 0.8
1 468 1 0.8
2 550 2 0.8
B2
0.5 36.2
B9
0.5 -
1 48.2 1 4.8
2 64.2 2 6.8
B3
0.5 3.8
C4
0.5 117.4
1 4.9 1 127.9
2 16.7 2 138.2
polyHIPE morphology also on the surface of the
membranes. This finding is important especially for
applications where convective mass transfer is preferred.
The high viscosity of the emulsions was occasionally
causing inhomogeneous cast on the substrate. In order to
address this issue, we attempted to reduce the viscosity
by the addition of an organic solvent, namely chloro-
benzene (CB). Improved homogeneity of the spread was
observed, especially when the share of the solvent as
high as the volume fraction of 40 % (in relation to oil
phase) had been used.
To characterize the permeability properties of poly-
HIPE membranes, the flux of deionised water through
the membranes at a given pressure was measured (see
Table 4). Membranes including ethylhexyl acrylate
(B1-B9) have a lower flow density than the basic
membranes (no 2-ethylhexylacrylate) and, generally,
their flow density decreases with the increasing share of
2-ethylhexylacrylate. The results of flow properties are
not in evident relation to pore sizes as measured from
SEM images. However, the actual flow characteristics
were measured from membranes immersed in water
while SEM images were taken from dry samples and real
"wet" morphology can differ from what is evident from
SEM.
4 CONCLUSIONS
Results of this work have shown that high internal
phase emulsions with internal phase volume ratio up to
85 % including styrene, divinylbenzene and ethylhexyl
acrylate in the continuous phase, can be used for
templating the porosity in thin polymer films. Addition
of a plasticizing monomer enabled the tailoring of
flexibility and did not compromise the precursor
emulsion stability. Resulting membranes were between
80 μm and 407 μm thick and had an open porous
structure which enabled the flow through applications;
furthermore, by changing the monomer composition, the
flow could be substantially affected.
Acknowledgement
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Tech-
nology of the Republic of Slovenia through the contract
No. 3211-10-000057 (Centre of Excellence Polymer
Materials and Technologies).
5 REFERENCES
1 K. J. Lissant (ed)., Emulsions and Emulsion Technology, Part 1, New
York 1974, Chap. 1
2 D. Barby, Z. Haq, Eur Pat 0060138, 1982
3 P. Krajnc, D. [tefanec, I. Pulko, Macromol. Rapid. Comm., 26
(2005), 1289–1293
4 S. Kova~i~, D. [tefanec, P. Krajnc, Macromolecules, 40 (2007),
8056–8060
5 A. I. Cooper, Adv. Mater., 15 (2003) 13, 1049–1059
6 A. R. Elmes, K. Hammond, D. C. Sherrington, Eur Pat 289238, 1988
7 P. Krajnc, N. Leber, D. [tefanec, S. Kontrec, A.Podgornik, J.
Chromatogr. A, 1065 (2005), 1, 69–73
8 D. Cummins, P. Wyman, C. J. Duxbury, J. Thies, C. E. Koning, A.
Heise, Chem. Mater. 19 (2007), 5285–5292
9 H. F. Zhang, A. I. Cooper, Soft Matter, (2005) 1, 107–113
10 Z. Bhumgara, Filtr. Separat., 32 (1995) 3, 245–251
11 I. Pulko, P. Krajnc, Chem. Commun., (2008), 4481–4483
12 C. J. C. Edvards, D. P. Gregory, M. Sharples, Eur Pat 239360, 1987
13 J. M. Williams, A. J. Gray, M. H. Wilkerson, Langmuir, 6 (1990),
437
14 I. Pulko, V. Smrekar, A. Podgornik, P. Krajnc, J. Chromatogr. A,
(2010), doi:10,1016/j.croma.2010. 11. 069
15 J. M. Williams, D. A. Wrobleski, Langmuir, 4 (1988), 656
16 N. Vogrin, ^. Stropnik, V. Musil, M. Brumen, J. Membr. Sci., 207
(2002) 1, 139–141
17 V. Kaiser, ^. Stropnik, Mater. Tehnol., 34 (2000) 3/4, 151–155
Materiali in tehnologije / Materials and technology 45 (2011) 3, 247–251 251
U. SEV[EK et al.: POLY(STYRENE-CO-DIVINYLBENZENE-CO-2-ETHYLHEXYL)ACRYLATE MEMBRANES ...

K. STANA - KLEINSCHEK et al.: MODIFICATION OF NON-WOVEN CELLULOSE FOR MEDICAL APPLICATIONS ...
MODIFICATION OF NON-WOVEN CELLULOSE FOR
MEDICAL APPLICATIONS USING NON-EQULIBRIUM
GASSIOUS PLASMA
MODIFIKACIJA CELULOZNIH KOPREN, UPORABNIH V
MEDICINSKE NAMENE, Z NERAVNOVESNO PLINSKO PLAZMO
Karin Stana - Kleinschek1, Zdenka Per{in2, Tina Maver2
1University of Maribor, Faculty of Mechanical Engineering, Laboratory for Characterisation and Processing of Polymers, Smetanova 17,
SI-2000 Maribor, Slovenia
2Centre of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, SI-1000 Ljubljana, Slovenia
karin.stana@uni-mb.si
Prejem rokopisa – received: 2011-02-09; sprejem za objavo – accepted for publication: 2011-03-04
This paper presents the use of a non-equilibrium gaseous plasma technique for the activation of regenerated non-woven
cellulose, as used in the preparation of wound-dressing materials. Plasma technology provides surface modification according to
the required quality in terms of speed, homogeneity, process stability, and efficiency.
In this study the non-woven cellulose was exposed to oxygen plasma (O2) in order to acquire the natural polymer’s
super-hydrophilicity which, among others, defines the materials’ usability for wound-dressing. The influence of the plasma
parameters on the material’s hydrophilicity was studied; and the optimal plasma conditions defined. Combinations of different
experimental techniques (contact angle, water retention value, and moisture content) were studied and correlated with the
mechanical properties, as a function of plasma modification.
The specific adsorption capacity of the non-woven cellulose using oxygen plasma treatment was achieved. In the next step, this
material with increased hydrophilicity and improved mechanical properties will be used in the preparation of multilayered
wound-dressing materials for specific functionalities (incorporation of drugs, specific functional properties).
Keywords: plasma, oxygen, regenerated non-woven cellulose, super-hydrophilicity, mechanical properties
V prispevku je predstavljena uporaba neravnovesne plinske plazemske tehnike za obdelavo regeneriranih celuloznih vlaken v
obliki netkanih kopren, ki se uporabljajo v materialih za oskrbo povr{inskih ran. Le-ta omogo~a modifikacijo/funkcionalizacijo
povr{ine polimerov oz. vlaken in poleg ekolo{ke neopore~nosti zagotavlja {e `eleno kakovost tako glede hitrosti, homogenosti,
stabilnosti kot tudi u~inkovitosti obdelave.
Za doseganja visoke stopnje hidrofilnosti, ki med drugim dolo~a uporabnost materialov za oskrbo ran, smo celulozne koprene
obdelovali s kisikovo (O2) plazmo ter dolo~ili njene optimalne pogoje obdelave.
Hidrofilnost oz. interakcijsko sposobnost plazemsko modificiranih kopren smo analizirali z razli~nimi eksperimentalnimi
metodami, kot so: dolo~anje sti~nih kotov, {tudij navzemanja vlage ter zadr`evanja vode. Analizirane so bile spremembe
mehanskih lastnosti plazemsko obdelanih kopren.
Kisikova plazemska obdelava je signifikantno vplivala na pove~anje hidrofilnosti oz. spremembo adsorpcijske kapacitete
celuloznih kopren ob so~asnih dobrih oz. izbolj{anih mehanskih lastnostih. Tako modificirani celulozni materiali se bodo v
naslednji stopnji uporabili za izdelavo ve~slojnih visoko funkcionalnih materialov za oskrbo ran (inkorporacija u~inkovin,
specifi~nih funkcionalnih lastnosti).
Klju~ne besede: plazma, kisik, regenerirana celulozna koprena, superhidrofilnost, mehanske lastnosti
1 INTRODUCTION
Several different materials are used for medicinal
products in wound-treatment and healing. By consi-
dering their composition and purpose, we can divide
them roughly into three groups. The first comprises
materials and products for the prevention of secondary
infections1–4, materials assuring restoration and regu-
lation of suitable micro- and macro- environments for
wound-healing are considered to be in the second
group,5–7 whilst all materials used for the mitigation of
patient trauma due to the acquired surface wounds or
their healing, belong the third group.8–13
Cellulose and its derivatives are very often used as a
functional part of different wound-dressing materials (in
different forms i.e. fibres, non-woven materials, hydro-
gels ...). Cellulose fibres’ crystalline/amorphous micro-
fibrillar structure (two-phase model) control the
accessibility of the surface, as well as those bulk polar
groups responsible for fibres’ hydrophilicity, an
important material characteristic which, amongst others,
influences the wound-healing process.
Wound-healing is a physiologic process involving a
series of stages: hemostasis, inflammation, proliferation
with repair and remodelling. The warm, moist micro-
environment created within the wound is essential for
further stages of healing.14 In the stage of re-epitheli-
sation, new keratinocytes must migrate onto the repair
site. Migration requires a fluid environment and this is
why the hydrophilicity of the material, as used in the
wound-dressing, is one of the more important agents.15
Materiali in tehnologije / Materials and technology 45 (2011) 3, 253–257 253
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Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)253(2011)
Characteristically different chemical approaches, i.e.
bleaching, treatment with alkali, are used for achieving
this.16 Thermodynamically non-equilibrium physical
processes, such as the use of gaseous plasma, promise
the most,7–21 as alternative to the usual chemical
reactions. The advantages of gaseous plasma such as its
ecological integrity and homogenous surface treatment
far outweigh the minor negative influences on the
materials’ mechanical properties. It has a vast variety of
possibly acquired final applications,22–23 the more often
used being cleaning or etching,24–26 sterilization,27–28
functionalization,29–38 and polymerization.39–40
This paper studies the effects of non-equilibrium
oxygen plasma for the modification of regenerated
non-woven cellulose. The desired final characteristics
are super-hydrophilicity, specific interaction along with
non-altered mechanical properties.
2 EXPERIMENTAL PART
2.1 Materials
Regenerated cellulose fibre was studied in its
non-woven form, i.e. viscose (CV), as produced by
KEMEX, The Netherlands. The surface mass of the
fabrics was 175 g/m2. Some construction parameters,
important for the sorption capacity, were studied, and are
presented in Table 1.
Table 1: Construction parameters of the non-woven cellulose sample
Tabela 1: Konstrukcijski parametri celuloznih netkanih kopren
Sample air permeability*(L/m2s)
thickness **
(mm)
Viscose 650 1.7
*DIN53 887 **SIST ISO 5084
2.2 Treatment procedures
The samples were vacuum dried (vacuum oven type
VS-50SC Kambi~; T = 20 °C, P = 100 mbar, t = 24 h)
before plasma treatment. Then the samples were exposed
to gaseous plasma within a discharge chamber. The
discharge chamber was a spherical cylinder with an inner
diameter of 36 cm, and positioned as an integral part of a
vacuum system pumped using two-stage oil rotary pump.
The plasma was excited with a radiofrequency generator
operating at 27.12 MHz, and a power of about 1 kW. The
plasma created by such a generator is a source of excited
particles including neutral oxygen atoms in the ground
state, excited oxygen atoms and molecules, as well as
positively and negatively-charged ions. Such particles
are determined by various techniques including electrical
and catalytic probes, titration, laser absorption fluore-
scence, and mass spectrometry.41–51
In order to find the best/optimal conditions for the
plasma modification process, several discharge para-
meters were varied: sample shape and dimension (fibres,
fabric; spherical, square), generator power of the plasma
system (between 100 W and 1000 W), frequency of the
plasma system (12.56 or 27.12 MHz, DC), gas flow and
pressure, and the activation time (1 s to15 min).
The optimal conditions used for the regenerated
cellulose samples’ treatment using plasma modification,
are summarised in Table 2.
2.3 Methods
Hydrophilic/hydrophobic properties determination
a) Water contact angle
The hydrophilic/hydrophobic character was studied
by contact angle measurements between polymer
material samples and water. The powder contact angle
method was used as developed for determining the
wetting properties of porous materials. The samples were
cut into (2 × 5) cm rectangular pieces and suspended in a
special sample holder of a Krüss K12 processor
Tensiometer. Immediately before measurement, the
container with the liquid (n-heptane; water) was raised
until the sample edge touched the liquid surface.
The samples’ mass (m) changes, as a function of time
(t) during the water adsorption phase, was monitored.
The initial slope of the function m2 = f (t) is known as the
capillary velocity, from which the contact angle between
the solid (polymer sample) and the water was calculated
using a modified Washburn equation:52
cos 

 
=
⋅ ⋅
m
t c
2
2 (1)
where  is the contact angle between the solid and liquid
phases, m2/t is the capillary velocity,  is the liquid
K. STANA - KLEINSCHEK et al.: MODIFICATION OF NON-WOVEN CELLULOSE FOR MEDICAL APPLICATIONS ...
254 Materiali in tehnologije / Materials and technology 45 (2011) 3, 253–257
Table 2: Optimal oxygen plasma conditions used for the modification of non-woven cellulose.  – frequency of RF plasma generator; p – oxygen
pressure; ni – density of ions; na – density of neutral atoms; Te – electron temperature, t – activation time
Tabela 2: Optimalni parametri plazemske obdelave celuloznih netkanih kopren.  – frekvenca RF plazemskega generatorja; p – tlak dovedenega
kisika; ni – gostota ionov; na – gostota nevtralnih atomov; Te – temperature elektronov; t – ~as obdelave
Sample Plasma treatmentOxygen gas
Shape Dimension Power Frequency Plasma parameters Activation time
non-woven (22 × 22) cm 500 W Í = 27.12 MHz
p = 75 Pa
ni = 10
15 m–3
na = 10
21 m–3
Te = 3 eV
t = 10 min
viscosity,  is the liquid density,  is the surface tension
of the liquid, and c is a material constant.
The constant c was determined for each sample from
contact angle measurements using n-heptane, for which
the contact angle on the non-woven was zero and  = 0.4
mPa s,  = 0.6836 g/cm3,  = 20.4 mN/m. The results
were statistically processed (a set of parallel measure-
ments until the standard deviation was less then 2°) and
represent the average value of ten measurements of the
water contact angle.
For more detailed description of this experimental
procedure, see Persin et al.53 and Fras Zemljic et al.54–55
b) Water retention value
The water retention value of the porous polymer
material was determined according to standard DIN 53
814. This method is based on determining the quantity of
water that the sample can absorb and retain, under
strictly controlled conditions. This property is expressed
as a ratio between the mass of water retained in the
sample after soaking (2 h) and centrifuging (20 min), and
the mass of an absolute dry sample (T = 105 °C, t = 4 h).
c) Moisture content
The moisture content of the porous polymer material
was analysed using a Halogen Moisture Analyser. This
was done using the thermo gravimetric principle: the
samples’ weight was measured before and after heating.
Mechanical properties’ determination
The tensile properties of fabrics were determined
according to standard SIST ISO 13934-1.
The maximum force and elongation at maximum
force i.e. at the moment of the samples’ tear, was deter-
mined, using the strip method.
3 RESULTS
Hydrophilic/hydrophobic properties
Figure 1 presents the square of the adsorbed mass
versus time for non-treated and oxygen plasma treated
samples. The slopes of these capillary velocities curves
characterise the rate of water sorption.
The non-treated material adsorbed the water very
slow. The slope even in 100 s did not reach the plateau,
respectively the non-treated material was not completely
wetted by the water. Within 100 s, the non-treated
sample was able to absorb 1.1 g of water. The capillary
velocity by the plasma treated samples was faster. After
15 s the sample was complete wetted and was able to
adsorb 2.6 g of water.
Water contact angles were calculated using equation
(1). The average results for the water contact angles,
water retention values, and moisture content on the
non-treated and oxygen plasma treated regenerated
cellulose sample, are given in Figure 2.
The significant effect of oxygen plasma modification
resulted in enhanced non-woven wettability. The water
contact angle decrease by 69 % followed by simulta-
neous increase in water retention value (21 % improve-
ment) and an increase in moisture content (22 %).
Mechanical properties
The tensile properties determined as breaking force
and elongations, are given in Table 3.
Table 3: Breaking force and elongation of non-treated and oxygen
plasma treated non-woven cellulose
Tabela 3: Pretr`na sila in raztezek neobdelanih in s kisikovo plazmo
obdelanih celuloznih netkanih kopren
Sample
treatment
Mechanical properties
Breaking force (N) Elongation (%)
horizontal longitudinal horizontal longitudinal
non-treated 54.8
±6.5
117.6
±4.7
41
±2.5
62.2
±1.2
plasma treated 87.4
±11.4
204.9
±25.0
22.2
±1.1
48.1
±0.6
Plasma treatment also affects the samples’ tensile
properties. The breaking force, as measured in both
directions, significantly increased (by 50 % horizontally
and by 74 % longitudinally). Elongation in both direc-
K. STANA - KLEINSCHEK et al.: MODIFICATION OF NON-WOVEN CELLULOSE FOR MEDICAL APPLICATIONS ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 253–257 255
Figure 2: Hydrophilic/hydrophobic properties of a non-treated and
oxygen plasma treated regenerated non-woven cellulose sample
Slika 2: Hidrofilno/hidrofobne lastnosti neobdelanih in s kisikovo
plazmo obdelanih celuloznih netkanih kopren
Figure 1: Capillary velocities of non-treated and oxygen plasma
treated regenerated non-woven cellulose sample
Slika 1: Kapilarne hitrosti neobdelanih in s kisikovo plazmo obde-
lanih celuloznih netkanih kopren
tions decreased but the effect was smaller (i.e. by 46 %
horizontally and 23 % longitudinally).
4 DISCUSSION
Applied oxygen plasma treatment significantly
improved the non-woven cellulose sorption capacity –
wetability. Surface modification resulted in the removal
or oxidation of contaminants on the surface,55 and in-
creased the amount of surface polar carboxyl groups.56–58
Thus, the plasma treatment resulted in a significantly
water contact angle decrease (see Figure 1). Longer
activation time (i.e. 10 min; see Table 2) also caused
some morphological changes in the bulk. Therefore, the
modified samples were able to retain larger amounts of
water, which remained in the porous structures even after
the applied mechanical stress. These changes are also
responsible for moisture content increase.
Oxygen plasma treatment also improved the tensile
properties. The significant increase in the breaking force
after plasma treatment was caused by the decreased
elongation. Low-temperature O2 treatment caused
etching and ablation action on the surface substrate,
resulting in a roughening effect on the fabric surface.
The rougher surface may have imparted more contact
points within the non-woven fibres, resulting in en-
hanced fibre friction. The increased friction produces a
great cohesive force between the fibres. The used plasma
treatment reduced the elongation of the material. This
can be explained by the increased interaction between
fibres in the non-woven, after plasma treatment. This
caused a reduction in the effective gap between the fibres
placed in longitudinal and horizontal directions, at their
crossover points, and the lateral-compressional abilities
of the fibres in the non-woven, leading to a reduction in
the extensibility.
5 CONCLUSION
Used optimised oxygen plasma treatment proved to
be an excellent tool for the surface, as well as for the
bulk modification, of regenerated non-woven cellulose
material. The wettability of the non-woven was drasti-
cally improved (decreased water contact angle by simul-
taneously increased water adsorption capacity). The
changes in morphology were, besides the improvement
in wetability, responsible for the better mechanical
bonding in the used non-woven. In this way the cellulose
material had been functionalised without changes in their
mechanical properties.
We have shown that the oxygen plasma modification
of regenerated cellulose samples in non-woven form,
potentially offers a flexible and environmentally friendly
activation process in order to obtain super-hydrophilic
matrices. Such functionalised super – hydrophilic rege-
nerated non-woven cellulose could be used as a func-
tional layer in wound-dressing materials and could,
among others, contribute to a better surface wound-
healing process.
Acknowledgement
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Tech-
nology of the Republic of Slovenia through the contract
No. 3211-10-000057 (Center of Excellence Polymer
Materials and Technologies).
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Materiali in tehnologije / Materials and technology 45 (2011) 3, 253–257 257

U. MAVER et al.: USE OF AFM FORCE SPECTROSCOPY FOR ASSESSMENT OF POLYMER RESPONSE ...
USE OF AFM FORCE SPECTROSCOPY FOR ASSESSMENT OF
POLYMER RESPONSE TO CONDITIONS SIMILAR TO THE
WOUND, DURING HEALING
UPORABA AFM-SPEKTROSKOPIJE SIL ZA SPREMLJANJE
ODZIVA POLIMERNIH MOLEKUL NA V RANI PODOBNA
OKOLJA MED CELJENJEM
Uro{ Maver1, Tina Maver2,3, Andrej @nidar{i~1, Zdenka Per{in2,3, Miran
Gaber{~ek1, Karin Stana-Kleinschek2,3
1 National Institute of Chemistry, Laboratory for materials electrochemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia
2 Centre of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, SI-1000 Ljubljana, Slovenia
3 University of Maribor, Faculty of Mechanical Engineering, Laboratory for Characterisation and Processing of Polymers, Smetanova 17,
SI-2000 Maribor, Slovenia
tina.maver@uni-mb.si
Prejem rokopisa – received: 2011-02-10; sprejem za objavo – accepted for publication: 2011-02-28
Force spectroscopy is a very promising technique for the evaluation of interactions within different environments. Knowledge
about them is especially important during the design and preparation of those modern wound dressings in contact with a
changing wound-environment over a prolonged time. Such exposure can cause a drastic decrease in the material’s mechanical
performance, and can lead to degradation, thus lowering the success of any healing process. Our study tries to establish a model
system, which would enable us to assess the applicability of the mentioned technique for the evaluation of any interaction
changes between polymer molecules and a chosen surface, after exposure to different environments. Our proposed experimental
setup consists of two representative polymers, a model silicon surface, and two solutions of various pHs and ionic strengths,
respectively. Within the chosen range of parameters, we are confident that we can prove the usefulness of force spectroscopy for
further research into polymer suitability, for the development of novel wound dressings.
Keywords: force spectroscopy, AFM, wound dressings, polymer materials, model system
Spektroskopija sil je zelo obetavna tehnika za uporabo pri dolo~anju interakcij v {tevilnih razli~nih okoljih. Poznanje le-teh je {e
posebej pomembno pri na~rtovanju in pripravi novodobnih obli`ev, ki morajo biti v stiku z rano dalj ~asa in so tako izpostavljeni
spreminjajo~emu se okolju. To lahko na samem materialu povzro~i drasti~ne spremembe in vodi v ote`en potek celjenja. V tem
delu smo `eleli pripraviti modelni sistem, s katerim bi preverjali mo`nost uporabe omenjene tehnike za ugotavljanje sprememb
interakcij med polimernimi molekulami ter izbrano povr{ino po izpostavitvi razli~nim okoljem. Eksperimentalni sistem je
vseboval dva vzor~na polimera, modelno silicijevo povr{ino ter po dve raztopini z razli~nimi pH in ionskimi mo~mi. S takim
naborom parametrov smo zajeli dovolj mo`nosti za ugotovitev uporabnosti tehnike za nadaljnje raziskave primernosti
polimernih materialov za nove obli`e.
Klju~ne besede: spektroskopija sil, mikroskopija na atomsko silo, obli`i, polimerni materiali, modelni sistem
1 INTRODUCTION
Using atomic force microscopy (AFM), a tip attached
to a flexible cantilever will move across the sample
surface to measure surface morphology on the atomic
scale. The force between the tip and the sample is
measured during scanning, by monitoring the deflection
of the cantilever.1 This force is a function of tip sample
separation and the material properties of the tip and the
sample. Further interactions arising between tip and
sample, can be used to investigate other characteristics of
the sample, the tip, or the medium inbetween.2 Such
measurements are usually known as force measurements.
The basics of a AFM force measurement is as follows:
the tip attached to a cantilever spring is moved towards
the sample in a normal direction, during this movement,
the vertical position of the tip and the deflection of the
cantilever are recorded and converted to force-versus-
distance curves, briefly called force curves.2
In addition to evaluating interaction forces between
the tip and model surfaces, AFM can also produce two-
dimensional chemical affinity maps by modifying the
cantilever tip with specific molecules. In this way, it is
possible to characterize differently responding regions
on the material’s surface, resulting in a better under-
standing and, consequently, application of the examined
materials.3–6
Mapping chemical functional groups and examining
their interactions with different materials is of significant
importance for problems ranging from lubrication and
adhesion, to the recognition of biological systems wide
spectrum of fields.7 A chemical force microscope has
been extensively used to monitor changes in the interac-
tions between different functional groups and surfaces,
whilst changing measurement conditions such as pH and
ionic strength.8,9 This technique is particularly useful
when applied in combination with electron spectroscopy
techniques for the determination of surface functional
Materiali in tehnologije / Materials and technology 45 (2011) 3, 259–263 259
UDK 533.53: 678 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)259(2011)
groups, such as X-ray photoelectron spectroscopy
(XPS).10–28
Additionally, some research has already been
performed in the field of monitoring the variations of
pull-of forces between differently functionalized tips and
surfaces.29 This methods main advantage is that it almost
always allows for measurement under those conditions
present in the system in which the sample will be later
used.
The ultimate use of AFM is for single-molecule
recognition, which can be achieved by applying force
spectroscopy. This technique was introduced many years
ago, but has only recently achieved most of its scientific
acclaim and is now one of the more used AFM modes.
Clausen-Schaumann et al. were the first to introduce
some of the possibilities, which enabled all the other
researchers to gain a thorough understanding of single-
molecule recognition measurements usingAFM.30
Additional weight to the measurements was introduced
by Kienberger et al., whilst introducing additional
blocking experiments to prove the specificity and
effectiveness of the measurements.31
Within the field of polymers sciences, AFM has been
used to quantify the entropic elasticity of single polymer
chains,32 the elastic moduli of nanowires,33 single poly-
mer chain elongation34, molecular stiffness of hyper-
branched macromolecules,35 friction of single polymers
on surfaces,36 influence of temperature on the stability of
single chain conformation,37 and surface glass transition
temperature.38 It has also been used to perform stretching
experiments on single carboxymehtylated amylase,39 and
to differentiate between sugar isomers.40
This work focuses on the AFM as a polymer
characterization method. It attempts to simulate those
conditions arising from changes in the solvent, the pH
and ionic strength, and possibility to use AFM force
spectroscopy to assess their influence on polymer
behaviour in such media. Such information would be
really crucial in the preparation of wound-dressings,
which are mostly polymer-based and are exposed to
drastically-changing environments in the wound during
the healing process.
2 EXPERIMENTAL
2.1 Preparation of functionalized AFM tips
Firstly AFM tips were decorated in order to show the
possibility of using them for force spectroscopy
measurements. Two different cellulose derivatives
(amylose-AM and carboxymethyl cellulose-CMC) were
chosen for adsorption onto the tip.
2.1.1 Adsorption of polymers to the tip surface
In order to achieve adsorption onto the tips, solutions
with 10 mmol concentrations were used for both poly-
mers (AM and CMC). Pure tips, out of the box, (NP-S10
contact-mode tips, Veeco) were gently dipped into the
solution using freshly-cleaned tweezers (they were
sonicated in acetone for 30 min and dried using clean
nitrogen), then left in the solution for 2 h. Afterwards
tips were removed from the solution and washed
carefully 6 times using two different solvents (water and
chloroform – CF), alternately.
All used solvents and other chemicals (when not
stated otherwise) were of analytical grade (high purity)
and bought from Sigma-Aldrich.
The preparation procedure is schematically depicted
in Figure 1.
2.2 Methods
2.2.1 AFM
We performed force spectroscopy measurements
using Agilent 5500 AFM. All measurements were done
in liquid medium (milli-Q water) to avoid capillary
forces, and to simulate different conditions. We
measured 100 curves over each time-scale at different
points on the sample’s surface and eliminated the
outliners. In order to achieve better statistics and prove
the measurements were performed correctly, some of the
measurements were repeated and included in the final
results. In all cases an atomically flat silicon surface was
used to avoid any discrepancies due to specific the
surface characteristics or different roughness at different
spots where the force spectroscopy was performed. Two
sets of measurements were performed for each medium
type. Two different pHs and two different ionic strengths
U. MAVER et al.: USE OF AFM FORCE SPECTROSCOPY FOR ASSESSMENT OF POLYMER RESPONSE ...
260 Materiali in tehnologije / Materials and technology 45 (2011) 3, 259–263
Figure 1: Preparation scheme for the attachment of polymer molecules to the AFM tip.
Slika 1: Shema pripenjanja polimernih molekul na AFM-konico
were used, in order to achieve a representative range of
measurement conditions. In order to achieve desired
pHs, citric acid (CA) was used as the sole reagent to
lower the pH, whilst KCl was used to alter the ionic
strengths. pHs were 3, 6, and ionic strengths were 5
mmol, 100 mmol, respectively.
Although measurements were also performed during
other than the mentioned time-scales, only determined
force using two especially representative durations were
used for further discussion. The first of them was used to
simulate the interactions involved in the first contact of
the polymer with the environment and the second,
simulating conditions after the polymer’s prolonged
exposure to the wound-environment.
3 RESULTS
3.1 AFM measurements
These measurements were performed to show their
possible use and assessment for the best scenario, predict
the behaviour of those polymer materials used for the
preparation of several modern wound dressings, after
their exposure to any changing environment within the
healing-wound.
In our attempt to prepare the right methodology to
tackle such a tough and complex problem, an attempt
was made to keep the experimental setup as simple as
possible. This would allow us to apply such methodolo-
gy to more complex and by far more realistic systems.
3.1.1 pH variation
Force spectroscopy was performed during the first set
of measurements whilst varying the pH. As mentioned in
the Experimental section, two different polymers were
used for the measurements, which are, in rough appro-
ximation, representatives of possible polymers for
wound-dressing preparation. One of them is the simple
amylose, undissociable under the measured conditions
but can, as all other cellulose derivatives, form intra- and
inter- molecular H-bonds. The second is carboxymethyl
cellulose which is, on the other hand, able to dissociate
in alkaline solution due to its additional carboxyl groups.
Two different measurement durations were used addi-
tionally one to show the forces and ranges involved upon
first contact with the media, and the second to show how
this changes after prolonged exposure. Figure 2 shows
the measured data for this set of force spectroscopy.
The results show that, with prolonged exposure,
interactions increased for both polymers. Also it was
observed that polymer nature plays a crucial role when
the pH changes, as in this study. Lowering pH causes a
decrease in the number of dissociated functional groups
in CMC molecules, which clearly lowers the interaction.
This effect is less pronounced with AM, which does not
possess such groups.
U. MAVER et al.: USE OF AFM FORCE SPECTROSCOPY FOR ASSESSMENT OF POLYMER RESPONSE ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 259–263 261
Figure 2: Force spectroscopy results for the measurements in two solutions with different pHs. TOP: retract force curves for amylose at two
different measurement durations with two pHs, BOTTOM: retract force curves for carboxymethyl cellulose at two different measurement
durations with two pHs.
Slika 2: Rezultati spektroskopije sil za meritve v raztopinah z dvema razli~nima pH. ZGORAJ: krivulje oddaljevanja pri uporabi amiloze;
meritve so bile izvedene na dveh ~asovnih skalah in v dveh raztopinah razli~nih pH, SPODAJ: krivulje oddaljevanja za meritve pri uporabi
karboksimetil celuloze; meritve so bile izvedene na dveh ~asovnih skalah in v dveh raztopinah razli~nih pH.
3.1.2 Ionic strength variation
In the second set of measurements, an attempt was
made to assess the influence of another important and
changing parameter during healing, namely the ionic
strength. Again the same two polymers were used and
the time was varied in order to gain some insight into the
effect of prolonged polymer exposure to the changed
environment over time. The results for this set of
measurement are shown in Figure 3.
The results show that ionic strength changes
influence the modified cellulose derivate, CMC more
rigorously. Its carboxyl groups are clearly more sensitive
to these changes, which lead to lower interactions. This
effect loses some of its magnitude, with increased
exposure, but the interactions stay lower when compared
to AM, which does not seem to be influenced by any
change in ionic strength.
4 DISCUSSION
Force spectroscopy, when used correctly, is a perfect
method for assessing any interactions over a wide range
of environmental conditions, especially in liquid media,
which allows for the exclusion of capillary forces
capable of hiding smaller interaction contributions.
When using them for the first time in any research area
or a new system, it is crucial that proof is found that the
prepared experimental setup works and yields correct
results. For this, our experiments had to be kept as
simple as possible to avoid any unexplainable discre-
pancies from the expected results. This presented study
tried to do precisely that. By simplifying the setup to
only two changing-parameters separately, it was possible
to show that our proposed technique could serve as a
good platform for assessing any changing wound-envi-
ronment during healing.
The experiments were performed over two sets of
experiments. The first used two different pHs to follow
any changes in the interaction force and range between
the polymer molecule and the model surface. After
comparing the two used polymers, it can be said that the
main difference in behaviour of CMC and AM arises due
to the acidic COOH group on the glucose rings. This
caused the appearance of negative charges on the chain.
Repulsion of the negative charges stretched the polymer
chain. When using the solution with pH 3, the H+ con-
centration was sufficiently high to prevent deprotonation
of COOH groups, and the formation of charges. These
enabled dense packing of CMC. Due to this effect, CMC
experienced lower forces (less functional groups to
interact with the surface), when put into the solution of
an acidic pH, This effect is far less significant for AM
interactions with the model surface, because it has no
such groups in its structure. Prolonged exposure times
lead to an increase in interaction for the used pHs.
U. MAVER et al.: USE OF AFM FORCE SPECTROSCOPY FOR ASSESSMENT OF POLYMER RESPONSE ...
262 Materiali in tehnologije / Materials and technology 45 (2011) 3, 259–263
Figure 3: Force spectroscopy results for the measurements in two solutions with different ionic strengths. TOP: retract force curves for amylose
at two different measurement durations in two solutions with different ionic strengths, BOTTOM: retract force curves for carboxymethyl
cellulose at two different measurement durations in two solutions with different ionic strengths.
Slika 3: Rezultati spektroskopije sil za meritve v raztopinah z razli~nima ionskima mo~ema. ZGORAJ: krivulje oddaljevanja za meritve pri
uporabi amiloze; meritve so bile izvedene na dveh ~asovnih skalah in v dveh raztopinah razli~nih ionskih mo~i, SPODAJ: krivulje oddaljevanja
pri uporabi karboksimetil celuloze; meritve so bile izvedene na dveh ~asovnih skalah in v dveh raztopinah razli~nih ionskih mo~i.
A similar structure-related explanation can be used to
explain the results of the second experimental set, in
which force spectroscopy was performed in two solu-
tions with different ionic strengths. Again it started with
the acidic carboxyl groups of CMC, which gave rise to
negative charges on the CMC polymer chain. The
stretching of a negatively-charged molecule is inevitable
due to electrostatic repulsion. But increased ionic
strength shields charged and, hence, decreased electro-
static repulsion, leading to closer packing of glucose
groups. The influence of increasing ionic strength on
amylose was, as expected, less drastic as no charges
appeared on the amylose chain. The results to a great
extent fit such an explanation. Lastly it could again be
seen that an increase in interactions took place during the
exposure to the medium.
5 CONCLUSIONS
We were able to design a simple test for an evaluation
regarding the possible use of force spectroscopy in the
assessment of polymer behaviour upon exposure to
different liquid environments. Such knowledge would be
of great use, as polymers are the main constituents of
most modern wound-dressings, which have to stay intact
upon contact with the changing environment within a
healing wound. Force spectroscopy proved to be the right
method for following such changes in our model system
consisting of two polymers and a model surface. The
results proved that such an approach is very promising
for future testing in order to find the correct combination
of materials for wound-dressing design regarding
specific wounds.
Acknowledgement
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Tech-
nology of the Republic of Slovenia through the contract
No. 3211-10-000057 (Center of Excellence Polymer
Materials and Technologies).
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U. MAVER et al.: USE OF AFM FORCE SPECTROSCOPY FOR ASSESSMENT OF POLYMER RESPONSE ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 259–263 263

I. POLJAN[EK et al.: BIODEGRADABLE POLYMERS FROM RENEWABLE RESOURCES ...
BIODEGRADABLE POLYMERS FROM RENEWABLE
RESOURCES: EFFECT OF PROTEINIC IMPURITYON
POLYCONDENSATION PRODUCTS OF
2-HYDROXYPROPANOIC ACID
BIORAZGRADLJIVI POLIMERI IZ OBNOVLJIVIH VIROV: VPLIV
PROTEINSKIH NE^ISTOT NA PRODUKTE POLIKONDENZACIJE
2-HIDROKSIPROPANOJSKE KISLINE
Ida Poljan{ek1, 2, Pavel Kucharczyk3,Vladimír Sedlaøík3,4*, Vìra Ka{párková5,
Alexandra [alaková6, Jan Drbohlav6
1Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, SI-1000 Ljubljana, Slovenia
2Biotechnical Faculty, Department of Wood Technology, University of Ljubljana, Jamnikarjeva ul. 101, SI-1000 Ljubljana, Slovenia
3Centre of Polymer Systems, Polymer Centre, Tomas Bata University in Zlin, nam. T. G. Masaryka 5555, 760 01 Zlin, Czech Republic
4Jo`ef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
5Department of Fat, Tenside and Cosmetics Technology, Faculty of Technology, Tomas Bata University in Zlin, nam. T. G. Masaryka 275,
76272 Zlin, Czech Republic
6Dairy Research Institute, Milcom a.s. Ke Dvoru 12a, 16000 Prague, Czech Republic
sedlarik@ft.utb.cz
Prejem rokopisa – received: 2011-02-13; sprejem za objavo – accepted for publication: 2011-03-05
The goal of the presented work is to describe the proteinic impurity (as possible residual component from biotechnological
lactic acid production from whey) in the reaction mixture with 2-hydroxypropanoic acid (lactic acid) for its direct
polycondensation into poly(lactic acid). The impurity presence study was carried out in the range from mass fractions 0 % up to
2 %. For an evaluation of the impurity effect on the values of molecular weight, melting and glass transition temperatures the
measurements of intrinsic viscosity, gel permeation chromatography and differential scanning calorimetry were used. Results
show slight reduction of molecular weight and increase in polydispersity with rising amount of the impurity. The thermal
properties investigation shows noticeable reduction of melting temperature and moderate decrease in glass transition
temperature. The results reveal considerable role of lactose on formation of structural irregularities in lactic acid
polycondensates.
Keywords: poly(lactic acid), whey protein, polycondensation, impurity effect
V prispevku je predstavljena {tudija vpliva proteinskih ne~istot, ki so v reakcijski me{anici, na polikondenzacijo
2-hidroksipropanojske kisline (mle~ne kisline) do polimle~ne kisline. Koncentracijsko obmo~je ne~istot (komponente, ki
ostanejo pri biotehnolo{ki pridelavi mle~ne kisline iz sirotke) je bilo od 0 % do 2 % glede na monomer. Z meritvami lastne
viskoznosti in z gelsko prepustnostno kromatografijo ter diferen~no dinami~no kalorimetrijo smo preu~evali vpliv ne~istot na
povpre~no molsko maso, na temperaturo tali{~a in na temperaturo steklastega prehoda nastalih polimerov. Z nara{~ajo~o
koncentracijo ne~istot vrednost povpre~nih molskih mas pada, nara{~a pa indeks polidisperznosti polimernih molekul. Rezultati
raziskave termi~nih lastnosti ka`ejo zni`anje temperature tali{~a in tudi zmerno zni`anje temperature steklastega prehoda z
nara{~ajo~o koncentracijo ne~istot. Iz rezultatov {tudije je razvidna pomembna vloga laktoze na tvorbo strukturnih nepravilnosti
pri polikondenzaciji mle~ne kisline.
Klju~ne besede: polimle~na kislina, protein sirotke, polikondenzacija, vpliv ne~istot
1 INTRODUCTION
Biodegradable and biocompatible polymers have
attracted increasing interest over the past two decades
both in the fundamental research and also in practical
use.1–3 Among them, the polymers from renewable re-
sources are paid a special attention due to environmental
concerns as well as sustainability factors. Poly(lactic
acid) (PLA) is of the polymers, which is considered as
biodegradable and obtainable form the not petroleum
based raw materials. PLA belongs in a group of biode-
gradable polyesters and it represents its applicability in
packaging technology but also in highly complex
medical field as materials for drug delivery, ortho-
paedics, sutures and scaffolds due to its biocompatible
properties.3–18
Production of the monomer for PLA preparation,
2-hydroxypropanoic acid, (known under trivial name
lactic acid) can be carried out through both synthetics
and biotechnological route. The latter way mostly
includes microbial fermentation of saccharidic substrates
into lactic acid and its subsequent polymerization. One
of the promising renewable resources for lactic acid
production is by-product of dairy industry – whey.
Liquid whey appears in large quantities and its annual
production rises continuously. Despite of its using in
several applications (e.g. food supplements), a lot of
whey is still wasted. Consequently, there is significant
interest in finding new utilization methods for that.19,20
The polymerization step represents the most difficult
part of PLA preparation process due to necessity of high
Materiali in tehnologije / Materials and technology 45 (2011) 3, 265–268 265
UDK 678:620.1/.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)265(2011)
monomer purity. However, biotechnologically prepared
lactic acid contains various impurities (co-products,
residual lactose, protein and inorganic components)21–25.
Purification of lactic acid represents additional econo-
mical loads, which reduce a competitiveness of this
polymer with petroleum based plastics. In spite of
extended research in the field of PLA synthesis, the
effect of impurities has not been studied.
This work is focused on investigation of the effects of
a residual proteinic impurity in lactic acid as monomer
for PLA production via direct polycondensation reaction.
This factor was quantified by observation of the changes
in molecular weight distribution, thermal properties of
the resulting product of the polycondensates.
2 EXPERIMENTAL PART
2.1 Materials
L-lactic acid (LA) C3H6O3, 80 % water solution,
optical rotation  = 10.6° (measured by the polarimeter
Optech P1000 at 22 °C, concentration of 10 %) was
purchased from Lachner Neratovice, Czech Republic.
Stannous 2-ethylhexanoate (Sn(Oct)2) (	95 %) was
supplied by Sigma Aldrich, Steinheim Germany. The
solvents acetone C3H6O, chloroform CHCl3 andmethanol
CH4O (all analytical grade) were bought from IPL
Lukes, Uhersky Brod, Czech Republic. Whey protein
concentrate (PROT) with a certified composition with
mass fractions of 75 % protein, 17.5 % lactose and 7.5 %
minerals was provided by Milcom a. s. Prague, Czech
Republic.
2.2 Polycondensation
A typical procedure was as follows: relevant portions
of LA and PROT (0, 0.25, 0.5, 0.75, 1.5 and 2) % –
related to LA. Samples are designed as PLAPROT X/%,
where X represents concentration of the PROT in the
reaction mixture) were added into a double necked flask
(250 mL) equipped with a Teflon stirrer. Total mass of
the mixture at the beginning of reaction was 50 g (water
is not included). The flask was then placed in an oil bath
heated by magnetic stirrer with heating and connected to
a laboratory apparatus for distillation under reduced
pressure. The dehydration step followed at 160 °C,
reduced pressure 15 kPa for 4 h. After that, the reactor
was disconnected from the vacuum pump and the
relevant amount (0.5 %, related to initial mass of the
reactants) of the catalyst (Sn(Oct)2) was added dropwise
under continuous stirring. The flask with dehydrated
LA/PROT/catalyst mixture was connected back to the
source of vacuum (100 Pa) and the reaction continued
for 24 h at the temperature 160 °C. The resulting product
was allowed to cool down at room temperature and then
dissolved in acetone. The polymer solution was
precipitated in a mixture of chilled methanol/distilled
water 1 : 1 (volume fraction) The obtained product was
filtrated, washed with methanol and dried at 45 °C for 48
h. The dissolving-precipitation procedure was repeated
three times.
2.3 Characterization methods
2.3.1 Intrinsic viscosity determination
Viscosity measurements were performed in chloro-
form at 30 °C in an Ubbelohde viscometer with capillary
0a. The intrinsic viscosity  was calculated using the
equation (1):
[ ] = −
→
lim
c c0
1rel
(1)
where rel is the relative viscosity, which is equal to the
ratio of polymer solution and pure solvent viscosities
and c is the concentration of the polymer solution (0.4,
0.8 and 1.2) %.
2.3.2 Determination of molecular weight by gel
permeation chromatography (GPC)
GPC analyses were performed using a chromato-
graphic system Breeze (Waters) equipped with a PLgel
Mixed-D column ((300 × 7.8) mm, 5 μm) (Polymer
Laboratories Ltd). For detection, a Waters 2487 Dual
absorbance detector at 239 nm was employed. Analyses
were carried out at room temperature with a flow rate of
1.0 mL min–1 in chloroform. The column was calibrated
using narrow molecular weight polystyrene standards
with molar mass ranging from 580 g mol–1 to 480 000
g mol–1 (Polymer Laboratories Ltd). A 100 μL injection
loop was used for all measurements. The sample
concentration ranged from 1.6 mg mL–1 to 2.2 mg mL–1.
Data processing was carried out using the Waters Breeze
GPC Software (Waters). The weight average molar mass
Mw, number average molar mass Mn and polydispersity
(Mw/Mn) of the tested samples were determined.
2.3.3 Differential scanning calorimetry (DSC)
For the determination of glass transition temperature
Tg, melting point Tm and crystallinity 
c of the poly-
condensates the differential scanning calorimetry was
used. Approximately 8 mg of the sample were placed in
an aluminium pan, sealed and analyzed on NETZSCH
DSC 200 F3, calibrated in terms of temperature and heat
flow, using indium. The experiments were performed
under nitrogen atmosphere (60 mL min–1) in two scans in
the temperature range of (0–180) °C and at the heating
rate of 10 °C min–1.
3 RESULTS AND DISCUSSION
The basic characteristics of the samples prepared by
melt polycondensation of LA and PROT are shown in
Figures 1 and 2. The value of  decreases with
increasing impurity content. While pure PLA shows 
0.47 dL g–1, PLAPROT 0.25 % proves almost 45 %
reduction (0.26 dL g–1). Interestingly,  of PLAPROT 0.5
% and 0.75 % fluctuates around that value. Further
addition of the PROT into the reaction mixture does not
influence  significantly and only slight decrease of 
can be noticed (Figure 1). It corresponds to results from
GPC analysis. The pure PLA sample has Mw 47.0 kg
I. POLJAN[EK et al.: BIODEGRADABLE POLYMERS FROM RENEWABLE RESOURCES ...
266 Materiali in tehnologije / Materials and technology 45 (2011) 3, 265–268
mol–1 and polydispersity index (PDI = Mw/Mn) 2.08. The
presence of PROT impurity is attended by slight Mw
reduction and rising PDI. The sample designated
PLAPROT 2 % (Mw = 23.4 kg mol–1, PDI = 3.0) can be
still considered as polycondensates (Figure 2). The yield
of the all polycondensations was over 60 % in all cases.
However, it should be mentioned that it represents pure
yield of the polycondensation products after purification
process described in the experimental part above. These
facts reveal that the presence of proteinic components do
not disturb the polycondensation reaction as significantly
as, for instance, other simple organic acids as we
reported in our previous works.26,27 The reason could be
found in, firstly, lactose presence and secondly proteins
denature at temperatures higher that 40 °C. The
polycondensation was carried outat 160 °C. It means that
all proteinic ingredients had denatured even before
polycondensation reaction started. The question is how
the low molecular products of protein denaturation
influences (if even) the polycondensation reaction.
The effect of PROT on thermal properties is pre-
sented in Figure 3. Pure PLA has a melting temperature,
Tm, positioned at 151.2 °C and glass transition tempe-
rature Tg at 53.8 °C (Figure 3). Increasing content of
PROT in the reaction mixture leads to reduction of both
Tm and Tg. The sample with the highest impurity content,
PLAPROT 2 %, shows Tm lower about almost 23 °C.
Reduction in Tg is about 4.6 °C, which is not as signi-
ficant as in case of Tm. It means that the impurity
presence effects mostly ability of the polycondensates
chains to organize into highly uniform structures. It can
be the result of molecular weight reduction.28 However,
the structural changes cannot be excludes due to low
difference between Tm values of pure PLA and PLA-
PROT 0.25 % whose Mw varies noticeably (Figure 2).
The same assumption can be done on the basis of Tg
measurements, where Fox-Flory (Tg = 58 °C and K =
5.5 · 104)29 for theoretical prediction of Tg from Mn fails
(Figure 4).
These facts can led to an assumption that the
impurity presence causes a non-selective branching of
the PLA chain as well as inhibition of the PLA chain
growth. On the basis of the fact that proteins are
supposed to undergo denaturation at high temperatures,
I. POLJAN[EK et al.: BIODEGRADABLE POLYMERS FROM RENEWABLE RESOURCES ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 265–268 267
Figure3: Dependence of melting temperature of PLA and PLAPROT
samples on concentration of the impurity
Slika 3: Odvisnost temperature tali{~a PLA- in PLAPROT-vzorcev od
koncentracije ne~istot
Figure 4: Glass transition temperature versus PROT concentration,
full symbols represents experimental data, empty symbols represents
Fox-Flory prediction of glass transition temperature (Tg = 58 °C and
K = 5.5 · 104)17
Slika 4: Odvisnost temperature steklastega prehoda od koncentracije
PROT-ne~istot; polni simboli pomenijo eksperimentalno pridobljene
vrednosti, prazni simboli pa vrednosti, izra~unane s Fox-Floryjeve
ena~bo za napoved temperatur steklastega prehoda (Tg = 58 °C in K
= 5,5 · 104)17
Figure 2: PROT concentration versus Mw (filled symbols)) and PDI
(empty symbols)
Slika 2: Odvisnost masnega povpre~ja molskih mas Mw, (polni
simboli) in indeksa polidisperznosti, PDI, (prazni simboli) od koncen-
tracije PROT-ne~istot
Figure 1: Dependence of intrinsic viscosity on PROT impurity
concentration
Slika 1: Odvisnost lastne viskoznosti od koncentracije PROT-ne~istot
the lactose can play important role here. This is
supported by our preliminary results when the effect of
pure lactose was studied30,31.
4 CONCLUSIONS
The aim of this work was to investigate the effect of
the proteinic impurities in the reaction mixture on
resulting properties of the lactic acid polycondensates.
The protein concentrate originating from the real
biotechnological production was used as a source of
impurities. The obtained results indicates that protein
components potentially presented in the reaction mixture
as residuals (up to the mass fraction 2 % related to lactic
acid) from biotechnological lactic acid production do not
effect polycondensation reaction significantly by means
of molecular weight reduction. However, slight decrease
was observed. On the other hand, noticeable decreases
were observed in melting temperatures of the samples.
Glass transition temperatures were reduced moderately
with increasing content of the whey protein concentrate.
It reveals possible structural modification of polyconden-
sates chains (random branching). The presence of
disaccharide seems to play important role here. Detailed
studies of this factor will be accomplished in our future
work.
Acknowledgements
This research was financially supported by the Mini-
stry of Education, Youth and Sports of the Czech Repub-
lic (project No. 2B08071) and Operational Program
Research and Development for Innovations co-funded by
the European Regional Development Fund (ERDF,
project No. CZ.1.05/2.1.00/03.0111). The authors
acknowledge the financial support from the Ministry of
Higher Education, Science and Technology of the
Republic of Slovenia through the contract No.
3211-10-000057 (Center of Excellence for Polymer
Materials and Technologies). Furthermore, this work was
also co-supported by the Ministry of Higher Education,
Science and Technology of the Republic of Slovenia
(programme P4-0015) and Slovenian Research Agency
(ARRS) and Science and Education Foundation of the
Republic of Slovenia within the "Ad futura" programme.
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Polym Eng Sci, 39 (1999), 1311–1319
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Cvelbar, Inf. MIDEM, 38 (2008) 4, 266–271
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Anal., 40 (2008) 11, 1444–1453
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man, M. Mozetic, H. Hägerstrand, V. Kralj - Iglic,The Open Auto-
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Processes Polym., 6 (2009) 10, 667–675
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Vesel, Mater. Tehnol., 44 (2010) 3, 153–156
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A. Popelka, Molecules, 15 (2010) 2, 1007–10027
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Junkar, P. Humpolicek, P. Saha, O. Valasek, Molecules,, 15(2010) 4,
2845–2856,
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Junkar, I. Chodak, Plasma Processes Polym., 7,(2010) 6, 504–514
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Salakova, J. Drbohlav, U. Cvelbar, Functionalization of polylactic
acid through direct melt polycondensation in presence of
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kova, J. Drbohlav, I. Junkar, P. Saha, Lactic acid polycondensation in
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I. POLJAN[EK et al.: BIODEGRADABLE POLYMERS FROM RENEWABLE RESOURCES ...
268 Materiali in tehnologije / Materials and technology 45 (2011) 3, 265–268
A. AN@LOVAR et al.: SUBMIKROMETRSKI IN NANO ZnO KOT POLNILO V PMMA-MATERIALIH
SUBMIKROMETRSKI IN NANO ZnO KOT POLNILO V
PMMA-MATERIALIH
SUB MICROMETER AND NANO ZnO AS FILLER IN PMMA
MATERIALS
Alojz An`lovar, Zorica Crnjak Orel, Majda @igon
Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, SI-1000 Ljubljana, Slovenia
alojz.anzlovar@ki.si
Prejem rokopisa – received: 2011-02-14; sprejem za objavo – accepted for publication: 2011-03-10
Podan je pregled sintetskih na~inov za pripravo cinkovega(II) oksida – ZnO z razli~nimi velikostmi delcev, oblikami,
morfologijo in s povr{insko modifikacijo in tudi pregled postopkov priprave nanokompozitov ZnO/poli(metil metakrilat) –
ZnO/PMMA. Nanostrukture ZnO so sintetizirali iz razli~nih prekurzorjev, medijev in z uporabo razli~nih katalizatorjev.
Isto~asno so potekale intenzivne aktivnosti pri pripravi ZnO/PMMA nanokompozitov. Homogene nanokompozite tega tipa so
pripravili po razli~nih postopkih z uporabo ZnO, modificiranega z razli~nimi povr{insko aktivnimi sredstvi in tudi z uporabo
nemodificiranega ZnO. V na{em laboratoriju smo sintetizirali ZnO nano- in submikrometrske delce z organofilno povr{ino v
razli~nih diolih in pripravili homogene nanokompozite ZnO/PMMA z radikalsko veri`no polimerizacijo MMA v masi.
Pripravljeni nanokompoziti absorbirajo UV-svetlobo in prepu{~ajo vidno ter imajo izbolj{ano termi~no stabilnost. Tak{ni
materiali so potencialno uporabni kot UV stabilizirani PMMA-materiali za zunanjo uporabo pri veliki izpostavljenosti son~ni
svetlobi.
Klju~ne besede: nano ZnO, ZnO kvantne to~ke, PMMA, nanokompoziti, NMR-spektroskopija, STEM-mikroskopija,
TGA-analiza, fotoluminiscen~na spektroskopija
A review of synthetic approaches towards zinc(II) oxide – ZnO of various particle sizes, shapes, morphology and surface
modifications is given as well as a review of preparation procedures for ZnO/poly(methyl methacrylate) – ZnO/PMMA
nanocomposites. ZnO nanostructures were prepared from various precursors, media and catalysts and homogeneous
nanocomposites were prepared by various preparation procedures using ZnO, modified by various surface active agents and also
unmodified ZnO. In our lab, ZnO nano and sub micrometer particles with organophillic surface were prepared in various diols.
Homogeneous ZnO/PMMA nanocomposites were formed by the radical chain polymerization of methyl metacrylate – MMA in
bulk by optimizing the preparation method. Resulting nanocomposite materials showed high UV absorption and high
transparency for visible light as well as enhanced thermal stability. Such materials have high potential as UV stabilized PMMA
materials for various outdoor applications with high sun light loads.
Key words: nano ZnO, ZnO quantum dots, PMMA, nanocomposites, NMR spectroscopy, STEM microscopy, TGA analysis,
photoluminescence spectroscopy
1 INTRODUCTION
Nano and micro structured materials are one of the
fastest growing fields in modern materials science,
especially the preparation of semiconductor materials.
Zinc oxide is very promising semiconductor due to
unique properties in near-UV region, electric conduc-
tivity and optical transparency and has drawn a consi-
derable attention of scientists in last few years.1 ZnO has
large direct band gap and high exciton binding energy
and it shows potential applications in catalysis, optoelec-
tronic devices, sensors, and photovoltaic. ZnO can be
synthesized in various shapes and particle sizes2–6 and
also by various synthetic approaches.7 ZnO is also an
environmentally friendly material, at least in the micro-
meter particle size range. Because of all this, it has
attracted high attention in various fields of science.
2 SUBMICROMETER AND NANO ZnO
The solution phase approach to the preparation of
nano-to-sub micrometer ZnO particles presents the low
cost preparation method and recently attracted a lot of
interest due to the low temperature particle growth
temperature (85–95 °C). It is much more favorable for
large-scale synthesis, especially nowadays, since the
consumption of energy is very low. ZnO nanoparticles
with controllable morphology have been prepared with
chemical precipitation method via a solution route.
Unfortunately, with this method either much different
morphology was obtained or the distribution range of
diameter is wide. From that reason the preparation of
ZnO nanostructures under low and moderate temperature
conditions still presents a great challenge. Below, we list
some representative routes of the preparation of ZnO
nanostructures.
Zinc oxide nanocrystals were prepared by homo-
geneous precipitation method using urea and zinc nitrate
as raw materials. The orientation adhesion of nano-
crystallites was discussed by the growth unit model of
anionic coordination polyhedrons. It was shown that zinc
oxide nanocrystal growth was more easily to occur along
c-axis, which was primarily formed by the connection of
positive hexagonal cone faces p(1 0 1 1) and negative
Materiali in tehnologije / Materials and technology 45 (2011) 3, 269–274 269
UDK 66.017:620.1/.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)269(2011)
hexagonal cone faces p(1 0 1 1), secondly by the positive
polar faces c(0 0 0 1) and negative polar faces c(0 0 0
1).8 Not only does the microwave irradiation of solutions
of Zn(NO3)2 and urea yield ZnO micro particles much
more rapidly than conventional heating methods, it also
produces ZnO with a quite different well-defined
needle-like morphology based on an unusual and
unexpected a-axis.3
The large-scale synthesis of uniform-sized hexagonal
pyramid-shaped ZnO nanocrystals can be performed by
the thermolysis of Zn-oleate complex, which was pre-
pared from the reaction of inexpensive and environmen-
tally friendly reagents such as zinc chloride and sodium
oleate.9 ZnO microspheres and hexagonal micro rods
with sheet like and plate like nanostructures were pre-
pared by the hydrothermal synthesis approach by using
trisodium citrate which plays a key role in directing the
formation of these microstructures. By increasing the
reaction time, these microspheres gradually dissolved to
form short hexagonal micro rods with stacked nanoplate
or nanosheet structure.10 The disk-like, flower-like and
nanorod flower-like ZnO nanostructures have been
controllably fabricated by citric acid assisted hydrother-
mal process.11,12 Different shapes of ZnO micro crystals
have been controllably prepared by a capping-molecule-
assisted hydrothermal process. The flowerlike, disk-like,
and dumbbell-like ZnO micro crystals of hexagonal
phase have been obtained respectively using ammonia,
citric acid, and poly(vinyl alcohol) as the capping
molecules.13A unique method was developed to produce
ZnO by kinetically controlled catalytic hydrolysis of a
molecular precursor (Zn nitrate) at low temperature,
operating in conjunction with the vectorial control of
crystal growth. Nucleation and growth of ZnO was
controlled by the local variation in the chemical potential
of Zn2+ resulting from accumulation of OH groups at the
gas-liquid interface.14 Anionic surfactant sodium
bis(2-ethylhexyl)sulfosuccinate – NaAOT can be made
to form micelles with diverse shapes by adjusting
experimental parameters. The self-assembled AOT-layers
at the interface of water and oil could act as template for
growing ZnO.15
ZnO nanostructures of different morphologies (nano-
particles, nanorods, and flowerlike ZnO structures) could
be synthesized by controlling the content of ethylene-
diamine, hexamethylenetetramine and triethanolamine
(soft surfactant) and the pH of the reaction mixture.16,17
Possible mechanisms for the variation of morphology
with synthesis parameters have been discussed.18 With
maleic anhydride-modified polystyrene (m-PS) micro-
spheres as a template (core) and zinc nitrate and
diethanolamine (DEA) as the starting materials the
m-PS/ZnO core-shell structure was synthesized by a wet
chemical route; the preferential growth of ZnO nuclei
resulted in a cup-shape shell around the m-PS core.19
Polymer-assisted control of particle morphology and
particle size of zinc oxide is another important synthetic
route to prepare ZnO nanostructures.20 For example,
water-soluble poly(ethylene oxide-block-methacrylic
acid) (P(EO-b-MAA) and poly(ethylene oxide-block-
styrene sulphonic acid) (P(EO-b-SSH)) diblock copoly-
mers,21,22 polyacrylamide,23 poly(vinyl alcohol),24
acrylates,25 PVP26 or even gelatin27 were used. In the
latest time dendritic28 and hyperbranched29 polymer
structures or mixed ligand coordination polymer30 were
also used in the controlled synthesis of nano ZnO. A
highly important ZnO structures are ZnO quantum dots
which are synthesized in various media such as alco-
hols31,32 or other solvents.33
Over the past years polyol method was used for
synthesis of different inorganic compounds as well as
pure metals.34–37 In case of less electropositive metal such
as cobalt, nickel copper and silver the reduction reaction
takes place and metallic nanoparticles are obtained while
the addition of water causes the hydrolysis reaction
resulting in the formation of oxides or hydroxides.35
Polyol acts simultaneously as solvent, reducing and
stabilizing agent, and medium for preventing particle
growth.37 Due to the mild reducing power, polyols are
unable to reduce a cation of an electropositive metal such
as zinc. Reaction of hydrolysis takes place in these
systems resulting in formation of ZnO. Using polyol
method nano ZnO was synthesized in various diols from
various precursors but DEG appears to be most suitable
medium and Zn(II) acetate most suitable precursor for
the preparation of ZnO with particle size between 50 nm
A. AN@LOVAR et al.: SUBMIKROMETRSKI IN NANO ZnO KOT POLNILO V PMMA-MATERIALIH
270 Materiali in tehnologije / Materials and technology 45 (2011) 3, 269–274
Figure 1: STEM micrographs ZnO nanoparticles synthesized by the
polyol method in: a) DEG b) TEG
Slika 1: STEM-mikrografije ZnO-nanodelcev, sintetiziranih po
poliolni metodi v: a) DEG, b) TEG
and 100 nm.34,38 Besides the choice of medium, heating
rate is crucial parameter which mostly determines the
particle size.
In our study, ZnO sub micrometer and nanoparticles
were synthesized in polyol media ( di(ethylene glycol) –
DEG and tetra(ethylene glycol) – TEG) using zinc(II)
acetate as a precursort39. STEM micrographs of ZnO
particles, synthesized in DEG and TEG without p-TSA,
are given in Figure 1. Micrographs show that 75 nm
ZnO particles formed in DEG, while those formed in
TEG have average particle size of 340 nm. Both ZnO
particles are actually agglomerates of smaller (10–30
nm) crystallites.39 Particle sizes and morphologies are
similar to the reported results.34,38 In other diols no
particulate ZnO was obtained and resulting agglomerates
were unsuitable for the preparation of nanocomposites.
IR spectra showed characteristic absorption band of ZnO
in the range of 430–470 cm–1 and weak absorption bands
of oxoacetate reaction intermediates at (1590, 1415 and
1340) cm–1. XRD diffractograms of the ZnO, synthesized
in TEG, shows characteristic diffraction pattern of
crystalline ZnO with hexagonal wurtzite structure
(Figure 2). Wide diffraction maxima indicate small size
of crystallites (18 nm – DEG, 25 nm – TEG) confirming
the observations from electron micrographs.39
3 ZnO/PMMA NANOCOMPOSITES
For organic-inorganic composites it is known that
they posses improved properties in comparison to pure
polymers such as better optical properties, thermal
stability, chemical resistance, mechanical properties as
well as flame-retardancy. The preferences in using ZnO
in the preparation of polymeric nanocomposites, are in
high transparency of the polymeric matrix in visible
spectral region as well as its UV-protection. Develop-
ment of simple procedures for the production of trans-
parent UV-protective coatings or the preparation of
transparent plates still represents a great scientific
challenge since complete compatibility of ZnO nano-
to-sub micrometer particles with the polymeric matrix
has not been achieved yet. Simply mixing nanoparticles
with most polymers leads to aggregation in most cases.
Small particles typically aggregate, thus nullifying any
benefits of nanoscopic dimensions. The particles are
frequently surface-modified or compounded in the
polymer in presence of surfactants although few authors
also report preparation of stable nano ZnO dispersions
without any surface modification40,41. To obtain misci-
bility of the inorganic particles and polymers usually
hydrophobic ligands are used such as: alkyl silanes,
oligomeric alkyl silicones, alkylphosphonic acid, hyd-
roxy propyl methyl cellulose, fatty acids, amphiphilic
statistical copolymer or block copolymers (diblock
copolymers, double-hydrophilic block and grafted
copolymers)42-49. Frequently used method of
ZnO/poly(methyl methacrylate) (ZnO/PMMA) nano-
composite preparation is surface modification of ZnO
nanoparticle using one of the mentioned ligands
followed by the subsequent grafting of methyl metha-
crylate or mixtures of acrylic monomers on the modified
surface of particles.50–55 Possible route towards
ZnO/PMMA nanocomposites is by coating the ZnO
particles with PMMA polymerized by gamma or electron
radiation.56 ZnO/PMMA nanocomposites were prepared
also by the sol-gel synthesis of nano ZnO in the presence
of PMMA.57 Transparent ZnO/PMMA with high
concentration of nano ZnO (up to 10 %) nanocomposites
A. AN@LOVAR et al.: SUBMIKROMETRSKI IN NANO ZnO KOT POLNILO V PMMA-MATERIALIH
Materiali in tehnologije / Materials and technology 45 (2011) 3, 269–274 271
Figure 3: STEM micrographs of ultra microtome sections of
ZnO/PMMA nanocomposites using ZnO synthesized in: a) DEG (part.
size = 75 nm), b) TEG (part. size = 340 nm)
Slika 3: STEM-mikrografije ultramikrotomskih rezin ZnO/PMMA-
nanokompozitov, pripravljenih z ZnO, sintetiziranim v: a) DEG (vel.
delcev = 75 nm), b) TEG (vel. delcev = 340 nm)
Figure 2: XRD diffractogram of nano ZnO synthesized in TEG
Slika 2: XRD-difraktogram nano ZnO, sintetiziranega v TEG
have been also prepared by in-situ bulk polymerization
in the presence of the prepared nanoparticles, which
gives improved dispersion of nanoparticles compared to
simple mixing.58–60 Bulk polymerization is the only
synthetic method of PMMA which gives material with
excellent optical properties (plexi glass) and is a widely
used industrial process giving this method a very high
potential in large scale production of ZnO/PMMA
nanocomposite materials. Alternative method of
ZnO/PMMA preparation is simultaneous formation of
ZnO nanoparticles and PMMA polymerization31 or
ATRP polymerization of MMA initiated from the surface
of the ZnO.61 In the latest time homogeneous ZnO QDs
nanocomposites were prepared by simply casting from
solution41,62–64 or even by melt processing.65
In our research work we focused on preparation of
nanocomposites using synthesized nano ZnO particles
with organophilic surface, synthesized by the polyol
method. ZnO/PMMA nanocomposites were prepared by
dispersing the unmodified ZnO nanoparticles in methyl
methacrylate and by the subsequent in-situ chain poly-
merization of MMA. ZnO nanoparticles were dispersed
in MMA by mixing and sonication. The dispersion was
subsequently transferred into a glass mold and MMA
was polymerized in a temperature controlled water bath
for 20 h. The ZnO particle size, surface modification and
concentration were varied.
The prepared nanocomposites were characterized by
the thermo gravimetric analysis – TGA, TEM micro-
scopy, UV-VIS spectroscopy and PLS – photolumine-
scence spectroscopy. STEM electron micrographs
confirmed that nano ZnO is uniformly distributed in the
PMMA matrix. (Figure 3).39 Thermal stability of
ZnO/PMMA nanocomposites was studied by the TGA.
Results showed that thermal stability is enhanced at ZnO
concentrations of 1 % and more (Figure 4). Enhanced
thermal stability was explained by the influence of nano
ZnO on the termination mechanism of MMA radical
polymerization causing reduced content vinylidene
double bonds in PMMA chains as confirmed by 1H NMR
spectroscopy (Figure 5).39 Double bonds are weak points
of MMA chains where decomposition starts and
reducing their content causes enhanced thermal stability.
UV-VIS spectroscopic measurements confirmed that
nano ZnO is extremely efficient UV absorber since 3 mm
thick plates containing the mass fraction of ZnO 0.01 %
absorb more than 90 % of the incident UV light (Figure
6). At the same time more than 70–80 % of the visible
light is transmitted through the material when material is
prepared by the prepolymer procedure. Our results are
different to those reported in the literature;61,62 concen-
trations of nano ZnO filler were much lower; mainly
because we used unmodified ZnO while in reported
nanocomposites ZnO, modified with t-butyl phosphonic
acid, was used. Nevertheless, changes in thermal stability
and UV-VIS absorption are similar to the reported ones.
Due to the absence of surface modification our ZnO
A. AN@LOVAR et al.: SUBMIKROMETRSKI IN NANO ZnO KOT POLNILO V PMMA-MATERIALIH
272 Materiali in tehnologije / Materials and technology 45 (2011) 3, 269–274
Figure 6: UV-VIS spectra of ZnO/PMMA nanocomposites using ZnO
prepared in DEG in dependence of ZnO concentration: A) 1.0 %, B)
0.1 %, C) 0.01 %, D) 0.01 % – prepolymer procedure
Slika 6: UV-VIS-spektri ZnO/PMMA-nanokompozitov na osnovi
ZnO, pripravljenega v DEG, v odvisnosti od masne koncentracije
ZnO: A) 1.0 %, B) 0.1 %, C) 0.01 %, D) 0.01 % – postopek preko
predpolimera
Figure 5: The intensity of vinylidene proton peaks in 1H NMR spectra
of PMMA in dependence of the ZnO concentration and size: A-ZnO
(75nm); A1) 0 %, A2) 1 %, A3) 0.1 %, A4) 0.01 % and B-ZnO (340
nm); B1) 0 %, B2) 1 %, B3) 0.1 %, B4) 0.01%
Slika 5: Intenziteta signalov vinilidenskih protonov v 1H NMR-spek-
trih PMMA v odvisnosti od ZnO-koncentracije in velikosti delcev:
A-ZnO (75 nm); A1) 0 %, A2) 1 %, A3) 0,1 %, A4) 0,01 % and
B-ZnO (340 nm); B1) 0 %, B2) 1 %, B3) 0,1 %, B4) 0,01 %
Figure 4: DTG curves of ZnO/PMMA nanocomposite using nano
ZnO, synthesized in TEG (part. size = 86 nm), in dependence of ZnO
concentration
Slika 4: DTG-krivulje ZnO/PMMA-nanokompozitov na osnovi nano
ZnO, sintetiziranega v TEG (vel. delcev = 86 nm), v odvisnosti od
koncentracije ZnO
nanofiller is, due to lower production costs, more price
competitive. Despite rather low ZnO concentration, there
is a substantial effect on UV absorption as well as on the
thermal stability of the prepared nanocomposites. Com-
parison with ZnO/PMMA nanocomposites, prepared by
other procedures, is rather difficult because those
nanocomposites are mostly in the form of powder or thin
films while our materials are 3.5 mm thick plates.52–59
Prepared ZnO/PMMA nanocomposites have, due to high
UV absorption, potential application as UV stabilized
PMMA materials for various outdoor applications.
4 CONCLUSIONS
Syntheses of sub micrometer and nano ZnO structu-
res as well as processing methods towards homogeneous
ZnO/PMMA nanocomposites have been reviewed. ZnO
nanostructures were synthesized by various hydrother-
mal and solvothermal synthetic routes, using various
precursors, media and catalysts, and we showed that
nano ZnO with organophilic surface can be successfully
prepared in gram quantities in various diol media.
Homogeneous ZnO/PMMA nanocomposites were pre-
pared by different synthetic strategies using surface
modified and unmodified nano ZnO. In our study, homo-
geneous ZnO/PMMA nanocomposites were prepared,
using unmodified nano ZnO, by the radical chain
polymerization of MMA in bulk and by optimizing the
preparation procedure. Prepared nanocomposites showed
enhanced thermal stability, when the concentration of
ZnO is at least 1 %. Enhanced thermal stability, when
ZnO is added in concentration of 1 % and higher, was
explained, using 1H NMR spectroscopy, as a conse-
quence of reduced concentration of vinylidene double
bonds in PMMA chains. Resulting nanocomposite
materials absorbed from 80–98 % of the incident UV
light by adding only the mass fraction of nano ZnO filler
0.01 %. Such materials have potential as UV stabilized
PMMA materials for various outdoor applications with
high sun light loads.
Acknowledgement
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Tech-
nology of the Republic of Slovenia through the contract
No. 3211-10-000057 (Center of Excellence for Polymer
Materials and Technologies).
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274 Materiali in tehnologije / Materials and technology 45 (2011) 3, 269–274
A. DOLI[KA, M. KOLAR: TUNING OF POLY(ETHYLENE TEREPHTALATE) (PET) SURFACE PROPERTIES ...
TUNING OF POLY(ETHYLENE TEREPHTALATE) (PET)
SURFACE PROPERTIES BY OXYGEN PLASMA
TREATMENT
PRILAGODITEV LASTNOSTI POVR[INE POLIETILEN
TEREFTALATA (PET) Z OBELAVO V KISIKOVI PLAZMI
Ale{ Doli{ka,1,2 Metod Kolar1
1Center of excellence for polymer materials and technologies (PoliMaT), Tehnolo{ki park 24, SI-1000 Ljubljana, Slovenia
2University of Maribor, Faculty of Mechanical Engineering, Laboratory for Characterization and Processing of Polymers, Maribor, Slovenia
ales.doliska@uni-mb.si
Prejem rokopisa – received: 2011-02-10; sprejem za objavo – accepted for publication: 2011-03-25
Modification of surface properties of poly(ethyleneterephtalate) (PET) thin films by treatment with weakly ionized oxygen
plasma was studied by contact angles of water and diiodomethane (DIM) drops. Samples were exposed to oxygen plasma with
the ion density of 5 · 1015/m3 and the neutral oxygen atom density of 3 · 1021/m3. Just after the treatment they were characterized
by contact angle measurements. Results showed a quick decrease of the water contact angle in the first few seconds of plasma
treatment, while prolonged treatment did not cause any substantiated modification. The contact angles of DIM, on the other
hand, remained rather constant for the first several seconds of plasma treatment, and increased after prolonged treatment. It was
found that the dispersion component of the surface free energy decreased with increasing treatment time, while the polar
component increased with treatment time. The results were explained by surface functionalization as well as by roughness
effects.
Keywords:poly(ethylene terephtalate), PET, oxygen plasma, contact angle, hydrophilic, functionalization
Povr{inske lastnosti polimera polietilentereftalata (PET) smo prilagodili specifi~nim zahtevam po visoki hidrofilnosti z
obdelavo v kisikovi plazmi. Stopnjo hidrofilnosti smo dolo~ali iz meritev sti~nih kotov dveh kapljevin, in sicer vode in
dijodometana. Vzorce smo obdelovali v kisikovi plazmi z gostoto ionov 5 · 1015/m3 in gostoto nevtralnih kisikovih atomov 3 ·
1021/m3. Meritve sti~nih kotov kapljic smo opravili takoj po plazemski obdelavi. Rezultati so pokazali hiter padec sti~nega kota
vodnih kapljic `e v prvih sekundah plazemske obdelave. Podalj{an ~as obdelave ni prinesel bistvenih sprememb. Sti~ni koti
dijodometana so ostali prakti~no nespremenjeni po prvih nekaj sekundah, z nadaljnjo obdelavo pa so za~eli padati. Ugotovili
smo, da disperzijska komponenta povr{inske energije po~asi pada s plazemsko obdelavo, medtem ko polarna komponenta raste.
Rezultate smo pojasnili s povr{insko funkcionalizacijo polimera in pove~ano hrapavostjo povr{ine.
Klju~ne besede: polietilentereftalat, PET, kisikova plazma, sti~ni kot, hidrofilnost, funkcionalizacija
1 INTRODUCTION
Polymer materials are nowadays widely used in
biology and medicine. They belong to organic materials
and often express fairly good biocompatibility. The
biocompatibility, however, is never optimal due to the
required chemical and mechanical properties of poly-
mers used for specific applications. In order to achieve
improved biocompatibility, the surface properties should
be modified. The most popular method for modification
of surface properties of any material is application of
non-equilibrium gaseous plasma treatment. The method
is nowadays widely used for the modification of surface
properties of bulk materials.1–18 Plasma of particular
interest is created in pure oxygen or oxygen-containing
gasses. Non-equilibrium plasma has the great advantage
over other techniques and the main one is the ability for
the modification of surface properties while leaving the
bulk properties intact. This ability is due to the fact that
plasma is found at room temperature while chemical
reactivity is extremely high. The chemical reactivity of
plasma depends on plasma parameters but is typically as
reactive as gas inthermal equilibrium would be at the
temperature of several 1000 °C. Extremely high chemi-
cal reactivity is due to the presence of gaseous particles
with a high potential energy.
2 EXPERIMENTAL PROCEDURES
Experiments were performed with model PET films
with a thickness of about 50 nm that were prepared as
follows. Foils of commercially available PET were
Mylar® foil, with a thickness of 175 μm. Mylar® foil
was used as a facsimile for PET used in medical devices
such as knitted or woven vascular grafts and PET
angioplasty balloons. The foil was dissolved in
1,1,2,2-tetrachloroethane. PET films were deposited onto
silicon wafers of rectangular shape with dimensions 1.5
cm × 3 cm × 0.20 cm. In order to achieve a uniform
coating the crystals were mounted into a spin coater
rotating at a maximum of 2000 r/min for 60 s. After this
procedure the crystals were dried in an oven at 105 °C
for 1 h. The surface free energy (SFE) of the samples
was calculated from measured contact angles of water
and diiodomethane drops according to OWRK (Owens,
Wendt, Rabel, Kaelble) approach. For contact angles
Materiali in tehnologije / Materials and technology 45 (2011) 3, 275–279 275
UDK 678:541 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)275(2011)
(CA) measurements we used a professional device OCA
35 from Dataphysics (Germany). The device comprises a
high speed camera that allows for pretty accurate
determination of contact angles for different liquid
drops. OWRK approach is one of the most common
methods for calculation of the SFE of polymer materials.
This model is based on assumption that the interfacial
energy can be split to disperse and polar interactions
equation (1). If liquid and solid surfaces come into
contact there would be an interaction between polar parts
as well as disperse parts of both phases at the interphase,
but not between polar and disperse parts.19–23Therefore,
one can determine the polar and disperse part of surface
free energy by measuring contact angle between a solid
and liquids with different polarity.
      sl s l s
d
l
d
s
p
l
p= + − ⋅ + ⋅2( ) (1)
Where sl is solid liquid interface free energy, s is
solid free energy and is surface tension of liquid, d and p
in superscript correspond to polar and dispersive part of
free energies. On the basis of Young equation, the work
of adhesion can be calculated from the measured contact
angle of a liquid on a solid.19–23 With rearranging
equation (1), the form as shown in equation (2) is
obtained and the solid surface free energy is determined
from measurement of the contact angle for water and
diiodomethane with accurately known polar and disper-
sive components of their surface tension (Table 1). In the
work approach, the contact angles of liquids with known
values of l, ld and lp are measured after which data are
fitted to straight line equation (3).
 





l
l
d s
p l
p
l
d s
d
( cos )1+
=
⎛
⎝
⎜
⎞
⎠
⎟ + (2)
y = k · x + b (3)
The equation (2) is the linear function, where the
slope of line, k (k =  s
p ) is the polar component and
intercept of resulting line, b (b =  s
d ) is dispersive
component of the solid’s surface free energy (Figure 5).
The surface free energy calculations were performed
using the software SCA 21, supplied with the measuring
device (Dataphysics, Germany).
At present experiments thevolume of all drops were
fixed to 3 μL. Measurements were performed at room
temperature with at least 5 repetitions and with an
experimental error within ±2 %.
Table 1: Surface tensions of test liquids for SFE determination
Tabela 1: Povr{inske napetosti preizkusnih raztopin za dolo~anje
povr{inske proste energije
l/(mN/m)  l
p/(mN/m)  l
d/(mN/m)
Water 72.8 51.0 21.8
Diiodomethane 50.8 0 50.8
Samples were exposed to oxygen plasma in a plasma
reactor which has been described previously.24–26 The
plasma reactor is pumped with a two stage rotary pump
and never baked, so the ultimate pressure was 6 Pa.
Experiments were performed at the pressure of 50 Pa.
The density of charge particles and neutral oxygen atom
density were measured with an electrical and a catalytic
probe.26–34 The density of charge particles was 5 ·
1015/m3, while the density of neutral oxygen atoms was 3
· 1021/m3.
3 RESULTS
The contact angles of water and DIM drops were
measured at different treatment times up to 24 s. The
photography of a water drop on an untreated sample is
presented in Figure 1. The contact angle is rather high at
77°, indicating reasonable hydrophobic character of PET.
A photo of a DIM drop is presented in Figure 2. The
contact angle of this liquid is much lower at about 23°.
Even a short exposure of a sample to oxygen plasma
causes a huge change of the water drop contact angle.
Figure 3 represents a photo of a water drop after
exposure to oxygen plasma for 1.5 s. On the other hand,
the contact angle of DIM drop remains practically the
same. Figure 4 is a photo of drop after 1.5 s of plasma
treatment. The contact angles have been measured at
various treatment times and the results are summarized
in Figure 6.
The contact angle of the water drops quickly
decreased from initial 77° to about 17°. This quick drop
happened in about 1 second of plasma treatment. With
A. DOLI[KA, M. KOLAR: TUNING OF POLY(ETHYLENE TEREPHTALATE) (PET) SURFACE PROPERTIES ...
276 Materiali in tehnologije / Materials and technology 45 (2011) 3, 275–279
Figure 2: A photography of the diiodomethane drop on untreated PET
sample
Slika 2: Fotografija kapljice dijodometana na neobdelanem PET-
vzorcu
Figure 1: A photography of the water drop on untreated PET sample
Slika 1: Fotografija vodne kapljice na neobdelanem PET-vzorcu
further treatment only small, but continuous decrease of
water drop contact angle is observed. The behavior the
DIM is completely different. The initial contact angle is
about 23° and it remains practically unchanged for the
first 6 s of plasma treatment. The contact angleincreases
up to about 33° for prolonged treatments. The results
summarized in Figure 6 allow for calculation of the
surface free energy (SFE) according to the procedure
described above. The total SFE versus the plasma
treatment time is plotted in Figure 7. The measurements
performed by two different liquids, i.e. water with the
surface tension of 72.8 mN/m and DIM with the surface
tension of 50.8 mN/m, allow for the determination of the
dispersion and polar components of the PET surface
energy. The polar and dispersion components of surface
tension for both liquids are presented in Table 1. The
calculated polar and dispersion components of surface
free energies versus the plasma treatment time are
presented in Figure 8.
4 DISCUSSION
The results summarized in Figures 6 to 8 reveal
some interesting features that are worth discussing. A
fast drop of the water drop contact angle can be
explained by characteristics of oxygen plasma applied at
our experiments. As mentioned earlier the density of
oxygen atoms in our plasma is 3 · 1021/m3. The flux of
neutral oxygen atoms on the surface of a sample is
calculated using basic equation
j nv=
1
4
(4)
Where the n is the density of neutral oxygen atoms in
the vicinity of our sample and v is the average of the
thermal velocity of neutral oxygen atoms which is
calculated as
v
kT
m
=
8
π
(5)
Here, k is the Boltzmann constant, T the neutral gas
kinetic temperature and m is the mas of one oxygen
atom. Taking into account numerical values and
assuming the gas is at room temperature the velocity is
630 m/s. The resultant flux of neutral oxygen atoms is
about 5 · 1023/(m2 s). This value is really high. The
typical surface density of atoms in solid materials is in
the order of 1019/m2. If all oxygen atoms reaching the
surface interacted with solid materials a monolayer of
oxygen atoms would form in less than 0.1 ms. The
reaction probability is, of course, never equal to 1, but
the huge flux of atoms onto the surface of our samples
allowed for a rapid saturation of polymer surface with
A. DOLI[KA, M. KOLAR: TUNING OF POLY(ETHYLENE TEREPHTALATE) (PET) SURFACE PROPERTIES ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 275–279 277
Figure 5: The OWRK model plot for SFE determination of PET
surface
Slika 5: Graf OWRK modela za dolo~itev povr{inske proste energije
PET-povr{ine
Figure 3: A photography of the water drop on oxygen plasma treated
PET sample for 1.5 s
Slika 3: Fotografija vodne kapljice na PET-vzorcu, ki je bil obdelan s
kisikovo plazmo 1,5 s
Figure 6: The contact angles of water and diiodomethane on model
PET surface at various oxygen plasma treatment times
Slika 6: Kontaktni koti kapljic vode in dijodometana na modelnih
PET- povr{inah pri razli~nih ~asih obdelave s kisikovo plazmo
Figure 4: A photography of the diiodomethane dropon oxygen plasma
treated PET sample for 1.5 s
Slika 4: Fotografija kapljice dijodometana na PET-vzorcu, ki je bil
obdelan s kisikovo plazmo 1,5 s
oxygen functional groups. Oxygen functional groups are
known to be very polar and this explains a rapid increase
of the polar component of SFE just after a brief exposure
of the samples to oxygen plasma. Figure 7 reveals that
the SFE keeps increasing slightly with plasma treatment
time even though the surface must have been already
saturated with functional groups. Such increase can be
explained by increased surface roughness. Namely,
numerous authors have shown that the treatment of any
material by oxygen plasma results in increased surface
roughness.32–34
The behavior of a DIM drop is completely different
as shown in Figure 6. The measurements reveal that the
contact angle of DIM drops remains practically
unchanged for the first several second of plasma
treatment and increases only slightly with prolonged
treatment time. This behavior is not unexpected taking
into account upper considerations. Namely oxygen
plasma causes formation of polar functional group that
definitely affects the polar component but has little effect
on the dispersion component of SFE. Since DIM a non-
polar liquid it cannot be affected by formation of polar
functional groups. Figure 8 even reveals a continuous
decrease of dispersion component. This effect can be
explained by increased surface roughness. Namely, while
the surface roughness increases, real surface area be-
comes larger and this causes a decrease of the dispersive
component.
The total SFE is just a sum of the dispersive and
polar components. It is presented in Figure 7. As
expected, the total SFE increases rapidly when the
samples are exposed to oxygen plasma but, since the
saturation occurs quickly, the total SFE remains barely
unchanged with prolonged treatment. From this point of
view it is obvious that the very brief exposure of PET
samples to oxygen plasma is efficient for modification of
the surface properties of our samples. Such a rapid
process allows for negligible modifications of the bulk
properties. Neutral oxygen atoms are thermal so they
cannot penetrate to any depth due to ballistic effects.
They could penetrate into bulk material at elevated
temperatures. As mentioned earlier, our plasma is kept at
room temperature (in terms of neutral gas kinetic tempe-
rature), so the samples cannot be heated by accommo-
dation of neutral oxygen atoms and molecules on the
polymer surface. The samples could be heated by other
effects. One of them is bombardment of the surface by
oxygen ions. The density of ions in our plasma, however,
is very low so this heating effect is easily neglected.
More important should be heterogeneous surface
recombination of neutral oxygen atoms. These atoms
may represent a pretty important source of heating as
showed by several authors.38–47 Happily enough,
however, the required treatment time is only about 1 s so
even this heating channel can neglected. The temperature
of our samples therefore remains close to the room
temperature so any bulk modifications are absent.
5 CONCLUSION
The surface free energy can be determined rather
precisely by measuring contact angles of testing liquids
with a known surface tension against polymer surface.
Surface free energy of PET surface was modified by
treatment with oxygen plasma. The results show an
increase of the surface free energy from around 47
mN/m to almost 75 mN/meven after a brief exposure of
the polymer to oxygen plasma. The increase is due to
increasing of polar component of surface free energy
because oxygen plasma causes formation of polar
functional group on a polymer. Such a surface is more
hydrophilic than untreated one and it is expected that
such a surface is by far more biocompatible than original
material. The results of our experiments showed that
optimal results were achieved even after a second of
plasma treatment so the method is suitable for modifi-
cation of surface properties of delicate organic materials
that do not stand heating to elevated temperatures.
A. DOLI[KA, M. KOLAR: TUNING OF POLY(ETHYLENE TEREPHTALATE) (PET) SURFACE PROPERTIES ...
278 Materiali in tehnologije / Materials and technology 45 (2011) 3, 275–279
Figure 8: The calculated polar and dispersion components of surface
free energies versus the plasma treatment time
Slika 8: Izra~unane vrednosti polarne in disperzne komponente
povr{inske energije v odvisnosti od ~asa obdelave s kisikovo plazmo
Figure 7: The surface free energies of PET versus the plasma
treatment time
Slika 7: Povr{inska energija PET-vzorcev v odvisnosti od ~asa
obdelave s kisikovo plazmo
ACKNOWLEDGMENTS
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Tech-
nology of the Republic of Slovenia through contract No.
3211-10-000057 (Centre of Excellence Polymer Mate-
rials and Technologies).
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A. DOLI[KA, M. KOLAR: TUNING OF POLY(ETHYLENE TEREPHTALATE) (PET) SURFACE PROPERTIES ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 275–279 279

A. DRENIK et al.: PROBABILITY OF RECOMBINATION AND OXIDATION OF O ATOMS ON a-C:H SURFACE
PROBABILITY OF RECOMBINATION AND OXIDATION
OF O ATOMS ON a-C:H SURFACE
VERJETNOST ZA REKOMBINACIJO IN OKSIDACIJO ZA ATOME
KISIKA NA POVR[INI a-C:H
1Aleksander Drenik, 2Kristina Eler{i~, 2Martina Modic, 2Peter Panjan
1Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, 1000 Ljubljana, Slovenia.
2Jo`ef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
aleksander.drenik@ijs.si
Prejem rokopisa – received: 2011-03-01; sprejem za objavo – accepted for publication: 2011-04-04
In the search for a sustainable energy source for future generations, thermonuclear fusion should not be left unconsidered. One
of the important problems of current and near-future fusion devices is the formation of amorphous hydrogenated carbon deposits
(a-C:H), which have to be removed regularly. Removal of a-C:H by atomic oxygen seems like a suitable candidate for a cleaning
method. Efficiency of the cleaning method will depend on the efficiency of atomic oxygen delivery. This in turn will depend on
the atom loss on reactor walls, which is predominantly governed by recombination. An experiment was performed to measure
the recombination coefficient of a-C:H for neutral oxygen atoms. The source of atomic oxygen was an inductively coupled RF
discharge, created in pure oxygen. The oxygen densities were measured by a nickel tipped FOCP. The a-C:H sample was
prepared by thermionic arc sputtering of a graphite target in a mixed argon / acetylene atmosphere. The recombination
coefficient was found to be of the order of 10–3. Moreover, it was discovered that the a-C:H deposition was eroded by O atoms
during the experiment. In a rough estimate, the probability of oxidation was found to be two orders of magnitude lower than the
probability of recombination.
Key words: weakly ionised plasma, atomic oxygen, recombination, a-C:H, Fiber Optic Catalytic Probe
Pri iskanju trajnostnega energijskega vira za prihodnje generacije ne smemo prezreti termonuklearne fuzije. Ena od
najpomembnej{ih te`av sodobnih in prihodnjih fuzijskih naprav je nalaganje amorfnih hidrogeniziranih ogljikovih nanosov
(a-C:H), ki jih je treba redno odstranjevati. Primeren kandidat za metodo ~i{~enja se ponuja odstranjevanje a-C:H z atomskim
kisikom. U~inkovitost te metode bo odvisna od u~inkovitosti dotoka atomskega kisika k povr{ini. Ta pa bo odvisna od izgub
atomov na stenah rekatorja, v katerih je najve~ji dele` rekombinacija. Izvedli smo eksperiment z namenom izmeriti
rekombinacijski koeficient a-C:H za nevtralne kisikove atome. Vir atomskega kisika je bila induktivno sklopljena
RF-razelektritev v ~istem kisiku. Gostote atomskega kisika smo merili z nikljevo opti~no kataliti~no sondo. Vzorec a-C:H je bil
pripravljen z razpr{evanjem grafitne tar~e v atmosferi me{anice argona in acetilena s termionskim oblokom. Ugotovili smo, da
je rekombinacijski koeficient reda velikosti 10–3. Nadalje smo ugotovili, da je med eksperimentom nastala erozija a-C:H-vzorca
zaradi delovanja O-atomov. Z grobo oceno smo ugotovili, da je verjetnost za oksidacijo dva velikostna reda ni`ja od verjetnosti
za rekombinacijo.
Klju~ne besede: {ibko ionizirana plazma, atomarni kisik, rekombinacija, a-C:H, Opti~na kataliti~na sonda
1 INTRODUCTION
In the effort to satisfy the ever growing energy needs
of the human civilization, mankind will have to look
beyond the horizon of currently used energy sources,
among which are fossil fuels are prevalent.1 Resources of
fossil fuels are known to be limited and moreover, the
use of fossil fuels is linked to the emission of greenhouse
gases, such as CO2, which is also a very significant
problem.2,3 While renewable sources offer production of
energy without the high environmental costs, they are
unfortunately not efficient enough to completely replace
the fossil fuels.4 Nuclear fission seems like a powerful
source of energy, which is also not linked to CO2
emissions. However, the fission fuel reserves are limited
and their use is associated to the problem of nuclear
waste. In contrast, when obtaining energy from nuclear
fusion, the reaction is fueled by isotopes of hydrogen and
lithium that are plentiful in nature. Moreover, the end
products of the fusion reaction are stable elements, so
there is no problem of radioactive waste. Unfortunately,
the controlling the fusion reaction is much more difficult
than the fission reaction. Despite the efforts invested in
this field, so far there are no operating fusion power
plants. However, individual experiments have shown that
fusion can indeed be both achieved in a laboratory and
controlled.5
The reaction that is most suitable for controlled
fusion is:
D + T  4He(3.5 MeV) + n(14.1 MeV) (1.1)
where a deuterium and tritium nucleus join to form a
helium nucleus and a neutron. In order to overcome the
electrostatic potential of the positively charged nuclei, a
substantial amount of energy must be invested into the
reaction (0.4 MeV). This is most easily achieved by
heating the fuel to a sufficiently high temperature, so that
the thermal kinetic energy of the hydrogen isotopes is
high enough – this is referred to as thermonuclear fusion.
The required temperature is several thousands of Kelvin,
at which the gas is ionized and becomes plasma.
Currently, the most promising reactor type for achieving
Materiali in tehnologije / Materials and technology 45 (2011) 3, 281–285 281
UDK 533 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)281(2011)
thermonuclear fusion is the tokamak. It is a toroidal
plasma reactor, where the plasma is confined by
magnetic field. The confining magnetic field is produced
both by external magnets, as well as the electric current
of the plasma itself.
While the plasma is magnetically confined, certain
contact between the plasma and reactor walls is required
for the normal operation of the reactor. In most modern
fusion devices, this takes place in the divertor. The heat
loads on the surfaces of plasma facing components can
be extremely high, in the forthcoming ITER, they are
expected to rise up to 10 MW/m2.6 For that reason, the
plasma facing components are constructed using mate-
rials which can sustain such high thermal loads. One of
such materials is carbon fiber composite (CFC), which
features excellent thermal conductivity and can also
withstand very high temperatures. The main drawback of
the CFC is that it is highly susceptible to chemical
erosion by hydrogen.
Hydrogen atoms from the plasma interact with
carbon atoms in the PFCs and form carbohydrate mole-
cules which are desorbed from the wall. This way, they
can migrate throughout the reactor before they are finally
re-deposited on the wall of the reactor. This gives rise to
the formation of amorphous hydrogenated carbon depo-
sits (a-C:H) in the walls of the reactor.7 Depending on
the position of deposit formation, the hydrogen content
in the deposit can be as high as 40 %. Because fusion
reactors operate on a deuterium-tritium fuel mixture, the
a-C:H layers also contain tritium, the radioactive isotope
of hydrogen. The buildup of a-C:H layers inside the
reactor cause the reactor vessel itself to become radio-
active, which is a very much undesired effect, and a
serious security risk. In order to ensure an uninterrupted
operation of a fusion device, these deposits should be
regularly removed.8
While there is – and most probably there will not be
– one definite method, but rather a combination of
various cleaning methods, oxidation of a-C:H deposits is
certainly a very suitable candidate. So far, thermal oxi-
dation (baking in O2 atmosphere) has shown promising
results, however the a-C:H removal rates are too low for
application in modern day and future fusion devices. A
much more efficient technique would be removal of
a-C:H by atomic oxygen. Neutral oxygen atoms, which
are the predominant species in weakly ionized oxygen
plasma, are known to be very reactive, especially in
interaction with organic (e.g. carbohydrate) materials.
The interaction of weakly ionized oxygen plasma
with organic materials is spread over a very broad
spectrum. The effect of oxygen atoms on materials can
either be very non-invasive, modifying only the topmost
atomic layers of the surface, such as is the case in
surface activation,9–17 or it can be destructive, as it is with
the application of weakly ionized plasma for destruction
of bacteria (sterilization).18,19 However, it should be noted
that even when atomic oxygen interacts destructively
with the processed material, the destruction is very
selective. Indeed, the selective etching by neutral oxygen
atoms is a well known and documented feature.20–24 The
selectivity is believed to stem from the different erosion
yields, based on different hydrogen content of the
processed material. The general rule is that the more
hydrogen the material contains (the "softer" it is), the
higher is the erosion yield for neutral oxygen atoms.25
The etching efficiency along with the selectivity of
etching could allow for successful removal of a-C:H
without causing significant damage to carbon based
reactor components. Weakly ionized oxygen plasma has
already been used as a source of atomic oxygen in
experiments of a-C:H removal, yielding removal rates as
high as 10 nm/s,26 and in experiments dealing with
interaction of oxygen atoms with tungsten containing
a-C:H deposits at extreme conditions.27
A successful technique for removal of a-C:H requires
efficient delivery of atomic oxygen to the contaminated
surface, which means that the density of oxygen atoms at
the contaminated surface should be relatively high, even
if that surface is not in the direct vicinity of the atom
source. The transmission of atoms from the source to the
target, so to speak, depends in a great way on losses of
atoms on the surrounding reactor walls. Among the
many possible reactions that can undergo during the
interaction of atomic oxygen with solid materials,28–40
heterogeneous recombination, reaction in which two
oxygen atoms join to form an oxygen molecule, is in
most cases the most probable reaction. Due to the laws
of conservation of momentum and energy, in gas phase it
requires a three body collision, which is an extremely
rare event at low pressures, so it is safe to assume it only
takes place on surfaces of solid materials. Since
recombination is the most important contribution to atom
losses, it has a very significant effect on the atomic
oxygen density inside the reactor. Thus, the probability
of heterogeneous recombination, or recombination
coefficient  is of key importance when planning a-C:H
removal systems.
In this paper, we present an experiment in which we
observed both recombination of oxygen atoms on an
a-C:H surface, as well as oxidation of a-C:H. Based on
these observations, we compare the probability of
recombination and the probability of oxidation.
2 EXPERIMENTAL
We performed an experiment in the afterglow of
weakly ionized oxygen plasma with the intent to
determine the recombination coefficient of a-C:H for
oxygen atoms. The a-C:H sample was prepared in a
thermionic arc sputtering system, by sputtering a
graphite target in an atmosphere of argon and acetylene.
The material used for the substrate, was aluminum foil,
due to its favorable mechanic properties. The thickness
of the deposited a-C:H film was 0.5 μm. The a-C:H
covered foil was inserted into a side-arm of our plasma
A. DRENIK et al.: PROBABILITY OF RECOMBINATION AND OXIDATION OF O ATOMS ON a-C:H SURFACE
282 Materiali in tehnologije / Materials and technology 45 (2011) 3, 281–285
reactor, so that the a-C:H covered side was facing the
inside of the tube, as seen in Figure 1. The reactor was
powered by an inductively coupled radiofrequency
generator, operating at 27.12 MHz with an approxi-
mately 100 W output power. Plasma was ignited in pure
oxygen, leaked into the plasma reactor at pressures
between 60 Pa and 180 Pa. The resulting atomic oxygen
densities in the direct vicinity of the a-C:H sample were
from 5 · 1021 m–3 to 2 · 1022 m–3. A spatially resolved
measurement of atomic oxygen densities was performed
using a nickel tipped fiber optic catalytic probe
(FOCP).41–47 The FOCP was used to measure the atomic
oxygen density at several discretely spaced points along
the axis of the side-arm, at different pressures. Simul-
taneously, another nickel tipped FOCP was kept at con-
stant position in the reactor to monitor the fluctuations of
the atomic oxygen density that might affect the results.
3 RESULTS AND DISCUSSION
An example of the recorded atomic oxygen density
profile, recorded at a fixed pressure, is shown in Figure
2. A physical model, based on the diffusion equation,48
was employed to determine the values of the recom-
bination coefficient from the recorded atomic oxygen
profiles. Fitting the model over the density profiles
obtained at all the different pressures, we get the value of
the recombination coefficient  = (1.0 ± 0,3) · 10–3.
When we removed the foil from the experimental
system, we noticed that the a-C:H film had been partially
eroded during the experiment. As seen in Figure 3, the
deposit was removed from the substrate up to 2 cm away
from the edge of the foil. This, of course, means that the
obtained values of the recombination coefficient are not
accurate, as during the measurement, the surface of the
sample changed from a-C:H to aluminum. Using only
the data obtained in the non-eroded region, the re-cal-
culated value of recombination coefficient is (9.9 ± 0,2) ·
10–4. As the value does not greatly differ from the first
calculation, we assume that the value of the recom-
bination coefficient of the substrate (Al) is similar to that
of a-C:H. However, that is not the only problem that has
arisen due to the erosion of the sample. The model we
used to determine the value of the recombination
coefficient assumes that the only mechanism of atom
loss on the walls is recombination when clearly, the
oxygen atoms engage in oxidation as well. However,
A. DRENIK et al.: PROBABILITY OF RECOMBINATION AND OXIDATION OF O ATOMS ON a-C:H SURFACE
Materiali in tehnologije / Materials and technology 45 (2011) 3, 281–285 283
Figure 3: Aluminum foil with the deposited a-C:H thin film after the
experiment. The foil used in the experiment was cut out from the piece
that is framing it in the picture. The difference between the eroded and
non-exposed a-C:H deposits are easily observable.
Slika 3: Aluminijeva folija z nanosom a-C:H po eksperimentu. Folija,
ki smo jo uporabili pri eksperimentu, je bila izrezana iz ve~jega kosa
folije, ki jo na sliki uokvirja. Razlika med pojedkanim in nespre-
menjenim vzorcem je o~itna.
Figure 1: Experimental set-up: a) manipulation of the a-C:H covered
foil, arrow indicates direction of folding; b) insertion of foil (2) and
the FOCP (4) into the side-arm (2). The FOCP is used to measure the
density of atomic oxygen along the axis (1) of the side-arm; c)
position of the additional FOCP (top side) that measures the atomic
oxygen density at a constant position.
Slika 1: Eksperimentalna postavitev: a) zvijanje folije, pokrite z
a-C:H, pu{~ica nakazuje smer zvijanja; b) vstavljanje folije (2) in
opti~ne kataliti~ne sonde (4) v stransko cev (2). Z opti~no kataliti~no
sondo merimo gostoto atomarnega kisika v osi (1) stranske cevi; c)
polo`aj dodatne opti~ne kataliti~ne sonde (zgoraj), s katero merimo
gostoto atomskega kisika na nespremenljivem polo`aju.
Figure 2: Atomic oxygen density profile recorded at 120 Pa. The term
z denotes the distance from the probe tip to the opening of the
side-arm.
Slika 2: Profil gostote atomskega kisika, posnet pri 120 Pa. Izraz z
ozna~uje razdaljo med ustjem stranske cevi in konico sonde.
while determination of the erosion rate was not the
primary aim of this experiment, we can nonetheless
make a rough estimate of the erosion rate and sub-
sequently the loss of atoms due to oxidation.
The topmost, eroded layer of the sample received, in
approximately two hours of total operation time, a dose
of neutral oxygen atoms:
G = ⋅15 10 28 3. / m (1.2)
The thickness of the deposit was h = 0.5 μm, which
results in an erosion rate of  	 0.06 nm/s. Assuming the
density of carbon atoms in the deposit to be the same as
the density of carbon atoms in graphite, n = 1.24 ·
1029/m3, we get the number of removed atoms per square
meter in those two hours, N = 6 · 1022/m2. Further
assuming that each eroded carbon atom reacts with one
oxygen atom from the gas phase, the probability for
oxidation is the ratio between the oxygen atoms that
engage in oxidation and the total number of impinging
oxygen atoms:
P
N
= =
⋅
⋅
= ⋅ −
G 6 10
15 10
4 10
22
28
6
.
(1.3)
which is for a factor of 250 lower than the probability of
recombination. Thus, we can safely conclude that pro-
bability of the oxygen atom engaging in recombination
is at least two orders of magnitude higher than the
probability it would engage in an oxidation reaction.
Moreover, since oxidation is only a minor contribution
to the atom loss on the a-C:H surface, we see that the
erosion hasn’t further affected the results of this
experiment.
4 CONCLUSION
An experiment was performed to determine the
recombination coefficient of a-C:H for neutral oxygen
atoms. The sample was prepared by thermionic arc
sputtering deposition on aluminum foil. An inductively
coupled radiofrequency discharge in pure oxygen was
used as the source of atomic oxygen. Neutral oxygen
atom densities were measured with a fiber optic catalytic
probe. After the experiment, it was found that the a-C:H
film had been eroded during the experiment. Using a
rough estimate, we conclude that an impinging oxygen
atom will take part in an oxidation reaction is 4 · 10–6.
The value of the recombination coefficient, obtained
using a diffusion model, was found to be (9.9 ± 0,2) ·
10-4. Thus, we found the probability of recombination
two orders of magnitude greater that the probability of
oxidation.
ACKNOWLEDGEMENT
The authors acknowledge the financial support from
the Ministry of Higher Education, Science and Tech-
nology of the Republic of Slovenia through the contract
No. 3211-10-000057 (Center of Excellence Polymer
Materials and Technologies).
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A. DRENIK et al.: PROBABILITY OF RECOMBINATION AND OXIDATION OF O ATOMS ON a-C:H SURFACE
Materiali in tehnologije / Materials and technology 45 (2011) 3, 281–285 285

M. BITENC, Z. CRNJAK OREL: HYDROTHERMAL GROWTH OF Zn5(OH)6(CO3)2 AND ...
HYDROTHERMAL GROWTH OF Zn5(OH)6(CO3)2 AND
ITS THERMAL TRANSFORMATION INTO POROUS ZnO
FILM USED FOR DYE-SENSITIZED SOLAR CELLS
HIDROTERMALNA RAST Zn5(OH)6(CO3)2 S TERMI^NO
TRANSFORMACIJO V POROZNO PLAST ZnO, UPORABLJENO ZA
ELEKTROKEMIJSKE SON^NE CELICE
Marko Bitenc, Zorica Crnjak Orel
Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, 1000 Ljubljana, Slovenia
marko.bitenc@ki.si
Prejem rokopisa – received: 2011-02-14; sprejem za objavo – accepted for publication: 2011-03-15
Zinc oxide (ZnO) films, composed of nano-sheets, were prepared by the thermal decomposition of hydrozincite film precursor
(Zn5(OH)6(CO3)2, ZnHC). The ZnO film kept the morphology on the microscale level during the heat treatment, while the
thermal decomposition caused the formation of a nano-porous structure of the nano-sheets’ surface. ZnHC precursor was
hydrothermally synthesized on the conductive glass from zinc nitrate and urea. The influence of media (water or mixture of
water/ethylene glycol), PVP-K additive and the concentration of the initial reagents, on the morphology of the film were
observed. The growth and morphology of the ZnHC film was followed with FE-SEM microscopy and the formation mechanism
of the ZnHC film, formed in the water, was proposed. The assumed growth mechanism follows the concept of a self-assembling
mechanism of the nanometer-sized building units into nano-sheets of ZnHC with a thickness of 	20 nm. These nano-sheets are
further agglomerating into porous film. The thickness of the film was around (5, 8, 10 and 20) μm after (2, 3, 4 and 24) h of
synthesis, respectively. The addition of PVP-K and ethylene glycol retards the growth of ZnHC film. Moreover, the addition of
PVP-K affects the interactions between particles and the substrate, which enable the parallel growth of the nano-sheets on the
substrate surface. As prepared ZnO films were used for DSSCs, where the conversion efficiency was 	0.6 %.
Keywords: ZnO, hydrozincite, hydrothermal, precipitation, particles growth, SEM, DSSCs
Cinkov oksid (ZnO) smo pripravili kot tako plast na prevodni podlagi s termi~no obdelavo prekurzorja hidrocinkita
(Zn5(OH)6(CO3)2, ZnHC). Tako pripravljena plast je bila sestavljena iz nanolisti~ev, ki so po termi~ni obdelavi obdr`ali svojo
obliko na mikroskopskem nivoju, medtem ko so na njihovi povr{ini nastale luknjice nanovelikosti. Plast ZnHC smo pripravili s
hidrotermalno precipitacijo cinkovega nitrata s se~nino. Ob tem smo opazovali vpliv razli~nih koncentracij za~etnih reagentov,
medija (voda in me{anica vode/etilen glikola) in dodatkov (PVP-K) na rast in morfologijo nastalega produkta, ki smo ju
spremljali z FE-SEM-mikroskopijo. Na osnovi rezultatov opazovanja vzorcev, pripravljenih v vodnem mediju, smo
predpostavili mehanizem rasti plasti ZnHC. Ugotovili smo, da ta mehanizem sledi konceptu povezovanja nanogradnikov v ve~je
urejene kristalne mikrostrukture z obliko nanolisti~ev z debelino 	20 nm. Nanolisti~i ZnHC se na podlagi aglomerirajo v
porozno plast. Debelina plasti je bila pribli`no 5 μm po 2 h, 8 μm po 3 h, 10 μm po 4 h in 20 μm po 24 h sinteze. Dodatek
PVP-K in etilen glikol upo~asnita rast plasti ZnHC. Poleg tega je dodatek PVP-K vplival na interakcije med listi~i in podlago, ki
omogo~ijo rast listi~ev vzporedno s povr{ino podlage. Plasti ZnO smo uporabili za izdelavo elektrokemijskih son~nih celic,
katerih u~inkovitost pretvorbe son~ne energije v elektri~no je 	0,6 %.
Klju~ne besede: ZnO, hidrocinkit, hidrotermalno, precipitacija, rast delcev, SEM, elektrokemijske son~ne celice
1 INTRODUCTION
ZnO nano-sheets have attracted increasing interest on
account of their specific microstructure and potential
applications in many areas, such as chemical sensors,
photocatalysts, phosphors and dye-sensitized solar cells
(DSSCs).1–4 For these applications the ZnO particles are
typically prepared as a film on various glass or polymer
substrates, covered with transparent conducting oxides
(TCO). ZnO films and powders, reported in the litera-
ture, are most commonly composed of particles with the
hexagonal rod-like morphology2,5–9 and rarely with
nano-sheet morphology prepared from the corresponding
layered ZnHC precursor with the thermal decompo-
sition.1,3,10 The hydrothermal precipitation of ZnHC from
zinc nitrate with urea was presented in our papers, where
the influence of the different additives on the final
morphology of the particles was studied in a closed
reactor system.11 The precipitated ZnHC decomposed
into ZnO in only one relatively sharp step. The growth
mechanism of the ZnHC particles based on the combi-
nation of in-situ SAXS measurement and ex-situ electron
microscopy was also presented in our recently published
paper.4
The interest in dye-sensitized solar cells (DSSCs) has
increased due to reduced energy sources and higher
energy production costs. For the most part, titania (TiO2)
has been the material of choice for dye-sensitized solar
cells and so far have shown the highest overall light con-
version efficiency 	11 %.12 However, ZnO has recently
been explored as an alternative material in DSSCs with
great potential since it has a bandgap similar to TiO2 at
3.2 eV and has a much higher electron mobility
(	(115–155) cm2/(V s)) than TiO2 (	10–5 cm2/(V s)). In
addition, ZnO has ability of simpler tailoring of the
Materiali in tehnologije / Materials and technology 45 (2011) 3, 287–292 287
UDK ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 45(3)287(2011)
particles’ morphology. Also it can not be neglected, that
the initial compounds for the production of ZnO are
much cheaper as compared to TiO2. The efficiency of the
DSSCs prepared from the ZnO nano-sheet film is lower
regarding DSSCs that use TiO2 for their preparation.1,3 In
the literature it was reported that DSSCs with the ZnO
nano-sheets film electrode, prepared from ZnHC pre-
cursor, have the efficiency around 4 %.
Figure 1 shows a schematic representation of DSSCs
applying vertically-aligned ZnO nanostructured elec-
trode. The typical basic configuration is as follows: At
the heart of the device is the mesoporous oxide layer
composed of ZnO film. Typically, the film thickness is
10 μm and the porosity is 50–60 %. The mesoporous
layer of ZnO is deposited on substrate (polymer or glass)
coated with TCO, such as indium tin oxide (ITO)
fluorine doped tin oxide (FTO) or aluminium doped zinc
oxide (AZO). Attached to the surface of the ZnO film is
a monolayer of the charge-transfer dye, mostly based on
ruthenium. Photoexcitation of the later results in the
injection of an electron into the conduction band of the
oxide, leaving the dye in its oxidized state. The dye is
restored to its ground state by electron transfer from the
electrolyte, which is usually an organic solvent con-
taining the iodide/triiodide redox system. The regene-
ration of the sensitizer by iodide intercepts the recapture
of the conduction band electron by the oxidized dye. The
I3– ions formed by oxidation of I– diffuse because of a
short distance (<50 μm) through the electrolyte to the
cathode, which is coated with a thin layer of platinum
catalyst, where the regenerative cycle is completed by
electron transfer to reduce I3– to I–.
In this work we present the preparation of ZnHC film
from zinc nitrate and urea in different media, such as
water, water/EG mixture and in water with addition of
PVP-K. The growth of the ZnHC film, which was later
thermally decomposed into ZnO, was followed with
FE-SEM microscopy. The formation mechanism of the
ZnHC film, in water, is presented. As prepared ZnO
films were used for DSSCs preparation.
2 EXPERIMENTAL
All reagents in the experimental work were of
analytical reagent grade. To avoid hydrolysis upon
storage, fresh stock solutions prepared from Zn(NO3)2 ×
6H2O (Aldrich) and urea (Aldrich) in millique water
were used. The detailed experimental conditions for the
preparation of samples are given in Table 1. The initial
concentrations of Zn2+ ions were 0.1 M and 0.01 M and
urea 0.5 M and 0.05 M. The concentration of urea was
five times higher than Zn2+ in all experiments. As sol-
vents water and a mixture of water and ethylene glycol
(EG, Merck) with the volume ration 1 : 1, were used.
Experiments were performed without and in the presence
of PVP-K additive. The additive was dissolved in the
initial reactive mixture before hydrolysis took place. The
concentration of additive was 10 mg/mL throughout the
experimental study.
Table 1: Experimental conditions for the sample preparations in water
and water/ethylene glycol (EG) mixture, with and without additive
Tabela 1: Pregled eksperimentov za pripravo tanke plasti na pre-
vodnem steklu v vodi in me{anici voda/etilen glikol (EG) brez dodatka
in z njim
Sample Medium SolutionmL
C (Zn2+)
M
PVP-K
mg/mL
Time
h
A water 50 0.1 / 2, 3, 4, 24
B water 50 0.01 10 24
C water +EG 25 + 25 0.01 / 72
The experiments were carried out in laboratory
bottles, where the total volume of reaction mixture was
50 mL. Borosilicate glass slides, with one side covered
with the SnO2 : F, were used as substrates for the film
deposition. The substrates were placed into laboratory
bottles filled with Zn/urea solution, sealed and kept at 90
°C for 2 h to 72 h in drying oven (Kambi~, Easy) as
presented in Figure 2a. During these experiments, the
temperature was measured in the center of laboratory
bottle with Pt100. The resulting temperature profile
shows that maximum temperature was reached after
around 100 min and was constant (85 °C) during the
synthesis (Figure 2b). After the deposition, the obtained
M. BITENC, Z. CRNJAK OREL: HYDROTHERMAL GROWTH OF Zn5(OH)6(CO3)2 AND ...
288 Materiali in tehnologije / Materials and technology 45 (2011) 3, 287–292
Figure 2: a – Schematic description of the film preparation. b – Tem-
perature profile in laboratory bottle
Slika 2: a – Shema priprave delcev na podlagi; b – Temperaturni profil
v laboratorijski steklenici
Figure 1: Schematic description of DSSCs applying ZnO nano
particles as the electron transport material, dye for light-harvesting
and electrolyte with a I–/I–3 redox couple
Slika 1: Shema elektrokemijske son~ne celice, sestavljene iz
ZnO-polprevodnika za transport elektronov, barvila na polprevodniku
in elektrolita z redoks parom I–/I–3
hydrozincite films were rinsed with ethanol and dried at
room temperature. The films were thermally treated at
300 °C for 30 min in air. As prepared ZnO film were
used for DSSCs preparation.
The DSSCs were prepared and characterized in
Laboratory of Photovoltaics and Optoelectronics of the
Faculty of Electrical Engineering (University of Ljublja-
na) as previously reported in their papers.13–15
Samples were characterized by scanning field
emission electron microscopy (FE-SEM, Zeiss Supra 35
VP). IR-spectra were obtained on an FTIR spectrometer
(Perkin Elmer 2000) in the spectral range between 4000
cm–1 and 400 cm–1 with a spectral resolution of 2 cm–1.
The KBr pellet technique was used for the sample prepa-
ration.
3 RESULTS AND DISCUSSION
The FE-SEM microstructure of the hydrozincite
(ZnHC) film marked as Samples A, which was prepared
on the glass substrate coated with SnO2 : F in water
medium after (2, 3, 4 and 24) h are presented in Figure 3
(top-view) and Figure 4 (cross-section). The porous
spherical structures composed of nano-sheets with
thickness around 20 nm was observed after 2 h of
synthesis as presented in Figures 3a and 4a. The
thickness of ZnHC film is round 5 μm after 2 h of the
synthesis as presented in Figure 4a. From the particles’
microstructure and morphology, we could propose that
ZnHC nano sheets begin to grow from one point (initial
nucleus) on a substrate and not separately, which is
typical for the growth of the pure hexagonal ZnO.16–18
The top-view and the cross-sectional micrographs of the
substrate’s microstructure of the Sample A, obtained
after 3 h (Figures 3b and 4b) and 4 h (Figures 3c and
4c) are presented, respectively. The nano-sheets cover
the entire surface of the substrate. The thickness of
ZnHC film is about 8 μm after 3 h and about 10 μm after
4 h of the synthesis. The thickness of the film increases
up to 20 μm after 24 h of synthesis (Figure 4d). We
found that nano-sheets grow in the length and width,
while the thickness of the nano-sheets was remaining
constant about 20 nm (Figure 3d). The ZnHC film was
found on substrate only on the conductive side and when
this side was facing down (Figure 2a). Scratches and dirt
on conductive side of the substrate retard and limited the
growth of the particles.
The proposed growth mechanism of the ZnHC film
on the TCO substrate is schematically presented in
Figure 5. We propose that growth of the nano-sheets
follows the growth mechanism of the ZnHC particles
which was recently presented by our group. We explain
the growth mechanism by the "non-classical crystalli-
zation" concept of a self-assembling mechanism. The
growth mechanism predicts the rapid formation of
nanometer-sized building units. The size of these nano
building units, stable only in the reaction medium,
remains nearly constant during the synthesis, as the
concentration of the nano building units increases
M. BITENC, Z. CRNJAK OREL: HYDROTHERMAL GROWTH OF Zn5(OH)6(CO3)2 AND ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 287–292 289
Figure 3: FE-SEM microstructure of the ZnHC of Sample A prepared on the glass substrate in water medium after (a) – 2 h, (b) – 3 h, (c) – 4 h in
(d) – 24 h
Slika 3: FE-SEM-mikrostruktura ZnHC-vzorca A, pripravljenega na prevodnem steklu v vodnem mediju pri razli~nih ~asih: (a) – 2 h, (b) – 3 h,
(c) – 4 h in (d) – 24 h
throughout the reaction. These leaves of ZnHC are
further agglomerated into porous, nano-sheet like film.
The thickness of the film is increasing with time.
The influence of the medium (a mixture of water/EG
at volume ratio 1/1), PVP-K additive and the concen-
tration of the initial reagents, on the morphology and
growth of the film were observed (Figure 6). Micro-
structure of Sample B, prepared with the addition of
PVP-K (10 mg/mL) after 24 h of synthesis, is presented
in Figure 6a. We found that the addition of PVP-K
noticeably retard the growth of ZnHC film, since the
microstructure of the particles is similar to the film of
Sample A prepared in water after 2 h of the synthesis
(Figure 3a). Additionally, we observed the parallel
(according to substrate surface) formation of nano-sheets
in the synthesis with the PVP-K, as it is presented in
Figure 6a. We assume the addition of PVP-K affects
M. BITENC, Z. CRNJAK OREL: HYDROTHERMAL GROWTH OF Zn5(OH)6(CO3)2 AND ...
290 Materiali in tehnologije / Materials and technology 45 (2011) 3, 287–292
Figure 6: FE-SEM microstructure of the ZnHC samples prepared on
glass substrate: (a) – Sample B, prepared in water with PVP-K
additive and (b) – Sample C prepared in water/EG mixture
Slika 6: FE-SEM-mikrostruktura delcev ZnHC, pripravljenih na
steklu: (a) – vzorec B, pripravljen v vodi z dodatkom PVP-K in (b) –
vzorec C, pripravljen v me{anici voda/EG
Figure 5: Schematic description of the growth mechanism of ZnHC
film on the substrate
Slika 5: Shemati~no predstavljen mehanizem rasti plasti ZnHC na
podlagi
Figure 4: FE-SEM microstructure of the ZnHC film cross-section of
Sample A prepared on glass substrate in water medium after (a) – 2 h,
(b) – 3 h, (c) – 4 h in (d) – 24 h
Slika 4: FE-SEM-mikrostruktura prereza plasti ZnHC-vzorca A,
pripravljenega na steklu v vodnem mediju ob razli~nih ~asih: (a) – 2 h,
(b) – 3 h, (c) – 4 h in (d) – 24 h
hydrophilic/hydrophobic interactions between particles
and the substrate, which enable the parallel growth of the
nano-sheets on the substrate surface.1 Microstructure of
Sample C, prepared in the mixture of water/EG 1/1, after
72 h of synthesis is shown in Figure 6b. Particle growth
is retarded even at a higher concentration of initial
reagents.
As prepared ZnHC films on the TCO substrate were
additional heat-treated for 30 min at 300 °C. The
FE-SEM microstructure of heat treated film of Sample A
(4 h) is presented in Figure 7 as an example. Similar
effects were observed also in other samples (micrographs
not presented in this paper). Generally the size and the
morphology of the particles observed by FE-SEM
remain practically identical on the micro scale during
heat treatment (Figure 7). However, the annealing
caused the formation of a nanoporous structure on the
particles’ surface, which revealed an increased specific
surface area.11
The samples were characterized by FTIR (Figure 8)
before and after heat treatment to determine the chemical
composition of the prepared films. The FTIR spectrum
of Sample A, prepared after 24 h of synthesis and before
heat treatment, is presented in Figure 8a. The presence
of carbonate groups in the ZnHC product was confirmed
by bands in the spectral range from 1600 cm–1 and 1200
cm–1, i.e. the frequency at 1507 cm–1 and 1390 cm–1, and
additionally with the frequency at 1044 cm–1, 956 cm–1
and at 708 cm–1.4,19 After thermal treatment (300 °C) of
this sample (Sample A, 24 h) the band at 421 cm–1 with a
pronounced shoulder at 530 cm–1 in the IR spectra
(Figure 8b) confirmed the presence of ZnO.
ZnO films, obtained after 4 h and 24 h of the syn-
thesis and additionally heat treated for 30 min at 300 °C,
were used for the preparation of DSSCs.12–15 Average
current density-voltage characteristics of electrochemical
solar cells under standard conditions (100 mW/cm2, 25
°C) are summarized in Table 2. The density-voltage
characteristic curve of the best DSSCs, prepared from
the ZnO film after 4 h of the synthesis is presented in the
Figure 9.
Table 2: An average short-circuit photocurrent density (JSC), open-
circuit voltage (VOC), fill factor (FF), and conversion efficiency () of
the DSSCs evaluated under STC (100 mW/cm2, 25 °C) for the ZnO
films prepared after 4 h and 24 h of synthesis
Tabela 2: Povpre~je meritev kratkosti~ne tokovne gostote (JSC),
napetosti odprtih sponk (VOC), polnilnega faktorja (FF) in u~inko-
vitost pretvorbe () za DSSCs, izmerjen pri standardnih pogojih (100
mW/cm2, 25 °C) in z uporabo plasti ZnO pripravljene po 4 h in 24 h
sinteze
Synthesis
time, h
JSC/
mA/cm2
VOC/
V
FF/
%
/
%
4 2.63 0.52 41.93 0.57
24 3.23 0.50 36.37 0.58
An average open-circuit voltage (VOC) 0.52 V and
0.50 V, and conversion efficiency () 0.57 % and 0.58 %
of the DSSCs, evaluated under STC (100 mW/cm2, 25
°C) and prepared from the ZnO films of the 4 h and 24 h
synthesis, are comparable. However, the short-circuit
photocurrent density (JSC) is higher (3.23 mA/cm2) in a
cell made from the thicker ZnO film (20 μm) prepared
after 24 h of the synthesis, compared to the JSC of the
M. BITENC, Z. CRNJAK OREL: HYDROTHERMAL GROWTH OF Zn5(OH)6(CO3)2 AND ...
Materiali in tehnologije / Materials and technology 45 (2011) 3, 287–292 291
Figure 8: FTIR spectra of Sample A prepared after 4 h of synthesis
(a) and with additional heat treatment (b)
Slika 8: FTIR-spekter vzorca A, pripravljenega po 4 h sinteze (a) in
po dodatni termi~ni obdelavi (b)
Figure 7: FE-SEM microstructure of ZnO film of Sample A (4 h)
after 30 min of heat treatment at 300 °C
Slika 7: FE-SEM-mikrostruktura plasti ZnO-vzorca A (4 h) po 30 min
termi~ni obdelavi na 300 °C
DSSCs prepared from the 10 μm thick ZnO film
obtained after 4 h of synthesis. Contrary, the fill factor
(FF) 41.93 % is higher in a DSSCs prepared from the
thinner ZnO film (4 h synthesis), compared to the FF
36.37 % obtained in the DSSCs with thicker ZnO film.
Results prove that larger film thickness increases the
amount of adsorbed dyes, which leads to higher JSC. On
the other hand, increasing the film thickness decreases
FF through diffusion-limited access.1
The VOC and FF of our DSC cells are comparable to
results reported in the literature.1,3 On the other hand, the
JSC of cells is between 2.63 mA/cm2 and 3.23 mA/cm2 in
a short circuit, which is almost one third of comparable
measurements in literature.1,3 The low JSC could be
explained by the generation of the Zn2+/dye aggregates.
Ru-complexes dyes, such as N-719 (used in our case),
are slightly acid, since they could have protons derived
from the complex.3 Consequently, the dye-loading solu-
tion dissolves the surface of ZnO and makes Zn2+/dye
aggregates, which influence on the lower electron
injection efficiencies and fill nano-scale pores. The
optimization of various parameters including ZnO film
fabrication, dyeing time, heating conditions, and so on,
will result in the improvement of the conversion efficien-
cy because the FF and VOC in this work are reasonably
high.
4 CONCLUSIONS
ZnO film with the nano-sheets porous microstructure
was prepared by the thermal decomposition of ZnHC
precursor, which was hydrothermally synthesized on the
conductive glass from zinc nitrate and urea. The growth
mechanism and the morphology of the ZnHC film was
followed with SEM microscopy and the formation of the
ZnHC film, is proposed by the "non-classical crystalli-
zation" concept of a self-assembling mechanism. The
influence of a mixture of water/EG, surfactant additive
PVP-K and the concentration of the initial reagents, on
the morphology of the film were observed. ZnO films
were used for the preparation of DSSCs with the average
conversion efficiency around 0.6 %.
ACKNOWLEDGMENT
The authors would like to thank the colleges from
Laboratory of Photovoltaics and Optoelectronics (Facul-
ty of Electrical Engineering, UL) for their help regarding
DSSC assembling and characterization. We acknowledge
the financial support from the Ministry of Higher
Education, Science and Technology of the Republic of
Slovenia through the contract No. 3211-10-000057 (Cen-
ter of Excellence for Polymer Materials and Techno-
logies).
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M. BITENC, Z. CRNJAK OREL: HYDROTHERMAL GROWTH OF Zn5(OH)6(CO3)2 AND ...
292 Materiali in tehnologije / Materials and technology 45 (2011) 3, 287–292
Figure 9: A current density-voltage curve of the dye-sensitized solar
cell prepared from ZnO films of Sample A (4 h of synthesis)
Slika 9: Tokovno-napetostni odziv elektrokemijske son~ne celice,
pripravljen iz plasti ZnO vzorca A (4 h sinteze)