C. E. BAN et al.: MULTI-WALLED CARBON NANOTUBES EFFECT IN POLYPROPYLENE NANOCOMPOSITES
11–16
MULTI-WALLED CARBON NANOTUBES EFFECT IN
POLYPROPYLENE NANOCOMPOSITES
VPLIV VE^STENSKIH OGLJIKOVIH NANOCEVK
V NANOKOMPOZITIH IZ POLIPROPILENA
Cristina-Elisabeta Ban1,2, Adriana Stefan1, Ion Dinca1, George Pelin1,2,
Anton Ficai2, Ecaterina Andronescu2, Ovidiu Oprea2, Georgeta Voicu2
1National Institute for Aerospace Research "Elie Carafoli" Bucharest, Materials Unit, 220 Iuliu Maniu Blvd, 061126 Bucharest, Romania
2University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Polizu St., 011061 Bucharest, Romania
ban.cristina@incas.ro
Prejem rokopisa – received: 2014-07-31; sprejem za objavo – accepted for publication: 2015-02-06
doi:10.17222/mit.2014.142
The paper presents a study concerning thermoplastic nanocomposites having polypropylene as the matrix and different contents
of carboxyl-functionalized multi-walled carbon nanotubes as the nanofiller. The materials are obtained by melt compounding
the nanofiller powder and polymer pellets through the extrusion process followed by injection molding into specific-shape
specimens. The materials are evaluated in terms of mechanical properties such as the tensile and flexural strengths and moduli,
the thermal stability under load (the heat deflection temperature) and the thermal-behavior properties using a TG-DSC analysis.
The fracture cross-section is analyzed using FTIR spectroscopy and SEM microscopy to evaluate the bulk characteristics of the
materials. The results show positive effects of the nanofiller addition to the thermoplastic polymer on the mechanical strength
and modulus of the materials during flexural and tensile tests, while in the case of the thermal stability under load, the nanofiller
addition has a minor influence on the heat-deflection-temperature values.
Keywords: polypropylene, melt mixing, carbon nanotubes, mechanical properties, thermal resistance
^lanek predstavlja {tudijo termoplasti~nih nanokompozitov s polipropilensko osnovo in razli~no vsebnostjo s karboksilom
obdelanih, ve~stenskih ogljikovih nanocevk kot nanopolnilom. Materiali so bili dobljeni iz taline, sestavljene iz prahu
nanopolnila in peletov polimerov, s postopkom ekstruzije, ki mu je sledilo tla~no litje vzorcev. Materiali so ocenjeni glede
mehanskih lastnosti, kot so natezna in upogibna trdnost in moduli, toplotne stabilnosti pri obremenitvi (deformacijska toplota)
in toplotne zna~ilnosti z uporabo TG-DSC analize. Za oceno zna~ilnosti osnovnega materiala je bil analiziran prelom s pomo~jo
FTIR spektroskopije in SEM mikroskopije. Rezultati ka`ejo pozitivne u~inke vpliva dodatka nanopolnila termoplasti~nemu
polimeru na mehansko trdnost in module materiala pri upogibnem in nateznem preizkusu, medtem ko ima dodatek nanopolnila
manj{i vpliv na vrednosti deformacijske toplote pri obremenitvi.
Klju~ne besede: polipropilen, me{anje taline, ogljikove nanocevke, mehanske lastnosti, toplotna obstojnost
1 INTRODUCTION
Polymer nanocomposites found applications in a
wide variety of fields, from microelectronics to aero-
space.1 Carbon nanotube-based polymer composites
combine the good processability of the matrix with the
remarkable functional properties of these nanofillers.
Multi-walled-carbon-nanotube (MWCNT) filled isotactic
polypropylene (PP) nanocomposites can be obtained
through several processing methods, such as melt mix-
ing, solution casting and in-situ polymerization, among
them, melt mixing having some major advantages as it
combines high speed and simplicity with the absence of
solvents and contaminants.2 For the production of these
nanocomposites, a double-screw extruder is a more
appropriate device than a single-screw extruder.3 The
formation of a filler network structure (Figure 1)
depends on several parameters, e.g., the concentration or
dispersion states of the nanotubes in the matrix. Carbon
nanotubes have a tendency to form agglomerates that
lead to a decrease in the surface area, consequently
hindering the structure formation.4 The screw speed is a
strong factor that influences the dispersion of the carbon
nanotubes in polypropylene; too high a speed rate can
generate a mechanical degradation of the final nanocom-
posite as a high shear stress can affect the nanotubes
structure, while too low a speed rate may be insufficient
for an aggregate disentanglement.4
Achieving a good dispersion is influenced also by the
surface optimization between the two phases. The
MWCNT-matrix interfacial-adhesion enhancement can
be obtained by modifying the MWCNT surface through
the non-covalent functionalization that maintains the
nanotube structure or the covalent functionalization, such
as acid treatment creating carboxyl and hydroxyl groups
on the surface, that enhances the load transfer to the
matrix.5 There are studies showing that properties
enhancements are achieved for smaller nanotubes con-
tents and moderate acid-treatment times.5,6
The study presents the characterization of isotactic
polypropylene filled with carboxyl-functionalized
MWCNT obtained through the simple melt-extrusion
technique. The results show an improvement in the
tensile and flexural strengths and moduli when adding
Materiali in tehnologije / Materials and technology 50 (2016) 1, 11–16 11
UDK 678.7:620.3:66.017 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(1)11(2016)
the nanofiller. In the case of a higher MWCNT loading
(w(MWCNT) = 4 %), the nanocomposites present a
higher stiffness, decomposition temperature and thermal
stability under load, while at a lower loading
(w(MWCNT) = 2 %) they exhibit a better mechanical
strength.
2 EXPERIMENTAL SECTION
2.1 Materials
The matrix was an isotactic polypropylene (TIPP-
LEN H 949 purchased from Basplast SRL) of the homo-
polymer type with a flow index of 45. The nanofiller was
carboxyl-functionalized multi-walled carbon nanotubes
of a 95 % purity (Chengdu Organic Chemicals Co. Ltd,
outer diameter: 10–20 nm, COOH content: w = 2.56 %,
length: 10–30 μm, specific surface area: 233 m2/g,
density: ~2.1 g/cm3).
2.2 Nanocomposites synthesis
The nanocomposite samples were obtained by direct
melt compounding using a twin-screw extruder (Leistritz
LSM 3034 with a 34 mm screw diameter). The poly-
propylene (PP) pellets and nanofiller powder were mixed
at a gradual temperature increase on the ten heating areas
of the extruder, with a temperature profile between
150–170 °C and at a screw speed rate of 220 min–1. The
filaments were cooled in water, chopped, dried and
injected at 165–185 °C into the specimens with a spe-
cific shape. Samples of pure PP, 2 and 4 % of mass frac-
tions of MWCNT (relative to the matrix) were obtained,
as higher contents favor agglomeration and lead to
economically non-viable materials.
2.3 Testing and characterization
The nanocomposites were subjected to a spectro-
scopy analysis (Thermo iN10 MX, mid-infrared FTIR
microscope/ATR mode) and scanning electron micro-
scopy (SEM – HITACHI S2600N microscope), in the
fracture cross-section to highlight the nanofiller pre-
sence. Tensile and flexural tests were performed using an
INSTRON 5982 machine, on a minimum of 5 specimens
per test, according to SR EN ISO 527-27 at a tensile rate
of 50 mm/min, on 1A-type specimens and SR EN ISO
1788 at a test speed of 2 mm/min, for conventional
deflection on rectangular specimens. The thermal-degra-
dation behavior was followed by TG-DSC (Netzsch TG
449C STA Jupiter) heating at 10 K/min, from 25–900 °C,
under a dried-air flow of 10 mL/min. The HDT thermal
stability under load was evaluated using Qualitest HDT1
according to SR EN ISO 75, using a 2 °C/min heating
rate and the standard deflection of 0.34 mm at a flexural
stress of 1.8 MPa. The density was calculated as the ratio
between mass and volume; the volume was measured
using the displacement method.
3 RESULTS AND DISCUSSION
3.1 FTIR spectroscopy
Figure 2 presents the spectra of the isotactic PP in
comparison with the samples nanofilled with 2 and 4 %
of mass fractions of MWCNT. All the spectra present the
characteristic peaks of PP: –CH2 and –CH3 stretching vibra-
tions (2800–2950 cm–1) 9, –CH3 and –CH2 bending
(1376, 1456 cm–1) 10,11, and C–CH3 stretching (841 cm–1) 10.
Calculating the ratio of the absorption bands at 998
and 973 cm–1 (A998/A973) 12 characteristic for the pure
isotactic PP, the isotacticity index for the PP used in this
study was 77.8 %.
There are some differences between the MWCNT
samples due to the matrix-nanofiller interaction.13
Between 1000-1100 cm–1 the signals are more intense,
probably due to the C=O bonding from the COOH
C. E. BAN et al.: MULTI-WALLED CARBON NANOTUBES EFFECT IN POLYPROPYLENE NANOCOMPOSITES
12 Materiali in tehnologije / Materials and technology 50 (2016) 1, 11–16
Figure 2: FTIR spectra of the polypropylene-based nanocomposites
Slika 2: FTIR-spektri nanokompozitov na osnovi polipropilena
Figure 1: Exfoliated nanocomposite formation during the polymer-melt mixing
Slika 1: Nastanek ekspandiranega nanokompozita med me{anjem taline polimera
functionalization.14 The increase in the signal intensity at
1078 cm–1 can be due to the stretching vibration of
C-O.15 Minor differences appear at 1500–1750 cm–1,
more visible in the 4 % of mass fractions of MWCNT-
COOH samples; the weak peak at approximately 1738
cm–1 can be due to the C=O stretching vibration from the
COOH group15 and the modifications at 1550 cm–1 to the
OH groups in C-OH from the nanotubes treatment.16
Minor differences between the FTIR spectra can be
due to the low MWCNT content or a weak connection
with the polymer, probably because of the non-polar
nature of the polypropylene.
3.2 SEM electronic microscopy
SEM highlights the sample morphology at different
magnifications. The cross-section is strongly influenced
by the nanofiller, presenting visible edges and cracks
prior to the tensile-test fracture. The fracture area of the
samples with w(MWCNT) = 2 % is rougher, more like
the simple PP than the ones with w(MWCNT) = 4 %,
due to the lower nanofiller content. At a 500× magni-
fication, a good dispersion of MWCNT can be noticed
(Figure 3). In PP/2 % MWCNT there are some pores,
probably as impurities from the carbon nanotubes. In the
case of PP/4 % MWCNT there are no visible pores, but
there are some non-uniform areas, most likely due to the
higher nanofiller content that increases the agglomera-
tion tendencies. In the case of PP/4 % MWCNT, frac-
ture-initiation sites can be observed (Figure 4).
3.3 Mechanical testing
Figure 5 presents the stress-strain curves of the
replicas of the mediated specimens corresponding to the
samples. Figure 6 presents the load-extension evolution,
illustrating that during the flexural test the materials
mainly exhibit the same behavior; in the first stage,
PP/2 % MWCNT followed the PP trend, then its line was
the same as the one for PP/4 % MWCNT.
Table 1 presents the summary of the mechanical and
heat-deflection results. The nanocomposites exhibit
C. E. BAN et al.: MULTI-WALLED CARBON NANOTUBES EFFECT IN POLYPROPYLENE NANOCOMPOSITES
Materiali in tehnologije / Materials and technology 50 (2016) 1, 11–16 13
Figure 3: SEM images of PP-based nanocomposites compared to simple PP, at 500× magnification
Slika 3: SEM-posnetki nanokompozitov na osnovi PP v primerjavi z enostavnim PP, pov. 500×
Figure 5: Stress-strain curves corresponding to PP-based nanocom-
posites
Slika 5: Krivulje napetost – raztezek nanokompozitov na osnovi PP
Figure 4: SEM images of PP-based nanocomposites at 1500× mag-
nification
Slika 4: SEM-posnetka nanokompozitov na osnovi PP, pov. 1500×
superior characteristics compared to the bare PP; the
increase is more significant for the Young’s modulus,
while the density remains low.
Table 1: Results for mechanical and thermal stability under load tests
Tabela 1: Rezultati mehanske in toplotne stabilnosti pri obremenitvi
Sample PP PP/MWCNT-COOH (2 %)
PP/MWCNT-
COOH (4 %)
Density, g/cm3 0.87 0.91 0.93
Tensile stress at
tensile strength, MPa 35.34 37.56 37.38
Young’s modulus,
MPa 2026.1 2307 2413.71
Flexure stress at
tensile strength, MPa 32.51 35.42 33.59
Young’s flexure
strain, MPa 1439 1547.66 1575.19
HDT, °C 66.9 69.2 70.3
Adding 2 % of mass fractions of MWCNT-COOH
generated an increase in the tensile modulus of about
15 % while 4 % of mass fractions of led to a 20 %
increase compared to the bare PP. The flexural modulus
of the w = 4 % sample presented an increase by 10 %,
proving that adding MWCNT-COOH leads to stiffer
materials.
For the tensile and flexural strengths, the increase
values are in the range of 5–9 % for both the 2 and 4 %
samples, with slightly higher values for the 2 % sample.
This fact can be due to the higher content of the
nanofiller, which favors agglomerations, an issue that
was probably not overcome with the mechanical disper-
sion using the extrusion. At the 2 % loading, there is a
significant increase in the elongation at break, while at
the 4 % loading, both the strength and the elongation at
break decrease compared to the 2 % loading, indicating a
decrease in the ductility.17 At the higher MWCNT con-
tent filler-filler agglomerates are likely to act as stress-
concentrating sites17 as observed in the SEM images,
resulting in lower strength values. The number of the
stress-concentration sites increases with the nanotubes
content, leading to a decrease in the elongation.18
The mechanical properties show that the composite
failure is dependent on the nanofiller content.
3.4 HDT thermal stability under load
The thermal stability under load is in concordance
with the other test results. There is a minor increase in
HDT for the MWCNT sample, from 66.9 to 69.2 °C for
the 2 % loading and 70.3 °C for the 4 % loading, which
is confirmed by the DSC curves of the PP-based mate-
rials that present no change in the thermal behavior up to
200 °C.
3.5 Thermal degradation behavior
The TG curves from Figure 7a indicate that the ma-
terials are thermally stable up to approximately 200–220
°C; after this temperature they undergo thermal-degra-
dation processes.
Table 2 summarizes the TG-DSC results, showing
that a MWCNT-COOH addition leads to an increase in
the initial temperature of the degradation process (Tonset)
on the TG curve of approximately 65 °C. The 50 %
weight loss is generally considered to be an indicator of
the structural destabilization19, which occurs up to
C. E. BAN et al.: MULTI-WALLED CARBON NANOTUBES EFFECT IN POLYPROPYLENE NANOCOMPOSITES
14 Materiali in tehnologije / Materials and technology 50 (2016) 1, 11–16
Figure 7: a) TG and b) DSC curves for PP/MWCNT nanocomposites
compared to simple PP
Slika 7: a) TG- in b) DSC-krivulje za PP/MWCNT kompozite v pri-
merjavi z enostavnim PP
Figure 6: Load-extension evolution during the flexural testing of
PP-based nanocomposites
Slika 6: Obna{anje obremenitev – raztezek med upogibnim preizku-
som nanokompozitov na osnovi PP
approximately 330 °C for PP, up to 370 °C for PP/2 %
MWCNT and 390 °C for PP/4 % MWCNT. The end of
the degradation process (Tendset) is shifted towards higher
values as the MWCNT content increases.
Table 2: TG-DSC analysis results
Tabela 2: Rezultati TG-DSC analiz
Sample Tonset,°C
Tendset,
°C
Tmelting,
°C
Tdecomp1,
°C
Tdecomp2,
°C
PP 263.3 390 167.9 279.2 410/461.5/496.7
PP/2 %
MWCNT 326.5 408.7 166.8 362.6
441.4/487.4/
546.4
PP/4 %
MWCNT 327.2 441.2 167.3 365.2
471.4/500.9/
561.4
In the 25–200 °C region, an endothermic effect is
registered on the DSC curve (Figure 7b), associated
with the polymer melting process. As melting is a
physical process, the effect is not accompanied by the
weight loss. The maximum rate of the endothermic effect
is reached at approximately the same temperature (167
°C) for all the samples, showing that the MWCNT
addition does not induce changes in the melting tempe-
rature, but the intensities of the corresponding peaks are
lower for the MWCNT samples.
In the regions above 200 °C, exothermic effects
appear, associated with the decomposition processes that
occur in two main stages, mainly between 200–400 °C
and 400–600 °C, divided into several secondary exother-
mic effects, accompanied by the weight loss. The re-
corded effects for the PP-based materials are similar, but
because of their different compositions, the peak inten-
sities differ.
In the 200–400 °C region, the maximum weight loss
occurs for all the samples, being approximately 94 %.
On the DSC curve, the peaks corresponding to the exo-
thermic effects are shifted towards higher values; the
peak for PP appears at 279 °C, for PP/2 % MWCNT it
appears at 363 °C and for PP/4 % MWCNT it appears at
365 °C, but with a lower intensity for the MWCNT
samples. The increase in the decomposition temperature
of the MWCNT nanocomposites could be due to the
barrier effect generated by the nanotubes, when they are
well dispersed into the matrix, hindering oxygen
diffusion and retarding the thermo-oxidative degradation
of polypropylene.20,21 MWCNT acts as the protective
agent against a thermal degradation of polypropylene.
Between 400–600 °C, the exothermic effects are
accompanied by a weight change of approximately 6 %
for all the samples. Also, in this region, the exothermic
effects recorded on the DSC curves of PP/MWCNT
appear at higher temperatures and with higher intensities,
with the difference increasing with the MWCNT content.
This shows that the protective effect is more pronounced
at higher temperatures and higher MWCNT contents.
The residual mass is extremely low (up to w = 0.5 %
for all the samples), proving that although the
PP/MWCNT nanocomposites burn slower than the bare
PP, they burn nearly completely, indicating that the
eventual flame-retardancy properties of these materials
are probably due to the chemical and physical processes
in the condensed phase rather than the gas phase.22
The TG-DSC analysis results prove that the addition
of MWCNT improves the decomposition temperature of
the polypropylene nanocomposites and, consequently,
the thermal-degradation resistance.
4 CONCLUSIONS
The study presents a characterization of an isotactic
polypropylene filled with carboxyl-functionalized
MWCNT prepared through the simple and quick way of
the melt-extrusion technique. The results show an
improvement in the mechanical strength and modulus,
thermal stability under load as well as decomposition
temperature when adding the nanofiller. In the case of
the higher MWCNT loading (w(MWCNT) = 4 %), the
nanocomposites exhibited higher stiffness, decompo-
sition temperature and thermal stability under load, while
in the case of the lower loading (w(MWCNT) = 2 %) the
nanocomposites exhibited better mechanical strengths.
The nanocomposites maintained their low-density
advantages showing a minor increase when adding the
nanofiller. The results prove that the thermoplastic
polymers loaded with carbon nanotubes might be a new
class of light and strong composites that could find
applications in a large variety of fields.
Further compatibilization of the polypropylene ma-
trix by grafting it with different agents such as maleic
anhydride, methylstyrene23 and copolymers based on
maleic anhydride24 can lead to nanocomposites with even
higher mechanical and thermal properties, due to an
increased matrix-filler adhesion.
Acknowledgments
This work was funded by the Romanian Ministry of
Education through the PN-II-PT-PCCA-168/2012 project
"Hybrid composite materials with thermoplastic matrices
doped with fibres and disperse nano-fillings for materials
with special purposes" and by the Sectoral Operational
Programme "Human Resources Development 2007–
2013" of the Ministry of European Funds through the
Financial Agreement POSDRU/159/1.5/S/132397.
5 REFERENCES
1 F. Hussain, M. Hojjati, M. Okamoto, R. E. Gorga, J. Compos. Mater.,
40 (2006) 17, 1511–1565, doi:10.1177/0021998306067321
2 E. Logakis, E. Pollatos, Ch. Pandis, V. Peoglos, I. Zuburtikudis, C.
G. Delides, A. Vatalis, M. Gjoka, E. Syskakis, K. Viras, P. Pissis,
Compos. Sci. Technol., 70 (2010) 2, 328–335, doi:10.1016/
j.compscitech.2009.10.023
3 A. Szentes, G. Horvath, Cs. Varga, Hungarian Journal of Industrial
Chemistry, 38 (2010) 1, 67–70
C. E. BAN et al.: MULTI-WALLED CARBON NANOTUBES EFFECT IN POLYPROPYLENE NANOCOMPOSITES
Materiali in tehnologije / Materials and technology 50 (2016) 1, 11–16 15
4 T. Y. Hwang, H. J. Kim, Y. Ahn, J. W. Lee, Korea-Australia Rheo-
logy Journal, 22 (2010) 2, 141–148
5 D. Bikiaris, A. Vassilou, K. Chrissafis, K. M. Paraskevopoulos, A.
Jannakoudakis, A. Docoslis, Polym. Degrad. Stab., 93 (2008) 5,
952–967, doi:10.1016/j.polymdegradstab.2008.01.033
6 S. P. Bao, S. C. Tjong, Mater. Sci. Eng. A, 458 (2007) 1–2, 508–516,
doi:10.1016/j.msea.2007.08.050
7 European Standard SR EN ISO 527-2: Determination of tensile
properties of plastics, Test conditions for moulding and extrusion
plastics, 2000
8 European Standard SR EN ISO 178: Plastics, Determination of
flexural properties, 2003
9 F. Ozmihci, Polypropylene – Natural Zeolite Composite Films,
Dissertation Thesis, Materials Science and Engineering Department,
Izmir Institute of Technology, 1999, p. 48
10 I. Karacan, H. Benli, The use of infrared-spectroscopy technique for
the structural characterization of isotactic polypropylene fibres,
Journal of Textile & Apparel, 21 (2011) 2, 116–123
11 G. Parthasarthy, M. Sevegney, R. M. Kannan, J. Polym. Sci., Part B:
Polym. Phys., 40 (2002) 22, 2539–2551, doi:10.1002/polb.10304
12 A. R. Horrocks, J. A. D’souza, J. Appl. Polym. Sci., 42 (1991) 1,
243–261, doi:10.1002/app.1991.070420129
13 L. V. Diyakon, O. P. Dmytrenko, N. P. Kulish, Yu. I. Prylutskyy, Yu.
E. Grabovskiy, N. M. Belyy, S. A. Alekseev, A. N. Alekseev, Yu. I.
Sementsov, N. A. Gavrylyuk, V. V. Shlapatskaya, L. Valkunas, R.
Ritter, P. Scharff, Functional Materials, 15 (2008) 2, 169–174
14 V. T. Le, C. L. Ngo, Q. T. Le, T. T. Ngo, D. N. Nguyen, M. T. Vu,
Adv. Nat. Sci.: Nanosci. Nanotechnol, 4 (2013) 3, 1–5, doi:10.1088/
2043-6262/4/3/035017
15 S. C. Her, C. Y. Lai, Materials, 6 (2013) 6, 2274–2284, doi:10.3390/
ma6062274
16 C. R. Biswal, K. Mishra, P. L. Nayak, Synthesis and Characterization
of Modified Multi-Walled Carbon Nanotubes Filled Thermoplastic
Natural Rubber Composite, Middle-East Journal of Scientific
Research, 18 (2013) 2, 168–176, doi:10.5829/idosi.mejsr.2013.18.
2.12431
17 S. A. Girei, S. P. Thomas, M. A. Atieh, K. Mezghani, S. K. De, S.
Bandyopadhyay, A. Al-Juhani, J. Thermoplast. Compos. Mater., 25
(2012) 3, 333–350, doi:10.1177/0892705711406159
18 S. R. Katti, B. K. Sridhara, L. Krishnamurthy, G. L. Shekar, Mecha-
nical Behaviour of MWCNT Filled Polypropylene Thermoplastic
Composites, Indian Journal of Advances in Chemical Science, 2
(2014), 6–8
19 E. M. Sabri, O. Emel, Fibres & Textiles in Eastern Europe, 21 (2013)
2, 22–27
20 F. Avalos-Belmontes, L. F. Ramos-deValle, E. Ramirez-Vargas, S.
Sanchez-Valdes, J. Mendez-Nonel, R. Zitzumbo-Guzman, Journal of
Nanomaterials, 2012 (2012), 1-8, doi:10.1155/2012/406214
21 N. Khelidj, X. Colin, L. Audouin, J. Verdu, C. Monchy-Leroy, V.
Prunier, Polym. Degrad. and Stab., 91 (2006) 7, 1593–1597,
doi:10.1016/j.polymdegradstab.2005.09.011
22 T. Kashiwagi, E. Grulke, J. Hilding, R. Harris, W. Awad, J. Douglas,
Macromol. Rapid Commun., 23 (2002) 13, 761–765
23 E. Manias, A. Touny, L. Wu, K. Strawhecker, B. Lu, T. C. Chung,
Chem. Mater., 13 (2001) 10, 3516–3523, doi:10.1021/cm0110627
24 A. Szentes, C. Varga, G. Horváth, L. Bartha, Z. Kónya, H. Haspel, J.
Szél, Á. Kukovecz, eXPRESS Polymer Letters, 6 (2012) 6, 494–502,
doi:10.3144/expresspolymlett.2012.52
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16 Materiali in tehnologije / Materials and technology 50 (2016) 1, 11–16