UDK 678.7	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 46(1)37(2012)
ENHANCEMENT OF MOLECULAR WEIGHT OF L-LACTIC ACID POLYCONDENSATES UNDER VACUUM
IN SOLID STATE
POVEČANJE MOLEKULARNE TEŽE POLIKONDENZATOV L-LAKTIČNE KISLINE V TRDNEM STANJU Z VAKUUMOM
Pavel Kucharczyk1, Vladimir Sedlarik1, Ita Junkar2, Darij Kreuh3, Petr Saha1
1Centre of Polymer Systems, Polymer Centre, Tomas Bata University in Zlin, nam. T. G. Masaryka 5555, 760 01 Zlin, Czech Republic
2Jozef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia 3Ekliptik d.o.o., Teslova 30, 1000 Ljubljana, Slovenia sedlarik@ft.utb.cz
Prejem rokopisa - received: 2011-06-05; sprejem za objavo - accepted for publication: 2011-07-18
Enhancement of L-lactic acid polycondensates by involving of postpolycondensation reactions in solid state under vacuum and catalyzed by stannous 2-ethylhexanoate were studied in the presented work. The catalyst was used only in postpolycondensation step. An effect of the catalyst concentration and postpolycondensation reaction time on resulting properties of poly(lactic acid) was studied by gel permeation chromatography, Fourier transform infrared spectroscopy and differential scanning calorimetry. Results show significant enhancement of molecular weight in case of the systems containing 2 wt. % of the catalyst due to the postpolycondensation procedure (M„~91.0 kg/mol; i.e. 1655 % after 24 hours). However, further progress can be assumed with increasing reaction time. This enhancement of poly(lactic acid) molecular weight had a response corresponding to changes of both physico-chemical and thermal properties of the solid state postpolycondensation products.
Keywords: poly(lactic acid), solid state postpolycondensation, molecular weight enhancement
V tem delu smo za izboljšanje lastnosti polikondenzata L-laktične kisline raziskali vpliv postpolikondenzacijskih reakcij v trdnem stanju z vakuumom. Za katalizo smo v post polikondenzacijskem koraku uporabili katalizator kositrov 2-etilheksanoat. Vpliv koncentracije katalizatorja in časa postpolikondenzacijskih reakcij na lastnosti poli(laktične kisline) smo raziskovali z gel prepustno kromatografijo, fourierjevo transformacijsko infrardečo spektroskopijo in z diferenčno dinamično kalorimetrijo. Zaradi postpolikondenzacijskega postopka smo opazil^bistveno povečanje molekulske mase v sistemu, kje smo uporabili 2 ut.% katalizatorja (Mw~91.0 kg/mol; 1655 % po 24 urah). Še dodatno povečanje bi najverjetneje lahko dosegli tudi s podaljšanjem časa reakcije. Izkazalo se je, da ima povečanje molekulske mase poli(laktične kisline) vpliv na fizikalno-kemijske, kot tudi na termične lastnosti trdnih postpolikondenzacijskih produktov.
Ključne besede: poli(laktična kislina), postpolikondenzacija trdnega stanja, izboljšanje molekulske teže
1 INTRODUCTION
groups intentionally introduced at the end of PLA chains (e.g. hydroxyl) and bifunctional compound (e.g. diiso-
Poly(lactic acid) (PLA) is one of the biodegradable	cyanate).13 The second method involves reaction between
polymers with promising applicability in the field of	ends of the PLA chains and/or residual monomer, which
environmentally friendly materials production (e.g.	is usually carried out in solid state under specific condi-
packaging, disposable items) as well as in medicine	tions.8 This postpolycondensation reaction is analogy to
(medical devices, drug carriers).17 Generally, PLA can	the method developed for molecular weight increasing of
be synthesized by three basic ways; (i) ring opening	poly(ethylene terephthalate)14, which has been also
polymerization; (ii) azeotropic distillation in solution;	studied for an usage in medical applications.15-17 The
(iii) polycondensation in molten state.8,9 First two	advantage of this method is fact that an addition of
methods are known to be experimentally difficult due to	external chemical agents is not required.
sensitivity to presence of impurities. The lastly	There are several works describing solid state
mentioned method allows relatively low-demanding PLA	postpolycondensation of PLA. For instance, Zhang et al.
preparation.1011 However, it is redeemed by low	reported two step (melt/solid) preparation of high
molecular weight of the polycondensation products,	molecular weight PLA.18 Effect of crystallization on the
which is not acceptable for most of the engineering	solid state polycondensation of PLA was investigated by
applications.12	Xu et al.19 These both works apply SnCl2 as a catalyst for
The disadvantage of the low molecular weight poly-	PLA prepolymer preparation and additional catalyst for
condensation product can be offset by introduction of	postpolycondensation reaction. The resulting products
subsequent techniques - postpolycondensation reactions,	had molecular weight in the range 20-150 kg/mol.
which can yield a product with high molecular weight.	Polydispersity index was mostly tightly below 2. How-
Basically, two approaches have been known. The first	ever, to the best of our knowledge, solid state postpoly-
one represents coupling reactions between functional	condensation of low molecular weight PLA prepared by
direct polycondensation of L-lactic acid without using of a catalyst has not been studied.
This work is dedicated to investigation of solid state postpolycondensation of low molecular weight L-lactic acid polycondensates prepared by non-catalyzed reaction. Various concentration of an organotin based compound was used for catalysis of the postpolyconden-sation step subsequently. Resulting products were characterized by gel permeation chromatography; differential scanning calorimetry and Fourier transform infrared spectroscopy.
2 EXPERIMENTAL PART
2.1	Materials
L-lactic acid (LA) C3H6O3, 80 % water solution, optical rotation a = 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 and methanol CH4O (all analytical grade) were bought from IPL Lukes, Uhersky Brod, Czech Republic.
2.2	Sample preparation
The procedure as samples preparation is summarized in Figure 1. Generally, PLA prepolymer was prepared by direct melt polycondensation of L-lactic acid. A typical procedure was as follows: 50 ml lactic acid solution 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
L-Lactic acid
•	clehydratation
•	polycondensation ' precipitation
•	drying
\/
•160°C,15kPa,4h
•	180 °C, no catalyst, 100 Pa, 24 h
•	from aceton to MeOH/HjO •55°C.15kPa.48 h
Low M^v PLA -Prepolymer
> dissolution ' catalyst addition • crystalization ' Dostßolvcondensation
•	toluene
•	Sn(0ct)2,0,5,1 and 2 wt. %
•	105 °C, 1 h
•	105 100 Pa. 6.12.18 and 24 h
High M„, PLA
Figure 1: Scheme of two step preparation of high Mw PLA through the postpolycondensation reaction in solid state Slika 1: Shema dvostopenjske priprave visoko Mw PLA z uporabo postpolikondezacijskih reakcij v trdnem stanju
°C, reduced pressure 15 kPa for 4 hours. The flask with dehydrated mixture was connected to the source of vacuum (100 Pa) and the reaction continued for 24 hours at the temperature 180°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, twice washed with hot distilled water (80 °C) and dried in at 55 °C for 48 hours under the pressure 15 kPa.
Solid state postpolycondensation was proceeded as follows: 6 g of the prepolymer was dissolved in 20 ml of toluene and the catalyst was added to the polymer solution (0.5, 1 and 2 wt. %). The solvent was evaporated at elevated temperature (80 °C) and the residuals were dried for 6 hours at 50 °C and 3 kPa. After that the prepolymer containing the catalyst was placed into a test tube and crystallized at 105 °C for 1 hour. Finally, the test tubes were connected to the vacuum source (100 Pa). The sampling was done after 6, 12, 18 and 24 hours.
2.3 Characterization of samples
2.3.1 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 x 7.8) mm, 5 ^m) (Polymer Laboratories Ltd). For detection, a Waters 2487 Dual absorbance detector at 254 nm was employed. Analyses were carried out at room temperature with a flow rate of 1.0 mL/min in chloroform. The column was calibrated using narrow molecular weight polystyrene standards with molar mass ranging from 580 g/mol to 480 000 g/mol (Polymer Laboratories Ltd). A 100 pL injection loop was used for all measurements. The sample concentration ranged from 1.6 mg/mL to 2.2 mg/mL. 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 (PDI=Mw/Mn) of the tested samples were determined. Relative enhancement of Mw (EMw) was calculated according as follows:
EM..,(%) =
M
M
-x100
(1)
where MwS represents Mw of the sample and Mw0 is Mw of the prepolymer.
2.3.2 Fourier transform infrared spectroscopy (FTIR)
Functional groups in LA polycondensation products were identified using FTIR analysis. The investigation was conducted on Nicolet 320 FTIR, equipped with attenuated total reflectance accessory (ATR) utilizing Zn-Se crystal and the software package "Omnic" over the range of 4000-650 cm1 at room temperature. The
uniform resolution of 2 cm1 was maintained in all cases
2.3.3 Differential scanning calorimetry (DSC)
For the determination of glass transition temperature Tg, melting point Tm the differential scanning calorimetry was used. Approximately 8 mg of the sample were placed in an aluminium pan, sealed and analyzed on Mettler Toledo DSC1 STAR System under nitrogen flow (20 cm3/min). The analysis was carried out according to the following programme: (i) first heating scan 0 - 195 °C (10 °C/min); (ii) annealing at 195 °C for 1 minute; (iii) cooling scan 195 - 0 °C (10 °C/min); (iv) annealing at 0 °C for 1 minute; (v) second heating scan 0 - 195 °C (10 °C/min). Melting point temperature (Tm) was obtained from the first heating cycle. The value of glass-transition temperature (Tg) was determined in the second heating scan at the midpoint stepwise increase of the specific heat associated with glass transition. The DSC results for prepolymer and the sample after 24 hours of postpolycondensation reaction are presented in this work.
3 RESULTS AND DISCUSSION
The results obtained from GPC analysis are summarized in Table 1. The prepolymer (see the scheme in Figure 1) had Mw = 5.5 kg/mol and low PDI. It shows relatively narrow distribution of molecule chain lengths. Subsequently proceeded postpolycondensation reactions led to enhancement of Mw. However, significant increase in Mw was observed mostly after 18 hours of the reaction. Further prolongation of the reaction time (24 hours) had positive effect on Mw enhancement as well as addition of the catalyst (Sn(Oct)2). It can be noticed in Figure 2 where dependence of relative enhancement of Mw, EMw, (Equation 1) is depicted as a function of postpolycon-
1-1-r
0	6	12	18	24
Time of postpolycx>ndensation reaction (hours)
Figure 2: Relative enhancement of Mw versus time of postpolycondensation reaction in presence of various catalyst concentrations Slika 2: Relativno povečanje Mw glede na čas polikondenzacijskih reakcij in v prisotnosti katalizatorja pri različnih koncentracijah
densation reaction time for various concentration of the catalyst. The lowest investigated concentration of the catalyst seemed to be not suitable for high Mw PLA preparation. Maximal EMw was only 167 % in this case (9.2 kg/mol). Better results were noticed for the systems containing 1 wt. % of the catalyst. EMw was 502 % after 24 hours of the postpolycondensation reaction. However, it represents Mw = 27.6 kg/mol, which is still not considerable as high Mw product. Significantly different results were observed when the concentration of the catalyst was risen to 2 wt. %. As can be seen in Figure 2, EMw for this system was higher already after 12 hours of postpolycondensation reaction that those described above (0.5 and 1 wt. %, 24 hours). In addition, its EMw steeply increases with the reaction time. The sample after 24 hours of the reaction showed EMw = 1655 %. It corresponds to 91.0 kg/mol. PDI was 1.6. It reveals narrow distribution of the chain lengths.
Table 1: GPC characteristics of the prepolymer and the samples after various times of postpolycondensation reaction Tabela 1: GPC karakteristika predpolimera in vzorcev z različnimi časi postpolikondenzacijskih reakcij
Catalyst concentration (wt. %)
Mw/PDI*
0.5
2
Time of
postpolycondensation reaction (hours)
0*
5.5/1,68
6
Mw/PDI
6.0/2.6
7.6/1.6
25.5/2.6
12
Mw/PDI
5.4/1.7
7.9/2.1
38.9/2.6
18
Mw/PDI
9.3/1.8
19.9/2.2
60.9/2.9
24
Mw/PDI
9.2/2.3
27.6/1.7
91.0/1.6
* prepolymer
**Mw - weight average molecular weight (kg/mol), PDI - poly-dispersity
Figure 3: FTIR-ATR spectra of PLA prepolymer and products after 6 and 24 hours of postpolycondensation reaction (catalyzed by Sn(Oct)2, 2 wt. %)
Slika 3: FTIR-ATR spekter PLA predpolimera in produktov po 6 in 24 urah postpolikondenzacijskih reakcij (katalizirano z 2 ut. % Sn(Oct) )
1
Figure 4: Normalized areas of the FTIR-ATR absorption peaks at 1080 cm-1 of PLA prepolymer and products after 6 and 24 hours of postpolycondensation reaction (catalyzed by Sn(Oct)2, 2 wt. %) Slika 4: Normalizirana površina FTIR-ATR adsorpcijskih vrhov pri 1080 cm-1 PLA predpolimera in produktov po 6 in 24 urah post-polikondenzacijskih reakcij (katalizirano z 2 ut. % Sn(Oct)2)
The process of postpolycondensation of PLA was also investigated by FTIR-ATR analysis. Figure 3 shows FTIR-ATR spectra of PLA precursor and the samples after 6 and 24 hours of postpolycondensation reaction in presence of 2 wt. % of the catalyst. A detailed description of PLA FTIR spectra can be found, for example, in our previous works.1112 From the observation of the postpolycondensation reaction point of view, it is interesting to focus on formation of new ester bonds, reduction of carboxylic and hydroxyl groups. Due to complexity of the PLA FTIR spectra, the most convenient could be the firstly mentioned ester bonds formation, which could be characterized by absorption at 1080 cm-1.20 Areas of this absorption peak, A1080, were normalized on the peak area representing absorption of
Figure 5: DSC curve (first heating scan) of PLA prepolymer and products after 24 hours of postpolycondensation reaction (catalyzed by Sn(Oct)2, 2 wt. %)
Slika 5: DSC krivulja (prvo gretje) PLA predpolimera in produktov po 24 urah postpolikondenzacijskih reakcij (katalizirano z 2 ut. % Sn(Oct)2)
methyl groups at 1453 cm-1 (A1453). Dependence of A1080/A1453 versus time of the postpolycondensation reaction is depicted in Figure 4. It can be seen that concentration of ester bonds increases with rising time of the reaction. It is in agreement with the GPC results.
The significant enhancement of Mw described above affects properties of the resulting PLA including thermal behavior of the polymer. Figure 5 shows DSC spectra (first heating scan) of the prepolymer and PLA postpoly-condensation product obtained after 24 hours in presence of 2 wt. % of the catalyst. It can be noticed that increasing Mw of the PLA samples (see Table 1) led to significant shift of the Tm towards higher values (from 147.3 to 175.7 °C). Enthalpy of melting, AH^, was 47.3 and 67.4 J/g, respectively (Figure 5). It means that a content of crystalline phase in the treated samples increased noticeably (from 50.5 to 71.9 %, AHm0 = 93.7 J/g)21 due to postpolycondensation reaction connected with rising of Mw. In addition, glass transition temperature, Tg, (taken from the second heating scan - not shown here) was also increased from 45.2 °C (PLA precursor) to 53.6 °C (after 24 hours of the reaction, 2 wt. % of the catalyst). These observations together with the GPC results reveal linear structure of the PLA macromolecules while the postpolycondensation process takes place at the end of the PLA precursor chains in the amorphous phase region8.
4 CONCLUSIONS
This work was dedicated to investigation of the high molecular weight poly(lactic acid) (PLA) synthesis through multistep process including postpolyconden-sation reactions in solid state. Unlike other already published works in this field, presented work involves using of catalyst (stannous 2-ethylhexanoate) only in postpolycondensation step. The main attention was paid to determination of the catalyst concentration and time of the postpolycondensation reaction on the molecular weight of the resulting products by using gel permeation chromatography. It was correlated with the results from Fourier transform infrared spectroscopy and differential scanning calorimetry.
The results show that the role of the catalyst concentration is crucial to enhancement of molecular weight of the PLA during the solid state postpolycon-densation process. While low concentration of the catalyst (up to 1 wt. %) do not provide sufficient results (enhancement up to 502 % after 24 hours), systems containing 2 wt. % of the catalyst showed noticeably higher molecular weight enhancement - 1655 % (from 5.5 to 91.0 kg/mol). Another important parameter is time of the postpolycondensation step. Obtained results reveal a possibility of an additional enhancement of PLA molecular weight for the postpolycondensation reaction times above 24 hours. However, finding of a time optimum must be further investigated.
Acknowledgements
This research was financially supported by the Ministry of Education, Youth and Sports of the Czech Republic (project No. 2B08071 and ME09072) 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).
5 REFERENCES
1 A. Gregorova, M. Hrabalova, R. Wimmer, J. Appl. Polym. Sci., 114
(2010)	5, 2616-2623
2V. Sedlarik, A. Vesel, P. Kucharczyk, P. Urbanek, Mater. Tehnol., 45
(2011)	3, 209-212
3 I. Poljansek, M. Gricar, E. Zagar, M. Zigon, Macromolecular
Symposia, 272 (2008), 75-80 4V. Sedlarik, N. Saha, J. Sedlarikova, P. Saha, Macromolecular Symposia, 272 (2008), 100-103
5	M. Hrabalova, A. Gregorova, R. Wimmer, V. Sedlarik, M. Machov-sky, N. Mundigler, J.Appl. Polym. Sci, 118 (2010) 3, 1534-1540
6	S. Jacobsen, P. H. Degee, H. G. Fritz, P. H. Dubois, R. Jerome, Polym Eng Sci, 39 (1999), 1311-1319
7	H. R. Kricheldorf, Chemosphere, 43 (2001), 49-54
8 T. Maharana, B. Mohanty, Y. S. Negi, Prog. Polym. Sci., 34 (2009), 99-124
9R. Auras, B. Harte, S. Selke, Macromol. Biosci., 4 (2004), 835-864 101. Poljansek, P. Kucharczyk, V. Sedlarik, V. Kasparkova, A. Sala-kova, J. Drbohlav, Mater. Tehnol., 45 (2011) 3, 261-264
11	V. Sedlarik, P. Kucharczyk, V. Kasparkova, J. Drbohlav, A. Salakova, P. Saha, J. Appl. Polym. Sci., 116 (2010) 2, 1597-1602
12	P. Kucharczyk, I. Poljansek, V. Sedlarik, V. Kasparkova, A. Salakova, J. Drbohlav, U. Cvelbar, P. Saha, J. Appl. Polym. Sci. DOI: 10.1002/app.34260
13	K. M. D. Silvia, T. Tarverdi, R. Withnall, J. Silver, Plast. Rubber. Compos., 40 (2011) 1, 17-24
14	J. R. Campanelli, D. G. Cooper, M. R. Kamal, J. Appl. Polym. Sci., 48 (1993), 443-451
15	A. Vesel, M. Mozetic, A. Zalar, Vacuum, 82 (2007) 2, 248-251
16	N. Krstulovic, I. Labazan, S. Milosevic, U. Cvelbar, A. Vesel, M. Mozetic, J. Phys. D: Appl. Phys., 39 (2006) 17, 3799-3804
17	A. Vesel, Mater. Tehnol., 45 (2011) 2, 121-124
18	W. X. Zhang, Y. Z. Wang, Chinese. J. Polym. Sci., 26 (2008) 4, 425-432
19	H. Xu, M. Luo, M. Yu, C. Teng, S. Xie, J. Macromol. Sci. B, 45 (2006), 681-687
20F. Achmad, K. Yamade, S. Quan, T. Kokugan, Chem. Eng. J., 151 (2009), 342-350
21E. W. Fischer, H. J. Sterzer, G. Wegner, Colloid. Polym. Sci., 251, (1973), 980-990