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Novel Semibiobased Copolyester Containing Poly(trimethylene-co-hexamethylene Terephthalate) and Poly(lactic Acid) Segments Zhu Xiong, Jian-Bing Zeng,* Xiu-Li Wang, Yu-Rong Zhang, Ling-Ling Li, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials (ERCPM-MoE), College of Chemistry, State Key Laboratory of Polymer Materials Engineering, Sichuan UniVersity, Chengdu 610064, China
Poly (L-lactic acid) (PLLA) is a biobased polymer, and poly(trimethylene terephthalate) (PTT) is a semibiobased polymer. However, PLLA is biodegradable, and PTT is not. In this paper, their copolymers (PTHT-PLLA) are synthesized via chain-extension reaction of hydroxyl terminated poly(trimethylene-co-hexamethylene terephthalate) (PTHT-OH) and hydroxyl terminated poly(L-lactic acid) (PLLA-OH) using toluene-2,4diisocyanate (TDI) as a chain extender. The structures and molecular weights of PTHT-OH, PLLA-OH, and PTHT-PLLA were characterized by Fourier transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance (1H NMR), and gel permeation chromatography (GPC). The thermal, crystalline, and mechanical properties of PTHT-PLLA were further studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and tensile testing. The resulting PTHT-PLLA copolyesters went through a two-stage thermal decomposition behavior and showed two glass transition temperatures. The tensile testing results showed that PTHT-PLLA copolyesters have excellent flexibility with a strain of more than 300%, which is much higher than that of PLLA. The copolyesters are expected to have a better biodegradability than PTT. 1. Introduction Nowadays, the cost of traditional polymer has been continuously increased because of the gradual reduction of petroleum resources. Therefore, it is urgent to develop biobased materials as substitutes for fossil-based materials. In the past decades, biobased materials have experienced fast growth, and the reported biobased materials included polyhydroxyalkanoate (PHA),1 poly(lactic acid) (PLA), poly(butylene succinate) (PBS), polypropylenecarbonate (PPC),2 biobased polyethylene (bioPE),3 starch-based materials, poly (trimethylene terephthalate) (PTT), etc. PTT, an aromatic polyester with three methylene moieties in the repeating unit, was first synthesized by Whinfield and Dickso through condensation polymerization of the terephthalic acid and 1,3-propanediol (PDO) in 1941.4 It was just commercially available in the 1990s when the cost of PDO was largely reduced.5 Although other aromatic polyesters, such as poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), have been extensively studied, PTT has been paid much less attention. Compared with PET and PBT, PTT has many special characteristics, such as excellent elastic recovery, low fiber modulus, and better dyeing ability, which makes PTT able to be used in fibers, films, carpets, clothing materials and engineering thermoplastics.6,7 PTT has attracted more and more attention from both academia and industry since DuPont developed a novel method to produce PDO with renewable resources. This attention mainly focused on the crystal structure, crystallization behavior, morphological structure, spinning technology of PTT, and physical properties of its fibers.8-16 Although PTT is not a fully biobased material, its application can partially reduce our dependence on petroleum resources. If PTT is combined with other biobased materials, the usage of fossil resources would further decrease. Poly(lactic acid) (PLA) is regarded as one of the most promising biobased and * Corresponding author. E-mail:
[email protected] (J.-B.Z.) or
[email protected] (Y.-Z.W.). Tel/Fax: +86-28-85410259.
biodegradable aliphatic polyesters, due to its excellent biodegradability, biocompatibility, and physical properties.17-20 In this study, we synthesized a novel biobased aromaticaliphatic copolymer consisting of PTHT and poly (L-lactic acid) (PLLA) via chain-extension reaction of PTHT-OH and PLLA-OH with TDI as a chain-extender. As the melting temperature of PTT is higher than 200 °C, PLLA would suffer from thermo-degradation if the melt chain-extension reaction is carried out at such a high temperature; consequently, it is difficult to obtain high-molecular-weight copolymers. A hexamethylene terephthalate segment was introduced to the PTT molecular chain to lower its melting point. In order to get high chain-extension efficiency, dihydroxyl terminated PTHT and PLLA were synthesized and used as the prepolymers because hydroxyl is more reactive than carboxyl when reacting with the isocyanate group. The PTHT-PLLA copolymers were characterized by means of proton nuclear magnetic resonance (1H NMR), Fourier transform infrared (FTIR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), wide-angle X-ray diffraction (WAXD), and tensile test. Up to now, only several articles on PTT copolymers have been reported, including PTT-PTMO, PETT, PBTT, PPT-Su, and P(TT-co-TN).21-25 No literature, as far as we know, has reported such a novel aromatic-aliphatic copolyester. 2. Experiment Section 2.1. Materials. 1,3-Propanediol (1,3-PDO) (purity >99.7%) produced by BASF Chemical Corporation was distilled under reduced pressure before use. Dimethyl terephthalate (DMT) (CP grade) was obtained from Sinopharm Chemical Reagent Corporation (Shanghai, China). L-Lactic acid with 85 wt % aqueous solution was bought from Guangshui Chemical Plant (Guangshui, China), and the free water was removed by reduced pressure distillation at 80 °C for 3 h. 1,4-Butanediol (AR grade) was purchased from Kelong Chemical Corporation (Chengdu, China) and was used without further purification. Tetrabutyl titanate provided by Kelong Chemical Corporation was dis-
10.1021/ie100817h 2010 American Chemical Society Published on Web 06/10/2010
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Scheme 1. Preparation of PLLA-OH, PTHT-OH, and PTHT-PLLA
solved in anhydrous toluene to prepare a 0.2 g/mL solution. TDI (AR grade) was received from Gansu Research and Development Institute of Polyurethane (Baiyin, China) and was used without further purification. Zinc acetate (Zn(CH3COO)2) (AR grade) from Sitong Chemical Corporation (Chengdu, China) and Dihydrate tin(II) chloride (SnCl2 · 2H2O) (AR grade) manufactured by Jinshan Chemical Plant (Chengdu, China) were used without further purification. All other chemicals with AR grade were used as received. 2.2. Preparation of PTHT-OH. PTHT-OH was prepared by a two-step procedure of transesterification and subsequent polycondensation. In the first step, 200.0 g (1.13 mol) of DMT, 110 mL (1.54 mol) of 1,3-PDO, 136.0 g (1.03 mol) of 1,6hexanediol, and 0.08 g (0.04 wt % of DMT) of Zn(CH3COO)2 were charged into a 500 mL three-necked round-bottom flask equipped with water separator, mechanical stirrer, and nitrogen inlet. The reaction mixture was stirred at 190 °C for 1.5 h under a nitrogen atmosphere, and then, the temperature was raised to 230 °C for 3 h to complete transesterifaction. In the second step, the polycondensation was carried out at 260 °C under the vacuum of 50 Pa for 2 h, with tetrabutyl titanate (0.1 wt % of DMT) as a catalyst. The resulting product was purified by dissolving in chloroform and then precipitating in excessive methanol. The white powder product was dried to constant weight in a vacuum oven at 80 °C. 2.3. Preparation of PLLA-OH. PLLA-OH was prepared via direct condensation polymerization of L-lactic acid and 1,4butanediol in the presence of SnCl2 · 2H2O, according to the synthetic process proposed by Hiltunen et al.26 417.35 g (3.48 mol) of L-lactic acid, 5.99 g (0.067 mol) of 1,4-butanediol, and 2.03 g (0.5 wt %) of SnCl2 · 2H2O were loaded in a 500 mL three-necked round-bottomed flask equipped with a mechanical stirrer. Polymerization was first carried out at 160 °C and 4000 Pa for 5 h and then continued at 180 °C and 150 Pa for another
5 h. The resulting product was purified using the same procedure as PTHT-OH. The white powder product was dried to a constant weight in a vacuum oven at 40 °C. 2.4. Chain-Extension Reaction. The chain-extension reaction was conducted in bulk under a nitrogen atmosphere. PTHT-OH and PLLA-OH were put into a glass reactor which was vacuumed and purged with nitrogen three times, and then, the reactor was immersed in a 160 °C silicone oil bath. The mass feed ratios of PTHT-OH and PLLA-OH were 100:0, 90:10, 80:20, 70:30, 60:40, and 0:100. When the reactants were completely molten, the desired amount of TDI was injected into the reactor under vigorous stirring. The chain-extension reaction was finished in an hour. The resulting polymer was purified using the same procedure as PTHT-OH. The white powder product was dried to constant weight in a vacuum oven at 60 °C. 2.5. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectra for the polymer samples were recorded on a Fourier transform infrared spectrometer in a range of wave numbers from 4000 to 400 cm-1. The specimens were milled into powders and then mixed and laminated with KBr. The resolution and scanning time were 4 cm-1 and 32 times, respectively. 2.6. Nuclear Magnetic Resonance (NMR) Spectroscopy. Chemical structures and compositions of polymers were characterized by 1H NMR, which was implemented on a Varian Inova 400 spectrometer at ambient temperature, using CDCl3 and tetramethylsilane (TMS) as the solvent and the internal standard, respectively. 2.7. Gel Permeation Chromatography (GPC). The molecular weights and polydispersity indices of prepolymers and PTHT-PLLA were determined at 35 °C by gel permeation chromatography (GPC), using a Waters GPC device equipped
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Table 1. Molecular Weights of PTHT-OH and PLLA-OH prepolymer
Mna(g · mol-1)
Mnb(g · mol-1)
Mwb(g · mol-1)
PDIb
PTHT-OH PLLA-OH
2880 2370
14 740 7000
19 620 8230
1.33 1.18
a Calculated by NMR analysis. 35 °C.
b
Obtained by GPC in chloroform at
with a 1515 pump, a Waters model 717 autosampler, and a 2414 refractive index detector. Chloroform was employed as the eluent at a flow rate of 1.0 mL/min, and a sample concentration of 2.5 mg/mL was used. The monodisperse polystyrenes were used as standards. 2.8. Thermogravimetric Analysis (TGA). TGA was used to determine the thermal stabilities of PTHT-OH, PLLA-OH, and PTHT-PLLA. The thermograms were recorded from room temperature to 650 °C at a heating rate of 10 °C/min under N2 atmosphere on a TA Instruments TGA-Q500. 2.9. Differential Scanning Calorimetry (DSC). DSC experiments were carried out on a TA Instrument DSC-Q200. Samples were quickly heated to 160 °C and kept for 5 min to remove thermal history, then cooled to -50 °C at a rate of 10 °C/min, and finally reheated to 160 °C at the same rate. 2.10. Wide-Angle X-ray Diffraction (WAXD). Wide-angle X-ray diffraction patterns of PTHT-OH, PLLA-OH, and PTHT-PLLA were recorded on a Philips X’Pert X-ray diffractometer with Cu Ka radiation. The equipment was operated at room temperature with a scan rate of 2°/min scanning from 2° to 50°. 2.11. Tensile Testing. Tensile testing was carried out on an Instron Universal Testing Machine at a crosshead speed of 50 mm/min under room temperature. The specimens were prepared by hot pressing (at 140 °C) and cutting with a dumbbell-shaped cutter. The thickness and width of the specimens were 0.3 mm and 5 mm, respectively, and the length of the sample between the two pneumatic grips of the testing machine was 25 mm. Six measurements were tested for each sample, and the results were shown by mean values. 3. Results and Discussion 3.1. Preparation and Characterization of PTHTPLLA Copolyesters. PTHT-OH was prepared from dimethyl terephthalate, 1,3-propanediol, and 1,6-hexanediol via a twostep procedure of transesterification and subsequent polycondesation. Zinc acetate and tetrabutyl titanate were used as the catalyst for the first and second steps, respectively (Scheme 1a). In order to obtain dihydroxyl terminated PTHT, the feed molar ratio of the total amount of 1,3-propanediol and 1,6-hexanediol to dimethyl terephthalate was fixed at 2.3:1. The number average molecular weight (Mn) of PTHT-OH calculated from the NMR spectrum was 2880 g/mol (shown in Table 1), and the calculating method was reported in our previous study.27 PLLA-OH was prepared by condensation polymerization of L-lactic acid in the presence of 1,4- butanediol (Scheme 1b). The Mn of PLLA-OH was also determined by NMR analysis with the method reported elsewhere,28 and the result is listed in Table 1. The molecular weights and polydispersity were determined by GPC, and the results are also shown in Table 1. The number average molecular weights for PTHT-OH and PLLA-OH are 14 740 g/mol and 7000 g/mol respectively, and their polydispersity indices are 1.33 and 1.18. Just like the results obtained in other literature, the Mns of PTHT-OH and PLLA-OH determined by NMR are much less than those obtained by GPC,
but Mn calculated by NMR analysis is more useful to determine the amount of chain extender, as reported elsewhere.27,28 PTHT-PLLA copolyesters were obtained via chain-extension reaction of PTHT-OH and PLLA-OH employing TDI as a chain extender under nitrogen at 160 °C for an hour. The preparation process is shown in Scheme 1c. The polymer only containing PTHT or PLLA segment was prepared and named as PTHT or PLLA. The copolymer containing both PTHT and PLLA segments was named as PTHT-PLLA1 to PTHT-PLLA4 in sequence, according to the mass feed ratio of PTHT-OH to PLLA-OH from 60:40 to 90:10. It was reported that the chainextended products with highest molecular weights would be obtained when the molar ratio of the prepolymers to chain extender was 1:1.27-29 Therefore, the same ratio of prepolymers to TDI was employed in the present study. The chemical structures and compositions of the polymers were determined by 1H NMR as shown in Figure 1. It is obvious that the characteristic shifts assigned to PTHT (δHa, δHg, δHc, δHd δHb, δHe) and PLLA (δHs, δHw, δHv) remained in the spectra of PTHT-PLLA, while the signals belonging to end groups of PTHT-OH (δHl, δHk, δHj) and PLLA-OH (δHs’) disappeared, illustrating that all the hydroxyl groups from the prepolymers were reacted completely. The signals belonging to TDI appeared at 2.20 ppm (δHt), 7.02 ppm (δHm), 7.15 ppm (δHn), and 7.52 ppm (δHz). According to the relationship between peak intensity and proton number, we can calculate the mass ratios and deduce the weight fraction (listed in Table 2) of each component in PTHT-PLLA copolyesters according to the following equations: 206 × Ia + 248 × Ig mPTHT ) mPLLA 4 × 72 × Is
(1)
174 × It mTDI ) mPLLA 3 × 72 × Is
(2)
FPTHT ) FPLLA )
mPTHT
mPTHT + mPLLA + mTDI
(3)
mPTHT
mPLLA + mPLLA + mTDI
(4)
FTDI ) 1 - FPTHT - FPLLA
(5)
where mPTHT/mPLLA and mTDI/mPLLA represent the mass ratios of PTHT to PLLA segment and TDI residue to PLLA segment in PTHT-PLLA copolyester, respectively. FPTHT, FPLLA, and FTDI mean the weight fraction of the related components in PTHT-PLLA. Ia, Ig, Is, and It represent the peak intensities of methylene protons in the repeating units and the corresponding protons in the end groups of the trimethylene terephthalate segment of the PTHT block, methylene protons in the units and terminus of hexamethylene terephthalate segment, methine protons in the units of PLLA block, and methyl protons in TDI residue of PTHT-PLLA copolyester, respectively. Numerical values 3 and 4 in eqs 1 and 2 are the numbers of the corresponding protons, respectively. 206, 248, 174, and 72 are the molecular weights of repeating units of the trimethylene terephthalate segment and the hexamethylene terephthalate segment of PTHT block, the residue of TDI, and PLLA block in PTHT-PLLA copolyester, respectively. The compositions of the polymers are listed in Table 2. f denotes the weight fraction in the feed ratio of the components, while F indicates the ratio of the components in resulting polymers, which was obtained by NMR calculation. It could be seen that the value
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Figure 1. 1H NMR spectrum of PTHT-PLLA. Table 2. Compositions and Molecular Weight of Resulting PTHT-PLLA Copolyesters sample
fPTHTa
PLLA PTHT-PLLA1 PTHT-PLLA2 PTHT-PLLA3 PTHT-PLLA4 PTHT
56.0 65.4 75.3 84.8 94.3
fPLLAa
fTDIa
FPTHTb
FPLLAb
FTDIb
Mn × 10-4 (g/mol)c
Mw × 10-4 (g/mol)c
PDIc
93.2 37.3 28.0 18.8 9.4
6.8 6.7 6.6 5.9 5.8 5.7
59.1 68.2 77.4 86.1
89.8 34.3 25.5 17.6 9.2 96.2
10.2 6.6 6.3 5.0 4.7 3.8
2.04 4.90 4.30 4.74 4.42 3.30
2.58 11.7 9.29 13.65 15.02 9.62
1.26 2.39 2.16 2.88 3.39 1.77
a Weight fraction in the feed ratio. with PS standards.
b
Weight fraction in PTHT-PLLA determined by NMR calculation. c Molecular weights were measured by GPC
Figure 2. GPC diagrams of PTHT-OH, PLLA-OH, PTHT, PLLA, and PTHT-PLLA.
of F is close to f, suggesting that the copolymerization was accomplished in the way of our design. GPC was carried out to determine the molecular weights of the resulting copolymers, and the results are summarized in Table 2 and Figure 2. The GPC traces of the prepared polymers show a unimodal peak, illustrating that the chain-extension reaction took place completely. Furthermore, the molecular weights of the resulting PTHT-PLLA copolyester went up 3 to 4 times higher than those of the prepolymers, indicating that the chain-extension reaction happened successfully. Moreover,
PTHT and PLLA have a narrower molecular weight distribution than PTHT-PLLA, which is because PTHT or PLLA was obtained by chain extension of only one prepolymer (i.e., PTHT-OH or PLLA-OH) with TDI; this system is simpler than that for synthesizing PTHT-PLLA, where two prepolymers were employed to react with TDI. The differences in the chemical structure and reactivity of PTHT-OH and PLLA-OH resulted in a more complicated reaction and wider molecular weight distribution of the PTHT-PLLA copolymers compared with PTHT or PLLA. From Table 2, we can see that the molecular weight of PTHT is higher than that of PLLA, which should be caused by the higher original molecular weight of PTHT, as shown in Table 1. It seems that PTHT-OH should have higher reactivity than PLLA-OH because the terminal OH group of the former is the first OH group, while that of the latter is the second OH group. However, from the chainextension efficiency, which could be described by the differences in the number average molecular weights between PTHT or PLLA and their prepolymers, we can see that the molecular weight of PLLA increased about three times while PTHT increased 2.2 times in comparison with their respective prepolymers. The results suggest that PLLA-OH is more active than PTHT-OH at the same reaction conditions; this might be caused by the higher molecular motion of PLLA-OH that resulted from the lower molecular weight in comparison with PTHT-OH. It is very interesting to find that the molecular weights of PTHT-PLLA are higher than those of both PLLA and PTHT; the results can be attributed to the addition of the low-viscosity PLLA-OH to the high-viscosity PTHT-OH, which could increase the molecular motion of PTHT-OH and make the first hydroxyl of PTHT-OH more active. Conse-
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Figure 3. FTIR spectra of PTHT-OH, PLLA-OH, and PTHT-PLLA.
Figure 4. TGA curves of PTHT, PLLA, and PTHT-PLLA.
quently, the mixture of prepolymers shows higher reactivity than both PLLA-OH and PTHT-OH alone. FTIR is a useful method to obtain information on the functional groups of polymers. Figure 3 shows FTIR spectra of the PTHT-PLLA copolyesters, PTHT, and their PTHT-OH and PLLA-OH precursors. From the spectra, we cannot find the absorptions attributed to the -NCO- stretching which would appear around 2200 cm-1. After the chain-extension reaction, the peaks occurring at about 3500 cm-1 (O-H vibration) in the spectra of prepolymers are replaced by other peaks appearing around 3360 cm-1(N-H vibration) in the spectra of PTHT-PLLA copolyesters. Moreover, the appearance of the characteristic peak of N-H vibration was also observed at about 1530 cm-1. This proves that the hydroxyl groups were entirely consumed and N-H assigning to urethane group was formed, i.e., the chain-extension reaction happened. Through NMR, FTIR, and GPC analysis, we can obtain much convincing evidence for the successful preparation of PTHT-PLLA copolyesters. 3.2. TGA Analysis. The thermal stability of PTHT-PLLA copolyesters was evaluated by TGA. Figure 4 shows the TGA thermograms of prepolymers. There was nearly no residue after degradation of PLLA, while the residue left more than 7 wt %
Figure 5. DSC heating thermograms of PTHT-OH, PLLA-OH, and PTHT-PLLA.
after thermal decomposition of PTHT and PTHT-PLLA. It seems that the residues would decrease with the increase in weight fraction of PLLA segments in the resulting polymers; however, the residue was almost unchanged with the composition. The thermo-degradation mechanisms of the resulting copolymers will be investigated, and explanations for the phenomenon will be given in our subsequent study. The degradation of PLLA started at 246 °C and finished at 265 °C, while that of PTHT started at 375 °C, and under this temperature, PLLA has decomposed entirely. PTHT-PLLA undergoes a two-stage thermal degradation, with the first stage happening between 246 and 293 °C, and the second stage occurring at 375 °C. It could be found that the curves of PTHT-PLLA overlapped with each other compactly. In comparison with the TGA curves of PTHT and PLLA, the first weight loss of PTHT-PLLA copolyesters was attributed to the decomposition of the PLLA segment and the second weight loss was ascribed to the decomposition of PTHT block. 3.3. Thermal Transition and Crystallization Behaviors. DSC was implemented to study the thermal transition and crystallization behaviors of the polymers. Figure 5 shows the heating scans of PTHT-OH, PLLA-OH, PTHT, PLLA, and PTHT-PLLA. PTHT and PLLA have only one glass transition temperature at 32.7 and 59.6 °C, respectively. All PTHT-PLLA copolyesters have two glass transitions, which belong to the PTHT and PLLA segment, respectively. The results suggest that PTHT and PLLA segments were not well compatible with each other in the amorphous phase of the copolymers. In addition, only PLLA-OH and PTHT-OH have melting peaks. The polymers did not show crystallization in the cooling scans which is not given in the paper. The results suggest that all the chainextended polymers are unable to crystallize at the heating rate of 10 °C/min, which should be caused by disturbance of chain regularity of resulting polymers after introduction of chain extender. Figure 6 shows the wide-angle X-ray diffraction patterns of PTHT, PLLA, PTHT-PLLA copolyesters, and their precursors. There are two obvious characteristic reflection peaks occurring at 2θ values of 16.7° and 19.1° for crystalline PLLA-OH and PLLA.28,30 However, the sharp peaks cannot be detected for PTHT-OH, PTHT, and PTHT-PLLA copolyesters. In comparison with the diffraction patterns of the homopolymers, we inferred that the 2θ diffraction peaks at 15.9°, 20.5°, 23.6°, and
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copolyester compositions on the biodegradability will be investigated systematically. Acknowledgment This work was supported by Natural Science Foundation of China (20904034). The Analytical and Testing Center of Sichuan University provided NMR analysis. Literature Cited
Figure 6. Wide-angle X-ray diffraction patterns of PTHT-OH, PLLA-OH, PTHT, PLLA, and PTHT-PLLA. Table 3. Tensile Properties of PTHT, PLLA, and PTHT-PLLA Copolyesters sample
tensile strength (MPa)
elongation at break (%)
PTHT-PLLA1 PTHT-PLLA2 PTHT-PLLA3 PTHT-PLLA4 PTHT
6.4 ( 0.5 14.0 ( 1.2 27.1 ( 1.8 19.7 ( 0.3 5.1 ( 0.1
313 ( 6.5 351 ( 8.0 596 ( 4.6 645 ( 17.2 1257 ( 23.4
27.3° are a consequence of crystalline domains associated to the PTHT block,22,25,31,32 and the diffraction peaks of PTHTPLLA copolyesters are generated by the PTHT block. The results suggest that the PLLA block could almost not crystallize in the copolymers. From the XRD patterns of samples, we can find that the diffraction peaks of the copolyesters are very weak. The results were the consequence of the poor crystallization capacity of the copolyesters caused by the disturbing chain regularity. 3.4. Tensile Properties. Tensile testing is usually used to evaluate the tensile strength and elongation at break of polymer materials. The results are listed in Table 3. Unfortunately, the neat PLLA sample for the test could not be gotten just due to its brittleness, therefore, its data of mechanical properties were unavailable. The tensile strength of the polymers increased first and then decreased with the increasing content of PTHT segments, while the elongation at break always increased due to the flexibility of the PTHT segment. The results suggest that PTHT-PLLA copolyesters can find their applications in the field of elastic materials. 4. Conclusion A series of novel semibiobased aromatic-aliphatic copolyesters have been successfully prepared through chain-extension reaction of dihydroxyl terminated prepolymers PTHT-OH and PLLA-OH employing TDI as a chain extender. The chemical structures and compositions of the copolyesters were well confirmed by 1H NMR and FTIR. The GPC results indicate that the molecular weights of the chain-extended products are increased to at least three times higher than those of the prepolymers. DSC and XRD results suggest that the copolyesters show a poor crystallization ability and PTHT-PLLA copolyesters have two glass transition temperatures. The copolymers show very high elongations at the break, which makes it possible to be used as potential elastic materials. The effect of the
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ReceiVed for reView April 5, 2010 ReVised manuscript receiVed May 21, 2010 Accepted May 24, 2010 IE100817H
