A Novel Multiblock Poly(ester urethane) Based on Poly(butylene

ARTICLE pubs.acs.org/IECR

A Novel Multiblock Poly(ester urethane) Based on Poly(butylene succinate) and Poly(ethylene succinate-co-ethylene terephthalate) Hong-Bing Chen, Xiu-Li Wang, Jian-Bing Zeng, Ling-Ling Li, Feng-Xia Dong, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China ABSTRACT: A novel multiblock poly(ester urethane) (PBESTU) was synthesized through chain-extension reaction of hydroxylterminated poly(butylene succinate) (PBS-OH) and poly(ethylene succinate-co-ethylene terephthalate) (PETS-OH), using toluene-2,4-diisocyanate (TDI) as chain extender. The chemical structure and molecular weight of PBESTUs were characterized by 1 H NMR, intrinsic viscosity, and GPC. The thermal behaviors were investigated by DSC and TGA. The resulting copolymers, PBESTUs, have a sole glass transition temperature (Tg), which indicated that the two segments, PETS and PBS, were well compatible in the amorphous phase. PETS segment enhanced the thermal degradation temperature of PBESTUs. The DSC and WAXD results showed that PBS segment made great contribution to the crystallization of PBESTUs. PBESTUs possessed excellent mechanical properties according to tensile testing.

1. INTRODUCTION In recent years, the amount of plastic wastes has obviously increased all over the world, which brings a serious environmental problem, and has drawn more and more concern from the public and scientists. Some measures have been taken to solve this problem, such as reducing the use of plastic products and recycling the plastic wastes. One of the feasible ways to solve the problem is to develop biodegradable plastics.1 Therefore, a series of biodegradable polymers have been developed in the past decades, most of which are aliphatic polyesters, such as poly(ε-caprolactone) (PCL), poly(lactic acid) (PLA), poly(3-hydroxybutyrate) (PHB), poly(butylene succinate) (PBS), poly(ethylene succinate) (PES), poly(propylene succinate) (PPSu), poly(propylene adipate) (PPAd), poly(propylene sebacate) (PPSe), and their copolyesters.2-7 Unfortunately, those aliphatic polyesters are either expensive or poor in physical and mechanical properties, which largely restrict their applications. Some lower-cost aromatic polyesters with excellent physical and mechanical properties such as poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) have been widely used in many areas. However, those aromatic polyesters have been proved unable to degrade in the environment, mainly due to their hydrophobic and compact chemical structures. However, if the molecular chain is short enough or some hydrophilic groups are introduced into the chain, the aromatic polyesters can also be degradable.8 To combine the advantages of both aliphatic and aromatic polyesters, some aliphatic-aromatic coplyesters have been intentionally designed and synthesized, and their physical properties and biodegradability have also been investigated.9-12 Kint et al.13 and Park et al.14 prepared a series of poly(ethylene terephthalate)/poly(1,4-butylene succinate) block copolymers via reactive blending of poly(ethylene terephthalate) (PET) and poly(1,4-butylene succinate) (PBS) at 290 °C. The PBS segment decreases the elastic modulus and tensile strength of the copolymers, while increasing the elongation at break, which also r 2011 American Chemical Society

endows the copolymers with pronounced hydrolytic degradability. Papageorgiou et al.15 synthesized random poly(propylene terephthalate-co-succinate) through polycondensation of dihydroxypropylene terephthalate and dihydroxypropylene succinate. The copolyesters showed improved physical and mechanical properties with increasing the content of aromatic units. In addition, the copolyesters with terephthalate content of less than 60 mol % are found to be biodegradable. We16 also synthesized a novel aliphatic-aromatic copolyester, poly(butylene succinateco-ethylene succinate-co-ethylene terephthalate) (PBEST), through melt polycondensation reaction. The thermal stability of the resulting copolyester could be improved after introduction of aromatic unit, but the degree of crystallinity and melting point of the copolyester were reduced badly because of the random structure of the copolymer. Furthermore, the molecular weight of the copolymer could not reach a very high value due to the limitation of direct polycondensation. To our knowledge, the aforementioned methods to synthesize aliphatic-aromatic copolyesters suffer from shortcomings. For example, for direct polycondensation, high efficient catalyst, high vacuum, and well-designed reactor are required to obtain high-molecular-weight copolyester, which would make the production processes more complicated and the product costs high, thus restricting the application of these polymers. In the case of the reactive blending, although it can produce block copolyester, thermal degradation cannot be avoided, which would result in reduced physical properties of the resulting copolymers. Chain-extension reaction is a simple but very efficient way to synthesize high-molecular-weight block copolymers with controlled physical and mechanical properties by the composition.17,18 Received: August 27, 2010 Accepted: December 21, 2010 Revised: December 20, 2010 Published: January 24, 2011 2065

dx.doi.org/10.1021/ie101798n | Ind. Eng. Chem. Res. 2011, 50, 2065–2072

Industrial & Engineering Chemistry Research

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Scheme 1. Synthesis Process of Hydroxyl-Terminated PBS Prepolymer (PBS-OH)

Figure 1. 1H NMR spectrum of PBS-OH.

Scheme 2. Synthesis Process of Hydroxyl-Terminated PETS Prepolymer (PETS-OH)

In this study, we aim to synthesize a series of multiblock poly(ester urethane)s (PBESTU) containing poly(butylene succinate) and poly(ethylene succinate-co-ethylene terephthalate) via the chain-extension reaction. Recently, we have reported other aliphatic-aromatic copolyesters synthesized efficiently in a similar procedure.19,20 The synthesis, chemical structure, thermal properties, as well as mechanical properties of PBESTU are systematically investigated in this article. A systematic investigation on the biodegradability of PBESTUs, especially the effect of the aliphatic-aromatic composition of the copolyesters on biodegradability, is underway and will be presented in future work.

2. EXPERIMENTAL SECTION 2.1. Materials. Dimethyl terephthalate (DMT, CP grade) was purchased from Sinopharm Chemical Reagent Co., Ltd.

Figure 2. 1H NMR spectrum of PETS-OH.

(Shanghai China). 1,4-Butanediol (BD, AR grade), succinic acid (SA, AR grade), triphenyl phosphate (TPP, CP grade), tetrabutyl titanate [Ti(OC4H9)4, AR grade], and zinc acetate [Zn(CH3COO)2, AR grade] were received from Kelong Chemical Reagent Factory (Chengdu, China). Ethylene glycol (EG, AR grade) was supplied by Jianjie Chemical Co. (Chengdu, China). Toluene2,4-diisocyanate (TDI, AR grade) was received from Bodi Chemical Plant (Tianjin, China). Among all the raw materials, only TDI was purified by distillation under reduced pressure; the others were used as received. 2.2. Synthesis of Hydroxyl-Terminated Poly(butylene succinate) (PBS-OH).21. BD and SA (at a mole ratio of BD/ SA = 1.2:1) were charged into a three-neck round-bottom flask. The reactants were stirred at 190 °C for 3-4 h under nitrogen to complete esterification. A certain amount of Ti(OC4H9)4 was then added into the flask with a syringe as catalyst. The reaction was carried out at 230 °C and reduced pressure (50 Pa) for about 2.5 h. 2.3. Synthesis of Hydroxyl-Terminated Poly(ethylene succinate-co-ethylene terephthalate) (PETS-OH). The synthetic route is similar to the preparation of PPTSu.15 DMT and EG (at a mole ratio of DMT/EG = 1:2.87) and the transesterification catalyst Zn(CH3COO)2 (1 10-3 mol/mol DMT] were charged into the reaction apparatus. The reactants were stirred at 180 °C under nitrogen for about 2 h until all the methanol was removed as byproduct. Next, SA (0.667 mol/mol DMT) was added into the apparatus and kept at 180 °C for another 2 h, and a certain amount of Ti(OC4H9)4 was added. Finally, the reaction was carried out under reduced pressure (50 Pa) at 250 °C for about 2 h. 2.4. Synthesis of Multiblock Poly(ester urethane)s Based on Poly(ethylene succinate-co-ethylene terephthalate) and Poly(butylene succinate) (PBESTU). All the PBESTUs were synthesized with appropriate molar ratios of NCO/OH (1:1.2). The glass flask was filled with an appropriate amount of PBSOH and PETS-OH, and vacuumed and purged with dry 2066

dx.doi.org/10.1021/ie101798n |Ind. Eng. Chem. Res. 2011, 50, 2065–2072


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Scheme 3. Synthesis Process of PBESTU

nitrogen three times to make sure it was a complete nitrogen atmosphere. Next, the flask was put into the oil bath and kept at 185 °C. After melting, the reactants were stirred vigorously, and meanwhile the predetermined amount of TDI was injected into the flask. The reaction was carried out for 1 h. The resulting products were dissolved in chloroform and precipitated with methanol, and then dried at 60 °C in a vacuum oven for 2 days. 2.5. Characterization. Intrinsic viscosities [η] of PBESTUs were determined using an Ubbelohde viscometer at 25 °C in chloroform. The solution was filtered before testing. Molecular weights of PBESTUs were performed by Waters GPC device equipped with a 1515 pump, a Waters model 717 autosampler, and a 2414 refractive index detector. Chloroform was used as an eluent (1.0 mL/min). The calibration curve was obtained using monodisperse polystyrene standard samples. 1 H NMR spectra of prepolymers and the PBESTUs were obtained with a Bruker spectrometer operating at a frequency of 400 MHz for protons. CDCl3 and tetramethylsilane (TMS) were used as the solvent and internal chemical shift standard, respectively. The scanning time was 16, and the sweep width was 8 kHz. Wide-angle X-ray diffraction (WAXD) measurements of PBS-OH, PETS-OH, and PBESTUs were performed using an X-ray diffractometer (Philips X’Pert X-ray diffractometer), using Cu K radiation in the angle 2θ range from 2° to 50° at room temperature. DSC curves were obtained with TA Q200 DSC apparatus, calibrated with pure indium and zinc standards, with a cooling attachment RSC 95 to satisfy the temperature changes. The 6 ( 0.5 mg samples were placed in the aluminum pans to test their thermal behaviors. Before being tested, the samples were placed in ambient air for days to get better crystallization. The samples were first heated from -50 to 200 °C at 10 °C/min to determine the melting temperature, and kept at 200 °C for 2 min to erase any thermal history, then quenched to -50 °C to obtain amorphous materials. After being held at -50 °C for another 2 min, the samples were heated again to 200 at 5 °C/min so as to acquire the glass transition temperature and crystallization properties. The testing curves and corresponding data were recorded. TGA curves were obtained with NETZSCH TG 209 F1 apparatus. 4.5 ( 0.5 mg samples were placed in Al2O3 pans to test their thermal stabilities. The samples were heated from 40 to 600 °C at 10 °C/min under dry nitrogen. The temperature T5%, at which 5 wt % of original weight was lost because of the decomposition, Td max, at which the PBESTUs possessed the maximum weight loss rate, and the char residue, were applied to study the thermal decomposing behaviors. Mechanical properties were investigated with an Instron Universal Testing Machine (model 4302, Instron Engineering Corp., Canton, MA) at a tensile rate of 50 mm/min according to

ASTM D638. Thin films were obtained using plate vulcanizer at 170 °C. Dumbbell-shaped tensile-test specimens (central part 4 0.6 mm) were prepared from these thin films and placed in ambient air for 24 h. The tests were carried out at 25 °C with at least five specimens for each sample. The average values together with the standard deviations of tensile strength, elongation at break, and Young’s modulus were recorded.

3. RESULTS AND DISCUSSION 3.1. Synthesis of PBS-OH. The synthesis is actually an esterification and polycondensation route (Scheme 1). In the reaction, Ti(OC4H9)4 is used as a catalyst for the polycondensation step. Under the aforementioned reaction condition, the removal of BD leads to the increase of molecular weights. The superfluous BD makes the prepolymer predominantly hydroxylterminated.22 As shown in the 1H NMR spectrum (Figure 1), the peaks occurring at 4.10(b), 2.60(c), and 1.68(a) ppm with the same integral area are attributed to three kinds of methylene in the repeating units. As for the peak at 3.68(d) ppm, it is assigned to methylene of terminal BD. The molecular weight (Mn,PBS-OH) is calculated to be 2000 g/mol according to the following equation:23 Ib DP ¼ ð1Þ Id

M n;PBS- OH ¼ 172DP þ 90

ð2Þ

where DP stands for the repeating unit number of the PBS chain, 172 is the molecular weight of this repeating unit, and 90 is the molecular weight of BD. Ib and Id are the relative intensities of the dyad peaks at 4.10 and 3.68 ppm. 3.2. Synthesis of PETS-OH. The synthetic route of PETSOH is shown in Scheme 2, which is a two-step reaction substantively. The purpose of introducing SA in PET chain is to decrease the melting point of PET.16,24,25 During the first step, dihydroxyethylene terephthalate and dihydroxyethylene succinate are formed by tansesterification and esterification. The second step is a polycondensation reaction. This synthetic route ensures that the molar ratio of resulting PETS-OH is nearly the same as that of feedings, which is due to the only elimination of EG in the polycondensation reaction. As shown in the 1H NMR spectrum (Figure 2), the resonance signals occurring at 8.10(e), 4.25-4.70(f), and 2.67(g) ppm are obviously ascribed to the protons of the terephthalate units, the methylene groups of the SA, and methylene of EG, respectively. Especially, the signal at 3.65-3.98(h) ppm should be assigned to the methylene of terminal EG, which is different from the methylene of repeating units. The microcosmic chemical structure of PETS-OH can be detected from the signal 4.25-4.70(f) ppm. The combinations TT (terephthalic acid-terephthalic acid), TS (terephthalic 2067



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Figure 3. 1H NMR spectra of (a) PBESTU0, (b) PBESTU100, and (c) PBESTU50.

acid-succinic acid), ST (succinic acid-terephthalic acid), and SS (succinic acid- succinic acid) correspond to the four peaks,

respectively.26 The randomness (R) is calculated through the relative intensities of the dyad peaks, ITT, ITS, IST, and ISS.27 2068



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Table 1. Chemical Composition and Molecular Weight of Prepared PBESTUs PBESTUsa

BS/(ETþES)/TDIb(mole)

PBS/PETS

PBESTU0

0/100

BS/(ETþES)/TDIc(mole)

[η] (dL/g)

Mn (g/mol)

Mw (g/mol)

PDI

0/95.79/4.21

0/95.57/4.29

0.43

35 600

74 800

2.10

PBESTU20

20/80

19.05/76.20/4.75

17.65/76.64/5.71

0.54

44 700

108 800

2.43

PBESTU40

40/60

37.89/56.83/5.29

34.04/58.90/7.07

0.78

55 500

149 300

2.69

PBESTU50

50/50

46.97/46.97/6.07

48.61/45.25/6.10

0.80

46 800

134 800

2.87

PBESTU60

60/40

56.19/37.46/6.36

57.96/34.28/7.76

1.42

93 500

300 600

3.21

PBESTU80

80/20

74.00/18.50/7.50

73.01/18.68/8.30

1.28

80 600

256 900

3.19

92.52/0/7.48

93.67/0/6.33

1.50

128 200

332 100

2.59

PBESTU100

100/0

a The number after PBESTU means the mole percent of feeding PBS. b Mole fraction in the feed ratio. c Mole fraction in PBESTU determined by 1H NMR.

Table 2. Thermal Properties of Prepared PBESTUsa ΔHm (J/g)

Tm (°C)

a

ΔHm1

ΔHm2

Tc (°C)

ΔHc (J/g)

T5%

Td max

char residue

n.d.

3.7

n.d.

n.d.

n.d.

298.3

428.3

13.3

n.d.

0.4

n.d.

n.d.

n.d.

295.1

423.2

11.3

156.6

86.16

1.2

13.2

n.d.

n.d.

299.8

393.1

8.8

156.0

91.4

0.9

21.9

n.d.

n.d.

280.7

393.0

8.3

0.5

31.0

70.3

4.8

293.8

393.1

5.4

h h

35.6 50.3

45.6 10.3

36.5 23.1

282.7 282.5

390.0 398.0

3.6 0.8

Tg (°C)

Tm1

Tm2

PBESTU0

36.1

163.4

PBESTU20

16.6

167.8

PBESTU40

1.1

PBESTU50

-7.96

PBESTUs

PBESTU60

-13.7

164.5

97.0

PBESTU80 PBESTU100

-17.8 -27.5

h h

101.2 100.1

n.d.: not detected.

R ¼ PST þ PTS

ð3Þ

PTS

ITS þ IST 1 2 ¼ ¼ ITS þ IST LnT þ ITT 2

ð4Þ

PST

ITS þ IST 1 2 ¼ ¼ ITS þ IST LnS þ ISS 2

ð5Þ

PTS and PST stand for the probability of TES and SET structure in the prepolymer chain, while LnS and LnT represent the number-average sequence length. From the above equation, R of PETS-OH is 1.006, indicating that it is a random copolymer. In fact, the random chain distribution was formed due to the thermodynamics equilibrium of the polycondensation. The molecular weight of PETS-OH (Mn,PETS-OH) is calculated as 3996 g/mol according to the following equation: Ie ð6Þ DP1 ¼ 2Ih DP2

Ig ¼ 2Ih

M n;PETS- OH ¼ 192DP1 þ 154DP2 þ 62

3.3. Chain-Extension Reaction. The synthetic route of PBESTUs is shown in Scheme 3. Reaction condition is well chosen: both the prepolymers can melt in a short time and avoid thermal degradation at 185 °C under dry nitrogen atmosphere. The amount of TDI is hard to calculate precisely because it might be affected by the activity of OH, reaction temperature, and especially the trace water in the reaction system. However, it is very important to the reaction. If excessive TDI is added, the reactants would end up with isocyano group, and the deficient amount of TDI leads to lower molecular weight. By the molar ratio of OH to NCO, the suitable amount of TDI is usually in the range of 1:1.2-1:1, according to our previous work.21,28,29 The structures of PBESTU0 (Figure 3a) and PBESTU100 (Figure 3b are similar to those of the prepolymers (Figures 1 and 2). The differences are the new signals appeared at chemical shift 2.10 (m) ppm and 6.50, 7.10(p) ppm, which is caused by methyl and phenyl groups of TDI. The 1H NMR spectrum of PBESTU50 (Figure 3 c) looks like the simple superposition of the 1H NMR spectra of PBESTU0 and PBESTU100, which proves the block structure of PBESTUs. The chemical composition of PBESTUs could be calculated from the peak intensities.

ð7Þ ð8Þ

where DP1 stands for the repeating unit number of ethylene terephthalate (ET), and 192 is molecular weight; DP2 stands for the repeating unit number of ethylene succinate (ES), and 154 is the molecular weight; 62 is the molecular weight of EG. Ie, Ig, and Ih are the relative intensities of the dyad peaks at 8.10, 2.67, and 3.65-3.98 ppm, respectively.

nET þ nES If ¼ nBS Ib

ð9Þ

nTDI 4Im ¼ nBS 3Ib

ð10Þ

where the (nET þ nES)/nBS stands for the molar ratio of ET and ES units to BS units, and the nTDI/nBS represents the molar ratio of TDI in PBESTU to BS units. If, Ib, and Im represent the peak intensities of the methylene group of EG, BD, and the methyl of TDI, respectively. The molar ratio of the compositions in PBESTUs varies little with that of feeding; the tiny changes seem to be stochastic. 2069



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Figure 6. TGA traces of PBESTUs.

Figure 4. DSC thermal diagrams of PBESTUs: (a) first heating and (b) second heating after melt quenching.

Figure 5. WAXD patterns of PBS-OH, PETS-OH, and PBESTUs.

The intrinsic viscosity [η], chemical composition, and molecular weight of PBESTUs are listed in Table 1. Molecular weight analysis confirms the feasibility of synthesizing such multiblock poly(ester urethane)s through chain-extension reaction. The PDI (Mw/Mn) is broader than that of copolyesters synthesized through the polycondensation reaction,15,30 which might be due to the quick reaction between the OH group and NCO group.

3.4. Thermal Behaviors and Crystallization. The thermal transition behaviors of the PBESTUs were investigated by DSC, and the corresponding data are summarized in Table 2. During the first heating scan (Figure 4a), all PBESTUs are semicrystalline. The melting peaks around 95 and 160 °C are reasonably assigned to the PBS segment and PETS segment, respectively. Multiple melting behaviors are observed for most PBESTUs containing PBS segment. The peak at about 40 °C is the so-called annealing peak of PBS segment. At ambient temperature, PBS segment would form crystals with defects. The large surface/ volume ratio leads to depressed melting temperature of metastable lamellae of crystals with defects. When heated, the metastable lamellae would melt and recrystallize so as to become more stable. The balance of the melting and recrystallization can be used to interpret the multi melting phenomenon.31-33 In addition, the melting temperature of PBESTU100 is lower than that of neat PBS (115 °C), which might be due to the low molecular weight of PBS segment. The crystallization of the PBS segment determines the enthalpy of fusion (ΔHm), which is also confirmed by WAXD. DSC traces of samples after melt quenching are illustrated in Figure 4b. Glass transition temperature (Tg), melting temperature (Tm), heat of fusion (ΔHm), crystallization temperature (TC), and enthalpy of crystallization (ΔHC) are summarized in Table 2. As shown in the second heating scan, one obvious glass transition is detected for each sample with different proportions of PETS and PBS segments, spanning from -27.5 °C of PBESTU100 to 36.1 °C of PBESTU0, which indicated a compatible system.21 For PBESTU60, PBESTU80, and PBESTU100, crystallization is observed. However, no crystallization peak is observed for samples PBESTU0, PBESTU20, PBESTU40, and PBESTU50. The results show that the PBS segment can enhance crystallization capacity, while the PETS segment decreases it. Meanwhile, if the sample can crystallize, the PETS segment will enhance the crystallization temperature (TC). This phenomenon can be explained by the fact that after erasing thermal history during the second heating, PETS segment is completely amorphous, the existence of which restricts the mobility of PBS segment and disturbs the chain regularity. When more PETS segments are introduced to PBESTU chain, the crystallization will appear at higher temperatures and in longer time. A similar phenomenon was also found in the PEU (PBS-bPLLA) system by Zeng et al.23 Figure 5 shows the wide-angle X-ray diffraction patterns of PETS-OH, PBS-OH, and all PBESTUs. Two main peaks of 2070



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Table 3. Mechanical Properties of Prepared PBESTUs sample

tensile strength

elongation at

modulus

(MPa)

break (%)

(MPa) 732 ( 70

PBESTU0

22.7 ( 2.0

447 ( 100

PBESTU20

33.7 ( 2.0

891 ( 100

PBESTU40

38.2 ( 2.0

1120 ( 77

34 ( 2 161 ( 2

PBESTU50

46.3 ( 1.5

1109 ( 90

196 ( 3

PBESTU60

46.9 ( 1.0

1107 ( 125

234 ( 35

PBESTU80

41.3 ( 0.8

1066 ( 50

255 ( 3

PBESTU100

51.1 ( 2.1

1110 ( 114

263 ( 13

Figure 7. Mechanical properties of PBESTU0, PBESTU20, and PBESTU40.

PBS-OH are observed at 2θ values of 19.42° and 21.75°, while the main peaks of PETS-OH are at 17.9°, 20.87°, 22.2°, and 26.0°. The peaks of PETS-OH are weak and obtuse, which represents its unsatisfactory crystallization ability. With the chain getting longer (PBESTU0), the crystallization capacity decreases. With the increase of PBS segments, the peak intensity gets stronger and stronger: PBESTU80 and PBESTU100 are well crystallized. The results are in accord with the DSC analysis; that is, the crystallization of PBESTU is mainly caused by the PBS segment. 3.5. Thermal Stabilities. The nonisothermal stabilities of PBESTUs were obtained with TGA. T5%, Td max, and char residue are summarized in Table 2. Figure 6 shows that PBESTUs have only one obvious thermal decomposition stage because the two blocks share a close decomposition temperature, which is different from many other block copolymers. All the PBESTUs possess good thermal stabilities; the initial decomposition temperatures T5% are higher than 280 °C, and Td max are over 390 °C. Aromatic units improve the thermal stabilities and enhance char residues of PBESTUs, as reported by others.16,34 3.6. Mechanical Properties. Mechanical properties are of high importance for applied polymers. The tensile properties of PBESTUs, such as tensile strength, elongation at break, and Young’s modulus, are obtained and listed in Table 3. It is found that PBESTUs have excellent mechanical properties, especially PBESTU50, PBESTU60, PBESTU80, and PBESTU100. Among the samples, PBESTU0 owns the lowest tensile strength and elongation at break, yet the highest Young’s modulus. It is believed that the lowest tensile strength is caused by its relative low molecular weight. PBESTU0 is almost amorphous at the testing temperature of 25 °C (ambient temperature), at which it

is in a glassy state (Tg = 36.1 °C), so it possesses the highest Young’s modulus value and lowest elongation at break. When the specimen is drawn, multiyield phenomenon occurs with the formation of “thin necks” in the specimen, which corresponds to the peaks of the tensile curve. A similar phenomenon was also reported by other authors.30,35,36 The composition of the samples does not seriously affect the tensile strength and elongation at break. However, the value of Young’s modulus increases with the increase of PBS segments, which is due to the better crystallization of PBESTUs. PBESTU20 is nearly amorphous, and its Tg is below testing temperature; its stress-strain curve accords with that of a typical elastomer. The curves of PBEST40 and PBEST100 are similar; they tally with that of crystalline polymers. Three different tensile curves are shown in Figure 7.

4. CONCLUSION A series of high molecular weight multiblock poly(ester urethane)s (PBESTUs) based on various proportions of poly(butylene succinate) (PBS) and poly(ethylene succinate-coethylene terephthalate) (PETS) are successfully synthesized via the chain-extension reaction. The molar ratios of PETS/PBS in PBESTUs are nearly the same as those of the feedings. The crystallization of PBESTUs is mainly determined by the PBS segment. The PETS segment has slightly enhanced thermal stabilities of PBESTUs and firmly increases the char residue. Multimelting phenomenon of PBS is also observed, indicating that the PBS segment maintained its thermal transition behaviors in PBESTUs. Tensile testing suggests that PBESTUs possess excellent mechanical properties. ’ AUTHOR INFORMATION Corresponding Author

*Tel./fax: þ86-28-85410259. E-mail: [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge the financial support of the National Science Foundation for Distinguished Young Scholars (Project No. 50525309). ’ REFERENCES (1) Rizzarelli, P.; Puglisi, C.; Montaudo, G. Soil burial and enzymatic degradation in solution of aliphatic co-polyesters. Polym. Degrad. Stab. 2004, 85, 855. (2) Anderson, J. M.; Hiltner, A.; Wiggins, M. J.; Schubert, M. A.; Collier, T. O.; Kao, W. J.; Mathur, A. B. Recent advances in biomedical polyurethane biostability and biodegradation. Polym. Int. 1998, 46, 163. (3) Decker, C.; Bendaikha, T. Interpenetrating polymer networks. II. Sunlight-induced polymerization of multifunctional acrylates. J. Appl. Polym. Sci. 1998, 70, 2269. (4) Umare, S. S.; Chandure, A. S.; Pandey, R. A. Synthesis, characterization and biodegradable studies of 1,3-propanediol based polyesters. Polym. Degrad. Stab. 2007, 92, 464. (5) Bikiaris, D. N.; Papageorgiou, G. Z. Crystallization and melting behavior of three biodegradable poly(alkylene succinates). A comparative study. Polymer 2005, 46, 12081. (6) Kim, J. K.; Park, D. J.; Lee, M. S.; Ihn, K. J. Synthesis and crystallization behavior of poly(L-lactide)-block-poly(epsilon-caprolactone) copolymer. Polymer 2001, 42, 7429. (7) Piao, L. H.; Dai, Z. L.; Deng, M. X.; Chen, X. S.; Jing, X. B. Synthesis and characterization of PCL/PEG/PCL triblock copolymers by using calcium catalyst. Polymer 2003, 44, 2025. 2071


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A Novel Multiblock Poly(ester urethane) Based on Poly(butylene

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