A Novel Aromatic−Aliphatic Copolyester of Poly(ethylene-co

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Ind. Eng. Chem. Res. 2010, 49, 9803–9810

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A Novel Aromatic-Aliphatic Copolyester of Poly(ethylene-co-diethylene terephthalate)-co-poly(L-lactic acid): Synthesis and Characterization Jun Li,† Zhi-Qiang Jiang,† Jian Zhou,† Ji Liu,† Wen-Tao Shi,† Qun Gu,† and Yu-Zhong Wang*,‡ Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, and Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM), College of Chemistry, Sichuan UniVersity, Chengdu 610064, China

Aromatic-aliphatic copolyesters of poly(ethylene-co-diethylene terephthalate)-co-poly(L-lactic acid) (PEDLT) with different compositions were synthesized by direct melt copolyesterization of L-lactic acid oligomers and poly(ethylene-co-diethylene terephthalate) (PEDT) oligomers. Chemical structures, thermal properties, and crystalline behaviors were studied. 1H NMR confirmed that the short lactyl units were incorporated into the PEDT chains. Thermogravimetric analysis showed that PEDLT copolyesters were thermally stable up to about 380 °C. Both wide-angle X-ray diffraction and Fourier transform infrared spectroscopy showed that no poly(L-lactic acid) crystal was formed during the crystallization of PEDLT copolyesters. Isothermal crystallization of PEDLT copolyesters was carried out, and it was found that both the crystallinity and crystal growth rate decreased after incorporation of lactyl units compared with those of PEDT. The obtained aromatic-aliphatic copolyesters are expected to find a potential application as biodegradable polymer materials. 1. Introduction Biodegradable copolyesters consisting of aromatic and/or aliphatic units in their structures are of interest for polymer chemists as they have potential applications in thermoplastic elastomers1,2 and biodegradable materials.3,4 The aromatic polyester poly(ethylene terephthalate) (PET) has been widely used in many fields, such as fibers, coatings, and packaging, due to its excellent mechanical and thermal properties. However, the wide usage of PET would exacerbate the reliance on the limited petroleum resources. With petroleum becoming scarcer, it has been increasingly urgent for us to divert from petroleum-based material to biobased resources, which could be conveniently obtained from and degraded into the natural environment. On the other hand, PET is very resistant to bacterial or fungal attack and does not degrade under environmental conditions; castoffs of PET products have brought a lot of troubles for us. Now research on potentially biodegradable and biobased PET has been paid more and more attention. One approach to improve the biodegradability of PET is to blend PET and natural or/and synthetic degradable and biobased polymers,5,6 and another important one is to copolymerize PET with biodegradable aliphatic polyesters. There are many reports on the biodegradable copolymers of PET, such as poly(ethylene adipate-co-terephthalate) copolyesters,7,8 poly(ethylene terephthalate-co-ε-caprolactone) copolyesters.9,10 PET-co-poly(butylene adipate-co-succinate) copolymer,11 PET-co-poly(1,4butylenesuccinate)copolyesters,12 PET-co-poly(succinicanhydrideco-ethylene oxide) copolyesters,13 and so on. PET-co-PLLA (PLLA ) poly(L-lactic acid)) copolyester is another new and interesting potentially biodegradable and partially biomass copolymer. It is well-known that PLLA is a typical biodegradable and biobased plastic obtained by direct condensation of lactic acid or by catalytic ring-opening polymerization of lactide.14 However, the application of PLLA is * To whom correspondence should be addressed. E-mail: yzwang@ nimte.ac.cn or [email protected]. Tel. and fax: +86-28-85410259. † Chinese Academy of Sciences. ‡ Sichuan University.

limited due to its poor thermal properties.15,16 Therefore, it should be a promising process to combine PLLA with other aromatic polymers, such as PET, to improve the thermal property and enrich the product variety of PLLA. Up to now, only a few research projects on copolymerization of PET with PLLA have been reported. Olewnik17 synthesized low molecular weight copolyester PET-co-PLLA. It was reported that the PET-co-PLLA copolyesters exhibited a melting point only when the PET block length was close to nine monomer units. Grzebieniak et al.18 studied the hydrolytic degradation and compost of copolyesters with different molar ratios of ethylene terephthalate (ET) to L-lactic acid (LLA), and it is shown that the shorter the ET block lengths in copolyesters, the more rapid the degradation in the composting tests. However, it was difficult to prepare high molecular weight PET-co-PLLA copolyesters due to the thermal degradation of PLLA when reacting with molten PET.19 PET-co-PLLA copolyesters obtained by direct transesterification of PET with oligomers of LLA (OLLA) were low molecular weight copolymers and showed poor thermal and mechanical properties. In this paper, the synthesis of a high molecular weight copolyester, poly(ethylene-co-diethylene terephthalate)-copoly(L-lactic acid) (PEDLT), is reported. Here, L-lactic acid was chosen to promote the biodegradability and to increase the biomass part of the copolyester; diethylene glycol was utilized to decrease the melting point of PET, which enables OLLA to react with PET at a relatively low temperature and produce high molecular weight copolyesters. The chemical and crystalline structures of the resulting copolyesters were characterized, and their thermal and crystalline behaviors were investigated. 2. Experimental Section 2.1. Materials and Methods. Purified terephthalic acid (PTA) was obtained from Yangzi Petrochemical Co. Ltd. Ethylene glycol (EG) and diethylene glycol (DEG) were purchased from Shanghai Petrochemical Co. Ltd.. L-Lactic acid aqueous solution was obtained from PURAC.

10.1021/ie100915y  2010 American Chemical Society Published on Web 09/10/2010

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Scheme 1. Reaction Route of PEDLT Copolyester

2.1.1. Synthesis of PEDT Copolyesters. PEDT copolyesters were synthesized by direct copolymerization of PTA, EG, and DEG with Sb2O3 (1 g/kg of PTA) as the catalyst. All the syntheses were carried out in a 1 L stainless steel reactor according to the usual two-stage polycondensation procedure. In the first stage reaction, the ester interchange was conducted by stirring at 100 rpm for 2.1 h at 240-245 °C. In the second stage, the polycondensation was carried out by stirring for 2.5 h at 255-260 °C under a vacuum of 15 Pa. PEDT copolyesters with different ethylene terephthalate/diethylene terephthalate (E/ D) molar ratios were studied before the PEDT composition was chosen. During each run, samples were taken from the bottom of the reactor at different times. 2.1.2. Synthesis of OLLA. L-Lactic acid aqueous solution (90 wt %) was dehydrated in a round-bottom flask at 120-125 °C under atmospheric pressure for 6 h, and then OLLA with an Mw of 4 kg/mol and PLLA with an Mw of 32 kg/mol were obtained after condensation under a vacuum of 0.03 kPa for 2 and 6 h, respectively. 2.1.3. Synthesis of PEDLT. Blends of PEDT oligomers (PEDTa; [η] ) 0.32 dL/g), OLLA (Mw ) 4 kg/mol), and composite catalysts composed of TiO2 (0.05 wt %), Sb2O3 (0.05 wt %), polyphospholic acid (0.1 wt %), and tetrabutyl titanate (0.02 wt %) were placed in a 1 L rotating steel reactor connected to a vacuum line (0.03 kPa). The blends were kept at 230 °C and under a vacuum of 15 Pa for about 4 h, and glycol and water were slowly distilled out. The reaction route for the synthesis of the copolyester PEDLT is shown in Scheme 1. The resulting copolyesters were dissolved in a mixture of chloroform/trifluoroacetic acid and filtered. The obtained solution was precipitated by petroleum ether to remove OLLA and low molecular weight PLLA and filtered. This was repeated three times, and then the filtered cake was collected and dried for the following characterization. The copolyesters obtained in this study are abbreviated as PEDLT, shown in Table 2. E, D, L, and T indicate the ethylene, diethylene, lactyl, and terephthalic units, respectively. 2.2. Molecular Parameters. The molecular weights of the PEDLT copolymers were measured by a Waters-1515 GPC instrument equipped with a refractive index detector and a PLGel 5 µm mixed-D-type column. CHCl3 was used as the mobile phase at a flow rate of 1 mL/min. A 200 µL volume of a 0.5 wt % solution of copolyesters in a CHCl3/o-chlorophenol (1/1, w/w) mixture was injected in all the runs. All the measurementa were performed at 25 °C. The molecular weights

and the molecular weight distributions were calculated with the Maxima 820 software, and calibration was performed using polystyrene standards with a narrow molecular weight distribution. The intrinsic viscosities ([η]) of solutions of copolyester PEDT or PEDLT (c ) 5 g/dL) in a mixture of phenol and 1,1,2,2-tetrachloroethane (1/1, w/w) were examined using an AV370 Ubbelohde viscometer at a constant temperature of 25 °C. The chemical structures of PEDT, PLLA, and PEDLT copolyesters were characterized using a Bruker AVIII400 NMR spectrometer at a resonance frequency of 400 MHz. All these samples were dissolved in the DMSO-d6/trifluoroacetic acid solvent mixture (3/1, v/v), and all experiments were carried out at 25 °C. 2.3. Thermal Properties. The thermal stabilities of PEDT, PLLA, and PEDLT copolyesters were evaluated both in the air and under a nitrogen atmosphere at a flow rate of 80 mL/min. The samples were heated to 650 °C at a heating rate of 10 °C/ min, which was performed with a Mettler Toledo TGA/DSC I digital thermogravimetric analysis (TGA) instrument. A Mettler Toledo DSC I differential scanning calorimetry (DSC) instrument was employed to detect the heat flow from the samples during isothermal crystallization as well as crystalline and melting behavior. All the experiments were conducted under a nitrogen atmosphere. Melting temperatures (Tm) were taken at the minima of melting endotherms and glass transition temperatures (Tg) at the inflection point. For isothermal crystallization kinetic studies, about 7 mg portions of the samples were heated at a rate of 60 °C/min to 250 °C, held for 4 min, and then quenched to the crystallization temperature (Tc); the exothermal curves as a function of time were then recorded. 2.4. Crystal Structure. X-ray diffractograms at room temperature were acquired by a Bruker D8 diffractometer, using Ni-filtered Cu KR radiation (IZ 0.154 nm) at 40 kV and 30 mA. All the specimens were made through three steps. First, the samples were made into films of 1 ( 0.2 mm by melting the samples between two pieces of polyimide films under 4 MPa. Then these films were placed on an HCS601 microscope hot/ cold stage equipped with an Instec STC200 temperature controller and melted at 30 °C above the melting point for 4 min to remove the thermal history. At last, the melted samples were quenched to a temperature of 30-60 °C below their melting point at a cooling rate of 250 °C/min and isothermally crystallized for 12 h under a nitrogen atmosphere. FTIR spectra were collected using a Nicolet 6700 spectrometer. The samples were placed in a homemade heating chamber; each sample was kept at 250 °C for 5 min to completely erase any thermal history and then quenched to the isothermal crystallization temperatures. The chamber was purged with nitrogen to reduce both possible thermal degradation of PLLA at elevated temperatures and the water content in the environment. Time-resolved FTIR spectra were then recorded after isothermal crystallization at intervals. 3. Results and Discussion 3.1. Composition and Microstructure. 3.1.1. Synthesis of PEDT Copolyesters. PEDT copolyesters with different compositions were synthesized and characterized before the last PEDT composition was chosen. The DSC curves of several obtained PEDT copolymers are shown in Figure 1. It was shown that PEDT copolyesters represented the amorphous state at room temperature when the E/D molar ratios were less than 75/25. Then the molar ratio of ethylene terephthalate to diethylene

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Table 1. Molecular Information of PEDT Copolyesters

sample

reaction time (h)

LnE

LnD

R

[η] (dL/g)

PEDTa PEDTb PEDTc

0.5a 1.5 2.4

9.6 5.5 4.8

2.1 1.5 1.2

0.58 0.85 1.04

0.32 0.61 0.69

a PEDT oligomers used in the latter melt polycondensation with OLLA.

Table 2. Compositions and Microstructures of the Copolyesters

Figure 1. DSC curves of PEDT copolyesters with different E/D molar ratios.

sample PEDTd PEDLT1a PEDLT1b PEDLT1c PEDLT2a PEDLT2b PEDLT2c PEDLT3a PEDLT3b PEDLT3c PLLA a

reaction time (h) 0.5 2.0 3.4 0.5 2.0 3.9 0.5 2.0 4.0

MLa (%)

nLL

[η] (dL/g)

Mw (kg/mol)

PDI

0 11.7 11.8 11.8 15.9 15.7 15.7 18.9 18.6 18.5 100

0.71 0.55 0.62 0.72 0.51 0.70 0.76 0.54 0.59 0.63

72.6

1.78

3.3 1.8 1.1 3.5 2.2 1.2 4.1 2.7 1.4

61.8

1.84

75.9

1.93

68.8 40.3

1.98 1.76

Lactyl mole ratio of the samples.

LnD )

fED + fDE 2 1 ) fED + fDE PDE 2

fD +

PED )

1 1 P ) LnE DE LnD

R ) PED + PDE 1

Figure 2. H NMR spectrum of PEDTd.

terephthalate was chosen as 80/20 for preparing PEDT oligomers with a low melting temperature and for maintaining the crystallizability of the repeating ET units. Several PEDT samples, named PEDTa, PEDTb, PEDTc, and PEDTd, were taken at reaction times of 0.5, 1.5, 2.4, and 2.5 h in the polycondensation step of the PEDT copolyester, respectively. 1H NMR of PEDT copolyesters was carried out to study the sequence changes of PEDT with reaction time, which was used to characterize the sequence changes of EDT in the following synthesis of PEDLT copolyesters. The 1H NMR spectrum of PEDTd with an E/D molar ratio of 80/20 (PEDTd), together with the corresponding chemical shift assignments, is shown in Figure 2. The PEDT compositions were calculated by the relative areas of the d protons of the ethylene units from ethylene glycol (E) and the f protons of the ethylene units from diethylene glycol (D). The degree of randomness (R) of the PEDT copolyesters can be calculated by the following equations:

LnE )

fDE + fED 2 1 ) fDE + fED PED 2

fE +

(1)

(2)

(3) (4)

where PED and PDE are the probability of finding a D unit next to an E unit and the probability of finding an E unit next to a D unit, respectively, while fE, fD, fED, and fDE represent the dyad fractions, which were calculated from the integral intensities of the resonance signals E, D, ED, and DE, respectively. LnE and LnD stand for the number-average sequence lengths, the socalled block lengths, of the E and D units, respectively. The calculated results are shown in Table 1. It can be seen that the number-average sequence lengths of the ET and DT units decreased with the reaction time and the degree of randomness increased from 0.58 to 1.2 when [η] increased from 0.32 to 0.69 dL/g, which suggests an increasing randomness of the PEDT copolyester with the reaction time. 3.1.2. Synthesis of PEDLT Copolyesters. PEDLT copolyesters with different compositions, named PEDLT1a-c, PEDLT2a-c, and PEDLT3a-c, were obtained by melt condensation of PEDTa oligomers and OLLA. PEDLT1a, PEDLT1b, and PEDLT1c were taken at reaction times of 0.5, 2.0, and 3.4 h in the polycondensation step of PEDLT1 copolyester. PEDLT2a, PEDLT2b, and PEDLT2c were taken at reaction times of 0.5, 2.0, and 3.9 h in the polycondensation step of PEDLT2 copolyester. PEDLT3a, PEDLT3b, and PEDLT3c were taken at reaction times of 0.5, 2.0, and 4.0 h in the polycondensation step of PEDLT3 copolyester. These copolyesters had intrinsic viscosities ranging from 0.51 to 0.76 dL/g and weight-average molecular weights ranging from 68 to 76 kg/mol with a polydispersity index of 1.78-1.98, shown in Table 2. GPC was used to evaluate the Mw and Mn of PEDLT1c, PEDLT2c, and PEDLT3c. Each of the GPC traces showed only one smoothing sample peak, indicating those samples are

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Figure 3. 1H NMR spectrum of PEDLT3c copolyester.

copolymers instead of blends. The chemical structures of those obtained PEDLT copolyesters were determined by 1H NMR spectroscopy. In all cases, the spectra were found to be consistent with the expected structures. The 1H NMR spectrum of PEDLT3c, together with the corresponding chemical shift assignments, is shown in Figure 3. The 1H NMR spectrum of PEDLT copolymers showed new signals at 1.48 and 5.39 ppm, compared with those of PEDT and PLLA, corresponding to the methyl and methine protons of lactyl groups in the terephthalyl/lactyl (T-L) structural units (-CO-Ar-CO-OCH(CH3)-CO-), respectively, which con-

firmed the incorporation of lactyl units in PEDT copolyester chains. Meanwhile, the relative intensity of these two new peaks to that of the aromatic protons increased with an increase of the initial OLLA/BHEDT mass ratio, which means the T-L structural units increase with the feeding OLLA. The actual terephthalate (T)/lactyl (L) molar ratios of those copolyesters can be determined from the aromatic proton (∼7.9 ppm) and lactyl methyl (1.2-1.55 ppm) resonances; the calculated results are shown in Table 2. As expected, the reactive blends of PEDT and OLLA were found to generate TEL, TDL (equivalent LET and LDT) and

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Scheme 2. Abbreviation of Sequences Containing L Units in PEDLT Copolyester

Table 3. All the Possible Sequences Containing L Units in PEDT-co-PLLA Copolyester

Figure 4. TGA curves of copolyesters under a nitrogen atmosphere. Table 4. Thermal Properties of Copolyesters

Scheme 3. Formation of Sequence C through PEDT and OLLA

sample

Tg (°C)

Tm (°C)

∆Hm (J/g)

Tid (°C)

Tdmax (°C)

PEDTd PEDLT1c PEDLT2c PEDLT3c PLLA

61.3 58.3 56.0 52.1 54.9

205.1 173.8 170.6 169.4 158.2

-26.8 -11.8 -2.3 -0.5 -17.9

383.6 382.9 382.0 377.0 201.0

437.8 438.3 437.2 436.9 275.6

maximum value of R, can be calculated by the following equation: Rmax ) ETL, DTL (equivalent LTE and LTD) dyads, in addition to the TE, TD, and LL sequences present in the initial copolymers. The relative intensives of the dyad peaks for PEDLT copolyesters can be used to calculate the number-average sequence lengths of all repeating units of the obtained copolyesters. Theoretically, L units can be separated into two groups, L units with an L unit next to them and L units with a different unit next to them. All the sequences containing L units in PEDLT copolyesters can be summarized as in Scheme 2, in which T and L represent terephthalate and lactyl units, respectively, X means an E or a D unit, a, b, d, and e are 0 or 1, and c is a natural number ranging from 1 to ∞, so all the possible sequences can be obtained, shown in Table 3. Then three groups of sequences can be obtained. The first (S1 ) S5). The second one one is sequence A: (S2 ) S3 ) S7). The third one is sequence B: (S4 ) S6 ) S8). The average is sequence C: sequence length of L units can be calculated by the equation nLL )

(NA + NB + NC) + NLL ) (NA + NB + NC) (N11 + N27 + N29 + NC) + N24 (N11 + N27 + N29 + NC)

(5)

where NA, NB, and NC represent the corresponding area integrals of the methenyl protons of the lactyl units in sequences A, B, and C, respectively. can only be formed through the Sequence C ester interchange reaction of PEDT with OLLA (shown as Scheme 3), and the number of end groups -OH did not change in the ester interchange reaction. Therefore, the relation of N11, N24, N27, N29, and NC can be calculated from the mole ratio (R) of end group -L-OH and -LL- units in the original OLLA homopolymer. The average Mw values of an OLLA chain and an L unit are 4000 and 72 g/mol, respectively, so Rmax, the

n-OH 1 ) 0.018 ) ) n-LL4000/72 (N11

NC + N27 + N29) + N24

(6)

In fact, only part of OLLA joined the reaction forming sequence C. Therefore, NC < Rmax , N11 + N27 + N29 + N24. From the results of 1H NMR, it can be seen that N11 + N27 + N29 ≈ N24. Then the equation of calculating the average sequence length of L units, eq 5, can be simplified as nLL =

N11 + N27 + N29 + N24 N11 + N27 + N29

(7)

The calculated values of the sequence distribution of L units of all PEDLT copolyesters are shown in Table 2. It was shown that the number-average sequence lengths of the lactyl units in these copolyesters were all less than 4.2 and decreased with the reaction time, which means an increasing randomness of PEDLT copolyesters in the reaction process. 3.2. Thermal Properties. The thermal properties of PEDTd, PEDLT1c, PEDLT2c, PEDLT3c, and PLLA copolyesters were characterized by DSC and TGA. The TGA traces under a nitrogen atmosphere are shown in Figure 4. In all cases, the weight loss of copolyesters took place in one step. The initial decomposition temperature (Tid, at which weight loss reaches 2%) and the temperature corresponding to the maximum weight loss rate (Tmax) were obtained and are collected in Table 4. It can be seen that PLLA started to degrade at about 160 °C, whereas all the PEDLT copolyesters appeared to be more stable with an onset degradation at about 380 °C, just as for PEDTd. For all of those PEDLT copolyesters, Tdmax turned out to be in the range of 436-438 °C, very close to that of PEDTd. This indicates that PEDLT copolyesters show a mechanism of thermal degradation similar to that of PEDT while the content of lactyl units is less than 19 mol %. The glass transition temperature (Tg) of copolyesters is shown in Table 4. It can be seen that the Tg of PEDLT copolyesters

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Figure 5. DSC curves around the melting of copolyesters.

Figure 7. WAXD patterns of PEDLT3c isothermally crystallized at various temperatures.

Figure 6. WAXD patterns of PLLA, PEDTd, and PEDLTc copolyesters.

decreased with increasing lactyl unit content. It is well-known that the incorporation of flexible units, such as ether oxygen, will cause a decrease in the Tg of the polymers.20 For PEDLT copolyesters, the decrease of Tg was caused by the plasticization effect21-23 generated by the increasing relatively flexible oxygen-containing segments of PLLA and decreasing relatively rodlike aromatic segments. Figure 5 shows the calorimetric curves of the melting of the copolyesters. It was found that both Tm and the heat of fusion (∆Hm) of PEDLT copolyesters decreased with increasing lactyl unit content, indicating a reduced crystallinity with respect to PEDTd. This is also reported in other copolymers made from two crystalline comonomers.24-26 The reason is that the incorporation of lactyl units decreased the chain regularity of the copolyesters, which resulted in a decrease of both the crystallinity and melting point of PEDLT. 3.3. Crystal Structure. The WAXD patterns of copolyesters are shown in Figure 6. It can be seen that the crystal unit cells of PEDT and PEDLT are all triclinic, just the same as crystal unit cells of PET,26 and diffraction peaks from (01j1), (010), (1j11), (1j10), (100), (11j1), and (02j1) are observed at 2θ ) 16.5°, 17.9°, 21.6°, 22.9°, 26.3°, 28.2°, and 32.9°, respectively. It is obvious that the intensity of d(100) of the PEDT crystals decreased sharply after lactyl units were incorporated into the copolyesters and the intensities of the other peaks were also relatively weakened, which means the regularity of the PEDT crystal structures was decreased when the lactyl units were introduced into the PEDLT chains. Figure 7 shows WAXD patterns of PEDLT3c crystallized isothermally between 85 and 165 °C. It is obvious that all the characteristic diffraction peaks became stronger and sharper as Tc increased, which can be attributed to the increasing regularity of the PEDT crystal structures when the crystallization temperature was increased. No matter whether the crystallization

Figure 8. FT-IR spectra (a, 3200-2000 cm-1; b, 1900-500 cm-1) of PLLA, PEDTd, and PEDLT3c in the quiescent melt and fully crystallized (C ) crystallized, A ) amorphous).

temperature was above or below the melting point of PLLA crystals, all the samples yielded the same diffraction peaks over the entire range of isothermal temperatures; no diffraction signals of crystalline PLLA blocks27 were detected, indicating that only one crystalline structure, such as triclinic PET, existed in PEDLT. It has been determined that it is difficult for these short chain segments of the minor component to form stable crystals when crystallizing if the minor component of this random copolyester is less than 25 mol %.28,29 Here, the PLLA crystal was absent in crystallized PEDLT because the number-average sequence length of the lactyl units in the copolyester PEDLT was only 3.3 and it was difficult for these short lactyl units to form stable crystallites. Parts a and b of Figure 8 show the FT-IR spectra of PLLA, PEDTd, and PEDLT3c in the quiescent melt and fully crystallized at 140, 150, and 185 °C, respectively. All examples exhibited mid-IR regions that are highly sensitive to the structural changes during crystallization. PEDTd and PEDLT3c

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Figure 10. Crystallization isotherms of copolyesters at the indicated temperatures (Tc). Table 5. Kinetic Parameters of Isothermally Crystallized PEDTd and PEDLT3c sample PEDTd

Figure 9. Time-resolved FT-IR spectra of crystallizing PLLA (a) and PEDLT3c (b).

showed the same changes during crystallization, which were different from those of PLLA. For PLLA, it is proved that the band at ∼920 cm-1 is ascribed to PLLA crystals in the R form with a distorted 103 helix conformation, arising from the coupling of C-C backbone stretching and the CH3 rocking mode.30,31 The time-resolved spectra of PLLA and PEDLT3c for this region are shown in parts a and b of Figure 9, respectively. Peak shifting and intensity changes can be observed during the crystallization period of PLLA at 920 cm-1. At the beginning of PLLA’s crystallization, no band can be observed at 920 cm-1, suggesting the absence of crystals in the sample. At some point during the crystallization period, this peak became visible and its intensity increased until the crystallization was complete. However, no peak shifting and/or intensity change of any peak at about 920 cm-1 was found during the crystallization of PEDLT3c, which confirmed the absence of PLLA crystals in the crystallized PEDLT3c. Together with the results obtained from WAXD, it can be concluded that no PLLA crystals were formed during the crystallization of PEDLT copolyesters. 3.4. Isothermal Crystallization Kinetic Analysis. It is wellknown that the expression mostly used for the analysis of the kinetics of isothermal crystallization is the Avrami equation:32 1 - X(t) ) exp(-Ktn)

(8)

where X(t) is the crystallization transformation developed after time t, K is the crystallization rate constant associated with the rate of nucleation and growth, and n is the Avrami exponent, the values of which depend on the primary nucleation and growth geometry of the crystalline entities. The values of n and K are usually obtained from the double logarithmic form of the above equation: ln[-ln(1 - X(t))] ) ln(K) + n ln(t)

(9)

When DSC is used to study isothermal kinetics, X(t) could be related to the ratio of exothermic enthalpy (∆Hexo(t)) at time t

PEDLT3c

Tc (°C)

n

135 141 149 157 161 163 165 167 169 104 107 111 115 121 125 129 131 133 137

2.7 2.6 2.5 2.6 2.5 2.6 2.4 2.3 2.7 2.7 2.7 2.3 2.9 2.7 2.3 2.4 2.7 2.3 2.4

K × 104 (s-n) ∆H (J/g) t1/2 (min) -24.5 -24.4 -24.0 -26.6 -27.3 -26.2 -23.3 -23.0 -23.3 -5.3 -3.8 -5.7 -6.1 -5.7 -5.3 -4.1 -2.8 -4.1 -2.9

26.3 42.4 47.6 50.7 46.1 25.7 22.9 17.0 7.3 3.0 5.5 7.1 10.8 16.2 28.2 29.4 25.3 22.0 19.7

7.7 7.0 6.9 6.5 7.0 7.9 9.2 10.2 11.8 16.7 13.5 12.7 11.4 10.2 9.6 9.2 11.6 10.7 11.9

r2 0.9998 0.9946 0.9959 0.9997 0.9959 0.9970 0.9908 0.9954 0.9941 0.9967 0.9993 0.9987 0.9984 0.9875 0.9992 0.9997 0.9922 0.9919 0.9875

over totally exothermic enthalpy (∆Hexo(∞)) after an infinite time period. While crystallization develops at a predetermined temperature T

∫ (dH ∫ (dH t

∆Hexo(t) ) X(t) ) ∆Hexo(∞)

0 ∞

0

exo(t)/dt)

dt

exo(∞)/dt)

(10) dt

According to the definition of the half-time of crystallization, when X(t) ) 50%, t1/2 ) (ln 2/K)1/n. Parts a and b of Figure 10 show DSC exothermic traces of PEDTd isothermally crystallized at temperatures ranging from 135 to 169 °C and PEDLT3c isothermally crystallized at temperatures ranging from 104 to 137 °C, respectively. The exothermic traces were integrated to get the crystallization enthalpy (∆H), and all the results of the isothermal crystallization of PEDTd and PEDLT3c are listed Table 5. It can be seen that, for both PEDTd and PEDLT3c, as the crystallization temperature (Tc) increased, the half-time of the final crystallinity (t1/2) and crystallization rate constant (K) were first shifted to shorter time and then shifted to longer time, indicating that, for both PEDTd and PEDLT3c, Tc was an important influencing factor determining the crystallization. The Avrami exponent n values of PEDTd and PEDLT3c were between 2.2 and 2.9, and PEDLT3c showed a lower crystallization enthalpy (∆H) and a longer t1/2 than PEDTd, indicating that incorporation of lactyl units into PEDT chains inhibits the crystallization of PEDT.

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4. Conclusions A series of PEDLT copolyesters were synthesized successfully. 1H NMR confirmed the incorporation of lactyl units into copolyester chains. Thermal analysis showed that all the PEDLT copolyesters showed excellent thermal stability and the melting points of copolyesters decreased with the incorporation of lactyl units. WAXD patterns revealed that the crystal structure of PEDLT isothermally crystallized between 85 and 165 °C was the characteristic triclinic crystalline structure of PET, and no diffraction signals of crystalline PLLA blocks were detected, which meant no PLLA crystal formed during the crystallization of PEDLT. This was also confirmed by time-resolved FTIR. Isothermal crystallization found that both the crystallinity and crystal growth rate decreased after incorporation of lactyl units into PEDT chains. The obtained PEDLT copolyesters are expected to be used as biodegradable polymer materials. Acknowledgment This work was supported by the National Key Technology R&D Program (Contract No. 2007BAE28B06), Zhejiang Province Science and Technology Project (Contract No. 2007C31031), Zhejiang Province Science Foundation (Contract No. Y4064417), Ningbo Municipal Natural Science Foundation (Contract No. 2006A610071), and Science and Technology Department of Zhejiang Province (Grant 2009C21016). Supporting Information Available: Synthesis, experimental procedures, and additional data on the assemblies. This information is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Parton, H.; Baets, J.; Lipnik, P.; Goderis, B.; Devaux, J.; Verpoest, I. Properties of poly(butylene terephthatlate) polymerized from cyclic oligomers and its composites. Polymer 2005, 46 (23), 9871–9880. (2) Hiki, S.; Miyamoto, M.; Kimura, Y. Synthesis and characterization of hydroxy-terminated [RS]-poly(3-hydroxybutyrate) and its utilization to block copolymerization with -lactide to obtain a biodegradable thermoplastic elastomer. Polymer 2000, 41 (20), 7369–7379. (3) Deng, L.-M.; Wang, Y.-Z.; Yang, K.-K.; Wang, X.-L.; Zhou, Q.; Ding, S.-D. A new biodegradable copolyester poly(butylene succinate-coethylene succinate-co-ethylene terephthalate). Acta Mater. 2004, 52 (20), 5871–5878. (4) Schiller, C.; Epple, M. Carbonated calcium phosphates are suitable pH-stabilising fillers for biodegradable polyesters. Biomaterials 2003, 24 (12), 2037–2043. (5) Voyfe, D. EP 0008717, Feb 24, 2005. (6) Smit, H. WO 0214430, Feb 21, 2002. (7) Monvisade, P.; Loungvanidprapa, P. Synthesis of poly(ethylene adipate) and poly(ethylene adipate-co-terephthalate) via ring-opening polymerization. Eur. Polym. J. 2007, 43 (8), 3408–3414. (8) Heidary, S.; Gordon, B., III. J. EnViron. Polym. Degrad. 1994, 2, 19. (9) Tokiwa, Y.; Ando, T.; Suzuki, T.; Takeda, T. Polym. Mater. Sci. Eng. 1990, 62, 988. (10) Zhang, R.; Luo, X.; Ma, D. Multiple melting endotherms from ethylene terephthalate-caprolactone copolyesters. Polymer 1995, 36 (22), 4361–4364. (11) Tserki, V.; Matzinos, P.; Pavlidou, E.; Panayiotou, C. Biodegradable aliphatic polyesters. Part II. Synthesis and characterization of chain extended poly(butylene succinate-co-butylene adipate). Polym. Degrad. Stab. 2006, 91 (2), 377–384.

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ReceiVed for reView April 19, 2010 ReVised manuscript receiVed July 10, 2010 Accepted July 11, 2010 IE100915Y