Isomorphism in Poly(butylene succinate-co-butylene fumarate) and Its

Jul 3, 2012 - Competition and miscibility of isodimorphism and their effects on band spherulites and mechanical properties of poly(butylene ...
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Isomorphism in Poly(butylene succinate-co-butylene fumarate) and Its Application as Polymeric Nucleating Agent for Poly(butylene succinate) Hai-Mu Ye, Rui-Dong Wang, Jin Liu, Jun Xu,* and Bao-Hua Guo* Advanced Materials Laboratory, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Strict isomorphism between butylene succinate and butylene fumarate in poly(butylene succinate-co-butylene fumarate) (PBSF) was revealed by DSC and wide-angle X-ray diffraction results. They adopt the same crystal modification, with only a little difference in crystal lattice parameters. The melting point of the copolymer increases linearly with increasing molar ratio of butylene fumarate and the melting enthalpy hardly changes, which meet the requirement of strict isomorphism. The introduction of unsaturated comonomer, fumaric acid, into PBS can enhance the total crystallization rate and the radial growth rate of spherulites. Consequently, PBF and PBSF are highly efficient polymeric nucleating agents for PBS and its copolymers. In this work, strict isomorphism provides us a new method to find polymeric nucleating agent.

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INTRODUCTION It is almost 200 years since the “discovery” of isomorphism,1 which is an important phenomenon in phylogeny of chemistry. Usually, the isomorphism is observed in inorganic and organic small molecules and now is playing an important role in supramolecular chemistry and crystal engineering fields.2,3 In 1959, the concept of isomorphism in polymer field was systematically arisen by Natta et al.,4,5 who divided this phenomenon into two cases: (a) isomorphism of macromolecular chains, where different polymer chains pack themselves in the same crystal lattice, and (b) isomorphism among monomeric units in copolymer system, where the different comonomer units exhibit the equal behavior during crystallization. Later Allegra and Bassi6 summarized two requirements for the formation of macromolecular isomorphism: “first, the different types of monomer units must approximately have the same shape, and occupy the similar volume; second, the conformation of the two types of monomeric units must be compatible with each other.” Many isomorphic systems have been reported so far. For example, some blends of copolymers show isomorphism of macromolecular chains, such as poly(R-3-hydroxybutyrate-co-R3-hydroxyvalerate)/poly(R-3-hydroxybutyrate-co-3-hydropropionate), poly(R-3-hydroxybutyrate)/poly(R-3-hydroxybutyrateco-R-3-hydroxyvalerate), polyethylene/poly(ethylene-co-butene), poly(4-methyl-1-pentene)/poly(4-methyl-1-hexene), poly(ethylene terephthalate)/poly(ethylene 2,6-naphthalate), poly(ether ether ketone)/poly(ether ketone),7−11 etc. Some copolymers reveal isomorphism of different monomer units, including poly(R-3hydroxybutyrate-co-R-3-hydroxyvalerate), poly(ethylene terephthalate-co-ethylene 2,6-naphthalate), poly(hexamethylene terephthalate-co-ethylene 2,6-naphthalate), poly(hexamethylene adipate-co-butylene adipate), poly(hexamethylene © 2012 American Chemical Society

sebacate-co-hexamethylene suberate), and poly(hexamethylene suberate-co-hexamethylene adipate).12−19 If we refer to strict isomorphism of different monomer units, an additional requirement must be met: only a unique crystal modification is observed for the whole composition. The crystalline phase of two homopolymers must be analogous, from the point of view of the chain conformation, the lattice symmetry, and dimensions. In fact, it is quite obvious that a single crystalline phase is possible only in this case, with small and continuous change of crystal lattice parameters with varying composition. Otherwise, if more than one crystalline phase containing two types of units is detected, depending on the composition and/or thermal or mechanical treatment, it is named as isodimorphism or isopolymorphism. So actually, most examples mentioned above are not ideal strict isomorphism in the whole composition range but isomorphism in a limited composition range or isodimorphism. Nucleation in polymer crystallization is an important research topic since it affects the crystallization rate, the crystalline structures, and the final properties of semicrystalline polymers. Among the methods to improve nucleation rate, nucleating agent, or nucleant, can create a large number of primary nuclei, resulting in smaller spherulites and improved optical and mechanical properties.20 On the other hand, due to the increase of crystallization rate, cycle time of processing can be significantly reduced so as to raise the output of product.21−23 Until now, two interpretations have been identified as the mechanism of nucleating agent. The first is epitaxial nucleation of polymers on inorganic, organic, or polymeric Received: April 3, 2012 Revised: June 15, 2012 Published: July 3, 2012 5667

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substrates, which was established by Lotz and Wittmann;24−34 another mechanism is chemical nucleation, which was proposed by Mercier and Nield.35 The former mechanism is widely operative in nucleating of polyethylene, polypropylene, polyesters, etc.36−39 The essential requirement for epitaxial nucleation is a two-dimensional lattice matching of polymer matrix and the nucleating agent, and the usually acceptable range of lattice mismatch is less than 15%.24 Comparing the isomorphism and epitaxial nucleation mechanism, we speculate that in an isomorphic system the polymer with higher crystallization temperature may work as nucleating agent for the component with lower crystallization temperature. Poly(alkyl dicarboxylates) are a family of biodegradable polyesters. In previous studies, we have synthesized poly(butylene succinate) (PBS), poly(butylene adipate) (PBA), and a series of copolymers.40−47 The detailed crystallization behavior has been investigated. In most cases, the melting point and degree of crystallinity of copolymers decrease with copolymerization when the molar ratio of comonomer units is less than 30%. In a recent review,48 we note that the results of poly(butylene succinate-co-butylenefumarate) (PBSF) reported by Nikolic et al.49 are an exception, which demonstrated a constant melting point with the copolymer composition up to 20 mol %. We speculate that PBSF is a strict isomorphic system, since the volume of succinate and fumarate is very similar and the trans configuration of fumarate is the same as that of succinate in the crystal lattice of PBS. Here we synthesized PBSF with a much wider range of composition and investigated the corresponding crystalline structures. The results agree with our speculation that PBS, poly(butylene fumarate) (PBF), and PBSF can form strict isomorphism. Furthermore, PBF and PBSF are found to be highly efficient polymeric nucleating agents for PBS and its copolymers, which is greatly beneficial for thermoprocessing of PBS.

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respectively. The wide-angle X-ray diffraction (WAXD) analyses were performed at room temperature using a Rigaku D/max2550HB+/PC X-ray diffractometer with Cu Kα radiation. The X-ray wavelength was 0.154 nm. The samples were scanned from 12° to 32° with a step interval of 0.02° at a scanning rate of 4°/min. Two-dimensional WAXD patterns of oriented polymer fibers were obtained on a Rigaku R-Axis Spider instrument with a Mo target. FTIR spectra were recorded on a Nicolet-560 IR spectrometer by signal averaging over 32 scans at a resolution of 4 cm−1 in the wavenumber range of 4000−400 cm−1. The spherulite morphologies and radial growth rates of polymers were measured under a polarized optical microscope (Weitu, Shanghai) with a temperature-controlled stage.

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RESULTS AND DISCUSSION Characterization of Copolyesters. The comonomer composition in PBSF copolymers was calculated from 1H NMR spectra using the relative intensities of proton peaks arising from butylene succinate and butylene fumarate repeating units. Figure 1 shows the 1H NMR spectrum of PBSF40, in which

EXPERIMENTAL SECTION

Poly(butylene succinate), poly(butylene fumarate), and the copolymers were synthesized from 1,4-butanediol, succinic acid, and fumaric acid with different feeding molar ratios by a two-step reaction of esterification and polycondensation in melt sate, as reported in previous literature.40−47 Tetra-n-butyl-titanate and p-hydroxyanisole (each was 0.5 wt % relative to the total reactants) were used as catalyst and free radical inhibitor during the reaction, respectively. The products were dissolved in chloroform and centrifuged to remove impurities and then precipitated in an excess amount of cold methanol. The precipitates were washed with methanol and dried in vacuum at 50 °C for 2 days before use. The molecular weights (Mn and Mw) and composition of the synthesized polymers were determined by gel permeation chromatography system (Viscotek, M302 TDA) and 1H NMR spectrometer (JEOL, ECA-300M). The oriented fibers of the homopolymers and copolymers were prepared using a homemade drawing mill. Single crystals of PBF were grown from o-dichlorobenzene solution with a concentration of 0.02 wt % at 84 °C by the self-seeding method. Poly(butylene succiniate-co-butylene maleate), poly(propylene succinate) (PPS), poly(propylene fumarate) (PPF), poly(propylene succinate-co-propylene fumarate) (PPSF), poly(hexamethylene succinate) (PHS), poly(hexamethylene fumarate) (PHF), and poly(hexamethylene succinate-co-hexamethylene fumarate) (PHSF) were obtained and purified through the same way as PBSF. A Shimadzu DSC-60 differential scanning calorimeter was used to study the thermal properties at a heating and cooling rate of 10 °C/ min under a nitrogen atmosphere. The peak temperature of the endotherm was taken as the melting point. Atomic force microscopy (Shimadzu, 9500-J3) and transmission electron microscopy (JEOL, JEM-2010) were utilized to characterize the morphologies and electron diffraction patterns of single crystals of different polymers,

Figure 1. 1H NMR spectrum of PBSF40 copolyester.

succinate and fumarate show characteristic peak at 2.617 and 6.844 ppm, respectively. The content of BF units in PBSF40 was calculated to be 38 mol % from the integrated area of 1H peak at the two positions. The compositions of the synthesized polymers are summarized in Table 1. The actual composition deviates only slightly from the succinic acid/fumaric acid feeding ratio. Herein, the samples are referenced using the feeding ratio of BF in copolymers. As noted in the previous studies,50,51 cofeeding of two types of monomer units using the same polycondensation procedure tends to produce nearly random distribution of comonomer units in the polymer chains, and the atacticities for all PBSF copolyesters are almost 1 calculated from 1H NMR spectra, as shown in Table 1. Figure 2 shows the variation of the glass transition temperature of copolymer versus composition, which fits well with the Fox equation.52 Therefore, it is reasonable to expect that the PBSF copolymers in this study are random copolyesters. The average molecular weight and PDI of the polymers were detected by GPC, with the number-average molecular weight and PDI ranges in 2.95− 4.46 × 104 and 1.47−1.94, respectively. Isomorphism between Butylene Succinate and Butylene Fumarate Units. Thermal properties are simple and convenient to check whether this series of polyesters are isomorphic or not. Nonisothermal melt crystallization and the 5668

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Table 1. Composition, Average Molecular Weight, Thermal Properties, and Atacticity Degree of the Synthesized Polyesters BS/BF molar ratio sample

feeding ratio

NMR result

Mn × 104

Mw × 104

PDI

Tg (°C)

Tc (°C)

Tm (°C)

ΔHm (J/g)

atacticity

PBS PBSF5 PBSF20 PBSF40 PBSF60 PBSF80 PBF

100/0 95/5 80/20 60/40 40/60 20/80 0/100

100/0 95/5 82/18 62/38 43/57 22/78 0/100

4.46 4.12 3.16 3.74 3.37 3.46 2.95

6.56 6.27 6.05 6.81 6.04 5.40 5.49

1.47 1.52 1.94 1.82 1.79 1.56 1.86

−44.6 −43.9 −40.7 −38.1 −32.9 −29.6 −24.2

76.4 78.6 81.2 88.0 94.8 97.9 105.9

113.3 114.9 117.8 122.0 127.2 133.4 138.6

77.5 76.2 76.5 77.2 78.1 76.4 77.3

0.93 0.97 0.95 1.02 1.04

PBSF40 exhibit four more diffraction spots, which are attributed to the (1̅11) lattice plane.55 With the increase of butylene fumarate content, the diffraction spots of (1̅11) shift closer to those of (021) plane and finally merge together when butylene fumarate content is higher than 60 mol % in the copolymer. All the above suggests that the crystal structure of PBSF is similar to that of PBS and the cell dimensions slightly change as the composition varies. The electron diffraction pattern of a PBF single crystal grown from solution is presented in Figure 5, which is very similar to that of PBS.55 The reflections in Figure 5 are the hk0 spots corresponding to the reflections on the equator of PBF in Figure 4. Figure 6a shows the WAXD diffractograms of PBS, PBF, and PBSF copolymers, which are quite similar to each other, but with gradual shift of the diffraction peaks. The cell parameters of PBS calculated from its WAXD diffractogram are a = 5.232 Å, b = 9.057 Å, c = 10.90 Å, and β = 123.87°, and the cell parameters of PBF are a = 5.012 Å, b = 9.258 Å, c = 10.93 Å, and β = 120.90°. On the basis of the space group and crystal structure of PBS, we calculate the cell parameters of PBSF copolysters, as displayed in Figure 6b. The dimension of a-axis decreases with increasing butylene fumarate content, while the b-axis increases and the c-axis remains almost constant. The sectional area of ab plane decreases from 47.39 Å2 of PBS to 46.40 Å2 of PBF; namely, PBF exhibits a little closer packing than PBS. Now we can conclude that all copolymers occupy the same crystal modification, only with a little difference in lattice parameters; namely, these are belonging to a strict isomorphic series in the whole composition range. The FTIR spectrum is sensitive to the chain conformation and packing state of polymer chains.56−60 Hence, it was utilized to characterize this isomorphic system. The FTIR spectra of PBS, PBF, and PBSF are presented in Figure 7. All samples show similar absorption bands at the range of 1500 to 1000 cm−1 (Figure 7a), but quite different bands in the region of 1800 to 1640 cm−1 (Figure 7b), which is originated from the stretching vibration of carbonyl groups. The absorption peak of PBS and PBF is at 1716 and 1706 cm−1, respectively. The copolymers show two absorption peaks at 1716 and 1706 cm−1, and the ratio of absorption strength (I1716/I1706) decreases with increasing content of fumarate units. It is interesting to note that the carbonyl groups of the copolymers display two absorption peaks; what is more, the position of the two peaks do not shift with copolymer composition. This demonstrates that there exist two independent types of hydrogen bonds, belonging to PBS and PBF. The red-shift of the absorption bands of the carbonyl groups in PBF may be result from the double bond in fumarate unit. Nucleating Effect Utilizing Isomorphism. Based on the above crystal structure analysis and epitaxial crystallization theory,24 PBF and PBSF are expected to show significant

Figure 2. Dependence of glass transition temperature on composition obtained from DSC. The dotted line is calculated through the Fox equation.

subsequent melting process of the samples were studied by DSC, as shown in Figures 3a and 3b, respectively. All polymers display similar crystallization and melting behaviors as PBS, showing one crystallization peak and two or more melting peaks. We take the highest peak temperature of the endotherms as the melting temperature of sample. Unlike the usual phenomenon that a depression of both crystallization and melting temperature occurs with increasing comonomer content, the crystallization and melting temperature of PBSF increase with the increase of butylene fumarate content, as plotted in Figure 3c. What’s more, the melting point versus copolymer composition is a perfect linear curve, with a slope of ∼2.5 °C/ (mol %). And the melting enthalpy per gram keeps almost constant (∼77 J/g) for all the compositions (see Table 1). The relative degree of crystallinity of the listed samples is all around 65% determined via WAXD diffractograms. A similar variation tendency of melting temperature dependent on copolymer composition was also discovered in ε-caprolactone and 2oxepane-1,5-dione random copolyesters,53 and ω-pentadecalactone and ε-caprolactone random copolyesters,54 but the melting enthalpy determined by DSC and the degree of crystallinity determined by WAXD changed obviously, which was due to the quite different molecular structure of monomers. From the DSC results, the novel phenomenon might arise from isomorphic crystal structure. But it still needs further confirmation from WAXD experiment. Figure 4 shows the two-dimensional WAXD patterns of PBS, PBF, and some copolyesters. All samples exhibit the similar crystal diffraction pattern, indicating the same crystal modification. But there is a little difference, as indicated by the arrows. PBS, PBSF20, and 5669

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Figure 3. DSC curves of (a) the first cooling and (b) the second heating of polyester samples. (c) Relation between melting temperature and butylene fumarate content.

Figure 5. AFM image and electron diffraction pattern of PBF single crystal.

Figure 4. Two-dimensional WAXD patterns of PBS, PBF, and PBSF fibers.

effect of the amount of PBF additive. When PBF content was (or more than) 0.5 wt %, the crystallization temperature of PBS could be increased to as high as 92 °C. The nucleating efficiency reported here is much higher than the previously reported nucleating agents, such as attapulgite and inclusion complex,61,62 even higher than carbon nanotube.63,64 Furthermore, isothermal crystallization experiments at 100 °C were

nucleating effect for PBS. The effects of PBF and PBSF on the crystallization behavior of PBS were detected by DSC measurement. Figure 8a shows the DSC curves of nonisothermal melt-crystallized PBS after adding 2 wt % PBF or PBSF copolyester. The crystallization temperature of PBS is raised from 77.5 °C to higher than 90 °C. Figure 8b further displays the 5670

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carried out in DSC and POM. As shown in Figure 9, the half crystallization time for PBS is shortened from 24.8 to 1.6 min, and the spherulite size significantly decreases when PBF is added into PBS matrix as nucleating agent. These results confirm that PBF and PBSF can be utilized as an great nucleating agent for PBS. Usually, comonomers are introduced into PBS to improve its mechanical properties, such as poly(butylene succinate-co-butylene adipate) (PBSA),65 poly(butylene succinate-co-propylene succinate) (PBSP),44 and poly(butylene succinate-co-butylene terephthalate) (PBST).66 But, meanwhile, the crystallization ability of the copolymers becomes lower. All these PBS copolymers have been reported to adopt PBS type crystal structure, when the content of comonomer unit is less than a certain value. According to the epitaxial crystallization theory, PBF and PBSF should also be useful to increase their crystallization rate. Similarly, DSC was utilized to measure the curves of nonisothermal melt crystallization kinetics of PBSA and PBSP with or without the addition of PBF, as displayed in Figure 10. The crystallization temperature of poly(butylene succinate-co-10 mol % butylene adipate) (PBSA10) and poly(butylene succinate-co-10 mol % propylene succinate) (PBSP10) increases from 65.6 to 80.9 and 81.8 °C, respectively. The crystallization temperatures of PBS copolymers nucleated with PBS during cooling are higher than that of pure PBS homopolymer (without nucleating agent), so we may be able to combine fumarate units into these copolyesters to improve the mechanical properties and crystallization rate simultaneously. The significant role of PBF and PBSF in accelerating crystallization of PBS and its copolymers should result from matching of the cell parameters, as a case of epitaxial crystallization mechanism. Thus, PBF and PBSF are highly efficient nucleating agents for PBS and its copolymers. Furthermore, these polymeric nucleating agents possess additional advantages, such as fine dispersion and low cost compared to small molecular nucleating agents. Our results demonstrate that isomorphism could be a method for seeking polymeric nucleation agents. Further Discussion. As revealed by the DSC and WAXD results, PBS, PBSF, and PBF display a strict isomorphism phenomenon: they maintain the same crystal modification. The melting temperature changes linearly with the composition, and the melting enthalpy remains almost constant. This is a new type of strict isomorphism by introducing the corresponding unsaturated monomer units into the saturated polyester.

Figure 6. (a) WAXD diffractograms of PBS, PBF, and PBSF copolymers isothermally crystallized at 70 °C; the crystallographic planes are indicated. (b) Variation of the unit cell parameters of crystal lattice with composition.

Figure 7. FTIR spectra of PBS, PBF, and PBSF collected at 25 °C. 5671

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Figure 8. (a) Effects of PBF and PBSF on nonthermal crystallization of PBS. (b) Effect of the amount of PBF additive on nonthermal crystallization of PBS.

Figure 9. (a) Isothermal crystallization kinetic curves of PBS with and without 2 wt % PBF at 100 °C. The POM images of (b) PBS and (c) PBS with 2 wt % PBF after isothermal crystallization at 100 °C completely.

Here, we still have two issues of concern: First, why can butylene succinate and butylene fumarate form strict isomorphism? Second, why does PBF have higher melting temperature and crystallization rate than PBS? Both fumarate and maleate are unsaturated forms of succinate with similar monomer size. However, fumarate adopts trans configuration, while maleate takes cis one, which are the essential difference between them. Once a polymer chain crystallizes, the energy barrier comes from both enthalpic and entropic origin.67,68 The latter arises from the adjustment of chain conformation during packing into the crystal lattice. In PBS crystal lattice, succinate units in polymer chain were reported to take trans conformation.19,69 Fumarate units just meet this requirement, so the butylene fumarate can cocrystallize easily with butylene succinate to form isomorphism. To further

confirm the importance of comonomer configuration, poly(butylene succiniate-co-20 mol % butylene maleate) (PBFM20) was prepared using the same procedure. PBSM20 shows the same crystal X-ray diffractions as PBS. Figure 11 shows that the melting temperature and crystallization temperature of PBSM20 are 102.9 and 60.7 °C, respectively. The melting enthalpy is 59.3 J/g. All these values of PBSM20 are much lower than those of PBS. In contrast, the melting and crystallization temperature of PBSF20 are considerably higher than those of PBS. The WAXD and DSC results reveal that maleate units play a similar role in PBSM as adipate units in PBSA;65 namely, the butylene maleate is excluded from the crystal lattice during crystallization. So the inability of cocrystallization of butylene succinate and butylene maleate must be due to their different conformations. 5672

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Figure 10. Melt-crystallization curves of poly(butylene succinate-co10 mol % butylene adipate) (PBSA10), poly(butylene succinate-co10 mol % propylene succinate) (PBSP10), and their blend with 2 wt % PBF at a cooling rate of 10 °C/min.

Figure 13. Crystallization rate (half-time of complete crystallization measured by DSC) of polyester samples at different temperatures.

Figure 11. DSC curves of the first cooling and the second heating of PBSM20 at a rate of 10 °C/min.

Figure 14. Radial growth rate of PBS and PBSF spherulites (a) and the morphology of PBF isothermally crystallized at 120 °C (b).

special properties of PBSF, we continued to prepare polyesters from other alkyl glycols with succinic acid and fumaric acid. Unlike PBS, when we introduce fumarate unit into poly(propylene succinate) (PPS) and poly(hexamethylene succinate) (PHS), they only show isomorphism in a limited composition range, as seen in Figure 12, which is quite different from PBSF. The only change of the methylene number in alkyl unit could generate so large difference. These might be due to that the introduction of fumarate units into PPS or PHS interrupt the conformation of their alkyl units in the backbone. In contrast, PBS is a special case, where the conformation of tetramethylene units keeps the same no matter copolymerization with fumaric acid or succinic acid. For brevity, more details of the properties of polyesters polymerized form different glycols with succinic acid and fumaric acid will be reported later. The trans conformation enables fumarate units form isomorphism with succinate units. What is more, the introduction

Figure 12. WAXD diffractograms of (a) poly(propylene succinate) (PPS), poly(propylene fumarate) (PPF), poly(propylene succinate-copropylene fumarate) (PPSF), (b) poly(hexamethylene succinate) (PHS), poly(hexamethylene fumarate) (PHF), and poly(hexamethylene succinate-co-hexamethylene fumarate) (PHSF).

So the conformation matching of monomer units and polymer chain is very important in isomorphic systems. Following the 5673

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Macromolecules of fumarate units improves the melting temperature and crystallization rate, as shown in Figures 3 and 13. Here, the crystallization rate is represented by the reciprocal of the half-time of complete crystallization (1/t1/2). We speculate that once more fumarate units exist in the backbone of the polymer chain, the chain conformation in melt state will take more probability to adopt the trans conformation (close to the crystal state) than PBS; namely, the entropic barrier during crystallization will be lowered down. At the same time, the melting enthalpy keeps almost constant, and as a result, the melting temperature increases. Once the entropic barrier during crystallization is lowered down, and the melting enthalpy keeps almost constant, the nucleating ability of polymer will be enhanced. We measured the radial growth rate of PBS and PBSF spherulites during isothermal crystallization at different temperatures ranging from 91 to 106 °C, as shown in Figure 14a. The radial growth rate was sharply increased when 20 mol % fumarate units were introduced into PBS polymer backbone, and the growth rate kept increasing with the increase of fumarate content. On the other hand, the introduction of double bond (fumarate) into PBS, which improved the stiffness of polymer backbone, should reduce its diffusion ability. Thus, the increasing of radial growth rate should result from the enhancement of secondary nucleation rate. Moreover, with the increase of BF content, the primary nucleation rate was greatly raised, which made it difficult to obtain the radial growth rate of PBSF with higher butylene fumarete content through optical microscope at the same temperature range. Figure 14b shows the spherulite morphology of PBF after melt crystallized at 120 °C. Even at such a high temperature, the primary nucleation ability of PBF is very strong. The enhancement of primary and secondary nucleation rate leads us to speculate that the energy barriers of both primary and secondary nucleation were lowered down with the introduction of butylene fumarate comonomer units. The behind mechanism of enhanced nucleation rate by incorporation of butylene fumarate units deserves further study and is still in progress.

ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National High-tech R&D Program of China (863 Program) (Grant 2011AA02A203) and the National Natural Science Foundation of China (Grant 20974060).

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CONCLUSION Here we introduce a new type of strict isomorphism via incorporating trans double-bond comonomer units into saturated polyester, PBS. PBS, PBF, and PBSF are proved to satisfy the requirements of strict isomorphism; namely, they adopt the same crystal modification, with only a little difference in crystal lattice parameters. The melting point of the copolymers varies linearly with the copolymer composition and the melting enthalpy hardly changes. The introduction of fumarate units into PBS can enhance the total crystallization rate and the radial growth rate of spherulite significantly. Furthermore, PBF and PBSF are found to be highly efficient nucleating agents for PBS and its copolymers. The isomorphism provides us a new method to find polymeric nucleating agents.

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*Tel +86-10-62784740, Fax +86-10-62784550, e-mail jun-xu@ mail.tsinghua.edu.cn (J.X.); Tel +86-10-62784550, Fax +86-1062784550, e-mail [email protected] (B.-H.G.). Notes

The authors declare no competing financial interest. 5674

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