Remarkable Melting Behavior of PLA Stereocomplex in Linear PLLA

Feb 6, 2015 - Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, ... College of Chemistry and Chemical Engineering, Jia...
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Remarkable Melting Behavior of PLA Stereocomplex in Linear PLLA/ PDLA Blends Jun Shao,†,‡ Sheng Xiang,† Xinchao Bian,† Jingru Sun,† Gao Li,*,† and Xuesi Chen*,† †

Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ College of Chemistry and Chemical Engineering, JiangXi Normal University, Nanchang 330022, China S Supporting Information *

ABSTRACT: The linear PLLA/PDLA blends were prepared by solution mixing method, and the melting behavior and structure evolution of neat PLLA and PLLA/PDLA specimens were investigated in this study. Results indicated that PLA stereocomplex crystallites (sc) preferentially formed in all blends, and the crystal structure of PLA sc and homochiral crystallites (hc) did not vary as molecular weights. The melting temperature of PLA neat specimens (Thm) increased monotonously with molecular weights. However, significantly different from the neat samples, the melting temperature of PLA sc (Tsc) increased at first, then decreased as the molecular weight of polymers increased from 4 to 100 kg/mol. When the molecular weights of PLLA and PDLA ranged from 23 to 50 kg/mol, multimelting behaviors observed at 230 °C in the blends. After annealing at a fix temperature (Tsc − 10 °C), the highest Tsc was observed at 249.9 °C in L32/D31 specimen, which was the highest report value until now. The WAXD and SAXS results attested that not crystal structure, but the variation of the thickness of lamellar crystal was the exterior reason, and the higher optical purity of PLLA and PDLA would be the inherent cause which resulted in the superior thermal properties. This investigation provides more potential for the application of PLA sc materials at higher temperature environments.

1. INTRODUCTION Poly(lactide) (PLA) is a widely investigated biobased renewable material; its raw material, lactic acid, generates from the renewable vegetable resources,1,2 and its product is degradable in nature and in the human body.3 Furthermore, PLA exhibits excellent physical properties comparable to some generalpurpose plastics.4 These properties have made PLA materials become a hot research area from the standpoint of both basic and applied research.5−7 It is known that a chiral carbon atom exists in lactic acid, and both L-lactic acid and D-lactic acid generate in nature. After polymerization, three types of PLA that is, PLA with L-configuration, PLA with D-configuration, and PLA with two configurations randomly distribute (PLLA, PDLA, and a-PLA)will develop from the two kinds of lactic acid. The PLLA and PDLA have optical activity due to the chiral structure, and the melting temperatures (Thm) of PLLA and PDLA do not exceed 180 °C. The Thm drops quickly, and the crystallization capacity reduces sharply with a decrease in optical purity.8 This issue limits the application temperature of PLA to an extent that is not higher than 60 °C (the glass transition temperature). In 1987, the PLA stereocomplex (sc) was first reported in the PLLA/PDLA blends.9 In contrast to neat PLLA or PDLA, the PLLA/PDLA blend is melted at a higher temperature (∼230 °C), and the blend develops crystallites at a faster rate.10 The PLA sc could form not only in the linear and branched PLLA/PDLA blends11−14 but also in the linear and multiarmed PLLA-b-PDLA block copolymers.15−18 Further, it also form in the substituted and unsubstituted poly(lactide)s.19,20 The melting temperature of PLA sc (Tsc) varies as different structures, most reports observe it at 230 °C in the linear PLLA/PDLA blends.9,21,22 In addition, © XXXX American Chemical Society

some higher Tsc is also reported, such as, Chen X. et al. reported the Tsc at 246 °C in the three-armed PLLA/linear PDLA blend,12 and 245 °C in the linear PLLA/PDLA blend with relative low molecular weights.23 This superior thermal properties also observe in PLLA/PDLA specimens with higher molecular weights, such as, Ikada et al. found the Tsc at 239 °C after aging for three years,21 Biela and co-workers reported the Tsc at 242 °C in the functionalized multiwalled carbon nanotubes poly(lactide),24 and Wang’s group detected it at 240 °C under stretched condition.25 In addition, similar results were also found in other research.14,26 In the reports with high molecular weights, the ordered sc structure observed or the methods for preparing the specimens (e.g., stirring for a long time, stretching at a proper temperature, etc.) are favorable to form fibrous crystallites, and the higher Tsc is ascribed to the formation of a structure with this orientation. Through these numerous investigations, the dependence of the Tsc on the molecular weights has not been investigated in detail yet, and the reason that the PLA sc with higher Tsc in moderate molecular weights is still not clear. In this manuscript, the linear PLLA and PDLA with different molecular weights were synthesized, and the PLLA/PDLA cast films with similar molecular weights were prepared. The melting behaviors and structure evolution of PLLA/PDLA blends together with PLLA specimens were discussed. It was interesting to find that the Tsc increased at first and then Received: November 13, 2014 Revised: January 23, 2015 Accepted: February 6, 2015

A

DOI: 10.1021/ie504484b Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Polystyrene was used as standard. The flow rate of the mobile phase was 1 mL/min. The wide-angle X-ray diffraction (WAXD) measurements were carried out on a Bruker D8 Advance X-ray diffractometer, which is equipped with Cu Kα radiation. The scanning angle ranged from 10 to 30°, and the measurement ran at a speed of 3°/min. Thermal properties of neat PLAs and PLLA/PDLA samples were performed on a differential scanning calorimeter (DSC, Q100, TA) apparatus under nitrogen atmosphere. The temperature and heat flow were calibrated with standard indium (Tm = 156.6 °C, ΔHm = 28.5 J/g). The specimens were scanned at a constant rate of 10 °C/min unless special illumination. The calculation methods of ΔHm were shown in Figure S1. For the modulated DSC (MDSC) protocol, the specimen was heated from 180 to 265 °C at a rate of 2 °C/min, and the total heat flow, reversible and nonreversible heat flow were recorded. The small-angle X-ray scatting (SAXS) experiments were performed using the NanoSTAR-U (Bruker AXS Inc.) with Cu Kα radiation, (wavelength, λ = 0.154 nm). The generator was operated at 40 kV and 650 μA. The distance from sample to detector was LSD = 1074 mm. The effective scattering vector q (q = 4π/λ sin θ, where 2θ was the scattering angle) at this distance was ranged from 0.044 to 2.0 nm−1.

decreased as the molecular weight increased; this was significantly different from the neat PLLA and PDLA specimens. The PLLA/PDLA had a Tsc of 249.9 °Cthe highest reported value until now.

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. L-Lactide (LLA) and D-lactide (D-LA), optical purity ≥99.5%, were kindly supplied by Changchun Sino Biomaterials Co., Ltd. (China), and they were recrystallized from dry ethyl acetate. Tin(II) 2ethylhexanoate (Sn(Oct)2, 95%) was purchased from Aldrich and was used directly as the catalyst. Linear PLLA and PDLA were obtained by ring-opening polymerization the L-LA and DLA, respectively, and all the polymerizations were initiated by dry isopropanol at 120 °C in anhydrous toluene.12 The molecular weights of polymers were controlled by the LA/ isopropanol weight ratio. PLLA and PDLA used in this study were listed in Table 1. Table 1. Characterization of PLLA and PDLA codea

Mn(GPC) (kg/mol)

PDI

Tm (°C)

ΔHm (J/g)

L4 L8 L15 L23 L32 L39 L50 L70 L100 D5 D7 D14 D22 D31 D42 D52 D67 D102

3.8 7.8 14.7 23.3 32.0 39.4 50.0 69.8 100.3 4.6 7.2 14.4 22.3 31.2 41.7 52.1 67.4 102.2

1.2 1.2 1.2 1.3 1.5 1.6 1.4 1.7 1.8 1.3 1.1 1.3 1.4 1.1 1.1 1.5 1.8 1.6

133.8 158.1 165.6 171.3 174.1 172.9 176.3 175.8 175.6 145.8 157.6 166.8 170.3 173.5 174.9 175.2 176.9 177.5

58.7 67.3 67.0 69.5 65.9 56.8 58.5 53.2 44.0 60.6 62.8 69.1 65.3 61.9 64.6 62.3 50.4 48.2

3. RESULTS AND DISCUSSION 3.1. Original Data of PLLA and PLLA/PDLA Specimens. Parts of the original WAXD profiles of PLLA were shown in Figure 1a. It was known that the crystal plane reflections located at 12.5, 14.9, 16.9, 19.3, 22.3° were separately assigned to the 103, 010, 200/110, 203, and 210 crystal planes of PLA α crystal.27,28 All these diffraction peaks vividly observed when the molecular weight was not higher than 15 kg/mol. As molecular weight increasing, the intensity of all diffraction peaks weakened gradually, and the location of these diffraction peaks tended to decrease. Only two major peaks at 16.7, 19.1° were detected in L70, indicating that the PLA α’ crystal (disordered α crystal) formed in the specimen. This variation suggested that the thermal stability of PLA homochiral crystallites formed in the cast films decreased as molecular weight increased, which would be due to the higher viscosity restricting the molecular chain rearrangement regularly. For the WAXD of the PLLA/PDLA blends (Figure 1b), as molecular weights increased from 4 to 50 kg/mol, all the diffractions assigned to PLA hc disappeared, and the diffraction peaks at 12.0, 21.0, 24.0° observed, which were ascribed to the reflection of 110, 300/030, and 220 planes of PLA sc, respectively.28 This result indicated that only PLA sc formed in the blends. These diffraction peaks weakened as molecular weight increasing. When the molecular weight of polymers were 70 kg/mol, the diffraction peaks at 16.9 and 19.1° as well as 12.0, 21.0, 24.0° presented, which implied that both the PLA hc and sc formed in the blend. Additionally, the diffraction peaks which belonged to PLA hc became more obvious, whereas the signals assigned to PLA sc weakened as the molecular weights further increased. The DSC curves of PLLA and PLLA/PDLA blends were shown in Figure 2. All the data were collected at the first heating, and and the higher one was recorded in Figure 2c if multimelting peaks were observed. From the DSC of PLLA specimens (Figure 2a), it was found that most of the PLLA samples presented a single melting peak. The melting

a

The PLLA is coded as L, and PDLA is abbreviated as D. The numbers after L or D are their number-average molecular weights.

The PLLA/PDLA blends were prepared according to previous studies.12,13 The PLLA and PDLA were completely dissolved in dichloromethane, separately, at a fixed concentration of 10 g/L. Then PLLA and PDLA solutions with similar molecular weights were mixed together under vigorous stirring for 3 h, and the weight ratio of PLLA/PDLA in all the blends was fixed at 1:1. Finally, the mixed solutions were casted onto Petri-dishes and followed by solvent evaporation at room temperature. The neat PLLA casted films were prepared with the same method and concentration. In order to prepare the PLLA/PDLA specimens without stirring, after the PLLA and PDLA solutions were mixed together, they were dispersed immediately under ultrasonic dispersion for 3 h at room temperature before solution evaporation. All the cast films were dried to constant weight at 50 °C prior to measurements. 2.2. Characterizations. The number-average molecular weights (Mn) and molecular weight distributions (PDI) of PLLA and PDLA were evaluated in chloroform at 35 °C, by using a Waters gel permeation chromatography (GPC) system with two styragel HR gel columns (HR2 and HR4). B

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Figure 1. WAXD of PLLA (a) and PLLA/PDLA specimens (b) with different molecular weights.

temperature of PLLA (Thm) increased monotonously as the molecular weight increased from 4 to 100 kg/mol. However, the melting enthalpy (ΔHhm) of PLLA increased before declining (see the Table S1). The DSC thermogram of PLLA/PDLA specimens unfolded in Figure 2b. Only one melting peak above 200 °C was detected when the molecular weights of polymers were not higher than 15 kg/mol, which should be attributed to the melting of PLA sc, and the Tsc increased steadily as molecular weight increased. When the molecular weights of PLLA and PDLA ranged from 23 to 50 kg/mol, multimelting behaviors observed around 230 °C, and the highest melting peak appeared at 248.9 °C in the L32/D31 sample. Two detached endothermic peaks presented after the molecular weights of PLAs were 70 kg/mol, and the intensity of the peak appeared at higher temperature reduced while the peak strength at lower temperature increased as molecular weights increased. Multimelting signals appeared at 230 °C when the molecular weights were between 23 and 50 kg/mol, this temperature was much higher than the equilibrium melting point (203 °C) of PLLA hc,29 and from the WAXD results, only PLA sc formed. Accordingly, the multimelting behavior should originate from the PLA sc. However, after the molecular weights of polymers were 70 kg/mol, besides the melting peaks around 220 °C, another lower melting peak was found around 175 °C, which was similar to the melting temperature of PLA hc. Combined with the WAXD results, the higher one should be assigned to the melting of PLA sc, and the lower one was ascribed to the melting of PLA hc.

Figure 2. DSC thermograms of PLLA (a), PLLA/PDLA (b), and the melting temperature of PLLA, PDLA (Thm), and PLLA/PDLA (Tsc) with different molecular weights (c). The Tsc versus molecular weight of PLLA was plotted (Figure 1c) and discussed in the whole study.

The effect of molecular weights on the Thm in the neat PLAs and Tsc in the PLLA/PDLA blends were shown in Figure 2c. As the molecular weight increased, both the melting temperature (Thm) of PLLA and PDLA specimens increased progressively. This regularities were observed in most of semicrystal polymers, including polyethylene,30 polypropylene,31 poly(ethylene oxide),32 and so forth. In the PLLA/PDLA blends, it was found that the Tsc increased first and then decreased as the molecular weight increased from 4 to 100 kg/mol, which was significantly different from the neat PLA specimens. C

DOI: 10.1021/ie504484b Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Because the CH3···OC interaction between enantiomeric PLA is the driving force for forming the PLA sc,28 this interaction only existed in the blends, not in the neat polymers. When the molecular weight was relatively low, the PLLA and PDLA polymers could move easily; if the interaction between PLLA and PDLA was sufficient, then only PLA sc formed. However, due to the lower molecular weight, the thickness of the lamellar was thin. Thus, the Tsc was lower. As molecular weight increased, the thickness of lamellar increased, and the viscosity was also elevated. However, the viscosity would restrict the mobility of the molecular chain and the stereocomplex formation became difficult. If the increments of viscosity did not retard the stereocomplex formation, the lamellar became thicker and Tsc increased as molecular weights increased, and only PLA sc formed. When the molecular weight further increased, the viscosity increased rapidly, which retarded the interaction between PLLA and PDLA molecular chains; therefore, the formation of racemic crystallites became difficult, then the thickness of lamellar of PLA sc reduced, and the Tsc declined. At the same time, the PLLA and PDLA homochiral molecular chain could aggregate by themselves and form PLA hc, respectively. Consequently, both the PLA hc and sc formed in the PLLA/PDLA blends with higher molecular weights. 3.2. Data Collected from the PLLA and PLLA/PDLA Specimens after Annealing. During the first heating, parts of the PLLA/PDLA specimens exhibited multimelting behaviors, and the melting temperatures and enthalpies could not be calculated exactly. Thus, all the PLLA and PLLA/PDLA specimens were first heated from room temperature to a fixed temperature, Tm − 10 °C, and then were annealed for 10 min. In the neat PLLA specimens, the Tm indicated the melting peaks in Figure 2a for each specimen, although it indicated the highest melting peak of PLA sc in the PLLA/PDLA blends, especially for the specimens which had multiple melting peaks. After cooling to room temperature at a rate of 10 °C/min, all these annealed specimens were analyzed in this section. The DSC of annealed PLLA and PLLA/PDLA specimens were presented at Figure 3, and the data calculated from DSC were collected in Tables S1 and S2, separately. During the heating, all the PLLA specimens showed a perfect melting peak, except the L4 (Figure 3a). Two melting peaks in the L4 specimen after annealing should be due to the melting of molecular chains with lower molecular weight in the specimen, and it formed crystallites during cooling and then was melted at lower temperature in the second heating. The variation of the Thm and ΔHhm with different molecular weights were similar to the first heating. However, the Thm ascended slightly and ΔHhm enhanced obviously after annealing. In the DSC of PLLA/ PDLA blends (Figure 3b), different from the original blends, most specimens exhibited single endothermic peaks between 190 and 260 °C. The influence of molecular weights on the Tsc and ΔHsc in the annealed PLLA/PDLA specimens were exhibited in Figure 3c. It was found that the Tsc and ΔHsc increased at first and then depressed as molecular weight increased from 4 to 100 kg/mol. This regularity was the same as the result shown in Figure 2c. However, multimelting behaviors were not observed obviously after annealing when the molecular weights of PLA polymers were ranged from 23 to 50 kg/mol. The Tsc observed around 249 °C when the molecular weights of polymers were between 32 and 50 kg/ mol, and the highest Tsc and ΔHsc, 249.9 °C and 124.7 J/g, appeared in the L32/D31 blend. The DSC of L32/D31 and L50/D52 annealed specimens were repeated and shown in

Figure 3. Second heating of PLLA (a) and PLLA/PDLA (b) with different molecular weights, and the Tsc and ΔHsc of PLA sc with different molecular weights (c).

Figure S2, and the results attested that these melting temperatures were easily repeated. According to the reported literature, the equilibrium melting temperature of PLA sc was 279 °C,29 and the 100% crystallinity of PLA sc was 142 J/g.33 The Tsc and ΔHsc in this investigation were the highest reported value until now although it was the value after annealing. The WAXD data of the annealed specimens were exhibited in Figure 4. For the PLLA samples (Figure 4a), all the D

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Figure 4. WAXD of PLLA (a) and PLLA/PDLA specimens (b) after annealing. Figure 5. SAXS of the PLLA (a) and PLLA/PDLA specimens (b) after annealing.

diffraction peaks assigned to the PLA α crystal were obviously detected, which implied that the stable crystallites formed in all the PLLA specimens after annealing. In the WAXD profile of PLLA/PDLA (Figure 4b), the diffraction peaks assigned to PLA sc vividly presented in all selected specimens, and all the diffraction peaks strengthened after annealing. However, when the molecular weight increased from 4 to 23 kg/mol, the diffraction peaks weakened, and these diffraction peaks strengthened when the molecular weight of polymers were between 32 to 50 kg/mol. Then, the diffraction intensity declined again as molecular weight further increased. The SAXS data of the annealed PLLA and PDLA specimens were presented in Figure 5. In the SAXS of PLLA, (Figure 5a), the q value decreased monotonously as the molecular weight of PLLA increased, which indicated that the lamellar thickness of PLLA hc increased continuously as molecular weight increased. However, in the case of PLLA/PDLA (Figure 5b), the q value increased first before it decreased as molecular weight increased, which suggested that the lamellar thickness of PLA sc increased then decreased. These regularities attested that the monotonous increase in the Thc of PLLA as molecular weight increased (as shown in Figure 3a) was due to the lamellar thickness increasing concurrent with molecular weights. However, in the PLLA/PDLA blends, the variation of the Tsc with molecular weights was ascribed to the fact that the lamellar thickness of PLA sc increased before it decreased.

4. STRUCTURE EVOLUTION OF PLA STEREOCOMPLEX DURING HEATING As interpreted from the WAXD profiles before and after annealing, no diffraction peaks varied in the PLLA/PDLA specimens, just the intensity varied, which suggested that the multimelting behaviors observing in some of PLLA/PDLA specimens originated from the same crystal structure (i.e., PLA sc). In order to investigate the structure evolution in the specimen, the DSC of the L32/D31 specimen with different heating rates was performed (Figure 6a). When the heating rate varied from 20 to 2 °C/min, all the curves showed multiple melting peaks, and the higher melting peak did not vary as scanning rates. When the specimen was heated at a rate of 20 °C/min, two melting peaks appeared. After the scanning rate was 5 or 2 °C/min, in addition to the two endothermic signals, another exothermal peak appeared, which implied that some structures changed during heating. The modulated DSC (MDSC) measurements of the L32/D31 were presented in Figure 6b. In the MDSC, the total heat flow in DSC signals would be separated into the nonreversible and reversible components, which enabled the visualization of the evolution of heat for the two processes. Two endothermic peaks were observed in the reversible heat flow, which indicated that two kinds of crystallites formed in the heating. An exothermic peak and an endothermic peak appeared in the nonreversible heat flow curve, implying that some structures formed at first, and E

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confine the interaction and movement of PLLA and PDLA chains. Therefore, the stereocomplex did not form adequately, and some disordered stereocomplex crystallites (or the thinner lamellar) formed in the later stage. During heating, the thinner lamellar crystal was melted at first (peak I in Figure 6b). However, it could reorganize into thicker lamellar under the existence of crystallites formed in the early stage (exothermic peak in Figure 6b). After the temperature further increased, all the crystallites were melted (peak II in Figure 6b). When the molecular weights were 70 kg/mol or higher, the viscosity became much higher and the entanglement between homochiral polymers became excessive, and the stereocomplex formation between PLLA and PDLA was restricted to a large extent even in the early stage. Thus, the PLA sc cannot form efficiently, and PLA hc formed. The above-mentioned PLLA/PDLA specimens were mixed in solution, followed by stirring for 3 h. Stirring for such a long time would form fibrous sc crystallites and which gave rise to the higher Tsc. In order to prepare the PLLA/PDLA specimens without stirring, the PLLA and PDLA solutions were mixed and then dispersed by ultrasonic dispersion, and the DSC thermograms of these casted films were shown in Figure 7.

Figure 6. DSC of L32/D31 specimen with different heating rates (a) and modulated DSC at a rate of 2 °C/min (b).

then were melted. The temperature of exothermic peak in the nonreversible heat flow was slightly higher than the first melting peak (peak I) in the reversible heat flow, which should indicate that some crystallites were melted at lower temperature and then reorganized into another crystallites rapidly (exothermic peak); all the crystallites were melted at higher temperature (peak II). However, the peak II was evidently larger than peak I in the reversible heat flow, which indicated that the heating transition signals at 249 °C incorporated into two parts (i.e., the existed crystallites formed in the solution mixing and the recrystallization parts). From the WAXD results (Figure 1 and Figure 4), it was found that no new crystallites formed before and after annealing in the PLLA/PDLA blends. Thus, it is rational to assign the peak I to the melting of the “imperfect” PLA sc, and the peak II was ascribed to the melting of the “perfect” PLA sc crystallites. The multimelting behaviors of PLA sc were observed when the molecular weights of PLLA and PDLA were around 23−50 kg/mol, which could be due to the specific viscosity in this molecular weight ranges. It was known that the molecular chains could diffuse easily, and therefore, the stereocomplex formed sufficiently in the PLLA/PDLA blends with low molecular weights. As molecular weights increased, the viscosity increased and chain entanglement became obvious, the interdiffusion of PLLA and PDLA would be inhibited. In the PLLA/PDLA mixed solutions with proper viscosity, the steroecomplex sufficiently formed and the perfect structure (or the thicker lamellar) formed in the early stage, but these formed stereocomplex crystallites and the viscosity would

Figure 7. DSC curves of PLLA/PDLA blends prepared by ultrasonic dispersion.

In the L15/D14 blend, the DSC curve was identical to the specimen prepared by stirring method. For the L32/D31 and L50/D52 specimens, the higher Tsc values were also found at 248.0 °C, and the ΔHsc values were also similar to the specimens prepared by the stirring method. This result strongly suggested that the unique higher Tsc in this study could not be ascribed to the formation of fibrous structure. In the previous reports, L-lactic acid with 98% optical purity and D-lactic acid with 97% optical purity were used, which resulted in the relative lower Tsc.9,22,34 In the reports which showed higher Tsc, the higher optical purities of L-LA and D-LA (≥99.5%) were used, and the PLLA and PDLA products were received with a higher optical purity. Accordingly, a higher ordered chiral sequential structure produced, which strengthened the stereocomplex capacity during blending; thus, thicker PLA sc lamellar crystallites formed, and higher Tsc was observed.

5. CONCLUSIONS In this study, the linear PLLA and PLLA/PDLA casted films with different molecular weights were prepared, and the thermal properties of neat PLLA and PLLA/PDLA blends were investigated. Results attested that the molecular weights F

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(11) Tsuji, H.; Bouapao, L. Stereocomplex formation between poly(L-lactic acid) and poly(D-lactic acid) with disproportionately low and high molecular weights from the melt. Polym. Int. 2012, 61, 442− 450. (12) Shao, J.; Sun, J.; Bian, X.; Cui, Y.; Li, G.; Chen, X. Investigation of poly(lactide) stereocomplexes: 3-armed poly(L-lactide) blended with linear and 3-armed enantiomers. J. Phys. Chem. B 2012, 116, 9983−9991. (13) Shao, J.; Sun, J.; Bian, X.; Cui, Y.; Zhou, Y.; Li, G.; Chen, X. Modified PLA homochiral crystallites facilitated by the confinement of PLA stereocomplexes. Macromolecules 2013, 46, 6963−6971. (14) Shao, J.; Sun, J.; Bian, X.; Zhou, Y.; Li, G.; Chen, X. The formation and transition behaviors of the mesophase in poly(Dlactide)/poly(L-lactide) blends with low molecular weights. CrystEngComm 2013, 15, 6469−6476. (15) Isono, T.; Kondo, Y.; Otsuka, I.; Nishiyama, Y.; Borsali, R.; Kakuchi, T.; Satoh, T. Synthesis and Stereocomplex formation of starshaped stereoblock polylactides consisting of poly(L-lactide) and poly(D-lactide) arms. Macromolecules 2013, 46, 8509−8518. (16) Kakuta, M.; Hirata, M.; Kimura, Y. Stereoblock polylactides as high performance bio-based polymers. Polym. Rev. 2009, 49, 107−140. (17) Fukushima, K.; Kimura, Y. A novel synthetic approach to stereoblock poly(lactic acid). Macromol. Symp. 2005, 224, 133−143. (18) Shao, J.; Tang, Z.; Sun, J.; Li, G.; Chen, X. Linear and fourarmed poly(l-lactide)-block-poly(d-lactide) copolymers and their stereocomplexation with poly(lactide)s. J. Polym. Sci. B: Polym. Phys. 2014, 52, 1560−1567. (19) Tsuji, H.; Okumura, A. Crystallization and hydrolytic/thermal degradation of a novel stereocomplexationable blend of poly(L-2hydroxybutyrate) and poly(D-2-hydroxybutyrate). Polym. J. 2011, 43, 317−324. (20) Tsuji, H.; Shimizu, K.; Sakamoto, Y.; Okumura, A. Heterostereocomplex formation of stereoblock copolymer of substituted and non-substituted poly(lactide)s. Polymer 2011, 52, 1318−1325. (21) Tsuji, H.; Hyon, S. H.; Ikada, Y. Stereocomplex formation between enantiomeric poly(lactic acid)s. 3. Calorimetric studies on blend films cast from dilute solution. Macromolecules 1991, 24, 5651− 5656. (22) Tsuji, H.; Hyon, S. H.; Ikada, Y. Stereocomplex formation between enantiomeric poly(lactic acid)s. 4. Differential scanning calorimetric studies on precipitates from mixed-solutions of poly(Dlactic acid) and poly(L-lactic acid). Macromolecules 1991, 24, 5657− 5662. (23) Sun, J.; Shao, J.; Huang, S.; Zhang, B.; Li, G.; Wang, X.; Chen, X. Thermostimulated crystallization of polylactide stereocomplex. Mater. Lett. 2012, 89, 169−171. (24) Brzeziński, M.; Bogusławska, M.; Ilčíková, M.; Mosnácě k, J.; Biela, T. Unusual thermal properties of polylactides and polylactide stereocomplexes containing polylactide-functionalized multi-walled carbon nanotubes. Macromolecules 2012, 45, 8714−8721. (25) Xiong, Z. J.; Liu, G. M.; Zhang, X. Q.; Wen, T.; de Vos, S.; Joziasse, C.; Wang, D. J. Temperature dependence of crystalline transition of highly-oriented poly (L-lactide)/poly(D-lactide) blend: In-situ synchrotron X-ray scattering study. Polymer 2013, 54, 964− 971. (26) Sakamoto, Y.; Tsuji, H. Stereocomplex crystallization behavior and physical properties of linear 1-arm, 2-arm, and branched 4-arm poly(L-lactide)/poly(D-lactide) blends: effects of chain directional change and branching. Macromol. Chem. Phys. 2013, 214, 776−786. (27) Zhang, J. M.; Duan, Y. X.; Sato, H.; Tsuji, H.; Noda, I.; Yan, S.; Ozaki, Y. Crystal modifications and thermal behavior of poly(L-lactic acid) revealed by infrared spectroscopy. Macromolecules 2005, 38, 8012−8021. (28) Brizzolara, D.; Cantow, H. J.; Diederichs, K.; Keller, E.; Domb, A. J. Mechanism of the stereocomplex formation between enantiomeric poly(lactide)s. Macromolecules 1996, 29, 191−197. (29) Tsuji, H.; Ikada, Y. Crystallization from the melt of poly(lactide)s with different optical purities and their blends. Macromol. Chem. Phys. 1996, 197, 3483−3499.

did not vary the crystal structures of PLA hc and sc, and the PLA sc preferentially formed in all the blends. The Thc in the PLLA and PDLA increased monotonously as molecular weight increased. However, the Tsc in the PLLA/PDLA blends increased at first and then decreased as the molecular weight increased from 4 to 100 kg/mol. When the molecular weights of PLLA and PDLA were between 23 and 50 kg/mol, the multimelting behaviors were observed around 230 °C. After annealing, the highest Tsc and ΔHsc, 249.9 °C and 124.7 J/g, were observed in the L32/D31 specimen, which were the highest values until now. The WAXD and SAXS results indicated that the thickness of crystal lamellar was the external reason, and the higher optical purity was the essential issue that resulted in this superior thermal property. This report will expand the knowledge of PLA sc, and the superior thermal properties will provide more application potential for PLA sc material.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from National Natural Science Foundation of China (Projects 51073154, 51273198, 51373169, 51033003, 51303176, 51321062, and 51403089) and 863 Program (2011AA02A202) from the Ministry of Science and Technology of China.

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