PDLA Blends - The

Mar 9, 2011 - Temperature-dependent XRD results indicated that even at 240 °C the ... (5-9) Ikada et al. first reported that the 1/1 blend of PLLA an...
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Crystallization Behavior of Asymmetric PLLA/PDLA Blends Jingru Sun,† Haiyang Yu,† Xiuli Zhuang,† Xuesi Chen,*,† and Xiabin Jing‡ †

Key Laboratory of Polymer Ecomaterials, ‡State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ABSTRACT: The effects of the addition of poly(D-lactide) (PDLA) on the crystallization behavior of poly(L-lactide)(PLLA) were investigated by means of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction(XRD). When the blends were cooled from different temperatures (250, 240, and 190 °C) at the rate of cooling of 5 °C/min, stereocomplex (sc) crystallites could stay at diverse states. Accordingly, the stereocomplexes acted as a nucleation agent exerting distinct effects on PLLA crystallization. The speculated mechanisms of the stereocomplex formation and the effectiveness as a nucleating agent are schematically described. Moreover, temperature-dependent XRD was carried out to further investigate the melt-crystallization behavior of PLLA/PDLA blends in real time. Temperature-dependent XRD results indicated that even at 240 °C the stereocomplex crystallites in all blend samples existed clearly, which could not be detected by DSC. These XRD results further suggest that the onset Tc values for the PLLA R-form crystals formation were 160, 120, 140, and 160 °C, respectively, for neat PLLA, PLLA/PDLA 95/5, 90/10, and 80/20 as well as 70/30 samples.

1. INTRODUCTION Poly(L-lactide) (PLLA) is a biocompatible and biodegradable polyester derived from renewable resources.1,2 The application area of PLLA has been limited until now because of its low crystallinity and low crystallization rate. Therefore, great efforts have been made to improve these properties of PLLA.3,4 Among these efforts, stereocomplexation between PLLA and PDLA is one of the most effective and promising methods for increasing the thermal stability of poly(lactic acid) (PLA)-based materials.5-9 Ikada et al. first reported that the 1/1 blend of PLLA and PDLA produced a stereocomplex whose crystal structure is different from that of PLLA.10 This stereocomplextype poly(lactic acid) (sc-PLA) showed its Tm at 230 °C, which is 50 °C higher than that of pure PLLA or PDLA, so that sc-PLA should accordingly have better thermal and mechanical properties than PLLA. Since this first report, the influences of the homopolymer molecular weight,11-13 blending ratio,11-15 blending condition,11-14 and optical purity11,15 on the formation and properties of stereocomplexes have been well investigated. Moreover, the crystal morphology and growth kinetics have also been widely studied.16-19 Recently, there has been growing attention to the crystallization behaviors of asymmetric blends including both homopolymer and stereocomplexes. Brochu et al. reported the crystallization behaviors of asymmetric PLLA/PDLA blends and proved that the stereocomplexation occurred with as little as 10 wt % PDLA.11 Schmidt et al. investigated the crystallization behavior in asymmetric blends of PLLA and low molecular weight PDLA.20 Yamane studied the thermal property and crystallization behavior of PLLA blended with a small amount of PDLA (1-5 wt %) and demonstrated that the stereocomplex crystallites acted as nucleation sites for PLLA and enhanced the crystallization of PLLA significantly r 2011 American Chemical Society

when the blend was cooled to a temperature below Tm of PLLA.21 Anderson et al. evaluated melt preparation and nucleation efficiency of polylactide stereocomplex crystallites. In this case, with only 3 wt % PDLA (Mn =14 kg/mol) in the mixture, a nucleation efficiency of nearly 100% was obtained.22 The stereocomplex crystallites were much more effective in enhancing the crystallization rate of PLLA compared with a common nucleating agent, talc.23 Tsuji observed the effects of incorporated PDLA as stereocomplex crystallites on the isothermal and nonisothermal crystallization behaviors of PLLA from the melt for a wide range of PDLA contents from 0.1 to 10 wt %.24 Their study revealed that addition of small amounts of PDLA is effective to accelerate overall PLLA crystallization when the PDLA content and crystallization conditions are scrupulously selected. However, to our knowledge, the crystallization behaviors of asymmetric PLLA/PDLA blends depending on the initial cooling states have not been explored in details. No research has been carried out on the correlation between the crystallization of stereocomplexes and the PLLA homopolymer, and the effects of stereocomplexation on the crystallization of PLLA by temperature-dependent X-ray diffraction (XRD) in real time. In the present work, we report enhanced stereocomplex nucleator formation by cooling the PLLA/PDLA blends from different temperatures and discuss its effect on the crystallization behavior of both PLLA and the stereocomplexes. The improved processability of PLA with a stereocomplex nucleator may lead to fast expansion of the commodity applications of the PLA products. Received: December 15, 2010 Revised: January 28, 2011 Published: March 09, 2011 2864

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Figure 1. DSC thermograms of neat PLLA and PLLA/PDLA blends during the first heating run from room temperature to 250 °C (a) and during the cooling run from 250 °C to room temperature, (b). The heating and cooling rates were both 5 °C/min.

2. EXPERIMENTAL SECTION L-Lactic acid (L-LA) and D-lactic acid (D-LA) were purchased from Aldrich. PLLA (Mw = 86 kg/mol, Mn = 68 kg/mol) and PDLA (Mw = 78 kg/mol, Mn = 61 kg/mol) were synthesized by the ring-opening polymerization of L-LA and D-LA, respectively.25 PLLA and PDLA were dissolved in chloroform separately and were solution-blended at different PLLA/PDLA ratios, coded as PLLA/PDLA x/y, where x and y represent the weight percentages of PLLA and PDLA, respectively. Thermal properties of the blends were examined by differential scanning calorimetry (DSC Q100, TA Instruments, USA) under N2 atmosphere. DSC scans were carried out in the cooling processes from 250, 240, and 190 °C to the room temperature after holding at that temperature for 2 min. Cooling rates were set to be 5 °C/min. To investigate the crystalline structure of the blend samples, the temperature-dependent X-ray diffraction measurement was carried out on a Bruker D8 Advance X-ray diffractometer by using Cu KR radiation in the scattering angle range of 2θ = 10-30° at a scan speed of 4°/min. The temperature was decreased at a rate of ca. 10 °C/min and XRD data were recorded at every 20 °C ranging from different temperature. Before each XRD measurement, the sample was maintained at that temperature for 5 min to make the sample equilibrated. The XRD patterns of the blend samples in the cooling processes were recorded in real time. 3. RESULTS AND DISCUSSION 3.1. DSC Measurements. Polylactides show a low melting temperature peak at about 180 °C, corresponding to the fusion of homochiral crystals (hc) of optically pure PLLA or PDLA type samples, and a higher melting temperature peak at about 230 °C, corresponding to the stereocomplex crystal (sc) fusion in PLLA/ PDLA blends of the two optically pure enantiomers of polylactide.14 In this paper, PLLA and PDLA polymer blends were likely to form both hc crystals and sc crystals. Figure1a shows the first heating DSC thermograms of pure PLLA and PLLA-rich blends (PLLA/PDLA = 100/0, 95/5, 90/10, 80/20, and 70/30 (wt %)). It was found that neat PLLA showed a single endothermic peak at 179 °C while the blends showed two broad

endothermic peaks (Tm) centered at 178 and 225 °C due to the simultaneous crystallization of the hc and sc crystals, respectively. The fact that the Tm positions do not shift with sample composition indicates that the PLLA/PDLA stereocomplex and the PLLA homopolymer crystallize almost independently. During the cooling runs of DSC measurements from 250 °C to room temperature (Figure1b), neat PLLA gave a single crystallization exotherm (Tc) around 106 °C, while three of the four blends exhibited a Tc at 100 °C and the Tc of PLLA70/PDLA30 was 122 °C. The Tc of PLLA70/PDLA30 was higher than other three blends mainly due to the higher content of PDLA, resulting in a significant concentration of the stereocomplex nucleating agents and subsequently higher sc crystallinity. By virtue of these sc nuclei, the difference of sample PLLA70/ PDLA30 from the other three may explained. This explanation is supported by the experimental observations in the next sections of XRD data. Influences of the PDLA chains on crystallization are thought to be 2-fold. First, the PDLA chains would represent obstacles to diffusion of the PLLA chains, and many of the blends showed a lower Tc than for neat PLLA, consistent with this diffusional limitation. Second, the PDLA promoted sc crystallization, and this effect was dominant in the case of PLLA70/ PDLA30 where the Tc increased. Because the Tm of sc crystals is near 225 °C (Figure 1a), the above DSC experiments were repeated with top temperatures of 240 and 190 °C, respectively. The different fractions of sc crystals were left in the melt at these temperatures and these residual sc crystals would play a role of nucleating agent for the sc and/or hc crystallization. As shown in Figure 2a, when the sample was cooled down from 240 °C, instead of 250 °C, all blend samples displayed obviously different crystallization behaviors; they showed up crystallization peaks near 180 °C and the intensities of the peaks depended on the PDLA contents in the samples. For special PLLA70/PDLA30, the 180 °C peak is stronger than that of the peak near 140 °C. Obviously, higher Tc peak is assigned to sc crystallization, and therefore, its appearance can be ascribed to the favorable influence of the residual sc crystals. Furthermore, these residual sc crystals promoted the crystallization of PLLA so that the Tc of PLLA shifted from 106 °C for neat PLLA to ca. 140 °C for PLLA70/PDLA30 (Figure 2a). 2865

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Figure 2. DSC thermograms of neat PLLA and PLLA/PDLA blends during cooling runs from a top temperature to room temperature at a cooling rate of 5 °C/min. The top temperatures were (a) 240 °C and (b) 190 °C, respectively.

Figure 3. Speculated crystallization mechanism of PLLA/PDLA blends during cooling runs from a top temperature to room temperature. The top temperatures were (a) 250 °C, (b) 240 °C, and (c) 190 °C, respectively (left, low PDLA content; right, high PDLA content).

When the melt was cooled from 190 °C, because this temperature was lower than the Tm of sc crystals, all sc crystals remained in the PLLA melt. They promoted the crystallization of PLLA as a nucleating agent so that the Tc of PLLA shifted from 125 °C for neat PLLA to 145 °C for PLLA90/PDLA10 and PLLA80/PDLA20. It is noticed that PLLA70/PDLA30 only showed a relatively weak peak near 138 °C, instead of 145 °C.

This is because PLLA/PDLA sc crystals took quite a fraction in the melt at 190 °C and neat PLLA chains were crystallizing under a constraint condition formed by PLLA/PDLA sc crystals. It is interesting to notice a common feature between parts a and b of Figure 2 that the Tc peak of PLLA90/PDLA10 is the strongest of the blend samples, indicating that as far as sc and hc crystallization is concerned, PLLA90/PDLA10 is the best for the practical application. Figure 3 describes the speculated mechanisms of the stereocomplex formation and the effectiveness as a nucleating agent schematically. When the blends were cooled from 250 °C, for PLLA 90 /PDLA 10 (low PDLA content), PDLA molecules are well dispersed in PLLA matrix and form small and imperfect stereocomplex. So the stereocomplex and neat PLLA crystallize simultaneously in the cooling process. At higher PDLA contents (for PLLA 70 /PDLA 30 ), the blends contain a lot of imperfect stereocomplex crystallites, resulting in a significant concentration of the stereocomplex nucleating agents and subsequently higher T c of blends. When the blends were cooled from 240 °C, for PLLA70/ PDLA30, these residual sc crystals would act as homogeneous nucleation sites for sc itself as well as heterogeneous nucleation sites for PLLA crystallization, or PLLA homocrystallites were formed epitaxially on the stereocomplex crystallites. At lower PDLA contents, such a tendency became obscure due to a small quantity of residual sc crystals. Stereocomplex crystallites stayed unmelted at 190 °C and embedded in the PLLA molten matrix. For PLLA90/PDLA10 (low PDLA content), quite a few stereocomplex crystallites act as a nucleation site of PLLA and enhance the crystallization of PLLA significantly (heterogeneous nucleation, higher Tc 145 °C). Such enhancement of the crystallization was distinct from the blend with higher PDLA content (for PLLA70/ PDLA30). This is mainly because a lot of stereocomplex nucleation sites disturbed the mobility of PLLA chains, resulting in lower Tc compared with that of PLLA90/PDLA10 blends. Above all, when the blend was cooled from different temperatures, stereocomplex crystallites could stay at diverse states. 2866

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Figure 4. In situ X-ray diffraction profiles of neat PLLA and PLLA/PDLA blends during cooling from a top temperature to 80 °C at an interval of 20 °C: (a) neat PLLA, (b) PLLA95/PDLA5, (c) PLLA90/PDLA10, (d) PLLA80/PDLA20, and (e) PLLA70/PDLA30.

Accordingly, the stereocomplexes exerted distinct effects on PLLA crystallization such as acting as a nucleation agent. Specially, crystallization enhancement was observed when the blend with lower PDLA content was cooled from 190 °C at which stereocomplex was unmelted and acted as the effective

nucleation agent. But such crystallization enhancement was indistinctive at which both PLLA crystal and the stereocomplex disappeared completely, for example when the blends cooled from 250 °C. For PLLA/PDLA blends which cooled from 240 °C, the residual sc crystals promoted the crystallization of 2867

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The Journal of Physical Chemistry B both themselves and PLLA homopolymers, which implies the importance of higher order structure of sc for the following crystallization of PLLA. 3.2. XRD Measurements. In order to better understand the PLLA/PDLA sc crystal formation and its influence on the crystallization of PLLA in the blends, the real-time XRD was performed when the samples were cooled from their melt temperatures. The XRD profiles are collected in Figure 4. The neat PLLA was examined from 200 to 80 °C. As shown in Figure 4a, PLLA was in its amorphous and molten state at 200 °C. At 160 °C, a weak crystalline diffraction peak appeared at 2θ = 16°. At 140 °C, the 16° peak became higher and two more peaks appeared at 2θ = 18.4 and 21.8°. These peaks grew with decreasing temperature from 140 to 100 °C. These peaks have been assigned to the (200) and/or (110) planes, (203), and (006) planes of PLLA crystals in R-phase (pseudoorthorhombic, a = 1.06 nm, c = 0.61 nm, c = 2.88 nm).26 Therefore, Figure 4a indicates that PLLA crystals were initially formed at 160 °C and grew from 160 to 100 °C under the experimental condition. The blend samples were measured from 260 or 280 °C, as shown in Figure 4b-e. As can be seen from Figure 3b, the XRD profiles of PLLA95/PDLA5 measured at 260 °C clearly reflect that both stereocomplexes and PLLA were in the amorphous state. As temperature decreased to 240 °C, the corresponding XRD pattern exhibited three main diffraction peaks at 2θ values of 11.6, 20.6, and 23.5°, which were characteristic peaks of the PLA stereocomplex crystallized in a trigonal unit cell of dimensions: a = b = 1.498 nm, c = 0.870 nm, R = β = 90°, and γ = 120°.16 On decreasing the temperature to 120 °C, the crystallization of PLLA homopolymer occurred. In the whole cooling process, a strong [110] reflection originating from the stereocomplex (2θ = 11.6°) was observed. A similar situation emerged in the case of PLLA90/PDLA10 while the latter had a higher crystallization temperature of the PLLA constituent (140 °C). For PLLA80/PDLA20 and PLLA70/PDLA30 samples, even at 240 °C, the stereocomplex crystals presented clearly. With decreasing temperature from 240 to 180 °C, only the reflections of [200], [110] and [006] appeared due to the sc crystallization. From 160 °C, the PLLA chain crystallization in the blends started. It must be noted that, for PLLA80/PDLA20 and PLLA70/PDLA30, the intensity of the peak around 2θ = 11.6° increased first and then decreased in the cooling process. Almost at 160 °C, the intensity of the peak achieved maximum, indicating that the crystallinity of the PLA stereocomplex reached the greatest extent. At the same time, the PLLA homopolymer began to crystallize. With the temperature decreasing, the intensity of the stereocomplex diffraction peak decreased, while that of the PLLA diffraction peak increased gradually. This phenomenon demonstrated that the increase of PLLA crystallization disturbed the sc crystallization because of the crystallization competition between the PLLA chains and stereocomplex chains. We may propose the mechanism as follows. In PLLA/PDLA blends, the sc structure develops as the sc crystallinity increases. This structure was slightly disturbed as the crystal growth of PLLA during cooling. These results were reasonably explained by the model where sc crystallites are not isolated in PLLA melt but connected like a physical gel.27 These XRD results further suggest that the onset Tc values for the PLLA R-form crystals formation were 160, 120, 140, and 160 °C, respectively, for neat PLLA, PLLA95/PDLA5, PLLA90/ PDLA10, PLLA80/PDLA20, and PLLA70/PDLA30 samples, which agrees well with the DSC data.

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4. CONCLUSION Both the DSC and XRD results suggested that the crystallization of PLLA homochiral crystals (hc) was affected by the blending with PDLA and the formation of the PLA stereocomplex crystals (sc) was favored in the blend systems. The crystallization behaviors of the PLLA/PDLA blends depended on their initial cooling states. When the blends were cooled from different temperatures, stereocomplex crystallites could stay at the different melting status. Accordingly, the stereocomplex crystals acted as a nucleation agent exerting distinct effects on PLLA crystallization. Compared with those cases cooled from other temperatures, the enhancement effects on the crystallization of PLLA were more significant when they were cooled from 190 °C (higher Tc, 145 °C for PLLA90/PDLA10). Temperature-dependent XRD results indicated that, even at 240 °C, the stereocomplex crystallites in all blend samples existed clearly, which could not be detected by DSC. The onset Tc values for the PLLA R-form crystals formation were changeable for neat PLLA as well as PLLA/PDLA blend samples. The XRD results reveal the completion and restriction of the PLLA homopolymer and stereocomplexes in real time. There was a competition balance that existed between the hc crystallization of PLLA chains and sc crystallization of PLLA/PDLA chains. ’ AUTHOR INFORMATION Corresponding Author

*Tel: þ86-431-85262112. Fax: þ86-431-85262112. E-mail: [email protected].

’ ACKNOWLEDGMENT This project was financially supported by the National Natural Science Foundation of China (Project No. 20974109 and 51073155, Key project No. 50733003), Support Project (2007BAE42B02) from the Ministry of Science and Technology of China, The Knowledge Innovation Project of Chinese Academy of Sciences (KGCX-YW-208), and Scientific and Technological Developing Project of Jilin Province (20095003, 20100107). ’ REFERENCES (1) Lim, L. T.; Auras, R.; Rubino, M. Prog. Polym. Sci. 2008, 33, 820. (2) Nair, L. S.; Laurencin, C. T. Prog. Polym. Sci. 2007, 32, 762. (3) Gan, Z. H.; Kuwabara, K.; Abe, H.; Iwata, T.; Doi, Y. Polym. Degrad. Stab. 2005, 87, 191. (4) Corradini, P.; Guerra, G. Adv. Polym. Sci. 1992, 100, 183. (5) Hirata, M.; Kimura, Y. Polymer 2008, 49, 2656. (6) He, Y.; Xu, Y.; Wei, J.; Fan, Z. Y.; Li, S. M. Polymer 2008, 49, 5670. (7) Zhang, J. M.; Tashiro, K.; Tsuji, H.; Domb, A. J. Macromolecules 2007, 40, 1049. (8) Xu, H.; Teng, C. Q.; Yu, M. H. Polymer 2006, 47, 3922. (9) Shirahama, H.; Ichimaru, A.; Tsutsumi, C.; Nakayama, Y.; Yasuda, H. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 438. (10) Ikada, Y.; Jamshidi, K.; Tsuji, H.; Hyon, S. H. Macromolecules 1987, 20, 904. (11) Brochu, S.; Prudhomme, R. E.; Barakat, I.; Jerome, R. Macromolecules 1995, 28, 5230. (12) Tsuji, H.; Horii, F.; Hyon, S. H.; Ikada, Y. Macromolecules 1991, 24, 2719. (13) Tsuji, H.; Hyon, S. H.; Ikada, Y. Macromolecules 1992, 25, 2940. (14) Tsuji, H.; Ikada, Y. Macromolecules 1993, 26, 6918. 2868

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