Stereocomplex Formation in Enantiomeric Polylactides by Melting

Dec 20, 2013 - Crystallization kinetics and crystal morphology of polylactide stereocomplex through melting recrystallization of homocrystals was inve...
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Stereocomplex Formation in Enantiomeric Polylactides by Melting Recrystallization of Homocrystals: Crystallization Kinetics and Crystal Morphology Bing Na,* Jie Zhu, Ruihua Lv, Yunhui Ju, Renping Tian, and Bibo Chen Fundamental Science on Radioactive Geology and Exploration Technology Laboratory, School of Biology, Chemistry and Materials Science, East China Institute of Technology, Nanchang, 330013, People’s Republic of China ABSTRACT: Crystallization kinetics and crystal morphology of polylactide stereocomplex through melting recrystallization of homocrystals was investigated in detail. Small homocrystals with the α′-form and large ones with the α-form were produced by annealing of amorphous 1:1 poly(L-lactide)/poly(D-lactide) blends at 80 and 120 °C, respectively. Small homocrystals with the α′-form were more favorable than large ones with the α-form for stereocomplex formation through melting recrystallization. Moreover, rod-like stereocomplex crystals were produced from melting of small homocrystals with the α′-form at the adopted crystallization temperatures. In contrast, plate-like or spherulitic stereocomplex crystals were generated by melting of large homocrystals with the α-form. The difference in the crystal morphology of sterecomplex was correlated with the variation of nucleation density regarding annealing and crystallization temperatures.

instance repeat casting,13 supercritical fluid technology,14 lowtemperature mixing,15 and so on. Besides, annealing of PLLA/PDLA blends at elevated temperatures is effective to generate stereocomplex due to melting recrystallization of homocrystals.16−18 It arises from the difference in the melting point between homocrystals and stereocomplex. This phase transition process has been well demonstrated by X-ray techniques and Fourier transform infrared spectroscopy.17,18 To date, crystallization kinetics and crystal morphology of stereocomplex upon melting recrystallization of homocrystals is less concerned yet. What is more, it is unclear that the effect of crystal form (α′ and α) and/or the size of homocrystals on the stereocomplex formation. This study clearly demonstrates that stereocomplex formation from small homocrystals with the α′-form is easer and rapider than that from large homocrystals with the α-form. And, rod-like stereocomplex crystals are produced from melting recrystallization of small homocrystals with the α′-form due to high nucleation density.

1. INTRODUCTION As a kind of biodegradable and biocompatible material, polylactide has been attracting much interest in the fields of polymer and biomedical research.1−5 The presence of a chiral carbon in the skeletal chain of polylactide yields two stereoregular enantiomers, namely poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA). The homocrystals formed by either PLLA or PDLA have a melting point of 160−180 °C, depending on the molecular weight and optical purity.6−8 However, blending of PLLA and PDLA can result in the formation of stereocomplex with a melting point of about 50 °C higher than that of homocrystals.9,10 The extremely high melting point of stereocomplex is originated from the strong interactions, i.e., hydrogen bonding in the unit cell.11,12 Thus, stereocomplexation opens a new way to enhance the properties such as thermal resistance, which benefits broader applications of polylactides. Stereocomplexation is competed with the formation of homocrystals, which is usually affected by the composition, molecular weight and preparation methods.10 It has been well demonstrated that stereocomplex formation is preferential in the 1:1 PLLA/PDLA blends, and apart from this composition homocrystals from either PLLA or PDLA are induced. Molecular weight is another factor determining stereocomplex formation. There exists a critical molecular weight of about 105 g/mol, above which sterecomplexation is significantly suppressed. From a practical view, however, high molecular weight is a prerequisite for superior mechanical performances of polylactides. Therefore, in the past, extensive efforts have been devoted to achieve high stereocomplexation from polylactides with high molecular weight by various preparation methods, for © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. PLLA, purchased from Natureworks, USA, had a Mn and Mw of 123 and 210 kg/mol, respectively. PDLA, having a Mn and Mw of 191 and 212 kg/mol, respectively, was provided by Changchun Sinobiomaterials Co., Ltd., China. Weighted PLLA and PDLA were dissolved in chloroform at room temperature to generate a transparent solution with a concentration of 0.1 g/mL; the mass ratio of PLLA and PDLA was Received: November 22, 2013 Revised: December 13, 2013

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fixed at 1:1. After then, the solution was slowly poured into a large amount of methanol under vigorous stirring to precipitate the PLLA/ PDLA blends. The obtained sediments were washed several times by methanol, followed by drying under vacuum at 40 °C overnight. Films were prepared by hot-pressing of the sediments at 240 °C and then quenching into ice water. Annealing of films was carried out in a hot stage at 80 and 120 °C for 5 and 1 h, respectively. 2.2. Characterizations. Differential scanning calorimetry (DSC) measurements were conducted at a heating rate of 10 °C/min in a nitrogen atmosphere using a Perkin-Elmer Pyris-1 DSC instrument. Fourier transform infrared spectra were measured by a Thermo Nicolet FTIR spectrometer with a resolution of 4 cm−1. To obtain in situ structural change at elevated temperatures, a hot stage was coupled with the FTIR spectrometer in its sample compartment. Samples were rapidly heated to 190 and 200 °C, respectively, and then held isothermally until the completion of crystallization. X-ray diffraction (XRD) measurements were conducted on the diffraction workstation in the Beijing Synchrotron Radiation Facility; the wavelength of the X-ray was 0.154 nm. The crystal morphology was disclosed by a small angle light scattering (SALS) setup with laser wavelength of 532 nm in the Hv mode and a polarized optical microscope (POM) under cross-polarization conditions, respectively.

dependence on the annealing temperatures. At 80 °C homocrystals with the α′-form is generated in the blends, indicated by the characteristic diffractions at 2θ of 16.4 and 18.7°, respectively.19 While being annealed at 120 °C, homocrystals still prevail in the blends but the crystal form is the α-form. It is confirmed by the diffractions at 2θ of 12.5, 15.0, 16.7, 19.1, and 22.3°, respectively.19 Note that the crystallinity, deduced from the XRD profiles, is 0.39 and 0.47 for the blends annealed at 80 and 120 °C, respectively. The above situation is same to that observed in the individual PLLA with respect to crystallization temperatures. There exists a temperature range for the formation of the α′- and α-form in the individual PLLA.20 At 120 °C and above, the α-form with dense 103 helical chain packing is induced. In contrast, loose 103 helical chain packing results in the α′-form due to limited molecular mobility while crystallization temperature is below 120 °C. Of note, the α′- and α-form have the same absorption band at 922 cm−1 in the FTIR spectra (Figure 1b).18,19 It suggests that PLLA and PDLA in the blends crystallize separately into homocrystals and the crystallization habit is same to that of individual PLLA. In other words, at low annealing temperatures there are little mutual chain interactions between PLLA and PDLA in the blends as regarding crystallization. It, in turn, is responsible for the absence of stereocomplex formation in the 1:1 PLLA/PDLA blends at low annealing temperatures due to limited molecular mobility. In such a sense, stereocomplex could be produced at higher annealing temperature where enough molecular mobility is gained. It is the exact fact observed in the PLLA/PDLA blends annealed at 160 °C for 1h. Stereocomplex manifests itself by the characteristic diffractions at 2θ of 12, 20.8, and 24.4° in the XRD profile9−18 (Figure 1a) and the characteristic absorption band at 908 cm−1 in the FTIR spectra18−22 (Figure 1b), respectively. At the same time, homocrystals with the α-form is also induced at this annealing temperature due to separated crystallization of PLLA and PDLA in the blends. Since our attention is focused on the stereocomplex formation by melting recrystallizaiton of homocrystals, the blends annealed at 160 °C will not be taken into account in the following section because of the partial presence of stereocomplex. In addition to crystal form, crystal morphology is also affected by annealing temperatures. Figure 2 gives the POM micrographs and SALS patterns of the PLLA/PDLA blends annealed at 80 and 120 °C, respectively. Spherulites are produced in both blends, judged from the four-leaf SALS patterns with apparent scattering peaks along scattering angles. Annealing at 80 °C produces smaller spherulites than that at 120 °C, as a result of higher supercoolings and nucleation density. That is, formation of tiny spherulites at 80 °C only involves local regulation of molecular chains, whereas molecular diffusion in a relatively broad range, due to high molecular mobility, prevails at 120 °C to generate large spherulites. Figure 3 presents DSC traces of the PLLA/PDLA blends annealed at 80 and 120 °C, respectively. The melting peaks in the temperature ranges between 160 and 180 °C correspond to the melting of homocrystals. In the blends annealed at 80 °C there exists an exothermic peak that corresponds to the α′ → α transition of homocrystals before dominant melting. It arises from the solid−solid reorganization of the α′-form with loose chain packing at elevated temperatures; this phase transition has been well disclosed by FTIR and X-ray measurements in the past.7,19,20 In contrast, the α-form in the blends annealed at

3. RESULTS AND DISCUSSION Upon quenching into ice water from melt, the PLLA/PDLA blends cannot crystallize and remain amorphous due to rapid cooling as well as high molecular weight. It is demonstrated by no diffractions from crystals in the XRD profile and no absorbance in the FTIR spectra ranged between 940 and 900 cm−1, as shown in Figure 1. After being annealed above glass transition temperature (∼60 °C), crystals are induced in the PLLA/PDLA blends as a result of cold crystallization. The crystal form of the PLLA/PDLA blends shows a significant

Figure 1. XRD profiles (a) and FTIR spectra (b) of the PLLA/PDLA blends annealed at the indicated temperatures. For comparison the ones of as-quenched samples without annealing are also included. The assignments of homocrystals (α′- and α-form) and stereocomplex (sc) are labeled in the legends. B

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80 °C during heating runs at 10 °C/min while its earlier stereocomplex formation is taken into account. The DSC results strongly suggest that small homocrystals with the α′form is more favorable for stereocomplex formation than large ones with the α-form through melting recrystallization. To confirm this, isothermal crystallization of the blends annealed at 80 and 120 °C was conducted by in situ FTIR measurements at 190 and 200 °C, respectively. Figure 4 shows

Figure 2. POM micrographs (a, b) and SALS patterns (c, d) of the PLLA/PDLA blends annealed at (a, c) 80 and (b, d) 120 °C, respectively.

Figure 4. Time-dependent FTIR spectra during isothermal crystallization at 200 °C of the PLLA/PDLA blends annealed at (a) 80 and (b) 120 °C, respectively.

Figure 3. DSC traces of the PLLA/PDLA blends annealed at 80 and 120 °C, respectively.

the examples of time dependent FTIR spectra recorded during isothermal crystallization at 200 °C. Once temperature reaches 200 °C, homocrystals are melted completely in both blends, indicated by the absence of the absorption band at 922 cm−1. Instead, stereocomplex formation with the characteristic absorption band at 908 cm−1 shows up with the elapse of time. It is further verified by the XRD profiles shown in Figure 5, where stereocomplex prevails with the absence of

120 °C undergoes direct melting without solid−solid reorganization due to its dense chain packing in the unit cell. On the other hand, further heating results in the appearance of an endothermic peak in the temperature ranges between 190 and 220 °C. It arises from the melting of stereocomplex with a high melting point; stereocomplex has a melting temperature higher about 50 °C than that of homocrystals due to side-byside dense molecular packing between PLLA and PDLA.21−23 Recalling that there is no stereocomplex in the blends annealed at 80 and 120 °C (see Figure 1), this stereocomplex must be induced from the melting recrystallization of homocrystals.17,18 Moreover, stereocomplex formation upon heating depends remarkably on the annealing temperatures. On the basis of the onset melting temperature of stereocomplex, as indicated by the small arrows in the Figure 3, it can be deduced that stereocomplex formation begins earlier in the blends annealed at 80 °C than that in the blends annealed at 120 °C. Its rationale lies in that crystals generated at low temperatures usually correspond to low melting temperatures. Besides, higher amount of stereocomplex, judged from the enthalpy of melting, is produced in the blends annealed at 80 °C than that in the blends annealed at 120 °C. It makes sense because more stereocomplexation period is available in the blends annealed at

Figure 5. XRD profiles of the PLLA/PDLA blends annealed at 80 and 120 °C after crystallization at 190 and 200 °C for 2 h, respectively. C

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homocrystals. Consistent with above DSC results, stereocomplex is produced earlier and faster in the PLLA/PDLA blends annealed at 80 °C, as compared with that in the blends annealed at 120 °C. Detailed analysis of stereocomplex formation with respect to crystallization period at 190 and 200 °C is shown in Figure 6. Note that the normalized

Figure 7. Avrami plots of normalized absorbance of 908 cm−1 band at 190 and 200 °C for the PLLA/PDLA blends annealed at 80 and 120 °C, respectively.

Figure 6. Evolution of normalized absorbance of the 908 cm−1 band with respect to isothermal period at 190 and 200 °C in the PLLA/ PDLA blends annealed at 80 and 120 °C, respectively.

absorbance represents the relative amount of stereocomplex, i.e., relative crystallinity. It is indicated that at either crystallization temperature stereocmplex formation is favored in the blends annealed at 80 °C. In addition, for both blends low crystallization temperature facilitates the stereocomplex formation because of large supercoolings. What is more, for the blends annealed at 80 °C stereocomplex formation is very rapid without the induction period at 190 °C. The crystallization kinetics of stereocomplex at 190 and 200 °C is further analyzed by Avrami equation as follows. 1 − X t = exp( −kt n)

(1)

log[− ln(1 − X t )] = log k + n log t

(2)

Figure 8. POM micrographs of the PLLA/PDLA blends annealed at (a, b) 80 and (c, d) 120 °C after crystallization at (a, c) 190 and (b, d) 200 °C for 2 h, respectively.

where Xt is the relative crystallinity, n is the Avrami exponent whose value usually depends on the dimension of crystal growth, k is the overall crystallization rate constant, and t is the crystallization time. Accordingly, the Avrami exponent n can be obtained from the slope in a plot of log[−ln(1 − Xt] versus log t, as shown in Figure 7. Note that the crystallization kinetics at 190 °C of the blends annealed at 80 °C is not included due to its absence of the induction period. The Avrami exponent n changes significantly with respect to annealing and crystallization temperatures. It is 0.73 and 2.82 for the blends annealed at 80 and 120 °C, respectively, while crystallization temperature is 200 °C. It corresponds to one and three-dimensional growth of stereocomplex in the blends annealed at 80 and 120 °C, respectively. At the same time, for the blends annealed at 120 °C two-dimensional growth of stereocomplex is realized at 190 °C while the Avrami exponent n of 2.07 is taken into account. In combination with the crystallization rate, it is expected that the variation in the dimension of stereocomplex growth could arise from the change of the nucleation density with respect to annealing and crystallization temperatures. In other words, low dimension of stereocomplex growth could be related to high nucleation density. As shown by the POM micrographs in Figure 8, it is not easy to resolve the stereocomplex crystals generated at 190 and 200

°C in the blends annealed at 80 °C by optical microscopy possibly because of the tiny size. On the contrary, relative large stereocomplex crystals are induced in the blends annealed at 120 °C, which becomes significant at crystallization temperature of 200 °C. To further disclose the crystal morphology, SALS technique that is powerful to detect crystals with size ranged from submicrometer to several micrometers was adopted. Figure 9 shows the corresponding SALS patterns. Interestingly, crystal morphology varies remarkably with respect to annealing and crystallization temperatures. Light scattering patterns with scattering streaks in horizontal and vertical directions (referred to +-type pattern) are observed at either 190 or 200 °C in the blends annealed at 80 °C. The difference with respect to crystallization temperatures is only the size of light scattering patterns, and larger light scattering pattern is produced at 190 °C than that at 200 °C. Since light scattering pattern is inverse to the crystal size, it means that crystallization at 190 °C produces smaller stereocomplex crystals than that at 200 °C. It is consistent with the supercoolings, i.e. low crystallization temperature favors high nucleation density and thus small stereocomplex crystals. As argued in previous D

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high nucleation density at elevated temperatures, responsible for rapid stereocomplex formation and rod-like stereocomplex crystals with low dimensions. As a comparison, low nucleation density is induced by the melting of large homocrystals with the α-form and thus stereocomplex formation is retarded. Correspondingly, plate-like and spherulitic aggregates with high dimensions are induced at 190 and 200 °C, respectively. The difference in the nucleation density for stereocomplex formation should be related to the segregation of PLLA and PDLA chains regarding formation of homocrystals during annealing process. In other words, crystallization from either PLLA or PDLA excludes the other component from the growth front of individual homocrystals, similar to that observed during crystallization in the miscible polymer blends.31−33 As demonstrated by the size of homocrystals in Figure 2, it is highly expected that the segregation between PLLA and PDLA chains is weaker in the blends annealed at 80 °C than that in the blends annealed at 120 °C. Therefore, at temperatures above the melting point of homocrytals, it is more possible for reaggregation of PLLA and PDLA chains to produce more nuclei of stereocomplex in the blends annealed at 80 °C, as compared with that in the blends annealed at 120 °C.

Figure 9. SALS patterns of the PLLA/PDLA blends annealed at (a, b) 80 and (c, d) 120 °C after crystallization at (a, c) 190 and (b, d) 200 °C for 2 h, respectively.

4. CONCLUSIONS Annealing of the amorphous PLLA/PDLA blends produces small homocrystals with the α′-form and large ones with the αform at 80 and 120 °C, respectively, as a result of separated crystallization of PLLA and PDLA. At elevated temperatures it is easier and rapider for small homocrystals with the α′-form to recrystallize into stereocomplex than large ones with the αform. What is more, rod-like and tiny stereocomplex crystals are generated from the melting of small homocrystals with the α′form because of high nucleation density. As a comparison, recrystallization from large homocrystals with the α-form results in plate-like or spherulitic sterecomplex crystals with relatively large size due to low nucleation density. The variation of nucleation density for stereocompelx can find the origin in the chain segregation regarding separated crystallization of PLLA and PDLA upon annealing. It is believed that weak chain segregation is involved during generation of small homocrystals with the α′-form, responsible for high nucleation density and rapid stereocomplex formation.

studies,24−26 the +-type SALS patterns under Hv mode suggest the formation of anisotropic rod-like crystals with the principal axis of the polarizability tilting at 45° to the long axis of the rods. Formation of rod-like crystals means one-dimensional growth of stereocomplex in the blends annealed at 80 °C, in line with the Avrami exponent n of about 1. As for blends annealed at 120 °C, the SALS patterns depend on the crystallization temperatures. The ×-type patterns with scattering streaks at azimuthal angle of odd multiples of 45° are generated at the crystallization temperature of 190 °C. It could arise from the scattering of anisotropic rods or plates with the principal axis of the polarizability parallel or perpendicular to the long axis.27 While the Avrami exponent n of about 2 is taken into account, it is expected that anisotropic plates are produced at 190 °C in the blends annealed at 120 °C. Rather, crystallization at 200 °C results in four-leaf patterns with remarkable scattering peaks along scattering angles. This is a typical scattering pattern for spherulites with spherical aggregates of crystallites. The formation of spherulites corresponds to three-dimensional growth of stereocomplex and thus the Avrami exponent n of about 3 is observed. The change of anisotropic plates to spherulites from 190 to 200 °C in the blends annealed at 120 °C should be related to the decrease in the nucleation density. (cf. Figure 8, parts c and d). It is widely accepted that spherulites with spherical symmetry are developed from sheaf-like aggregates in the early stage of polymer crystallization.28 This morphological transformation has been confirmed by microscopic observations.29,30 Therefore, in the presence of a large number of nuclei sheaf-like aggregates cannot develop well into spherulites due to impingement of neighboring aggregates in the early stage of crystallization. It, in turn, results in rod-like stereocomplex crystals in the blends annealed at 80 °C. Of course, reducing the number of nuclei could lead to perfect development of sheaf-like aggregates into spherulites, as illustrated by the crystallization at 200 °C of the blends annealed at 120 °C. On the basis of the above results, it can be safely deduced that melting of small homocrystals with the α′-form results in

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AUTHOR INFORMATION

Corresponding Author

*(B.N.) Fax: +86 794 8258320. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Nos. 21364001 and 21004010), the Program for Young Scientists of Jiangxi Province (No. 20112BCB23023) and the Major Program of Natural Science Foundation of Jiangxi, China (No. 20133ACB21006).

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