Stereocomplex Formation in Enantiomeric Polylactides by MeltingRecrystallization of Homocrystals: Crystallization Kinetics and CrystalMorphologyBing 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 MaterialsScience, East China Institute of Technology, Nanchang, 330013, People’s Republic of China

ABSTRACT: Crystallization kinetics and crystal morphology of polylactidestereocomplex through melting recrystallization of homocrystals was investigatedin detail. Small homocrystals with the α′-form and large ones with the α-form wereproduced by annealing of amorphous 1:1 poly(L-lactide)/poly(D-lactide) blends at80 and 120 °C, respectively. Small homocrystals with the α′-form were morefavorable than large ones with the α-form for stereocomplex formation throughmelting recrystallization. Moreover, rod-like stereocomplex crystals were producedfrom melting of small homocrystals with the α′-form at the adopted crystallizationtemperatures. In contrast, plate-like or spherulitic stereocomplex crystals weregenerated by melting of large homocrystals with the α-form. The difference in thecrystal morphology of sterecomplex was correlated with the variation of nucleationdensity regarding annealing and crystallization temperatures.

1. INTRODUCTION

As a kind of biodegradable and biocompatible material,polylactide has been attracting much interest in the fields ofpolymer and biomedical research.1−5 The presence of a chiralcarbon in the skeletal chain of polylactide yields twostereoregular enantiomers, namely poly(L-lactide) (PLLA)and poly(D-lactide) (PDLA). The homocrystals formed byeither 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 theformation of stereocomplex with a melting point of about 50°C higher than that of homocrystals.9,10 The extremely highmelting point of stereocomplex is originated from the stronginteractions, i.e., hydrogen bonding in the unit cell.11,12 Thus,stereocomplexation opens a new way to enhance the propertiessuch as thermal resistance, which benefits broader applicationsof 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 welldemonstrated that stereocomplex formation is preferential inthe 1:1 PLLA/PDLA blends, and apart from this compositionhomocrystals from either PLLA or PDLA are induced.Molecular weight is another factor determining stereocomplexformation. There exists a critical molecular weight of about 105

g/mol, above which sterecomplexation is significantly sup-pressed. From a practical view, however, high molecular weightis a prerequisite for superior mechanical performances ofpolylactides. Therefore, in the past, extensive efforts have beendevoted to achieve high stereocomplexation from polylactideswith high molecular weight by various preparation methods, for

instance repeat casting,13 supercritical fluid technology,14 low-temperature mixing,15 and so on.Besides, annealing of PLLA/PDLA blends at elevated

temperatures is effective to generate stereocomplex due tomelting recrystallization of homocrystals.16−18 It arises from thedifference in the melting point between homocrystals andstereocomplex. This phase transition process has been welldemonstrated by X-ray techniques and Fourier transforminfrared spectroscopy.17,18 To date, crystallization kinetics andcrystal morphology of stereocomplex upon melting recrystal-lization of homocrystals is less concerned yet. What is more, itis unclear that the effect of crystal form (α′ and α) and/or thesize of homocrystals on the stereocomplex formation. Thisstudy clearly demonstrates that stereocomplex formation fromsmall homocrystals with the α′-form is easer and rapider thanthat from large homocrystals with the α-form. And, rod-likestereocomplex crystals are produced from melting recrystalliza-tion of small homocrystals with the α′-form due to highnucleation density.

2. EXPERIMENTAL SECTION2.1. 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 atroom temperature to generate a transparent solution with aconcentration of 0.1 g/mL; the mass ratio of PLLA and PDLA was

Received: November 22, 2013Revised: December 13, 2013

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fixed at 1:1. After then, the solution was slowly poured into a largeamount of methanol under vigorous stirring to precipitate the PLLA/PDLA blends. The obtained sediments were washed several times bymethanol, followed by drying under vacuum at 40 °C overnight. Filmswere prepared by hot-pressing of the sediments at 240 °C and thenquenching into ice water. Annealing of films was carried out in a hotstage 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 anitrogen atmosphere using a Perkin-Elmer Pyris-1 DSC instrument.Fourier transform infrared spectra were measured by a ThermoNicolet FTIR spectrometer with a resolution of 4 cm−1. To obtain insitu structural change at elevated temperatures, a hot stage wascoupled with the FTIR spectrometer in its sample compartment.Samples were rapidly heated to 190 and 200 °C, respectively, and thenheld isothermally until the completion of crystallization. X-raydiffraction (XRD) measurements were conducted on the diffractionworkstation in the Beijing Synchrotron Radiation Facility; thewavelength of the X-ray was 0.154 nm. The crystal morphology wasdisclosed by a small angle light scattering (SALS) setup with laserwavelength of 532 nm in the Hv mode and a polarized opticalmicroscope (POM) under cross-polarization conditions, respectively.

3. RESULTS AND DISCUSSIONUpon quenching into ice water from melt, the PLLA/PDLAblends cannot crystallize and remain amorphous due to rapidcooling as well as high molecular weight. It is demonstrated byno diffractions from crystals in the XRD profile and noabsorbance in the FTIR spectra ranged between 940 and 900cm−1, as shown in Figure 1. After being annealed above glasstransition temperature (∼60 °C), crystals are induced in thePLLA/PDLA blends as a result of cold crystallization. Thecrystal form of the PLLA/PDLA blends shows a significant

dependence on the annealing temperatures. At 80 °Chomocrystals with the α′-form is generated in the blends,indicated by the characteristic diffractions at 2θ of 16.4 and18.7°, respectively.19 While being annealed at 120 °C,homocrystals still prevail in the blends but the crystal form isthe 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 thecrystallinity, deduced from the XRD profiles, is 0.39 and 0.47for the blends annealed at 80 and 120 °C, respectively. Theabove situation is same to that observed in the individual PLLAwith respect to crystallization temperatures. There exists atemperature range for the formation of the α′- and α-form inthe individual PLLA.20 At 120 °C and above, the α-form withdense 103 helical chain packing is induced. In contrast, loose103 helical chain packing results in the α′-form due to limitedmolecular mobility while crystallization temperature is below120 °C. Of note, the α′- and α-form have the same absorptionband at 922 cm−1 in the FTIR spectra (Figure 1b).18,19

It suggests that PLLA and PDLA in the blends crystallizeseparately into homocrystals and the crystallization habit issame to that of individual PLLA. In other words, at lowannealing temperatures there are little mutual chain interactionsbetween PLLA and PDLA in the blends as regardingcrystallization. It, in turn, is responsible for the absence ofstereocomplex formation in the 1:1 PLLA/PDLA blends at lowannealing temperatures due to limited molecular mobility. Insuch a sense, stereocomplex could be produced at higherannealing temperature where enough molecular mobility isgained. It is the exact fact observed in the PLLA/PDLA blendsannealed at 160 °C for 1h. Stereocomplex manifests itself bythe characteristic diffractions at 2θ of 12, 20.8, and 24.4° in theXRD profile9−18 (Figure 1a) and the characteristic absorptionband at 908 cm−1 in the FTIR spectra18−22 (Figure 1b),respectively. At the same time, homocrystals with the α-form isalso induced at this annealing temperature due to separatedcrystallization of PLLA and PDLA in the blends. Since ourattention is focused on the stereocomplex formation by meltingrecrystallizaiton of homocrystals, the blends annealed at 160 °Cwill not be taken into account in the following section becauseof the partial presence of stereocomplex.In addition to crystal form, crystal morphology is also

affected by annealing temperatures. Figure 2 gives the POMmicrographs and SALS patterns of the PLLA/PDLA blendsannealed at 80 and 120 °C, respectively. Spherulites areproduced in both blends, judged from the four-leaf SALSpatterns with apparent scattering peaks along scattering angles.Annealing at 80 °C produces smaller spherulites than that at120 °C, as a result of higher supercoolings and nucleationdensity. That is, formation of tiny spherulites at 80 °C onlyinvolves local regulation of molecular chains, whereas moleculardiffusion in a relatively broad range, due to high molecularmobility, 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 inthe temperature ranges between 160 and 180 °C correspond tothe melting of homocrystals. In the blends annealed at 80 °Cthere exists an exothermic peak that corresponds to the α′ → αtransition of homocrystals before dominant melting. It arisesfrom the solid−solid reorganization of the α′-form with loosechain packing at elevated temperatures; this phase transitionhas been well disclosed by FTIR and X-ray measurements inthe past.7,19,20 In contrast, the α-form in the blends annealed at

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

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120 °C undergoes direct melting without solid−solidreorganization 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 190and 220 °C. It arises from the melting of stereocomplex with ahigh melting point; stereocomplex has a melting temperaturehigher about 50 °C than that of homocrystals due to side-by-side dense molecular packing between PLLA and PDLA.21−23

Recalling that there is no stereocomplex in the blends annealedat 80 and 120 °C (see Figure 1), this stereocomplex must beinduced from the melting recrystallization of homocrystals.17,18

Moreover, stereocomplex formation upon heating dependsremarkably on the annealing temperatures. On the basis of theonset melting temperature of stereocomplex, as indicated bythe small arrows in the Figure 3, it can be deduced thatstereocomplex formation begins earlier in the blends annealedat 80 °C than that in the blends annealed at 120 °C. Itsrationale lies in that crystals generated at low temperaturesusually correspond to low melting temperatures. Besides, higheramount of stereocomplex, judged from the enthalpy of melting,is produced in the blends annealed at 80 °C than that in theblends annealed at 120 °C. It makes sense because morestereocomplexation period is available in the blends annealed at

80 °C during heating runs at 10 °C/min while its earlierstereocomplex formation is taken into account. The DSCresults strongly suggest that small homocrystals with the α′-form is more favorable for stereocomplex formation than largeones 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 FTIRmeasurements at 190 and 200 °C, respectively. Figure 4 shows

the examples of time dependent FTIR spectra recorded duringisothermal crystallization at 200 °C. Once temperature reaches200 °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 characteristicabsorption band at 908 cm−1 shows up with the elapse oftime. It is further verified by the XRD profiles shown in Figure5, where stereocomplex prevails with the absence of

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

Figure 3. DSC traces of the PLLA/PDLA blends annealed at 80 and120 °C, respectively.

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

Figure 5. XRD profiles of the PLLA/PDLA blends annealed at 80 and120 °C after crystallization at 190 and 200 °C for 2 h, respectively.

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homocrystals. Consistent with above DSC results, stereo-complex is produced earlier and faster in the PLLA/PDLAblends annealed at 80 °C, as compared with that in the blendsannealed at 120 °C. Detailed analysis of stereocomplexformation with respect to crystallization period at 190 and200 °C is shown in Figure 6. Note that the normalized

absorbance represents the relative amount of stereocomplex,i.e., relative crystallinity. It is indicated that at eithercrystallization temperature stereocmplex formation is favoredin the blends annealed at 80 °C. In addition, for both blendslow crystallization temperature facilitates the stereocomplexformation because of large supercoolings. What is more, for theblends annealed at 80 °C stereocomplex formation is very rapidwithout the induction period at 190 °C.The crystallization kinetics of stereocomplex at 190 and 200

°C is further analyzed by Avrami equation as follows.

− = −X kt1 exp( )tn

(1)

− − = +X k n tlog[ ln(1 )] log logt (2)

where Xt is the relative crystallinity, n is the Avrami exponentwhose value usually depends on the dimension of crystalgrowth, k is the overall crystallization rate constant, and t is thecrystallization time. Accordingly, the Avrami exponent n can beobtained from the slope in a plot of log[−ln(1 − Xt] versus logt, as shown in Figure 7. Note that the crystallization kinetics at190 °C of the blends annealed at 80 °C is not included due toits absence of the induction period.The Avrami exponent n changes significantly with respect to

annealing and crystallization temperatures. It is 0.73 and 2.82for the blends annealed at 80 and 120 °C, respectively, whilecrystallization temperature is 200 °C. It corresponds to one andthree-dimensional growth of stereocomplex in the blendsannealed at 80 and 120 °C, respectively. At the same time, forthe blends annealed at 120 °C two-dimensional growth ofstereocomplex is realized at 190 °C while the Avrami exponentn of 2.07 is taken into account. In combination with thecrystallization rate, it is expected that the variation in thedimension of stereocomplex growth could arise from thechange of the nucleation density with respect to annealing andcrystallization temperatures. In other words, low dimension ofstereocomplex growth could be related to high nucleationdensity.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 microscopypossibly because of the tiny size. On the contrary, relative largestereocomplex crystals are induced in the blends annealed at120 °C, which becomes significant at crystallization temper-ature of 200 °C. To further disclose the crystal morphology,SALS technique that is powerful to detect crystals with sizeranged from submicrometer to several micrometers wasadopted.Figure 9 shows the corresponding SALS patterns. Interest-

ingly, crystal morphology varies remarkably with respect toannealing and crystallization temperatures. Light scatteringpatterns with scattering streaks in horizontal and verticaldirections (referred to +-type pattern) are observed at either190 or 200 °C in the blends annealed at 80 °C. The differencewith respect to crystallization temperatures is only the size oflight scattering patterns, and larger light scattering pattern isproduced at 190 °C than that at 200 °C. Since light scatteringpattern is inverse to the crystal size, it means that crystallizationat 190 °C produces smaller stereocomplex crystals than that at200 °C. It is consistent with the supercoolings, i.e. lowcrystallization temperature favors high nucleation density andthus small stereocomplex crystals. As argued in previous

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

Figure 7. Avrami plots of normalized absorbance of 908 cm−1 band at190 and 200 °C for the PLLA/PDLA blends annealed at 80 and 120°C, respectively.

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.

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studies,24−26 the +-type SALS patterns under Hv mode suggestthe formation of anisotropic rod-like crystals with the principalaxis of the polarizability tilting at 45° to the long axis of therods. Formation of rod-like crystals means one-dimensionalgrowth of stereocomplex in the blends annealed at 80 °C, inline 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 withscattering streaks at azimuthal angle of odd multiples of 45° aregenerated at the crystallization temperature of 190 °C. It couldarise from the scattering of anisotropic rods or plates with theprincipal axis of the polarizability parallel or perpendicular tothe long axis.27 While the Avrami exponent n of about 2 istaken into account, it is expected that anisotropic plates areproduced at 190 °C in the blends annealed at 120 °C. Rather,crystallization at 200 °C results in four-leaf patterns withremarkable scattering peaks along scattering angles. This is atypical scattering pattern for spherulites with sphericalaggregates of crystallites. The formation of spherulitescorresponds to three-dimensional growth of stereocomplexand thus the Avrami exponent n of about 3 is observed. Thechange of anisotropic plates to spherulites from 190 to 200 °Cin the blends annealed at 120 °C should be related to thedecrease 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 ofpolymer crystallization.28 This morphological transformationhas been confirmed by microscopic observations.29,30 There-fore, in the presence of a large number of nuclei sheaf-likeaggregates cannot develop well into spherulites due toimpingement of neighboring aggregates in the early stage ofcrystallization. It, in turn, results in rod-like stereocomplexcrystals in the blends annealed at 80 °C. Of course, reducingthe number of nuclei could lead to perfect development ofsheaf-like aggregates into spherulites, as illustrated by thecrystallization 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

high nucleation density at elevated temperatures, responsiblefor rapid stereocomplex formation and rod-like stereocomplexcrystals with low dimensions. As a comparison, low nucleationdensity is induced by the melting of large homocrystals with theα-form and thus stereocomplex formation is retarded.Correspondingly, plate-like and spherulitic aggregates withhigh dimensions are induced at 190 and 200 °C, respectively.The difference in the nucleation density for stereocomplexformation should be related to the segregation of PLLA andPDLA chains regarding formation of homocrystals duringannealing process. In other words, crystallization from eitherPLLA or PDLA excludes the other component from the growthfront of individual homocrystals, similar to that observed duringcrystallization in the miscible polymer blends.31−33 Asdemonstrated by the size of homocrystals in Figure 2, it ishighly expected that the segregation between PLLA and PDLAchains is weaker in the blends annealed at 80 °C than that inthe blends annealed at 120 °C. Therefore, at temperaturesabove the melting point of homocrytals, it is more possible forreaggregation of PLLA and PDLA chains to produce morenuclei of stereocomplex in the blends annealed at 80 °C, ascompared with that in the blends annealed at 120 °C.

4. CONCLUSIONSAnnealing of the amorphous PLLA/PDLA blends producessmall homocrystals with the α′-form and large ones with the α-form at 80 and 120 °C, respectively, as a result of separatedcrystallization of PLLA and PDLA. At elevated temperatures itis easier and rapider for small homocrystals with the α′-form torecrystallize into stereocomplex than large ones with the α-form. What is more, rod-like and tiny stereocomplex crystals aregenerated 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-formresults in plate-like or spherulitic sterecomplex crystals withrelatively large size due to low nucleation density. The variationof nucleation density for stereocompelx can find the origin inthe chain segregation regarding separated crystallization ofPLLA and PDLA upon annealing. It is believed that weak chainsegregation is involved during generation of small homocrystalswith the α′-form, responsible for high nucleation density andrapid stereocomplex formation.

■ AUTHOR INFORMATIONCorresponding Author*(B.N.) Fax: +86 794 8258320. E-mail: bna@ecit.edu.cn,bingnash@163.com.NotesThe authors declare no competing financial interest.

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

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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.

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