Crystal Growth and Solid-State Structure of Poly(lactide

Biomacromolecules 2005, 6, 457-467

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Crystal Growth and Solid-State Structure of Poly(lactide) Stereocopolymers Hideki Abe,*,†,‡ Mariko Harigaya,‡ Yoshihiro Kikkawa,† Takeharu Tsuge,‡ and Yoshiharu Doi†,‡ Polymer Chemistry Laboratory, RIKEN Institute, Hirosawa, Wako-shi, Saitama 351-0198, Japan, and Department of Innovative and Engineered Materials, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8501, Japan Received August 24, 2004; Revised Manuscript Received October 27, 2004

Solid-state structure and melting behavior for random stereocopolymers of L-lactide with meso-lactide (P(L-LA-co-meso-LA)) with different meso-LA compositions of 0, 2, 4, and 10 mol % were investigated under various isothermal crystallization conditions. The crystalline morphology of P(L-LA-co-meso-LA) samples changed from the spherulitic aggregates to hexagonal lamellae stacking with a rise in crystallization temperature. Under each crystallization condition, P(L-LA-co-meso-LA) samples formed R-crystal modifications for homopolymer of L-LA. By using the atomic force microscopy and small-angle X-ray scattering, the stacking structure of lamellar crystals was examined for the isothermally crystallized P(L-LA-co-mesoLA) thin films. The lamellar thickness of P(L-LA-co-meso-LA) ranged from 6.2 to 15.5 nm, and the values increased with crystallization temperature. Melting profiles of crystalline regions were examined by the differential scanning calorimetry (DSC) for the P(L-LA-co-meso-LA) samples. Distinct two melting peaks were detected in the DSC thermograms of several samples. Investigations on the time-dependent changes in lamellar structure and melting temperature of the P(L-LA-co-meso-LA) samples under isothermal crystallization conditions provided the evidence that a small amount of D-lactyl units was trapped in the crystalline regions during early stage of crystallization process under the certain crystallization condition. In addition, it was found that the D-lactyl units trapped in crystalline regions were excluded from crystalline lamellae to form the thermally stable crystals without changes in crystal thickness during further isothermal storage at a crystallization temperature. The equilibrium melting temperature (T0m) of P(L-LA-co-meso-LA) samples was estimated by using modified Hoffman-Weeks methods, and the obtained T0m values decreased from 215 to 184 °C as the meso-LA composition was increased from 0 to 10 mol %. Furthermore, the crystal growth kinetics of the P(L-LA-co-meso-LA) samples was analyzed by using the secondary nucleation theory. Transitions of crystalline regime both from regime III to regime II and from regime II to regime I were detected for each sample. The transition temperature from regime II to regime I of each of the P(L-LA-co-meso-LA) samples was very close to the temperature region revealed the morphological changes in the crystalline aggregates from the spherulitic aggregates to hexagonal lamellae stacking. Introduction Poly(L-lactide) (PLLA), produced by chemosynthetic methods from either L-lactic acid or its cyclic dimer,1-6 is a biodegradable and biocompatible thermoplastic with a melting point around 180 °C.7 PLLA has been extensively investigated as a material for medical devices such as controlled drug release matrixes, degradable sutures, and implantation for bone fixation.8-12 It has been found that PLLA can be degraded in natural environments and that the monomer and oligomers of L-lactic acid generated as degradation products are metabolized by various microorganisms.7 Therefore, recently, there is significantly increasing interest in using PLLA for environmentally degradable plastic materials. In addition, since the L-lactic acid as monomer of * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +81-48-467-9404. Fax: +81-48-462-4667. † RIKEN Institute. ‡ Tokyo Institute of Technology.

PLLA can be produced by biosynthetic method from renewable carbon sources, PLLA materials have attracted industrial attention as environmentally recyclable thermoplastics. The crystal structure, physical properties, and biodegradability of PLLA have been investigated in detail. Biodegradation of PLLA materials in natural environments is progressed due to both the enzymatic and nonenzymatic hydrolysis reactions.13-20 In both cases, the rate of biodegradation was strongly dependent on the molecular weight and on the solid-state structure of the samples. Especially, crystallinity and lamellar crystal size play a decisive role in the degradation process. Copolymerization of PLLA with comonomer units is one of the useful techniques to produce PLLA-based materials with a wide range of physical properties and biodegradability. PLLA-based copolymers with various comonomers have been synthesized, and the structure and properties of these

10.1021/bm049497l CCC: $30.25 © 2005 American Chemical Society Published on Web 12/17/2004

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copolymers were characterized.21 Copolymers of L-lactide with glycolide (P(L-LA-co-GA)) have been intensively investigated by many researchers, and it has been reported that the composition and sequential distribution of monomer units in P(L-LA-co-GA) copolymers affect the solid-state structures such as crystallinity and crystal size and that the physical properties and biodegradability of P(L-LA-co-GA) can be varied with both the composition and sequential distribution.22 Lactic acid has an asymmetric carbon atom and includes the stereoisomers of L and D forms. Cyclic dimers of lactic acids form three enantiomers of L-lactide (L-LA), D-lactide (D-LA), and meso-lactide (meso-LA). By using these stereoisomers as monomers, stereocopolymers consisting of lactyl units can be synthesized.23 Physical properties and biodegradability of the stereocopolymers have been studied as well as the PLLA-based copolymers.15,17,24-33 McCarthy’s group determined the rate of enzymatic degradation for stereocopolymers of L-LA and D-LA and of L-LA and meso-LA by proteinase K.17,24-26 They reported that the degradation rate of stereocopolymers is regulated not only by the substrate specificity of enzyme for stereostructure of lactyl units but also by the crystallinity and lamellar organization of samples. In addition, it has been suggested that, for the L-LA-rich stereocopolymers, the crystallinity and lamellar crystal size play a decisive role in the degradation process. The crystallization kinetics, lamellar microstructure, and melting behavior of stereocopolymers of L-LA and D-LA and of L-LA and meso-LA were most intensively investigated by Runt’s group.29-32 They demonstrated that the introduction of D-lactyl units into PLLA molecules induced the decrease in crystallinity and the depression of melting temperature. In addition, from the characterization of lamellar structure and analysis of equilibrium melting temperature of stereocopolymers, they have concluded that the D-lactyl units are excluded from crystalline lamellae. In this paper, random stereocopolymers of L-LA and mesoLA (P(L-LA-co-meso-LA)) having different meso-LA compositions of 0, 2, 4, and 10 mol % are isothermally crystallized at a wide range of temperature, and the crystalline morphologies of P(L-LA-co-meso-LA) samples are characterized by means of optical microscope, atomic force microscope, wide-angle X-ray diffraction, and small-angle X-ray scattering. Time-dependent changes in melting behavior of lamellar crystal are determined in detail to investigate the crystallization mechanism of P(L-LA-co-mesoLA). In addition, kinetic analysis of P(L-LA-co-meso-LA) crystal growth is carried out to determine the relationship between the crystalline morphology and the regime of crystal growth. Experimental Section Materials. L-LA and meso-LA (Purac Co.) were recrystallized from toluene solution under isothermal conditions at 65 and 50 °C, respectively. ZnEt2/H2O(1/0.6) catalyst was prepared as follows. ZnEt2 was reacted with deoxygenated water at a molar ratio of 1/0.6 (ZnEt2/H2O) in dry 1,4dioxane, followed by freeze-drying of the reaction mixture. A yellow powder was obtained.

Abe et al. Table 1. Molecular Weights and Thermal Properties of Poly(lactide) Stereocopolymers molecular weight

thermal properties

sample

Mn

Mw/Mn

Tg, °C

T0m, °C

P(L-LA-co-2mol %-meso-LA) P(L-LA-co-4mol %-meso-LA) P(L-LA-co-10mol %-meso-LA) PLLA

78 000 88 000 65 000 89 000

1.6 1.7 1.9 1.7

59 58 57 62

209 200 184 215

Preparation of P(L-LA-co-meso-LA) with Different meso-LA Contents. P(L-LA-co-meso-LA) samples with different meso-LA contents were synthesized by the ringopening polymerization of a mixture of L-LA and meso-LA in the presence of ZnEt2/H2O as a catalyst. Monomer and ZnEt2/H2O catalyst were admitted into a reactor under nitrogen atmosphere, and then dichloromethane was added into a reactor to dissolve the monomer and catalyst. Polymerization was carried out in the homogeneous solution at 40 °C under nitrogen atmosphere. The reacted solution was diluted in excess chloroform and poured into methanol. The P(L-LA-co-meso-LA) precipitated in methanol was recovered by filtration. The precipitate was dried in vacuo at room temperature. Molecular weights of P(L-LA-co-meso-LA) were determined from gel permeation chromatography using polystyrene standards, and the data are presented in Table 1. For the P(L-LA-co-meso-LA) samples with low meso-LA compositions (meso-LA ) 2-10 mol %), it was difficult to determine the sequential distributions of L-LA and mesoLA monomers. Therefore, P(L-LA-co-meso-LA) samples with high meso-LA composition (meso-LA ) 50 mol %) were synthesized through the similar procedures by changing the monomer feed ratios and then characterized by 1H NMR analysis. Based on the 1H NMR analysis, the sequence distribution of L-LA and meso-LA monomers of P(L-LAco-50 mol % meso-LA) was found to be statistically random. From this result, it was considered that the P(L-LA-co-mesoLA) samples used in this study were random copolymers of L-LA and meso-LA units. Preparation of Melt-Crystallized Films. P(L-LA-comeso-LA) thin films of 100 nm thickness were initially prepared by solvent-cast technique from chloroform solutions. A 10 µL droplet of the polymer solution (1.0% (w/v)) was placed on a glass cover slip (substrate dimensions: 18 × 18 mm) and sandwiched with another cover slip to spread the solution. The cover slips were then slid against each other, which results in the formation of a thin P(L-LA-co-mesoLA) layer on the substrate surface. Subsequently, the thin film on the substrate was heated on a hot-stage (Linkam LK600PM) to 200 °C at a rate of 30 °C/min. Samples were maintained at 200 °C for 30 s, and then the temperature was rapidly lowered to a crystallization temperature (Tc) in the range of 90-160 °C. The samples were crystallized isothermally at the given Tc for a certain period of time. P(L-LA-co-meso-LA) thick films of 100 µm thickness were initially prepared by conventional solvent-cast techniques from chloroform solutions using glass Petri dishes as casting surfaces. The solvent-cast films were inserted between two Teflon sheets with a Teflon sheet (0.05 mm thickness) as a

Poly(lactide) Stereocopolymers

spacer and were compression-molded on a Mini Test Press (Toyoseiki) by heating at 200 °C for 30 s under a pressure of 75 kg/cm2. After melting, the samples were kept at a given crystallization temperature (Tc) and isothermally crystallized for 3 days. Enzymatic Etching Treatment for Melt-Crystallized Thin Films. Proteinase K from Tritirachium album was purchased from Beohringer mannheim GMBH Biochemica and used without further purification. The P(L-LA-co-mesoLA) thin film was immersed into 5 mL of 0.05 M Tris-HCl buffer (pH 8.5) containing proteinase K (200 µg/mL). The enzymatic degradation of thin films by proteinase K was carried out at 25 °C. The thin film was removed from enzyme solution after reaction for a periodic time, and then the thin film was washed with distilled water and dried to constant weight in vacuo before analysis. Analytical Procedures. Crystalline morphology of meltcrystallized thin films and growth rate of the P(L-LA-comeso-LA) crystals were determined by using an optical microscope (Nikon OPTIPHOTO-2) equipped with phase contrast lens. Melting temperature and enthalpy of fusion were determined from differential scanning calorimetry (DSC) measurements. The DSC data were recorded in a temperature range of 0-200 °C on a Perkin-Elmer Pyris 1 equipped with a cooling accessory operated at a nitrogen flow of 20 mL/ min. Samples (3 mg) were encapsulated in aluminum pans and heated from 0 to 200 °C at a rate of 20 °C/min. Samples were maintained at 200°C for 30 s, and then the temperature was rapidly lowered to a given crystallization temperature. After a certain period of time, the samples were reheated from the crystallization temperature to 200 °C at a rate of 20 °C/min. The melting temperature was taken as the temperature corresponding to the curve peak. Wide-angle X-ray diffraction patterns of melt-crystallized films were recorded at 25 °C on a Rigaku RINT 2500 system using nickel-filtered Cu KR radiation (λ ) 0.154 nm; 40 kV; 200 mA) in the 2θ range of 6-40° at a scanning speed of 2.0°/min. Small-angle X-ray scattering analyses of films were carried out at 27 °C using a Rigaku RINT 2500 system in the 2θ range of 0.1-3.0° at a scan speed of 0.05°/min. Radiation of wavelength 0.154 nm (Cu KR) was employed at a generator power of 40 kV and 200 mA. Atomic force microscopy (AFM) was performed with a SPI3800/SPA400 (Seiko Instruments Inc.). A pyramid-like Si3N4 tip mounted on a 200 µm long microcantilevers with a spring constant of 0.02 N/m was applied for the contact mode measurements. Simultaneous registration was performed for height and deflection images. Results and Discussion Crystalline Morphologies of Melt-Crystallized P(L-LAco-meso-LA) Films. P(L-LA-co-meso-LA) thin films with a thickness of 100 nm were crystallized isothermally from the melt state at 200 °C. Crystalline features of the P(L-LAco-meso-LA) thin films were characterized by AFM. Figure 1 shows typical AFM deflection images of P(L-LA-co-meso-

Biomacromolecules, Vol. 6, No. 1, 2005 459

LA) thin films on the glass substrates crystallized at various temperatures, together with AFM images of PLLA thin films. For each P(L-LA-co-meso-LA) sample, spherulitic morphology was observed in thin films crystallized at lower temperature region, whereas the hexagonal crystals were formed in the thin films crystallized at higher temperature region. The crystalline morphologies of melt-crystallized P(L-LAco-2mol %-meso-LA) thin films revealed a distinct change from the spherulitic aggregates to hexagonal crystals in the 140-150 °C regions of the crystallization temperature. Similar morphological changes from the spherulitic aggregates to hexagonal crystals were observed in the P(L-LAco-4mol %-meso-LA) and P(L-LA-co-10mol %-meso-LA) thin films crystallized in the 140-150 °C regions and in the 130-140 °C regions of the crystallization temperature, respectively. The lamellar crystals formed in both the spherulitic aggregates and hexagonal crystals of all P(L-LAco-meso-LA) samples were flat-on lamellae on the basis of electron diffraction patterns of these crystals. Lamellar crystals developed on the folding surface of the hexagonal crystals. Overgrowth of lamellar crystals occurred not only on the folding surface of hexagonal mother crystals but also under the crystals. Number of overgrowth crystals increased with an increase in the meso-LA composition of P(L-LAco-meso-LA). Even if after the isothermal crystallization for sufficient period, the amorphous regions remained on the surface of P(L-LA-co-meso-LA) thin films and at the interlamellar regions. Therefore, the accurate value of lamellar thickness cannot be estimated directly from the P(L-LA-co-meso-LA) thin films without any treatment. It is known that the proteinase K from Tritirachium album is a serine endopeptidase and hydrolyzes PLLA molecules in the amorphous region much faster than those in the crystalline region.17,24-26 In addition, the enzymatic degradation in the crystalline region takes place from the edge of crystalline lamellae rather than from the chain-folding surface.18 Previously, we have demonstrated that the only amorphous regions can be removed from the melt-crystallized PLLA thin films by the enzymatic degradation with proteinase K.34 Also, in this study, the enzymatic etching treatment with proteinase K was applied for melt-crystallized P(L-LA-co-meso-LA) thin films to determine the lamellar thickness. Figure 2 shows the AFM height images and line profile data of the P(L-LA-co-2mol %-meso-LA) thin films crystallized at 150 °C before and after enzymatic etching treatment with proteinase K. Before degradation, the hexagonal lamellar crystals were observed in the P(L-LA-co-2mol %-mesoLA) thin film. The heights of the hexagonal crystal and of the surrounding amorphous materials were almost similar. After enzymatic etching, the hexagonal crystal stood out about 100 nm from the surface of surround area, as can be seen from Figure 2B. The shape of the hexagonal crystal remained almost unchanged during enzymatic treatment. As shown in Figure 2C, the remained hexagonal crystals of P(LLA-co-2mol %-meso-LA) formed at 150 °C revealed the periodical steps with around 15 nm corresponding to the stacking of lamellar crystals. The lamellar thickness (lc) was determined from the height images of the flat-on lamellae.

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Figure 1. AFM deflection images of melt-crystallized thin films of P(L-LA-co-meso-LA) with different meso-LA composition crystallized at various crystallization temperatures. PLLA crystallized at 160 °C for 60 min (A), at 140 °C for 10 min (B), at 120 °C for 1 min (C), P(L-LA-co-2mol %-meso-LA) crystallized at 150 °C for 7 min (D), at 140 °C for 3 min (E), at 120 °C for 2 min (F), P(L-LA-co-4mol %-meso-LA) crystallized at 150 °C for 80 min (G), at 140 °C for 10 min (H), at 115 °C for 3 min (I), P(L-LA-co-10mol %-meso-LA) crystallized at 140 °C for 40 min (J), at 130 °C for 15 min (K), at 110 °C for 15 min (L).

Figure 2. AFM height image of P(L-LA-co-2mol %-meso-LA) (Tc ) 150 °C for 60 min) before (A) and after (B) enzymatic etching with proteinase K. (C) High magnification image of white rectangular area in image (B). Graphs under the height images are line profile data measured from the white line region in each image.

The lc (AFM) value was averaged on 150 data points obtained from individual lamellar crystals. Table 2 lists the average values of lamellar thickness for melt-crystallized P(LLA-co-meso-LA) samples. The lc values of P(L-LA-co-2mol %-meso-LA) crystallized at a temperature from 110 to 150 °C were in the range from 7.1 to 15.3 nm. The lc values of P(L-LA-co-4mol %-meso-LA) and of P(L-LA-co-10mol

%-meso-LA) were in the ranges from 6.7 to 15.5 nm and from 6.4 to 12.9 nm, respectively. As shown in Table 2, for all P(L-LA-co-meso-LA) samples, the lc (AFM) value increased with the crystallization temperature. The lamellar structure of P(L-LA-co-meso-LA) was also characterized by using small-angle X-ray scattering (SAXS) measurements for melt-crystallized thick films with a thick-


Poly(lactide) Stereocopolymers Table 2. Thickness Data of Lamellar Crystals for Poly(lactide) Stereocopolymers Crystallized at Various Temperatures AFM sample P(L-LA-co-2mol %-meso-LA)

Tc, °C

150 145 140 130 120 110 P(L-LA-co-4mol %-meso-LA) 150 145 140 135 130 120 110 P(L-LA-co-10mol %-meso-LA) 140 135 130 125 120 110 100 PLLA 170 160 150 140 130 120 110

SAXS

WAXD

lc (nm)

L p, nm

lc, nm

la, nm

Xc, %

15.3 ( 0.9 14.3 ( 1.0 10.4 ( 0.9 8.4 ( 1.0 7.7 ( 0.8 7.1 ( 0.8 15.5 ( 0.7 15.2 ( 1.2 11.3 ( 0.9

24.2 23.6 22.7 20.2 19.0 18.8 26.2 23.5 22.7 20.9 18.9 18.8 18.9 24.9 22.1 21.9 19.1 19.7 17.6 18.8

14.9 14.0 12.2 8.4 7.4 7.2 15.0 13.5 11.8 9.5 7.8 7.1 6.2 12.7 11.8 10.4 7.7 7.4 6.7 6.5

9.3 9.6 10.5 11.9 11.6 11.6 11.2 10.0 10.9 11.4 11.1 11.7 12.7 12.2 10.3 11.5 11.4 12.3 10.9 12.4

63 64 57 51 50 45 56 56 54 50 47 42 39 38 42 37 35 29 30 28

11.9 9.7 11.2 11.9 12.2

67 64 57 52 47

8.1 ( 0.6 7.6 ( 0.7 6.7 ( 0.6 12.9 ( 1.0 12.2 ( 1.7 11.9 ( 1.0

7.3 ( 0.7 6.9 ( 0.6 6.4 ( 0.6 20.5 ( 1.0 18.9 ( 0.8 14.7 ( 0.9 29.8 17.9 13.7 ( 1.1 24.8 15.1 24.6 13.4 8.8 ( 0.9 21.9 10.0 8.1 ( 0.9 20.7 8.5

Figure 3. Variation in small-angle X-ray scattering (SAXS) intensity with q for P(L-LA-co-2mol %-meso-LA) films crystallized for 3 days at 120, 140, and 150 °C, respectively.

ness of 100 µm. Figure 3 shows the typical variation in relative SAXS intensities as a function of the magnitude of the scattering vector q for the samples of P(L-LA-co-2mol %-meso-LA). The scattering vector magnitude is defined by q ) 4π sin θ/λ, where θ is one-half the scattering angle and λ is the wavelength of the radiation. To estimate the long period distance (Lp) and lamellar thickness (lc) of the meltcrystallized P(L-LA-co-meso-LA) samples, the scattering data were analyzed according to the pseudo two-phase model using a one-dimensional correlation function which can be taken directly as the Fourier transform of the scattering intensity.35,36 Figure 4 shows the long period distance (Lp), lamellar thickness (lc), and amorphous thickness (la) of melt-

crystallized P(L-LA-co-meso-LA) samples, and the values are also listed in Table 2. The Lp value of P(L-LA-co-2mol %-meso-LA) increased from 18.8 to 24.2 nm as crystallization temperature was increased from 110 to 150 °C. The lc value of P(L-LA-co-2mol %-meso-LA) samples also increased from 7.2 to 14.9 nm with an increase in crystallization temperature. The Lp values of P(L-LA-co-4mol %-mesoLA) and of P(L-LA-co-10mol %-meso-LA) ranged in 18.926.2 nm and 18.8-24.9 nm, respectively. The lc values of P(L-LA-co-4mol %-meso-LA) and of P(L-LA-co-10mol %-meso-LA) ranged in 6.2-15.0 nm and 6.5-12.7 nm, respectively. In both samples, the Lp and lc values tended to increase with crystallization temperature. For all P(L-LAco-meso-LA) samples, the la values ranged in 9.3-12.7 nm. A detailed comparison of the lc values obtained by SAXS with those by AFM is shown in Figure 5. For each sample, the lc values of AFM were in good agreement with the lc values of SAXS. The crystalline structure of melt-crystallized P(L-LA-comeso-LA) films was characterized by the wide-angle X-ray diffraction (WAXD) analysis. The diffraction patterns of all melt-crystallized P(L-LA-co-meso-LA) samples showed the reflections arising from the R-form of the PLLA crystalline lattice.37 The X-ray crystallinities of polyester films were calculated from the diffraction patterns, and the data are listed in Table 2. The crystallinity of melt-crystallized P(L-LAco-meso-LA) films ranged from 28 to 63%, and the values tended to increase with crystallization temperature. In addition, the crystallinity of P(L-LA-co-meso-LA) films crystallized at an identical temperature decreased with an increase in the meso-LA composition. Thermal Properties of Melt-Crystallized P(L-LA-comeso-LA) Films. The thermal properties of melt-crystallized P(L-LA-co-meso-LA) films were characterized by differential scanning calorimetry (DSC). Figure 6 shows the DSC curves of P(L-LA-co-meso-LA) films crystallized at various temperatures for 3 days. The Tm value increased with crystallization temperature, suggesting that the thickness of crystalline lamellae in melt-crystallized P(L-LA-co-meso-LA) films increases with an increase in the crystallization temperature. When the Tm values were compared to different P(L-LA-co-meso-LA) samples crystallized at an identical temperature, the Tm value decreased with an increase in the meso-LA composition. As shown in Figure 6A, two endothermic peaks were detected in the thermograms for the P(LLA-co-2 mol %-meso-LA) films crystallized at temperatures above 145 °C. The DSC thermograms of P(L-LA-co-4 mol %-meso-LA) films crystallized at temperatures above 140 °C also revealed the presence of two endothermic peaks. For the P(L-LA-co-10 mol %-meso-LA) films, two endothermal peaks were detected in the thermograms of samples crystallized at temperatures below 120 °C (see Figure 6C). However, only single melting peak was observed in the DSC thermogram of any PLLA samples crystallized at a temperature ranging from 110 to 160 °C. To examine whether the peak at higher temperature arises from a recrystallization phenomenon during heating process, the DSC curves of melt-crystallized films were recorded at different heating rates of 5-40 °C/min. Figure 7A shows

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Figure 4. Variations in long period (Lp), lamellar thickness (lc), and amorphous thickness (la) as a function of crystallization temperature (Tc) for melt-crystallized P(L-LA-co-meso-LA) films. (A) PLLA, (B) P(L-LA-co-2mol %-meso-LA), (C) P(L-LA-co-4mol %-meso-LA), and (D). P(L-LA-co10mol %-meso-LA). (9): long period (Lp), (b): lamellar thickness (lc), (O): amorphous thickness (la).

Figure 5. Relationship between the lamellar thickness values determined from AFM and from SAXS. ([) PLLA, (b) P(L-LA-co-2mol %-meso-LA), (9) P(L-LA-co-4 mol %-meso-LA), and (2) P(L-LA-co10mol %-meso-LA).

the DSC curves of P(L-LA-co-2 mol %-meso-LA) films crystallized at 150 °C recorded at different heating rates. Two endothermic peaks were detected in all thermograms, and the peak temperatures at two endotherms were unchanged with the heating rate. Also, the peak intensities of two endotherms revealed the same values independent of the heating rate. This result indicates that the peak at higher temperature is not attributed to a recrystallization process and that both the higher and lower endothermic peaks represent the melting of original crystals formed at crystallization temperature of 150 °C. Figure 7B shows the DSC curves of P(L-LA-co-2mol %-meso-LA) films crystallized at 150 °C for different crystallization time. Only one peak was detected in the DSC thermograms of P(L-LA-co-2mol %-meso-LA) films crystallized at 150 °C for 1 day. When the crystallization time exceeded 2 days, P(L-LA-co-2mol %-meso-LA) samples revealed distinct two melting peaks in the DSC curves. The peak intensity of the higher temperature region increased with crystallization time, whereas the lower melting peak became

Figure 6. DSC thermograms of P(L-LA-co-meso-LA) films crystallized at various crystallization temperatures. (A) P(L-LA-co-2mol %-meso-LA), (B) P(L-LA-co-4mol %-meso-LA), and (C) P(L-LA-co-10mol %-meso-LA).


Figure 7. Typical DSC thermograms of P(L-LA-co-2mol %-mesoLA) films crystallized at 150 °C. (A) Sample was crystallized at 150 °C for 3 days. Thermograms were recorded at different heating rates. (B) Samples were crystallized at 150 °C for different crystallization time. Thermograms were recorded at 20 °C/min.

Figure 8. WAXD (A) and SAXS (B) patterns of P(L-LA-co-2mol %-meso-LA) films crystallized at 150 °C for different crystallization time.

to be small. Total amounts of enthalpy of fusion for P(LLA-co-2mol %-meso-LA) samples were around 62 ( 5 J/g and the values remained almost unchanged during isothermal crystallization process. From these results, it is suggested that the crystals with higher melting temperature have been formed during isothermal crystallization process at 150 °C, resulting from the rearrangement of crystals with lower melting temperature. In addition, both the higher and lower melting temperatures rose gradually with crystallization time. Similar trends were observed for the samples of P(L-LAco-4mol %-meso-LA) crystallized at temperatures above 140 °C and P(L-LA-co-10mol %-meso-LA) crystallized at temperatures below 120 °C. In contrast, for the P(L-LA-co-mesoLA) films showing a single melting endotherms in Figure 6, only one melting peak was detected in the DSC thermograms of samples crystallized at a given temperature throughout for all crystallization periods in the range from 6 h to 5 days. Time-dependent changes in crystalline structure and lamellar thickness of P(L-LA-co-meso-LA) samples were examined by using WAXD and SAXS, respectively. Figure 8 shows the WAXD and SAXS patterns of P(L-LA-co-2 mol %-meso-LA) samples crystallized at 150 °C for different crystallization periods. As shown in Figure 8A, the WAXD patterns of all P(L-LA-co-2 mol %-meso-LA) samples were identical, and only the reflections arising from the R-form


of the PLLA crystalline lattice were detected. The height and width of each reflected peak were hardly changed with time, and the values of crystallinity were also the same as 62 ( 3%. From the SAXS data of P(L-LA-co-2 mol %-mesoLA) crystallized for different crystallization periods (Figure 8B), it was found that both the long period distance (Lp) and lamellar thickness (lc) were unchanged with time, and that the values were Lp ) 24.1 ( 0.3 nm and lc ) 15.0 ( 0.2 nm. These results indicate that the formation of crystals with higher melting temperature is proceeded without changes in crystalline structure and crystal thickness. Furthermore, despite the presence of two melting peaks in the DSC curves, the Lp and lc values of P(L-LA-co-meso-LA) samples were almost unchanged with crystallization time. Such phenomenon for crystallization of P(L-LA-co-mesoLA) may be explained by considering following crystallization mechanisms. The P(L-LA-co-meso-LA) molecules form the R-form of PLLA crystal using the long sequential segments of L-lactyl units. Since the D-lactyl units in P(LLA-co-meso-LA) molecules act as defects of crystals, these stereoisomers are normally excluded from the PLLA crystalline region. However, under certain crystallization conditions, small amounts of D-lactyl units may be initially trapped into the crystalline regions, and the thermally unstable crystals are formed during the early stage of primary crystallization process. At the relatively lower crystallization temperature regions, P(L-LA-co-2 mol %-meso-LA) and P(L-LA-co-4 mol %-meso-LA) molecules formed lamellar crystals with a thickness ranging from 7.2 to 12.2 nm and from 6.2 to 9.5 nm, respectively. Considering the 2.78 nm of the fiber repeat distance of the PLLA crystal,37 the average numbers of sequential L-lactyl units in lamellar crystals are in the range from 26 to 44 and from 23 to 35. Under such conditions, P(L-LA-co-2 mol %-meso-LA) and P(L-LA-co-4 mol %-mesoLA) molecules formed stable crystalline lamellae excluding the D-lactyl units. When the P(L-LA-co-2 mol %-meso-LA) and P(L-LA-co-4 mol %-meso-LA) samples were crystallized at higher temperature regions, these molecules formed lamellar crystals with a thickness ranging from 11.8 to 15.0 nm and from 14.0 to 14.9 nm, respectively. The average numbers of sequential L-lactyl units in lamellar crystals are ranged from 43 to 54 and from 51 to 54. Under such conditions for P(L-LA-co-2 mol %-meso-LA) and P(L-LAco-4 mol %-meso-LA) samples, a small portion of randomly distributed D-lactyl units may be trapped into the crystalline regions during the early stage of crystallization process. During further storage of P(L-LA-co-meso-LA) samples at isothermal crystallization temperature, the D-lactyl units trapped in crystalline regions are gradually excluded from crystalline lamellae by the chain relaxation phenomenon along the crystallographic c axis to form the thermally stable crystals. Since the P(L-LA-co-2 mol %-meso-LA) and P(LLA-co-4 mol %-meso-LA) samples contain relative low meso-LA units, the amount of noncrystallizable D-lactyl units trapped into the crystalline region were very small. Under such situation, the exclusion process of D-lactyl units from crystalline region leads the formation of two populations of crystalline region with different thermal stability. As a result, distinct two melting endotherms were observed in the DSC

464


thermograms of P(L-LA-co-2 mol %-meso-LA) and P(L-LAco-4 mol %-meso-LA) samples crystallized at higher temperature regions. It is of interest to note that the two endothermic peaks were detected only in the DSC thermograms of P(L-LA-co10 mol %-meso-LA) crystallized at temperatures below 120 °C. Lamellar thickness of P(L-LA-co-10 mol %-meso-LA) samples crystallized at temperatures below 120 °C was varied from 6.5 to 7.5 nm, and the average numbers of sequential L-lactyl units in lamellar crystals were ranged from 24 to 27. Here, the number average length of sequential L-lactyl units in P(L-LA-co-10 mol %-meso-LA) can be calculated to be 20 assuming that the P(L-LA-co-10 mol %-meso-LA) is a random stereocopolymer. From this result, it is considered that the lamellar crystals of P(L-LA-co-10 mol %-mesoLA) formed at temperatures below 120 °C include the portion of D-lactyl units in the molecules. These D-lactyl units included in crystalline regions are also excluded from crystalline lamellae under isothermal crystallization conditions. At crystallization temperatures above 130 °C, the P(LLA-co-10 mol %-meso-LA) molecules formed more thick lamellar crystals. From the lamellar thickness data, the average numbers of sequential L-lactyl units in lamellar crystals formed at temperatures above 130 °C were calculated to be larger than 38. In the lamellar crystals of P(L-LA-co10 mol %-meso-LA) formed at temperatures above 130 °C, large amounts of D-lactyl units were trapped. Most of trapped D-lactyl units could not be excluded from crystalline lamellae under isothermal crystallization conditions. As a result, a single melting peak may be detected in the DSC thermograms of P(L-LA-co-10 mol %-meso-LA) samples crystallized at temperatures above 130 °C. The trapped D-lactyl units into crystalline lamellae act as defects of the crystalline region, and the unstable regions with irregular chain-packing are developed in the crystalline lamellae. It can be expected that the overgrowth lamellar crystals are derived from such irregular crystalline regions. The number of irregular region increases with the fraction of trapped D-lactyl units. As a result, the number of overgrowth crystals increased with an increase in the mesoLA composition of P(L-LA-co-meso-LA) (see Figure 1). As shown in Figure 7B, the higher melting peak shifted toward higher temperature regions with time during the storage at isothermal conditions. However, the thickness of crystalline lamellae formed at a given temperature remained unchanged during the isothermal storage (see Figure 8B). The temperature shift of melting peak during the storage at isothermal conditions was also observed for the P(L-LA-comeso-LA) samples revealed a single melting endotherm. Furthermore, it was confirmed that the melting temperature of PLLA samples shifted toward higher temperature regions with crystallization time without thickening of lamellar crystal. From these results, it is suggested that the rearrangement at the folding surface structure also take place during the isothermal storage to reduce the folding surface free energy of lamellar crystals. The fold surface rearrangement may progresses successively after the exclusion process of the crystalline defects. As a result, the higher melting

Abe et al.

Figure 9. Spherulite growth rates for P(L-LA-co-meso-LA) samples with different meso-LA composition at various crystallization temperatures. ([) PLLA, (b) P(L-LA-co-2mol %-meso-LA), (9) P(L-LA-co4mol %-meso-LA), and (2) P(L-LA-co-10mol %-meso-LA).

temperature shifted toward higher temperature regions with crystallization time. Kinetic Analysis of P(L-LA-co-meso-LA)Crystal Growth. P(L-LA-co-meso-LA) thin films with a thickness of 100 nm were crystallized isothermally from the melt state at 200 °C and viewed under phase-contrast in the optical microscope. Spherulitic morphology similar to the AFM images was observed in thin films crystallized at lower temperature regions, whereas the hexagonal crystals were formed in the thin films crystallized at higher temperature regions. The P(LLA-co-meso-LA) spherulite radius increased linearly with time during isothermal crystallization from the melt. The crystal growth rates (G) of P(L-LA-co-meso-LA) spherulites were derived from the slopes of the lines obtained by plotting the spherulite radius against time. For the hexagonal lamellar crystals, the distance from the crystal center to the vertexes of the hexagonal crystal along the crystallographic b axis was measured with time. Figure 9 shows the rates of crystal growth (G) of P(L-LA-co-meso-LA) thin films (around 100 nm) with different meso-LA composition at various crystallization temperatures. The rates of crystal growth were dependent on both the meso-LA composition and the crystallization temperature. The crystal growth rate of P(LLA-co-meso-LA) crystals formed at identical temperature decreased as the meso-LA composition was increased from 0 to 10 mol %. Based on the nucleation theory established by Hoffman et al.,38 the crystal growth rate, G, at crystallization temperature Tc can be expressed by the following equation:

{

G ) G0 exp -

} {

Kg U* exp Tc∆Tf R(Tc - T∞)

}

(1)

where Kg is the nucleation constant, Tc is the crystallization temperature, ∆T is the undercooling T0m - Tc where T0m is the equilibrium melting temperature, f is the factor expressed by 2Tc/(T0m + Tc) that accounts for the change in heat of



fusion as the temperature is decreased below T0m, U* is the activation energy for transportation of segments to the crystallization site, R is the gas constant, T∞ is the hypothetical temperature where all motions associated with viscous flow ceases, and G0 is the front constant. The nucleation parameter Kg has variable values depending on the crystallization regime, and is given by nbσsσeT0m Kg ) k∆H

(2)

where b is the layer thickness, σs is lateral surface free energy, σe is folding surface free energy, ∆H is heat of fusion per unit volume, and k is the Boltzman constant. The value of n is dependent on the regime of crystallization, and the n values are theoretically as 4, 2, and 4 for regimes I, II, and III, respectively. As shown in eqs 1 and 2, the equilibrium melting temperatures (T0m) is one of the most important parameters for kinetic analyses of the crystal growth rate. To determine the equilibrium melting temperature, the Hoffman-Weeks method has been widely used. The Hoffman-Weeks method involves linear extrapolation of experimental melting temperatures (Tm) observed for various crystallization temperatures (Tc) toward the equilibrium line Tm ) Tc.38 However, the experimental melting temperatures of isothermally crystallized P(L-LA-co-meso-LA) samples varied with crystallization time. Therefore, the equilibrium melting temperature of P(L-LA-co-meso-LA) samples cannot be estimated from a conventional Hoffman-Weeks method. In our previous study,39 we have estimated the T0m value of PLLA from the experimentally observed melting temperatures according to the method of Marand et al.40,41 In their method, it is assumed that the thickening of lamellar crystals cause a shift of observed melting temperature during isothermal crystallization process and that the thickening coefficient is varied with crystallization temperature. Therefore, by employing the melting of nonthickened lamellar crystals to eliminate the effect of isothermal thickening process on experimentally observed melting temperature, the equilibrium melting temperature is extrapolated from the nonlinear relationship between crystallization temperature (Tc) and melting temperature of the nonthickened lamellar crystals (T′m). By using this method, the equilibrium melting temperature of PLLA was determined to be T0m ) 227.1 °C. However, as shown in Figures 7B and 8B, the thickness of crystalline lamellae formed at a given temperature was unchanged during the isothermal storage, whereas the melting temperature shifted toward higher temperature regions with time. We suggest that the shift of observed melting temperature with time is owing to both the exclusion of the crystalline defects and the rearrangement of the folding surface structure. As mentioned above, the P(L-LA-co-mesoLA) crystals including the crystalline defects were formed under the certain conditions, and the exclusion process of crystalline defects leaded the bimodal melting peaks in the DSC thermograms. On the other hand, the rearrangement of the folding surface structure took place for each P(L-LA-

Figure 10. Estimation of equilibrium melting temperatures of P(L-LA-co-meso-LA) samples. (A) Plots of the observed melting temperature (Tm) against the crystallization time for P(L-LA-co-2mol %-meso-LA) sample. (B) Relationship between the melting temperatures of pseudo-equilibrium crystal (T′′m) and crystallization temperature (Tc). ([) PLLA, (b) P(L-LA-co-2mol %-meso-LA), (9) P(L-LAco-4mol %-meso-LA), and (2) P(L-LA-co-10mol %-meso-LA). Open symbols refer to the T′′m values detected at higher temperature regions for the samples with two melting endotherms.

co-meso-LA) sample during isothermal storage. Therefore, we have applied a new method to evaluate the T0m values from the experimentally observed melting temperatures mainly elucidating the effect of surface rearrangement as follows. In this model, the melting temperature of pseudoequilibrium lamellar crystals, which are formed at a given crystallization temperature after the crystallization time reached infinity, was employed. The pseudo-equilibrium lamellar crystals have relatively ordered folding surface. Figure 10A shows the observed melting temperature of P(L-LA-co-meso-LA) crystallized at various crystallization temperatures as a function of time. As shown in Figure 10A, the observed melting temperature rose with time during the isothermal crystallization process for a given crystallization temperature. The plot of observed melting temperature

466


Abe et al.

Table 3. Crystal Growth Kinetic Data of Poly(lactide) Stereocopolymers

G0 , µm/min

KgI × 10-5, K2

KgII × 10-5, K2

KgIII × 10-5, K2

KgI/KgII

TtI/II, °C

KgIII/KgII

sample

TtII/III, °C

P(L-LA-co-2mol %-meso-LA) P(L-LA-co-4mol %-meso-LA) P(L-LA-co-10mol %-meso-LA) PLLA

17.0 × 109 16.8 × 109 13.4 × 109 20.8 × 109

4.52 4.37 2.36 6.32

2.41 2.38 1.28 3.46

4.78 4.58 2.33 7.01

1.88 1.83 1.84 1.82

145 142 136 147

1.98 1.92 1.82 2.03

118 113 110 120

against inverse crystallization time shows the linear relationship. The melting temperature of pseudo-equilibrium lamellar crystals (T′′m) at a given crystallization temperature was extrapolated from the intercept of the line. Then, the equilibrium melting temperatures of P(L-LA-co-meso-LA) samples were calculated from the extrapolated melting temperatures (T′′m) at various crystallization temperatures (Tc) by using Hoffman-Weeks method. Figure 10B shows the evaluation of the extrapolated melting temperature of crystals with the crystallization temperature for P(L-LA-comeso-LA). The T0m values of all P(L-LA-co-meso-LA) samples are listed in Table 1. The T0m value of P(L-LA-co-mesoLA) samples decreased from 215 to 184 °C as the meso-LA composition was increased from 0 to 10 mol %. Crystal growth rate data were analyzed using the growth rate expression. For the kinetic analysis of crystal growth, the universal empirical values of U* ) 1500 cal/mol and T∞ ) Tg - 30 K were used for the analysis.38 Figure 11 shows the plots of ln G + 1500/R(Tc - Tg + 30) against 1/Tc∆Tf for P(L-LA-co-meso-LA) samples with different meso-LA composition. The plot of ln G + 1500/R(Tc Tg+30) against 1/Tc∆Tf will give Kg as the slope and the intercept being lnG0. The Kg and G0 values were calculated from the plots, and the values are listed in Table 3. The KgII value decreased from 3.46 × 10-5 K2 to 1.28 × 10-5 as the meso-LA composition was increased from 0 to 10 mol %. In contrast, the G0 value decreased with an increase in the meso-LA composition of P(L-LA-co-meso-LA). For all P(LLA-co-meso-LA) samples, both the transition temperatures from regime III to regime II and from regime II to regime I were detected. The transition temperatures from regime III to regime II were detected at around 120, 118, 113, and 110 °C for the P(L-LA-co-meso-LA) samples with meso-LA content of 0, 2, 4, and 10 mol %, respectively. In addition, the transition temperatures from regime II to regime I were detected at around 147, 145, 142, and 136 °C for the copolymers with meso-LA content of 0, 2, 4, and 10 mol %, respectively. In both regime changes, the transition temperatures slightly decreased with an increase in the mesoLA composition. Moreover, the transition temperature from regime II to regime I of each P(L-LA-co-meso-LA) samples was very closed to the temperature region revealed the morphological changes in the crystalline aggregates from the spherulitic aggregates to hexagonal lamellae stacking (see Figure 1). Conclusions For each P(L-LA-co-meso-LA) sample, spherulitic morphology was observed in the thin films crystallized at lower temperature region, whereas the hexagonal crystals were

Figure 11. Plots of ln G + 1500/R(Tc - Tg + 30) against 1/Tc∆Tf for P(L-LA-co-meso-LA) samples with different meso-LA composition. (A) PLLA, (B) P(L-LA-co-2mol %-meso-LA), (C) P(L-LA-co-4mol %-meso-LA), and (D) P(L-LA-co-10mol %-meso-LA).

formed in the thin films crystallized at higher temperature region. From the AFM observation combined with enzymatic etching treatment with proteinase K, the accurate stacking structure of lamellar crystals was investigated for the isothermally crystallized P(L-LA-co-meso-LA) thin films. It was found that the lamellar thickness (lc) determined from AFM was in good agreement with the value from smallangle X-ray scattering measurement for each sample. Based on the results of time-dependent changes in lamellar structure and melting temperature of isothermally crystallized samples, it is suggested that the P(L-LA-co-meso-LA) molecules normally forms the PLLA crystalline lamellae of L-lactyl units, whereas that under the certain crystallization condition,


small amount of D-lactyl units is trapped or included into the crystalline regions during early stage of crystallization process. The D-lactyl units trapped in PLLA crystalline regions are gradually excluded from crystalline lamellae to form the thermally stable crystals during further isothermal storage at a crystallization temperature. As a result, in the DSC thermograms of several P(L-LA-co-meso-LA) samples, distinct two melting peaks were detected. The crystallization condition forming the crystals including D-lactyl units is strongly dependent on the temperature and meso-LA contents responsible for the lamellar thickness and the segmental length of sequential L-lactyl units. Kinetic analysis of crystal growth of P(L-LA-co-meso-LA) demonstrated the existence of transitions of crystalline regime both from regime III to regime II and from regime II to regime I. Acknowledgment. This work was supported in part by a grant for Ecomolecular Science Research provided by RIKEN Institute and by the SORST (Solution Oriented Research for Science and Technology) grant from Japan Science and Technology Agency (JST). References and Notes (1) Kohn, F. E.; Van der Berg, W.; Ridder, G.; Feijen, J. J. Appl. Polym. Sci. 1984, 29, 4265. (2) Leenslag, J. W.; Pennings, A. J. Makromol. Chem. 1987, 188, 1909. (3) Kricheldorf, H. R.; Sumbe¨l, M. Eur. Polym. J. 1989, 25, 585. (4) Dubois, P.; Jacobs, C.; Je´roˆme, R.; Teyssie, P. Macromolecules 1991, 24, 2266. (5) Ajioka, M.; Enomoto, K.; Suzuki, K.; Yamaguti, A. Bull. Chem. Soc. Jpn. 1995, 68, 2125. (6) Libiszowski, J.; Kowalski, A.; Duda, A.; Penczek, S. Macromol. Chem. Phys. 2002, 203, 1694. (7) Tsuji, H. Biopolymers, 4, Polyesters; Doi Y.; Steinbu¨chel A., Eds.; WILLEY-VCH Verlag GmbH: Weinheilm, Germany, 2002; p 129. (8) Jackanicz, T. M.; Nash, H. A.; Wise, D. L.; Gregory, J. B.Contraception 1973, 8, 227. (9) Arshady, R. J. Controlled Release 1991, 17, 1. (10) Penning, J. P.; Dijkstra, H.; Pennings, A. J. Polymer 1993, 34, 942. (11) Vainionpaa, S.; Rokkamen, P.; Tormala, P. Prog. Polym. Sci. 1989, 14, 679. (12) Ikada, Y.; Shikinami, Y.; Hara, Y.; Tagawa, M.; Fukuda, E. J. Biomed. Matter. Res. 1996, 30, 553.

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Crystal Growth and Solid-State Structure of Poly(lactide

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