Synthesis, Solid-State Structure, and Surface Properties of End

Biomacromolecules 2004, 5, 530-536

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Synthesis, Solid-State Structure, and Surface Properties of End-Capped Poly(L-lactide) Yuka Kobori,† Tadahisa Iwata,† Yoshiharu Doi,†,‡ and Hideki Abe*,†,‡ 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 226-8502, Japan Received September 29, 2003; Revised Manuscript Received December 4, 2003

End-capped poly(L-lactide) (PLLA) samples with dodecyl or 2-(2-(2-methoxyethoxy)ethoxy)ethyl (MEEE) ester were synthesized by ring-opening polymerization of L-lactide in the presence of zinc dodecanoxide or zinc 2-(2-(2-methoxyethoxy)ethoxy)ethoxide as a catalyst, respectively. On the basis of NMR analysis, it was confirmed that the carboxylic acid chain ends of PLLA molecules were selectively substituted by dodecyl or MEEE ester groups. To evaluate the wettability on the surface of end-capped PLLA films, the advancing contact angle (θa) with water was measured. The amorphous PLLA films showed relatively similar θa values regardless of the chemical structure of the polymer chain end. In contrast, the θa values of semicrystalline films were varied over a wide range, dependent on the chemical structure of the chain end. In addition, the θa values of dodecyl ester end-capped PLLA film with low molecular weight increased with an increase in the crystallization temperature. Both the crystallinity and lamellar thickness of dodecyl ester end-capped PLLA films increased with the crystallization temperature. These results suggest that the segregation of the chain ends on the PLLA film surface was strongly affected by the crystallization conditions. Introduction Poly(L-lactide) (PLLA) is synthesized from either lactic acid1,2 or its cyclic dimer (L-lactide)3-9 and is a biodegradable and biocompatible thermoplastic with a melting point around 180 °C.10 PLLA has been investigated as a material for medical devices such as controlled drug release matrixes11 and degradable sutures12 and has been implanted for bone fixation.13 Because of all of the potential applications for PLLA in biomedicine, it is important to regulate the surface properties of PLLA materials, since these are strongly related to the protein adsorption associated with biocompatibility. To obtain polymeric materials with desired surface properties, both physical and chemical modification techniques have been applied to many types of polymer systems. It is wellknown that the component of a low surface free energy in multicomponent polymeric systems is preferentially concentrated at the air surface region in order to minimize the air/ material interfacial free energy. The most common techniques to modify the surface properties of polymeric materials include blending,14 copolymerization,15 grafting,16 coating, and plasma treatment. Although blending, copolymerization, and grafting are attractive methods to modify surface properties of PLLA, these methods also cause radical changes in the bulk properties, such as melting temperature and crystallinity of PLLA. End-capping with functional groups for polymers is a good technique to modify the surface properties while limiting the * To whom correspondence should be addressed. E-mail: habe@postman. riken.go.jp. Phone: +81-48-467-9404. Fax: +81-48-462-4667. † RIKEN Institute. ‡ Tokyo Institute of Technology.

changes to the bulk properties of polymeric materials. Lee et al.17 have reported that the film surface of end-capped PLLA with short fluorocarbon was segregated by the short fluorocarbon segments and that the thermal properties of fluorocarbon end-capped PLLA were similar to those of nonend-capped PLLA. Recently, many types of PLLA samples having various end groups, such as R-glucose,18 lactose,19 and aminoethanol,20 have been synthesized to vary the surface wettability of PLLA films. PLLA molecules form crystalline regions consisting of chain-folded lamellar crystals and amorphous regions. Most of the chain end groups are excluded from the lamellar crystals during crystallization, since these end groups migrate to the lamellar surface and amorphous regions. The solidstate structure of PLLA materials, such as the crystallinity, size, and thickness of lamellar crystals, is strongly dependent on the crystallization conditions, for example, crystallization temperature and crystallization time. Therefore, it is presumed that the surface properties of PLLA with functional chain ends can vary with the crystallization conditions due to the differences in solid-state structure. However, there have been few reports on the influence of the crystallization conditions on the surface properties of PLLA materials. In this study, we prepared PLLA samples end-capped at the carboxylic acid chain ends with two types of functional groups, dodecyl ester or 2-(2-(2-methoxyethoxy)ethoxy)ethyl (MEEE) ester, either the hydrophobic compound or the hydrophilic compound. The solid-state structure of endcapped PLLA samples was characterized by differential scanning calorimetry, wide-angle X-ray diffraction, and atomic force microscopy. In addition, the surface wettability

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of the end-capped PLLA samples was evaluated by contact angle measurements, and the influence of functional groups at the PLLA chain end on the surface properties is discussed. Experimental Section Preparation of Zinc-Based Catalysts. Diethylzinc/water (ZnEt2/H2O) (1/0.6) catalyst was prepared by a reported method.21 ZnEt2 was reacted with deoxygenated water at a molar ratio of 1/0.6 (ZnEt2/H2O) in dry 1,4-dioxane (distilled over Na), followed by freeze-drying of the reaction mixture. A yellow powder was obtained and used as a catalyst. Zinc didodecanoxide (Zn(OD)2) and zinc di(2-(2-(2methoxyethoxy)ethoxy)ethoxide) (Zn(OMEEE)2) catalysts were prepared as follows: ZnEt2 was allowed to react with n-dodecanol or 2-(2-(2-methoxyethoxy)ethoxy)ethanol (tri(ethyleneglycol) monomethyl ether) at a molar ratio of 1/2 (ZnEt2/alcohol) in 1,2-dichloroethane at room temperature for 5 h, followed by freeze-drying of the reaction mixture. White powders were obtained and used as reaction catalysts. Synthesis of Polymers. L-Lactide (L-LA) (Puronic Co.) was recrystallized from toluene solution isothermally at 65 °C. Poly(L-lactide) (PLLA) samples with dodecyl ester or 2-(2-(2-methoxyethoxy)ethoxy)ethyl ester chain ends at the carboxylic acid termini were synthesized by the ring-opening polymerization of L-LA in the presence of Zn(OD)2 or Zn(OMEEE)2 as a catalyst, respectively. The monomer, catalyst and 1,2-dichloroethane were placed into a reactor under nitrogen. The polymerization was carried out at 40 °C for several hours under nitrogen. The polyester produced was dissolved in chloroform and precipitated in methanol. The precipitate was dried in vacuo at room temperature. Nonend-capped PLLA samples were synthesized by the ring-opening polymerization of L-LA in the presence of ZnEt2/H2O catalyst. The monomer, ZnEt2/H2O catalyst, and 1,2-dichloroethane were placed into a reactor under nitrogen. The polymerization was carried out at 40 °C for several hours under nitrogen. PLLA thin films of 2-3 µm thickness were initially prepared by solvent-cast technique from chloroform solutions of polymers. Droplets of 10 µL of polymer solution (1.0% (w/v)) were placed on one glass substrate (substrate dimensions: 18 × 18 mm). The polyester layer on the substrate was heated on the hot-stage (Linkam LK-600PM) at 200 °C. The sample was maintained at 200 °C for 30 s, and then the temperature was rapidly lowered to a given temperature. The semicrystalline film was prepared by isothermal crystallization at the given crystallization temperature (Tc) ranging from 80 to 150 °C for 48 h. The amorphous film was obtained by quenching of the melt sample in the atmosphere at room temperature. Analytical Procedures. All molecular weight data were obtained by size exclusion chromatography (SEC) at 40 °C, using a Shimadzu 10A GPC system and 10A refractive index detector with Shodex K-800M and K-802 columns. Chloroform was used as an eluent at a flow rate of 0.8 mL/min, and a sample concentration of 1.0 mg/mL was applied. Polystyrene standards with a low polydispersity were used to make a calibration curve.

1

H NMR analysis of end-capped PLLA samples was carried out on a JEOL R-400 spectrometer. The 400 MHz 1 H NMR spectra were recorded at 23 °C in a CDCl3 solution of polymer (5 mg/mL) with a 5.5 µs pulse width (45° pulse angle), 5 s pulse repetition, 8,000 Hz spectral width, and 16 K data points. Differential scanning calorimetry (DSC) data of PLLA samples were recorded in the temperature range of 0-200 °C on a Perkin-Elmer Pyris 1 equipped with a cooling accessory under a nitrogen flow of 20 mL/min. Samples of 3 mg were encapsulated in aluminum pans and heated from 0 to 200 °C at a rate of 20 °C/min. The melting temperature (Tm) and enthalpy of fusion (∆Hm) were determined from the DSC endotherms. The Tm was taken as the peak temperature. For the measurement of the glass transition temperature (Tg), the samples were maintained at 200 °C for 1 min and then rapidly quenched to 0 °C. They were heated from 0 to 200 °C at a heating rate of 20 °C/min. The Tg was taken as the midpoint of the change in heat capacity. The X-ray diffraction patterns of PLLA films were recorded at 23 °C on a Rigaku RINT2500 system using nickel-filtered Cu KR radiation (λ ) 0.154 nm; 40 kV; 110 mA) in the 2θ range 6-60° at a scan speed of 2.0°/min. Wettability of the PLLA film surface was estimated by the advancing contact angle (θa) measurement with water, using a FACE Contact Angle Meter CA-X (Kyowa Interface Science Co.). Distilled water (pharmaceutical injectable grade) was purchased from Fuso Pharmaceutical Industries, Ltd. and used without further purification. The θa value was averaged on at least 10 data obtained at different points on the film surface. Atomic force microscopy (AFM) was performed with a SPI3800/SPA400 (Seiko Instruments Inc.). Pyramid-like SiN4 tips, mounted on 200 µm length microcantilevers with spring constants of 0.2 N/m were applied for the contact mode experiments. Simultaneous registration was performed to determine the height and deflection image of the sample. Results and Discussion Synthesis of End-Capped PLLA Samples. End-capped PLLA samples were synthesized by the ring-opening polymerization of L-LA in the presence of zinc-based catalyst at 40 °C. Zn(OD)2 or Zn(OMEEE)2 were used as catalysts to prepare the PLLA with dodecyl or MEEE ester chain ends, respectively. To obtain PLLA samples with narrow polydispersity, the reaction was stopped during the course of the polymerization process. The number-average molecular weight (M(SEC) ) and polydispersity (Mw/Mn) of the obtained n PLLA samples were determined by SEC measurements. The results are shown in Table 1. All of the end-capped PLLA samples had a relatively narrow polydispersity (Mw/Mn ) 1.18-1.29). The M(SEC) values of PLLA samples ranged n from 8000 to 39 000, depending on both the monomer/ catalyst ratio and the polymer yield. Assuming that the polymerization reaction took place at all points of zinc) alkoxide linkage in catalyst, the molecular weight (M(calc) n of the obtained samples was calculated from the monomer/ catalyst ([L-LA]0/[Zn]) ratio and the polymer yield as

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Table 1. Polymerization Conditions of L-Lactide with Zinc-Based Catalysts and Molecular Weights of End-Capped PLLA Samples sample entry 1a entry 2 entry 3 entry 4 entry 5b entry 6c

PLLA-D4K PLLA-D6K PLLA-D11K PLLA-D19K PLLA-MEEE5K PLLA-5K

[L-LA]0/[Zn]

reaction time (h)

polymer yield (%)

DSd

e M(SEC) n

Mw/Mn f

g M(calc) n

h M(NMR) n

92 92 246 246 190 15

12 20 28 72 12 48

67 78 69 95 33 70

0.95 0.95 0.93 0.95 0.94

7800 11 000 25 000 39 000 8000 9400

1.26 1.21 1.19 1.18 1.29 1.76

4400 5200 13 000 17 000 4500

4000 5500 11 000 19 000 5400 5300

a Entry 1-4 were synthesized by polymerization of L-LA with zinc dodecanoxide as a catalyst. Polymerization conditions; reaction temperature at 40 °C, in CH2Cl2. b Entry 5 was synthesized by polymerization of L-LA with zinc 2-(2-(2-methoxyethoxy)ethoxy)ethoxide as a catalyst. Polymerization conditions; reaction temperature at 40 °C, in CH2Cl2. c Entry 6 was synthesized by polymerization of L-LA with diethylzinc/water as a catalyst. Polymerization conditions; reaction temperature at 40 °C, in CH2Cl2. d Degree of substitution with functional groups at the carboxylic acid chain end was estimated from 1H NMR spectra. e The number-average molecular weight was determined by SEC analysis using a polystyrene standards. f The polydispersity was determined by SEC analysis. g The number-average molecular weight was calculated from [L-LA]0/[Zn] ratio and polymer yield. h The number-average molecular weight was determined from 1H NMR spectra.

presented by following equations: DP )

[L-LA]0 polymer yield (%) 100 2[Zn]

) DP × ML-LA M(calc) n where DP and ML-LA represent the degree of polymerization and molar mass of L-LA (ML-LA ) 144 g/mol), respectively. values of end-capped PLLA Table 1 also lists the M(calc) n samples. As shown in Table 1, the M(calc) values of endn values. It has capped PLLA samples were lower than M(SEC) n been reported that such disagreement between M(SEC) and n (calc) results from the difference of the exclusion volume Mn of PLLA and polystyrene of the same molar mass.22 Baran et al.23 reported that the pertinent correction factor to convert based on polystyrene to the actual values for the M(SEC) n PLLA was equal approximately to 0.58. In this study, the and M(SEC) ranged from 0.44 to 0.56, and ratios of M(calc) n n these values were relatively consistent with the reported value of the correction factor. Figure 1 shows the typical 1H NMR spectra of end-capped PLLA samples with dodecyl ester (entry 1) and MEEE ester (entry 5) together with the chemical shift assignment for each proton resonance. In Figure 1A, peaks (a) (1.59 ppm) and (b) (5.17 ppm) were assignable to methyl and methine proton resonances of lactic acid in main chain. Peaks (d) (4.35 ppm) and (e) (4.13 ppm) were assigned to the methine proton in the hydroxyl terminal lactic acid unit and the methylene proton in the dodecyl ester group, respectively. From the peak intensities of the methine proton at the hydroxyl-end and the methylene proton of the dodecyl ester, the molar ratio of thye dodecyl ester end group to the hydroxyl end group was calculated as 0.95. It can be concluded that the majority of the carboxylic acid chain ends of the PLLA molecules were substituted by the dodecyl ester group. The ) of end-capped number-average molecular weights (M(NMR) n PLLA samples were determined from the peak intensities of the methine proton (d) at the hydroxyl end and the main chain methine proton (b) in the 1H NMR spectra. The M(NMR) values were consistent with the M(calc) values (see n n Table 1). These results indicate that all of the Zn-OD groups act as active sites of polymerization of L-LA and that chain transfer reactions rarely occurred during the polymerization reaction.

Figure 1. 1H NMR spectra of PLLA samples end-capped with dodecyl ester (entry 1) (A) and with MEEE ester (entry 5) (B).

In Figure 1B, the major peaks at 1.59 and 5.17 ppm were assigned to the methyl and methine proton resonances of the lactic acid unit in the main chain. Peak (d) (4.36 ppm) was assigned to the methine proton in the hydroxyl terminal lactic acid unit. Peaks (i) (3.39 ppm) and (e-h) (3.50-4.30 ppm) were assigned to the methoxy and ethoxy protons in the MEEE ester group, respectively. The M(NMR) value of n MEEE ester end-capped PLLA was determined from the peak intensities of the methoxy proton (i) at the MEEE ester end group and the main chain methine (b) proton in the 1H NMR spectrum. These values were consistent with the


End-Capped Poly(L-lactide) Table 2. Thermal Properties of End-Capped PLLA Samples sample

Tga (°C)

Tmb (°C)

∆Hmc (J/g)

PLLA-D4K PLLA-D6K PLLA-D11K PLLA-D19K PLLA-MEEE5K PLLA-5K

48 47 51 54 43 45

160 162 169 178 150 164

58 60 51 60 45 65

a Glass-transition temperature; measured by DSC (second scan) from 0 to 200 °C at a rate of 20 °C/min. b Melting temperature; measured by DSC (first scan) from 0 to 200 °C at a rate of 20 °C/min. c Enthalpy of fusion; measured by DSC (first scan).

M(calc) value. These results indicate that all of Zn-OMEEE n groups also act as active sites for the polymerization of L-LA. Thermal Properties and Crystalline Structure of EndCapped PLLA Samples. The glass-transition temperature (Tg), melting temperature (Tm), and enthalpy of fusion (∆Hm) of end-capped PLLA samples were determined from DSC thermograms. The results are shown in Table 2, together with the values of nonend-capped PLLA. The Tg of PLLA with dodecyl ester ends ranged from 47 to 54 °C, and increased with an increase in molecular weight. The Tm values of PLLA samples with dodecyl ester end increased from 160 to 178 °C as the Mn(NMR) value was increased from 4000 to 19 000. The ∆Hm values were 51 to 60 J/g. The Tg, Tm, and ∆Hm values of PLLA samples with dodecyl ester ends were nearly identical with those of nonend-capped PLLA with similar molecular weights. For the PLLA sample with MEEE ester ends, the Tg, Tm, and ∆Hm values were 43 °C, 150 °C and 45 J/g, respectively, and these values were appreciably lower than those of the nonend-capped PLLA sample. It has been reported that PLLA and poly(ethylene oxide)s with low molecular weights were miscible in the amorphous region and that the Tg and Tm values of PLLA components were depressed by blending with poly(ethylene oxide).24-26 Therefore, it is suggested that the MEEE ester groups at the chain ends are miscible with the PLLA main chain and acts as a plasticizer of PLLA to prevent the crystallization of PLLA main chain. The crystalline structures of end-capped PLLA samples were characterized by wide-angle X-ray diffraction. Figure 2 shows the typical X-ray powder patterns of end-capped PLLA samples. As shown in Figure 2, diffraction patterns of all end-capped PLLA samples showed reflections arising from the R-form of the PLLA crystalline lattice.27 This result indicates that the introduction of functional groups at the carboxylic acid chain end of PLLA has little effect on the formation of the PLLA crystalline region and that nearly all of the chain ends may be excluded from the crystalline region of PLLA. Wettability of End-Capped PLLA Films. Wettability on the film surface for PLLA samples was examined by measuring the advancing contact angle (θa) with water. It was expected that the wettability of the film surface would be affected by the roughness of the film surface. To minimize the effect of roughness of the film surface, PLLA thin films of 2-3 µm thickness were used for contact angle measurements. Amorphous films of PLLA samples were prepared by quenching from the melt at room temperature, whereas

Figure 2. Wide-angle X-ray powder patterns of end-capped PLLA samples. (A) PLLA-D4K, (B) PLLA-MEEE5K.

Figure 3. Advancing contact angles (θa) with water on the surface of PLLA thin films.

the semicrystalline films of PLLA samples were obtained by isothermal crystallization at 135 °C from the melt. Both the melt-quenched amorphous films and melt-crystallized semicrystalline films were used for contact angle measurements. Figure 3 shows the advancing contact angle (θa) with water for PLLA films. The amorphous film of the nonend-capped PLLA sample (PLLA-5K) showed a θa value of 88 ( 1°, whereas the θa value of semicrystalline film was 94 ( 2°. A slight increase in the θa value by the isothermal crystallization may be attributed to the formation of a hydrophobic PLLA crystalline region. For the dodecyl ester end-capped PLLA samples with different molecular weights, the θa values of amorphous films ranged from 85 to 92°, and the value tended to decrease with an increase in molecular weight. The isothermal crystallization of film at 135 °C resulted in an increase of θa value for all PLLA samples with a dodecyl ester end. The θa values of semicrystalline films ranged from 93 to 112°, and these values also tended to decrease with an increase in molecular weight. The gap in θa values between the amorphous and semicrystalline films decreased with an increase in the molecular weight. When the gaps in θa values were compared to PLLA samples with similar molecular weights, the value of the dodecyl ester endcapped PLLA sample (PLLA-D4K) was much larger than that of the nonend-capped PLLA sample (PLLA-5K). This

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Figure 4. Advancing contact angles (θa) of PLLA films crystallized at various temperatures. (O): PLLA-5K, (b): PLLA-D4K.

result indicates that the addition of the dodecyl esters at the PLLA chain ends enhances the hydrophobicity on the film surface. The θa value of the amorphous film for the MEEE ester end-capped PLLA sample (PLLA-MEEE5K) was 86 ( 3°, and this value was nearly identical to that of PLLA5K. However, the θa value of the PLLA-MEEE5K film decreased to 78 ( 4° by isothermal crystallization at 135 °C. This result indicates that the surface of the semicrystalline PLLA film became hydrophilic by the introduction of MEEE esters. From these results, it can be concluded that the chain ends of polymers are distributed homogeneously in the amorphous film. However, the chain ends are segregated to the film surface after the crystallization. As a result, the surface wettability of the melt-crystallized PLLA film is reflected by the hydrophilicity of the compounds substituted at the PLLA chain end. In addition, the wettability of the film surface varied with the molecular weight of PLLA due to the changes in the number of end-capped chain-end groups. Surface Structure and Properties of End-Capped PLLA Films. To further investigate the correlation between surface structure and properties of end-capped PLLA samples, the PLLA-D4K were melt-crystallized at different temperatures and characterized by various techniques. Figure 4 shows the θa values of melt-crystallized PLLA-D4K films crystallized at different temperatures, together with the data of PLLA-5K films. The θa values of PLLA-D4K films crystallized at temperatures below 125 °C ranged from 92 to 97 °. The θa values of melt-crystallized films of PLLAD4K revealed a distinct change from 97 ° to 115 ° in the 125-130 °C regions of the crystallization temperature. The θa values of films crystallized at temperatures above 130 °C were nearly identical (115°), and the value was relatively similar to that of the polyethylene film.28 In contrast, the θa of melt-crystallized nonend-capped PLLA-5K films was nearly identical (90 ( 7°) throughout the crystallization temperature (110-145 °C). The melting temperature and crystallinity of the PLLA films crystallized at various temperatures was determined by DSC measurement. Figure 5A shows the melting temperature of the PLLA films isothermally crystallized at various temperatures. The melting temperature of PLLAD4K tended to increase with an increase in the crystallization

Figure 5. Melting temperature and crystallinity of PLLA films crystallized at various temperatures. (O): PLLA-5K, (b): PLLA-D4K.

Figure 6. AFM deflection images for PLLA-D4K films crystallized at various temperatures. (A) Tc ) 80 °C, (B) 125 °C, (C) 135 °C, and (D) 145 °C.

temperature. The melting temperature of PLLA-5K also increased from 158 to 175 °C. As shown in Figure 5B, the crystallinity of PLLA-D4K films increased from 42 to 60% as the crystallization temperature increased from 110 to 150 °C. The crystallinity of PLLA-5K films also increased from 52 to 70% with a rise in crystallization temperature. Therefore, this sample achieved higher crystallinity than that of PLLA-D4K film crystallized at the same temperature. The features of the crystalline surface for PLLA thin films were characterized by AFM. Figure 6 shows the typical AFM deflection images of the surface for the PLLA-D4K film isothermally crystallized at various temperatures. At a low magnification, the spherulitic morphologies of PLLA were observed. As shown in Figure 6 (parts A and B), both the flat-on and edge-on lamellar crystals were formed in PLLA

End-Capped Poly(L-lactide)


crystallization temperature is due to the decrease in the amount of chain-folding in lamellar crystals. In the case of PLLA-D4K films crystallized at a lower temperature region, most of the dodecyl ester chain ends may be dispersed in the chain-folding region of the crystal surface or amorphous region on the film surface. In contrast, the dodecyl ester chain ends may be concentrated in the facile region on the top of film surface for PLLA-D4K films crystallized at higher temperatures. Such differences in the distribution of chain ends on the crystalline surface due to crystalline morphology can also affect the wettability of semicrystalline films of PLLA end-capped with functional groups. Conclusion Figure 7. Lamellar periodicities of PLLA-D4K films crystallized at various temperatures determined from AFM images.

films crystallized at temperatures below 125 °C. In contrast, the flat-on lamellae were detected in the images of films crystallized at temperatures above 135 °C (Figure 6 (parts C and D)). From the height images of the flat-on lamellae in spherulites, the lamellar periodicity was measured. Figure 7 shows the lamellar periodicity of crystalline lamellae formed in the melt-crystallized PLLA-D4K film. The lamellar periodicity values of PLLA-D4K films crystallized at temperatures below 110 °C were around 8 ( 2 nm, whereas those above 140 °C were around 15 ( 3 nm. Similar discontinuous changes in lamellar thickness against the crystallization temperature have been found for several polymer, such as polyethylene29 and poly(ethylene oxide),30 samples with low molecular weights. In this study, the average number of degree of polymerization for PLLA-D4K determined from the M(NMR) value was around 56. On the n basis of this value combined with the 2.78 nm of the fiber repeat distance of the PLLA crystal, the chain folding at the lamellar surface may occur in the crystals formed below 110 °C, but not above 140 °C. It is of interest to note that two types of lamellar crystals with 8 nm thickness and with 1215 nm thickness were detected in the films crystallized at 125-135 °C. The lamellar crystals with 8 nm and 12-15 nm thickness correspond to the folded-chain crystals with single folded and nonfolded chain crystals with extended chains, respectively. Formation of both the folded- and nonfolded-chain lamellar crystals at same the temperature may be due to the distribution of the molecular weight of the sample. Based on these results, the amounts of chain ends in a unit area of crystalline surface decreased with an increase in the number of chain folds in one chain. The concentration of dodecyl ester chain ends of PLLA-D4K on the surface of nonfolded-chain crystals is 2-fold higher than that of single folded-chain crystals. As a result, the hydrophobicity on the surface of PLLA-D4K film composed of nonfolded-chain crystals was much higher than that of single folded-chain crystals. As shown in Figure 5B, the crystallinity of PLLA films increased with an increase in crystallization temperature. The chain-folding region in lamellar crystals corresponds to the pseudocrystalline region. An increase in crystallinity with

Two types of PLLA samples end-capped at the carboxylic acid chain ends with either dodecyl or MEEE ester were prepared by ring-opening polymerization of L-LA in the presence of zinc-based catalysts. The thermal properties and crystalline structures of end-capped PLLA samples were characterized by DSC measurement and X-ray analysis. The surface morphology and surface wettability of end-capped PLLA films were characterized by observation via AFM and contact angle measurements, respectively. Based on the result from contact angle measurements for melt-crystallized films, it has been concluded that the chain ends of PLLA molecules are segregated on the film surface during the crystallization process. The wettability of the PLLA film is reflected by the hydrophobicity or hydrophilicity of the compounds substituted at the chain ends. By the DSC measurement and AFM observation of low molecular weight PLLA films, it can be concluded that the surface wettability of PLLA films is strongly affected by the distribution of the chain end group on the surface of crystals due to crystalline morphology. Acknowledgment. This work was supported by the Ecomolecular Science Research Project of the RIKEN Institute. We appreciate the assistance provided by Dr. C. Nomura for correcting the English of our manuscript. References and Notes (1) Ajioka, M.; Enomoto, E.; Suzuki, K.; Yamaguchi, A. Bull. Chem. Soc. Jpn. 1995, 68, 2125. (2) Moon, S. I.; Lee, C. W.; Miyamoto, M.; Kimura, Y. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 1673. (3) Kohn, F. E.; Van der Berg, W.; Ridder, G.; Feijen, J. J. Appl. Polym. Sci. 1984, 29, 4265. (4) Kricheldorf, H. R.; Sumbe¨l, M. Eur. Polym. J. 1989, 25, 585. (5) Leenslag, J. W.; Pennings, A. J. Makromol. Chem. 1987, 188 1809. (6) Libiszowski, J.; Kowalski, A.; Duda, A.; Penczek, S. Makromol. Chem. Phys. 2002, 203, 1694. (7) Eguiburu, J. L.; Fernandez-Berridi, M. J.; Cosiio, F. P.; San Roman, J. Macromolecules 1999, 32, 8252. (8) Hans, R.; Kricheldorf, M. B.; Nico, S. Macromolecules 1988, 21, 286. (9) Dubois, P.; Jacobs, C.; Je´roˆme, R.; Teyssie, P. Macromolecules 1991, 24, 2266. (10) Tsuji, H. Biopolymers, 4, Polyesters; Doi, Y., Steinbu¨chel, A., Eds.; Willey-VCH Verlag GmbH: Weinheim, Germany, 2002; p 147. (11) Ouchi, T.; Hamada, A.; Ohya, Y. Macromol. Chem. Phys. 1999, 200, 436. (12) Lam, K. H.; Nijenhuis, A. J.; Bartels, H. J. Appl. Biomater. 1995, 6, 191. (13) Agrawal, C. J.; Athanasiou, K. A.; Heckman, J. D. Mater. Sci. Forum 1997, 250, 115.

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Synthesis, Solid-State Structure, and Surface Properties of End

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