Effect of Water on the Surface Molecular Mobility of Poly(lactide) Thin

Apr 10, 2004 - Physical properties associated with molecular mobility on the surface of thin films with 300 nm thickness for poly(lactide)s (PLAs) wer...
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Biomacromolecules 2004, 5, 1187-1193

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Articles Effect of Water on the Surface Molecular Mobility of Poly(lactide) Thin Films: An Atomic Force Microscopy Study Yoshihiro Kikkawa,*,† Masahiro Fujita,† Hideki Abe,†,‡ and Yoshiharu Doi†,‡ Polymer Chemistry Laboratory, RIKEN Institute, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan, and Department of Innovative and Engineered Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Received December 2, 2003; Revised Manuscript Received February 24, 2004

Physical properties associated with molecular mobility on the surface of thin films with 300 nm thickness for poly(lactide)s (PLAs) were studied under vacuum conditions as well as under aqueous conditions by using friction force mode atomic force microscopy (AFM). Two types of PLAs were applied for the experimental samples as uncrystallizable PLA (uc-PLA) and crystallizable PLA (c-PLA). The friction force on the surface of thin films was measured as a function of temperature to assess the surface molecular mobility both under vacuum and under aqueous conditions. A lower glass-transition temperature of the uc-PLA surface in water was detected than that under vacuum conditions. In the case of the c-PLA thin film, change in friction force was detected at a lower temperature under aqueous conditions than in vacuo. A morphological change was observed in the c-PLA thin film during heating process from room temperature to 100 °C by temperature-controlled AFM. The surface of the c-PLA thin film became rough due to the cold crystallization, and the crystallization of c-PLA molecules in water took place at a lower temperature than in vacuo. These friction force measurements and AFM observations suggest that molecular motion on the surface of the both uc- and c-PLA thin films is enhanced in the presence of water molecules. In addition, in situ AFM observation of the enzymatic degradation process for the c-PLA thin film crystallized at 160 °C was carried out in buffer solution containing proteinase K at room temperature. The amorphous region around the hexagonal crystal was eroded within 15 min. It has been suggested that the adsorption of water molecules on the PLA film surface enhances the surface molecular mobility of the glassy amorphous region of PLA and induces the enzymatic hydrolysis by proteinase K. Introduction Poly(lactide)s (PLAs), which can be produced from renewable carbon sources, are of great interest due to their degradability as well as potential applications such as drug delivery systems, sutures, and surgical implants.1-8 PLA with high molecular weight is synthesized by the ring-opening polymerization of the lactide monomer in the presence of catalysts.9-14 Since the lactide monomer has three enantiomers, PLAs of L and D homopolymers as well as stereocopolymers with different stereochemical compositions can be prepared by the polymerization of L-, D-, and mesolactide. Tsuji and Ikada15 have studied on the effect of stereochemical composition on the crystallization of PLA. They found that the melting temperature and crystallinity reduced with increasing the composition of D units in the sequence, and that PLAs with an optical purity below 70% were uncrystallizable. * To whom correspondence should be addressed. Phone: +81-48-4679404. Fax: +81-48-462-4667. E-mail: [email protected]. † RIKEN Institute. ‡ Tokyo Institute of Technology.

It is well-known that hydrolysis of PLA materials can be accelerated by the function of enzymes such as proteinase K produced from the mold Tritirachium album.16-21 Williams16 first reported that poly(L-lactide) (PLLA) was degraded by proteinase K. McCarthy and co-workers17-20 have investigated the effect of stereochemical composition on the enzymatic degradation of PLA by proteinase K. They have found that proteinase K preferentially hydrolyzes PLLA rather than poly(D-lactide) (PDLA) and that preferential hydrolysis takes place from the amorphous regions of PLLA. Recently, Tsuji and Miyauchi21 have studied the effect of crystallinity on the enzymatic degradation of PLLA films in the presence of proteinase K and found that the amorphous region outside of the spherulite is predominantly degraded rather than that between the crystalline regions inside of the spherulite. Atomic force microscopy (AFM) has demonstrated its ability to determine the crystalline morphology and enzymatic degradation behavior of PLA.22-24 Iwata and Doi22,23 have studied the enzymatic degradation in the presence of proteinase K and alkaline hydrolysis of PLLA single crystals

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by using transmission electron microscopy and AFM. They found that the lamellar thickness of the PLLA crystal remained unchanged during the enzymatic degradation, whereas that the lamellar thickness decreased during the alkaline hydrolysis. They have concluded that enzymatic degradation of the PLLA single crystal preferentially takes place from the crystal edge rather than the chain-folding surface, whereas that the alkaline hydrolysis erodes both the edge and chain-folding surface of the single crystal. In a previous work,24 we studied the enzymatic degradation behavior of PLLA thin films by using AFM. Preferential erosion of the free amorphous region around the crystal was directly observed in the early stage of enzymatic degradation by proteinase K, whereas the restricted amorphous region between the crystal and the glass substrate was degraded at slower rate. The AFM observations have suggested that the state of the amorphous phase affects the degradability of PLLA. AFM is also known as a sensitive tool for studying the surface properties of polymer materials. The surface molecular mobility of polymer thin films has been studied by viscoelastic mode AFM,25,26 friction force mode AFM,27,28 and the force-distance curve of AFM.29 Kajiyama’s group25 has studied the surface molecular mobility of thin films of atactic polystyrene (PS) by using friction force mode AFM and suggested that the alteration of friction force against temperature is attended by the change in the dynamic loss modulus corresponding to the glass-transition temperature (Tg) of the near surface region. Bliznyuk et al.29 have determined the surface Tg of amorphous PS by the forcedistance curve of AFM during the heating process and suggested that depression of the surface Tg compared with the bulk Tg is derived from the variation of polymer chain entanglement rather than the end group segregation on the free surface of the thin film. In this study, we study the surface molecular mobility on the thin films of uncrystallizable PLDLA (uc-PLA) and crystallizable PLLA (c-PLA) in order to gain insight into the enzymatic degradation behavior of the amorphous region. The bulk Tg of PLA materials determined by differential scanning calorimetry (DSC) is generally around 45-60 °C. Therefore, it can be considered that the mobility of the main chain of PLA is negligible at room temperature. Interestingly, proteinase K easily hydrolyzes the PLA molecules in the glassy amorphous region at room temperature under aqueous conditions. Therefore, molecular mobility on the surface of PLA is determined by friction force mode AFM under vacuum and under aqueous conditions. In addition, in situ observation of enzymatic degradation by proteinase K is carried out by AFM in a buffer solution. Experimental Section Materials. Uncrystallizable poly(L,D-lactide) (uc-PLA; PLDLA, Lot No. #9800, Shimadzu Inc.: the optical purity of L-lactyl unit is 50-60%) and crystallizable poly(L-lactide) (c-PLA; PLLA, Polyscience Inc.) were used in this study. These samples were purified by precipitation in methanol from chloroform solutions and dried in vacuo for 1 week.

Kikkawa et al. Table 1. Molecular Weights and Thermal Properties of PLA Samples Used in This Studya sample uc-PLA c-PLA a

molecular weight Mn × 10-3 Mw/Mn 86 420

2.1 1.7

Tg, °C

Tm, °C

heat of fusion, ∆Hm, J/g

53 60

n.d. 180

0 36

n.d.: not detected.

The number-average molecular weight (Mn) and polydispersity (Mw/Mn) of PLA samples were evaluated by gel permeation chromatography using a polystyrene standard. The glass-transition temperature (Tg), the melting temperature (Tm), and the heat of fusion (∆Hm) of the samples were evaluated by DSC (Perkin-Elmer, Pyris 1) at a heating rate of 20 °C/min. Table 1 shows the Mn, Mw/Mn, Tg, Tm, and ∆Hm values of the PLA samples. Preparation of PLA Thin Films. Thin films with approximately 300 nm thickness were prepared on a cover glass (18 × 18 mm) by the spin-cast method. The chloroform solutions of PLA were prepared with the concentration of 3.0% (w/v). 20 µL of the chloroform solution was dropped on the glass substrate with rotation of 4000 rpm. The cast thin films of PLA samples were annealed at 220 °C for 1 min, and then quenched at room temperature in order to prepare the complete amorphous thin film with smooth surface. AFM Imaging of PLA Thin Films. Surface morphologies of PLA thin films were observed by AFM with contact mode. The morphological change during the heating process from room temperature to 100 °C was observed under vacuum conditions and in Milli Q water. Height and deflection images were simultaneously obtained. In situ observation of the enzymatic degradation of the c-PLA thin film with a 120 nm thickness was performed in the presence of proteinase K at room temperature by dynamic force mode (tapping mode) AFM (Seiko Instruments Inc., SPI3800/SPA400) in a buffer solution. The c-PLA thin film crystallized at 160 °C for 20 min was placed on the bottom of reaction vessel. 1.5 mL of 50 mM Tris-HCl buffer (pH ) 8.5) was poured into the vessel. The surface morphology of the c-PLA thin film before enzymatic degradation was observed by AFM in the buffer solution at room temperature. To initiate the enzymatic reaction by proteinase K from Tritirachium album (Boehringer Mannheim GMBH Biochemica), 30 µL of concentrated enzyme solution (10 mg/ mL) was added into the buffer solution in the vessel. The final concentration of enzyme solution in the vessel was ca. 0.2 mg/mL. A rectangular Si tip cantilever with spring constants of 1.5 N/m was applied for the dynamic force mode (tapping mode) imaging. Simultaneous registration was performed for height and deflection images. Friction Force Measurement on PLA Thin Films. Surface molecular mobility on the surface of PLAs thin films was evaluated as the friction force on the film surface against temperature by friction force mode AFM with heating stage (FFM; Seiko Instruments Inc., SPI3800/SPA300V). The friction force was measured either under vacuum conditions of 10-3 Torr or in Milli Q water. For the friction force measurements under vacuum conditions, thin films were

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fixed on the heating stage with a silver paste to transmit the heat effectively. The surface temperature of the thin film was monitored with thermocouple glued on the surface of thin film. In the case of the experiment under aqueous conditions, the thin film was glued with silver paste on the bottom surface of small vessel for AFM operation in liquid. An 1.5 mL of Milli Q water was poured into the vessel just before the experiment. The temperature of water was recorded with a thermocouple. A triangular cantilever mounting Si3N4 tip with spring constant of 0.02 N/m was applied for the friction force measurement. The normal load to the cantilever tip was set to be between 5 and 15 nN in the repulsive region. The scanning rate for the friction force measurement was fixed at 103 nm/s. Results Enzymatic Degradation of PLLA Thin Film by Proteinase K. In situ AFM observation of the enzymatic degradation process by proteinase K was carried out for a crystallizable PLLA (c-PLA) thin film crystallized at 160 °C. Figure 1 shows the continuous AFM height images of the c-PLA thin film before and during enzymatic degradation, which were obtained in a buffer solution. Before enzymatic degradation (Figure 1A), a flat-on crystal with a hexagonal appearance was observed in the buffer solution without enzyme. There were concave areas at the periphery of the crystal. The plane of the hexagonal crystal was level with that of the amorphous region around the crystal. To initiate the enzymatic hydrolysis, the concentrated enzyme solution was introduced into the buffer solution in the reaction vessel. On adding the concentrated enzyme solution of proteinase K, the amorphous region around the hexagonal crystal was gradually degraded (Figure 1, parts B and C), and the crystalline region of ca. 120 nm thick stood out on the glass substrate after 15 min of enzymatic degradation (Figure 1D). From the AFM observation, it was found that the free amorphous region around the crystal was completely eroded by the function of proteinase K, whereas that of the crystalline region remained unchanged. In our previous paper,24 the enzymatic degradation of the PLLA thin film has been studied from ex situ AFM observations. After enzymatic degradation, the free amorphous region around the hexagonal crystal was first eroded, and then the restricted amorphous region between the crystal and solid substrate was followed to degrade. The thickness of the lamellar crystal formed at 160 °C was estimated as 25 nm. In Figure 1D, the hexagonal crystal with 120 nm thickness was observed after enzymatic degradation for 15 min. This hexagonal region may include the restricted amorphous region between the crystal and based substrate. In any cases, enzymatic degradation preferentially occurred from the free amorphous region around the hexagonal crystal, and the amorphous region was eroded extremely fast. Evaluation of Friction Force for Glass Substrate. The friction force derived from the glass substrate was first determined at various temperatures by FFM. Figure 2 shows the relationship between the friction force and temperature

Figure 1. AFM images of the c-PLA thin film with 120 nm thickness (crystallized at 160 °C for 20 min) before (A) and during enzymatic degradation by proteinase K at 20 °C (B-D), and line profile data at the white line region in each image. The first image (A) was taken in a buffer solution before addition of the enzyme solution. The following frames were recorded in the enzyme solution (0.2 mg/mL) during enzymatic degradation for 5 min (B), 10 min (C), and 15 min (D), respectively. Before enzymatic degradation, the heights of the hexagonal crystal and surrounding amorphous region were almost similar (A). On starting the enzymatic degradation by proteinase K, the height difference between the hexagonal crystal and the surrounding amorphous region was increased to 50-60 nm (B), 90100 nm (C), and 110-120 nm (D), respectively.

for the glass substrate under vacuum and in water conditions. As shown in Figure 2, the glass substrate itself showed no significant change in friction force throughout the temperature range applied in this study. Molecular Mobility on the Surface of uc-PLA Thin Films. Molecular motion on the surface of uncrystallizable PLDLA (uc-PLA) thin films with 300 nm thickness was evaluated by using FFM. Figure 3 shows the temperature dependence of the friction force for the uc-PLA thin film both under vacuum and aqueous conditions. As shown in Figure 3A, the friction force revealed the almost constant value in the range of room temperature to 70 °C under vacuum. In the region of 70-80 °C, the friction force increased with a rise in annealing temperature. In the case of water conditions, the friction force was almost constant up to 58 °C. However, increases in friction force were observed above 58 °C, as shown in Figure 3B. Molecular Mobility and Morphology on the Surface of c-PLA Thin Films. The surface molecular mobility on

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Figure 2. Temperature vs friction force plots of the glass substrate measured by FFM in vacuum conditions (A) and in water (B). Normal load to the cantilever tip was set to be 5 nN ((), 10 nN (9), 15 nN (2) in a repulsive force region.

Figure 3. Temperature dependence of friction force for uc-PLA thin films with 300 nm thickness measured in a vacuum condition (A), and in water (B). Normal load to the cantilever tip was set to be 5 nN ((), 10 nN (9), 15 nN (2) in a repulsive force region.

the c-PLA thin film with 300 nm thickness was also determined by FFM both under vacuum and under aqueous conditions. Before the friction experiment, the c-PLA thin

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Figure 4. Temperature dependence of friction force for c-PLA thin films with 300 nm thickness. The data in (A) was obtained in a vacuum condition, while the data (B) was measured in water. Normal load to the cantilever tip was set to be 5 nN ((), 10 nN (9), 15 nN (2) in a repulsive force region.

film was melted at 220 °C and quenched at room temperature in order to prepare an amorphous thin film. Figure 4 shows the plots of friction force against the annealing temperature for the c-PLA thin film under vacuum and aqueous conditions. The onset of the increase in friction force could be detected at 74 °C under vacuum, whereas the change in friction force occurred at 57 °C in water. The surface morphology of the c-PLA thin film during the heating process was observed by dynamic force mode (tapping mode) AFM. The thin film of the amorphous state was heated on the AFM heating stage from room temperature to 100 °C under vacuum and aqueous conditions. Figure 5 shows the AFM deflection images of c-PLA thin films obtained at various temperatures. As shown in Figure 5A, the thin film of the amorphous state at room temperature exhibited a smooth surface. At 61 °C in water, the surface of the c-PLA thin film became rough due to cold crystallization. In contrast to the aqueous condition, the surface of the thin film remained unchanged at 65 °C under vacuum. Further heating toward 78 °C under vacuum has induced the crystallization of small crystals throughout the thin film. Both FFM measurements and AFM observations suggest that molecular mobility on the surface of c-PLA in water is enhanced compared with that under vacuum conditions, resulting in the formation of small crystals in the c-PLA thin film of the amorphous state. Discussion Effect of Substrate on the Friction Force of the PLA Thin Film. The friction force of the glass substrate against

AFM Study of PLA Molecular Mobility

Figure 5. AFM images of c-PLA thin films with 300 nm thickness recorded at an ambient condition (A), at 61 °C in water (B), at 65 °C in vacuum conditions (C), and at 78 °C in vacuum conditions (D).

temperature was evaluated by FFM under vacuum and in aqueous conditions. In Figure 2, changes in the friction force on the glass substrate were hardly detected within the temperature range (25-80 °C) used in this study. In contrast, the friction force on the surface of uc-PLA and c-PLA showed the alteration at the temperature around 60-75 °C under vacuum and aqueous conditions (Figures 3 and 4), indicating that the change in friction force is derived from the nature of the polymer surface rather than the glass substrate. Effect of the Normal Load to the Cantilever Tip on Friction Force. Friction forces at various temperatures were measured with different normal loads (5, 10, and 15 nN) to the cantilever tip. It has been reported that Tg of the surface region increases when the place of the measurement is deepened.30 As shown in Figures 3 and 4, a similar tendency of the friction force against temperature was obtained even if the normal load to the cantilever tip was selected within the range from 5 to 15 nN in the repulsive force region, suggesting that the molecular mobility of PLA is determined in the surface region of thin film. Validity of the Enhanced Molecular Mobility on the Surface of uc-PLA by Water. The surface molecular mobility of the amorphous uc-PLA thin film was measured by FFM. As shown in Figure 3, the constant friction force under vacuum and aqueous conditions could be detected up to 70 and 58 °C, respectively. Then, the friction force was increased with a rise in annealing temperature. Kajiyama and co-workers27,28 have reported the series of papers on the friction force measurement to determine the surface molecular mobility on the amorphous PS thin films. They have reported that surface Tg is estimated from the plots of the friction force against temperature and that a sudden increase in the friction force against temperature indicates the onset of glass transition.

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In the case of thin films for amorphous uc-PLA, an increase in friction force was detected at around 70 °C under vacuum conditions, whereas the change in the friction force was found at around 58 °C in aqueous conditions, as shown in Figure 3. These results indicate that the Tg of the uc-PLA thin film measured in water is lower than that in vacuo. Therefore, it is concluded that depression of Tg in 12 °C takes place on the uc-PLA surface in the presence of water and that the surface water molecules function as a plasticizer. The Tg of uc-PLA measured by FFM was around 70 °C in vacuum conditions (Figure 2), whereas that determined by DSC was 53 °C (Table 1). This may be derived from the relaxation processes of polymer chains determined by different methods of FFM and DSC. Hence, in this study, it is impossible to compare the surface and bulk Tg as intrinsic values. It has been reported that the surface Tg of the amorphous polymer is influenced by molecular weight, polydispersity, and film thickness. Tanaka et al.27 have reported the effect of molecular weight and polydispersity on the surface molecular motion of polystyrene (PS). They found that the Tg of monodisperse PS with low molecular weight (Mn < 30k) is lower than that of PS with a higher molecular weight due to the segregation of the polymer chain end group. In addition, they found that the monodisperse PS blended with the low molecular weight component and polydisperse PS show a significant depression of the Tg relative to the similar molecular weight PS. It has been suggested that a decrease in Tg for polydisperse PS is derived from the segregation of the low molecular weight component. The effect of film thickness on the Tg has been investigated by Keddie et al.31 They have evaluated the glass transition temperature by elipsometry and found that the Tg of PS thin films is decreased with a decrease in film thickness. A similar result obtained from the force-distance curve of AFM has been reported by Bliznyuk et al.29 In the present study, the molecular mobility on the surface of the amorphous uc-PLA thin film under vacuum conditions was compared with that under aqueous conditions. The molecular weight and film thickness of uc-PLA were almost the same for each sample. A lower Tg was just detected for the uc-PLA thin film under aqueous conditions than under vacuum. Thus, molecular weight, polydispersity, and film thickness of the uc-PLA sample have no relation to the depression of Tg in water, but the depression is derived from the existence of water. Validity of Enhanced Molecular Mobility on the Surface of c-PLA by Water. On the surface of the amorphous c-PLA thin film, the molecular mobility was also measured by FFM. As shown in Figure 4, a sudden increase of the friction force under vacuum and under aqueous conditions was detected at 74 and 57 °C, respectively. The change in the friction force of the amorphous uc-PLA film surface during heating process was due to the relaxation phenomena of the glass transition. For the amorphous c-PLA surface, in contrast, it is difficult to judge whether the change in the friction force is derived from the glass transition or not, because the amorphous c-PLA thin film crystallizes above Tg. Therefore, morphological change during the

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heating process was determined by AFM. It was revealed that amorphous c-PLA crystallized at a lower temperature in water than that under vacuum (Figure 5). In the family of biodegradable polyesters, the effect of water on the crystallization of isotactic poly[(R)-3-hydroxybutyrate] (i-PHB) has been determined by Hobbs and Barham.34 They found that the existence of water in i-PHB enhances the growth rate of spherulite and that the Tg of i-PHB decreased ca. 7 °C on contact with water. On the basis of crystallization and thermal data, they proposed that water acts as a weak plasticizer for i-PHB. As shown in Figure 5, the formation of small crystals in water was found at a lower temperature than that in vacuo. In addition to the morphological investigation, the onset of the increase in the friction force in water was found to be lower than that in vacuo (Figure 4). These results indicate that surface water molecules show the plasticizer effect for PLA molecules on the film surface. As a result, the molecular mobility of PLA molecules on the surface of c-PLA is enhanced by water molecules enough to crystallize at a lower temperature (61 °C). Thus, cold crystallization on the c-PLA surface under an aqueous environment took place 17 °C lower than that in a vacuum condition. Plausible Mechanism of Fast Enzymatic Degradation for the Amorphous Region by Proteinase K. Enzymatic degradation of the partially crystallized c-PLA thin film with 120 nm thickness (crystallized at 160 °C for 20 min) was performed in the presence of proteinase K at room temperature (20 °C). As shown in Figure 1, the free amorphous region around the hexagonal crystal was completely degraded by the enzyme within 15 min. Taking into account the Tg data measured by AFM, the amorphous region around the hexagonal crystal exists as the glassy state in buffer solution of 20 °C. However, the enzymatic degradation of the glassy amorphous region occurs very fast. Focarete et al.35 have studied the enzymatic degradation of chemosynthesized atactic PHB (a-PHB) blended with PLLA with different compositions in the presence of PHB depolymerase. PHB can be hydrolyzed by the specific enzyme of PHB depolymerae, whereas the other component of PLLA is not. They have suggested that the blend of 10% a-PHB with PLLA showed nonbiodegradability due to the higher Tg (ca. 50 °C) of the blend than enzymatic degradation temperature (37 °C), causing the insufficient molecular mobility of a-PHB chains in the PLLA matrix. He et al.36 determined the effect of blend compositions of a-PHB and a-PLA on the enzymatic degradability of the blend by PHB depolymerase. They have concluded that the a-PHB component in the blend is enzymatically hydrolyzed at the degradation temperature close to the Tg, whereas enzymatic degradation is restricted when the Tg of the miscible blend of a-PHB and a-PLA is high compared with the temperature of enzymatic degradation experiment because the a-PHB molecular chains are too rigid to be hydrolyzed. In this study, enzymatic degradation of the amorphous region in c-PLA was observed at 20 °C, which is far below the Tg (57 °C) of c-PLA in water measured by AFM. Nevertheless, we would like to propose that the fast enzymatic degradation of the amorphous PLA region is

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related to the enhanced surface molecular mobility of PLA molecules due to the plasticizer function of surface water molecules. In Figure 3, the depression of the surface Tg for uc-PLA was detected in the presence of water, and in Figure 4, the crystallization temperature during the heating process was decreased in water. These results indicate that water molecules enhance the molecular mobility of PLA molecules on the surface of uc- and c-PLA thin films. Enzymatic degradation of amorphous PLA by proteinase K occurs at 20 °C in buffer solution so that the mobility of glassy amorphous PLA chains may be further enhanced in the presence of proteinase K. Conclusions The enzymatic degradation behavior of the amorphous region and the surface molecular mobility of PLA molecules were evaluated in PLA thin films by using AFM and FFM. The amorphous region around the hexagonal crystal in a partially crystallized c-PLA thin film was completely degraded at 20 °C within 15 min, whereas the crystalline region remained unchanged in the presence of proteinase K. Thus, the amorphous region around the crystal was preferentially degraded by the enzyme at a fast rate. The surface glass-transition temperature of the amorphous uc-PLA (PLDLA) thin film was determined from the friction vs temperature curves. The glass-transition temperature (58 °C) for uc-PLA in water was lower than that (70 °C) in vacuum conditions, suggesting that water molecules function as a plasticizer and enhance the molecular mobility of PLA molecules on the surface of PLAs. Friction force measurement was also performed on the surface of the amorphous c-PLA (PLLA) thin film as a function of temperature. In addition, the morphological change of c-PLA during heating process was observed by AFM. A cold crystallization temperature for c-PLA was decreased in the presence of water compared with that under vacuum conditions. On the basis of the results from the enzymatic degradation experiment and FFM measurements, a plausible model of fast enzymatic degradation of PLA molecules in the amorphous region has been proposed; that is, surface water molecules induce and enhance the movement of molecular chains in the amorphous region and help the enzymatic degradation of PLA molecules by proteinase K. Acknowledgment. This work has been supported by the grants of Ecomolecular Science Research from RIKEN Institute and SORST (Solution Oriented Research for Science and Technology) from Japan Science and Technology Agency (JST). Y.K. is a recipient of the Special Postdoctoral Researchers Program of the RIKEN Institute. References and Notes (1) Jacknicz, T. M.; Nash, H. A.; Wise, D. L.; Gregory, J. B. Contraception 1973, 8, 227. (2) Leenslag, J. W.; Pennings, A. J.; Bos, R. R. M.; Rozema, F. R.; Boering, G. Biomaterials 1987, 8, 311. (3) Vanionpaa, S.; Rokkamen, P.; Tormala, P. Prog. Polym. Sci. 1989, 14, 679. (4) Arshady, R. J. Controlled Release 1991, 17, 1. (5) Penning, J. P.; Dijikstra, H.; Pennings, A. J. Polymer 1993, 34, 942. (6) Vert, M.; Li, S.; Garreau, H. Macromol. Symp. 1995, 98, 633.

AFM Study of PLA Molecular Mobility (7) Ikada, Y.; Shikinami, Y.; Hara, Y.; Tagawa, M.; Fukuda, E. J. Biomed. Mater. Res. 1996, 30, 553. (8) Ikada, Y.; Tsuji, H. Macromol. Rapid. Commun. 2000, 21, 117. (9) Lillie, E.; Schultz, R. C. Macromol. Chem. 1975, 176, 1901. (10) Kohn, F. E.; Van Den Berg, J. W. A.; Van De Ridder, G.; Feijen, J. J. Appl. Polym. Sci. 1984, 29, 4265. (11) Leenslag, J. W.; Pennings, A. J. Makromol. Chem. 1987, 188, 1809. (12) Trofimoff, L.; Aida, T.; Inoue, S. Chem. Lett. 1987, 991 (13) Kricheldorf, H. R.; Berl, M.; Scharnagl, N. Macromolecules 1988, 21, 286. (14) Nijenhuis, A. J.; Grijpma, D. W.; Pennings, A. J. Macromolecules 1992, 25, 6419. (15) Tsuji, H.; Ikada, Y. Macromolecules 1992, 25, 5719. (16) Williams, D. F. Eng. Med. 1981, 10, 5. (17) Reeve, M. S.; McCarthy, S. P.; Downy, M. J.; Gross, R. A. Macromolecules 1994, 27, 825. (18) MacDonald, R. T.; McCarthy, S. P.; Gross, R. A. Macromolecules 1996, 29, 7356. (19) Cai, H.; Dave, V.; Gross, R. A.; McCarthy, S. P. J. Polym. Sci.: Polym. Phys. 1996, 34, 2701. (20) Li, S.; McCarthy, S. P. Macromolecules 1999, 32, 4454. (21) Tsuji, H.; Miyauchi, S. Polymer 2001, 42, 4463. (22) Iwata, T.; Doi, Y. Macromolecules 1998, 31, 2461. (23) Iwata, T.; Doi, Y. Sen′i Gakkaishi 2001, 57, 172. (24) Kikkawa, Y.; Abe, H.; Iwata, T.; Inoue, Y.; Doi, Y. Biomacromolecules 2002, 3, 350.

Biomacromolecules, Vol. 5, No. 4, 2004 1193 (25) Kajiyama, T.; Tanaka, K.; Takahara, A. Macromolecules 1997, 30, 280. (26) Satomi, N.; Takahara, A.; Kajiyama, T. Macromolecules 1999, 32, 4474. (27) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 1997, 30, 6626. (28) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 2000, 33, 7588. (29) Bliznyuk, V. N.; Assender, H. E.; Briggs, G. A. D. Macromolecules 2002, 35, 6613. (30) Kajiyama, T.; Tanaka, K.; Takahara, A. Macromolecules 1995, 28, 3482. (31) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Europhys. Lett. 1994, 27, 59. (32) Doi, Y. Microbial Polyesters; VCH Publishers: New York, 1990. (33) Lauzier, C.; Revol, J.-F.; Marchessault, R. H. FEMS Microbiol. ReV. 1992, 103, 299. (34) Hobbs, J. K.; Barham, P. J. Polymer 1997, 38, 3879. (35) Focarete, M. L.; Ceccorulli, G.; Scandola, M.; Kowalczuk, M. Macromolecules 1998, 31, 8485. (36) He, Y.; Shuai, X.; Kasuya, K.; Doi, Y.; Inoue, Y. Biomacromolecules 2001, 2, 1045.

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