Annealing and Melting Behavior of Poly(l-lactic ... - ACS Publications

In situ annealing and melting of folded-chain single crystals of poly(l-lactic acid) (PLLA) was examined by temperature-controlled atomic force micros...
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Biomacromolecules 2003, 4, 1301-1307

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Annealing and Melting Behavior of Poly(L-lactic acid) Single Crystals as Revealed by In Situ Atomic Force Microscopy Masahiro Fujita*,† 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 226-8501, Japan Received March 25, 2003; Revised Manuscript Received June 29, 2003

In situ annealing and melting of folded-chain single crystals of poly(L-lactic acid) (PLLA) was examined by temperature-controlled atomic force microscopy (AFM). Prominent changes in the crystal appearance during annealing could be followed in real time by the AFM at temperatures above the original crystallization temperature. Thickening of the crystal edges could be occasionally observed, and this indicates that the crystal edges are less perfect than the central, well-ordered regions. At higher annealing temperatures, melting of the unthickened part started. The melting of the unthickened region progressed from the boundaries of the thickened portion normal to the growth face, rather than to the folding surfaces. In addition, it is suggested that melting also initiates at defective or distorted sites in the crystal as revealed by transmission electron microscopy (TEM) and AFM. Introduction Poly(L-lactic acid) (PLLA), synthesized by chemical methods,1-4 has been investigated and widely used as a material for biomedical applications such as resorbable sutures, surgical implants, and so on, because of the hydrolytic degradation to nontoxic products in the human body and of good mechanical properties.5-10 On the other hand, there is increasing interest in using PLLA as an environmentally degradable plastic material. In natural enviroments, PLLA materials can be degraded by both enzymatic and nonenzymatic hydrolysis reactions. Generally, the rate of degradation strongly depends on the solid state structure of the materials. The amorphous regions are preferencially hydrolyzed so that the crystalline regions in the materials play a decisive role in the degradation process.11 Thus, the crystal structure and biodegradability of PLLA have been investigated in detail using fibers, films, and single crystals.12-16 Among them, the solution-grown single crystals with well-defined structure have been used as a useful model substrate in order to elucidate the mechanism of enzymatic degradation on the crystalline region in the material. PLLA single crystals can be grown from dilute solution in p-xylene, acetonitrile, and so on.17-20 The resulting crystals are lozenge- and hexagonal- (truncated lozenge) shaped lamellae with folded-chains and exhibit R-modification21 (orthorhombic, a ) 1.07 nm, b ) 0.645 nm and c (chainaxis) ) 2.78 nm). The enzymatic degradation behavior of the single crystal by proteinase-K for a certain period demonstrated that the hexagonal PLLA single crystal is degraded from its edge to form a rounded apperance without * 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.

decreasing both the lamellar thickness and the molecular weights.20 This biodegradation behavior is identical to those of other biodegradable polymer single crystals, e.g., poly[(R)-3-hydroxybutyrate] (P(3HB)) single crystal.22,23 Therefore, it has been widely accepted that the degradation by enzymatic hydrolysis progresses preferentially from the crystal edge and the defective or less perfect sites within the single crystal. The present work deals with annealing experiments using solution-grown single crystals of PLLA because the less perfect sites will be more sensitive to thermal treatment as well as enzymatic hydrolysis. The annealing behavior of solution-grown polymer single crystals have been the subject of numerous studies in the past.24,25 For the solution-grown PLLA single crystals, annealing experiments were also reported by Miyata et al.19 They reported that the crystal surface becomes rough and the lamellar thickness increases by annealing.19 Because most microscopic studies reported so far were performed ex situ by TEM and AFM, it has not been able to follow the behavior without the influence of sample cooling from the annealing temperature to room temperature on the crystal appearance. Recently, using an AFM equipped with a heating stage, annealing analyses for solution-grown single crystals of long alkanes and polyethylene (PE) have been reported by Winkel et al.26 and Tian et al.,27 respectively. The dynamic process during thermal treatment has been visualized in real-time, and the thermal stability and chain mobility within the isolated crystal have been revealed. In our previous work, annealing experiments of P(3HB) single crystals were performed using AFM with a heating stage.28 The in situ AFM experiment successfully visualized that the morphological change progress heterogeneously from the lateral outer faces of the crystal, suggesting that the melting of P(3HB) crystal initiates

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preferentially at defective sites at the crystal edges and the chains at the edges are activated more easily than those at central portion of the crystal by annealing.28 In this paper, in situ annealing experiments of solutiongrown single crystals of PLLA are performed by AFM with a heating stage, to investigate the less perfect sites within the crystal. In addition, thermal properties of the crystal are examined by differential scanning calorimetry (DSC) of a sedimented mat of single crystals. We also discuss the correlation between thermal behavior and enzymatic degradation one of PLLA single crystals. Experimental Sections Preparation of PLLA Single Crystals. PLLA was purchased from Polysciences, Inc. The number-average molecular weight (Mn) and polydispersity (Mw/Mn) were estimated at 62 000 and 1.7, respectively, determined by gel permeation chromatography (Shimadzu 10A GPC System). According to a similar procedure in previous work,20 single crystals of PLLA were isothermally grown at 90 °C for 24 h from a 0.025 w/v% p-xylene solution. The resulting single crystals were collected by centrifugation, washed several times with methanol at room temperature and resuspended in methanol. The single crystals were mounted on a freshly cleaved mica sheet and then dried. Both of ex situ and in situ annealing AFM experiments were carried out. For ex situ experiments, two procedures were adopted: (1) the specimen was annealed on a hot stage (Linkam LK-600PM) under nitrogen atmosphere at a desired temperature for a certain time (mostly 10 min), and then observed at room temperature (≈ 25 °C), (2) the specimen was annealed under vacuum in AFM chamber, and then observed after the specimen was cooled 25 °C. Transmission Electron Microscopy. The single crystals were mounted on a carbon-coated Cu grid for TEM. The specimens were not shadowed with Pt-Pd alloy or other metals. Bright-field images and selected-area electron diffraction patterns were recorded onto Mitsubishi MEM film with a JEOL JEM-2000FX II TEM (accelerating voltage: 120 kV) at room temperature. Atomic Force Microscopy. Morphologies of as-grown and annealed PLLA single crystals on mica sheet were observed at room temperature (≈25 °C) with an SPA400/ SPI3800N AFM (Seiko Instruments Inc.) with operating in a dynamic force microscope (DFM) mode. A 20 µm scanner and a rectangular Si cantilever with a spring constant of 20 N/m was applied. In situ annealing experiments of the specimens (PLLA/ mica) were performed on a heating stage under vacuum (10-2-10-3 Torr) with an SPA300HV/SPI3800N AFM (Seiko Instruments Inc.) with operating in a DFM mode, as similar to our previous work.28 A 20 µm scanner and a rectangular Si cantilever with a spring constant of 40 N/m was applied in all annealing experiments. Height and deflection images were recorded simultaneously. The specimen was fixed on the heating stage with silver paste. Both temperatures of the specimen surface and of the heating stage (corresponding to the bottom of specimen) were monitored.

Figure 1. (a) TEM bright-field image of PLLA single crystals grown at 90 °C for 24 h from a 0.025 (w/v)% p-xylene solution and (b) the selected-area electron diffraction pattern. Highly enlarged image from the rectangular area in (a) is shown in (c).

The former was measured by a thermocouple glued on the mica surface using silver paste. Temperature lag between the former and the latter increased with elevating temperature. The lag was 5-10 °C above 100 °C. In the present study, the surface temperature of the specimen was regarded as annealing temperature. The specimen was heated at 30 °C/ min to the desired temperature (in the temperature range from room temperature (≈25 °C) to about 170 °C), and then images were recorded at their temperatures. All images were taken after given Ta’s became stable. Differential Scanning Calorimetry. Thermal properties of the single crystals used were evaluated by DSC (PerkinElmer Pyris 1) of the sedimented single-crystal mats of PLLA under nitrogen. The DSC thermograms were obtained at a heating rate of 10, 20, and 30 °C/min under nitrogen atmosphere. Results and Discussion Solution-Grown PLLA Single Crystal. Figure 1a shows a TEM bright-field image of PLLA single crystals grown at 90 °C for 24 h from a 0.025 (w/v)% p-xylene solution. Mostly, hexagonal (or truncated-lozenge) shaped apperances with multistacked or spiral overgrowths were observed as reported previously.20 Occasionally, monolamellar single crystals with same lateral growth faces were observed. The lateral dimension of the crystals is in the micrometer scale (about 2-3 µm). Figure 1b is a selected-area electron diffraction pattern corresponding to the PLLA single crystal. All of the reflections were well indexed with the R-modification (orthorhombic, a ) 1.07 nm, b ) 0.645 nm, and c (chain-axis) ) 2.78 nm).21 This diffraction pattern represents the hk0 net-pattern of the R-modification. The chain stems in the crystal are thus set perpendicular to its basal surface. Figure 1c is a highly enlarged image from the rectangular area in Figure 1a. As can be seen in this figure, the striations with brighter or darker contrast are running perpendicular to the growth face in each sector. The width of the striation could be estimated at about 50 nm from the image. Such a bright-field TEM image with diffraction contrast, shown in Figure 1, was also reported in other polymer single crys-

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Figure 2. AFM height images of (a) spiral overgrowths and (b) monolamella of PLLA grown at 90 °C for 24 h from a 0.025 (w/v)% p-xylene solution. The cross section profile data corresponding to the dotted lines are shown below each image.

tals.29,30 The darker contrast, which region satisfies the Bragg condition, arises from cutting scattered/diffracted electrons off by objective aperture in TEM. This figure means that the PLLA single crystal has a mosaic nature,30 maybe because of structural defects, mechanical bending, and so on. It was hence deduced that the orientation of the crystallite with darker contrast and that with brighter one are slightly deviated from each other. The interface between the crystallites can be considered to be defective, that is, their defective regions run perpendicular to the growth face of PLLA single crystal. Figure 2 shows AFM height images of spiral overgrowths (Figure 2a) and monolamella (Figure 2b). Both appearances are hexagonal or truncated-lozange shape with the (110) and (200) growth faces as indicated in Figure 2a. The lamellar thickness is about 11-12 nm, estimated by the cross section profile data of height image with indicated dotted line in each figure. Taking the molecular weight into account, the solution grown single crystal of PLLA is a folded-chain lamella which has uniform lamellar thickness with a smooth surface. In Figure 2a, spiral overgrowths with both clear and fuzzy outlines can be recognized. Because the top surface of the specimen is directly followed by cantilever tip of AFM, the terraces with clear outlines exist on and those with fuzzy ones under basal lamella. It is indicated that the spiral overgrowths can grow on both top and bottom sides of basal lamella. Such crystals are thus distorted because of adhesion of lamellae on the mica substrate after removing solvent. Thermal properties of PLLA single crystals prepared here were investigated by DSC under nitrogen atmosphere. In this study, the DSC thermograms were obtained using a sedimented single crystal mat of PLLA (ca. 2 mg). The sedimented mat of the single crystals grown at 90 °C for 24 h were prepared by filtering the solution suspension. The DSC thermograms, which were obtained by heating the mats in the DSC from 0 to 200 °C at 10, 20, and 30 °C/min, are shown in Figure 3. As can be seen, double melting peaks

Figure 3. DSC thermograms which were obtained under nitrogen atmosphere by heating PLLA single-crystal mats from 0 to 200 °C at 10, 20, and 30 °C/min. The TL and TH indicate temperatures of lower (169 °C) and higher (179 °C) melting peaks, respectively.

can be recognized in the endotherms; the lower peak (TL) is about 169 °C and higher one (TH) about 179 °C. This is because melting of the original crystal, subsequent recrystallization, and melting processes during heating are involved. It was also found that the melting peaks are broad and the lower endotherms start from around 150 °C, independent of heating rate. Recently, calorimetry of an isolated PE single crystal was reported.31 The melting peak appears lower than that of the bulk sample obtained by conventional DSC. Therefore, it is deduced that the melting of isolated PLLA single crystal with 11-12 nm in thick may occurs below the onset temperature obtained from the sedimented mat (around 140-150 °C). Ex Situ Anealing Behaviors of PLLA Single Crystals. In this study, morphological changes of PLLA single crystal by annealing were observed with static AFM at room temperature (25 °C). The annealing experiments were performed above the glass transition temperature of PLLA (ca. 60 °C), and the AFM observations were carried out at room temperature, and the chain mobility below the glass transition temperature is considered negligible. In practice, no morphological changes were observed below the original crystallization temperature (90 °C).

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Figure 4. AFM deflection images of PLLA single crystals after annealing at 145 °C for 10-15 min under nitrogen atomosphere (a and b) and under vaccuum (c and d). The images were taken at 25 °C. The arrows in (d) indicate cracks due to melting.

Figure 4 shows some typical AFM deflection images of PLLA single crystals after annealing at 145 °C for 10-15 min under nitrogen atomosphere (Figure 4, parts a and b) and under vaccuum (Figure 4, parts c and d), and the images were taken at 25 °C by static AFM. The morphological changes due to heating were independent of atomosphere (nitrogen and vacuum) in the present work. In Figure 4a, some striation-like crevaces perpendicular to each growth face, small holles, and erosion at the crystal edges were also recognized. The depth of the crevace was estimated at about 7-8 nm, this is close to the one lamellar thickness. This figure also shows that the crystals are surrounded with nodule- and needlelike objects. Nodule-like objects were frequently formed at the crystal edges as shown in Figure 4b-d. The height of the nodule was estimated at 15-17 nm which is higher than the thickness of the original crystal. In Figure 4b, an interesting feature is recognized. The morphological change of the (200) sector by annealing was prominent. This fact means that the (200) sector is less stable thermally than the (110) sector. Recently, it was reported by Kikkawa et al. that the (200) sectors of the truncatedlozenge crystal grown in PLLA thin film melt more preferentially than the (110) sectors, suggesting that the chain-folding in the former sectors is energetically unstable.16 Therefore, the morphological feature in Figure 4b may be associated with the difference in the chain folding manner. Another prominent feature is shown in Figure 4d. As indicated by arrows in this figure, wide cracks run almost

parallel to the growth face. This feature was observed mostly for the multistaked crystals or spiral overgrowths after annealing. The above experiments were performed at an annealing temperature of 145 °C. According to the results obtained from the DSC of PLLA single crystal mat, it can be assumed that the morphological changes of isolated PLLA single crystal shown in Figure 4 are due to melting. The melting event will preferentially initiate at defective or less perfect sites in crystal.24,28,32,33 The interface between the crystallites, as mentioned above, is responsible for the formation of the crevices running perpendicular to the growth faces by heating, as shown in Figure 4a. Distortion along the terrace of multistacked lamellae formed in sample preparation when the lamellae adsorb onto substrate should also act as a trigger of melting so that the wide crack along its growth face are observed by heating, as shown in Figure 4d. The nodulelike feature at the crystal edges is probably formed by melting and subsequent crystallization, though it is hard to determine whether the recrystallization proceeds at 145 °C or during the cooling process after annealing. For solution-grown polymer single crystals, a thin edge is occasionally formed even in isothermal crystallization. Probably because the overgrowth may take place on cooling from the crystallization temperature. Therefore, the edges would be expected to melt and recrystallize at a lower temperature, leading to the formation of nodules. In Situ Annealing and Melting Behavior of PLLA Single Crystals. In situ annealing experiments of PLLA

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Figure 5. Series of AFM height images for the morphological changes of mono-lamellar single crystal during heating (25-170 °C) in AFM chamber taken at (a) 25, (b) 126, (c) 139, (d) 145, (e) 158, and (f) 168 °C, respectively.

Figure 6. Cross section profile data of PLLA single crystals corresponding to the dotted lines indicated in Figure 5a: (a) 25, (b) 126, (c) 139, (d) 145, (e) 158, and (f) 168 °C, respectively.

single crystals were performed under vacuum on a heating stage in the AFM chamber. A series of AFM height images showing morphological changes of monolamellar single crystal at elevated temperatures (25 - 170 °C) is shown in Figure 5. The hexagonal crystal has a smooth surface and uniform lamellar thickness 12 nm before annealing (25 °C) (Figure 5a). Figure 5b-f were under vacuum taken at elevated annealing temperatures in AFM. The present in situ experiments demonstrated that no remarkable morphological changes are observed at temperatures below the original crystallization temperature (90 °C), as expected in the ex situ annealing experiments. When the temperature of the hotstage in AFM rose to about 125 °C, the crystal edges were suddenly thickened as shown in Figure 5b. The corresponding cross section height profile data of the edges are shown in Figure 6. Compared between the profiles taken at 25 °C (Figure 6a) and 126 °C (Figure 6b), the thicknesses of the crystal edges increased from 12 to 15-20 nm and the rest remained unthickened with smooth surface. In addition, Figure 5b-d show that ridges perpendicular to each growth faces are formed. Interestingly, the widths of the thickened

edge and ridge were little changed by annealing. Further annealing, at 140-150 °C, shown in Figure 5d-f, the morphological change in the unthickned part with 12 nm thickness started from the boundaries of the thickend regions. As can be seen in Figure 6d,e, the boundaries gradually caved in and reached the substrate surface. Eventually, the thickend edges remained, whereas the area of unthickend part decreased shown in Figure 5e,f. The newly formed edges thickened in some places. In this annealing temperature range, the unthickend part seems to thicken gradually (Figure 6f). The present annealing and melting behavior of PLLA single crystal is schematically represented in Figure 7. The original crystal has a uniform lamellar thickness with a smooth surface (Figure 7a). Nevertheless, only the crystal edge preferentially thickened when the crystal was heated to 125-140 °C. It is known that the polymer crystals thicken irreversibly in the direction of the chain axis toward a stable state by annealing.24,25 The following mechanisms have been suggested to account for the lamellar thickning: (1) sliding diffusion of the chain along its axis in the solid-state and (2) melting or partial melting followed by recrystallization. In Figure 5c, a thin film appearance which seems to be in the molten state can be observed at the surrounding crystal when thickning of the edge takes place. In situ AFM annealing experiments for crystals of poly(ethylene oxide)34 and P(3HB)28 revealed that melting of the crystal starts at the edges. As illustrated in Figure 7b,c, thus, it is interpreted that the preferential thickenning of the edge can be attributed to melting and subsequent recrystallization of the lateral side. This means that the crystal edges are thermally less stable than the central region or the well-ordered crystalline lattice. This instability probably arises from the chain-packing state rather than the thickness of the edge. The preferential thicknning of the edge was recognized in annealing of long alkane single crystals reported by Winkel et al.26 They suggested that the molecules at the edge have a higher level of mobility because of the limited extent of the lattice. So, the chain mobility at the edge may be more easily activated,

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Figure 7. Schematic illustration of annealing and melting behavior of PLLA single crystals on the basis of the present in situ AFM annealing experiments: (a) Before annealing, (b) thickening of crystal edge, (c) melting of the unthickened part at the boundaries of the thickened part, (d) progress of melting toward the center of the crystal.

even at lower annealing temperature. Furtheremore, it is deduced that the nodule-like morphology recognized in the present ex situ annealing experiments is due to the melting and subsequent recrystallization of the edge at the annealing temperature. As a result, the thickened regions become thermally more stable. The unthickend part started to melt from the boundaries of thickened regions, which remain unchanged with increasing annealing temperature (Figure 5d at 145 °C). This temperature is almost consistent with the onset temperature of the endothermic DSC peak of the singlecrystal mat. Therefore, the morphological changes above 145 °C are due to melting. The melting from the boundaries of the ridges running perpendicular to the growth face is likely to lead to the formation of a crevice-like appearance, as recognized in Figure 4a. The erosion from the boundaries of the thickened edge progresses toward the central portion of the crystal is also attribute to melting of the unthickened part (Figure 7d). Morphological changes due to annealing or melting are preferentially initiated at defective or thermally instable sites in crystal.24,28,32,33 In terms of enzymatic degradation of the PLLA single crystal by proteinase-K,20 it has been suggested that chain mobility at the edge of the crystal and defective sites appears to be higher than that within the crystal. On the basis of both the thermal behavior in this study and the enzymatic degradation behavior reported so far, we conclude that the annealing and melting of PLLA single crystals is preferentially initiated at defective sites or phases with higher chain mobility such as crystal edges. Concluding Remarks In the present study, morphological changes of PLLA single crystals during annealing were followed in situ by a temperature-controlled AFM. Remarkable changes in crystal appearance were observed at temperatures above the original crystallization temperature (90 °C). The crystal edges suddenly thickened when the crystals were heated at 125-140 °C, suggesting that the crystal edge is thermally less stable. With increasing annealing temperature, the unthickened

region started to melt at 145 °C, whereas the thickened regions remained unchanged. The melting of the unthickened region progressed from the boundaries of the thickened portion normal to the growth face, rather than to the folding surface. On further annealing, the melting progressed heterogeneously toward the central part with simultaneous thickening of the central portion. In addition, it is suggested that melting initiates at defective and distorted sites in the crystal as revealed by the bright-field TEM and AFM images. On the basis of both the present experimental evidences and the enzymatic degradation manner of PLLA single crystals reported so far, we conclude that the chains at edges and defective sites in PLLA single crystals have a higher mobility than those in well-ordered crystalline regions and that melting and enzymatic degradation initiate at the edge and such defective sites. Acknowledgment. This research was supported by grants for Ecomolecular Science Research from RIKEN Institute, and for SORST (Solution Oriented Research for Science and Technology) from the Japan Science and Technology Corporation (JST). References and Notes (1) Kleine, J.; Kleine, H.-H. Macromol. Chem. 1959, 30, 23-38. (2) Leenslag, J. W.; Pennings, A. J. Macromol. Chem. 1987, 188, 18091814. (3) Sipos, L.; Zsuga, M.; Kelen, T. Polym. Bull. 1992, 27, 495-502. (4) Ajioka, M.; Enomoto, K.; Suzuki, K.; Yamaguti, A. Bull. Chem. Soc. Jpn. 1995, 68, 2125-2131. (5) Leenslag, J. W.; Pennings, A. J.; Bos, R. R. M.; Rozema, F. R.; Boering, G. Biomaterials 1987, 8, 311-314. (6) Vainionpa¨a¨, S.; Rokkanen, P.; To¨rma¨la¨, P. Prog. Polym. Sci. 1989, 14, 679-716. (7) Arshady, R. J. Controlled Release 1991, 17, 1-22. (8) Penning, J. P.; Dijkstra, H.; Pennings, A. J. Polymer 1993, 34, 942951. (9) Ikada, Y.; Shikinami, Y.; Hara, Y.; Tagawa, M.; Fukada, E. J. Biomed. Mater. Res. 1996, 30, 553-558. (10) Ikada, Y.; Tsuji, H. Macromol. Rapid. Commun. 2000, 21, 117132. (11) Joziasse, C. A. P.; Grijpma, J. E.; Cordewener, F. W.; Bos, R. R. M.; Pennings, A. J. Colloid Polym. Sci. 1998, 276, 968-975. (12) Reeve, M. S.; McCarthy, S. P.; Downey, M. J.; Gross, R. A. Macromolecules 1994, 27, 825-831.

Annealing and Melting of PLLA Single Crystals (13) MacDonald, R. T.; McCarthy, S. P.; Gross, R. A. Macromolecules 1996, 29, 7356-7361. (14) Li, S.; McCarthy, S. Macromolecules 1999, 32, 4454-4456. (15) Tsuji, H.; Miyauchi, S. Polymer 2001, 42, 4463-4467. (16) Kikkawa, Y.; Abe, H.; Iwata, T.; Inoue, Y.; Doi, Y. Biomacromolecules 2002, 3, 350-356. (17) Fischer, E. W.; Sterzel, H. J.; Wegner, G. Kolloid Z. Z. Polym. 1973, 251, 980-990. (18) Kalb, B.; Pennings, A. J. Polymer 1980, 21, 607-612. (19) Miyata, T.; Masuko, T. Polymer 1997, 38, 4003-4009. (20) Iwata, T.; Doi, Y. Macromolecules 1998, 31, 2461-2467. (21) De Saintis, P.; Kovacs, A. J. Biopolymers 1968, 6, 299-306. (22) Nobes, G. A. R.; Marchessault, R. H.; Chanzy, H.; Bries, B. H.; Jendersek, D. Macromolecules 1996, 29, 8330-8333. (23) Iwata, T.; Doi, Y.; Kasuya, K.; Inoue, Y. Macromolecules 1997, 30, 833-839. (24) Geil, P. H. Polymer Single Crystals; John Willey and Sons: New York, 1963. (25) Bassett, D. C. Principles of polymer morphology; Cambridge University Press: Cambridge, 1981.

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