The crystal structure of poly(l-lactic acid) (PLLA) α form has been analyzed in detail by utilizing the 2-dimensional wide-angle X-ray (WAXD) and neutron diffraction (WAND) data measured for the ultradrawn sample. The WAXD data were collected using a synchrotron-sourced high-energy X-ray beam of wavelength 0.328 Å at SPring-8, Japan and the WAND data were measured using a neutron beam of wavelength 1.510 Å with a cylindrical imaging-plate camera of BIX-3 system at Japan Atomic Energy Agency. The initial crystal structure model was extracted successfully by a direct method under the assumption of the space group P212121 using about 700 X-ray reflections observed at −150 °C, the number of which was overwhelmingly large compared with the data reported by the previous other researchers and allowed us to perform more precise structural analysis. The crystal structure model obtained by the direct method was refined so that the best agreement between the observed and calculated integrated intensities was obtained or the reliability factor (R) became minimal: R was 18.2% at −150 °C and 23.2% at 25 °C. The thus-obtained chain conformation took the distorted (10/3) helical form with 21 helical symmetry along the chain axis. However, the symmetrically forbidden reflections 003, 007, 009 etc. were detected in a series of the 00L reflections, requiring us to erase the 21 screw symmetry along the molecular chain. By assuming the space group symmetry P1211, the structural refinement was made furthermore and the finally obtained R factor was 19.3% at −150 °C and 19.4% at 25 °C. Although the structural deviation from the 21 screw symmetry was only slightly, this refined model was found to reproduce the observed reflection profiles well for all the layer lines. The thus X-ray-analyzed crystal structure was transferred to the WAND data analysis to determine the hydrogen atomic positions. The R factor was 23.0% for the 92 observed reflections at 25 °C. The agreement between the observed and calculated layer line profiles was good. In this way the crystal structure of PLLA α form has been established on the basis of both the X-ray and neutron diffraction analyses.

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INTRODUCTIONPoly(L-lactic acid) (PLLA) is now attracting many attentions because of its possibility for the practical usages as one of the general multipurposes polymer materials. However, there are many problems to be clarified including the relatively poor mechanical property, the relatively low melting point and so on. 1,2 Among these problems, the slow crystallization rate in the melt processing is one of the most serious problems in the industrial point of view. 1,2 As one important factor, the existence of D-lactic acid units in PLLA chains as a contaminant is said to reduce the melt-crystallization rate by one order or higher. 3 It might be easy to speculate that the inclusion of different type of comonomer units disturbs the formation of regular helical sequences along the chain axis, resulting in the poor crystallization. However, there have been quite limited number of reports which describe concretely and in detail the structural evolution process itself in the crystallization of PLLA. Most of these papers investigated the crystallization behaviors from the melt or glass by measuring the thermal behavior or by observing the optical microscopic images of the spherulites. 4À31 At first let us review these previous studies about the cold crystallization from the glassy sample, which is prepared by quenching the melt into iceÀwater temperature. 13À15,17,21,24À26,28 The thus-obtained glassy sample is not necessarily in the perfectly amorphous state but some parts are more or less ordered in the chain aggregation mode. 32À37 This sample is called the mesomorphic phase or mesophase. 36 Stoclet et al. measured the X-ray diffraction pattern of the stretched PLLA glass containing 4 mol % D component and proposed the partially ordered liquid-crystal-like structure for the mesophase of PLLA. 32,33 But the detailed structure ABSTRACT: Structural evolution has been traced in the crystallization process of poly-(L-lactic acid) (PLLA) from the melt or from the glass by measuring the FTIR spectra, wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) patterns as functions of temperature or time. Using the infrared bands characteristic of the melt, mesomorphic phase (mesophase) and crystalline phase as well as the WAXD 200/110 and 0010 reflections and the SAXS long-period peak, the regularization processes were analyzed systematically from the different points of view. In the case of cold-crystallization from the glass, the mesophase was found to regularize into the disordered α (α 0 or δ) form in the temperature region from just above the glass transition point (Tg, 70°C) to 120°C, whereas it changed to the regular α form in the higher temperature region. On the other hand, in the melt-crystallization process, the crystallization to the α or δ form occurred not directly from the melt but via the mesophase from the melt. In this way the isothermal crystallizations from the glass and from the melt were found to occur always with the first appearance of the mesophase from the amorphous phase foll...

The crystallite modulus or ultimate Young's modulus along the chain axis in the crystal lattice was estimated theoretically and experimentally for poly-(L-lactic acid) (PLLA) α and δ (disordered α) forms and the mesophase (isolated chain model) on the basis of the refined crystal structures reported in previous papers (Macromolecules 2011, 44, 6441, 9650; Polymer, 2011, 52, 6097). The calculated modulus was 14.7 (α form), 12.9 (δ form), and 6.7 GPa (mesophase), which are in good agreement with the values obtained by the X-ray diffraction method, 13.76 ± 0.17, 12.58 ± 0.15, and 7.47 ± 0.20 GPa, respectively. (The experimental data were obtained under the assumption of homogeneous stress distribution, and so the experimentally obtained crystallite modulus should be assumed as the apparent crystallite modulus. But the observed tendency might be kept even if the true crystallite modulus is clarified in future.) The difference in Young's modulus among the three phases has been discussed in terms of the chain conformational regularity. The anisotropy in Young's modulus and linear compressibility in the ab plane perpendicular to the chain axis has been also calculated and compared with those of polyoxymethylene (POM), isotactic polypropylene, orthorhombic polyethylene, and poly(vinyl alcohol) crystals. They show more or less different anisotropy and magnitude of modulus (and compressibility) depending on the difference in chain conformation, chain packing mode and intermolecular interactions between the neighboring chains with and without bulk side groups. The difference in crystallite modulus may reflect on Young's modulus of the bulk sample. In fact, dynamic Young's modulus was measured for the three kinds of oriented PLLA samples of the α form, the δ form, and the mesophase in the heating process. In the temperature regions corresponding to the phase transitions from the mesophase to the δ form and to the α form, Young's modulus was found to change by reflecting the difference in the crystallite modulus of each phase.

の総数の 3-4 倍の実測回折点が必要である．(たと えば，等方性温度因子 B を仮定した場合，N 個の原子に 対して決定すべきパラメーター総数は凡そ 4N (つまり * 1 豊田工業大学大学院工学研究科極限材料専攻 (〠 468-8511 名古屋市天白区久方 2-12-1) * 2 茨城大学工学部生体分子機能工学科 (〠 316-8511 日立市 中成沢町 4-12-1) * 3 総合科学研究機構 (CROSS) 東海事業センター (〠 319-1106 茨城県那珂郡東海村白方 162-1) * 4 日本原子力研究開発機構量子ビーム応用部門 (〠 319-1106 茨城県那珂郡東海村白方 2-4) * 5 クラレ (株) くらしき研究センター (〠 710-1801 倉敷市酒 津 2045-1) * 6 名古屋大学大学院工学研究科科学・生物工学専攻 (〠 464-8603 名古屋市千種区不老町) * 7 日本大学文理学部化学科 (〠 156-8550 東京都世田谷区桜 上水 3-25-40)