Crystal structure analysis of poly(L-lactic acid) form on the basis of the 2-dimensional wide-angle synchrotron X-ray and neutron diffraction measurements

Wasanasuk, K.*; Tashiro, Koji*; Hanesaka, Makoto*; Ohara, Takashi*   ; Kurihara, Kazuo; Kuroki, Ryota; Tamada, Taro; Ozeki, Tomoji*; Kanamoto, Tetsuo*

The crystal structure of poly(L-lactic acid) (PLLA) form has been analyzed in detail by utilizing the 2-dimensional wideangle 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 at SPring-8 and the WAND data were measured using a neutron beam of wavelength 1.510 with a BIX-3 detector at JRR-3, JAEA. The initial crystal structure model was extracted successfully by a direct method under the assumption of the space group 222 using about 700 X-ray reflections observed at -150C. 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 () became minimal: was 18.2% at -150C and 23.2% at 25C. The obtained chain conformation took the distorted (10/3) helical form with 2 helical symmetry along the chain axis. However, the symmetrically forbidden reflections were detected in a series of the 00l reflections, requiring us to erase the 2 screw symmetry along the molecular chain. By assuming the space group symmetry 2, the structural refinement was made furthermore and the finally obtained R factor was 19.3% at -150C and 19.4% at 25C. This refined model was found to reproduce the observed reflection profiles well for all the layer lines. The X-ray-analyzed crystal structure was transferred to the WAND data analysis to determine the hydrogen atomic positions. The factor was 23.0% for the 92 observed reflections at 25C. The agreement between the observed and calculated layer line profiles was good.