Crystal Structure Analysis of Poly(L-lactic Acid) α Form On the basis of the 2-Dimensional Wide-Angle Synchrotron X-ray and Neutron Diffraction Measurements

The crystal structure of poly(L-lactic acid) (PLLA) alpha 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 angstrom at SPring-8, Japan and the WAND data were measured using a neutron beam of wavelength 1.510 angstrom with a cylindrical imaging-plate camera of BLX-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 P2(1)2(1)2(1) using about 700 X-ray reflections obgerved at -150 degrees C, the number of which was over-whelmingly 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 degrees C and 23.2% at 25 degrees C. The thus-obtained chain conformation took the distorted (10/3) helical form with 2(1) 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 2(1) screw symmetry along the molecular chain. By assuming the space group symmetry P12(1)1, the structural refinement was made furthermore and the finally obtained R factor was 19.3% at -150 degrees C and 19.4% at 25 degrees C. Although the structural deviation from the 2(1) 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 degrees C. The agreement between the observed and calculated layer line profiles was good. In this way the crystal structure of PLLA alpha form has been established on the basis of both the X-ray and neutron diffraction analyses.