Application of neutron imaging-plate system to crystal structure analysis of deuterated polymers

Abstract

In order to elucidate the intermolecular interactions involved in complex formation of chitosan with hydrogen halides, crystal structures of chitosan complexes with HBr and HI were analyzed based on synchrotron X-ray fiber diffraction data (BL40B2, SPring-8, Hyogo, Japan). Both crystals are isomorphous and belong to the monoclinic space group P21. The unit cell constants are a = 9.299(9), b = 9.504(8), c(fiber axis) = 10.41(1) Å and beta = 106.93(8) deg, and a = 9.46(2), b = 9.79(2), c(fiber axis) = 10.33(2) Å and beta = 105.1(2) deg for HBr and HI complexes, respectively. The final packing models were obtained by the linked-atom least-squares refinement, which gave R-factors of 0.192 for HBr complex (93 observed spots) and 0.193 for HI complex (44 observed spots). The halide ions are aligned along the c-axis at intervals of about 5Å and are surrounded by four polymer chains. In an asymmetric unit, there are two halide ions. One ion accepts three hydrogen bonds from NH3 + groups (N2 nitrogen). The other one participates in one hydrogen bond from N2 and two hydrogen bonds from primary hydroxyl groups (O6 oxygen). In addition, the chitosan chains are linked by N-H...O and C-H...O hydrogen bonds along the b-axis direction. The crystal structure of the hydrated form of chitosan was reanalyzed using synchrotron X-ray fiber diffraction data (BL38B1, SPring-8). The chitosan chains make sheet structures parallel to the bc-plane and these sheets stack along the a-axis. Water molecules form columns between these sheets. The sites of the halide ions in the complex crystals are similar to those of the water molecules in the hydrated form. It was suggested that the columnar structure of water in the hydrated form plays an important role for the complex formation.

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

“…As well-known, a preparation of fully deuterated polymer sample is ideal since the deuterium atom has a larger neutron scattering cross-sectional area than that of the normal hydrogen atom. Some successful results were reported for polyethylene, poly(oxymethylene), poly(ethylene oxide), isotactic polypropylene, and atactic poly(vinyl alcohol) where the fully deutereated samples were used. But the preparation of such fully deuterated polymer samples is quite difficult at the present stage.…”

“…The thus-obtained 2-dimensional diffraction patterns were analyzed using a homemade software, by which the positions and integrated intensities of all the observed reflections were evaluated. 31 The peak positions of the observed reflections (x, y) on the cylindrical camera were read out and transformed to the cylindrical coordinates (ξ, ζ) in the reciprocal lattice. The indexing of the observed reflections and the determination of the unit cell parameters were performed using these (ξ, ζ) data.…”