X-ray Crystal Structure Analysis of Poly(3-hydroxybutyrate) β-Form and the Proposition of a Mechanism of the Stress-Induced α-to-β Phase Transition

Abstract

The crystal structure of poly(3-hydroxybutyrare) β crystal form has been analyzed on the basis of two-dimensional X-ray diffraction data. The all-trans zigzag chains are packed in the hexagonal unit cell of a = b = 9.22 A, c (chain axis) = 4.66 A, and γ = 120° with the space group P3221. The upward and downward chains are statistically located at one lattice site at 50% probability. By combining the thus-analyzed structure information of the β-form with the previously reported structure information of the α-form, the geometrical relation between these two crystalline phases has been clarified in detail. The tension-induced α-to-β phase transition affects the higher-order structural change, as known from the small-angle X-ray scattering (SAXS) data collected in the tensile deformation process. By analyzing all experimentally obtained wide-angle X-ray diffraction and SAXS data, the following transition model has been proposed: the high tension to the oriented sample causes the increase of the long period of...