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

The crystal structure of poly(3-hydroxybutyrare) beta 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 angstrom, c (chain axis) = 4.66 angstrom, and gamma = 120 degrees with the space group P3(2)21. The upward and downward chains are statistically located at one lattice site at 50% probability. By combining the thus analyzed structure information of the beta-form with the previously reported structure information of the alpha-form, the geometrical relation between these two crystalline phases has been clarified in detail. The tension-induced alpha-to-beta 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 the a-form lamellae. As the tensile force is increased, the local stress concentration starts to occur at the short tie chain segmental parts in the amorphous region sandwiched between the stacked lamellae. These highly tensioned tie chains induce the alpha-to-beta structural change in both amorphous and directly connected crystalline regions and generate the 40 angstrom-wide bundles of zigzag chain segments passing through several neighboring lamellae along the draw direction. These bundles are repeated with the averaged period of about 90 angstrom in the lateral direction perpendicular to the draw axis. Further stretching causes the cut of the highly tensioned extended chain parts, resulting in the formation of voids and finally the breakage of the whole sample before the completion of the phase transition from the alpha- to beta-form.