The Structural Evolution of β-to-α Phase Transition in the Annealing Process of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

In this study, the structural and property changes induced in the highly ordered structure of preoriented poly(3-hydroxybutyrate-co-3-hydroxyvalerate) PHBV films containing the beta-form during annealing were investigated. The transformation of the beta-form was investigated by means of in situ wide-angle X-ray diffraction (WAXD) using synchrotron X-rays. The comparison of PHBV films with the beta-form before and after annealing was performed using small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The evolution mechanism of beta-crystal transformation was elucidated. It was revealed that most of the highly oriented beta-form directly transforms into the highly oriented alpha-form, and there might be two kinds of transformations: (1) The beta-crystalline bundles may be transformed one by one rather than one part by one part during annealing before a certain annealing time. (2) The beta-crystalline bundles crack or the molecular chains of the beta-form are separated from the lateral side after annealing after a certain annealing time. A model to describe the microstructural evolution of the ordered structure during annealing was established based on the results obtained.