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 β- form during annealing were investigated. The transformation of the β- 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 β- 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 β- crystal transformation was elucidated. It was revealed that most of the highly oriented β- form directly transforms into the highly oriented α- form, and there might be two kinds of transformations: (1) The β -crystalline bundles may be transformed one by one rather than one part by one part during annealing before a certain annealing time. (2) The β -crystalline bundles crack or the molecular chains of the β- 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.