Investigation of the phase structure of poly(3-hydroxybutyrate)/poly(vinyl acetate) blends by dielectric relaxation spectroscopy

Abstract

Blends of poly(3-hydroxybutyrate) (PHB) and poly(vinyl acetate) (PVAc) were isothermally cold crystallized from homogeneous samples obtained by quenching. Dielectric relaxation spectroscopy has been used to investigate the glass–rubber relaxation characteristics and corresponding phase separation in cold-crystallized PHB/PVAc blends as a function of blend composition and crystallization conditions. All isothermally cold-crystallized blends displayed two glass–rubber relaxations corresponding to the coexisting of a mixed amorphous interlamellar phase, and a pure PVAc phase residing in interfibrillar/interspherulitic regions. No PHB relaxation was observed, indicating the absence of a pure crystal–amorphous interphase. The composition of the interlamellar phase was enriched in PVAc with increasing overall PVAc content in the blend. Lowering the crystallization temperature led to an increase in the amount of PVAc trapped in the interlamellar regions, which was consistent with kinetic control of the evolving morphology: as crystallization temperature decreased, the crystallization rate increased and the diffusion of PVAc molecules decreased. Comparison of the relaxation characteristics of interfibrillar/interspherulitic phase with those of pure PVAc indicated a much broader spectrum of local relaxation environments of PVAc in the blend, consistent with PVAc segregation across a wide range of size scales.