Crystallisation of poly(3-hydroxybutyrate)/polyvinyl acetate blends

Poly(3-hydroxybutyrate) (PHB) has been blended with poly(vinyl acetate) (PVAc) and the system has been found to be miscible over the complete composition range studied, i.e. 0-100 wt%. The blends exhibited a single glass transition temperature, which varied with composition between the limits of the two homopolymers. Positive deviation from a linear dependence of the glass transition temperature with composition was observed and a molecular interaction parameter determined. At temperatures close to the melting point, the rates of bulk crystallisation as measured by DSC were found to increase with PVAc content. This was not observed in crystallising close to the glass transition temperature, where addition of PVAc not only increased the T-g but also retarded the crystallisation. Hot-stage microscopy confirmed that the spherulitic radial growth rates were reduced by the addition of PVAc, but this was not due to the lowering of the equilibrium melting point. This study also showed that the nucleation density was greater in the blends than in PHB itself. The anomalous increase in the bulk crystallisation rates has been attributed to an increase in the nucleation density produced in blending with PVAc. PHB is a bacteriological polymer and is low in impurities which can act as nuclei for crystallisation, such that few spherulites are observed in the light microscope. Blending with PVAc was considered to add impurities with a resulting increase in nucleation density in the blends. (C) 2000 Elsevier Science Ltd. All lights reserved.