Crystallization and thermal behaviour of poly( d(—)-3-hydroxybutyrate)-based blends

The influence of molecular structure and characteristics of an uncrystallized rubbery second component (ethylene-propylene rubber (EPR) and poly(vinyl acetate) (PVAc)) on the melt miscibility, phase structure, morphology, thermal and crystallization behaviour of blends based on poly( d(—)-3-hydroxybutyrate) (PHB) has been investigated by differential scanning calorimetry and optical microscopy. PHB and EPR are immiscible in the melt, whereas PHB and PVAc are compatible. Consequently PHB-PVAc blends show a single glass transition and a drastic depression of equilibrium melting temperature of PHB. Conflicting results are obtained when the Flory-Huggins and Kwei-Frisch equations are used to describe and analyse this melting-point depression. In PHB-EPR blends no change in the radial growth rate G of PHB spherulites occurs with increasing EPR content. The PHB spherulites grow in the presence of a PHB melt containing EPR domains as the dispersed phase. During growth the EPR particles are first ejected and then occluded in intraspherulitic regions. In the case of PHB-PVAc blends, at a given T c, G decreases with increasing PVAc content. PHB spherulites grow in equilibrium with a one-phase melt. The phase structure in the solid state is characterized by the presence of a homogeneous amorphous phase situated mainly in interlamellar regions of crystalline PHB and consisting of PVAc molecules and uncrystallized PHB chains.