CRYSTALLIZATION KINETICS AND MORPHOLOGY OF POLY(3-HYDROXYBUTYRATE) CELLULOSE ESTER BLENDS

Isothermal crystallization from the melt of blends of bacterial poly(3-hydroxybutyrate) (PHB) with two cellulose acetobutyrates (CE1 and CE2) has been investigated by hot-stage optical microscopy. Space-filling spherulites are observed at all compositions (50-100% PHB) and crystallization temperatures (50-130-degrees-C) explored. The spherulite radius increases linearly with time, and the radial growth rate (G) strongly depends on the cellulose ester content of the blend. G decreases with increasing CE content: the crystallization rate of the blend containing 50% CE2 is 2.5 orders of magnitude lower than that of pure PHB. The spherulites show banding whose spacing increases with increasing T(c) for a given blend, while at constant T(c) banding decreases with increasing cellulose ester concentration. In PHB/CE1 blends, owing to the ability of CE1 to crystallize from the melt concomitantly to PHB at certain compositions and crystallization temperatures, a very unusual deviation from linearity of the radial growth of PHB spherulites is observed. The rate is seen to increase with time, reflecting the compositional changes which occur in the melt as a consequence of CE1 crystallization. Apart from the cases where crystallization of CE1 occurs, the morphology of melt-crystallized PHB/CE1 and PHB/CE2 blends is rather independent of the cellulose ester identity.