Crystallization behaviour of poly(ethylene oxide) from poly(3-hydroxybutyrate)/poly(ethylene oxide) blends: phase structuring, morphology and thermal behaviour

Abstract

The influence of composition and thermal history on the phase structure and crystallization processes of poly( d(−)-3-hydroxybutyrate) (PHB) and poly(ethylene oxide) (PEO) blends at temperatures below the melting temperature of PHB has been investigated by differential scanning calorimetry (d.s.c.) and scanning electron microscopy. PHB and PEO, both crystallizable polymers, are compatible in the melt, where only one homogeneous phase is formed. But complex phase separation can take place when PHB crystallizes isothermally in the presence of the one-phase melt blend at temperatures higher than the melting temperature of PEO. During growth of the PHB spherulites, some PEO molecules are trapped in interlamellar regions, forming a homogeneous solution with uncrystallized PHB. Other PEO molecules are probably rejected into interfibrillar regions, forming almost pure PEO domains. Such domains of PEO crystallized on cooling, as shown by d.s.c. and scanning electron micrographs at temperatures between 45 and 35°C, close to the crystallization temperature of pure PEO. On the contrary, the PEO molecules trapped with uncrystallized PHB in interlamellar regions of PHB spherulites can crystallize only after cooling to lower temperatures, between −20 and −30°C. This behaviour is related to the fact that the amorphous PHB/PEO solution has a composition-dependent glass transition temperature higher than that of pure PEO: so the diffusion of PEO molecules is lowered. This complex phase separation is also influenced by the growth rate of PHB spherulites.