Spherulitic morphology and crystallization kinetics of melt-miscible blends of poly(3-hydroxybutyrate) with low molecular weight poly(ethylene oxide)

The spherulitic morphology and crystallization kinetics of the blends of poly(3-hydroxybutyrate) (PHB) with the low molecular weight poly(ethylene oxide) (PEO) prepared by the solution casting have been investigated by differential scanning calorimetry and polarized optical microscopy. The blend was a crystalline/amorphous system when temperatures lie between the melting point of PEO (TmPEO=ca. 60 °C) and that of PHB (TmPHB=ca. 170 °C), while it became a crystalline/crystalline system below TmPEO. With PHB crystallization at the crystallization temperature (Tc) of 70 °C, the blends showed PHB banded spherulitic texture, which could be perturbed by the subsequent PEO crystallization in PEO-rich compositions upon further cooling to room temperature. When PHB and PEO were allowed to crystallize upon quench from melt to room temperature, fairly competitive crystallization emerged where crystallization and segregation of PHB and PEO may occur simultaneously, leading to a complicated spherulitic morphology. As for the results of crystallization kinetics, the PHB crystallization exothermic temperature (TfPHB) on cooling, PHB spherulitic growth rate (GPHB), and overall crystallization rate of PHB (kn) exhibited maxima in their dependences of PEO composition. This was attributed to the coupling between enhanced chain mobility and depression in equilibrium melting point (Tm0), since the low molecular weight PEO has a very low glass transition temperature (Tg) over the Tc range studied, GPHB increased with Tc but kn was found to decrease. The drop of kn with increasing Tc was therefore predominately governed by the large depression in nucleation rate at higher Tc.