Phase behavior and morphology of poly(ethylene oxide) blends

Crystallization studies on blends of high and low molecular weight poly(ethylene oxide) (PEG) have been carried out to characterize phase behavior and morphology. At low crystallization temperatures, both components cocrystallize into a common crystal lattice. The thickness of the cocrystal is found to vary with the blend composition. At high fractions of the low molecular weight polymer, the cocrystal is thin, suggesting a folding of the long-chain polymer into the lattice of the low molecular weight chain. At lower fractions of the low molecular weight polymer, the cocrystal is thicker, resembling the higher molecular weight polymer. At higher crystallization temperatures, the components form a defective cocrystal which dynamically phase splits into its components. The crystal transformation in the blend occurs by thinning of low molecular weight chains and thickening of high molecular weight chains. The formation of crystal templates of both components eventually results in phase-segregated growth. Models for cocrystallization and dynamic phase separation of components are discussed. A time-resolved small-angle X-ray scattering study along with complementary calorimetry and optical microscopy results confirms the mechanism of cocrystallization and dynamic phase separation in PEO blends.