Phase Behavior of Partially Miscible Ethylene−Styrene Copolymer Blends

Blends of ethylene−styrene copolymers were studied as a model system for miscibility of α-olefin copolymer blends. The phase behavior of partially miscible copolymers, with styrene content difference of 9−10 wt %, was characterized by phase diagrams. Blends were rapidly quenched from the melt to retain the phase morphology, and the volume fractions of the two phases were obtained from AFM phase images. Assuming monodisperse polymers, the phase composition was approximated by extrapolation of the relationship between blend composition and phase volume fraction. The blends exhibited an upper critical solution temperature (UCST). With decreasing molecular weight and decreasing styrene content difference, the blends became more miscible and the UCST decreased. However, the χ interaction parameter extracted from the phase compositions varied with molecular weight, contrary to expectations, and did not show the expected squared dependence on styrene content difference. Much better results were obtained with an approach that considered the molecular weight distribution. Although the polydispersity of ethylene−styrene copolymers is relatively small, the analysis indicated that phase composition depended on initial blend composition. The χ parameter obtained with this approach was independent of molecular weight and was proportional to the square of the styrene content difference. The solubility parameter difference extracted from the χ parameter agreed with literature values.