Rheology and phase separation in a model upper critical solution temperature polymer blend

Abstract

The viscoelastic properties of a model binary polymer blend exhibiting an upper critical solution temperature phase diagram were investigated by utilizing small amplitude oscillatory and steady shear measurements. A mixture of unentangled monodisperse polystyrene and poly(phenyl methyl siloxane), exhibiting Newtonian shear viscosity, was used, and its phase diagram was established by turbidity and dynamic light scattering measurements. In the miscible region, the concentration dependence of the viscosity was adequately described by a mixing rule accounting for the surface fractions instead of volume fractions. Near the phase separation temperature and far from the glass transition, critical concentration fluctuations dominated the linear viscoelastic response and were responsible for the observed thermorheological complexity. An appropriate quantitative account of these fluctuations resulted in the accurate rheological determination of both the binodal and spinodal temperatures, extending thus the applicability of the relevant procedure earlier applied to lower critical solution temperature blends involving higher molecular weight entangled polymers. In the phase separated regime, the normal stress of the dispersed phase undergoing spinodal decomposition followed a recent scaling proposed for molecular mixtures with large viscosity difference.