Rheology and phase transitions of an upper critical solution temperature (UCST) polymer blend

The viscoelastic properties of a model binary polymer blend exhibiting an Upper Critical Solution Temperature (UCST) phase diagram were investigated by utilizing small amplitude oscillatory and steady shear measurements. A mixture of unentangled monodisperse poly(styrene) and poly (phenyl methyl siloxane) was used, and its phase diagram was established by turbidity and light scattering measurements. In the miscible region the concentration dependence of the viscosity was adequately described by a mixing rule accounting for the molecular surfaces. 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 relevant procedure originally developed for blends exhibiting lower critical solution temperature (LCST) behavior. In the phase separated regime, the elasticity of the dispersed phase undergoing spinodal decomposition was due to the interfacial tension, and the resulting normal stresses followed the scaling recently predicted by Onuki for molecular mixtures with large viscosity difference.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat