| Summary: | 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 of === Chemical and Biological Engineering, Department of === Graduate
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