Rheology of molten polystyrene with dissolved supercritical and near-critical cases

The viscosities of polystyrene melts containing three different dissolved g ases, carbon dioxide, and the refrigerants R134a (1,1,1,2-tetrafluoroethane ) and R152a (1,1-difluoroethane) are investigated at pressures up to 20 MPa . These pressures reach near-critical and supercritical conditions for the three gas components, and produce polymer-gas solutions containing up to 10 wt% gas. The measurements are performed in a sealed high-pressure capillar y rheometer at 150 and 175 degrees C, and at shear rates ranging from 1-2,0 00 s(-1). Very large reductions in melt viscosity are observed at high gas loading; at 150 degrees C, 10 wt% R152a reduces the Newtonian viscosity by nearly three orders of magnitude relative to pure polystyrene. The viscosit y data for all three polystyrene-gas systems follows ideal viscoelastic sca ling, whereby the set of viscosity curves for a polymer-gas system can be s caled to a master curve of reduced viscosity vs. reduced shear rate identic al to the viscosity curve for the pure polymer. The Viscoelastic scaling fa ctors representing the effect of dissolved gas content on rheological behav ior are found to follow roughly the same variation with composition for all three polystyrene gas systems. (C) 1999 John Wiley & Sons, Inc.