Apparent thickening behavior of dilute polystyrene solutions in extensional flows

Abstract

An experimental investigation was undertaken to study the apparent thickening behavior of dilute polystyrene solutions in extensional flow. Among the parameters investigated were molecular weight, molecular weight distribution, concentration, thermodynamic solvent quality, and solvent viscosity. Apparent relative viscosityη was measured as a function of wall shear rate$\dot \gamma$ for solutions flowing from a reservoir through a 0.1 mm I.D. tube. As$\dot \gamma$ increased, slight shear thinning behavior was observed up until a critical wall shear rate$\dot \gamma _c$ was exceeded, whereupon either a large increase inη or small-scale thickening was observed depending on the particular solution under study. As molecular weight or concentration increased,$\dot \gamma _c$ decreased andΔη, the jump inη above$\dot \gamma _c$, increased.Δη increased as thermodynamic solvent quality improved. These results have been interpreted in terms of the polymer chains undergoing a coil-stretch transition at$\dot \gamma _c$. The observation of a drop-off inη at high$\dot \gamma$ (above$\dot \gamma _c$) was shown to be associated with inertial effects and not with chain fracture due to high extensional rates.