Apparent thickening behavior of dilute polystyrene solutions in extensional flows

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 viscosity η was measured as a function of wall shear rate {Mathematical expression} for solutions flowing from a reservoir through a 0.1 mm I.D. tube. As {Mathematical expression} increased, slight shear thinning behavior was observed up until a critical wall shear rate {Mathematical expression} was exceeded, whereupon either a large increase in η or small-scale thickening was observed depending on the particular solution under study. As molecular weight or concentration increased, {Mathematical expression} decreased and Δη, the jump in η above {Mathematical expression}, 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 {Mathematical expression}. The observation of a drop-off in η at high {Mathematical expression} (above {Mathematical expression}) was shown to be associated with inertial effects and not with chain fracture due to high extensional rates.