RHEOLOGICAL AND RHEOOPTICAL STUDIES OF SHEAR-THICKENING POLYACRYLAMIDE SOLUTIONS

The shear-thickening behavior of partially hydrolyzed polyacrylamide (HPAM) solutions was investigated as a function of polymer concentration, solvent composition (glycerol/water ratio), temperature, and prior shear history. Delayed-step rheological and rheooptical methods were used to generate new insights into kinetic aspects of the shear-thickening process. When a critical shear rate is exceeded after an initial overshoot upon a start-up shear, a second slow growth in stress and now birefringence is observed, and the solution achieves a higher shear viscosity. Delayed-step flow experiments support the existence of shear-induced interchain structures in the shear-thickening state. By varying the precritical now history of the solution, we identify two stages of the shear-thickening mechanisms, i.e., chain stretching and disruption of preexisting network association, followed by convective buildup of the shear-thickened state. With increasing glycerol content, the critical shear rate first decreases and then the trend is reversed at a specific solvent composition where the solution shows a discontinuous change in its theological properties. The relative viscosity at low shear rates and the critical shear rate both increase with temperature. These observations are interpreted to indicate that the solvent quality is poorer at higher glycerol contents and at higher temperatures.