CONCENTRATION FLUCTUATION ENHANCEMENT IN POLYMER-SOLUTIONS BY EXTENSIONAL FLOW

When subjected to flow, various polymer solutions undergo an 'apparent'' change of phase, which is manifested by a dramatic increase in the turbidity. This phenomenon is the result of anisotropic flow-induced growth of concentration fluctuations, which can be predicted qualitatively by the coupled equations of motion for the concentration and velocity fields. We have investigated the growth of concentration fluctuations of poor and near-THETA semidilute solutions of polystyrene in dioctyl phthalate (PS/DOP) subject to plane extensional flow at temperatures above the quiescent cloud point. Scattering dichroism and small-angle light scattering (SALS) indicate that concentration fluctuations grow perpendicular to the principal axis of extension for low strain rates. For higher extensional rates, fourfold symmetry appears in the structure factor, with intensity maxima on the axes at 45-degrees to the principal axes. This fourfold symmetry is predicted by a recent theoretical model. We also show that the strength of flow-induced scattering is linear to first order in the ratio of viscoelastic stress to osmotic pressure.