Time-dependent small-angle light scattering of shear-induced concentration fluctuations in polymer solutions

Abstract

A small-angle light-scattering experiment probing the time-dependent behavior of a semidilute polymer solution under simple shear is presented. Specifically, this paper investigates time-dependent anisotropic growth and orientation of concentration fluctuations in a semidilute solution of polystyrene in dioctyl phthalate above the quiescent cloud-point temperature. Two flow cells, which permitted investigation of the structure and dynamics of concentration fluctuations in both the flow-shear and the flow-vorticity planes, were used. Concentration fluctuations were found to be enhanced anisotropically by the shear flow in both planes. In the flow-vorticity plane, shear-enhanced concentration fluctuations orient along the vorticity direction, while in the flow-shear plane, the axis of average orientation is at an angle between 135° and 45° to the direction of flow. Qualitative trends of this anisotropic growth of concentration fluctuations are in agreement with the predictions of the Helfand–Fredrickson model in the flow-shear plane. Furthermore, the Onuki–Doi theory relating the scattering dichroism to the structure factor has been verified experimentally by comparing scattering dichroism calculated from scattering patterns with previous experimentally measured dichroism. Scattering dichroism, both calculated from structure factors and experimentally determined over a range of shear rates and temperatures, is demonstrated to depend only on the Weissenberg number. Relaxation of shear-enhanced fluctuations on cessation of flow allowed for the calculation of a diffusion coefficient.