A RHEOOPTICAL STUDY OF NEAR-CRITICAL POLYMER-SOLUTIONS UNDER OSCILLATORY SHEAR-FLOW

The flow-induced structure and dynamics of concentration fluctuations in near-critical polymer solution have been investigated using the tec hnique of polarimetry. Specifically, this article presents scattering dichroism of a single-phased, semidilute solution of polystyrene in di octyl phthalate (PS/DOP) subjected to small amplitude oscillatory shea r flow. The measured frequency response is compared to results obtaine d using the Onuki-Doi theory, which relates dichroism to structure fac tor, and the Helfand-Fredrickson model, from which the structure facto r is calculated. The predictions are in good qualitative agreement wit h the experimental data when stress relaxation is taken into account i n the constitutive equation relating stress to strain in the system. O ur results demonstrate that the strength of the coupling between visco elastic stress and flow governs the behavior of concentration fluctuat ions in semidilute polymer solutions under oscillatory shear flow. (C) 1995 Society of Rheology.