Rheo-Optical Studies of Shear-Induced Structures in Semidilute Polystyrene Solutions

Shear-induced structure formation in semidilute solutions of high molecular weight polystyrene (PS) was investigated by rheology, shear-dichroism, shear-birefringence, shear-small-angle light-scattering (SALS) and shear-optical-microscopy methods. At the shear rate γc < γ < γa, where γc and γa are the critical shear rates for the onset of the shear-enhanced concentration fluctuations and the onset of the anomalies in the rheological and scattering behaviors respectively, we observed negative dichroism, almost zero birefringence, and butterfly type LS patterns. All these results are consistent with the optical images captured by shear-microscopy and show the shear-induced concentration fluctuations with the following characteristics: the amplitude of the fluctuations is enhanced along the flow direction, and the domain structure (regions rich in polymer concentration) formed is extended normal to flow. However, at the shear rate γ > γa, the dichroism became positive and the absolute values of both the dichroism and the birefringence increased drastically. The changes in the dichroism and the birefringence are believed to be related to formation of the stringlike structures and molecular orientation both parallel to flow.