Time Evolution of Shear-Induced Structures in Semidilute Polystyrene Solutions

Abstract

We investigated time evolution of structures induced by imposing shear flow on semidilute solutions of high molecular weight polystyrene (PS) by means of both flow light scattering and rheology. PS with weight-average molecular weights of 5.48 × 106 and 2.89 × 106 were dissolved in dioctyl phthalate (DOP). After imposing shear flow with a shear rate γ̇ > γ̇c (the critical shear rate for shear-induced concentration fluctuations), the unique butterfly-type scattering pattern appeared. The pattern had a scattering maximum along the flow direction. The wave number qm and the intensity Im at the scattering maximum decreased and increased, respectively, with time and eventually reached steady state values. The rheological experiments that were carried out on the same solution under the same shear flow revealed two stress overshoots. The second overshoot was found to be related to the development of butterfly pattern and its time change, i.e., to the formation and growth of shear-induced structures.