Structure Evolution of a Polymer Solution at High Shear Rates

We utilized large angle light scattering to study the structure of a sheared semidilute polymer solution. Below a critical shear rate A o c we observe an enhancement of concentration fluctuations in accordance with previous experimental and theoretical work. Above A o c we find dramatic changes in the transient and steady state scattering behavior which indicate a shear-induced phase separation. A new four-peaked structure which coarsens in time is observed which is superimposed upon the scattering characteristic of low shear. The four-peaked scattering structure evolves into two peaks along the gradient direction, implying the formation of phase-separated domains elongated along the shear direction. In the present paper, we measure the static structure factor under shear flow in the shear plane and introduce a new analysis of the light scattering data in which the structure factor is decomposed into symmetric and antisymmetric components with respect to the shear flow. These two components exhibit vastly different dynamical behavior. The most important difference is that one component does not coarsen in time and resembles previously observed scattering patterns which are described by the HF model while the other compo- nent exhibits considerable coarsening behavior which occurs on relatively slow time scales and is not described by the HF model. The different dynamics of these two components suggests that the decomposition we employ has physical significance and may be able to provide guidance for the development of new theoretical models which can describe the high shear rate behavior of these polymer solutions. We critically discuss the implications of our results with respect to the suggestion that shear flow can induce some kind of phase separation. We also correlate our light scattering results with rheological data which show shear thinning and transient stress overshoots above A o c.