(573f) Non-Homogeneous Flows of Entangled Polymer Solutions Driven By Flow-Concentration Coupling

Models for the rheological behavior of entangled polymeric fluids often assume that both the flow and polymer concentration remain uniform. It is nevertheless well-established that coupling of viscoelastic and osmotic stresses can produce flow-enhanced concentration fluctuations that lead to local heterogeneities in fluid structure. Here, we explore the possible influence of such heterogeneities on the rheology and flow behavior entangled polymer solutions. Polystyrene and polybutadiene solutions in a marginal solvent are used as a model system in which the osmotic compressibility, and accordingly the degree of flow-concentration coupling, can be systematically varied with temperature. Simultaneous rheology, velocimetry and microscopy measurements in Taylor-Couette flow reveal a wealth of complex, nonhomogeneous flow behavior coupled with nonhomogeneous shear-enhanced concentration fluctuations. In some systems, flow-concentration coupling produces transient shear banding in which the local shear rates within bands oppose the imposed stress gradient. In other systems, steady state shear banding that is qualitatively similar to other reports of banding in polymer solutions is observed. We employ a recently developed two-fluid theory that incorporates flow-concentration coupling with the Rolie-Poly constitutive to rationalize how shear-induced local and non-local concentration heterogeneities arise and influence rheology and flow kinematics. The results suggest that significant departures from uniform flow can occur during startup shear of highly entangled polymer solutions in the presence of flow-enhanced concentration fluctuations, and provide a potential mechanism to explain shear banding of polymeric liquids.