Flow Field Visualization of Entangled Polybutadiene Solutions under Nonlinear Viscoelastic Flow Conditions

Using self-designed particle visualization instrumentation, startup shear and step strain tests were conducted under a series of systematically varied rheological and geometrical conditions, and the velocity profiles of three different well-entangled polybutadiene/oligomer solutions were obtained. For startup shear tests, in the regime of entanglement densities up to 89 and Weissenberg numbers up to 18.6, we generally observe either wall slip and a linear velocity/strain profile or simply the linear profile with no wall slip unless a massive edge fracture or instability has occurred in the sample. Meanwhile, step strain tests were conducted under similar and higher step Weissenberg numbers, and little motion was observed upon cessation. These results lead us to a conclusion that there is no compelling evidence of shear banding or non-quiescent relaxation in the range of entanglement density and Wi investigated; we interpret the results to imply that any observed banding probably correlates with edge effects, and these probably are also present in the work of the S. Q. Wang group, and they lead to shear banding.