Startup shear of a highly entangled polystyrene solution deep into the nonlinear viscoelastic regime

We have employed optical particle tracking velocimetry to directly visualize the deformation field of a highly entangled polystyrene/diethyl phthalate (PS/DEP) solution (entanglement density Z = 61) in a parallel plate geometry. To probe the existence of shear banding, startup shear tests were carried out deep into the nonlinear viscoelastic (NLVE) regime. By roughening the surface of the rheometer plates, wall slip was effectively suppressed. In the presence of edge fracture and prior to it, steady-state shear banding was not seen as evidenced by velocity profiles that were linear for most of the shear tests. An exception was for an extremely high Weissenberg number Wi = 255, for which weak transient banding was observed. For the PS/DEP system studied, the results suggest that in the absence of wall slip, shear banding is not a steady-state phenomenon and weak shear banding is observed only in the transient state and this could possibly correlate with the edge fracture effects.