Banding in simple steady shear of entangled polymer solutions

Velocity profiles of six entangled polybutadiene solutions (PBD) have been determined during startup shear using a particle tracking velocimetric (PTV) technique, where the number Z of entanglements per chain in these solutions varies from 13 to 119, depending on the PBD molecular weight and solution concentration. Flow behavior of these solutions at various rates in the stress plateau region has been investigated in both parallel-disk and cone-plate cells. For the least entangled solution with Z = 13, homogeneous shear was observed under all flow conditions. The solution with Z = 27 displayed inhomogeneous shear after the stress maximum before returning to a linear velocity profile at long times. For solutions with Z >= 40, shear banding was observed in both transient and steady states for a range of shear rates in the stress plateau region. At sufficiently high rates, shear homogeneity returns in steady state for these solutions (Z >= 40) after initial banding.