How polymeric solvents control shear inhomogeneity in large deformations of entangled polymer mixtures

This work aims to elucidate how molecular parameters dictate the occurrence of inhomogeneous cohesive failure during step strain and large amplitude oscillatory shear (LAOS) respectively in entangled polymer mixtures. Based on three well-entangled polybutadiene (PB) mixtures, we perform simultaneous rheometric and particle-tracking velocimetric (PTV) measurements to illustrate how the slip length controls the degree of shear banding. Specifically, the PB mixtures were prepared using the same parent polymer (M _w ∼ 10⁶ g/mol) at 10 wt.% concentration in respective polybutadiene solvents (PBS) of three different molecular weights 1.5, 10, and 46 kg/mol. After step strain, the entangled PB mixture with PBS-1.5 K displayed interfacial failure whereas the PB mixture with PBS-10 K showed bulk failure, demonstrating the effectiveness of our strategy to suppress wall slip by controlling PBS’ molecular weight. Remarkably, the PBS-46K actually allows the elastic yielding to occur homogeneously so that no appreciable macroscopic motions were observed upon shear cessation. PBS is found to play a similar role in LAOS of these three PB mixtures. Finally, we demonstrate that in case of the slip-prone mixture based on PBS-1.5 K the interfacial failure could be drastically reduced by use of shearing plates with considerable surface roughness.