Molecular Processes Leading to Shear Banding in Well Entangled Polymeric Melts.

We have performed hi-fidelity dissipative particle dynamics (DPD) simulations of shear flow of polymeric melts in a broad range of system sizes and two entanglement densities to determine the critical conditions for occurrence of both transient and steady shear banding. Here, we report, for the first time, simulation results that clearly demonstrate the consecutive steps leading to shear banding, that is, the stress overshoot drives locally inhomogeneous chain deformation and thus spatially inhomogeneous chain disentanglement; in turn, the localized jump in the entanglement density along the velocity gradient direction results in a considerable jump in normal stress and viscosity, which ultimately leads to shear banding. Overall, our observations are consistent with prior experimental studies, and an explanation for the stability of steady and transient shear banded flows is postulated based on the well-known interfacial stability mechanism of stratified polymeric fluids.