Surface Disentanglement and Slip in a Polymer Melt: A Molecular Dynamics Study

We perform nonequilibrium molecular dynamics shear flow simulations of an entangled polymer melt consisting of flexible linear chains. A steady-state rectilinear shear flow is imposed by sliding explicit walls with permanently grafted chains in a planar Couette flow geometry. As the channel average shear rate is increased, a rapid coil stretch transition of the surface end-grafted chains is observed. The corresponding primitive path network properties are investigated, revealing a disentanglement between surface grafted and nongrafted chains during the coil stretch transition. Changes in slip length and surface friction are also measured. Grafted chains develop a trumpet-like conformation at high shear rates, which correlates with an increased relative density of entanglements near the free ends, a phenomenon that has already been considered by scaling models. The same mechanisms leading to slip in the current system may remain relevant for polymer melts of much higher (and more experimentally relevant) molecular weights. Therefore, we use the simulation results to examine the predictions and assumptions of some existing theoretical models. The conclusions drawn from the simulation may be used in the future to further develop theoretical models for the surface rheology of polymer melts.