Molecular Dynamics Reveals a Dramatic Drop of the Friction Coefficient in Fast Flows of Polymer Melts

We here revisit existing results of Kremer-Grest molecular dynamics simulations of polymer melts that provide values of the friction coefficient ζ of the Kremer-Grest bead for the coarse-grained Fraenkel chain model, both at equilibrium, and in very fast uniaxial extensional flows. The Fraenkel chain model is chosen because it can guarantee chain inextensibility in the flow-induced fully aligned conformation. Since the value of ζaligned for the fully aligned conformation is found to be much smaller than the equilibrium value ζeq, a controversial issue of the nonlinear rheology of polymer melts is here resolved. Indeed, this finding confirms previous evidence emerging from the rheology of several polymers, where seemingly the friction coefficient decreases with increasing flow-induced molecular coalignment. Often, however, such evidence was indirect, and the friction coefficient reduction has not been accepted by a large fraction of the scientific community.