Chain tension reduces monomer friction in simulated polymer melts

The monomer friction coefficient ζ is known to vary with monomer structure, solvent, and concentration; its variation with chain conformation is less well known and appreciated. We explore the decrease of the friction coefficient in the extensional flow of polymer liquids, during which chains become partially stretched and aligned. To induce stretching and alignment similar to the extensional flow without a nonequilibrium flow simulation, we apply equal and opposite forces to the chain ends. We measure the friction coefficient by observing the diffusion of chains along the direction of pull. Our results for friction coefficient versus stretching qualitatively support recent arguments from rheology that friction reduction is necessary to account for smaller than predicted stress overshoots in the startup of the flow.