In-plane and out-of-plane rotational motion of individual chain molecules in steady shear flow of polymer melts and solutions

Abstract

Recent nonequilibrium molecular dynamics (NEMD) simulations of mildly entangled C400H802 and moderately entangled C700H1402 linear polyethylene melts undergoing steady shear flow have revealed that several inconsistencies between theory and experiment could be rectified by consideration of the rotational motion of individual polymer chains that occurs at moderate to high flow strengths. In this study, we investigated the configurational dynamics of the individual molecular chains that allow these once-entangled, long-chain molecules to execute retraction/extension semi-periodic cycles in response to the imposed shear via NEMD simulations. Brownian dynamics simulations were also performed to extract dynamical and configurational information about the similar cycles of polymer chain behavior that occur in dilute solutions of macromolecular chain liquids dissolved in low molecular weight solvents. Results revealed that the configurational motions of the individual chains in both melt and solution were essentially the same and governed by a single timescale that scaled exponentially with the magnitude of the shear rate. This configurational motion contained both in-plane and out-of-plane components with respect to the flow-gradient plane, with the out-of-plane component playing a much larger role during the retraction phase of the cycle than during the extension phase. This was determined to be caused by the enhancement of the retraction motion by the out-of-plane entropic Brownian forces; however, these entropic forces were detrimental to the in-plane hydrodynamic diffusive forces during the extension phase of the cycle and were thus suppressed. Consequently, the configuration of a rotating chain was significantly more compact during the retraction stage than during the extension stage, wherein the latter phase most molecules were more preferentially distributed in the flow-gradient plane.