Test of Orientation/Stretch-Induced Reduction of Friction via Primitive Chain Network Simulations for Polystyrene, Polyisoprene, and Poly(n-butyl acrylate)

Abstract

Entanglement dynamics of polymers under fast elongation has not been fully understood. Namely, the steady-state uniaxial elongational viscosity of entangled polystyrene (PS) solutions increases with an increase of strain rate above the reciprocal Rouse time, whereas the viscosity of PS melts monotonically decreases even at such high rates. This qualitative difference between solution and melt has been hypothesized to result from the orientation/stretch-induced reduction of friction (SORF). This study examines universality of the mechanism of SORF for narrowly distributed PS, polyisoprene (PI), and poly(n-butyl acrylate) (PnBA). For this purpose, SORF was incorporated in the multichain slip-link simulation through an empirical relationship between the magnitude of friction reduction and the stretch/orientation order parameter for PS [reported in Macromolecules 2012, 45, 2773–2782]. This empirical SORF relationship adopted a mean-field view for contacts between polymer backbone and solvent, and the simulation having this mean-field feature was confirmed to reproduce satisfactorily the transient and steady elongational viscosity data for PS melts and solutions. Assuming that the magnitude of SORF is insensitive to the chemical structure of polymers, we further made the simulation with the same SORF relationship for PI and PnBA. For PI, the simulation reasonably described the viscosity data of a melt and solutions. However, for PnBA melt, the data were well described by the simulation but without SORF. This result suggests that the magnitude of SORF is not universal but changes with the chemical structure of polymers. Specifically, for PnBA, the flexible side chains always surround the PnBA backbone and may behave as a solvent to screen the direct interaction between the backbone segments even in melt. For this case, the mean-field type SORF relationship obviously fails. Nevertheless, a non-mean-field type SORF could result in negligible friction reduction even under fast elongation, thereby allowing the simulation without SORF to describe thet data for PnBA.