Individual chain dynamics of a polyethylene melt undergoing steady shear flow

摘要

Individual molecule dynamics have been shown to influence significantly the bulk rheological and microstructural properties of short-chain, unentangled, linear polyethylene liquids undergoing high strain-rate flows. The objective of this work was to extend this analysis to a linear polyethylene composed of macromolecules of a much greater length and entanglement density; i.e., a liquid consisting of C400H802 molecules, with approximately ten kinks per chain at equilibrium, as calculated by the Z1 code of Kroger [Comput. Phys. Commun. 168, 209-232 (2005)]. To achieve this, we performed nonequilibrium molecular dynamics (NEMD) simulations of a model system using the well-established potential model of Siepmann et al. [Nature 365, 330-332 (1993)] for a wide range of Weissenberg numbers (Wi) under steady shear flow. A recent study by Baig et al. [Macromolecules 43, 6886-6902 (2010)] examined this same system using NEMD simulations, but focused on the bulk rheological and microstructural properties as calculated from ensemble averages of the chains comprising the macromolecular liquids. In so doing, some key features of the system dynamics were not fully elucidated, which this article aims to highlight. Specifically, it was found that this polyethylene liquid displays multiple timescales associated with not only the decorrelation of the end-to-end vector (commonly related to the Rouse time or disengagement time, depending on the entanglement density of the liquid), but also ones associated with the retraction and rotation cycles of the individual molecules. Furthermore, when accounting for these individual chain dynamics, the "longest" relaxation time of the system was higher by a factor of 1.7, independent of shear rate, when calculated self-consistently due to the coupling of relaxation modes. Brownian dynamics (BD) simulations were also performed on an analogous free-draining bead-rod chain model to compare the rotation and retraction dynamics of a single chain in dilute solution with individual molecular motions in the melt. These BD simulations revealed that the dynamics of the free-draining chain are qualitatively and quantitatively similar to those of the individual chains comprising the polyethylene melt at strain rates in excess of Wi approximate to 50, implying a possible breakdown of reptation theory in the high shear limit. An examination of the bulk-average properties revealed the effects of the chain rotation and retraction cycles upon commonly modeled microstructural properties, such as the distribution function of the chain end-to-end vector and the entanglement number density. (C) 2015 The Society of Rheology.