Flow Effects on Melt Structure and Entanglement Network of Linear Polymers: Results from a Nonequilibrium Molecular Dynamics Simulation Study of a Polyethylene Melt in Steady Shear

Abstract

We present detailed results about the structural, conformational, rheo-optical, and topological properties of an entangled of C 400H802 linear polyethylene (PE) melt over a wide range of shear rates (covering both the linear and the highly nonlinear viscoelastic regimes) from direct nonequilibrium molecular dynamics (NEMD) simulations of a large system containing 192 chains (corresponding to 79200 interacting atomistic units). We discuss results for (i) the probability distribution of the mean-square chain end-to-end distance and its radius of gyration, (ii) the conformation tensor, (iii) the material functions in steady shear (viscosity, normal stress differences, nonequilibrium shear compliance, hydrostatic pressure), (iv) the flow birefringence, (v) the orientation angle and order parameter, (vi) the interaction energies and their relative importance, (vii) the intermolecular pair distribution function, and (viii) the intrinsic molecular shape of the chains (represented by the isosurface plots in terms of their monomer number density), all as a function of flow strength. A detailed primitive path (PP) analysis has allowed us to examine how the flow field alters the statistical properties of the underlying topological network of the melt (probability distribution functions and mean values of PP contour length, of the number and size of entanglement strands, etc.). Our results reveal significant distortions of all these distributions due to applied flow. One of the most important results of our work is that as the shear rate is increased, the average value of the contour length goes through a maximum and the number of entanglements per chain exhibits a shear-thinning behavior which bears many similarities with the corresponding behavior of the shear viscosity. Overall, most of the computed rheological properties of the C400H 802 melt change in a nonlinear way with the applied shear rate due to the simultaneous effect of (a) chain orientation and stretching, (b) chain rotation and tumbling under shear, and (c) chain disentanglement.