A molecular dynamics study of a short-chain polyethylene melt.: I. Steady-state shear

Abstract

Utilizing a united atom potential model and reversible reference system propagator algorithm (rRESPA) multi-timestep dynamics, we have performed equilibrium and nonequilibrium molecular dynamics simulations of a monodisperse C 100H 202 polyethylene melt at 448 K and 0.75 g/cm 3. We report a variety of properties calculated at equilibrium including rotational relaxation time and self-diffusion coefficient as well as shear-enhanced diffusion and rheological properties calculated under steady-state shearing conditions. Shear thinning is observed in the viscosity and normal stress coefficients over the range of strain rates studied. A minimum in the hydrostatic pressure is observed at an intermediate strain rate that is associated with a minimum in the intermolecular Lennard–Jones potential energy as well as transitions in the strain-rate-dependent behavior of several other viscous and structural properties of the system. The shear field also imposes significant alignment of the chains with the flow direction, approaching a limiting angle of approximately 3° at high strain rate. In addition, the self-diffusion coefficients (calculated in terms of the unconvected positions according to the Cummings–Wang formalism) are markedly enhanced under shear compared to the equilibrium state (up to two orders of magnitude at the highest shear rate studied).