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

Utilizing a united atom potential model and reversible reference system pro pagator algorithm (rRESPA) multi-timestep dynamics, we have performed equil ibrium and nonequilibrium molecular dynamics simulations of a monodisperse C100H202 polyethylene melt at 448 K and 0.75 g/cm(3). We report a variety o f properties calculated at equilibrium including rotational relaxation time and self-diffusion coefficient as well as shear-enhanced diffusion and rhe ological properties calculated under steady-state shearing conditions. Shea r thinning is observed in the vis cosity and normal stress coefficients ove r the range of strain rates studied. A minimum in the hydrostatic pressure is observed at an intermediate strain rate that is associated with a minimu m in the intermolecular Lennard-Jones potential energy as well as transitio ns in the strain-rate-dependent behavior of several other viscous and struc tural properties of the system. The shear field also imposes significant al ignment of the chains with the flow direction, approaching a limiting angle of approximately 3 degrees at high strain rate. In addition, the self-diff usion coefficients (calculated in terms of the unconvected positions accord ing to the Cummings-Wang formalism) are markedly enhanced under shear compa red to the equilibrium state (up to two orders of magnitude at the highest shear rate studied). (C) 2000 Elsevier Science B.V. All rights reserved.