Rheological and structural studies of linear polyethylene melts under planar elongational flow using nonequilibrium molecular dynamics simulations

Abstract

We present various rheological and structural properties of three polyethylene liquids, C50 H102, C78 H158, and C128 H258, using nonequilibrium molecular dynamics simulations of planar elongational flow. All three melts display tension-thinning behavior of both elongational viscosities, η1 and η2. This tension thinning appears to follow the power law with respect to the elongation rate, i.e., η∼ ε̇ b, where the exponent b is shown to be approximately -0.4 for η1 and η2. More specifically, b of η1 is shown to be slightly larger than that of η2 and to increase in magnitude with the chain length, while b of η2 appeared to be independent of the chain length. We also investigated separately the contribution of each mode to the two elongational viscosities. For all three liquids, the intermolecular Lennard-Jones (LJ), intramolecular LJ, and bond-stretching modes make positive contributions to both η1 and η2, while the bond-torsional and bond-bending modes make negative contributions to both η1 and η2. The contribution of each of the five modes decreases in magnitude with increasing elongation rate. The hydrostatic pressure shows a clear minimum at a certain elongation rate for each liquid, and the elongation rate at which the minimum occurs appears to increase with the chain length. The behavior of the hydrostatic pressure with respect to the elongation rate is shown to correlate with the intermolecular LJ energy from a microscopic viewpoint. On the other hand, 〈 Rete2 〉 and 〈 Rg2 〉 appear to be correlated with the intramolecular LJ energy. The study of the effect of the elongational field on the conformation tensor c̃ shows that the degree of increase of tr (c̃) -3 with the elongation rate becomes stronger as the chain length increases. Also, the well-known linear reaction between σ and c̃ does not seem to be satisfactory. It seems that a simple relation between σ and c̃ would not be valid, in general, for arbitrary flows.