Local viscoelastic properties and shear stress propagation in bulk and confined polymer melts and low-molecular weight liquids

Through the analysis of the spatial correlations of local stress, we detect the propagation of long-ranged liquid-elasticity-mediated shear stress waves in polymeric and low-molecular weight liquids. The propagation of shear waves is effectively planar; i.e., σαβ propagates in the αβ plane. The autocorrelation functions of the local stress of a region are affected by both the relaxation of stress in that region and the propagation of stress from the region to the rest of the sample. However, due to the planar propagation of shear waves, the transfer of σαβ from those slices of the simulation box that are periodic in the αβ plane is negligible. This allows direct probing of the position-dependent local stress relaxation modulus of liquid in the vicinity of a confining surface.Goodyear Tire & Rubber Company Horizon 2020 (European Union)11 página

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

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, ??1 and ??2. This tension thinning appears to follow the power law with respect to the elongation rate, i.e., ????? ??̇ b, where the exponent b is shown to be approximately -0.4 for ??1 and ??2. More specifically, b of ??1 is shown to be slightly larger than that of ??2 and to increase in magnitude with the chain length, while b of ??2 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 ??1 and ??2, while the bond-torsional and bond-bending modes make negative contributions to both ??1 and ??2. 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 ?? and c̃ does not seem to be satisfactory. It seems that a simple relation between ?? and c̃ would not be valid, in general, for arbitrary flows.open29