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

Parametrizing coarse grained models for molecular systems at equilibrium

Hierarchical coarse graining of atomistic molecular systems at equilibrium has been an intensive research topic over the last few decades. In this work we (a) review theoretical and numerical aspects of different parametrization methods (structural-based, force matching and relative entropy) to derive the effective interaction potential between coarse-grained particles. All methods approximate the many body potential of mean force; resulting, however, in different optimization problems. (b) We also use a reformulation of the force matching method by introducing a generalized force matching condition for the local mean force in the sense that allows the approximation of the potential of mean force under both linear and non-linear coarse graining mappings (E. Kalligiannaki, et al., J. Chem. Phys. 2015). We apply and compare these methods to: (a) a benchmark system of two isolated methane molecules; (b) methane liquid; (c) water; and (d) an alkane fluid. Differences between the effective interactions, derived from the various methods, are found that depend on the actual system under study. The results further reveal the relation of the various methods and the sensitivities that may arise in the implementation of numerical methods used in each case

Molecular simulation via connectivity-altering Monte Carlo and scale-jumping methods: application to amorphous polystyrene

Well-equilibrated atactic-polystyrene (aPS) samples are obtained through the end-bridging Monte Carlo (EBMC) algorithm. A coarse-grained (CG) description of aPS is used; monomers are represented by two CG beads. The algorithm produces correct polymer conformations on all length scales, beyond the size of the CG beads. The code is very efficient, even though the acceptance of 0.001-0.005% is approximately 10-100 times lower than in the original EB code for PE. Systems of aPS of the order of 5000 monomers (50 chains of 100 monomers on average) can be equilibrated on all length scales within a week, in a single-processor run. The computer code is also adequate for simulations of other polymers that have the same regularity in their sequence of chemical groups and that are modeled at the same or at a coarser level of description