An Enhanced Scheme for Multiscale Modeling of Thermomechanical Properties of Polymer Bulks

Abstract

While multiscale modeling significantly enhances the capability of molecular simulations of polymer systems, it is well realized that the systematically derived coarse-grained (CG) models generally underestimate the thermomechanical properties. In this work, a charge-based mapping scheme has been adopted to include explicit electrostatic interactions and benchmarked against two typical polymers, atactic poly(methyl methacrylate) (PMMA) and polystyrene (PS). The CG potentials are parameterized against the oligomer bulks of nine monomers per chain to match the essential structural features and the two basic pressure−volume−temperature (PVT) properties, which are obtained from the all-atomistic (AA) molecular dynamics (MD) simulations at a single elevated temperature. The so-parameterized CG potentials are extended with the MD method to simulate the two polymer bulks of one hundred monomers per chain over a wide temperature range. Without any scaling, all the simulated results, including mass densities and bulk moduli at room temperature, thermal expansion coefficients at rubbery and glassy states, and glass transition temperatures (T g ), compare well with the corresponding experimental data. The proposed scheme not only contributes to realistically simulating various thermomechanical properties of both apolar and polar polymers but also allows for directly simulating their electrical properties.

Temperature-Transferable Coarse-Grained Models for Volumetric Properties of Poly(lactic Acid)

“…Generally, coarse graining makes the potentials smooth and thus yields the lower T g , which applies to those in most of the previous studies. In my previous studies, ,,,,,− in particular, the nonbonded CG potentials were parametrized using the same method, and the simulated T g was near to or lower than the experimental value. However, this cannot explain the current result that the CG simulations lead to the higher T g than the AA simulations, implying a different mechanism.…”

“…The nonbonded CG potentials were initially parametrized against the mass densities of decamer bulk at several (high) temperatures instead of the mass density at a single (high) temperature. This choice of pressure–volume–temperature (PVT) properties transferred thermodynamics information that combined features of multiproperties and multistates, which contributed to improving temperature transferability of the nonbonded CG potentials . Moreover, such volumetric properties could be readily obtained by simulations, experiments, and theoretical calculations, and they were very sensible to the nonbonded interactions potentials parameters.…”

“…Typically, each molecular segment or monomer of a polymer chain containing four heavy atoms on average is mapped as one CG bead, and the interaction site is placed at the position of some true atom, the center of mass (COM) or center of geometry (COG) for these atoms. Seldom, the interaction site was also placed at the center of charge (COC), which was found to be superior to other alternatives in some cases. − As the basic logic is similar to the AA force field, some assumptions are made for CG potentials: the total potential energy can be decomposed into two parts, bonded and nonbonded; the bonded potential energy can be separated into three items, bond, angle, and dihedral, which are applied to 1–2, 1–3, and 1–4 CG beads along the same chain, respectively. While the CG dihedral potentials are excluded in some cases, the CG bonded potentials beyond 1–4 are introduced in other cases .…”