Effects of Coarse-Graining on Molecular Simulations of Mechanical Properties of Polymers

Conventional coarse-graining methods for polymers are typically based on matching equilibrium thermodynamic properties in the melt state, such as the aveage structural correlations or forces. It is not clear whether the resulting coarse-grained potentials can be used to simulate non-equilibrium properties of melts at high rates or the mechanical properties of low-temperature cyrstals or glasses. As a case study, we simulate polystrene using models with different levels of coarse-graining but the same strucutal correlations at thermal equilibrium. The elastic modulus, yield stress, and strain hardening of polystyrene in the glassy state show a steady decrease with increasing degree of coarse-graining. This reflects that the configurational average of fine-grained structure leads to a smoother coarse-grained potentail with lower energy barriers between local configurations. We find that the stress-strain curves with different degrees of coarse-graining can be collapsed with a simple rescaling factor in some cases, but too much coarse-graining leads to qualitiative changes of the curves. We develop a multi-scale method that uses properly selcted coarse-grained models to accelerate the simulation of mechanical response while fine-grained models to capture the accurate stress value.