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

Conventional coarse-graining methods for polymers are typically based on matching equilibrium thermodynamic properties in the melt phase, such as average structural correlations or forces. It is not clear that 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 crystals or glasses. Glasses are inherently far from equilibrium and local regions are unable to sample an ensemble of fine-grained configurations on practical time scales. The mechanical response reflects the properties of these frozen local configurations and the transition states at which they become unstable. This talk will present simulations of polystyrene with different levels of coarse graining. Results for the elastic modulus, yield stress, and strain hardening show a steady decrease in stress with increasing degrees of coarse- graining. This reflects the fact that configurational averaging of fine-grained structure leads to a smoother coarse-grained potential with lower energy barriers between local configurations. This smoothing has been viewed as an advantage in melt simulations because it often accelerates diffusion by a constant factor. We find that in some cases the stress-strain curves of solid phases with different degrees of coarse-graining can also be collapsed with a simple rescaling factor, but too much coarse-graining leads to qualitative changes in the curves. We conclude by presenting a multi-scale simulation method that uses coarse-grained models to accelerate dynamics and fine-grained models to obtain accurate stress values.