(726g) 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.