Effects of Coarse-Graining on Molecular Dynamics Simulations of Craze Formation in Polystyrene Glass

Crazing is a unique failure mechanism of glassy polymers. Craze forms through a fibril-drawing process at constant stress S, and its density ρ_craze is lower than the density ρ_glass of the glass. We perform molecular dynamics simulations of craze formation in glassy polystyrene using a United-Atom (UA) model and a Coarse-Grained (CG) model. The extension ratio Λ=ρ_glass/ρ_craze is almost the same for the two models and agrees with the experimental value. This is consistent with the argument that Λ is determined by the entanglement molecular weight M_e, which is identical in the simulations and experiments. The structure of a craze as characterized by the distribution of fibril diameters and the structure factor is almost identical in the UA and CG simulations, showing that the CG model preserves the structural correlations. The drawing stress S_CG in the CG simulation is smaller than S_UA at the same drawing velocity in the UA simulation. This reduction reflects that the coarse-graining leads to a smoother potential with lower energy barriers for craze formation. The same large fraction (70%-80%) of S_CG and S_UA is dissipative stress, showing that the coarse-graining preserves the relative contributions from the energy dissipation and internal energy change during craze formation.