Effects of Coarse-Graining on Molecular Simulation of Craze Formation in Polymer Glass

Crazing precedes the crack propagation in polymer glass and greatly increases the fracture toughness. We perform molecular dynamics simulations to study craze formation in glassy polystyrene (PS). The use of a structure-based coarse-grained (CG) model allows us to create and equilibrate a large-scale sample (approximate to 71 nm x 71 nm x 71 nm) of well-entangled PS chains with molecular weight 10 times the entanglement threshold. The back-mapping of the CG sample to the united-atom (UA) representation generates a PS sample with fine atomistic details. The structural features of the craze fibrils in the CG and UA simulations are almost the same, and both correlate with the underlying entanglement network as in the traditional theoretical description, reflecting the preservation of structural correlations during the coarse-graining. The stress level in the CG simulation is reduced compared with the UA simulation, as the coarse-graining with fine atomistic details removed leads to a smoother potential energy landscape for craze formation. In both CG and UA simulations, the same large fraction (70%-80%) of the stress during craze formation is dissipative stress, suggesting the coarse-graining preserves the relative contributions of the energetic and dissipative components to the overall stress. The constant drawing stress is related to the surface tension and the average spacing between craze fibrils in the simulations, as in the traditional models of crazing. We also demonstrate a scale-bridging simulation protocol where the CG simulation is used to accelerate the craze formation, and the subsequent back-mapping to the UA simulation is used to recover the stress level.