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

We simulate the mechanical response of polystyrene glasses using the models with different levels of coarse-graining but the same structural correlations at thermal equilibrium. The stress level during the strain hardening at the same temperature and deformation rate decreases as the degree of coarse-graining increases, reflecting that the configurational average of fine-grained structure leads to a smoother coarse-grained potential with lower energy barriers between local configurations. However, the stress-strain curves with different degrees of coarse-graining can be collapsed with a simple rescaling factor. The decomposition of the total stress σ into the energetic component σU and the dissipative component σQ shows that the same rescaling factor can be used to collapse σU and σQ. The range of strain for the collapses is before the significant increase in σU, which is affected by the interaction details of a model. In the same strain range, the conformational changes in local chain segments are almost the same for different models. The strain rate dependencies of the stress levels in different models are universal as well. We demonstrate the feasibility of a scale-bridging simulation method that is based on the universal features of strain hardening in different models of polystyrene. The method uses the coarse-grained model to accelerate the simulation of mechanical response, while the atomistic model is used to capture the accurate stress level.