Characterization of Polystyrene Under Shear Deformation Using Molecular Dynamics

Abstract

Nano-filled polymers are becoming more and more important to meet the continuously growing requirements of modern engineering problems. The investigation of these composite materials at the molecular level, however, is either prohibitively expensive or just impossible. Multiscale approaches offer an elegant way to analyze such nanocomposites by significantly reducing computational costs compared to fully molecular simulations. When coupling different time and length scales, however, it is in particular important to ensure that the same material description is applied at each level of resolution. The Capriccio method (Pfaller et al, 2012, 2013), for instance, couples a particle domain modeled with molecular dynamics (MD) with a finite element based continuum description and has been used i.a. to investigate the effects of nano-sized silica additives embedded in atactic polystyrene (PS), cf. Pfaller et al (2016); Liu et al (2017). However, a simple hyperelastic constitutive law is used so far for the continuum description which is not capable to fully match the behavior of the particle domain. To overcome this issue and to enable further optimization of the coupling scheme, the material model used for the continuum should be derived directly from pure MD simulations under thermodynamic conditions identical to those used by the Capriccio method. To this end, we analyze the material response of pure PS under uniaxial deformation using strain-controlled MD simulations (Ries et al, 2019). Analogously, we perform simulations under pure shear deformation to obtain a comprehensive understanding of the material behavior. As a result, the present PS shows viscoelastic characteristics for small strains, whereas viscoplasticity is observed for larger deformations. The insights gained and data generated are used to select a suitable material model whose parameters have to be identified in a subsequent parameter optimization.