Shock Induced Virtual Glass Transition to Rapidly Reduce Deviatoric Stress in Polystyrene

Shock compression introduces a near-instantaneous increase in temperature and deviatoric stress as the shockwave propagates within a condensed matter system. When polymers are subjected to strong shocks, relaxation mechanisms, such as cooperative intramolecular motion and chain rearrangements, operate to reduce deviatoric stress towards a hydrostatic equilibrium state. However, these mechanisms and their associated rates are poorly understood, especially challenging are the initial response following shock loading. Therefore, we simulate shock loading on glassy polystyrene using molecular dynamic simulations with the multiscale shock technique (MSST) and characterize the stress relaxation processes. For strong shocks, the relaxation of deviatoric (Von Mises) stress exhibits two regimes: i) an initial fast relaxation lasting 2-5 ps, ii) followed by a more gradual process. Analysis of the torsional transition events in the polymer backbone bonds (dihedral angles switching between low-energy states) indicate that the fast relaxation is associated with shock-induced virtual melting. The second regime corresponds to glassy dynamics.