Molecular dynamics simulations of several linear homopolymers: Assessment and comparison of shock properties

Understanding the behavior of polymers under shock loading is essential for their applications in car equipment, aircraft, space structure and plastic bonded explosives. Despite much effort, both experimentally and numerically over the last twenty years, there are specificities of the shock behavior of polymers that are still unclear. These include the relation of the shocked state to the dynamic glass transition and the deviation of the Hugoniot locus from the usual linear relation us = c + Sup, with us the shock velocity, c, the bulk sound velocity, up the particle velocity and S an empirical fitting parameter. To address these questions this work studies direct shock simulations in molecular dynamics (MD) of three different polymers, cis-1,4-polybutadiene, polystyrene and phenoxy resin. After thoroughly assessing that the polymer configurations created in this work are correct with respect to mass density, structure factor, chain dimensions and Hugoniot locus, this work focuses on two aspects of the behavior under shock loading. (i) The deviation of the Hugoniot locus from the linear relation us = c + Sup is related to a change in the relative contribution to the shock energy of the bonding and non-bonding potential energies as the shock strength increases. (ii) The shear stress relaxations behind the shock front are compared for the three polymers. It is found that the polymers with the highest glass transition temperature have the most slowly relaxing shear stress behind the shock front.