Molecular insights into shock responses of amorphous polyethylene

Abstract

Abstract Shock responses of amorphous polyethylene (APE) were characterized utilizing two different types of methodology, direct non-equilibrium molecular dynamics (NEMD) and multi-scale shock technique (MSST). Providing a detailed physical view of the shock front itself, pico-second time resolved evolution of plasticity behind the shock front was explored by NEMD through simulating piston driven shock compression. The induced-shock propagation and reflection were visualized according to the evolution of the particle displacement, particle velocity field and pressure field. Exponential relations between the compression rate in a shock wave and the hydrodynamic pressure, in addition, the thickness of shock front and the hydrodynamic pressure were clarified, which quantitatively indicate the shrinkage of shock front resulted from higher compression strength under larger piston velocity. On the other hand, in addition to reproducing the final compressed states, the thermo-dynamical state variables behind the leading shock front were captured by MSST with a much smaller computational cell with enough efficiency and accuracy. Hugoniot relations were obtained to predict the bulk sound speed and two material constants indicating the compressibility with reliable values compared with the existing results. Temperature-dependency was clarified as that high temperature reduces the bulk sound speed with low density and improves the compressibility of material. The temperature-sensitivity of compressibility weakens or even disappears during the transition from glassy state to rubbery state. The critical shock velocity, which equals to the bulk sound speed at a given temperature, was specified to guarantee stable shock wave instead of quasi-isentropic wave propagation in APE. Only a single plastic shock wave with a steep front travelling at a constant velocity greater than the bulk sound speed generates in APE, resulting in the over-driven in the material.