Molecular dynamics simulations of shock waves in cis-1,4-polybutadiene melts

Abstract

Molecular dynamics simulations of supported shock waves in monodisperse melts of cis-1,4-polybutadiene initially at atmospheric pressure and T = 413 K were performed to study the shock-induced structural changes and post-shock relaxation. Simulations were performed for Rankine-Hugoniot shock pressures between 7.22 GPa and 8.26 GPa using the united-atom force field due to Smith and Paul [G. D. Smith and W. Paul, J. Phys. Chem. A 102, 1200 (1998)] for systems composed of chains containing 32, 64, or 128 united atoms. The sensitivity of the results to the non-bonded interaction potential was studied by comparing results obtained using the Lennard-Jones 12–6 potential from the original Smith and Paul force field to ones obtained when the 12–6 potential was replaced by the Buckingham exponential–6 potential. Several structural and mechanical properties were studied as functions of distance (time) behind the shock front. Bulk relaxation was characterized by calculating profiles of temperature, density, and principal and shear stress. Microscopic shock-induced structural rearrangement and relaxation were studied by calculating the ratio of Cartesian components of the mean-squared radius of gyration to corresponding values for the equilibrated material; dihedral angle distributions; and the distribution of, and second Legendre polynomial order parameter for, the angle formed by covalent bond vectors and the shock propagation direction.