Molecular dynamics simulation of shock compression in polyisobutylene: Effect of chain orientation

The main objective of this work was to investigate how the orientation of polyisobutylene (PIB), employed in this study as a model system, affected shockwave behavior and, in particular, the kinematic ${U}_{s}\ensuremath{-}{U}_{p}$ hugoniot using MD simulations. There is a gap in knowledge on how the orientation of polymer chains in the shock direction influences the shockwave propagation and the state of shock compression behind the shockwave front. To achieve this objective, a fully equilibrated cubic amorphous cell (AC) consisting of 131 randomly oriented PIB chains with 60 repeat units was created first and used to simulate the ${U}_{s}\ensuremath{-}{U}_{p}$ hugoniot in unoriented control using the equilibrium MD multiscale shock technique (MSST), which a priori assumes that the Rankine-Hugoniot (RH) jump conditions are satisfied. Then, the cubic AC was stretched to form a long square prism. The stretching resulted in a strong orientation of the polymer chains along the long axis of the prism. After equilibration, this elongated AC was used to generate shockwaves at different piston velocities, ${U}_{p}$, by the nonequilibrium molecular dynamics (NEMD) method and to measure the corresponding shockwave velocities, ${U}_{s}$, and directly calculate the ${U}_{s}\ensuremath{-}{U}_{p}$ dependence. To ensure accurate calculation of the shockwave velocity ${U}_{s}$, a machine learning (ML) algorithm was used to automate and optimize multiple curve fittings of the shockwave profiles for various times. This protocol allowed unprecedentedly accurate measurements of the ${U}_{s}\ensuremath{-}{U}_{p}$ relationship for the oriented PIB system. It was shown that the ${U}_{s}\ensuremath{-}{U}_{p}$ shock hugoniots simulated for unoriented and oriented PIB systems overlapped perfectly in the range above 5 km/s but diverged noticeably more and more at lower ${U}_{p}$ values (1--5 km/s), with the ${U}_{s}$ values for the oriented system being larger for the same ${U}_{p}$. The rationale for this difference is discussed, including whether the RH conditions are satisfied for the oriented PIB system. This study also reports on the anisotropy of normal stresses (pressures) along the AC axes in the oriented system, the density behavior, and the behavior of intramolecular (angular, bond, dihedral) and intermolecular (pairwise) contributions to the total potential energy in oriented PIB behind a shock front in a compressed state.