Effects of chain length, cross-linking density, and initial temperature on the shock Hugoniot locus and post-shock relaxations in polydimethylsiloxane

We report classical molecular dynamics predictions of the shock Hugoniot locus and aspects of post-shock stress and structure relaxation in amorphous, inert polydimethylsiloxane (PDMS), as functions of degree of polymerization (\(𝑛_{\text{DOP}}\)), cross- linking density (\(\rho_{xl}\)), and initial temperature (\(T_0\)). The results were obtained using a nonreactive united-atom force field comprising harmonic covalent bonds, angles, and dihedrals, and 12-6-1 interactions for non-bonded pairs. Starting from initially monodisperse systems with \(n_{\text{DOP}}\) = 32, 64, 128 or 256, spatial proximity-based cross linking was performed at a prescribed \(𝑇_0\) until the desired \(\rho_{xl}\) was achieved (\(0.0 \le \rho_{xl} \le 10^{-3} \; \text{mol} \; \text{cm} ^{-3}\)). Shocks were simulated using explicit reverse-ballistic impacts and/or the Multi Scale Shock Technique (MSST), focusing on supported shocks with pressures \(𝑃_{shock} \lesssim 10 \; \text{GPa}\). The physical quantities of interest are the Hugoniot loci in various projections, chain-level structural changes caused by shock passage, and the spatiotemporal scales and mechanisms for post-shock stress relaxation and recovery of structural isotropy behind the shock. We also assess the steadiness of the explicit-shock spatial property profiles over time, the degree to which MSST replicates explicit-shock predictions, and the sensitivity of the results to whether long-range electrostatics are included in the force evaluations.