A novel reaction kinetics model with metastable end states for shock-induced decomposition of polystyrene

Recent renewed interest in shock-induced decomposition of hydrocarbon polymers has exposed limitations of Arrhenius kinetics in describing in situ particle velocity profiles measured via embedded electromagnetic gauges or photonic Doppler velocimetry. To address this, we develop a novel reaction kinetics model for shock-compressed polystyrene, as a representative hydrocarbon polymer, above its decomposition threshold. By incorporating a reactant equation of state (EOS) developed within the SESAME framework and a product EOS constructed from a thermochemical model, this model supersedes the conventional Arrhenius kinetics model by permitting decomposition to terminate at thermodynamically metastable states, with relaxation timescales controlling the thermodynamic evolution path. Hydrodynamic simulations demonstrate that the framework provides a clear physical explanation for two key phenomena observed in embedded electromagnetic gauge measurements: rounded particle velocity profiles and the absence of distinct separation between reactant and product waves. Consequently, compared to Arrhenius models, our kinetic formulations take a major step toward resolving the recently identified discrepancies between experimental data and theoretical simulations, offering a superior interpretation of shock-induced polymer decomposition kinetics.