Role of Molecular Disorder on the Reactivity of RDX

Shock initiation of heterogeneous high-energy materials is often preceded by the loss of crystalline order around hotspots where mechanical energy is localized and chemical reactions start. We use molecular dynamics (MD) simulations with the reactive force field ReaxFF to determine the impact of molecular disorder on the reactivity of the high-energy material RDX under fast homogeneous heating and hotspots. Under fast heating to identical temperatures, amorphous samples exhibit faster decomposition and reaction than their crystalline counterparts. Following heating, the crystalline samples undergo fast endothermic processes associated with the loss of crystalline order that occur in timescales shorter than chemical decomposition and reduce the actual temperature of the reaction. Once this process is accounted for and actual decomposition temperatures are determined, both amorphous and crystalline samples follow identical kinetics. We also characterize the critical temperature required for a hotspot 10 nm in diameter to become critical and turn into a deflagration wave. In both crystalline and amorphous samples, hotspots with initial temperatures of 1650 K and higher result in self-sustained deflagration waves and those at 1600 K quench. We observe slightly faster propagation in the amorphous samples with initial velocities increasing with temperature. The higher reactivity of amorphous samples is not large enough to explain the significantly increased reactivity in hotspots formed after shock-induced pore collapse observed recently in large-scale MD simulations.