Highly shocked polymer bonded explosives at a nonplanar interface: Hot-spot formation leading to detonation

We report reactive molecular dynamics simulations using the ReaxFF reactive force field to examine shock-induced hot-spot formation followed by detonation initiation in realistic (2.7 million atoms) models of polymer bonded explosives (PBX) with nonplanar interfaces. We considered here two energetic materials (EMs) pentaerythritol tetranitrate (PETN), a common EM for PBX, and silicon pentaerythritol tetranitrate (Si-PETN), which is so extremely sensitive that it has not been possible to characterize its shock properties experimentally. In each case the EM was embedded in a hydroxyl-terminated polybutadiene (HTPB) based polymer binder matrix to form a model of PBX that has a periodic sawtooth nonplanar interface. For the cases in which the shock wave propagates from the EM to polymer (EM→poly), we observed that a hot spot arises from shear localization at the convex polymer asperity. For the case in which the shock direction is inverted (shock wave propagates from the polymer to the EM, EM←poly), we find that a hot spot is initiated at the concave polymer asperity and a second more significant hot spot forms at the convex polymer asperity. This second hot spot is enhanced due to converging shock wave interactions with the nonplanar interface. Under the same shock conditions, the first step in the Si-PETN decomposition is the Si-C-O-X rearrangement to Si-O-C-X through a five centered transition state on the Si that releases 45 kcal/mol of energy that leads to a continuous increase of temperature and pressure in the hot-spot region, until detonation. By contrast, the first step for PETN is NO₂ release, which is endothermic by 39 kcal/mol, with the consequence that the hot spot is attenuated by the polymer binder, reaching a steady temperature state involving NO₂ dissociation and HONO formation.