Pressure and Polymorph Dependent Thermal Decomposition Mechanism of Molecular Materials: A Case of 1,3,5,-Trinitro-1,3,5,-triazine.

1,3,5,-Trinitro-1,3,5,-triazine (RDX) serves as an important energetic material and is widely used as various solid propellants and explosives. Understanding the thermal decomposition behaviors of various polymorphs of RDX at high pressure and high temperature is significantly important for safe storage and handling. The present work reveals the early thermal decay mechanisms of two polymorphs (α- and ε-forms) of RDX at high pressure and high temperature by ReaxFF reactive molecular dynamic simulations and climbing image nudged elastic band (CI-NEB) static calculations. It is found that the thermal decomposition rate has positive and negative effects on the pressure for α- and ε-RDX, respectively. This difference originates from the difference of pressure effect on the intermolecular H transfer of the two polymorphs, as we confirm that the bimolecular H transfer rather than the NO partition initiates the decay with a significantly lower energy barrier therein. This finding may be beneficial to understand the pressure and polymorph dependent effect on the decay of RDX and to develop a kinetic model for the combustion of solid RDX.