Polymorph-Dependent Initial Thermal Decay Mechanism of Energetic Materials: A Case of 1,3,5,7-Tetranitro-1,3,5,7-Tetrazocane
Guangrui Liu, Beibei Tian, Su‐Huai Wei, Chaoyang Zhang
Abstract
Polymorphism is universal in energetic materials (EMs), and polymorphic transition may lead to variations of structure and performance. This work exemplifies such variations by revealing the polymorph-dependent mechanism of initial thermal decomposition of a typical EM, 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), using density functional tight-binding molecular dynamics simulations. This polymorphic dependence is reflected by the differences in starting steps and their weights and the evolutions of potential energy, reactants, and some important intermediates and products. Our simulations show that δ-HMX, thermally transitioned from α- or β-HMX, possesses the lowest thermostability; the C–N cleavage and the NO2 partition initiate the thermal decomposition of all three polymorphs, and their weights increase and reduce with the increasing temperature; and the intermediate-initiated thermal decomposition of the HMX molecules occurs only in the β-formed polymorph. The polymorphic dependence is originated from the differences of molecular conformers and stacking mode of different polymorphs, which further result in the differences in internal energy, intermolecular contact, and intermolecular reaction. This work presents a molecular insight into the polymorph-dependent thermal decomposition mechanism and exemplifies the importance of thinking highly of the structure transitioned, instead of the most stable structure under common conditions only.