Initial Decay Mechanism of the Heated CL-20/HMX Cocrystal: A Case of the Cocrystal Mediating the Thermal Stability of the Two Pure Components

XG Xue and Y Ma and Q Zeng and CY Zhang, JOURNAL OF PHYSICAL CHEMISTRY C, 121, 4899-4908 (2017).

DOI: 10.1021/acs.jpcc.7b00698

Energetic cocrystallization, by combining existing molecules together, is thought to be new strategy for creating energetic materials. Nevertheless, the underlying mechanism of its influences on properties and performances in comparison with their pure components remains unclear. The present work reveals the cocrystallization influence of a typical energetic cocrystal of CL-20/HMX on thermal stability, by ReaxFF molecular reactive dynamic simulations and kinetics calculations on the pure and cocrystals. As a result, we find that the cocrystal mediates the cocrys thermal stability of pure crystalsand this is in agreement with experimental observations. The initial decay steps in pure crystals remain still in the cocrystal, that is, the independent and intramolecular reactions of N-N bond cleavage governing the initial decay of the pure CL-20 and HMX crystals also dominate in the cocrystal of CL-20/HMX. Meanwhile, during the thermal decomposition of the cocrystal, CL-20 releases heat faster than HMX, thus the heat is transferred from CL-20 to WAX, and further the decay rate of HMX increases while that of CL-20 decreases, relative to the pure crystals. This leads to a moderate decay rate of the cocrystal and a small difference in decay barrier after cocrystallization. Besides, the moderated decay rate is also attributed to the small variation in intermolecular interactions after cocrystallization and the intrinsic weak stability of both component molecules of CL-20 and HMX. Thus, the intrinsic molecular stability of components and intermolecular interactions should be noted as two main factors in a strategy for increasing stability by energetic cocrystallization.

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