Thermal Decomposition Mechanisms of Insensitive High Energy qy-HMX Using ReaxFF MD Simulation Supported with TG-DSC and Pyro-GC/MS Experiments

The two-dimensional high nitrogen insensitive energetic polymer TAGP has been used in intercalation of HMX crystals, resulting in novel hybrid crystals (so called qy-HMX) with better stability and higher density. By adjusting the content of TAGP, various qy-HMX crystals with different thermal reactivity and density can be obtained. In this paper, theoretical calculations based on reactive force field (ReaxFF) molecular dynamics have been used to demonstrate the stabilization and decomposition mechanisms of TAGP on HMX, which have been further proved by TG-DSC and Pyro-GC/MS experiments. Six types of HMX molecule-based models with different TAGP contents have been constructed for the simulations. The results suggest that the conformation of HMX molecule could be changed under the strong constraint of TAGP structure, which could make the decomposition of qy-HMX difficult under lower temperature due to enhanced intramolecular interactions, so that more solid-state decomposition reaction occurs than that of pristine β-HMX. The TAGP and HMX may decompose simultaneously at relatively low TAGP content, whereas the TAGP decomposes earlier than HMX molecules, if its mass content reaches over 50%. This phenomenon has been proved by the TG-DSC experiments. The isothermal decomposition activation energies of raw β-HMX and modified HMX crystals have been obtained, indicating that qy-HMX crystal is more stable but can release heat more quickly than of β-HMX crystals. Besides, the Pyro-GC/MS experiments prove that there is little change for the major decomposition pathways for HMX under the effect of TAGP. The experimental gaseous products differ only in relative content, which depends on the pyrolysis temperature, and some products are more productive under temperature of 350 °C, such as the HCN, H2O and NH3; but some products are produced less, such as CO2, HNO and NO2, which are in agreement with the simulating results.