Mechanism of the reduced shock sensitivity of CL-20/MTNP co-crystals from reactive molecular dynamics simulations

Elucidating the reasons for the reduced shock sensitivity of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane/1-methyl-3,4,5-trinitropyrazole (CL-20/MTNP) compared with CL-20 is crucial for understanding the shock initiation of CL-20/MTNP co-crystalline explosives. In this study, molecular dynamics based on the ReaxFF-lg reactive force field was employed to simulate the propagation of the shock wavefront in the CL-20/MTNP co-crystal and corresponding single crystals. Thermodynamic properties and initial chemical reactions were analyzed using in-house programs. The results indicated that the pressure and temperature at the shock wavefront in the CL-20/MTNP co-crystal were not significantly different from those in the CL-20 crystal. During low- and medium-velocity impacts, the reactant molecules of the CL-20/MTNP co-crystal decayed slower than those of the corresponding single crystals. However, during high-velocity impacts, the decay rates of reactant molecules in the three crystals were similar. After the shock wavefront passed through, multiple CL-20 molecules of the CL-20 crystal simultaneously generated a large amount of NO₂. In the co-crystal, polymerization occurred primarily between CL-20 and MTNP molecules, which likely suppressed polymerization between CL-20 molecules, leading to the prolonged presence of NO₂ and extended generation of the intermediate product, which delayed the appearance of N₂. This may be the reason for the reduced impact sensitivity of CL-20/MTNP co-crystalline explosives.