Is the impact sensitivity of RDX polymorph dependent?

Abstract

Impact-sensitivity predictions based on the vibrational up-pumping model show a strong polymorph dependency for RDX and highlight that one of the high-pressure forms, which has been postulated to form during shock-wave experiments, is appreciably more susceptible to mechanical initiation. The origin of the predicted impact sensitivity variation can be attributed to vibrational mode hardening by pressure and to differences in the molecular conformation of RDX in the four polymorphs studied. These polymorphs present different distributions of molecular vibrations within their respective up-pumping windows, which leads to their varying ability to up-pump and trap the vibrational energy that arises from mechanical insult.

John N. Sherwood: Studies of Energetic Materials

“…Although accidental initiation may not occur via impact, these materials must have some form of inherent sensitivity to insult. In the case of impact, this is being keenly studied, and recently some great strides have been made in developing a fundamental understanding of impact sensitivity and crystal structure. − Currently none of the hazard sensitivity tests require a knowledge of the crystal polymorph. What about other sensitivity tests? For small scale testing, some empirical observations and thoughts can be made for both chemical compatibility and electrostatic discharge tests.…”

High-pressure structural studies and pressure-induced sensitisation of 3,4,5-trinitro-1H-pyrazole

“…First, the predicted IS for Form I 3,4,5-TNP fits very well with experiment and correctly sits between the predicted sensitivities of α-RDX and α-FOX-7 using the same computational up-pumping model. 13 Second, the model indicates a substantially increased sensitivity for Form II, with its IS predicted to be greater than that of β-HMX. Third, because of mode hardening, the predicted IS for Form I compressed to 5.3 GPa is almost identical to that obtained for Form II.…”

“…Variations in crystal density directly affect calculated heats of formation and the detonation velocities and pressures, 1 but these properties are seldom reported for different polymorphs. Mechanical impact sensitivity can also be polymorph-dependent, as recently demonstrated by computational modelling studies of FOX-7 (1,1-diamino-2,2-dinitroethene), 12 RDX, 13 and by combined experimental and computational studies of CL-20 and HMX. 14,15 These studies emphasise the importance of studying the structural evolution of EMs over operational conditions to understand better the structure/property relationships of these technologically important materials.…”

Grammar of Impact Sensitivity: An Incremental Theory

“…17−19 At the same time, the objects for impact sensitivity modeling are mainly vacuum-isolated molecules, despite the fact that a number of recent works stress the significant role of polymorphs in molecular engineering of impact sensitivity. 20,21 Therefore, modeling of this quantity using properties of the crystalline state is crucial for mechanistic consideration. In this aspect, some efforts were made earlier to develop solid-state descriptors.…”

“…In recent works, the question of the physical backgrounding of the phenomenon and the corresponding modeling are increasingly raised. − At the same time, the objects for impact sensitivity modeling are mainly vacuum-isolated molecules, despite the fact that a number of recent works stress the significant role of polymorphs in molecular engineering of impact sensitivity. , Therefore, modeling of this quantity using properties of the crystalline state is crucial for mechanistic consideration. In this aspect, some efforts were made earlier to develop solid-state descriptors. − Meanwhile, calculations of periodic systems are very expensive, especially when using nonempirical methods, like density functional theory (DFT); therefore, the data sets in such cases are relatively small (tens of compounds). ,− …”