Detonation Performance Analysis of Cocrystal and Other Multicomponent Explosives

Development of novel energetic molecules is a challenging endeavor. Successful discovery and synthesis of a novel viable energetic molecule is an even more challenging endeavor. To qualify for scale-up in production, the molecule must undergo extensive characterization at the small scale and meet criteria for sensitivity, stability, toxicity, lifetime, etc. A failure to qualify for further scale-up can result in significant wasted investment. Cocrystallization of energetic materials is a potentially attractive route to development of new energetic materials because existing molecules can be used to create new materials that have tailored properties different from either coformer. A cocrystal is a combination of two crystalline monomolecular materials that yields a material with a unique crystal structure. While cocrystallization reduces the front-end investment ordinarily required for discovery of new energetic molecules, discovery of energetic cocrystals is not trivial. A number of energetic cocrystals have been reported that display attractive properties such as high density and improved thermal stability. However, the effect of cocrystal formation on larger scale properties, particuarly detonation properties, is not well-understood. Knowledge of these properties is important for understanding the potential improvements gained from pursuing discovery of cocrystals. \\\\

A challenge with obtaining detonation properties is that most techniques typically require anywhere from hundreds of grams to several kilograms of material. For example, rate stick experiments typically have an L/D (length to diameter) ratio between 12 and 20. Even for ideal explosives, diameters used are typically at least two centimers in diameter. Such experimental configurations are poorly suited for materials in the early stages of development. \\\\

In this work, comparative detonation velocity measurements were performed for select hexanitrohexaazaisowurtzitane (CL-20) cocrystals that have been reported in the past five years along with corresponding formulations or physical mixtures of the components. The detonation velocity measurements were performed using microwave interferometry, a well-established detonation velocity diagnostic. Using precision-machined hardware and appropriate matching of booster charge to sample charge, it was shown with statistical analysis that well-resolved measurements of detonation velocity could be obtained with shot-to-shot variation in the range of 130 m/s. The detonation velocity for cyclotetramethylene tetranitramine (HMX) was obtained using this experimental technique to validate the method and estimated variation. It was demonstrated that detonation tests with good repeatability could be performed for the nearly ideal explosives considered. \\\\

The experimental technique described above was performed first for a cocrystal of 1-methyl-3,5-dinitro-1,2,4-triazole (MDNT) and CL-20. Comparative measurements were performed for the cocrystal and physical mixture at a loading density of 1.4 $\gcc$. We chose a fixed loading density in order to isolate isolate effects other than loading density. The cocrystal was observed to detonate about 500 m/s faster than the physical mixture. In comparison, thermochemical equilibrium predictions showed that the cocrystal would detonate about 230 m/s faster than the physical mixture at this density. The enthalpy of formation for this cocrystal was double that of the physical mixture and this difference resulted in the predicted difference. Similar measurements were performed for the cocrystal of cyclotetramethylene tetranitramine (HMX) and CL-20 and CL-20/hydrogen peroxide (HP) solvate at the same loading density. The HMX/CL-20 cocrystal was observed to detonate about 300 m/s faster than the physical mixture. The CL-20/HP solvate was observed to detonate about 300 m/s faster than CL-20. \\\\

Using the Kamlet scaling laws, it was determined that the differences in detonation velocity observed are attributable to differences in enthalpy of formation. That is, the energy state is different between the configurations. The enthalpy of formation for MDNT/CL-20 was measurably larger than its physical mixture. The CL-20/HP solvate was also measurable larger than that of CL-20. This result has implications for intermolecular bond and configurational energies formed in cocrystals that affects their energy content.

Fully explaining the precise reason for this, and perhaps exploiting this in future cocrystals and multimolecular systems is a challenge for modelers, theoreticians, and synthesis chemists.