Cocrystallization of Energetic Materials.

Here cocrystallization, the formation of crystals containing two or more neutral molecular components in a defined ratio within a crystal lattice, is explored as a means to alter and improve the properties of energetic materials, including explosives, propellants and pyrotechnics. To develop cocrystal engineering principles for energetic materials, model cocrystal systems between energetic and non-energetic materials were studied. Series of cocrystals were formed with 2,4,6-trinitrotoluene (TNT) and 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX) to investigate cocrystallization of aromatic and aliphatic energetic materials respectively. These model systems demonstrated the ability to cocrystallize energetic materials and provided insight on favorable interactions for the formations of these cocrystals. TNT formed cocrystals exclusively with electron-rich aromatic compounds through donor acceptor pi-pi interactions, while HMX formed cocrystals with a wide variety of cocrystal formers primarily through electrostatic interactions. The structures of these cocrystals were dictated by the size, shape and electronic character of the cocrystal formers. Properties relevant to energetic materials were altered through cocrystallization including the melting point, decomposition temperature, density, oxygen balance and impact sensitivity. The principles developed from these two model energetic cocrystal systems led to the formation of novel energetic-energetic cocrystals between diacetone diperoxide (DADP) and a series of halogenated trinitrobenzenes: 1,3,5-trichloro-2,4,6-trinitrobenzene (TCTNB), 1,3,5-tribromo-2,4,6-trinitrobenzene (TBTNB) and 1,3,5-triiodo-2,4,6-trinitrobenzene (TITNB). Like the TNT and HMX cocrystals, the DADP cocrystals showed altered materials properties. Unlike the previously discovered energetic cocrystals, impact sensitivity was not improved with respect to the most sensitive component, DADP, even after cocrystallization, suggesting that the peroxide moiety is inherently unstable. This result suggests that cocrystallization can be used as an experimental means to probe causes of sensitivity for energetic materials. Finally, the DADP/TBTNB cocrystal was discovered to be a kinetic cocrystal. The existence of a kinetic cocrystal is relevant for the development of cocrystallization because it highlights the need to ensure that potential kinetic cocrystals are not overlooked during the cocrystal screening process by employing methods that only favor thermodynamically stable forms.