Polymorphic Transition in Traditional Energetic Materials: Influencing Factors and Effects on Structure, Property, and Performance

Polymorphism is universal in energetic materials, and polymorphic transformation (PT) causes variations in the structure, properties, and performance. This article reviews the polymorphs of six traditional energetic compounds (ECs), including 2,4,6-trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), 1,3,5-trinitro-1,3,5-triazinane (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), and 2,2-dinitroethylene-1,1-diamine (FOX-7), and the PT-induced variations of molecular and packing structures and energies, crystal morphology, sensitivity and detonation performance, and defects, as well as factors influencing the PT and strategies for controlling PT. In all of the polymorphs determined experimentally, there is a small difference in molecular volume, whereas a large difference can appear in molecular conformation, intermolecular distance, packing pattern, crystal density, sensitivity, and detonation performance. Moreover, in addition to temperature and pressure, crystal quality and additive can seriously affect the PT. In addition, recrystallization, coatings, and addition of additives are available to control the PT. Finally, some issues are raised, such as the determination of new polymorphs at high temperature and high pressure, clarifying the PT boundary and mechanism, considering polymorphs at extremes in understanding an EC, and paying attention to the PT of newly thriving energetic cocrystals and energetic ionic salts.