Effects of Solution Conditions on Polymorph Development in 2,4,6-Trinitrotoluene

The explosive performance of crystalline energetic materials is strongly related to the material's crystal structure. For example, 2,4,6-trinitrotoluene (TNT), one of the most common secondary explosives, is known to predominantly exist as one of two polymorphs-monoclinic or orthorhombic-with the former being more thermodynamically stable. The production of different polymorphs is commonly achieved via crystal growth from solution in which the outcome is highly dependent on the absolute solubility. In the present work, effects related to both the nature and temperature of different solvents used for TNT crystallization are investigated via two separate crystal growth techniques: solvent evaporation and solvent/antisolvent (water) precipitation. The resulting polymorphism for each crystal sample was independently characterized using differential scanning calorimetry and single crystal X-ray diffraction. For both methods of crystallization, results demonstrate a strong correlation between the obtained polymorph and both TNT's relative solubility in each solvent and the crystal growth temperature. The results are mostly consistent with the theoretical viewpoint of Ostwald's rule of stages. Furthermore, an investigation of TNT crystals produced from solutions at temperature in excess of 25 degrees C is reported for the first time. Interestingly, TNT exhibited the formation of a supercooled liquid after undergoing solvent evaporation at 70 degrees C which is described from the viewpoint of two-step nucleation.