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

Abstract

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 °C is reported for the first time. Interestingly, TNT exhibited the formation of a supercooled liquid after undergoing solvent evaporation at 70 °C which is described from the viewpoint of two-step nucleation.

Structural Origins of Elastic and 2D Plastic Flexibility of Molecular Crystals Investigated with Two Polymorphs of Conformationally Rigid Coumarin

“…A change in the molecular structure inevitably leads to changes in the crystal structure. For this reason, organic polymorphs are effective model systems for probing crystal structure–mechanical property relationships since they enable the elimination of the influence of molecular structure while probing very different physical properties, such as color, solubility, conductivity, and explosibility, − and mechanical properties. − A few polymorphic pairs exhibited distinct plastic and elastic mechanical behaviors. ,,, However, all these molecules undergo conformational changes in different polymorphs. Crystals of rigid organic molecules can be either plastic, where molecules stack to form rigid columns through π–π stacking interactions and only weak interactions are present between columns, ,,− or elastic, where molecules stack (π–π) along the crystal long axis combined with crisscross arrangements of molecules in adjacent columns. ,− These observations would suggest that molecular conformational flexibility is not required for molecular crystals to exhibit either plastic or elastic flexibility.…”

“…24−30 A few polymorphic pairs exhibited distinct plastic and elastic mechanical behaviors. 19,20,23,24 However, all these molecules undergo conformational changes in different polymorphs. Crystals of rigid organic molecules can be either plastic, where molecules stack to form rigid columns through π−π stacking interactions and only weak interactions are present between columns, 12,20,31−33 or elastic, where molecules stack (π−π) along the crystal long axis combined with crisscross arrangements of molecules in adjacent columns.…”

Polymorphism in a Nonsensitive-High-Energy Material: Discovery of a New Polymorph and Crystal Structure of 4,4′,5,5′-Tetranitro-1H,1′H-[2,2′-biimidazole]-1,1′-diamine

“…In order to balance these two requirements, crystal engineering is an effective strategy. − After many years of research, it has been found that the macroscopic properties of energetic materials are related not only to the microscopic molecular structures but also to the stacking mode of the crystal. − By means of controlling crystal shape and morphology or eutectic preparation, the properties of various explosives have been modified. CL-20, HMX, FOX-7, and LLM-105 are the most commonly investigated explosives.…”

Study of Physicochemical and Explosive Properties of a 2,4,6-Trinitrotoluene/Aniline Cocrystal Solvate

“…TNT has been shown to mostly occur as either a monoclinic (more stable) or orthorhombic polymorph under ambient conditions. , Sherwood and co-workers reported that it was possible to reliably produce either polymorph depending on the solvent (i.e., relative solubility) that was used during the crystallization process . While a comparable investigation was performed related to the polymorphic crystallization of TNT in a wider array of different solvents, it was unexpectedly discovered that TNT and aniline cocrystallized to produce a cocrystal solvate with a ruby red appearance (Figure A,B). A similar red color transition is also observed in aniline solutions containing relatively high concentrations of TNT (Figure C).…”