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 polymorphsmonoclinic
or orthorhombicwith 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.
In the development of crystal engineering and supramolecular chemistry, cocrystallization has been used as a way to develop novel explosives with tailored properties. We present a novel cocrystal solvate composed of 2,4,6trinitrotoluene (TNT) and aniline that exhibits unique physicochemical and explosive properties. X-ray diffraction studies reveal the crystallographic structure to contain TNT and aniline in a 1:1 molecular ratio. The crystals themselves exhibit a vibrant, ruby red color that likely results due to a charge-transfer interaction between the overlapping π-orbitals of the aromatic rings. The most notable evidence for a chargetransfer complex is the appearance of a broad absorbance peak in the visible region which is not present in the spectrum of either pure component. Comparisons of the cocrystal solvate to that of pure TNT are conducted to determine thermodynamic and kinetic parameters using both experimental and theoretical techniques. The desolvation of aniline from the cocrystal solvate was also investigated using both in situ powder X-ray diffraction and atomic force microscopy measurements to monitor changes in the crystal structure and surface topography, respectively.
The present work presents results from an investigation of the glass transition and crystallization behaviors of HNAB tested over more than five orders of magnitude of cooling rate from 0.005 °C/s to 600 °C/s (0.3 to 36000 °C/ min) by a combination of conventional and Flash differential scanning calorimetry (DSC). The work quantifies the influence of the thermal amorphization route on the properties of this high explosive. Cooling rates faster than 100 °C/s (6000 °C/min) result in amorphous HNAB as expected from prior work, but we also find that amorphization of the HNAB occurs at cooling rates slower than 0.008 °C/s (0.5 °C/min). The behavior of the amorphous HNAB made by slow cooling is compared with that of amorphous HNAB made by fast cooling, as well as with that made by solvent casting in terms of glass transition temperature, apparent activation energy of glass transition, and dynamic fragility parameter m. Besides, the non-isothermal crystallization response as a function of cooling rate is also reported. The thermal stability and decomposition energy of amorphous HNAB are compared with those of the crystalline counterpart, being similar heats of decomposition of 3295 and 3392 J/g, respectively; suggesting that the amorphous HNAB will have similar thermal stability and chemical energy to the crystalline form.
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