Layers can be deceiving: A hopping model for small molecule diffusion in TATB crystal

Many material properties of the insensitive high explosive TATB (1,3,5-triamino-2,4,6- trinitrobenzene) exhibit pronounced anisotropy due to its graphitic-like layered crystal packing structure. This structure evokes a mental schema in which the layers form nanoscopic channels, but very little is known regarding the permeability of the lattice by small molecules. We use molecular dynamics (MD) simulations to obtain predictions for the diffusion of small molecules through TATB single crystal. An approach to fit classical MD force fields is developed to accurately model interactions between TATB and small molecules including H2O, He, Ne, and Ar. Fitted force fields are combined with a steered MD approach to probe transport of these small molecules along selected directions in the crystal. We find that small molecule transport occurs via a hopping mechanism exhibited by distinct jumps between interstitial sites. Perhaps counter-intuitively, we find that transport via the hopping mechanism is substantially faster normal to the layers as compared to along them. This result stems from the finding that intralayer junctions between adjacent TATB molecules are the most stable interstitial sites, which leads to lower energetic barriers along directions that have a component normal to the layers. An empirical model for diffusion rate based on the MD data shows that the rate decays exponentially with increasing molecular radius and is negligibly small for for all molecules larger than He, including common atmospheric gases. These findings have implications on the interpretation of experiments that measure surface area, material response to extreme conditions, and material aging.