Predicted Cohesive Properties of a TATB Polymer-Bonded Insensitive High Explosive

Polymer-bonded explosives (PBXs) are composite materials whose structure-property relationships are poorly understood due to their complex microstructure. Microstructural adhesion between the energetic molecular crystals and the polymer binder in a PBX affects key properties including mechanical strength, safety, and aging characteristics, but is difficult to measure experimentally. To this end, we develop a molecular dynamics (MD) modeling framework to predict the cohesive properties of a prototypical PBX based on the insensitive high explosive TATB (1,3,5-triamino-2,4,6-trinitrobenzene) and a fluoropolymer binder. The MD framework rests on two key capabilities: (1) general protocols to construct MD simulations of crystal-polymer and crystal-crystal interfaces in low-symmetry materials, and (2) a numerically robust approach to compute the surface and interface energies. Formal analysis of the surface energy equation shows that its direct evaluation is poorly conditioned for systems involving polymers. In this respect, we adopt a regression-based approach for computing surface energies and extend it to interfaces. Application to idealized, defect-free TATB facets interfaced with a fluoropolymer reveals several energetic drivers, including a thermodynamic driving force for grain coarsening and a greater propensity for TATB crystallites to adhere to each other than to the polymer binder. These data indicate that the thermodynamic landscape governing formulation design and aging of PBXs is complex and involves additional variables besides crystal-polymer adhesion. While the present study focuses on explosives, the MD methodology is general and can be readily adapted to model other material systems involving low-symmetry crystals, including organic electronics, pharmaceuticals, and advanced polymer composites.