Elastic Anisotropy of RDX Studied by the Supramolecular Structural Unit

The elastic property-crystal structure relations provide a foundation to design new materials with desired properties and understand the chemical decomposition and explosion of energetic materials. The supramolecular structural unit was proposed as the smallest chemical unit to quantitatively characterize the elastic anisotropy of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX). The supramolecular structural unit refers to the nearest-neighbor coordination polyhedron of one molecule. The supramolecular structural unit of RDX was composed of 15 molecules, which were analyzed by the total molecular number density and the density of intermolecular interactions. The elastic modulus model was established based on the following assumptions: (i) the RDX molecules were spherical and rigid-body; (ii) the intermolecular interactions were viewed as the linear spring, i.e., bond-spring model; (iii) the molecules were close-packed with the series mode. The elastic modulus model based on the supramolecular structural unit demonstrated that the elastic modulus was intrinsically determined by the total molecular number, the equilibrium distance of the molecular pair, the intermolecular force constant, and the angle between the intermolecular interactions and the normal to crystal face. The intermolecular force constant was calculated as the second derivative of the intermolecular interactions with regard to the equilibrium centroid distances. The intermolecular interactions were expressed as the sum of van der Waals and electrostatic interactions calculated by COMPASS (condensed-phase optimized molecular potentials for atomistic simulation studies) II forcefield. The calculated elastic moduli were 21.7, 17.1, 20.1, 19.1, and 15.3 GPa for RDX (100), (010), (001), (210), and (021) crystal faces, respectively. The calculation results were consistent with the theoretical values computed by the density functional theory. Excluding RDX(001), the calculated elastic moduli agreed with the experimental results measured by the resonant ultrasound spectroscopy (RUS), impulsive stimulated thermal scattering (ISTS), Brillouin spectroscopy, and nanoindentation methods. The theoretical values (20.1 GPa) of RDX(001) overestimated the experimental values with the range of 15.9~16.6 GPa. The reason can be attributed to the rigid-body approximation for flexible molecules, which ignored the motion and deformation of the ring and NO2 groups when the external loads were applied to RDX(001). The results suggested that the supramolecular structural unit can be the smallest chemical unit to quantitatively characterize the elastic anisotropy of RDX and the elastic anisotropy was mainly attributed to the angle between the intermolecular interactions and the normal to crystal face.