Elastic Anisotropy of Molecular Crystals Calculated by an Elastic Model Established on a Supramolecular Structural Unit

The crystal structure-elastic property relationship provides a foundation for the design of new materials. The supramolecular structural unit, i.e., the nearest neighbor coordination polyhedron, is proposed to describe the elastic deformation of molecular crystals. The elastic model established on the assumptions of bond-spring and spherical molecule demonstrates that the elastic modulus is intrinsically dependent on the coordination number (N), equilibrium centroid distance (R-ij), intermolecular force constant (k(ij)), and the angle (theta(ij)) between the intermolecular interactions and the normal to the crystal face. The supramolecular structural units are composed of 15, 13, 9, 7, 8, and 6 molecules, and the elastic moduli are in the ranges of 15.3-21.7, 14.3-19.5, 17.6-23.0, 6.2-9.1, 5.7-10.1, and 2.3-16.2 GPa for alpha-RDX, beta-HMX, epsilon-CL-20, I-Aspirin, II-Aspirin, and IV-Aspirin, respectively. The predicted elastic moduli are consistent with previous theoretical and nanoindentation values, and the elastic anisotropy may be attributed to the change in the molecular conformation and arrangement of the intermolecular interactions. The supramolecular structural unit may fail to describe the long-range intermolecular interactions such as hydrogen bonds, halogen bonds, and pi-packing. Therefore, The proposed method should be further optimized to apply to molecular crystals with planar molecules and graphite-like layered structures.