The mechanical response of molecular materials is important in a variety of applications, from powder processing and tablet pressing in pharmaceuticals, to cycling behavior of optoelectronics, to detonation initiation in explosives. Incredibly, despite the importance of crystal mechanics in diverse applications, irrefutable knowledge of such processes in these materials is rudimentary at best.
As a class of materials, molecular materials are poorly understood. Cubic and hexagonal metals, and even covalent or ionic structures, have clear general rules for how deformation occurs, and for how impurities can be incorporated to influence deformation or other properties. In molecular materials, which almost always have lower symmetry crystal structures, there is little data and understanding of their mechanics. The balance between symmetry and lattice deduced pathways, molecular flexibility, and chemical interaction is a mystery.
We have recently demonstrated a method to quantify the elastic-plastic response of molecular materials using nanoindentation for the pharmaceutical paracetamol and the explosive cyclotrimethylene trinitramine (RDX). Both specific material properties (modulus, hardness, yield point, shear strength) and the physical mechanisms of deformation are quantified for different polymorphs, orientations, purities, and co-crystals. This technique promises to offer both a quantitative and conclusive understanding of molecular crystal mechanics since rational changes to structure are possible. The technique is also an extremely sensitive probe of surface quality, impurities, and phase stability. Complementary experimental methods reveal dislocation mobility and rate-dependent pathways. Large-scale molecular dynamics simulations with molecular flexibility reveal impurity locations and confirm elastic-plastic mechanisms.
This is joint work with K. J. Ramos, S. R. Vigil, M. J. Cawkwell, and D. F. Bahr.