Energy dissipation pathways in molecular organic crystals under low-velocity impact.

With the adoption of mechanical processing to synthetic organic chemistry, mechanochemistry is undergoing a new phase of significant growth. The prospect of developing more sustainable, alternative synthetic routes to high-value organic and pharmaceutical compounds has provided a formidable boost to experimental studies. These experimental advances are not accompanied by a comparable understanding of the phenomena involved. In particular, the application of mechanical forces to solid systems, mostly in granular form, is frequently assumed to directly perturb chemical bonds. This work was conceived expressly to address this misconception by elucidating the mechanisms of mechanical energy transfer and subsequent dissipation in organic molecular crystals by a deliberately general theoretical investigation. The system investigated consists of a 0.5-mm thick layer of monodisperse organic powder with an average particle size of 10 µm, arranged in a random dense packing, impacted by a 12-g steel sphere traveling at 10 m s-1. The impact energy is progressively scaled down to the mesoscopic and microscopic regimes considering contact mechanics, crystal slip plasticity, thermal constraints associated with localised melting, and the energy transfer from external lattice phonons to intramolecular vibrational modes. The analysis shows that particle rearrangement, plastic shear sliding, phase change, and low-frequency lattice phonon thermalization collectively dissipate the mechanical energy, preventing intramolecular bonds to be altered by the impact. In other words, in the case of organic molecular crystals, mechanical energy from ball milling can be dissipated almost entirely through collective intermolecular processes, leaving the intramolecular covalent framework chemically intact.