Polymorph-Dependent Mechanical Behavior of 2-Bromo-3-Chlorbenzoic Acid Crystals: Plastic Bending, Twisting, and Shearing

Polymorphism plays a critical role in determining the mechanical behavior of molecular crystals. In the present study, we have investigated the polymorphism-dependent mechanical behavior of 2-bromo-3-chlorobenzoic acid (2B3CBA). During evaporative crystallization, 2B3CBA was found to crystallize in two polymorphic forms, namely, Form I (monoclinic) and Form II (triclinic). Form I crystals were also obtained through the sublimation of 2B3CBA. Form I crystals, which exhibit a needle-shaped morphology, were found to undergo one-dimensional plastic bending along with delamination of crystal strands. These crystals were also found to be twistable. On the other hand, Form II crystals, which have block-shaped morphology, underwent mechanical shearing upon application of mechanical stress. Detailed structural, thermal, and mechanical investigations were performed using powder X-ray diffraction, DSC, scanning electron microscopy, single-crystal X-ray diffraction, confocal Raman spectroscopy, Hirshfeld surface analysis, interaction energy calculations, and nanoindentation studies. Structural analysis revealed that the distinct mechanical responses of the two polymorphs originate from differences in slip-plane arrangements, intermolecular interaction energies, and molecular layer organization.