Impact of polymorphism on mechanical properties of molecular crystals: a study of p-amino and p-nitro benzoic acid with nanoindentation

We report on nanoindentation data for two pairs of polymorphic compounds of p-aminobenzoic acid (pABA) and p-nitrobenzoic acid (pNBA). Variations of 60 % or higher are observed in both stiffness and hardness data between the two forms of each pair. We interpret these observations through variations of Periodic Bond Chains (PBCs), i.e. variations in continuous intermolecular interactions strength and nature. Aided with hardness data of polymorphs from the literature, we then further assess how PBCs can be used to assess and compare mechanical properties of polymorphs. We define a new parameter, s-PBC which quantifies the strongest PBC interaction that needs breaking so as to allow a switch of the PBC network from 3D to 2D/1D. We found that higher values of s-PBCs, normalised by the mean molecular volumes, led to higher values of mechanical properties. Although not infallible, this trend seemed to hold well when comparing polymorphs of a given compound, but not so much when comparing across different compounds.