Abstract:
The crystal structures of four INZ cocrystals with analogous crystal
coformers were probed to understand the relationships among molecular packing,
H-bonding dimensionality, single-crystal plasticity, and bulk mechanical behavior.
These structurally analogous coformers coherently directed H-bonds by “philic”
functionalities (−OH and −COOH) and vdW interactions by a “phobic” scaffold
(C6H5−n, where n = 0, 1, 3). In comparison to INZ:2HBA and INZ:4HBA,
INZ:BA and INZ:GA exhibited higher plasticity and, hence, better tableting
performance due to larger bonding area and higher tensile strength. The rank order
of apparent yield pressure and incipient plasticity quantified from “in-die” Heckel
analysis of the bulk phase, INZ:2HBA > INZ:4HBA > INZ:GA > INZ:BA,
however, does not match that of nanomechanical hardness and elastic modulus,
INZ:BA > INZ:2HBA > INZ:4HBA > INZ:GA. The discrepancy may be
attributed to the anisotropy in crystal mechanical properties, where the stiffness of
the dominant crystal faces probed with nanoindentation may grossly deviate from
the bulk mechanical behavior. Therefore, nanomechanical attributes are more predictive of more isotropic molecular crystals,
such as 3D H-bonded or interlocked structures, in comparison to those exhibiting gross structural anisotropy, such as crystals
with distinct molecular layers that favor facile slip. Hence, the accurate prediction of bulk behavior on the basis of
nanomechanical characterization requires the incorporation of crystal shape and packing as well as knowledge of facet-specific
mechanical properties. Moreover, the prediction of bonding strength on the basis of molecular packing is still warranted when
the crystallographic molecular slip may cause a deviation in the proposed relationship.
