Structure–Thermomechanical Property Correlation in Polymorphic Molecular Crystals Probed by the Nanoindentation Technique

Recent
progress in understanding of the structure–mechanical
property relationship in organic materials has opened up many new
opportunities in materials science. Although the thermal and mechanical
properties of materials are well known, their correlation from a structural
point of view, which is crucial in the application of numerous functional
materials, remains poorly explored, particularly in organic crystals.
Here, we report the distinct thermal expansion behavior of three polymorphic
forms of a pharmaceutical salt, bazedoxifene acetate. Detailed analysis
reveals
the anomalous negative thermal expansion behavior in one of the forms
(form D), a consequence of the “spring-like” thermal
motion of structure. Further, our nanoindentation results reveal that
the thermal expansion coefficients are inversely proportional to the
elastic moduli of the polymorphic forms. The correlation between the
structure and thermomechanical properties of polymorphs is important
in the context of crystal engineering for the design and manufacture
of functional crystalline materials.