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.