Characterisation of Structural Order in Intergrowth Polymorphs

Kumar, Mudda Deva (2022) Characterisation of Structural Order in Intergrowth Polymorphs. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Mudda Deva Kumar (17MS132)) 17MS132_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (3MB)

Abstract

Despite the great deal of developments in the field of Crystal Engineering, there has been no or little attention paid to “intergrowth polymorphism” which has been observed in Metals, Alloys, Zeolites, etc. There have been only a few examples of organic molecules showing this interesting intergrowth polymorphism reported in the literature. The expression “intergrowth” refers to the coexistence of two or more polymorphic components in a single entity (single crystal) due to a higher degree of structural similarity between two polymorphs. Thus, in this study, I have chosen a reported system of Aspirin, an Active Pharmaceutical Ingredient (API) which has been extensively studied and was observed to show the intergrowth polymorphism. I intended to investigate the changes in the structural order of the Aspirin crystals along its crystal length using different experimental and characterisation techniques. While exploring the intergrowth polymorphism in this system, I have also observed the novel plastic bending deformation in the aspirin single crystals. These aspirin crystals were contrarily reported to be showing brittle behaviour upon application of a mechanical stress. Aspirin crystals here not only showed plastic deformation but also the exfoliation of the crystal layers. Due to arrangement of slip planes in such a way that they are nearly perpendicular to relatively robust C–H···O hydrogen bonding interactions which resulted in anisotropy in crystal packing. Thus, the kind of packing seen in aspirin single crystals very well establishes the reasoning for the discovered plastic bending upon inspection with tweezers and needle while scrutinizing their mechanical properties beneath optical microscope. In this case, existence of these weakly stabilized low energy slip planes in crystal structures facilitate shearing movement along the direction of the applied stress causing irreversible bending in the aspirin crystals. This study aimed to exploit a state-of-the art Dark-Field Mueller Matrix Spectroscopy (Mueller Matrix Imaging Polarimetry) and Raman Spectroscopy techniques. As these are effective tools for probing the structural order with high spatial resolution. Furthermore, the plastic deformation and exfoliation nature in aspirin single crystals was not reported so far in the literature, hence we investigated this system with diverse characterization techniques such as PXRD, DSC, Raman spectroscopy, etc which has also aided in confirming the presence of metastable form-II along with form-I of aspirin confirming the intergrowth polymorphism in aspirin crystals.

Actions (login required)

View Item