Interaction anisotropy and shear instability of aspirin polymorphs established by nanoindentation

Varughese, Sunil and Kiran, MSRN and Solanko, Katarzyna A and Bond, Andrew D and Ramamurty, U and Desiraju, Gautam R (2011) Interaction anisotropy and shear instability of aspirin polymorphs established by nanoindentation. In: Chemical Science, 2 (11). pp. 2236-2242.

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Abstract

Nanoindentation is applied to the two polymorphs of aspirin to examine and differentiate their interaction anisotropy and shear instability. Aspirin provides an excellent test system for the technique because: (i) polymorphs I and II exhibit structural similarity in two dimensions, thereby facilitating clear examination of the differences in mechanical response in relation to well-defined differences between the two crystal structures; (ii) single crystals of the metastable polymorph II have only recently become accessible; (iii) shear instability has been proposed for II. Different elastic moduli and hardness values determined for the two polymorphs are correlated with their crystal structures, and the interpretation is supported by measured thermal expansion coefficients. The stress-induced transformation of the metastable polymorph II to the stable polymorph I can be brought about rapidly by mechanical milling, and proceeds via a slip mechanism. This work establishes that nanoindentation provides ``signature'' responses for the two aspirin polymorphs, despite their very similar crystal structures. It also demonstrates the value of the technique to quantify stability relationships and phase transformations in molecular crystals, enabling a deeper understanding of polymorphism in the context of crystal engineering.

Item Type:	Journal Article
Publication:	Chemical Science
Publisher:	Royal Society of Chemistry
Additional Information:	Copyright of this article belongs to Royal Society of Chemistry.
Department/Centre:	Division of Chemical Sciences > Solid State & Structural Chemistry Unit Division of Mechanical Sciences > Materials Engineering (formerly Metallurgy)
Date Deposited:	17 Nov 2011 09:45
Last Modified:	17 Nov 2011 09:45
URI:	http://eprints.iisc.ac.in/id/eprint/42106

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