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Issue 11, 2011
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Interaction anisotropy and shear instability of aspirin polymorphs established by nanoindentation

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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.

Graphical abstract: Interaction anisotropy and shear instability of aspirin polymorphs established by nanoindentation

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Publication details

The article was received on 06 Jul 2011, accepted on 05 Aug 2011 and first published on 25 Aug 2011


Article type: Edge Article
DOI: 10.1039/C1SC00430A
Citation: Chem. Sci., 2011,2, 2236-2242
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    Interaction anisotropy and shear instability of aspirin polymorphs established by nanoindentation

    S. Varughese, M. S. R. N. Kiran, K. A. Solanko, A. D. Bond, U. Ramamurty and G. R. Desiraju, Chem. Sci., 2011, 2, 2236
    DOI: 10.1039/C1SC00430A

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