Interaction anisotropy and shear instability of aspirin polymorphs established by nanoindentation
Interaction anisotropy and shear instability of aspirin polymorphs established by nanoindentation
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DOI：
10.1039/c1sc00430a
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发表时间：
2011-10
期刊：
影响因子：
8.4
通讯作者：
S. Varughese;M. Kiran;K. Solanko;A. Bond;U. Ramamurty;G. Desiraju
中科院分区：
文献类型：
--
作者：
S. Varughese;M. Kiran;K. Solanko;A. Bond;U. Ramamurty;G. Desiraju
纳米压痕是应用到两个多晶型阿司匹林检查和区分它们的相互作用的各向异性和剪切不稳定性。阿司匹林提供了一个很好的测试系统的技术，因为：（i）多晶型I和II在两个维度上表现出结构相似性，从而有利于明确检查机械响应的差异，这与两种晶体结构之间的明确差异有关;（ii）亚稳态多晶型II的单晶最近才变得容易获得;（iii）剪切不稳定性已被提出用于II。两种多晶型物测定的不同弹性模量和硬度值与其晶体结构相关，并且测量的热膨胀系数支持了解释。亚稳多晶型物II向稳定多晶型物I的应力诱导转化可以通过机械研磨快速实现，并且通过滑移机制进行。这项工作确立了纳米压痕提供了两种阿司匹林多晶型物的“签名”响应，尽管它们的晶体结构非常相似。它还展示了该技术的价值，以量化稳定性关系和分子晶体中的相变，使更深入地了解晶体工程的背景下的多态性。
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.