New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates.
Department of Chemistry, Bioscience and Environmental Engineering, Faculty of Science and Technology, University of Stavanger, POB 8600, 4036 Stavanger, Norway.
J Am Chem Soc. 2020 Jun 24;142(25):11219-11231. doi: 10.1021/jacs.0c03990. Epub 2020 Jun 12.
The melting of any pure crystalline material at constant pressure is one of its most fundamental properties, and it has been used to identify organic compounds or to verify their chemical or phase purity since the early times of chemistry. Here, we report that a mechanical deformation of plastic organic single crystals such as bending results in a small yet significant decrease in their melting point of about 0.3-0.4 K. The bent section of the crystal was found to be mechanically softer relative to the straight sections, and the softening temperature preceding the melting was also lower on the convex (outer) side of the bent crystal. Melting of the bent crystal starts at the kink and often appears as splitting of the respective endothermic peak in its thermal (DSC) fingerprint, while unilateral compression of the crystal results in multiple peaks. These thermomechanical effects become more pronounced with heavier mechanical damage due to an increased concentration of defects and ultimately result in a large temperature spread of the associated phase change in addition to melting-point depression in deformed or damaged crystals relative to their pristine counterparts. Within a broader context, the results show that mechanical treatment during sample preparation has a profound effect on the melting of a pure substance, and this could be critically important where the exact melting point is used as a means for polymorph identification.
在恒定压力下，任何纯晶态物质的熔化都是其最基本的性质之一。自化学早期以来，它就被用于识别有机化合物或验证其化学或物相纯度。在这里，我们报告说，对塑料有机单晶（如弯曲）进行机械变形会导致其熔点小幅度但显著降低，约为 0.3-0.4 K。发现弯曲晶体的弯曲部分相对于直段机械上更软，并且在熔化之前的软化温度在弯曲晶体的凸（外）侧也更低。弯曲晶体的熔化始于拐点，并且在其热（DSC）指纹中经常表现为各自的吸热峰分裂，而晶体的单侧压缩会导致多个峰。随着由于缺陷浓度增加而导致的更严重的机械损伤，这些热机械效应变得更加明显，并且除了变形或损坏的晶体相对于其原始晶体的熔点降低之外，还会导致相关相变的温度范围增大。在更广泛的背景下，结果表明，在样品制备过程中的机械处理对纯物质的熔化有深远的影响，而在将精确的熔点用作多晶型体识别手段的情况下，这可能至关重要。
