From Mechanical Effects to Mechanochemistry: Softening and Depression of the Melting Point of Deformed Plastic Crystals

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.