Thermal and mechanical responsiveness of some 4-substituted benzonitriles
Thesis (PhD)--Stellenbosch University, 2018.
ENGLISH ABSTRACT: This work is based on three publications. The first manuscript describes the large volumetric thermal expansion of an organic cocrystal over a wide range of temperature (100–300 K). The novel organic multicomponent crystal (cocrystal) (ABN·2DMABN) consisting of 1:2 molar ratio of 4-aminobenzonitrile (ABN) and 4-(dimethylamino)benzonitrile (DMABN) was prepared. It shows linear positive thermal expansion (PTE) along all its three principal axes over the temperature range 100–300 K, which is exclusively dependent on the intermolecular interactions that govern the crystal packing. The associated volumetric thermal expansion coefficient (αv) of 222 MK-1 is the largest reported to date for a cocrystal over such a wide temperature range. The second manuscript explains the mechanical response/behaviour of a rigid small organic molecule, 4-bromobenzonitrile (4-BBN) that has been crystallised using sublimation under dynamic vacuum (0.02 mbar). Interestingly, the crystals show highly flexible plastic behaviour and bends along two orthogonal faces under mechanical stress− a rare phenomenon, resulting in helical twisting, or coiling, which is also rare for crystals of such rigid small organic molecules. The third manuscript discusses the thermal behaviour of the purely organic material 4-aminobenzonitrile (ABN). The single crystals of ABN undergo reversible thermosalience upon cooling from 300 to 100 K and subsequent heating to 300 K. In this study we have demonstrated that the release of accumulated strain in these crystals upon cooling, which results in rapid structural rearrangement, is due to the interplay between the directional and non-directional intermolecular interactions in the system.
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