Novel Phase Behaviour in Bent-Core Nematic Liquid Crystals and their Mixtures

Studies of bent-core liquid crystals have provided a fascinating insight into these systems, not only for their potential use in applications but also for fundamental research. It is known that the elastic behaviour in bent-core liquid crystals is different than in calamitic liquid crystals; the splay elastic constant K11 < K33 (bend elastic constant) in bent-cores, compared to K33 < K11 in calamitics. This thesis will investigate the elastic behaviour in oxadiazole based bent-core liquid crystals and in their mixtures, aiming to gain an insight into the phase behaviour that is exhibited in these systems. An oxadiazole based bent-core liquid crystal was found to exhibit anomalously low twist (K22) and bend elastic constants. Computational and theoretical calculations were carried out on this material; the elastic behaviour was explained by considering the contributions of spontaneous chirality and polarity. Fluctuations of the chiral conformers of the molecule cause a negative correction to K22 which was found to be large in this material. In a similar fashion, fluctuations in the local polar order cause a negative correction to K33 which was also found to be large in this material. Binary mixtures of two bent-core liquid crystals were created and the elastic constants measured. It was found that the values measured for the mixtures lie in between those measured for the two pure materials, as expected when considering simple mixing rules. Mixtures of calamitic liquid crystals doped with 10% of bent-core mesogens were also investigated. In all of these mixtures the splay constant was unaffected by the bent-core dopant, with the bend constant reducing by 10-20% and the twist constant reducing by up to 40%. The splay results are explained by considering that the bent-core dopants have no scope to exhibit any wedge-shaped conformers, thus having no significant impact on K11. The bend results are in line with simple mixing rules and existing literature. The twist results suggest that the fluctuations in chiral conformers of the bent-core molecules enhance fluctuations of chirality in the conformers in the calamitic host, thus providing a large negative correction to K22. Investigating the phase behaviour of the binary mixtures of two bent-core liquid crystals showed that two different underlying phases were exhibited, dependent on the concentrations of the bent-core materials. It was found that the elastic behaviour in the nematic phases of the mixtures had no discernible influence on the formation of the underlying phases, and thus the elastic constants are not the sole driving factor in the formation of the underlying phases. In mixtures of calamitic liquid crystals doped with bent-core mesogens, it was found that some of the mixtures exhibited self-assembling �filament structures that grow in phases below the isotropic phase. Results showed that the �lament growth is dependent on the director, and exhibited time-dependent behaviour that is analogous to that observed in certain lyotropic systems. This thesis forms part of a project funded by the EPSRC CASE award with Merck Chemicals Ltd., aiming to investigate the potential of bent-core liquid crystals for both display and non-display applications. The results described here aid in understanding the novel elastic and phase behaviour in bent-core liquid crystals and their mixtures.