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Title: Templating novel thermotropic liquid crystal phases
Author: Trbojević, Nina
ISNI:       0000 0004 9352 3352
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2020
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Templating is a bottom-up approach used in materials science, whereby intricate nanoscale features of a ‘host’ medium are transferred onto a ‘guest’ matrix through a series of chemical and physical mechanisms. The past few decades have seen a great deal of research on the topic of templating lyotropic liquid crystal phases, mainly for biomedical applications. More recently, thermotropic liquid crystal templating has also sparked research interests, particularly the cholesteric and blue phases, for their applications in tuneable photonic structures. With thermotropic liquid crystal templating being a relatively recent field of research, little is known about the nanoscopic interactions between host liquid crystals and guest polymers that result in the templating effect. This thesis aims to gain a better understanding of how templates are created through the process of surface modification of polymers and which properties of the host liquid crystal can be templated. Two novel thermotropic mesophases were templated for the first time, namely the twist-bend nematic (NTB) phase and the ferroelectric SmA (SmAPF) phase. The host liquid crystal used to template the NTB phase was a mixture of the dimer CB7CB with the calamitic liquid crystal 5CB. Refilling the NTB template with a nematic mesogen allowed for a better understanding of the electrical and mechanical properties of the NTB phase. A carbosilane-tethered bent-core mesogen exhibiting the SmAPF phase was synthesised by adapting procedures available in literature, and this mesogen was used for templating the SmAPF phase. Refilling the SmAPF template enabled an insight into which properties of the host mesophase can be templated, such as the layered structure, biaxiality and ferroelectric switching. Samples composed of a monomer and host liquid crystal were thermodynamically-stabilised (photopolymerised) and metastable room temperature NTB phases were formed using this process. All photopolymerised phases showed similar thermal and dielectric properties to the pure host liquid crystals. Residual birefringence was observed in the polymer networks of all polymerised phases when the liquid crystal was in the isotropic state. Templates were created by washing the host liquid crystal from the polymer matrix. Raman spectroscopy was used for the first time to confirm that the templates were not contaminated with unwashed host liquid crystal. Visualisation of the templates using SEM revealed a clear difference between the template morphologies formed from different host liquid crystals, whereby the general mesoscopic structure of the host mesophase had been templated. The thermal and dielectric properties of the refilled templates showed that order was being induced in the refilled liquid crystal. The distinction between physical properties of the refilled templates and pure 5CB was especially clear in measurements of permittivity and threshold electric fields. For NTB templates, it was suggested that the nanofeatures of the NTB phase had been templated. The field of thermotropic liquid crystal templating is still in its infancy, yet it offers a great deal of potential in understanding the elegant interplay of chemical and physical processes in soft matter systems at the nanoscopic scale.
Supervisor: Nagaraj, Mamatha ; Read, Daniel J. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available