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Title: The Search for the Elusive Biaxial Nematic Phase
Author: Martin, Pamela Jean
ISNI:       0000 0001 3620 1646
Awarding Body: University of York
Current Institution: University of York
Date of Award: 2008
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Liquid crystals are ordered fluids formed ofanisotropic molecules, and there are a number of different mesophases within the liquid crystal state. This thesis is concerned with the search for a specific liquid crystal mesophase, known as the biaxial nematic phase, which has largely eluded experimentalists since it was first hypothesised. The re~earch is based on two, different strategies that use synthesised molecular candidates designed as a result of the interpretation of theory. Deuterium NMR spectroscopy is used to determine the phase symmetry. Chapter 1 commences with an introduction to the liquid-crystalline state, with a general description of the structures and of their constituent molecules. The structure and topology of the nematic phase and biaxial nematic phase are discussed in detail while only some of the more common mesophases are described. The general methods utilised in the characterisation of liquid crystals are discussed and this is followed by a discussion of techniques specific to the identification ofthe biaxial nematic phase. The successful discovery of biaxial nematic phases in some systems is highlighted, after which the theory behind strategies employed in forming a biaxial nematic phase in lowmolar- mass thermotropic systems are considered along with possible applications. Chapter 2 deals with one of the approaches in forming a biaxial nematic phase, namely combining rods and discs. The importance of preventing phase separation is highlighted and strategies discussed. The synthesis of a series of laterally attached rod-disc dimers and their component parts is described and their mesophase behaviour is presented. The amphiphilic nature of covalently linked rod-disc dimers is discussed and a detailed miscibility study of the dimer with its component parts is presented. Intermediary conclusions are drawn as to the nature of the phase behaviour. Chapter 3 presents the synthesis undertaken to produce selectively deuterated analogues of the rod-disc dimer, and its component parts. Deuterium NMR spectroscopic studies on some of the mixtures described in chapter 2 and aspects of the phase behaviour of these mixtures are investigated. Chapter 4 discusses the results from the preceding two chapters and draws conclusions about the phase behaviour of the rod/disc/dimer mixtures. A detailed phase diagram is suggested. Chapter 5 deals with bent-core mesogens as candidates for the biaxial nematic phase and the structures of some mesophases specific to bcnt-core mesogens a~e reviewed. Mesogens with an oxadiazole central core are discussed and the problems associated with the high transition temperatures highlighted. The introduction of flexibility into the mesogen via hydrogen-bonding is proposed to generate materials with much lower transition temperatures. The synthesis of three bent-core oxadiazole systems is described and the effect of both flexibility and core structure are discussed. In addition, the selective deuteration of a fuBy covalent oxadiazole biaxial ncmatic candidate is described and results from deuterium NMR spectroscopy performed on this matcrial are presented.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available