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Title: The effects of structural complexity on the properties of low molar mass liquid crystals
Author: Douglass, Andrew Gordon
ISNI:       0000 0001 3430 6479
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 1993
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This thesis investigates the effect of molecular flexibility on the mesogenic behaviour of dimeric and trimeric liquid crystals. Chapter 1 describes the structures and properties of mesogenic materials and the experimental techniques by which they are studied. In chapter 2 the mesogenic properties of a series of benzene-1,3,5-tri(4-carboxyhexyloxybenzylidene-4'-n-alkylanilines) in which three rod-like mesogenic units are attached to a disc-like core via flexible spacers are examined. A complex and unusual smectic polymorphism is noted for this series. Optical and X-ray diffraction studies reveal that the phases formed are calamitic rather than discotic or columnar. One possible explanation for this behaviour is that the flexible spacer chains preferentially adopt conformations that give a high molecular length to breadth ratio in the liquid crystal phase. In chapter 3 the properties of homologous series of novel structurally isomeric trimeric liquid crystals are presented. These molecules have a phenyl diester core and two pendant Schiff's bases attached via flexible spacers. The mesophase behaviour has a strong dependence on spacer chain parity, particularly at short terminal chain lengths. Ortho disubstituted derivatives have been shown by X-ray diffraction to form bilayer smectic phases whilst meta and para disubstituted derivatives form monolayer smectic phases. In chapter 4 a 1,1'-ferrocene unit replaces the disubstituted benzene as the core of a trimeric molecule. These materials form nematic and smectic phases as a function of terminal chain length. The spacer chain parity has little effect on the mesophase stability which is reduced by incorporation of the ferrocene. The dependence of the odd-even behaviour for dimeric liquid crystals on the geometry of the linkage between the spacer and mesogenic units is studied in chapter 5. The orientational order of cyanobiphenyl dimers with methylene or ether links is probed by NMR spectroscopy via an anthracene-d₁₀ solute. The enhanced odd-even effect observed for methylene linked dimers relative to those with ether links is shown to be in accord with theoretical predictions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Solid-state physics