X-ray and neutron diffraction studies of monomeric and dimeric thermotropic liquid crystals
X-ray and neutron diffraction studies of monomeric and dimeric thermotropic liquid crystals
This thesis is primarily concerned with the preparation of a homologous series of dimeric thermotropic liquid crystal compounds and with the characterisation of the mesophases by X-ray diffraction studies and the phase transitions by differential scanning calorimetry. Dimcr compounds consist of two semi-rigid groups connected by a flexible alkyl chain or spacer. Such materials are of particular interest because of the dramatic odd even effect in the transition temperature and entropy to the isotropic phase on variation of the length of the spacer chain. Those compounds with a spacer chain containing an even number of methylcne units arc found to have an apparently higher degree of orientational order when compared to compounds containing an odd number of mcthylene units or the corresponding monomer compounds, which contain only one semi-rigid unit. The compounds studied are the a,w-bis(4-alkylaniline benzylidenc-4'-oxy)alkancs and are the first dimers found to exhibit an extensive smectic polymorphism. Chapter 1 contains a review of the structures of thermotropic liquid crystal phases and a discussion of structure and liquid crystal property relationships in monomer and dimer compounds. In chapter 2 the transition temperatures and entropies of transition for the various phases of these compounds are presented. Chapter 3 describes X-ray and neutron diffraction theory and the form of X-ray diffraction patterns expected from the various thermotropic liquid crystal phases. In chapter 4 several X-ray monodomain diffraction patterns are shown as confirmation of the identification based on optical textures for some of the dimer mesophases. In addition the trends in the variation of the layer spacing, obtained from powder diffraction patterns, in the various mesophases across the homologous series are discussed. In chapter 5 the phase diagrams of several binary mixtures of a,w-bis(4-alkylanilinebenzilidinc-4'-oxy)alkanes with a,w-bis(4,4'-cyanobiphenyloxy) alkanes are shown in which an injected smectic A phase can result, with an intercalated molecular packing. Finally in chapter 6 the results of neutron diffraction experiments to measure orientational order parameters of differing ranks by employing mixtures of perdcuteriated and protonated molecules to enhance the single molecule scattering, are presented for monomeric 4-methoxy-4'-cyanobiphenyl (1-OCB) and 4,4'-dimethoxyazoxyanisole (PAA).
University of Southampton
1991
Date, Richard William
(1991)
X-ray and neutron diffraction studies of monomeric and dimeric thermotropic liquid crystals.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
This thesis is primarily concerned with the preparation of a homologous series of dimeric thermotropic liquid crystal compounds and with the characterisation of the mesophases by X-ray diffraction studies and the phase transitions by differential scanning calorimetry. Dimcr compounds consist of two semi-rigid groups connected by a flexible alkyl chain or spacer. Such materials are of particular interest because of the dramatic odd even effect in the transition temperature and entropy to the isotropic phase on variation of the length of the spacer chain. Those compounds with a spacer chain containing an even number of methylcne units arc found to have an apparently higher degree of orientational order when compared to compounds containing an odd number of mcthylene units or the corresponding monomer compounds, which contain only one semi-rigid unit. The compounds studied are the a,w-bis(4-alkylaniline benzylidenc-4'-oxy)alkancs and are the first dimers found to exhibit an extensive smectic polymorphism. Chapter 1 contains a review of the structures of thermotropic liquid crystal phases and a discussion of structure and liquid crystal property relationships in monomer and dimer compounds. In chapter 2 the transition temperatures and entropies of transition for the various phases of these compounds are presented. Chapter 3 describes X-ray and neutron diffraction theory and the form of X-ray diffraction patterns expected from the various thermotropic liquid crystal phases. In chapter 4 several X-ray monodomain diffraction patterns are shown as confirmation of the identification based on optical textures for some of the dimer mesophases. In addition the trends in the variation of the layer spacing, obtained from powder diffraction patterns, in the various mesophases across the homologous series are discussed. In chapter 5 the phase diagrams of several binary mixtures of a,w-bis(4-alkylanilinebenzilidinc-4'-oxy)alkanes with a,w-bis(4,4'-cyanobiphenyloxy) alkanes are shown in which an injected smectic A phase can result, with an intercalated molecular packing. Finally in chapter 6 the results of neutron diffraction experiments to measure orientational order parameters of differing ranks by employing mixtures of perdcuteriated and protonated molecules to enhance the single molecule scattering, are presented for monomeric 4-methoxy-4'-cyanobiphenyl (1-OCB) and 4,4'-dimethoxyazoxyanisole (PAA).
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Published date: 1991
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Local EPrints ID: 460471
URI: http://eprints.soton.ac.uk/id/eprint/460471
PURE UUID: 98f23178-dcfd-4cb8-b160-228777d211cf
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Last modified: 04 Jul 2022 18:23
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Richard William Date
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