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Biaxial nematic liquid crystals: fact or fiction?

Biaxial nematic liquid crystals: fact or fiction?
Biaxial nematic liquid crystals: fact or fiction?
Mesogenic molecules invariably deviate from the cylindrical symmetry usually assumed for them. One consequence of this is that, in addition to the ubiquitous uniaxial nematic phase, there should also be a biaxial nematic phase. For this there are three different principal components. of a second rank tensorial property, such as the magnetic susceptibility, as opposed to two for a uniaxial phase. Each of these components is associated with a separate director; the three directors correspond to the directions about which three orthogonal axes set in the molecule tend to be aligned. Although the existence of the biaxial nematic phase was predicted over 30 years ago it was not until 10 years later that the first claim to have found this phase appeared. This system was, in fact, a lyotropic liquid crystal. Surprisingly, the search for a thermotropic biaxial nematic has proved to be especially challenging. This review is concerned with the thermotropic materials for which a biaxial nematic have now been claimed. Of particular importance is the technique used to establish the symmetry of the nematic phase and it is suggested that deuterium NMR spectroscopy is a powerful method with which to determine this. However, the biaxial nematics claimed to be formed by certain compounds are shown by NMR to have uniaxial symmetry. The reasons why the biaxial nematic phase proves to be so elusive are explored using molecular field theory and used to inform the design of thermotropic mesogens which are likely to form this phase.
uniaxial nematic phase, nmr spectroscopy, thermotropic mesogens cylindrically symmetric molecules, orientational order, computer-simulation, anisotropic systems, phase-transitions, lyotropicsystem, monte-carlo, mixtures, diagram, dimers
0040-6090
40-52
Luckhurst, G.R.
7807d1c6-54a1-414f-9abe-22de4d9c30f7
Luckhurst, G.R.
7807d1c6-54a1-414f-9abe-22de4d9c30f7

Luckhurst, G.R. (2001) Biaxial nematic liquid crystals: fact or fiction? Thin Solid Films, 393 (1-2), 40-52. (doi:10.1016/S0040-6090(01)01091-4).

Record type: Article

Abstract

Mesogenic molecules invariably deviate from the cylindrical symmetry usually assumed for them. One consequence of this is that, in addition to the ubiquitous uniaxial nematic phase, there should also be a biaxial nematic phase. For this there are three different principal components. of a second rank tensorial property, such as the magnetic susceptibility, as opposed to two for a uniaxial phase. Each of these components is associated with a separate director; the three directors correspond to the directions about which three orthogonal axes set in the molecule tend to be aligned. Although the existence of the biaxial nematic phase was predicted over 30 years ago it was not until 10 years later that the first claim to have found this phase appeared. This system was, in fact, a lyotropic liquid crystal. Surprisingly, the search for a thermotropic biaxial nematic has proved to be especially challenging. This review is concerned with the thermotropic materials for which a biaxial nematic have now been claimed. Of particular importance is the technique used to establish the symmetry of the nematic phase and it is suggested that deuterium NMR spectroscopy is a powerful method with which to determine this. However, the biaxial nematics claimed to be formed by certain compounds are shown by NMR to have uniaxial symmetry. The reasons why the biaxial nematic phase proves to be so elusive are explored using molecular field theory and used to inform the design of thermotropic mesogens which are likely to form this phase.

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More information

Published date: 1 August 2001
Keywords: uniaxial nematic phase, nmr spectroscopy, thermotropic mesogens cylindrically symmetric molecules, orientational order, computer-simulation, anisotropic systems, phase-transitions, lyotropicsystem, monte-carlo, mixtures, diagram, dimers

Identifiers

Local EPrints ID: 19564
URI: http://eprints.soton.ac.uk/id/eprint/19564
ISSN: 0040-6090
PURE UUID: c98c4cd4-283b-4783-8e38-0db6c136b15a

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Date deposited: 15 Feb 2006
Last modified: 15 Jul 2019 19:27

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Author: G.R. Luckhurst

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