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Molecular field theory for biaxial smectic A liquid crystals

Molecular field theory for biaxial smectic A liquid crystals
Molecular field theory for biaxial smectic A liquid crystals
Thermotropic biaxial nematic phases seem to be rare, but biaxial smectic A phases less so. Here we use molecular field theory to study a simple two-parameter model, with one parameter promoting a biaxial phase and the second promoting smecticity. The theory combines the biaxial Maier-Saupe and McMillan models. We use alternatively the Sonnet-Virga-Durand (SVD) and geometric mean approximations (GMA) to characterize molecular biaxiality by a single parameter. For non-zero smecticity and biaxiality, the model always predicts a ground state biaxial smectic A phase. For a low degree of smectic order, the phase diagram is very rich, predicting uniaxial and biaxial nematic and smectic phases, with the addition of a variety of tricritical and tetracritical points. For higher degrees of smecticity, the region of stability of the biaxial nematic phase is restricted and eventually disappears, yielding to the biaxial smectic phase. Phase diagrams from the two alternative approximations for molecular biaxiality are similar, except inasmuch that SVD allows for a first-order isotropic-biaxial nematic transition, whereas GMA predicts a Landau point separating isotropic and biaxial nematic phases. We speculate that the rarity of thermotropic biaxial nematic phases is partly a consequence of the presence of stabler analogous smectic phases
0021-9606
134902-[14pp]
To, Tung B.T.
07914a27-ca25-4e9d-a855-bf5db2fd7ac3
Sluckin, Timothy J.
8dbb6b08-7034-4ae2-aa65-6b80072202f6
Luckhurst, Geoffrey R.
7807d1c6-54a1-414f-9abe-22de4d9c30f7
To, Tung B.T.
07914a27-ca25-4e9d-a855-bf5db2fd7ac3
Sluckin, Timothy J.
8dbb6b08-7034-4ae2-aa65-6b80072202f6
Luckhurst, Geoffrey R.
7807d1c6-54a1-414f-9abe-22de4d9c30f7

To, Tung B.T., Sluckin, Timothy J. and Luckhurst, Geoffrey R. (2013) Molecular field theory for biaxial smectic A liquid crystals. The Journal of Chemical Physics, 139, 134902-[14pp]. (doi:10.1063/1.4820555).

Record type: Article

Abstract

Thermotropic biaxial nematic phases seem to be rare, but biaxial smectic A phases less so. Here we use molecular field theory to study a simple two-parameter model, with one parameter promoting a biaxial phase and the second promoting smecticity. The theory combines the biaxial Maier-Saupe and McMillan models. We use alternatively the Sonnet-Virga-Durand (SVD) and geometric mean approximations (GMA) to characterize molecular biaxiality by a single parameter. For non-zero smecticity and biaxiality, the model always predicts a ground state biaxial smectic A phase. For a low degree of smectic order, the phase diagram is very rich, predicting uniaxial and biaxial nematic and smectic phases, with the addition of a variety of tricritical and tetracritical points. For higher degrees of smecticity, the region of stability of the biaxial nematic phase is restricted and eventually disappears, yielding to the biaxial smectic phase. Phase diagrams from the two alternative approximations for molecular biaxiality are similar, except inasmuch that SVD allows for a first-order isotropic-biaxial nematic transition, whereas GMA predicts a Landau point separating isotropic and biaxial nematic phases. We speculate that the rarity of thermotropic biaxial nematic phases is partly a consequence of the presence of stabler analogous smectic phases

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Published date: October 2013
Organisations: Applied Mathematics

Identifiers

Local EPrints ID: 369793
URI: http://eprints.soton.ac.uk/id/eprint/369793
ISSN: 0021-9606
PURE UUID: fb267d20-141a-4143-9b59-345b974d60d7
ORCID for Timothy J. Sluckin: ORCID iD orcid.org/0000-0002-9163-0061

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Date deposited: 13 Oct 2014 10:38
Last modified: 15 Mar 2024 02:32

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Author: Tung B.T. To
Author: Geoffrey R. Luckhurst

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