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Direct Observation of Biaxial Nematic Order in Auxetic Liquid Crystal Elastomers

Auxetic materials exhibit a negative Poisson’s ratio, i.e., they become thicker rather than thinner in at least one dimension when strained. Recently, a nematic liquid crystal elastomer (LCE) was shown to be the first synthetic auxetic material at a molecular level. Understanding the mechanism of th...

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Autores principales: Wang, Zhenming, Raistrick, Thomas, Street, Aidan, Reynolds, Matthew, Liu, Yanjun, Gleeson, Helen F.
Formato: Online Artículo Texto
Lenguaje:English
Publicado: MDPI 2022
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9822019/
https://www.ncbi.nlm.nih.gov/pubmed/36614732
http://dx.doi.org/10.3390/ma16010393
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author Wang, Zhenming
Raistrick, Thomas
Street, Aidan
Reynolds, Matthew
Liu, Yanjun
Gleeson, Helen F.
author_facet Wang, Zhenming
Raistrick, Thomas
Street, Aidan
Reynolds, Matthew
Liu, Yanjun
Gleeson, Helen F.
author_sort Wang, Zhenming
collection PubMed
description Auxetic materials exhibit a negative Poisson’s ratio, i.e., they become thicker rather than thinner in at least one dimension when strained. Recently, a nematic liquid crystal elastomer (LCE) was shown to be the first synthetic auxetic material at a molecular level. Understanding the mechanism of the auxetic response in LCEs is clearly important, and it has been suggested through detailed Raman scattering studies that it is related to the reduction of uniaxial order and emergence of biaxial order on strain. In this paper, we demonstrate direct observation of the biaxial order in an auxetic LCE under strain. We fabricated ~100 μm thick LCE strips with complementary geometries, exhibiting either planar or homeotropic alignment, in which the auxetic response is seen in the thickness or width of the sample, respectively. Polarized Raman scattering measurements on the planar sample show directly the reduction in the uniaxial order parameters on strain and suggest the emergence of biaxial order to mediate the auxetic response in the sample thickness. The homeotropic sample is studied via conoscopy, allowing direct observation of both the auxetic response in the width of the sample and increasing biaxiality in the LCE as it is strained. We verified that the mechanism of the auxetic response in auxetic LCEs is due to the emergence of the biaxial order and conclude such materials can be added to the small number of biaxial nematic systems that have been observed. Importantly, we also show that the mechanical Frèedericksz transition seen in some LCEs is consistent with a strain-induced transition from an optically positive to an optically negative biaxial system under strain, rather than a director rotation in a uniaxial system.
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spelling pubmed-98220192023-01-07 Direct Observation of Biaxial Nematic Order in Auxetic Liquid Crystal Elastomers Wang, Zhenming Raistrick, Thomas Street, Aidan Reynolds, Matthew Liu, Yanjun Gleeson, Helen F. Materials (Basel) Article Auxetic materials exhibit a negative Poisson’s ratio, i.e., they become thicker rather than thinner in at least one dimension when strained. Recently, a nematic liquid crystal elastomer (LCE) was shown to be the first synthetic auxetic material at a molecular level. Understanding the mechanism of the auxetic response in LCEs is clearly important, and it has been suggested through detailed Raman scattering studies that it is related to the reduction of uniaxial order and emergence of biaxial order on strain. In this paper, we demonstrate direct observation of the biaxial order in an auxetic LCE under strain. We fabricated ~100 μm thick LCE strips with complementary geometries, exhibiting either planar or homeotropic alignment, in which the auxetic response is seen in the thickness or width of the sample, respectively. Polarized Raman scattering measurements on the planar sample show directly the reduction in the uniaxial order parameters on strain and suggest the emergence of biaxial order to mediate the auxetic response in the sample thickness. The homeotropic sample is studied via conoscopy, allowing direct observation of both the auxetic response in the width of the sample and increasing biaxiality in the LCE as it is strained. We verified that the mechanism of the auxetic response in auxetic LCEs is due to the emergence of the biaxial order and conclude such materials can be added to the small number of biaxial nematic systems that have been observed. Importantly, we also show that the mechanical Frèedericksz transition seen in some LCEs is consistent with a strain-induced transition from an optically positive to an optically negative biaxial system under strain, rather than a director rotation in a uniaxial system. MDPI 2022-12-31 /pmc/articles/PMC9822019/ /pubmed/36614732 http://dx.doi.org/10.3390/ma16010393 Text en © 2022 by the authors. https://creativecommons.org/licenses/by/4.0/Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
spellingShingle Article
Wang, Zhenming
Raistrick, Thomas
Street, Aidan
Reynolds, Matthew
Liu, Yanjun
Gleeson, Helen F.
Direct Observation of Biaxial Nematic Order in Auxetic Liquid Crystal Elastomers
title Direct Observation of Biaxial Nematic Order in Auxetic Liquid Crystal Elastomers
title_full Direct Observation of Biaxial Nematic Order in Auxetic Liquid Crystal Elastomers
title_fullStr Direct Observation of Biaxial Nematic Order in Auxetic Liquid Crystal Elastomers
title_full_unstemmed Direct Observation of Biaxial Nematic Order in Auxetic Liquid Crystal Elastomers
title_short Direct Observation of Biaxial Nematic Order in Auxetic Liquid Crystal Elastomers
title_sort direct observation of biaxial nematic order in auxetic liquid crystal elastomers
topic Article
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9822019/
https://www.ncbi.nlm.nih.gov/pubmed/36614732
http://dx.doi.org/10.3390/ma16010393
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