The influence of structure on the elastic, optical and dielectric properties of nematic phases formed from bent-core molecules
The influence of structure on the elastic, optical and dielectric properties of nematic phases formed from bent-core molecules
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DOI：
10.1039/c3tc31545b
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发表时间：
2013-01-01
影响因子：
6.4
通讯作者：
Gleeson, H. F.
中科院分区：
文献类型：
--
作者：
Kaur, S.;Liu, H.;Gleeson, H. F.
报道了四种弯核恶二唑基材料形成的向列相的物理性质。特别是，描述了材料的张开（K-11）、扭曲（K-22）和弯曲（K-33）弹性常数、双折射和介电各向异性，并确定了链长和芳族核心结构的外部亚苯基上氟取代基的存在对这些参数的影响。发现双折射和有序参数与分子结构的修饰无关。介电各向异性很大程度上取决于分子结构；氟取代材料的介电各向异性最大，而烷基取代材料的介电各向异性最小。研究发现，在同等降低的温度下，弯曲核材料中分子长度和氟取代的变化对张开、扭曲和弯曲弹性常数影响很小。然而，与烷氧基取代的化合物相比，用烷基末端链取代的材料表现出更小的弹性常数和更不显着的温度依赖性。通过遵循 Berreman 和 Meiboom 提出的分析，可以深入了解与暗砾岩相形成相关的弹性常数的行为，暗砾岩相是所研究化合物之一的向列相的基础。重要的是，利用分子场论和原子模型，我们计算出的弹性常数与实验值非常吻合。我们的结论是，由弯曲核心分子形成的向列相的弹性不会受到末端链的变化或芳香核心结构的外部亚苯基上氟取代基的存在的强烈影响，这与我们之前的工作一致，表明主要参数是弯曲角度。
The physical properties of the nematic phases formed by four bent-core oxadiazole based materials are reported. In particular, the splay (K-11), twist (K-22) and bend (K-33) elastic constants, the birefringence and the dielectric anisotropy of the materials are described and the effect of chain length and the presence of fluoro-substituents at the outer phenylene group of the aromatic core structure on these parameters is determined. The birefringence and order parameter are found to be independent of the modification of molecular structure. The dielectric anisotropy is quite strongly dependent on molecular structure; the fluoro-substituted material has the largest magnitude of dielectric anisotropy while the alkyl-substituted compound has the smallest. Changes in the molecular length and fluoro-substitution in the bent-core materials are found to have little influence on the splay, twist and bend elastic constants at equivalent reduced temperatures. However, the material substituted with an alkyl terminal chain exhibits both smaller elastic constants and a less marked dependence on temperature than the alkoxy-substituted compounds. A possible insight into the behaviour of the elastic constants relevant to the formation of the dark conglomerate phase, which underlies the nematic phase in one of the compounds studied, is suggested by following the analysis proposed by Berreman and Meiboom. Importantly, using molecular field theory and atomistic modelling, we calculate elastic constants that are in excellent agreement with the experimental values. Our conclusion that the elasticity in the nematic phase formed from bent-core molecules is not strongly influenced by changes to the terminal chains or the presence of fluoro-substituents at the outer phenylene group of the aromatic core structure is in agreement with our previous work showing that the dominant parameter is the bend angle.