Dielectric, calorimetric and mesophase properties of 1"-(2′,4-difluorobiphenyl-4′-yloxy)-9"-(4-cyano-biphenyl-4′-yloxy) nonane: an odd liquid crystal dimer with a monotropic mesophase having the characteristics of a twist-bend nematic phase
Dielectric, calorimetric and mesophase properties of 1"-(2′,4-difluorobiphenyl-4′-yloxy)-9"-(4-cyano-biphenyl-4′-yloxy) nonane: an odd liquid crystal dimer with a monotropic mesophase having the characteristics of a twist-bend nematic phase
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DOI：
10.1039/c4cp03462g
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发表时间：
2014-10-21
影响因子：
3.3
通讯作者：
Jackson, D. J. B.
中科院分区：
文献类型：
--
作者：
Sebastian, N.;Lopez, D. O.;Jackson, D. J. B.
本文报道了一种新型液晶相，具有由非对称醚连接液晶二聚体形成的扭转弯曲向列相的特征。二聚体1''-(2',4-二氟联苯-4'-基氧基)-9''-(4-氰基联苯-4'-基氧基)壬烷(FFO9OCB)在以足够高的速率从各向同性相冷却时表现出两个液晶相。高温中间相在文献中被报道为向列相，并在本研究中得到证实。另一个中间相是亚稳态的，可以过冷而产生玻璃态。其识别和表征基于光学织构、宽带介电光谱、量热法、向列相中张开和弯曲弹性常数的测量以及混溶性研究。结论是低温中间相表现出扭转弯曲向列相的特征。介电测量使我们能够获得静态介电常数以及有关各向同性相、向列中间相以及各向同性到向列相转变的分子动力学信息。已经研究了与指向矢平行和垂直的两个方向。在高温向列中间相中，发现介电各向异性为正。向列二聚体介电弛豫的分子理论可以很好地解释介电常数并联分量的测量（M. Stocchero、A. Ferrarini、G. J. Moro、D. A. Dunmur 和 G. R. Luckhurst、J. Chem. Phys., 2004, 121, 8079）。二聚体被建模为顺式和反式构象异构体的混合物，并且该模型允许估计它们在每个温度下的相对数量。根据热容和静态介电常数数据的精确演化，对向列相到各向同性相变进行了详尽的研究。得出的结论是，该转变本质上是一阶的，但接近三临界的。由于精度不够，向列相到新型液晶相变的性质很难以同样的程度进行分析。只能在 10 K min(-1) 或更高的冷却速率下进行观察，因为从玻璃态加热时，扭转弯曲向列中间相在远低于向列-向列相变的温度下结晶。
This paper reports a novel liquid crystal phase having the characteristics of a twist-bend nematic phase formed by a non-symmetric ether-linked liquid crystal dimer. The dimer 1 ''-(2',4-difluorobiphenyl-4'-yloxy)-9 ''-(4-cyanobiphenyl-4'-yloxy) nonane (FFO9OCB) exhibits two liquid-crystalline phases on cooling at a sufficiently high rate from the isotropic phase. The high temperature mesophase has been reported in the literature as nematic and confirmed in this study. The other mesophase is metastable and can be supercooled giving rise to a glassy state. Its identification and characterization are based on optical textures, broadband dielectric spectroscopy, calorimetry, measurements of both splay and bend elastic constants in the nematic phase and miscibility studies. It is concluded that the low temperature mesophase exhibits the characteristics of a twist-bend nematic phase. Dielectric measurements enable us to obtain the static permittivity and information about the molecular dynamics in the isotropic phase, in the nematic mesophase and across the isotropic-to-nematic phase transition. Two orientations, parallel and perpendicular to the director, have been investigated. In the high temperature nematic mesophase, the dielectric anisotropy is found to be positive. Measurements of the parallel component of the dielectric permittivity are well-explained by the molecular theory of dielectric relaxation in nematic dimers (M. Stocchero, A. Ferrarini, G. J. Moro, D. A. Dunmur and G. R. Luckhurst, J. Chem. Phys., 2004, 121, 8079). The dimer is modelled as a mixture of cis and trans conformers and the model allows an estimate of their relative populations at each temperature. The nematic-to-isotropic phase transition has been exhaustively studied from the accurate evolution of the heat capacity and the static dielectric permittivity data. It has been concluded that the transition is first order in nature, but close to tricritical. The nature of the nematic-to-the novel liquid crystal phase transition is difficult to analyze to the same extent because of insufficient precision. Only observations at cooling rates of 10 K min(-1) or higher were possible because on heating from the glassy state, the twist-bend nematic mesophase crystallizes at temperatures far below the nematic-nematic phase transition.