标题： 对称氰基联苯二聚体及其与5CB的二元混合物向列相中的偶极-偶极相关性 显示英文标题

标题： Dipole-dipole correlations in the nematic phases of symmetric cyanobiphenyl dimers and their binary mixtures with 5CB

摘要： 我们报告了在一系列由对称的、弯曲形状的1",9"-双(4-氰基联苯-4'-基)壬烷(CB9CB)二聚体与单体向列相物质5CB之间的二元混合物中，双折射率和静态介电常数张量的温度依赖性。在研究的组成范围内，这些混合物表现出两个具有不同双折射率和介电特性的向列相。利用双折射率测量来估算低温度向列相中定义纳米尺度螺旋调制的轴与氰基联苯单元最大排列方向之间的倾斜角的温度依赖性。平面以及磁性和/或电场对齐的样品用于测量两个向列相中介电常数的垂直和平行分量。引入了一种自洽的分子场理论，该理论考虑了组成介晶的柔性和对称性，用于计算柔性二聚体的序参数和分子内取向偶极相关性随温度/浓度的变化。利用从双折射率测量中计算得到的倾斜角和分子理论的预测，在各向异性Kirkwood-Frohlich理论框架下对介电常数进行了建模。使用分子间Kirkwood相关因子作为可调参数，在整个温度范围和混合物组成范围内，理论与介电常数测量结果之间获得了很好的一致性。讨论了取向的分子内和分子间偶极相关性的重要性，它们对静态介电性质的相对影响，以及它们与弯曲形状双分子向列相局部结构的联系。 显示英文摘要

摘要： We report on the temperature dependence of birefringence and of the static dielectric permittivity tensor in a series of binary mixtures between the symmetric, bent-shaped, 1",9"-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB) dimer and the monomeric nematogen 5CB. In the studied composition range the mixtures exhibit two nematic phases with distinct birefringence and dielectric features. Birefringence measurements are used to estimate the temperature dependence of the tilt between the axis defining the nanoscale helical modulation of the low temperature nematic phase with the (local) direction of the maximal alignment of the cyanobiphenyl units. Planar as well as magnetically and/or electrically aligned samples are used to measure the perpendicular and parallel components of the dielectric permittivity in both nematic phases. A self-consistent molecular field theory that takes into account flexibility and symmetry of the constituent mesogens is introduced for the calculation of order parameters and intra-molecular orientational dipolar correlations of the flexible dimers as a function of temperature/concentration. Utilising the tilt angle, as calculated from the birefringence measurements, and the predictions of the molecular theory, dielectric permittivity is modelled in the framework of the anisotropic version of the Kirkwood-Frohlich theory. Using the inter-molecular Kirkwood correlation factors as adjustable parameters, excellent agreement between theory and permittivity measurements across the whole temperature range and composition of the mixtures is obtained. The importance of the orientational, intra- and inter-molecular, dipolar correlations, their relative impact on the static dielectric properties, as well as their connection with the local structure of the nematic phases of bent-shaped bimesogens, is discussed.