The formation of biaxial nematic phases in binary mixtures of thermotropic liquid-crystals composed of uniaxial molecules

ABSTRACT

Monte Carlo simulations in the isothermal-isobaric ensemble are used to investigate the formation of an ordered, biaxial nematic phase in a binary mixture of thermotropic liquid crystals. The orientational dependence of the interaction between molecules of each pure component is the same as in the well-known Maier-Saupe model; each pure component of the mixture is therefore capable of forming a uniaxial nematic phase. For the interaction between molecules of different components, we use the same Maier-Saupe model but change the sign of the coupling constant. As a consequence a T-shaped arrangement of these molecules is energetically favoured. The formation of the biaxial phase occurs in two steps. At higher temperatures T, one of the components forms a uniaxial nematic phase whereas the other is in a quasi two-dimensional restricted isotropic liquid state. We develop a simple theoretical model to understand the high degree of (ostensible) nematic order in the latter. At lower T, the second component becomes nematic and then the entire mixture of the two compounds has biaxial symmetry. The biaxial nematic phase does not demix into domains rich in molecules of one or the other species.     

双轴性向列相液晶的相变理论

假设一双轴性向列相液晶分子简单相互作用模型，利用平均场理论，得到从各向同性相至单轴向列相、单轴向列相至双轴向列相的相变. 结果表明，在一定温度范围内，分子形状的非轴对称性可导致系统双轴向列相的产生. 在核磁共振实验中，导出双轴向列相序参数与谱线分裂频率间的一般关系，并讨论序参数测量性的可能性. 关键词： 向列相液晶 相变     

A biaxial nematic liquid crystal composed of matchbox-symmetric molecules

By means of Monte Carlo simulations in the isothermal-isobaric ensemble, we investigate the structure and phase behaviour of a thermotropic liquid crystal composed of matchbox-symmetric (or board-like) molecules. Besides the isotropic phase the liquid crystal exhibits also uniaxial and biaxial nematic phases. The interaction potential is derived through an expansion in terms of Stone's rotational invariants [A. J. Stone, Mol. Phys. 78, 241–256 (1978).] that can be reexpressed in terms of Cartesian tensors. This latter formulation is particularly well suited for computer simulations. We analyse the orientation distribution function which allows us to distinguish between intrinsic and extrinsic biaxiality. In addition, we study the orientation-dependent correlation functions. In the limit of large intermolecular separations, the value of the orientation correlation function corresponds to the uniaxial and biaxial order parameters which are coupled in a complex fashion.     

Asymmetric dynamic phase holographic grating in nematic liquid crystal

A new scheme for recording a dynamic phase grating with an asymmetric profile in C60-doped homeotropically aligned nematic liquid crystal(NLC) was presented.An oblique incidence beam was used to record the thin asymmetric dynamic phase holographic grating.The diffraction efficiency we achieved is more than 40%,exceeding the theoretical limit for symmetric profile gratings.Both facts can be explained by assuming that a grating with an asymmetric saw-tooth profile is formed in the NLC.Finally,physical mechanism and mathematical model for characterizing the asymmetric phase holographic grating were presented,based on the photo-refractive-like(PR-like) effect.     

Anchoring transitions of nematic liquid crystals

A crystalline substrate such as mica or gypsum can orient a thin layer of nematic liquid crystal spread on top. By adsorbing one or more other kinds of molecules onto the surface, we have observed transitions in the orientation of the molecules on the surface. These “anchoring transitions” are analyzed via Landau-Ginzburg theory and may be first or second-order. The second-order anchoring transition is the first two-dimensional system where critical fluctuations may be observed optically.     

A biaxial nematic gel phase in aqueous vanadium pentoxide suspensions

Aqueous suspensions of V₂O₅ ribbons are one of the very few examples of mineral liquid crystals. In the concentrated regime, we show that these ribbons organize in a biaxial nematic gel phase. A Couette shear cell was used to produce a well oriented sample for in situ synchrotron X-ray scattering studies. We observed two perpendicular anisotropic sections of reciprocal space, which proves the biaxial symmetry of the nematic order. The thermodynamic and flow properties of the biaxial nematic are well described by hard-core theories. We suggest the use of a shear geometry to produce and investigate single domains of other biaxial nematics, reported but still questioned in the literature. Received 26 May 1999     

Short range orientational order in nematic liquid crystals

The different approximations that have been used in applying Bethe’s cluster model to the nematic-isotropic phase transition are examined. It is shown that the introduction of a higher order term in the mean field potential of an outer molecule of the cluster improves the consistency of the theory considerably. In particular, the importance of satisfying Chang’s relation is emphasized. Calculations are presented of the long and short range order parameters, heat of transition and specific heat for different values ofz, the number of nearest neighbours around any given molecule, for both nonpolar and antiparallel near neighbour correlations. Even the new mean field potential appears to be inadequate forz=3.     

Convection in relatively thin layers of nematic liquid crystals

Laser-induced convective motion in thin layers (less than a millimeter) of nematic liquid crystals is investigated experimentally. It is shown that convection arising in the presence of the vertical gradient of temperature is toroidal. Velocities of convective motions in the liquid crystal are measured for two different diameters of the incident laser beam. Comparison is made of the threshold of the temperature gradient of convective motions obtained by theoretical estimations and with use of experimental parameters.     

向列相液晶中光学相位共轭技术的研究进展

对向列相液晶中光学相位共轭技术的研究进展进行了介绍,分析总结了向列相液晶中光学相位共轭过程的机理与实验方法,并对向列相液晶中的光学相位共轭技术的应用前景进行了展望。     

向列相液晶中空间光孤子的观测

分别以He-Ne激光器、半导体激光器和Ar⁺激光器作为光源照射液晶材料，在低光强入射的情况下均观察到了光束的自聚焦现象以及空间光孤子的形成，观察到了长度l>5mm的孤子波和多模空间光孤子，并对空间孤子的产生条件及特性进行了讨论. 关键词： 自聚焦 空间光孤子 向列相液晶     

Influence of the molecular structure of components on the induction of mesophases in nematic liquid crystal systems

The dielectric properties of nematic liquid crystals (NLC) are studied in the temperature range of mesophase existence with taking into account the peculiarities of the molecular structure of components. The dependence of the ordering parameter and activation energy on the temperature and composition of an LC system are obtained and investigated by analysis of EPR spectra based on the spin relaxation theory. The values of the local viscosity of the systems are estimated within the framework of the hydrodynamic model. The appearance of induced and reentrant mesophases is established. It is shown that in NLC systems with non-center-symmetric molecules the correlation between the activation energy and thermostability should be inverse to that in the Maier-Saupe model.     

Orientational order parameters of a binary mixture showing both induced smectic and re-entrant nematic phases

The phase diagram of a new binary system of two polar nematic compounds, 4-cyanophenyl [4′ (4″-n-heptylphenyl)] benzoate(7CPB) and 4-cyanophenyl 4-nonylbenzoate (9.CN), shows the presence of both the induced smectic and re-entrant nematic phases in a certain concentration range 0.4 < x _9.CN < 0.87. The results of X-ray diffraction and density measurements are reported here. Orientational order parameters have been determined in the induced smectic, normal nematic and re-entrant nematic phases from X-ray diffraction measurements. The order parameters have been fitted to those calculated from modified McMillan's theory as proposed by Luckhurst and Timimi. The agreement is found to be closer to the modified model. The experimental variation of the layer thickness with mole fraction of 9.CN has also been explained fairly well by assuming the presence of two types of homo dimers (7CPB + 7CPB, 9.CN + 9.CN) and one type of hetero dimer (7CPB + 9.CN).     

向列相液晶中弱光引致各向异性衍射图样的研究

研究了弱线偏振光(≈0.16 W/cm²)通过垂直排列C₆₀掺杂的向列相液晶(5CB)薄膜的远场衍射图样.基于取向光折变机理,二波耦合使液晶分子进行二次取向之后,强度为高斯分布的光束通过样品时将形成高斯分布的空间电荷场,偏振光束通过样品时将产生偏振衍射图样.衍射图样的轮廓是同心圆环,在垂直于光的偏振方向有对称缺口.改变入射光的偏振方向可以看到衍射图样也随之改变, 有效非线性折射率系数n₂≈0.3cm²/W 关键词： 向列相液晶 取向光折变效应 自相位调制 衍射图样     

Determining the sum of flexoelectric coefficients in nematic liquid crystals by the capacitance method

A detailed theoretical analysis of determining the sum of flexoelectric coefficients in nematic liquid crystals using the capacitance method is given. In the strong anchoring parallel aligned nematic （PAN） and hybrid aligned nematic （HAN） cells, the dependences of the capacitance on the sum of flexoelectric coefficients and the applied voltage are obtained by numerical simulations, and the distributions of the director and the electric potential for different applied voltages and flexoelectric coefficients are also given. Based on this theoretical analysis, we propose an experimental design for measuring the capacitance of a liquid crystal cell using the improved precision LCR meter E4980A （Agilent）. Through comparing the experimental data with the simulated results, the sum of flexoeletric coefficients can be determined.     

Electric-field-induced steering in soliton direction of propagation in chiral nematic liquid crystals

In this work we demonstrate the properties of spatial solitary waves in chiral nematic liquid crystals with an external electric field. Such self-trapped beams, called nematicons, were created due to the optical reorientation nonlinearity for a light power of a few tens of milliwatts. We show that the direction of propagation of such nematicons can be changed by applying an external electric field. Additionally, this effect can be modified by changing the input polarization of the light beam. The experimental results were obtained in four independent guiding layers created by a chiral nematic structure.     

Electroconvective instabilities in nematic liquid crystals

Electroconvective instabilities in nematic liquid crystals are caused by the electric force acting on charge densities generated in the medium due to a distortion in the orientation field of the anisotropic medium. The conventional view is that anisotropic conductivity provides the mechanism for the charge accumulation. Several experimental results however can be accounted for only by invoking an additional mechanism involving flexoelectric polarization for the charge accumulation. In this article we discuss the recent experimental and theoretical developments in this field.     

Molecular theory of nematic liquid crystals viewed as effect of collective excitation in ferromagnetic systems

We develop a microscopic theory of the nematic phase with consideration of the effect of the collective excitation on properties of nematic liquid crystals. The model is based on the Heisenberg's exchange model of the ferromagnetic materials. Since the orientation of the molecular long axis and the angular momentum of the molecule rotating around its long axis have the same direction, operators can be introduced to research the nematic liquid crystals. Using the lattice model and the Holstein--Primakoff transformation, the Hamiltonian of the system can be obtained, which has the same form as that of the ferromagnetic substance. The relation between the order parameter and reduced temperature can be gotten. It is in good agreement with the experimental results in the low temperature region, the accordance is better than that of the molecular field theory and the computer simulation. In high temperature region close to the transition point, by considering the effect of the higher-order terms in the Hamiltonian, theoretical prediction is in better agreement with the experiment. That indicates the many-body effect is important to nematic liquid crystals.     

Unusual physical properties in B phase of polar bent-core thioester compound

Bent-core compound, 4-chloro-1,3-phenylene bis{4-[(n-undecyloxybenzoyl)-sulfanyl]benzoate}, has been studied by dielectric spectroscopy, DSC calorimetry, and electro-optic methods. As found, the spontaneous polarization of the B phase is by one order of magnitude smaller than for B ₂ phases of nOSOR series studied by us before. Its temperature dependence is characteristic for B ₁ phase, observed for shorter homologues of nOSORs. Measurements of the complex electric permittivity were carried out in the frequency range from 40 to 15?MHz using gold-coated electrodes. Electro-optic studies have been done applying AWAT HG 1.9?µm cells with strongly rubbed polymer layers. Such cells facilitate inhomogeneous planar alignment, which under strong electric field transforms into a mono-domain exhibiting switching behavior. Dielectric and electro-optic data of bent-core compounds will be discussed taking into account molecular structure and phase structure as well.