梳型高分子聚合物向列相液晶的相变

本文根据Freiser关于向列相液晶分子相互作用模型,利用平均场理论,建立了梳型高分子聚合物向列相液晶分子的哈密顿量,导出序参数自洽方程,得到各向同性相至单轴向列相以及单轴至双轴向列相的相变,结果表明,当分子主、侧链耦合项中排斥作用及主、侧链中较强向列相场的场强增大时,单轴至双轴向列相相变温度升高,并从一级相变逐渐变为二级相变。 关键词：     

双轴分子构成向列型液晶中向列序的计算

通过格胞理论研究双轴分子构成的向列型液晶。发展新的数值方法液晶中的向列序。不用任何函数展开,通过迭代方法求解平衡态方程得到精确的取向分布函数。计算了序参数和单分子内能对温度的依赖关系。给出了有关物理量在向列相一各向同性相相变点的值。数值结果较分子场理论更接近于Ｍｏｎｔｅ Ｃａｒｌｏ模拟的结果。     

分子双轴性与向列型液晶在各向同性相的最低过冷温度

在分子场近似下,研究分子双轴性对向列型液晶在各向同性相最低过冷温度T^*的影响.理论结果表明,通过考虑分子双轴性,各向同性相最低过冷温度T^*与向列相—各向同性相相变温度T_c的比值向实验值靠近.当分子双轴性参数δ取到0.3时,T^*/T_c达到0.9817,已很接近0.99的实验值. 关键词：     

双轴向列相液晶的Monte Carlo模拟

利用Monte Carlo方法模拟棒状液晶分子随温度变化的相变行为,采用热力学统计方法得到能量与温度的关系,得出比热随温度的变化规律,通过图像的峰值确定相变点;重点模拟计算序参数矩阵元与温度的关系,得到不同分子结构参数下液晶系统的温度相图,模拟结果显示,当两个短棒长度相等时,不存在双轴相.     

丝状相液晶由表面相互作用产生的相变

导出双轴丝状相液晶在指向矢与易取向轴重合时的表面相互作用能.应用Ritz方法,对半无 界液晶体得出沿表面法向的双轴相(B_c)至沿表面某一轴的双轴相(B_a)的相变.随着表面锚泊强度双轴性部分的增大,B_a的产生机制逐渐由降低温度 转为表面锚泊,因此B_a相可出现在较高温度和较大温度范围内.在表面锚泊强度 单轴性部分α值的一定范围内,增大α值可使B_a在较高温度产生,但α值的进 一步增大则使液晶又从B_α返回B_c相. 关键词： 液晶 相变 表面能     

液晶近晶C相双轴特性的统计理论

采用格胞模型,定义了分子取向序参量、双轴序参量、质心位置序参量以及取向与质心位置耦合序参量.由自由能极小求得指向矢倾斜角与双轴序参量间的关系.对典型Sc相液晶物质TBBA,TBSA和NOBA的双轴序参量随温度的变化以及取向序参量和倾斜角对双轴序参量的影响作了数值计算,并与实验结果进行比较.表明这种双轴特性主要产生于Sc相的指向矢倾斜结构和分子取向的有序性. 关键词：     

向列型液晶理论评述

本文评述了向列型液晶的唯象理论和微观理论。由简单的关于对称性的讨论,推得了以序参数张量为变量的唯象的Landau自由能密度展开式。然后在下列三方面阐明了Landau-deGennes理论的应用:计算了向列型—各向同性型相变的热力学性质,研究了大块向列型液晶对于外界干扰的响应,并描述了涨落和界面效应。关于向列型液晶微观理论的讨论又分为两部分,一是讨论基于位致排斥相互作用而给出的理论,另一是讨论基于分子间的吸引相互作用而给出的理论。本文将表明,虽然每一类型的理论都能阐明向列序存在的物理基础的某些方面,但没有一个理论能够全面地说明向列型液晶的状态和它向各向同性相的转变。最后,本文指出了某些尚未解决的问题及其可能的解决途径。     

向列型液晶理论评述

本文评述了向列型液晶的唯象理论和微观理论。由简单的关于对称性的讨论,推得了以序参数张量为变量的唯象的Landau自由能密度展开式。然后在下列三方面阐明了Landau-deGennes理论的应用:计算了向列型—各向同性型相变的热力学性质,研究了大块向列型液晶对于外界干扰的响应,并描述了涨落和界面效应。关于向列型液晶微观理论的讨论又分为两部分,一是讨论基于位致排斥相互作用而给出的理论,另一是讨论基于分子间的吸引相互作用而给出的理论。本文将表明,虽然每一类型的理论都能阐明向列序存在的物理基础的某些方面,但没有一个理论能够全面地说明向列型液晶的状态和它向各向同性相的转变。最后,本文指出了某些尚未解决的问题及其可能的解决途径。     

双轴向列相液晶的表面能

利用双轴向列相液晶表面能对指向矢和序参量矩阵的旋转不变性,构造了与指向矢^a,^b,^c对易取向轴^a0,^b0,^c0的偏离有关的表面能Fs(a,b,c),以及与序参量矩阵有关的表面能Fs(Q),并得到两种表达式的等价形式.在Fs(a,b,c)和Fs(Q)中分别有四个和两个独立参量.为讨论上述独立参量的物理意义,假设一液晶分子对表面相互作用的简单模型,将液晶分子沿易取向轴^b0,^c0的双轴性取向看成是受到沿^b0,^c0两相互垂直的平均场作用的结果.为测量表面锚泊系数,考虑了三种简单形变位型,即在外磁 关键词： 液晶 弹性能 表面能     

外磁场作用下液晶中向列序的计算

在格胞理论的基础上计算外磁场作用下液晶中的向列序，考虑分子短程关联后，序参数和内能对磁场强度的依赖关系与计算机模拟结果较好地符合。克服了平均场理论遇到的困难。     

A Hierarchy of Defects in Biaxial Nematics

We study the topological properties of the manifolds which describe uniaxial and biaxial nematics in order to compare and classify all types of bulk and surface defects that can arise in a nematic liquid crystal. We explain how topological charges can be assigned both to isotropic defects in uniaxial nematics and to uniaxial defects in biaxial nematics; we further see how it is possible to distinguish between isolated surface defects and surface defects which extend in the bulk, and between surface defects which can relax in the bulk and defects forced to stay on the surface. Received: 28 November 1994 / Accepted: 30 September 1996     

Effects of fields and anchoring on biaxial nematic ordering

Symmetry of a nematic liquid crystal phase is broken by an anchoring wall and also by an external field. Nematic system sandwiched between biaxial anchoring walls is introduced as a correspondent to a bulk nematic system exposed to a couple of fields, an electric field and a magnetic one in directions perpendicular to each other, and thermal behaviours of the system are studied. The crossover between a homeotropic structure and homogeneous one occurs, similarly to the bulk system in the fields, in which the anchoring condition of coexistence is shown to have the same expression as the one at the bulk. As to a characteristic phenomenon at the sandwiched system, it is proved that an appearance of a biaxial nematic order suppresses a uniaxial nematic order. A surface transition, i.e., a wetting phenomenon is shown to occur also in the biaxial nematics, even though the uniaxial order is suppressed therein.     

Investigation of the influence of molecule deformation on the dynamics and collective excitation spectra in nematic liquid crystals

The generalization of dynamic equations in uniaxial and biaxial nematic liquid crystals is obtained yielding an approach accounting for the deformation of structural elements of these media. Collective excitation spectra are calculated and their angular dependence is established. The possibility of propagation of one, two, and three acoustic waves in nematics is predicted, and extreme angular characteristics of these waves are elucidated. Results of investigation are compared with available experimental data and satisfactory agreement between them is demonstrated. Low-frequency asymptotics of two-time Green’s functions of uniaxial nematics are calculated. It is found that their structure depends essentially on the character of spatial anisotropy of the considered condensed medium and the shape of molecules. Polarization specific features of acoustic waves in uniaxial nematics are studied. It is shown that the first and second sounds are the superposition of the longitudinal and transverse components. The ratio of amplitudes of these components as a function of angle between the wave vector and the anisotropy axis is established.     

弹性形变对丝状相液晶相变的影响

应用Saupe形式的弹性自由能,研究了简单的扭曲,展曲和弯曲形变对相变的影响.结果表明,弹性形变可以产生从各向同性相至双轴丝状相的一级相变,对螺状相液晶,当系统手征性增加并超过某一临界值时,一级相变可过渡到二级相变. 关键词：     

The elastic anisotropy of nematic elastomers

We examine the robustness of order in nematic elastomers under mechanical strains imposed along and perpendicularly to the director when director rotation is prohibited. In contrast to electric and magnetic fields applied to conventional nematics, mechanical fields are shown theoretically and experimentally to greatly affect the degree of nematic order and related quantities. Unlike in liquid nematics, one can impose fields perpendicular to the director, thereby inducing biaxial order which should be susceptible to experimental detection. Nematic elastomers with unchanging director and degree of order should theoretically have the same elastic moduli for longitudinal and transverse extensions. This is violated when nematic order is permitted to relax in response to strains. Near the transition we predict the longitudinal modulus to be smaller than the transverse modulus; at lower temperatures the converse is true, with a crossover a few degrees below the transition. The differences are ascribed to the different temperature dependence of the stiffness of uniaxial and biaxial order. We synthesised side chain single-crystal nematic polymer networks, performed DSC, X-ray, birefringence, and thermo-mechanical characterisations, and then obtained linear moduli from stress-strain measurements. Received 29 September 2000     

Thermotropic biaxial nematic liquid crystals

We have synthesized liquid crystal (LC) mesogens based on a nonlinear oxadiazole unit that exhibit nematic phases near 200 degrees C. Polarized microscopy and conoscopy indicate that these LCs are biaxial nematics. Unambiguous and quantitative evidence for biaxiality is achieved using 2H NMR spectroscopy. "2D powder" spectra, obtained by rotating 2H-labeled samples about an axis perpendicular to the magnetic field at approximately 200 Hz, yield phase biaxiality parameters of approximately 0.1 when coupled with rigorous and proven simulations.     

Enhancement by polydispersity of the biaxial nematic phase in a mixture of hard rods and plates

The phase diagram of a polydisperse mixture of uniaxial rodlike and platelike hard parallelepipeds is determined for aspect ratios kappa=5 and 15. All particles have equal volume, and polydispersity is introduced in a highly symmetric way. The corresponding binary mixture is known to have a biaxial phase for kappa=15, but to be unstable against demixing into two uniaxial nematics for kappa=5. The phase diagram for kappa=15 is qualitatively similar to that of the binary mixture, regardless of the amount of polydispersity, while for kappa=5 a sufficient amount of polydispersity stabilizes the biaxial phase. This provides clues for designing an experiment to observe this long searched biaxial phase.     

Light transmission at the uniaxial-biaxial lyotropic nematics phase transitions

Results are reported on light transmission versus the temperature near the phase transitions between lyotropic nematics. Strong pretransitional effects are observed when the phase transitions occur between biaxial nematic and uniaxial phases. The light is strongly depolarized both at the calamitic-biaxial and at biaxial-discotic phase transitions. The results are qualitatively explained.     

Dynamics of nematics with conformational degrees of freedom

On the basis of Hamilton approach the dynamics of the biaxial nematics is considered. All hydrodynamic parameters, connected with broken symmetry, are introduced in terms of the distortion tensor. The equations of ideal hydrodynamics are obtained and the three spectra of collective excitations of biaxial nematics are considered, taking into account the rod-shape of molecules.