Properties of Liquid Crystals

Active-Matrix-Addressed Liquid Crystal Displays. . 127 

Liquid crystals consist of rigid rodlike molecules whose interaction tends to align them parallel to each other (for more details, see de Gennes, 1974  

Chandrasekhar, 1977). This cooperative behavior results in weak elastic properties. Then, the application of an electric field can easily change the molecular orientation, which is initially fixed by the mechanical boundary conditions. The concomitant changes in the optical properties form the basis of liquid crystal displays (LCD). 

Liquid crystals can display different degrees of long-range order, dependent on temperature, chemical composition, and the presence or absence of electric fields. In the nematic phase, the molecular axes point in a common direction, denoted by the director n but the molecular centers are otherwise arranged randomly. Because of the low degree of long-range order, nematic LCs have viscosities typical of ordinary liquids, and displays based on nematic LCs can operate at television frame rates. The most popular nematic-based display, the twisted nematic (TN), will be discussed in more detail below. 

In cholesterics, the structure is similar to nematics, but the director rotates in a corkscrewlike fashion along n. Electric-field-induced transitions between the cholesteric and nematic phases are used in the dye phase change display discussed below. 

---

We are all familiar with tire tliree states of matter gases, liquids and solids. In tire 19tli century the liquid crystal state was discovered [1 and 2] tliis can be considered as tire fourtli state of matter [3].The essential features and properties of liquid crystal phases and tlieir relation to molecular stmcture are discussed here. Liquid crystals are encountered in liquid crystal displays (LCDs) in digital watches and otlier electronic equipment. Such applications are also considered later in tliis section. Surfactants and lipids fonn various types of liquid crystal phase but this is discussed in section C2.3. This section focuses on low-molecular-weight liquid crystals, polymer liquid crystals being discussed in tire previous section. 

The label liquid crystal seems to be a contradiction in tenns since a crystal cannot be liquid. However, tire tenn refers to a phase fonned between a crystal and a liquid, witli a degree of order intennediate between tire molecular disorder of a liquid and tire regular stmcture of a crystal. Wlrat we mean by order here needs to be defined carefully. The most important property of liquid crystal phases is tliat tire molecules have long-range orientational order. For tliis to be possible tire molecules must be anisotropic, whetlier tliis results from a rodlike or disclike shape. 

Blinov L M 1983 Electro-optical and Magneto-optical Properties of Liquid Crystals (Chichester Wiley)... 

Simoni F 1997 Nonlinear Optical Properties of Liquid Crystals and Polymer-Dispersed Liquid Crystals (Singapore World Scientific)... 

In Chapter 3 it was pointed out that certain rod-like polymers showed many of the attributes of liquid crystals in the melt. In particular, these molecules were oriented in shear to such an extent that interchain entanglement was small and the melts had a low viscosity. On cooling of the melt these rod-like molecules remained oriented, effectively self-reinforcing the polymer in the direction of flow. The essential differences in the properties of liquid crystal polymers... 

Iizuka, E. Properties of Liquid Crystals of Polypeptides with Stress on the Electromagnetic Orientation. Vol. 20, pp. 79 — 107. 

General reviews of the structure and properties of liquid crystals can be found in the following G. H. Brown, J. W. Doane, and V. D. Neff. "A Review of the Structure and Physical Properties of Liquid Crystals." CRC Press, Cleveland, Ohio, 1971 P. J. Collings and M. Hind, Introduction to Liquid Crystals. Nature s Delicate Phase of Matter," Taylor and Francis, Inc., Bristol. Pennsylvania, 1997 P. J. Collins, "Liquid Crystals. Nature s Delicate Phase of Matter," Princeton University Press. Princeton. New Jersey, 1990. A thermodynamic description of the phase properties of liquid crystals can be found in S. Kumar, editor, "Liquid Crystals in the Nineties and Beyond, World Scientific, Riven Edge, New Jersey, 1995. 

The maintenance of a connection to experiment is essential in that reliability is only measurable against experimental results. However, in practice, the computational cost of the most reliable conventional quantum chemical methods has tended to preclude their application to the large, low-symmetry molecules which form liquid crystals. There have however, been several recent steps forward in this area and here we will review some of these newest developments in predictive computer simulation of intramolecular properties of liquid crystals. In the next section we begin with a brief overview of important molecular properties which are the focus of much current computational effort and highlight some specific examples of cases where the molecular electronic origin of macroscopic properties is well established. 

It is clear from the forgoing discussions that the important material properties of liquid crystals are closely related to the details of the structure and bonding of the individual molecules. However, emphasis in computer simulations has focused on refining and implementing intermolecular interactions for condensed phase simulations. It is clear that further work aimed at better understanding of molecular electronic structure of liquid crystal molecules will be a major step forward in the design and application of new materials. In the following section we outline a number of techniques for predictive calculation of molecular properties. 

Intramolecular Bonding and Electronic Properties of Liquid Crystal Molecules... 

A number of bulk simulations have attempted to study the dynamic properties of liquid crystal phases. The simplest property to calculate is the translational diffusion coefficient D, that can be found through the Einstein relation, which applies at long times t ... 

In the previous sections, we have seen how computer simulations have contributed to our understanding of the microscopic structure of liquid crystals. By applying periodic boundary conditions preferably at constant pressure, a bulk fluid can be simulated free from any surface interactions. However, the surface properties of liquid crystals are significant in technological applications such as electro-optic displays. Liquid crystals also show a number of interesting features at surfaces which are not seen in the bulk phase and are of fundamental interest. In this final section, we describe recent simulations designed to study the interfacial properties of liquid crystals at various types of interface. First, however, it is appropriate to introduce some necessary terminology. 

Gray GW (1962) In Molecular structure and the properties of liquid crystals. Academic Press, New York, p 186... 

Crain J, Clark SJ (1999) Calculation of Structure and Dynamical Properties of Liquid Crystal. 94 1-39... 

The wetting and spreading properties of liquid crystals (LC) on solid substrates are of interest due to their use in display devices. In addition, these interesting liquids provide a good testing ground for fundamental theories of wetting. Films of 8CB (4 -n-octyl-4-... 

The thickness of the membrane phase can be either macroscopic ( thick )—membranes with a thickness greater than micrometres—or microscopic ( thin ), i.e. with thicknesses comparable to molecular dimensions (biological membranes and their models, bilayer lipid films). Thick membranes are crystalline, glassy or liquid, while thin membranes possess the properties of liquid crystals (fluid) or gels (crystalline). 

Metallomesogens are unique compounds which combine the properties of liquid crystals with that of metal complexes to generate novel electric or... 

The unique properties of liquid crystals have also provided opportunity for study of novel nonlinear optical processes. An example involves the ability to modify the pitch of cholesteric liquid crystals. Because a pseudo-wave vector may be associated with the period of pitch, a number of interesting Umklapp type phasematching processes (processes in which wave vector conservation is relaxed to allow the vector addition to equal some combination of the material pseudo-wave vectors rather than zero) are possible in these pseudo-one-dimensional media. Shen and coworkers have investigated these employing optical third harmonic generation (5.) and four-wavemixing (6). 

Monomer which can impart the properties of liquid crystals to the polymer , formed by its polymerization. 

D. Demus, J.W. Goodby, G.W. Gray, H.-W. Speiss and V. Vill (Eds.), Physical Properties of Liquid Crystals, Wiley-VCH, Weinheim, 1999. 

Thanks to this auto-organization faculty, some amphiphilic sugars have properties of liquid crystals. This tendency is pronounced in the case of fluoroalkyl sugars. The... 

Physical properties of liquid crystals are generally anisotropic (see, for example, du Jeu, 1980). The anisotropic physical properties that are relevant to display devices are refractive index, dielectric permittivity and orientational elasticity (Raynes, 1983). A nematic LC has two principal refractive indices, Un and measured parallel and perpendicular to the nematic director respectively. The birefringence An = ny — rij is positive, typically around 0.25. The anisotropy in the dielectric permittivity which is given by As = II — Sj is the driving force for most electrooptic effects in LCs. The electric contribution to the free energy contains a term that depends on the angle between the director n and the electric field E and is given by... 

We have omitted discussing such interesting properties of liquid-crystal solutions as the Frank elastic constants, the Leslie viscosity coefficients, cholesteric pitch, textured structure (or defects), and rheo-optics. Some of them are reviewed in recent literature [8,167], but the level of their experimental and theoretical studies still remains largely qualitative. 

Synthesis and Physical Properties of Liquid Crystals An Interdisciplinary Experiment 118... 

---