Liquid crystal definition

Thennotropic liquid crystal phases are fonned by rodlike or disclike molecules. However, in the following we consider orientational ordering of rodlike molecules for definiteness, although the same parameters can be used for discotics. In a liquid crystal phase, the anisotropic molecules tend to point along the same direction. This is known as the director, which is a unit vector denoted n. 

Many solid substances (camphor, iodine, naphthalene, etc.), are known which are appreciably volatile at ordinary temperatures. Others, such as the metals, are apparently quite fixed, but they probably possess a definite, although very small vapour-pressure, even at ordinary temperatures. Thus, if magnesium is heated to 550° for a few hours in a magnesia boat enclosed in a vacuous tube it sublimes in beautiful crystals on the cool part of the tube. The vaporisation of a solid without previous fusion is called sublimation the vapour-pressure (like the vapour-pressure of a liquid), is definite for each temperature, is independent of i the volume of the vapour space, and increases with rise of temperature. 

A distinction between a solid and liquid is often made in terms of the presence of a crystalline or noncrystalline state. Crystals have definite lines of cleavage and an orderly geometric structure. Thus, diamond is crystalline and solid, while glass is not. The hardness of the substance does not determine the physical state. Soft crystals such as sodium metal, naphthalene, and ice are solid while supercooled glycerine or supercooled quartz are not crystalline and are better considered to be supercooled liquids. Intermediate between the solid and liquid are liquid crystals, which have orderly structures in one or two dimensions,4 but not all three. These demonstrate that science is never as simple as we try to make it through our classification schemes. We will see that thermodynamics handles such exceptions with ease. 

For many chemists LCs are mysterious and complex materials, their very definition defying a simple understanding. The basic idea behind this discussion of stereochemistry in LCs is that molecules and LCs represent the same phenomenon. Liquid crystals are supermolecules in a different way than are supramolecular assemblies. Indeed, LCs can be composed of supramolecular... 

FIGURE 5.11 Supramolecular, helical architecture and definition of pitch length p of chiral nematic liquid crystals. 

Definitions of Basic Terms Relating to Low-Molar-Mass and Polymer Liquid Crystals... 

This document provides definitions of the basic terms that are widely used in the field of liquid crystals and in polymer science (See references 1-39). It is the first publication of the Commission on Macromolecular Nomenclature dealing specifically with liquid crystals. 

Note 4 The tilt direction varies in a random manner from layer to layer in conventional smectic C mesophases. However it can alternate from layer to layer, as in an antiferro-electric chiral smectic C mesophase (see Definition 5.9, Note 7) and in the smectic C mesophase formed by certain liquid crystal dimers with an odd-number of carbon atoms in the spacers. The recommended symbol for this type of mesophase is SmCa. 

Note 4 A thin sample of a smectic C phase with the layers parallel to the sample surfaces gives schlieren textures with centers that have four brushes. However, a smectic C phase formed by odd-membered liquid-crystal dimers (see Definition 2.11.2.9, Note 5) has schlieren textures with two or four brushes. 

Note 4 The pendant groups in these polymers have structures compatible with liquid-crystal formation, that is, they are mesogenic but not intrinsically mesomorphic. See the examples given in Definitions 2.10 2.11.2.1. 

Definitions of basic terms relating to low-molar-mass and polymer liquid crystals (lUPAC Recommendations 200 ), Pure Appl. Chem. 73, 845-895 (2001). 

The term meso will be used to describe local chain organizations that occur within the nano- and microscale regions (Figure 18.2). While the terms mesophase and mesoregions have been employed in describing order within liquid crystals, the definition will be... 

The dodecahydrate is efflorescent at room temperature in air. At 22° C. it becomes damp and the dampness increases as the temperature rises until finally the salt becomes completely liquid. A definite melting point is not exhibited.3 The liquid becomes clear at 56 2° C. and is then a true solution of the heptahydrate. When this is supercooled to about 40° C. and seeded with a crystal of the salt, crystals of the heptahydrate are deposited and the temperature rises to 56-2° C. A similar evolution of heat, but less marked, occurs at 22° C. The heptahydrate does not effloresce appreciably at room temperature in air. 

Polarized light is the must powerful tool for investigating liquid crystals, all of which exhibit characteristic optical properties. A smectic liquid crystal transmits light more slow ly perpendicular to the layers than parallel to them. Such substances are said to be optically positive. Nematic liquid crystals are also optically positive, bui their action is less definite than that of smectic liquid crystals. However, the application of a magnetic field to nematic liquid crystals lines up their molecules, changing their optical properties and even their viscosity. 

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