Liquid crystal phase 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. 

Nematic liquid crystals have anisotropic molecules, and they are arranged in space in more or less in one direction called director, but there is no regularity in their positions like liquids. They are only more viscous than ordinary liquids and like liquids they do not possess definite shape. It is the most type of liquid crystal phase [5,6]. 

When polymerization is effected at the air-water interface, monolayer polymer films may result. Such films have attracted a great deal of attention in recent years by virtue of their similarity to biological membranes. Polymeric films do not really fall within the strict definition of liquid crystal polymers. However, the molecular arrangement of the side chains resembles that found in many liquid crystal phases, most notably the lamellar phase, and could therefore realistically be described as one half of a lamellar sheet. Bilayer films, on the other hand, may be thought of as a single lamellar layer, and therefore the comparison becomes even more appropriate. The technical applications of polymer membranes are extremely diverse and for these reasons a short section will be devoted to their history, their uses and their current status. 

In the isoelectronic B3H7 LB species, the liquid phase structure (VII-A13) is definitely indicated to be correct by Parry and Paine 39, 109), rather than a structure with elements of both VII-A13 and II-A13 which may be more stable in the crystal phase 97). 

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. 

An alternative and perhaps more accurate definition of thermodynamic fragility that can be determined experimentally for almost all liquids would be to determine the fraction of entropy of fusion lost by T/Tm = 0.8 [37,88]. In both the definitions, the property of the crystal phase enters this disadvantage is missing in the kinetic measures of fragility [37,88]. 

The term liquid crystal was first applied to compounds that, unlike most compounds that melt in a single step at a definite temperature, show one or more well-defined phases between the solid and the true liquid. Otto Lehmann in 1888 was contacted by Friedrich Reinitzer who had observed that crystalline cholesteryl benzoate, on heating, seemed to have two melting points.At 145.5° C a cloudy liquid forms that, on further heating, changes sharply to a clear liquid at 178.5° C. On cooling the reverse order of phases was found. The turbid liquid is doubly refracting, like the anisotropic crystals described in Chapter 5, hence the... 

Liquid crystals (LCs) are molecules that have the ability to self-assemble into organized mesophases with properties intermediate between those of crystalline solids and isotropic liquids [1,2]. In LC phases, the molecules are dynamic and collectively behave as a viscous liquid but retain on average a degree of organization reminiscent of an ordered, crystalline solid. Consequently, they can be considered ordered fluids, as a more accurate definition. LCs can be subdivided into two general classes—thermotropic LCs and lyotropic LCs—depending on the environmental and molecular factors that govern how they form ordered fluid phases. 

There are many metal complexes, metallomesogens, which deomonstrate liquid crystal or mesomorphic properties, and the organization of metal complexes in fluid phases is definitely supramolecular behaviour. These compounds were well reviewed in 1991 [20a], 1992 [20b] and 1993 [20c] the strength of this research in Europe and Japan is evident. 

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