Liquid Crystal Classification

Figure C2.2.7. Schematic illustrating tire classification and nomenclature of discotic liquid crystal phases. For tire columnar phases, tire subscripts are usually used in combination witli each otlier. For example, denotes a rectangular lattice of columns in which tire molecules are stacked in a disordered manner (after [33])...

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. 

Thermotropic liquid crystals may be classified as either low molar mass (i.e. non-polymeric), or high molar mass (i.e. polymeric) and within each of these broad classifications, there are several sub-classifications. 

There are now three major shape classifications of low molar mass liquid crystals - rod-like (calamitic), disc-like (discotic) and bent-core. The last of these is the most recent, and while examples of bent mesogens have been known for some years, it is only since the mid-1990s that the area has attracted widespread attention [2],... 

In many cases, these polymer chains take on a rod-like (calamitic LCPs) or even disc-like (discotic LCPs) conformation, but this does not affect the overall structural classification scheme. There are many organic compounds, though not polymeric in nature, that exhibit liquid crystallinity and play important roles in biological processes. For example, arteriosclerosis is possibly caused by the formation of a cholesterol containing liquid crystal in the arteries of the heart. Similarly, cell wall membranes are generally considered to have liquid crystalline properties. As interesting as these examples of liquid crystallinity in small, organic compounds are, we must limit the current discussion to polymers only. 

Thus, polymers with mesogenic groups in side chains form structural mesophases of the same types as low-molecular liquid crystals. This makes it possible to apply traditional mesophase classification for the description of the structure of LC polymers. At the same time, the structure of some of comb-like polymers (see Table 5) considered as crystalline, may probably be treated as one of highly-ordered smectic mesophases (SH or Sj), whose study is only started74). 

The optical texture of mesophase and resultant carbons is observed readily by means of a reflected polarized light microscope and may be classified according to the shape and size of the isochromatic units. Such a classification is useful to evaluate the properties of mesophase and carbons such as needle cokes. The mesophase has been defined as the intermediate state which shows optical anisotropy and is quinoline-insoluble at room temperature (5,51) (liquid crystal glass), although it is a viscous liquid crystal during the carbonization process (6). ... 

We do not give a detailed classification of the types of liquid crystals, since the systems under consideration mainly form nematic or in some cases cholesteric phase. The latter phase belongs, in principle, to the same range of liquid crystals as the nematic phase, since there is no phase transition between them (unlike smectic-nematic phase transition). 

We do not give a detailed classification of the types of liquid crystals, since the systems under consideration mainly form nematic or in some cases cholesteric phase. 

Another classification of methods for measuring anisotropies is that of partially oriented molecules, the most important examples of which are the liquid crystals. The utility of this method lies in the fact that many molecules can be dissolved in liquid crystals without destroying the liquid crystal phase.12-15 The solute molecules are able to translate relatively freely in one direction-along the optic axis. As a result of this motion the intermolecular dipole-dipole interactions are averaged to zero but the intramolecular interactions are not. Also since the solute molecules cannot rotate equally in all directions, there will be an anisotropic contribution to the observed shielding. 

Over the past two decades, liquid crystal polymers (LCP s) have received a considerable amount of attention in both academic and industrial laboratories. Often termed mesomorphic (meaning having "middle form"), liquid crystalline phases have a degree of order between that of the zero ordered liquid and that of the three dimensional crystal lattice. Recent reviews of liquid crystal polymers have provided a fundamental understanding of the synthesis, classification, morphology, and rheology of this unique class of materials (52-541. 

We now introduce the classification of the liquid crystal phases and their structural features. 

In the Sl phase, the molecules are perpendicular to layers and are arranged in a hexatic lattice within layers. This symmetry is the same as that of the Sb phase. Both of the Sl and Sb phases are optically uniaxial, their molecules within smectic layers rotate more freely around their long axes or rotate in groups simultaneously. In some of the literature, the Sb phase is called the hexatic B phase while the Sl phase is named the Sb phase or crystal B phase. The molecules within the Sq and Si phases are tilted with respect to the layers while the molecules of the Sl phase are aligned more or less perpendicularly to the layers. The stacking of molecules within the layer of the Sl, Sq and Sj phases is similar to those shown in Figure 1.7 as well. The crystallography classifications of these three liquid crystal phases are the same as those of the Sb, Sp and Si phases. 

Polarizing optical microscopy is often the method of first choice. It was with POM that the textures of liquid crystals were observed and the classification of liquid crystals was first made according to these observations. In this field, two books are recommended The Textures of Liquid Crystals by Demus and Richter (1978) and Smectic Liquid Crystals — Textures and Structures by Gray and Goodby (1984). While the latter provides readers with a practical and useful experimental guideline to the textures and classification of smectic liquid crystals of different polymorphic types together with as many as 124 reference photographs of typical textures, the former... 

Goodby J W and Gray G W 1998 Guide to the nomenclature and classification of liquid crystals Handbook of Liquid... 

Thermotropic liquid crystals can be divided into three principal types through the mesophases they form, two of which will be introduced at this stage. This classification is based on the essential shape of the molecules and while,... 

Leadbetter, A. J. Structural Classification of Liquid Crystals. This classification was proposed in Thermotropic Liquid Crystals Gray, G. W., Ed. Wiley Chichester, 1987 Chapter 1, pp 1-27. 

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