Classification of liquid crystals

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... 

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. 

The basic classification of "Liquid-Crystals" is divided into two major groups the thermotropic and lyotropic. In the first case the structure is modified from the crystalline state to the amorphous liquid, passing trough the "Liquid-Crystals" state as the temperature is increased in the second case the order related to the crystalline texture is decreased trough the non ordered amorphous structure as a solvent is added to the crystalline materials. In both cases the adjective mesomorphic refers to the occurence of the new state of matter in the range of stability interposed, for a particular compound, between that of the crystalline type, which prevails at low temperature or in the pure state, and that of die amorphous type which, in turn, is found at higher temperatures or in diluite solutions. 

A. J. Leadbetter, Structure classification of liquid crystals, in Thermotropic liquid crystals, ed. G. W. Gray... 

Second, the ability of completely aromatic polyesters to form mesomorphic structures is interesting. Obviously, in such cases, the appearance of nematic LC states seems most probable. Furthermore, some polybenzoates (3-8) show mesophases, which cannot be described in terms of the conventional classification of liquid crystals, with respect to their arrangement, and which are similar to mesomorphic structures of flexible-chain polyorganosiloxanes (9) and polyphosphazenes (10). Finally, studies of relaxations and phase transitions in rigid-chain LC polyesters, in particular, their molecular mobility in the solid state, i.e., below the melting temperature of crystalline phase, are of great interest. 

The liquid ciystalline phase is a distinet phase of matter, but there are many different types of liquid ciystalline phases. The various liquid crystalline phases and other mesophases are characterised and then classified according to the molecular ordering that constitutes the phase stracture. Not surprisingly, the difference between the many different liquid ciystal phases and mesophases is generally minimal. Such minimal differences in stracture mean that the precise classification of liquid crystals often requires the use of several analytical techniques and a great deal of experience. However, in some cases, classification is relatively simple. Each individual liquid crystal phase has been characterised as a distinct phase of matter by a number of different physical techniques and new liquid crystal phases continue to be discovered as the identification techniques improve. The identification and classification of liquid ciystalline and other mesophases is of vital importance to those working in any discipline of the wide field of liquid ciystals. The techniques that are used to characterise and identify liquid crystalline phases are also very relevant to a wide range of other scientific areas. The aim of this chapter is to consider the major methods of liquid crystal phase characterisation and identification. 

Thermotropic liquid crystals were studied for the first time in the late 19th century. Initially, research focused on the structural characterization and classification of liquid crystals [11]. Also, various theories [12] were implemented on viscosity [13], elasticity constant [14], etc. Major progress has been recorded in the year 1960, driven by practical applications of liquid crystals with main [15, 16] and side chains [16, 17]. 

Figure 11.2 Classification of liquid crystals. Reproduced from reference 6.

Demus, D. Types and classification of liquid crystals, in Liquid Crystals, Applications and Uses, B. Bahadur, Ed. World Scientific, New Jersey, 1990, Vol. 1, pp. 1-36, and references therein. 

The period from their discovery in the latter part of the nineteenth century through to about 1925, the years during which the initial scepticism by some that a state of matter was possible in which the properties of anisotropy and fluidity were combined, through to a general acceptance that this was indeed true, and publication of a first classification of liquid crystals into different types. 

In addition to containing a wealth of information on microscopic techniques and materials, Friedel s article represented in 1922 the first classification of liquid crystals into types, i.e., nematic, smectic and cholesteric. Today, of course, cholesterics are known simply as chiral nematics with no need that they be derived from cholesterol, and we recognize the existence of several polymorphic smectic forms, whereas Frie-del allowed for only one (today s smectic A SmA). 

Chapter II Guide to the Nomenclature and Classification of Liquid Crystals.17... 

Liquid crystals are now best known for their applications in Hquid crystal displays (LCDs), but in fact the kind of Hquid crystal used in such displays represents a small subgroup of the family of Hquid crystal phases and materials. In this chapter, we will learn about the main classifications of liquid crystals, their properties, and their applications, including a description of standard experimental techniques for phase identification. 

L2 General Classification of Liquid-Crystal Polymers and Networks... 

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