Classification of mesophases

There are several different phases in thermotropic liquid crystals. The structural nature of mesophases is influenced by the molecular shape and therefore depends on whether the liquid crystal is formed by rod-like or disc-like molecules. Thermotropics of rod-like molecules may be divided into two main categories nematic and smectic phases. There exist many types of smectic phases, labeled as 5, 5b, S /. When an ordered solid of a liquid crystal melts (see Fig. 1.1), it may melt into a nematic phase or a smectic A phase. Upon further heating, it eventually turns into an isotropic liquid. First, classical thermotropic liquid crystals are described, and then a group of more exotic liquid crystals like discotic thermotropics, lyotropics, and liquid crystalline polymers. 

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A tentative classification of mesophases was reported by Wunderlich [2], who divided mesophases into six different types of phases hquid crystals (LC), condis crystsds (CD), plastic crystals (PC) and the corresponding LC, CD and PC glasses. 

The aggregation of surfactants into clusters or micelles in dilute solutions, as we will see, is a direct consequence of the thermodynamic requirements of the particular surfactant-solvent system under consideration. It has been suggested that phases occurring between the simplest micelles and true crystals are natural consequences of the removal of water from the micellar system, but do not constitute thermodynamically distinct states. In other words, the factors determining the structures of the mesophases are identical to those that control the formation of micelles in the first place. The same would be true of aggregates other than micelles, which do not fall under the classification of mesophases. 

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 have shown that this classification is sufficiently simple and general to include most important polymer solid mesophases identified so far. We are aware, however, that our classification of mesomorphic polymers, as well as alternative ones based on the different kinds of disorder which may occur in crystalline polymers, may not be exhaustive, since in many cases different kinds of disorder are interrelated and simultaneously present. For instance, in the case of polyacrylonitrUe, the irregular configuration of chains deter-... 

Like other states of matter, thermotropic mesophases are indefinitely stable at defined temperatures and pressures. Moreover, a thermotropic liquid-crystal-line material exhibits reversible phase transitions at well-defined temperatures. For example, the liquid crystal 4-n-pentyl-4 -cyanobiphenyl (5CB) melts from the solid to a nematic liquid crystal at 22.5°C and then from the nematic phase to the liquid at 35.0°C. As a consequence, the characterization and classification of thermotropic phases by microscopy also requires the use of an accurately controlled oven. 

In the following sections some of the more common liquid crystal textures utilized in the classification of thermotropic calamitic mesophases are described. It should be noted that similar examinations of defects are used to classify discotic, polymeric, and lyotropic phases. 

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. 

Identification and systematic classification of scientific phenomena is vital in any area of research. Liquid crystals are no exception and many different liquid crystalline phases and other mesophases have been identified and classified according to their distinct phase... 

To conclude this discussion of the thermotropic mesophases composed of rod-like molecules we would like to compile Table 1.1 from [18], where the structural classification of all known phases is given. Among them, the cubic smectic D phase is mentioned which was not discussed above. It is formed by rod-like molecules with specific intermolecular interactions between them [6]. Other types of cubic mesophases were discussed theoretically by Katz [19]. 

The classification of polymer mesophases have been considered in many reports (P-70). On analyzing the published data there arises much controversy in the identification. In order to avoid confusion in terminology it is necessary to define the basic terms relating to polymer columnar mesophases before further discussion. Columnar polymer systems have both a correlation of the centers of gravity and molecular orientation, but have mesomorphic properties due to the conformational disorder both of the polymer back-bone and side chains. The structural unit of the polymeric columnar mesophase is a macromolecule. In columnar phases macromolecules form regular 2D-periodic arrays. The two-dimensional symmetry of the column packing and the parameters of two-dimensional lattice are strongly dependent on the form and dimensions of the cross-section of a polymer molecule. 

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