Nematic liquid crystals formed from

NEMATIC LIQUID CRYSTALS FORMED FROM FLEXIBLE MOLECULES A MOLECULAR FIELD... 

Mesomorphic phases have properties between those of solids and liquids. For example, rod-shaped or disk-shaped molecules often melt to form liquid crystals, which are fluid, but contain some order. One form of liquid crystals formed from rod-shaped molecules, called a nematic liquid crystal, is shown in Fig. 11. The molecules in the nematic phase are partially ordered in one dimension, with an orientation angle that can vary from one domain to another in the fluid. The forces... 

We consider first the Maier-Saupe theory and its variants. In its original formulation, this theory assumed that orientational order in nematic liquid crystals arises from long-range dispersion forces which are weakly anisotropic [60. 61 and 62]. However, it has been pointed out [63] that the form of the Maier-Saupe potential is equivalent to one in... 

Discotic nematic liquid crystal A liquid crystal formed from discotic (disk-shaped) molecules, which are more or less planar and which are stacked. 

Discotic liquid crystals arise from disk-shaped molecules as nematic or cholesteric mesophases. Their structural characteristics are similar to the respective ealamitie mesophases, that is, the normals of the disks are oriented parallel. Instead of the smectic mesophases, diseotie columnar liquid crystals arise from eonnecting the disks to each other. The columns of the discotic columnar mesophase form a two-dimensional lattice whieh is in a hexagonal or rectangular modification. In addition, the columns may be tilted (Fig. 2f,g). 

The cubic amphiphilic mesophases (Sic, Vi, and V2) from their interposition in the succession of mesophases Sic, Mi, Vi, G, V2, and M2, have generally been termed liquid crystalline like the optically anisotropic amphiphilic mesophases Mi, G, and M2. The cubic mesophases formed by non-amphiphilic globular molecules have however usually been termed plastic crystals. This nomenclature has obscured the fact that these plastic crystals are fundamentally liquid crystals rather than solid cyrstals and bear a relationship to the optically anisotropic non-amphiphilic smectic and nematic liquid crystals similar to that born by the amphiphilic cubic mesophases to the optically anisotropic neat (G) and middle (Mi and M2) liquid crystalline phases. 

One compound may exist in more than one liquid crystal form. The structure of cholestryl myristate changes from solid to smectic A at 71 °C, to chiral nematic at 79 °C, and finally to an isotropic liquid at 85 °C. Figure 16.10 shows the transitions in p-methoxybenzoic acid. Note that the order increases with increased pressure and lower temperature. Liquid crystal phases are more densely packed than the... 

Schematic diagrams of die cell and apparatus for measuring die temperature dependence of die optical rotation of an aligned nematic liquid crystal cell. The birefringence cell is formed from microscope slides and die lines in the cell denote the rubbing direction for the glass plates and thus the direction for the nematic director n.

In general, cholesteric liquid crystals are found in optically active (chiral) mesogenic materials. Nematic liquid crystals containing optically active compounds show cholesteric liquid crystalline behavior. Mixtures of right-handed and left-handed cholesteric liquid crystals at an adequate proportion give nematic liquid crystals. From these results cholesteric liquid crystals are sometimes classified into nematic liquid crystals as twisted nematics . On the other hand, cholesteric liquid crystals form batonnet and terrace-like droplets on cooling from isotropic liquids. These behaviors are characteristic of smectic liquid crystals. Furthermore, cholesteric liquid crystals correspond to optically negative mono-axial crystals, different from nematic... 

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