ampersand c period circ semicolon"/>. In the case of smectic‐A, which possesses rotational symmetry around By convention,
1.3.4. Functionalized and Discotic Liquid Crystals
Through various chemical synthesis techniques as well as nanotechnologies, an entire class of novel or so‐called functionalized liquid crystals have emerged [11–13]. Figure 1.14 shows, for example, the shuttlecock‐shaped liquid crystal formed by incorporating fullerene C60 to various crystals and liquid crystals reported by Sawamura et al. [11]. Others have investigated a special class of liquid crystals comprising disc‐like molecules, discotic liquid crystals, that possess interesting and useful semiconducting properties suitable for optoelectronic applications [12, 13].
1.4. MIXTURES, POLYMER‐DISPERSED, AND DYE‐DOPED LIQUID CRYSTALS
In general, temperature ranges for the various mesophases of single constituent liquid crystals are quite limited. Therefore, while many fundamental studies are still conducted on such liquid crystalline materials, industrial applications employ mostly mixtures, composites, or specially doped liquid crystals with large operating temperature range and tailor‐made physical and optical properties.
Figure 1.14. A shuttlecock‐shaped liquid crystal formed by incorporating fullerene C60 into various liquid crystals was reported [11].
There are many techniques for modifying the physical properties of a liquid crystal. At the most fundamental level, various chemical groups such as bonds or atoms can be introduced to modify the LC molecule. A good example is the cyanobiphenyl homologous series nCB (n = 1,2,3…). As n is increased through synthesis, the viscosities, anisotropies, molecular sizes, and many other parameters are greatly modified. Some of these physical properties can also be modified by substitution. For example, the hydrogen in the 2, 3, and 4 positions of the phenyl ring may be substituted by some fluoro (F) or chloro (Cl) group [14].
Besides these molecular synthesis techniques, there are other ways to dramatically improve the performance characteristics of liquid crystals. In the following sections, we describe three well‐developed methods, focusing our discussion on nematic liquid crystals as they exemplify the unique characteristics of liquid crystals widely used in optical and photonic applications.
1.4.1. Mixtures
A large majority of liquid crystals in ubiquitous devices are eutectic mixtures of two or more mesogenic substances. A good example is E7 (from EM Chemicals), which is a mixture of four liquid crystals (cf. Figure 1.15). The optical properties, dielectric anisotropies, viscosities of E7 are very different from those of the individual mixture constituents.
Figure 1.15. Molecular structures of the four constituents making up the liquid crystal E7 (from EM Chemicals).
Creating mixtures is an art, guided, of course, by some scientific principles. One of the guiding principles for making the right mixture can be illustrated by the exemplary phase diagram of two materials with different melting (i.e. crystal → nematic) and clearing (i.e. nematic → isotropic) points, as shown in Figure 1.16. Both substances have small nematic ranges (Ti–Tn and
Figure 1.16. Phase diagram of the mixture of two liquid crystals.
If
