Liquid Crystals. Iam-Choon Khoo

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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 (T_i–T_n and ). When mixed at the right concentration [2], however, the nematic range () of the mixture can be orders of magnitude larger. To date, there is an assortment of nematic liquid crystals, for example, with an operating temperature range from sub freezing (<−30 °C) to well over 100 °C that found their way in ubiquitous display devices (Figure 1.16).