Applications chiral nematics

By far the most important commercial applications of nematic liquid crystals are in the a multi-billion dollar display industry. Cholesteric, or chiral nematic liquid crystals have been used in coloured guest-host displays and in thermography/ther-mochromic applications. 

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

The first application described was as temperature sensors by using a chiral nematic liquid crystal, which displays different colors at different temperatures. It is also worth noting that many common fluids are in fact liquid crystals. Soap, for instance, is a liquid crystal, and forms a variety of liquid crystal phases depending on its concentration in water. 

The application of an electric field above the threshold value results in a reorientation of the nematic liquid crystal mixture, if the nematic phase is of negative dielectric anisotropy. The optically active dopant then applies a torque to the nematic phase and causes a helical structure to be formed in the plane of the display. The guest dye molecules are also reoriented and, therefore, the display appears coloured in the activated pixels. Thus, a positive contrast display is produced of coloured information against a white background. The threshold voltage is dependent upon the elastic constants, the magnitude of the dielectric anisotropy, and the ratio of the cell gap to the chiral nematic pitch ... 

The pitch of the helix for compound 42 was found to be approximately 0.2-0.3 xm, thus the material selectively reflects visible light over a wide temperature range. Moreover, the pitch is relatively temperature insensitive thus the material can be used in large area non-absorbing polarizers, or in optical notch filters or reflectors. In addition, in the glassy state the helical macrostructure of the chiral nematic phases is retained, thus similar applications are possible. 

In general, the term liquid crystal is used to describe an intermediate phase between liquid and solid occurring in some organic compounds. The phase of liquid crystal can be divided into two mesophases smectic and nematic. Nematic liquid crystals can be further divided as chiral nematic or archiral nematic. In chiral nematic liquid crystals, sterol-related compounds are called cholesteric, and non-sterol-based compounds are termed chiral nematic. For heat transfer applications, encapsulated forms of chiral nematic [71] or the composite liquid crystal sheets of the cholesteric type [72] are commonly used. Recently, the application of micro-encapsulated liquid crystals has become more popular in heat transfer measurements because of the fast response and easy paintbrush or spray application to the test surface. 

In view of the unique roles of cholesteric GLCs in material synthesis and device application, the commonly practiced statistical synthesis and the continuing effort in deterministic synthesis are highlighted. All factors considered, a hybrid chiral-nematic precursor is much preferred over separate chiral and nematic precursors in terms of synthesis and subsequent product separation and purification. 

The linear electro-optic effect in a cholesteric, i.e. a hard-twisted chiral nematic (the helical pitch must be less than the wavelength of visible light) was a very original proposal for using the flexoelectric effect in a new display, shutter or modulator device. The patent application by R.B. Meyer and J.S. Patel dates from 1987 and was granted in 1990. The physics was developed in a series of papers by these authors and later elaborated by others.It is now commonly called the flexoelectro-optic effect. 

Chiral nematic mesoporous films of Eu " doped Zr02 have been produced via a hard-templating approach using nanoctystalline cellulose-templated silica (Fig. 11). It was found that these chiral nematic nanostructures are capable of modulating the spontaneous emission of the Eu ions. The emission lines of the Eu " at 596 nm, 613 and 625 nm were significantly suppressed, and an increase in the luminescence lifetime is observed. It was suggested that these new chiral luminescent nanomaterials could find potential applications in sensing and new optical nanodevices. 

---