Cholesteric LCs

Liquid crystals (LCs) are organic liquids with long-range ordered structures. They have anisotropic optical and physical behaviors and are similar to crystal in electric field. They can be characterized by the long-range order of their molecular orientation. According to the shape and molecular direction, LCs can be sorted as four types nematic LC, smectic LC, cholesteric LC, and discotic LC, and their ideal models are shown in Fig. 23 [52,55]. 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

As compared to the cholesteric LC, the lyotropic LC consists of two or more components that exhibit liquid-crystalline properties (dependent on concentration, temperature, and pressure). In the lyotropic phases, solvent molecules fill the space around the compounds (such as soaps) to provide fluidity to the system. In contrast to thermotropic liquid crystals, these lyotropics have another degree of freedom of concentration that enables them to induce a variety of different phases. A typical lyotropic liquid crystal is surfactant-water-long-chain alcohol. 

The cholesteric phase in hquid crystals is analogous to the nematic phase but it is formed by materials that contain a chiral centre, initially derivatives of cholesterol (5.3), hence the name cholesteric LCs. Since synthetic chiral molecules can also be used on their own or as dopants for nematic LCs, e.g. (5.4), chiral nematic is probably a more appropriate term for these materials. 

Cholesteric LC materials are able to reflect visible light and also respond to temperature changes causing variations in the shade of the reflected colour. The... 

The thermochromic behaviour of cholesteric LCs results from the fact that the pitch length, and hence the wavelength of the reflected light, is dependent on the temperature. 

Cholesteric LCs can act as hosts for dyes to produce coloured displays (see section 5.2.2.1 below) " their temperature dependent colour change has found applications in thermochromic inks, °" and as pigments and copy safe colours their selective reflecting capabilities have been applied in colours and filters for reflective displays and projection systems, " reflective polarisers and their electrical field induced switching in displays and smart reflectors, in colour patterning for full-colour recording. ... 

It is possible to make the colour of a cholesteric LC independent of temperature by locking it covalently into a polymer matrix. This can be achieved by cross-linking parts of the sample at different temperatures or by quenching locally at temperatures below the glass transition temperature. 

Figure 5.13 Prototype full-colour re-writable recording with low MW cholesteric LC.

Copolymerization thus presents an effective method for modification of LC polymer properties aimed at diversification of their thermal characteristics as well as at the creation of new types of LC polymers, cholesteric LC polymers in particular. 

Table 13. Cholesteric LC copolymers containing chiral monomer units... 

A different phenomenon is the appearance of the so-called blue phases in case of chiral LC s, first observed at cholesteric LC s. Sometimes, one or more blue phases occur close below the clearing point with a very small phase width and bright blue or green colours. Figure 15 shows some typical representatives containing one or more X=Y groups in... 

Fig. 8 The self-assembly of chiral networks presenting cholesteric LC phases [87]...

Fig. 16 Reversible photochemical cholesteric LC induction promoted in a nematic liquid crystal (4 -pentyloxy-4-biphenylcarnonitrile)...

A bisthienylethene D-7 with imines of (S)-l-phenylethylamine on C-5 both thiophene rings showed a slight diastereoselectivity. It was proved by induction of cholesteric LC phase [38] or H NMR spectra with a shift reag< [39]. 

Fig. 8a-c. Textures Observed for a Smectic, b Nematic and c Cholesteric LC Polymers on a Polarizing Microscope... 

Fig. 7. Viscosity of crosslinkable cyclosiloxane (n = 4.5, x = 0.45) and the crosslinkable cholesteric LC siloxane mixtures CC390L and CC680L.

In the 1960s, there was already a forerunner of photochemically induced phase transition in LC (20). When mixtures of cholesteryl iodide and cholesteryl bromide with cholesteryl nonanoate were exposed to UV irradiation, the helical pitch of cholesteric LC changes as a result of photodecomposition of the halides. The reflected color shifts gradually to red with progression of photodecomposition. Pattern-wise imaging was demonstrated but the image was blurred within 15 min. since LC of small molecules is a viscous fluid. 

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