Cholesteric nematics

Fig.1.16. Common mesophases of rodlike mesogens A nematic, B smectic-A, C smectic-C, D cholesteric nematic...

Sensitized for blue-green or red light, photoconductive polyimides and liquid crystal mixtures of cyanobiphenyls and azoxybenzene have been used in spatial light modulators [255-261]. Modulation procedure was achieved by means of the electrically controlled birefringence, optical activity, cholesteric-nematic phase transition, dynamic scattering and light scattering in polymer-dispersed liquid crystals. 

Fig. 32a, b. Volt-contrast (a) and current-voltage (b) characteristics for polyimide modulator with cholesteric-nematic phase transition without (/) and with switch (2) on the recording light [254]... 

Most of the molecules of interest in vision and biology are prepared lyotropically. There are three main classes, the nematic, the smectic, and the cholesteric-nematic (frequently simplified to just cholesteric) When deposited on a substrate, the chromophores of vision assume a stmcture of smectic class. This class is further subdivided into types A through H. Type A liquid crystals have their major molecular axis perpendicular to the surface of the film. Type C crystals have their major molecular axis at some other angle. Assuming an analogy of similar chromophores in photography, it is most likely that the chromophores of vision are of type C. 

Cholesteric-Nematic Phase Change Effect (CNPC)... 

Figure 3.3 Schematic representation of a cholesteric-nematic phase-change effect (CNPC) LCD. ...

Thus, side-chain systems can exhibit many properties in between, well-oriented and solid materials. Many applications for cholesteric, nematic, and smectic cyclic siloxanes have been proposed. Most of them use cholesterics. Cholesteric liquid crystals (n ) or tilted smectic phases reflect the incident light in a specific wavelength range and with circular polarization. The... 

Cholesteric liquid crystals show many electro-optic effects. Among them the cholesteric-nematic phase transition effect is the most interesting one which was addressed in the early part of this chapter. Others are the square grid effect, storage effects and color effects, etc. 

Square grid effect Before the electric field becomes great enough to cause the cholesteric-nematic phase transition, a periodic deformation may appear in cholesteric liquid crystals. The layer undulation occurs in two orthogonal directions so that a square pattern is observed. This effect is more likely to happen for cholesteric liquid crystals of large pitch (about microns). 

Cholesteric-nematic phase transition change from negative uniaxiality to positive uniaxiality The electric field is parallel to the helix axis. ... 

Display devices can also be constructed using the field effect, the cholesteric memory effect and the cholesteric-nematic phase change effect [259, 262]. The recognition of the useful electro-optical properties of liquid crystals has stimulated efforts in synthesis of new mesomorphic materials. Today, more than 6000 compounds are available but an ideal liquid crystal is still elusive. 

Magnetic field normal to the helical axis the cholesteric-nematic... 

It is interesting to note that this threshold field is lower than that for a conical distortion or that for cholesteric-nematic (unwinding) transition. For a conical distortion, the theory is closely similar to that discussed in 3.4.2 and has been treated by Leslie the critical field is given by... 

A direct method of studying the translational order (or the amplitude of the density wave) is by measuring the intensity of the Bragg scattering from the smectic planes. McMillan s experimental results on cholesteryl myr-istate are shown in fig. 5.2.6 and as can be seen there is excellent agreement with the refined model. The X-ray intensities reveal an appreciable pretransitional smectic-like behaviour in the cholesteric (nematic) phase. This aspect of the problem will be dealt with in a later section. 

Observed structures of a lyotropic material are classified into three categories nematic, smectic, and cholesteric. Nematic and cholesteric mesophases can be readily identified by microscopic examination. The existence of a smectic mesophase is not well defined and is only suggested in some cases. Solvent, solution concentration, polymer molecular weight, and temperature all affect the phase behavior of lyotropic polymer solutions. In general, the phase transition temperature of a lyotropic solution increases with increasing polymer molecular weight and concentration. It is often difficult to determine the critical concentration or transition temperature of a lyotropic polymer solution precisely. Some polymers even degrade below the nematic isotropic transition temperature so that it is impossible to determine the transition temperatures. Phase behavior is also affected by the polymer molecular conformation and intermolecular interactions. 

C. Mioskowski, J. Bourguignon, S. Candau, G. Solladie, Photochemically induced cholesteric-nematic transition in liquid-crystals. Chem. Phys. Lett. 38, 456-459 (1976)... 

De Gennes-Meyer Model for Field Induced Cholesteric-Nematic Transition... 

Early work on this technology was performed at Bell Labs and in 1972 a paper was presented describing a LCLV system. A cholesteric/nematic mixture was used in a transmissive cell having... 

There are two types of electro-optic effects which occur with cholesteric materials. The materials which possess negative dielectric anisotropy exhibit the so-called reflective optical storage mode while positive materials undergo field-induced cholesteric-nematic phase changes. 

Emphasis has been placed on nematic materials because of the predominance of the twisted nematic effect in display devices. However, its disadvantages have caused considerable interest in the cholesteric-nematic phase change effect and the dye-phase change device in particular.61>62... 

With an increasing external field a series of the field-induced phase transitions BP I — cholesteric, BP II cholesteric, and then cholesteric —> nematic are observed. This is illustrated by Fig. 6.27 [91] where the voltage-temperature phase diagram is presented for a mixture (47-53 mol.%) of... 

Light valves were first produced on the basis of the classical semiconductors, ZnS, CdS, ZnSe, CdTe, and GaAs, in contact with nematic or chiral nematic liquid crystal [18]. The basic effects in liquid crystals included electrically controlled birefringence, dynamic scattering, and the cholesteric-nematic phase transition with the frequency response limited to a few Hertz. 

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