Cholesteric material

Some cholesteric materials show the blue phase as the temperature increases from that of the cholesteric phase and before it reaches that of the isotropic phase. The blue phase is a cubic phase. There have been three blue phases found so far BP I, BP II and BP III phases. It is now understood that the BP I phase is a body-centered cubic, the BP II phase is a primitive cubic and the BP III phase is a fog phase with no structural symmetry. Generally the temperature range of the blue phase is quite narrow, less than 1 degree... 

The cholesteric material was then doped with a small quantity of a non-mesomorphic epoxy compound, Lixon, which lowered the cholesteric-isotropic transition temperature and gave rise to a broad two-phase region. Because the glass plates have greater affinity for Lixon than for the liquid crystal compound, the cholesteric drops were surrounded on all sides by the isotropic phase. With thick cells, spherical drops with the characteristic /-line of strength 2 were formed (fig. 4.4.2). In thin cells ( 8 pm thick) flattened drops were obtained in which the central portion... 

Dependence of pitch on temperature applications to thermography In most pure cholesteric materials, the pitch is a decreasing function of the temperature. An elementary picture of the temperature dependence of the pitch can be given in analogy with the theory of thermal expansion in crystals. " Assuming anharmonic angular oscillations of the molecules about the helical axis, the mean angle between successive layers... 

Liquid crystals have interesting electro-optical properties. When subjected to small electric fields, reorientation and alignment of the liquid crystal molecules takes place, which produces striking optical effects because light travels more slowly along the axes of the molecules than across them. This has led to their use in optical display devices for electronic instruments such as digital voltmeters, desk calculators, clocks, and watches. Nematic liquid crystals are most commonly used in these applications. Cholesteric materials are added to provide memory effects. 

SPECTRAL REFLECTANCE OF TWO CHOLESTERIC MATERIALS IN THE MESOMORPHIC STATE. 

Kent Display is a pioneer of cholesteric liquid crystal displays (ChLCDs) in which the director of the liquid crystal twists around a helical axis [3]. The remarkable property is that the cholesteric material reflects light of certain wavelengths depending on the pitch over which the director rotates. When an electric held is applied. 

The l,4 3,6-dianhydroalditols (mainly) and other saccharide derivatives were found to be useful chiral components of cholesteric materials with interesting optical properties, capable of forming Grandjean textures [8]. Chirality plays an important role in combination with the liquid crystalline (LC) character of both low- or high-molecular weight... 

Fig. 8.20 Volterra process. A stack of the cholesteric quasi-layers with vertical helical axis and a cut S shown by the solid line terminated in point L (a). The cut is open up-down and the cholesteric material is added on the right of the cut (b). The final structure of the quasi-layers after relaxation leaving a line defect (c)...

I will present some of the early work on the development of nematic compounds and discuss briefly cholesteric materials and their application to displays. Dr. Harrison will follow with a... 

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. 

As a result of work to produce stable colorless nematics the area of nonsteroid cholesteric materials has been opened up with many useful compounds being reported.By introduction of an asymmetric center in the form of a branched alkyl group specifically the readily available 2-methylbutyl. substituent, cholesterics with wide temperature range, different pitch lengths and helical sense may be prepared (e.g., 32a,b,c). An interesting structural influence on sense and pitch is observed. The closer the position of branching to the central core, the... 

Fig. 14 Transmittance and reflection of linearly polarized light by a low molar mass cholesteric material. Lasing happens at the band-edges black boxes at 500 and 580 nm), where the density of states is highest. There, two standing waves evolve out-of-phase (left) and in-phase (right). See [101, 102,105] for details...

Today s challenge is the search for robust cholesteric materials with a low threshold value for lasing, but a high tolerance for pumping. As mentioned above, lasing from cholesteric structures is mostly the domain of low molar mass liquid crystals. That is because it is essential to obtain a monodomain, which is as defect-free as... 

Phase-matched third-harmonic generation would of course occur in any cholesteric liquid crystal as long as the helical pitch can be adjusted to the correct value. We tried the experiment on an entirely different cholesteric material. Poly-y-benzyl-L-glutamate (PBLG), a synthetic a-helix protein, dissolved in dioxane is cholesteric for concentrations from 0.1 to - 0. 5 g of PBLG per g of solvent. ... 

As with nematic liquid crystals, cholesteric materials may show electrooptical effects which are related to the orientation of the molecules by the hydrodynamic flow induced by a space charge motion in a material with a rather large conductivity. The sign and magnitude of Ae are not very important for the electrohydrodynamic phenomena, since they are induced by the anisotropy of the electrical conductivity. 

A model to determine the birefringence of a cholesteric material was proposed by de Vries [20] for a planar cholesteric organized in a stack of a large number of layers. Each layer is considered to be rotated by a few degrees in relation to the ones immediately under and above in such a manner that a helical structure is formed. Illuminating... 

In the cholesteric phase, the helical configuration of the director is the consequence of the molecular chirality. From the microscopic point of view, unlike in the nematic phase where the molecules have a tendency to be locally parallel to each other, in cholesteric materials, the adjacent molecules have a tendency to form a small angle as shown in Figure 2.6. As we will point out in Section 2.6, such a local tendency cannot be satisfied globally and the system is frustrated. 

As already mentioned, the layered structure of cholesteric materials imposes certain limitations on the topological classification of defects based on ho-motopy groups a more general theory is still lacking. In this section we discuss macroscopic defects such as focal conic domains and oily streaks whose existence depends crucially on the layered character of ordering. 

Cholesteric materials are temperature sensitive and show a thermochromic effect. The reason for this is that the pitch length of the helix and the refractive index are temperature dependent ... 

The texture change or memory effect is observed in cholesteric materials with negative dielectric anisotropy [71]. The liquid crystal layer is homogeneously oriented by boundary forces to form the planar texture which is completely transparent if the band of selective light reflection is outside the visible spectrum. The substrates are covered with conducting films that are in contact with the liquid crystal. When a d.c. or low frequency field is applied, the sample is transformed to the so-called focal conic texture. In this texture, the liquid crystal is broken up into small domains which are randomly oriented and have diameters of a few microns. Since these domains are optically anisotropic, they act as scattering centers for visible light. Therefore the focal conic texture exhibits a milky white appearance. 

However, as cholesteric material has a minimum reaction time which is in the order of 0.1 s, it is only suitable for displaying color information when the information changes slowly. It is unsuitable for reproducing fast changing, moving pictures. 

The time constant of ordinary cholesteric materials is in the order of 0.1 s. For example, for cholesteryl nonanoate it is 0.1 s and for cholesteryl oleyl carbonate 0.2 s. These response times do not indicate a limitation if the materials are to be observed by eye motion pictures at 64 frames/s have been successfully made and information determined from single frames. 

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