Liquid cholesteric materials

Liquid Crystallinity. The Hquid crystalline state is characterized by orientationaHy ordered molecules. The molecules are characteristically rod-or lathe-shaped and can exist in three principal stmctural arrangements nematic, cholesteric, and smectic (see Liquid crystalline materials). 

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... 

In Chapter 7, Gottarelli and Spada extend further the theme of liquid crystalline materials. They note that although much is understood, important mysteries remain about the relationships between the handedness of cholesteric liquid crystalline phases and the chirality of the constituting building blocks. This may seen surprising given the intense interest in these materials, including their... 

Doping of nematic liquid crystal materials ZLI-389 and K15 with 30a resulted in stable cholesteric phases. The cholesteric phase was induced by the addition of 0.7 wt% 30a to ZLI-389 at 51-54 °C, and the phase was stable for many hours. When... 

The following table lists the liquid crystalline materials that are useful as gas chromatographic stationary phases in both packed and open tubular column applications. In each case, the name, structure, and transition temperatures are provided (where available), along with a description of the separations that have been done using these materials. The table has been divided into two sections. The first section contains information on phases that have either smectic or nematic phases or both, while the second section contains mesogens that have a cholesteric phase. It should be noted that each material may be used for separations other than those listed, but the listing contains the applications reported in the literature. 

While you will synthesize and characterize cholesterol nonanoate, you will actually study mixtures of two liquid crystalline materials each of which is a derivative of cholesterol. The other derivative is cholesteryl chloride (ChCl) whose structure along with that of cholesterol nonanoate is shown in Figure C.4. The reason to study mixtures here is for convenience of handling. Though both pure materials exhibit liquid crystalline phases (ChNon at 74" C, solid smectic 80°C smecticicholesteric 93" C cholesteric isotropic, and ChCl about 70 "C, solid cholesteric), appropriate mixtures are liquid crystalline at much lower and more manageable temperatures, for example, room temperature. 

Nucleation studies on supercooled cholesteric liquid crystalline materials (cholesteryl caproate and nonanoate) are reported. The presence of sodium deoxycholate induces optical activity in bilirubin, with significant Cotton effects near 410 and 460 nm. ... 

A similar approach has been used to produce materials with a chiral (cholesteric) structure by performing the experiments described above in the presence of a low molecular weight chiral liquid crystalline material (Figure 9.6). The chiral material is not covalently attached to the network and can be removed subsequently to produce an imprinted chiral structure. As before, the polymer displays a nematic mesophase between the glass transition (Tg 33°C) and the transition to an isotropic fluid (rN, 128°C). 

Cholesteric liquids can rotate polarized light to a large degree, such as some thousand degrees per 1 mm layer thickness for visible light [258]. Because of different absorption of the two polarized components in the cholesteric liquid, the material shows dichroism. Most of the effects are applied practically. The colour change in reflection with temperature of a cholesteric liquid can be used for very sensitive temperature measurements of 0.001°C It is even possible to construct infra-red/visible image converters with cholesteric liquid crystals. 

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... 

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. 

An acid chloride functionalized liquid crystalline material was tried to bond to the silanol group in silica. The direct reaction does not seem to be successful also, a spacer with a substituted dichlorodimethyl silane is not successful. The reaction scheme is shown in Figure 16.8. However, a separation performance is observed rather by deposition than by bonding of the liquid crystals. The liquid crystalline materials used in this study was based on cholesteric moieties. It is suspected that the reaction failed because of the large size of the cholesteryl group. In another study, dimeth-ylchlorosilane was added to the allyl group of 4-methoxyphenyl-4-allyloxy benzoate. This intermediate could be bonded to silica. Several other routes to fix liquid crystalline moieties on silica have been reviewed. ... 

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... 

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... 

It will be appreciated that visualization of the structure of liquid crystalline materials is a particularly difficult task as the phase being studied has liquid-like character. The technique of freeze fracture transmission electron microscopy allows examination of most systems however, lyotropic materials which contain greater than 85% water still prove to be difficult. The technique involves the fast freezing of the material and then examination of the fracture surface. Despite the obvious attraction of this method it appears to be still in its infancy. Studies of cholesteric, smectic " phases have been reported and show that it is possible to identify stacks of well-ordered materials which are often bananashaped but do conform to the concepts that have been developed above. 

Figure 12.6 Color change in a cholesteric liquid crystalline material as a CQ function of temperature. 

In our experiments we have used a number of different cholesteric liquid-crystalline materials in order to observe the various predicted phase-matching peaks in third-harmonic generation at a convenient temperature. Most of the phase-matching conditions (1-11) in Table I require a cholesteric liquid crystal with a pitch less than 1. 5 pm. 

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. ... 

Since the microscopic structure of nematics and cholesterics is similar, these two phases are mixed like two nematics. Thus the thermodynamic behavior of nemato-cholesteric mixtures needs no special discussion (see Section 1.2.1). What is of interest, especially for liquid crystalline materials for technological applications, is the concentration dependence of the helical pitch and the physical parameters relevant to this dependence. 

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. 

This book was conceived as a renewed version of the earlier published original book, Electro-Optical and Magneto-Optical Properties of Liquid Crystals (Wiley, Chichester, 1983) written by one of us (L.M. Blinov). That book was first published in Russian (Nauka, Moscow, 1978) and then was modified slightly for the English translation. Since then new information on electrooptical effects in liquid crystals has been published. Novel effects have been discovered in nematics and cholesterics (such as the supertwist effect), and new classes of liquid crystalline materials, such as ferroelectric liquid crystals, appear. Recently, polymer liquid crystals attracted much attention and new electrooptical effects, both in pure polymer mesophases and polymer dispersed liquid crystals, were studied. An important contribution was also made in the understanding of surface properties and related phenomena (surface anchoring and bistability, flexoelectricity, etc.). 

The book is subdivided into three parts. The first three introductory chapters include consideration of the nature of the liquid crystalline state of matter, the physical properties of mesophases related to their electroop-tical behavior, and the surface phenomena determining the quality of liquid crystal cells giving birth to many new effects. The second part (Chapters 5-7) is devoted to various electrooptical effects in nematic, cholesteric, and smectic mesophases including ferroelectric compounds. Here major emphasis is given to explaining the physical nature of the phenomena. The last part (Chapter 8) is a rather technical one. Here recent applications of liquid crystalline materials in electrooptical devices are discussed. 

In this book the authors present a complete and readily understood treatment of virtually all known phenomena occurring in liquid crystals under the influence of an electric field. In the first three chapters (Chapters 1-3) bulk and surface properties of liquid crystalline materials are discussed. The next two chapters (4, 5) are devoted to consideration of the electrooptical effects due to the formation of uniform and spatially modulated structures in nematics. In Chapters 6 and 7 the electrooptical properties of the cholesteric and smectic mesophases are presented, including a discussion of ferroelectric materials. Major emphasis is given to explaining the qualitative aspects of the phenomena and to portraying their physical basis. The prospects for the practical application of electrooptical effects are also discussed (Chapter 8). 

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