Cholesteric crystals

FIGURE 5.2 Liquid crystals, (a) Smectic crystals the ends of the molecules are on a plane, (b) Nematic crystals the ends of the molecules do not match, (c) Cholesteric crystals the molecules in each layer are arranged in a manner similar to nematic crystals, but the angle changes from plane to plane of the molecules, forming a helix of pitch length p. 

Cholesteric liquid crystals are compounds that go through a transition phase in which they flow like a liquid, yet retain much of the molecular order of a crystalline solid. Liquid crystals are able to reflect iridescent colors, depending on the temperature of their environment. Because of this property they may be applied to the surfaces of bonded assemblies and used to project a visual color picture of minute thermal gradients associated with bond discontinuities. Cholesteric crystals are potentially a simple, reliable, and economical method for evaluating bond defects in metallic composite structures.f Materials with poor heat-transfer properties are difficult to test by this method. The joint must also be accessible from both sides. ... 

Radley and co-workers [133] reported that when chiral dopants—decyl esters of amino acids, serine, alanine, leucine, and methionine—were added to an aqueous solution of alkyl methyl ammonium bromides, mixed with decanol, amphiphilic cholesteric crystals were formed. They found that the sense and magnitude of the induced helix by decyl ester of amino acid hydrochlorides except decyl ester of alanine are dependent on the achiral cationic surfactant, alkyl methyl ammonium bromides. The formation of amphiphilic cholesteric crystals was interpreted in terms of the trans and cis ro-tamers of the chiral ester of amino acid, associated with the ester linkage. Also, they reported [134] that in the case of the ester hydrochloride of proline as a chiral dopant, a concentration-dependent reversal in helical twist was observed. 

The reactions were carried out at room temperature in two different cholesteric mesophases with solute concentrations less than 5% in weight, in order not to disturb the anisotropic arrangement. The reaction products were separated properly from the liquid crystal and then carefully purified to eliminate any further contamination from the chiral phase. In all cases, the products did not show significant optical rotation, thus indicating the formation of racemic mixtures and the absence of asymmetric induction by cholesteric crystals. 

Figures 3 a, b, c show the temperature dependences of viscosity for the solutions under study. The above dependences are described by curves with well-pronounced sharp maxima. This behavior is typical of the solutions with LC transitions (Kulichikhin Golova, 1985, Vshivkov Rusinova, 2008, Gray, 1962). According to Gray (1962), this profile of the temperature dependences of viscosity corresponds to the (isotropic liquid)-(nematic liquid crystal) phase transition. Therefore, upon cooling of HPC, CEC and PBG solutions under deformation conditions, no cholesteric crystals are formed in other words, under dynamic conditions, a liquid crystal changes its type from cholesteric to nematic. The results obtained are in good agreement with the data of other authors (Volkova et al., 1986), who showed that the shear deformation of CEC solutions (c= 30%) in trifluoroacetic acid and a 2 1...

Any planar wave entering the cholesteric crystal in a direction parallel to the helix axis is split up into two elliptically polarized components which are rotating in opposite directions with somewhat different velocities. 

Licjuid Crystals. Ferroelectric Hquid crystals have been appHed to LCD (Uquid crystal display) because of their quick response (239). Ferroelectric Hquid crystals have chiral components in their molecules, some of which are derived from amino acids (240). Concentrated solutions (10—30%) of a-helix poly(amino acid)s show a lyotropic cholesteric Hquid crystalline phase, and poly(glutamic acid ester) films display a thermotropic phase (241). Their practical appHcations have not been deterrnined. 

FIGURE 5.51 The cholesteric phase of a liquid crystal. In this phase, sheets of parallel molecules are rotated relative to their neighbors and form a helical structure. 

The three classes of liquid crystals differ in the arrangement of their molecules. In the nematic phase, the molecules lie together, all in the same direction but staggered, like cars on a busy multilane highway (Fig. 5.49). In the smectic phase, the molecules line up like soldiers on parade and form layers (Fig. 5.50). Cell membranes are composed mainly of smectic liquid crystals. In the cholesteric phase, the molecules form ordered layers, but neighboring layers have molecules at different angles and so the liquid crystal has a helical arrangement of molecules (Fig. 5.51). 

See Chapters on TGB phases and cholesteric liquid crystals in this volume... 

Ito et al. [152] described the crystal structure of 4-[(S)-2-methylbutyl]phe-nyl 4 -hexylbiphenyl-4-carboxylate which shows a smectic A phase and a cholesteric phase. The molecules are arranged in a tilted smectic-like layer structure. Within the layers, the long molecular axes are tilted (30°). However, the compound exhibits no smectic C phase. 

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]. 

The unique properties of liquid crystals have also provided opportunity for study of novel nonlinear optical processes. An example involves the ability to modify the pitch of cholesteric liquid crystals. Because a pseudo-wave vector may be associated with the period of pitch, a number of interesting Umklapp type phasematching processes (processes in which wave vector conservation is relaxed to allow the vector addition to equal some combination of the material pseudo-wave vectors rather than zero) are possible in these pseudo-one-dimensional media. Shen and coworkers have investigated these employing optical third harmonic generation (5.) and four-wavemixing (6). 

Several 4-(3-alkyl-2-isoxazolin-5-yl)phenol derivatives that possess liquid crystal properties have also been obtained (533-535). In particular, target compounds such as 463 (R = pentyl, nonyl) have been prepared by the reaction of 4-acetoxystyrene with the nitrile oxide derived from hexanal oxime, followed by alkaline hydrolysis of the acetate and esterification (535). A homologous series of 3-[4-alkyloxyphenyl]-5-[3,4-methylenedioxybenzyl]-2-isoxazolines, having chiral properties has been synthesized by the reaction of nitrile oxides, from the dehydrogenation of 4-alkyloxybenzaldoximes. These compounds exhibit cholesteric phase or chiral nematic phase (N ), smectic A (S4), and chiral smectic phases (Sc ), some at or just above room temperature (536). 

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