Cholesterics Ch

Cholesteric LCs can be formed by optically active compounds or optically active mixtures. Locally a cholesteric is very similar to a nematic material. Again, the molecular centres of gravity have no long-range order but the molecules tend to be parallel to some common axis which defines the director n. However, the structure is helicoidal with a precession of the director about the helical axis Z (Fig. 2.6(b)). The structure is periodic along Z and the spatial period L is equal to one half of the pitch, P. Typical values of L are in the 3000 A range, i.e. much longer than the molecular dimensions. 

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Figure 1.1 Cartoon representations of molecular packing in various smectic (S), nematic (N) and cholesteric (Ch) liquid crystalline phases comprised of rod-like molecules [26].

A chiral dopant in a nematic phase can cause a helical pitch and the resulting phase is called cholesteric (Ch = N ). This phase is continuously miscible with the nematic phase (Effect (b)). In some special cases, the helical pitch might be infinitely long as found for the compensated cholesteric phase. Even this phase is different from the achiral nematic phase, because it has polar properties, i.e., pressure can induce chiral ordering (Effect (a)). A chiral... 

The nematic phases of chiral molecules (N ) are often called cholesteric (Ch), because the first chiral nematic phases consisted of cholesterol... 

Figure 2.7. State diagram of a three-dimensional system, a - isotropic (I) state b - transition to the cholesteric ordering c - cholesteric (Ch) state d - transition to nematic ordering e - nematic (N) state - stable state, o - unstable state...

Figure 4.6-10 Infrared spectra of transmittance T, reflectance R, and optical rotation p of an induced cholesteric solution at 22 °C (Ch) and as isotropic liquid at 63 °C (Is). Solvent eutectic mixture of the isomeric N-oxides of p-methoxy-p -n-butylazobenzene, see 3 of Table 4.6-1 (Nematic Phase IV Licristal E. Merck) solute 17-3-acetoxy-5-/3-androst-l-en-3-one molar fraction x = 0.05.5 (Korte and Schrader, 1981).

Subsequently, the kinetics of intramolecular pyrenyl excimer formation were examined in CH for a series of PnP which includes polymet - lene chains much longer than that of P3P (i.e., n-3, 5, 6, /, " 10, 11, 12, 13, 22). The dynamic behavior of these compo in normal isotropic solvents of low viscosity is known to be Vm j complex(45). However, in both the isotropic and cholesteric phases of CH, all of the temporal emission data for the PnP could be accommodated by Scheme 2. The pyrenyl decay waveforms were monoexponential and the excimer waveforms could be expressed as the difference between two exponentials (representing the growth and decay of excimer emission). Temperature dependent ki+k2 values for P3P were obtained as before from Z the ki+k2 for the other PnP were taken from experiments with 1-dodecyIpyrene (DP). 

Sc Sa, Sa->Ch, and Ch->I respectively. The smectic D phase is observed in the hexyl series but not in the decyl analogs. This observation suggests that the increased flexibility provided by the longer alkyl group destabilizes the higher temperature smectic phases, while stabilizing the smectic C. Optical micrographs of the PDBPB polymer exhibit a fine broken focal conic fan texture for Sq phase, and focal conic fan textures for the Sa phase and cholesteric texture. 

Fig. 6.2 Representative GLCs from early efforts accompanied by DSC data from second heating scans. G glassy, Ch cholesteric, N nematic, I isotropic. Used with permission [22]...

The sense of cholesteric twist, in the liquid crystal of PBLG in dioxane, is opposed to that in CH Cl, as pointed out by Robinson. We measured the cholesteric pitch of PBLG in various mixed solvent systems, and estimated the sense of cholesteric twist in the individual solvent. If two solvents which make the sense of cholesteric twist of PBLG opposite to each other, are mixed, the cholesteric pitch of PBLG in mixed solvent will diverge at the critical composition. It is found that the sense of cholesteric twist of PBLG in dioxane and chloroform is opposite to that in dichloromethane, dichloroethane and benzene. 

Table III, on the other hand, shows some copolymers in which the mesophase can be locked-in for use till temperatures of about 200 C. These are novel cholesteric compounds resulting from the copolymerization of meso-gen 9 with the mixed spacer MAA /TAA, where MAA is the optically active 3-methyladipic acid and TTA is trans-traumatic acid, H0C0-CH=CH-(CH2)g-COOH.

A specific feature of ch selective light reflection an trophotometric study of the f homopolymers in the 20-150 C ing temperatures of homopolym the existence of a broad refl 300 nm region. Circular dich same wavelength region (Fig. homopolymers produce a choles sic selective UV-light reflec molecular cholesterics, chole play a tendency to layer pack the structuring effect of the... 

Lin CH, Hsu CS. Synthesis and characterization of cholesteric liquid crystalline copolysilox-anes containing 4-biphenyl 4-allyloxybenzoate and -2-naphthoate side groups. J Polym Res 2000 7 167-73. 

Kricheldorf H.R., Sun S.J., Gerken A., Ch. Chang T., Polymers of carbonic add 22. Cholesteric polycarbonates derived from (S)-[(2-methylbutyl)thio]hydroquinone or isosorbide. Macromolecules, 29, 1996, 8077—8082. 

Kricheldorf H.R., Sun Sh.J., Chen Ch.R, Chang T.Ch., Polymers of carbonic acid. XXIV. Photoreactive, nematic or cholesteric polycarbonates derived from hydroquinone-4-hydroxybenzoate 4,4 -dihydroxychtilcone and isosorbide, J. Polym. Sci. Part A Polym. Chem., 35, 1997, 1611-1619. 

Sun S.J., Schwarz G., Kricheldorf H.R., Ch. Chang T., New polymers of cabonic acid. XXV. Photoreactive cholesteric polycarbonates derived from 2,5-bis(4 -hydroxybenzylidene)cyclopentanone and isosorbide, J. Polym. ScL Part A Polym. 

N or Ch Dqo T(3) Chiral nematic or cholesteric phase with twist axis perpendicular to the director and macroscopic periodicity... 

Matsui T, Ozaki M, Yoshino K (2004) Tunable photonic defect modes in a cholesteric liquid crystal induced by optical deformation of helix. Phys Rev E 69 061715 Yoshida H, Lee CH, Fujii A, Ozaki M (2007) Tunable chiral photonic defect modes in locally polymerized cholesteric liquid crystals. Mol Cryst Liq Cryst 477 255... 

Outside the cholesteric cell, the medium is an isotropic medium with the refractive index rig. On top of the Ch film (incident side), there is incident hght and reflected light, and the actual Berreman vector is the sum of the Berreman vectors of the incident light and reflected light. From Equation (3.166) we know that for the incident Ught, the Berreman vector is... 

The flexoelectric effect also exists in cholesteric liquid crystals because the orientational order is locally the same as in nematic liquid crystals. Here we consider a ceU geometry shown in Figure 4.6 [2,10-12]. The Ch liquid crystal is sandwiched between two parallel substrates with transparent electrode. A homogeneous alignment layer is coated on the inner surface of the substrates. When the liquid crystal is cooled down from the isotropic phase under an external... 

In a cholesteric display with a single layer of cholesteric hquid crystal, only a single color can be displayed. In order to make multiple color displays, Ch LCs with a variety of pitches must be... 

Multiple color displays from one layer can be made from pixelation of colors. The displays have three alternating types of stripes of Ch liquid crystals with three different pitches reflecting blue, green and red light. Partition or some other means of preventing inter-stripe diffusion must be used. Polymer walls, especially field-induced polymer walls, are good candidates. The different pitches can be achieved by two methods. In the first method, three cholesteric liquid crystals with different pitches are filled into empty cells with partitions. The second method is photo color tuning [62,63]. A photosensitive chiral dopant is added to the liquid... 

When cholesteric liquid crystals are encapsulated in droplet form, the bistability can be preserved when droplet size is much larger than the pitch [64]. There arc two methods which are used to encapsulate Ch liquid crystals phase separation and emulsification. In phase separation [69], the Ch liquid crystal is mixed with monomers or oligomers to make a homogeneous mixture. The mixture is coated on plastic substrates and then another substrate is laminated on. The monomers or oligomers are then polymerized to induce phase separation. The liquid crystal phase separates from the polymer to form droplets. In the emulsification method [70-73], the Ch liquid crystal, water, and a water dissolvable polymer are placed in a container. Water dissolves the polymer to form a viscous solution, which does not dissolve the liquid crystal. When this system is stirred by a propeller blade at a sufficiently high speed, micron-size liquid crystal droplets are formed. The emulsion is then coated on a substrate and the water is allowed to evaporate. After the water evaporates, a second substrate is laminated to form the Ch display. 

The encapsulated cholesteric liquid crystals are suitable for flexible displays with plastic substrates. They have much higher viscosities than pure cholesteric liquid crystals and can be coated on substrates in roll-to-roll process [71,72]. The polymers used for the encapsulation have good adhesion to the substrates and can make the materials self-adhesive to sustain the cell thickness. Furthermore, the encapsulated Ch liquid crystals can no longer flow when squeezed, which solves the image-erasing problem in displays from pure cholesteric liquid crystals where squeezing causes the hquid crystal to flow and to be switched to the planar state. 

The state of a cholesteric liquid crystal is mainly determined by smface anchoring, cell thickness, and apphed fields. The hquid crystal can be switched from one state to another by applying electric fields. There are many possible transitions among the states, as shown in Figure 10.24 [50,54]. In order to design drive schemes for the bistable Ch reflective display. 

In bistable Ch reflective display applications, it is desirable that the threshold of the transition from the planar state to the focal conic state be high, so that the cholesteric liquid crystal can remain in the planar state and the display does not exhibit flicker under colimin voltage in addressing. 

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