Cholesteric range

In main chain cholesteric liquid crystalline polymers, the mesogenic groups and flexible spacers are linked alternatively. The flexible units contain asymmetrical carbon atoms which enable the polymers to possess chirality and thus form cholesteric liquid crystals. By varying the ratio of chiral to non-chiral parts, the cholesteric temperature range and pitch can be changed. The cholesteric range depends on the mol fraction of the polymers. A typical main chain cholesteric liquid crystalline polymer is shown in Figure 6.27. 

Dynamic mechanical measurements of the storage modulus G and the loss modulus G" in polydomain samples of cholesteric LCE have been described [9]. It turns out that G >G" throughout the whole cholesteric range and that both G and G" increase when approaching the cholesteric-isotropic and cholesteric-chiral SmC transition, respectively, thus indicating the presence of substantial pretransitional effects at both phase transitions. 

A series of optically active aromatic Schiff bases containing p-alkoxy and p-acyloxy groups were prepared and their mesomorphic properties studied, Chirality was achieved by introducing an asymmetric carbon atom into the hydrocarbon chain of either the alkoxy or acyloxy group. Structures (21), (22) and (23) are some examples. Structure (23), with 2 asymmetric centers, has a cholesteric range of nearly 96 ... 

The cholesteric range of cholesteryl oleyl carbonate increases with pressure. There is evidence of a tricritical point. In the case of methoxy- and ethoxybenzoic acid a pressure-induced nematic and a smectic phase are observed. 

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 introduction of a second chiral atom in the system leads to a reduction in the mesogenic properties and only a monotropic chiral nematic transition is observed for compound 23. However, when this compound is cooled down from the isotropic liquid state at a cooling rate of 0.5 °Cmin , very unusual blue phases BP-III, BL-II and BP-I are observed in the range 103-88 °C. Blue phases usually require pitch values below 500 nm. Hence the pitch value of the cholesteric phase for 23 must be very short, suggesting that the packing of two chiral carbons forces a faster helical shift for successive molecules packed along the perpendicular to the director. 

The main factor in determining the handedness of the cholesterics induced by bridged 1,1 -binaphtliyls is the helicity (P or M) of the solute, and this observation is the basis of many configurational studies of chiral binaphthyls. All the homochiral (aP)-binaphthyls 15-19 have an M helicity of the core, and all induce, in biphenyl nematics, M cholesterics.65,75 By systematic structural variations of the covalent bridge, it is possible to obtain I J -binaphthalenes with dihedral angles ranging from 60° to 96° (see series 20-24) the handedness of the cholesteric phase always matches the helicity... 

Similar to cholesteric liquid crystals, the nematics have an orientational long-range order with the deviation that the direetion of the preferred orientation does not rotate (Fig. 2a). If, however, a chiral mesogen is dissolved in a nematic liquid crystal, the latter will be transformed into a eholesterie liquid crystal. 

The cholesteric mesophase is a helically disturbed nematic phase. As in the nematic phase, the centers of gravity of the mesogenic molecules are statistically disordered, whereas the long molecular axes possess an orientational long range order with respect to the director. The director, however, is not constant in space, but continuously... 

As the cholesteric phase is a twisted nematic phase, the orientational long range order of the mesogenic molecules, characterized by Eq. (3) ... 

The photoresolution of JIN proceeds most efficiently In chiral media without singlet quenchers and with macroscopic order. Several factors may contribute to the necessity of long-range order. One of these Is the ability of cholesteric media to... 

It was really only a matter of time until researchers in the field started doping blue phases with quasi-spherical nanoparticles. This area is very much in its infancy, but the few recent reports already show some promising results. Yoshida et al., for example, reported on an expansion of the temperature range of cholesteric blue phases from 0.5 to 5°C by doping blue phases with gold nanoparticles (average diameter of 3.7nm) as well as a decrease in the clearing point of approximately 13°C [427]. A similar effect was also observed by Kutnjak et al. for CdSe quantum dots simultaneously capped with oleyl amine and TOP (diameter of the core 3.5 nm) in CE8 (Merck) and CE6 (BDH). The authors found that particularly blue phase III was stabilized in these mixtures, blue phase II destabilized, and... 

For example, the selective reflection of a cholesteric mixture of derivative 1 ( 45 wt %)/monomer 2 (diethylene glycol dimethacrylate) covered the whole visible spectrum in a narrow temperature range i.e., the mixture was bluish at 20 °C, greenish at 30 °C, and reddish at 50 °C. Therefore, by successive photo-crosslinking of the liquid-crystalline sample at different temperatures through a mask, a colored picture or pattern could be fixed onto the resulting composite film. 

The valency of the hydrogen bond donors and acceptors can be varied to produce chiral mesophases [85]. Mixtures of the divalent 4,4 -bipyridine with 4-[(S)-2-methylbutoxy]benzoic acid in ratios between 1 9 and 4 6 show LC behaviour, but the chiral mesophases are exhibited for only a small range of compositions [86]. Here, the cholesteric and a blue LC phase were observed. The association of the acid to the bipyridine was confirmed by a crystal structure of the 1 2 complex. 

Another saturated tetrahydrofuryl core has found application as a component of liquid crystals. Cholesteric liquid crystal polymers are useful as photostable UV filters in cosmetic and pharmaceutical preparations for the protection of human epidermis and hair against UV radiation, especially in the range 280-450nm <2000DEP19848130>. Fused bifuran 81 is a suitable monomer for the preparation of these desired polymers as it contains the requisite characteristics of having more than one chiral, bifunctional subunit type which is capable of forming a cholesteric liquid crystal phase with a pitch of <450 nm. It also contains an achiral aromatic or cycloaliphatic hydroxyl or amino carboxylic acid subunit, achiral aromatic or cycloaliphatic dicarboxylic acids, and/or achiral aromatic or cycloaliphatic diols or diamines. Polymers prepared from suitable monomers, such as diol 81, can also be used as UV reflectors, UV stabilizers, and multilayer pigments. 

Liquid crystalline solutions of KB LG (or of RBDG) and films prepared from such srdutions also ow significant CD in the wavelength range of the aromatic absorption bands 46,47) the CD for PBDG in methylene chloride is positive (46). CD bands are also induced when dye mdecules are introduced in liquid crystal films of polypeptide (PMDG). These induced CD bands are interpreted as arising from the dissymmetric field of the cholesteric structure 48). 

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