Cholesteric compensated mixtures

The field-induced cholesteric-nematic transition is most important for the application of compensated mixtures of cholesteryl derivatives as anisotropic solvents. If electric fields of the order of 10,000 V/cm are used for the orientation of the hquid crystal it is not necessary to adjust the temperature, T, of the sample exactly to the characteristic nematic temperature, of the mixture. Good alignment can still be achieved if T - < 5°C. 

Quasi-nematic or compensated cholesteric phases were formed by CTC dissolved in mixtures of methylpropyl ketone (MPK) and DEME. CTC/MPK has a right-handed twist but CTC/DEME a left-handed one (109). Siekmeyer et al. (IIQ) studied the phase behavior of the ternary lyotropic system CTC/3-chlorophenylurethane/triethyleneglycol monoethyl ether. 

Mixtures dependence of pitch on composition We have seen in 4.1.6 that a mixture of right- and left-handed cholesterics adopts a helical structure whose pitch is sensitive to temperature and composition. This result was first described by Friedel. For a given composition, there is an inversion of the rotatory power as the temperature is varied, indicating a change of handedness of the helix. The inverse pitch exhibits a linear dependence on temperature, passing through zero at the nematic point where there is an exact compensation of the right- and left-handed forms (fig. 4.1.16). 

N.V. Madhusudana, R. Pratibha and H.P. Padmini, Electromechanical effect in cholesteric liquid crystals with fixed boundary conditions. Mol. Cryst. Liq. Cryst. 202(1), 35-49, (1991). doi 10.1080/00268949108035658 H.P. Padmini and N.V. Madhusudana, Electromechanical effect in cholesteric mixtures with a compensation temperature, Liq. Cryst. 14(2), 497-511, (1993). doi 10.1080/02678299308027665... 

The Goldstone mode in an achiral SmC tries to restore the symmetry of the smectic A phase Cooh —> Dooh by free rotation of the director along the conical surface with the smectic layer normal as a rotation axis. Thus, like chiral molecules convert a nematic into a cholesteric, they convert an achiral SmC into chiral SmC without any phase transition. In addition, mixing left (L)- and right (R)-handed additives results in a partial or complete compensation of the helical pitch both in cholesterics and chiral smectic C. For example, the L- and R- isomers of the same molecule taken in the equal amounts would give us a racemic mixture, that is achiral SmC without helicity and polarity. 

The liquid crystal parameters K2, K3/Kx> na and cell design parameters 3g OCg, d and the orientation of polarizers were varied one at a time. The ratio K3/KX was chosen as a parameter in order to keep the Freedericksz transition threshold voltage constant while varying the elastic constants. Variations in the threshold voltage due to the natural cholesteric pitch were not compensated for. Several calculations were also conducted for the case of infinite cholesteric pitch (no cholesteric compound in the liquid crystal mixture). 

A typical case of the nonmonotonic behavior of the pitch (in fact, the inversed pitch Pq ) on mixture composition is shown in Fig. 1.18 [45, 46]. It is worthwhile to note that all left-hand cholesterics (like cholesteryl acetate, pelargonate, and oleate) form left-hand mixtures with BBBA. On the other hand, for right-twisted cholesterics (like Ch, chloride) we can observe a change in the sign of the helix from left to right. There are also compensation points where a mixture is achiral. In this case, a nematic matrix itself, not being twisted initially behaves like a left-hand cholesteric. This problem was the subject of a hot theoretical discussion [47, 48]. 

One explanation is based on consideration of the molecular structure of the cholesteryl esters [47]. The steroid skeleton of the molecule is right-twisted, but a hydrocarbon chain is left-twisted and partially compensates for the helical power of the skeleton. In mixtures with nematics, the nematic molecules are aligned in parallel with the cholesteric molecules and reinforce the action of the hydrocarbon chains, resulting in a decrease and change of sign of the twist. However, this treatment cannot account for the whole set of the observed experimental data. Probably, the more realistic approach has to involve multiparticle interaction [49]. 

When L/p I, the cholesteric does not differ much from the nematic phase. No wonder therefore that optical observations for weakly twisted cholesterics reveal thick (nonsingular) and thin (singular) line defects —disclinations similar to that in the nematic phase. Moreover, in droplets of the so-called compensated cholesteric mixtures with extremely small Ljp one can observe point defects [6] which, from the topological point of view, are allowed only in a nematic phase. 

An electric field induced cholesteric-to-nematic transition is demonstrated in Fig. 4 for a partially compensated cholesteric mixture of cholesteryl chloride and cholesteryl nonanoate. The sample has been sandwiched between two glass plates in such a way that the helix axis is parallel to the glass surface. The distance between two adjacent dark (or bright) lines is therefore a measure for the pitch fo the cholesteric phase. It is clearly seen that the pitch increases with increasing field strength. At a critical field strength of = 10,000 V/cm the sample has become nematic with the director oriented parallel to the electric field. 

As noted in Section 2 the d- and fi-isomers of optically active molecules exhibit somewhat different order parameters in liquid crystals possessing a local screw sense (such as cholesteryl derivatives). Accordingly d- and C-isomers should separate on cholesteric substrates. For 3,3,3-trichloropropylene oxide the difference in the order parameter is of the two isomers is AS z = 0.00015 (cf Section 2) for a compensated nematic mixture of cholesteryl derivatives. On the basis of Equ. (42) one would expect a separation factor of a 1.002. Up to the present the separation of optically active isomers on cholesteric substrates has not been achieved. 

For UV and fluorescence measurements, the most commonly used liquid crystals are mixtures of the nematic 4 -alkylbicyclo-hexyl-4-carbonitrile s (CCH) (e.g., ZLI 1167 and 1695), which are transparent down to 200 nm and exhibit nematic ranges between =30 and 80 °C (see, for example, [315, 331, 333, 334]), and various cholesteric or compensated nematic phases of cholesteryl chloride/cholesteryl ester mixtures, which are transparent to =240 nm [310, 313, 326, 329]. Some use has also been made of 4 -(4-alkylcyclohexyl)benzo-nitriles (PCH-n), which are transparent to =290 nm [330, 335]. Several other meso-phases, including thermotropic smectics, discotics, and lyotropic phases, have low absorption in the UV region and have been used from time to time as well. The most commonly used liquid crystals in FTIR studies are the CCH-mixtures ZLI 1167 and 1695 [314, 321, 336, 337]. The orientation of the liquid crystalline solution is most commonly achieved either by cell surface treatment or the application of an electric or magnetic field. 

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