Compensated nematic mixtures

The use of the Merck bicyclohexyl eutectic mixture, (ZLI 1167) instead of the compensated nematic mixture of cholesteryl chloride... 

Due to the extreme sensitivity of the order parameters to small changes in the molecular geometry it is also possible to distinguish between the nmr spectra of the d- and 1-isomers of optical active molecules [56]. The two isomers of a racemic mixture of trichloropropylene oxide (IV) yield different nmr spectra in a compensated nematic mixture of cholesteryl derivatives (Section 1). 

Figure 10. (b) Esr-spectrum of naphthalene in an oriented glass (a compensated nematic mixture of cholesteryl chloride and cholesteryl laurate) taken at 77K. 

For the experiments with the compensated nematic mixtures of the cholesteryl derivatives one has to prepare cells which allow the application of electric fields of the order of 10,000 V/cm. Depending on the solubility of the solute the light path of the cells may be varied between several pm and 1 cm. Electrode distances between 0.4 and 1.0 cm are possible. For measurements of the order parameters, the light path of the cells should be made as small as possible in order to minimize depolarization effects caused by thermal fluctuations of the solvent order [101]. Depolarization effects may also be minimized by placing polarizers on either side of the cell [7]. A schematic representation of the experimental arrangement for the absorption and polarization experiments is shown in Fig. 11. The preparation of ordered... 

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. 

The electro-optical characteristics of multiplexed STN-LCDs exhibit a significant dependence on temperature. This has to be compensated in order to avoid variations of the optical performance of the display with temperatures. This can be achieved electronically. However, this problem can also be solved by the use of optically active, chiral dopants. The capacitive threshold voltage of a chiral nematic mixture depends on the pitch of the mixture ... 

Figure 10.20 shows polarizing microscopic photographs of the mixture of E44/chiral/w-azo-8 (80/9.8/10.2 wt%), changing to a Ch phase from a compensated nematic phase on UV irradiation. A schlieren texture in the initial state... 

For some applications, nematic mixtures have to be doped with chiral compounds, such dopants must not deteriorate the chemical stability and thermodynamic parameters of the mixtures. To this end, compounds with a high twisting power and good solubility are used. In special cases a dopant can be chosen which can compensate for the temperature drift of some important physical parameters of the nematic matrix. Compounds (l.xxxia) and (l.xxxib) are examples of the left- and right-handed chiral dopants, respectively. 

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. 

Polymers, e.g., polypeptides, form chiral lyotropic liquid crystalline phases T. and E. Samulski calculated the mutual van der Waals-Lifshitz forces between rod-like particles with chiral polarizability in an isotropic dielectric medium and therewith explained the phase chirality of polypeptides in organic solvents like dioxane. Furthermore, they showed that the use of different solvents can cause different handedness of the helix for the same solutes and that suitable mixtures of such solvents can result in compensated (i.e., nematic) mixtures although the polypeptide is chiral [4]. Solvent-... 

By using suitable cholesteryl derivatives and by varying the composition, nematic mixtures can be prepared between lO C and 150°C. Several compensated mixtures of cholesteryl chloride with some cholesteryl esters and with one achiralic compound, namely 4,4 -di-n-hexyloxyazoxybenzene (Ic), are summarized in... 

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. 

Figure 16. Relationship between the average orientations, of the z-axes of some aromatic solute molecules and their principal polarizabilities Oix x - Solid curve Theoretical relation calculated from Equ. (30) for Q = 0.115 kT. Dots ( ) experimental values of 8 2 as observed in a compensated nematic, 1.8 1 by weight mixture of cholesteryl chloride and cholesteryl laurate for 30° C.

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. 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

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. 

In order to produce black-and-white as well as full-colour STN-LCDs, the monochrome interference colours must first be eliminated. This was achieved initially by using two STN-LCDs in a combined double-layer (DSTN) LCD configuration. This involves the use of another non-addressed, passive STN cell in addition to the active display STN-LCD. However, the non-addressed cell has an opposite sense of twist of the nematic director in the cell to that of the addressed STN-LCD. The second STN-LCD, which is identical to the first, but not addressed at all, acts as a retardation compensation layer. The use of an identical second STN-LCD in combination with the active STN-LCD has the advantage that both displays exhibit exactly the same temperature dependence of the birefringence with the same dispersion, assuming that both cells are filled with the same liquid crystal mixture. The second STN-LCD is not addressed and, therefore, there is no increase in power consumption. However, the use of two identical STN-LCDs instead of only one clearly increases the cost and weight of the final product significantly. 

Figure 4.6-8 Optical rotation exhibited by a 0.2 mm thick sample of a mixture of cholesteryl chloride and cholesteryl myristate (molar ratio 1.67) at 1900 cm Scanning the temperature changes the pitch. At 59.5 °C the pitch corresponds to 1900 cm , at about 48 °C the twisting influences of the mixture components are mutually compensated so that the sample is nematic, at lower temperatures the structure is countercurrent. Above and below T em the rotatory dispersion follows a curve as derived by de Vries (1951).

Sackmann et have investigated the temperature variation of the pitch of a mixture of right-handed cholesteryl chloride and left-handed cholesteryl myristate by this method. At a certain temperature (7 ) there is an exact compensation of the two opposite helical structures and the sample becomes nematic. At this temperature only the central spot (zero order) is observed, while at the other temperatures, polarized diffraction maxima of higher order make their appearance. The inverse pitch varies almost linearly with temperature passing through zero at 7 (fig. 4.1.16). 

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). 

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. 

At the present time, as shown by the pocket calculator, materials are available with adequate electro-optic performance where the duty cycle is 1 in 2, 3 or 4. Where temperature compensation is used, 1 in 7 duty cycle is possible O with mixtures of cyanobiphenyls and benzoate esters. For a duty cycle of 1 in 10, temperature compensation is a must and the electro-optic performance is determined principally by the threshold sharpness. By way of example. Fig. 36 shows a viewing cone plot for ZLl 1253 (dotted line curves) compared with a sharper threshold material (1.65) (solid line curves) in an 8y, low tilt cell. By single frequency addressing, a 1 in 10 duty cycle is approaching the maximum for an acceptable performance in a twisted nematic display. 

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]. 

The compensated mixtures provide nematic phases with a local screw sense which orient d- and 1-isomers of optically active molecules somewhat differently. 

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. 

Such a compensated mixture provides an environment which exhibits both anisotropy and helicity. The two isomers have somewhat different order parameters in a solvent with a local screw sense and correspondingly exhibit different values of the direct coupling constants. In an ordinary nematic solvent (such as n-hexyloxyazoxybenzene) the two isomers give identical spectra. [56]. 

When a chiral material is added to a nematic liquid crystal at low concentrations, the pitch p appears to vary linearly with the concentration. A mixture of two compounds of opposite chirality can produce a nematic phase at a certain composition. At this compensation point, the pitch becomes infinite. Unwinding of helical structures can be achieved by external fields. Finally, it is mentioned that, for chiral nematics with relatively short pitch, there exist several intermediate phases known as the blue phases between the isotropic and the chiral nematic phases. These blue phases are... 

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