Twist, cholesteric

To summarize, in the field of lyotropic cholesterics formed by helical polymers of known geometry, it is possible to predict qualitatively only the entropic steric contribution to the cholesteric twist The prediction relies on the model of Figure 7.4 and should be compared to the experimental Sq data obtained by extrapolation of the twist to l/T = 0. A practical model for calculating the often dominant dispersion contribution is not yet available. 

Guo and Gray (114) foimd that acetylation of the imsubstituted groups in ethyl cellulose changes the sense of the helicoidal cholesteric twist from leff-handed to right-handed in either CHCI3 or m-cresol. 

Cholesteric Twisted Structure in Solid Films of Polypeptides.66... 

However, there is a structure consistent with both the required space group and the optical properties. The gyroid surface, which occurs frequently in lipid-water systems, provides such a possibility. If we assume that cholesterol skeletons form rod-like infinite helices, this structure represents an effective three-dimensional packing of such helices. Thus, the rods form a body-centered arrangement as shown in Fig. 5.5. In this structure, there is a helical twist between the rods, in addition to the cholesteric twist within each rod. The h)rperbolic structure is a consequence of the chirality of the esters, which induces torsion into the packing arrangement. A racemic mixture does not exhibit this phase natural cholesteric esters contain a single enantiomer only. 

The Volterra process for creating these disclinations is the same as for nematic disclinations. For the screw disclination the plane of cut is parallel to the cholesteric twist axis while for the edge disclination it is perpendicular to it. 

As with thermotropic nematics, the addition of optically active species to lyotropic nematic phases gives lyotropic cholesteric phases. Whilst details of their structures are not fully established they appear to follow the general pattern outlined above. The cholesteric twist would appear to derive from the packing of optically active mole-... 

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. 

As stated already, the sign of cholesteric twist of the solution of PBLG in CHCl is opposite to that in dichloroethane. The cholesteric twist of PALG also behaves in the same manner, that is, the sign is different to each other in the two solvents. However, the nematic temperature of PALG in EDC increases with the increase of alkyl chain length in the side chain. The constant n, then can be obtained from the plot of log 1/S against log C in EDC. It... 

Many synthetic polymers form cholesteric phases, and even solids showing certain of the fundamental symmetries of cholesteric liquids. The purpose of this paper is to review the main examples of biological polymers assembling into cholesteric liquids or into more or less solid analogues. We will present them according to the main chemical classes of polymers to which they belong. We will also indicate the main forces involved in creating the cholesteric twist. 

There are several related phenomena that will not be treated in this chapter. They include the well documented transformation of nematic phases into cholesteric (twisted nematic) phases by adding small amounts of optically active molecules to a nematogen [171], creation of smectic phases from mixtures of molecules which alone form only nematic phases [172,173], and the presence of reentrant phases [174] due to molecular reorganizations based upon the relative importances of various short- and long-range intermolecular interactions in different temperature regimes (as mediated by the interplay of entropy and enthalpy terms) [175 -177]. Each has been exploited to create interesting and novel systems and devices based upon mesomorphism. 

If the mesogens are chiral, a twisted nematic, suprarmolecular, cholesteric (twisted) phase can form [51, 52]. The achiral nonlinear mesogens can also form chiral supramolecular arrangements in tilted smectic phases. 

The liquid crystal was assumed to be doped with an optically active material that gives it a quarter turn of cholesteric twist in the cell thickness. This prevents untwisting of the liquid crystal even if anchoring is weak. It also prevents walls due to twist reversal. 

FIGURE 12 Unwinding of the cholesteric twist. The lines denote the helical twist of the cholesteric. In (a) the field is zero, in (b) the field is finite but below the critical value for unwinding, and in (c) the field completely unwinds the cholesteric twist. 

Figure 31 d shows how the presence of a -n disclination eliminates very acute conical shapes of nested layers. This arrangement of layers creates a rhombic and conical domain in the midpart of most of the polygon edges in Fig. 6 a. Careful examination of the layers under natural light at a level close to the coverslip generally shows a structure like the one shown in Fig. 38 f. The disclination forms a helical half-loop [20] due to the cholesteric twist, a situation very similar to that described in other helical cholesteric patterns (see Sec. 7.3.4). 

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