Ordering cholesteric

Ganapathy and Welss(22), demonstrated that the selective photoproduction of one atropisomer from a racemic solution of 1,1 -binaphthyl (BN.) requires the presence of both a chiral and macroscopically ordered cholesteric reaction medium. No measurable change in atropisomeric content was observed upon heating racemic N in a variety of cholesteric mixtures. Thus, photoequilibration is more selective than thermal equilibration BN the isomerlzing excited state, and solvent are able to interact more strongly than ground state N. and the solvent matrix. Since the maximum atropisomeric excess observed upon irradiation of Bfi. was 1.1%, once again the solvent influence on the solute motions is very weak. 

In nematic liquid crystals, the molecular axes are ligned up in the same direction without any 2 dimensional ordering. Cholesteric and smectic liquid crystals are composed of layers of molecules with long molecular axes parallel and perpendicular to the planes, respectively. 

A very simple optical model of a perfectly ordered cholesteric liquid crystal was first studied by Oseen and later by HI. de... 

Demus, Goodby and Gray [16] present different systems based on cellulose derivatives, in which the ordered cholesteric liquid crystalline phase appears only in specific solvents at high concentrations, remaining isotropic in other solvents (Table 14.1). In addition, one can notice that the flexibility of side-chain substituents can generate orientational equilibrium of the main chains and also that the numerous large substituents from the cellulose backbone increase the hydrodynamic volume, influencing chain conformation. 

Another method to obtain a uniformly-ordered cholesteric network was applied by Vallerian et al. [15]. They started with a linear cholesteric side-chain polymer which was prepared between glass slides. By surface effects, a Grandjean texture becomes stable. Because the linear polymer additionally contains polymerizable groups, a thermally-induced cross-linking reaction forms the network. A disadvantage of this method is the limited size of the sample and problems in analyzing the mechanical properties of the network. 

Cholesteric liquid crystals composed of molecules with negative diamagnetic anisotropy (lxnl>lxil) i ay also be ordered uniformly by magnetic fields. Since molecules of this type tend to orient perpendicular to the magnetic field direction, the helix axes of the spontaneously ordered cholesteric domains are turned in a direction perpendicular to the magnetic field. Such an experiment will be described in more detail below. 

Figure 5. Diffraction pattern of an ordered cholesteric glass (pitch - 10 urn) obtained with a He-Ne-laser (632.8 nm). The light beam was directed perpendicular to the helix axis. The glass has been prepared by rapid transfer of the sample (cqntained in flat cells of 0.2 mm thickness) from the orienting magnetic field (30,000 Gauss) to a coolant of -50 C (reprinted from Sackmann et al. [8]).

Finally, a comment on nomenclature is in order. Cholesteric liquid... 

Until this point, the discussion has dealt with relatively dilute polymer solutions at low temperature, where chain conformation has been extensively studied. The knowledge of the structure of concentrated systems is of utmost importance in respect to swelling of initially dry polymers. In fact, lyotropic mesostructures with one-dimensional order (cholesteric phase) have been observed in recent years for most of the water-soluble cellulose derivatives [23-25]. The HPC-water system has been extensively investigated and Fortin and Charlet [25] have, for instance, established the phase diagram for the 0-100% polymer weight fraction range (Figure 3). 

Liquid Crystallinity. The Hquid crystalline state is characterized by orientationaHy ordered molecules. The molecules are characteristically rod-or lathe-shaped and can exist in three principal stmctural arrangements nematic, cholesteric, and smectic (see Liquid crystalline materials). 

The three classes of liquid crystals differ in the arrangement of their molecules. In the nematic phase, the molecules lie together, all in the same direction but staggered, like cars on a busy multilane highway (Fig. 5.49). In the smectic phase, the molecules line up like soldiers on parade and form layers (Fig. 5.50). Cell membranes are composed mainly of smectic liquid crystals. In the cholesteric phase, the molecules form ordered layers, but neighboring layers have molecules at different angles and so the liquid crystal has a helical arrangement of molecules (Fig. 5.51). 

Analytical approaches to understanding the effect of molecular flexibility on orientational order have concentrated on both the isotropic-nematic and the nematic-smectic transition [61, 62] and mean field theory has shown that cholesteric pitch appears not to depend on the flexibility of the molecule [63]. 

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]. 

Similar to the sitnation with DNA structures formed under osmotic stress, DNA strands in cation-condensed bundles were found to be hexagonally packed and to possess liqnid crystalline order. For example, spermine and spermidine-condensed samples were fonnd to contain a cholesteric phase [70]. Surprisingly, DNA condensed with (Co(NH3)6) failed to exhibit a liquid crystalline ordering [47]. 

When the mesogenic compounds are chiral (or when chiral molecules are added as dopants) chiral mesophases can be produced, characterized by helical ordering of the constituent molecules in the mesophase. The chiral nematic phase is also called cholesteric, taken from its first observation in a cholesteryl derivative more than one century ago. These chiral structures have reduced symmetry, which can lead to a variety of interesting physical properties such as thermocromism, ferroelectricity, and so on. 

The mixing of nematogenic compounds with chiral solutes has been shown to lead to cholesteric phases without any chemical interactions.147 Milhaud and Michels describe the interactions of multilamellar vesicles formed from dilauryl-phosphotidylcholine (DLPC) with chiral polyene antibiotics amphotericin B (amB) and nystatin (Ny).148 Even at low concentrations of antibiotic (molar ratio of DLPC to antibiotic >130) twisted ribbons are seen to form just as the CD signals start to strengthen. The results support the concept that chiral solutes can induce chiral order in these lyotropic liquid crystalline systems and are consistent with the observations for thermotropic liquid crystal systems. Clearly the lipid membrane can be chirally influenced by the addition of appropriate solutes. 

Reinitzer discovered liquid crystallinity in 1888 the so-called fourth state of matter.4 Liquid crystalline molecules combine the properties of mobility of liquids and orientational order of crystals. This phenomenon results from the anisotropy in the molecules from which the liquid crystals are built. Different factors may govern this anisotropy, for example, the presence of polar and apolar parts in the molecule, the fact that it contains flexible and rigid parts, or often a combination of both. Liquid crystals may be thermotropic, being a state of matter in between the solid and the liquid phase, or they may be lyotropic, that is, ordering induced by the solvent. In the latter case the solvent usually solvates a certain part of the molecule while the other part of the molecule helps induce aggregation, leading to mesoscopic assemblies. The first thermotropic mesophase discovered was a chiral nematic or cholesteric phase (N )4 named after the fact that it was observed in a cholesterol derivative. In hindsight, one can conclude that this was not the simplest mesophase possible. In fact, this mesophase is chiral, since the molecules are ordered in... 

Figure 6.2 Discotic molecules in a (a) nematic state ND, (b) twisted nematic discotic state Np (P/2 is half of cholesteric pitch), (c) columnar state, ordered D0 and disordered Dd, (d) hexagonal ordered columnar state Dho two-dimensional packing arrays for columnar structures in (e) hexagonal, Colh rectangular, Colr oblique, Col0b.

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 study of mesophases of cellulose and cellulose derivatives is an active field which has expanded rapidly since the initial observation of liquid crystms of hydroxy-propyl cellulose in 1976. There are two areas that warrant turther investigation recent observations regarding the influence of solvent and/or substituents on the cholesteric helicoidal twist await a theoretical explanation there is a lack of careful studies to permit a theoretical treatment of the behavior of ordered celltdose phases. To date, no applications have been developea where the unusual properties of cellulose derivatives are utilized. 

Navard and Haudin studied the thermal behavior of HPC mesophases (87.88) as did Werbowyj and Gray (2), Seurin et al. (Sp and, as noted above, Conio et al. (43). In summary, HjPC in H2O exhibits a unique phase behavior characterized by reversible transitions at constant temperatures above 40 C and at constant compositions when the HPC concentration is above ca. 40%. A definitive paper has been recently published by Fortin and Charlet ( who studied the phase-separation temperatures for aqueous solutions of HPC using carefully fractionated HPC samples. They showed the polymer-solvent interaction differs in tiie cholesteric phase (ordered molecular arrangement) from that in the isotropic phase (random molecular arrangement). 

Sixou et al. (101) showed the circular dichroism of cholesteric CTA solutions in TFA depends on the CTA molecular weight. The intensity of the circular dichroic peak increases with molecular weight. Meeten and Navard (97) studied gel formation and liquid crystallinity in TFA-H2O solutions of CTA. When water was added to a liquid crystalline solution of CTA in TFA a gel phase formed presumably by the formation of crosslinks due to hydrogen bonding. They interpreted their results that liquid crystalline ordering involves both inter- and intramolecular forces. 

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