Lyotropic cholesteric phase

Fig.10 Guanosine derivative tetramers that form lyotropic cholesteric phases [91]...

The derivatives of cellulose esters exhibit the lyotropic cholesteric phase in many solvents. Cellulose is less rigid than PBG. The persistence length of PBG is about 2000 A while the latter is between 130 and 330 A, depending on the chemical formula of the substitute and the solvent. The cellulose derivatives are cheap in cost and easy to process, and hence were studied extensively. Among the derivatives, hydroxypropyl cellulose (HPC) is one of most well studied. 

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-... 

Fig. 15 Representation of the optimized system. Cellulose caihanilates are used as long mesogens [112], They form a lyotropic cholesteric phase in mono- and bis-aaylales as solvents. Their photochemical polymerization freezes the helical cholesteric structure. Later on, fluorescent materials can be incorporated into this matrix by swelling and deswelling...

Fig. 16 Optical properties of different lyotropic cholesteric phases of cellulose carbanilates,...

Fig. 4.6 a Sketch of the Cano preparation showing the disclination lines as black vertical lines separating integer values N of the pitch p, the latter is depicted as gray double helix, b Exemplary picture of a Cano preparation of the lyotropic cholesteric phase formed by cellulose tricarbanilate and diethylene glycol butyl ether (adapted fixrm [15] with permission of the author)... 

Sugars are a commonly used source for amphiphilic liquid crystals [10]. These materials show lamellar, columnar, and cubic phases, but chiral phases are very rarely observed. Thermotropic cholesteric phases are never observed and lyotropic cholesteric phases based on asymmetric micelles only in a few cases [11]. The bicontinuous cubic phase of these glycolipids may have macroscopic chiral ordering, but this has not been resolved hitherto [12], [13]. Thus, alkylated sugars are chiral compounds, but not effective... 

Chiral mesophases can be obtained from sugars by several strategies. Many cellulose derivatives show thermotropic and lyotropic cholesteric phases [16]. Peracylated sugars can be used as chiral dopants for discoid nematic phases [17]. Also classical cholesteric and ferroelectric phases can be obtained from carbohydrate-based compounds [18]. In this case, chiral oxa-heterocycles are prepared from sugars. Figure 4.8 shows a chiral twin compound prepared from mannitol [19]. 

Liquid crystals can be used as sensors for stereoelectronic effects of molecules or as detectors of chirality. Even a small enantiomeric excess of less than 1% can create a texture change in a nematic phase and a measurable pitch. Dopants of type C2a can be used to determine the absolute configuration of chiral centers and changes of aggregation or conformation of molecules can be displayed by pitch changes. More common is the use of lyotropic cholesteric phases as solvent for nmr studies [40]. 

In the first instance, poly-a-amino acids with carbazole-substituted side chains were chosen for this purpose. Poly-a-amino acids are well known to be capable of forming lyotropic cholesteric phases due to their a-helical conformation. The formation of a homeotropic nematic phase, as shown schematically in Fig. 10, would give the desired morphology with the stacking of the carbazole rings in the plane of the cell. This is in essence the same morphology obtained with the carbazole-doped thermotropic... 

Licjuid Crystals. Ferroelectric Hquid crystals have been appHed to LCD (Uquid crystal display) because of their quick response (239). Ferroelectric Hquid crystals have chiral components in their molecules, some of which are derived from amino acids (240). Concentrated solutions (10—30%) of a-helix poly(amino acid)s show a lyotropic cholesteric Hquid crystalline phase, and poly(glutamic acid ester) films display a thermotropic phase (241). Their practical appHcations have not been deterrnined. 

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... 

Several natural10 and synthetic (e.g., polyisocyanates11) polymers form lyotropic cholesterics with the appropriate solvent also micellar systems formed by amphiphilic molecules and water, if chirality is introduced by either using a chiral amphiphile or adding a chiral dopant, can give cholesteric phases.12... 

Recently, a promising theoretical treatment was introduced by Ferrarini et al.22 which, in selected cases, leads to the effective calculation of the helical sense and pitch of the induced cholesteric phases.23 Attempts to relate the cholesteric handedness of lyotropic cholesterics to the helical sense of the polymers were first reported by Sato and co-workers.11... 

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. 

Lyotropic (in solvent) cholesteric mesophases have been observed for self-assembled guanosine derivatives. In water, the compounds shown in Fig. 10 generate left-handed columnar aggregates [91]. When the concentration is sufficiently high, cholesteric phases are formed which also have a left-handed twist, as determined by CD spectroscopy. The same tetrameric motif is present in the lyomesophases formed by more lipophilic guanosine deriva-... 

Both with thermotropics and lyotropics, a large number of properties can be studied. For example, NMR can be used to obtain the order parameter of the cholesteric phase (2 ). Rheooptic studies (27) (for example, the variation of the refractive index under shear) describe the way cholesteric phase reorganizes to nematic after orientation by a strong shear. 

Unfortunately, there is no report on the detailed physical characterization of these polymers. Such information as unidirectional twist angle and form optical rotation, as well as their dependence on chemical structures and temperature, can be very useful in further understanding the molecular orientations of the polymers in the cholesteric phase. In contrast, a number of studies have been made on the physical-chemical properties of cholesteric lyotropic polymer systems, especially polypeptides. 

The potential of nuclear magnetic resonance spectroscopy for studying liquid crystalline systems is discussed. Typical spectra of nematic, smectic, and cholesteric mesophases were obtained under high resolution conditions. The observed line shape in the cholesteric phase agrees with that proposed on the basis of the preferred orientation of this phase in the magnetic field. The line shapes observed in lyotropic smectic phases appear to be the result of a distribution of correlation times in the hydrocarbon portions of the surfactant molecules. Thermotropic and lyotropic phase transitions are easily detected by NMR and agree well with those reported by other methods. The NMR parameters of the neat and middle lyotropic phases allow these phases to be distinguished and are consistent ivith their proposed structures. 

Helpful tools for this structurization of liquid crystal research were temperature dependent X-ray investigations [36] of natural and synthetic lipids, and the discovery that mesophases may be identified by their different textures appearing in the microscope using crossed polarizers [37]. In the decade starting in about 1957 systematic screening of the concentration and temperature dependency of the major lyotropic mesophases was done and models of the molecular arrangement in the different phases were developed [38-45] (e.g., the so-called middle or neat phases [38], the cholesteric phase of polypeptides and nucleopep-tides [44]). 

Only a few solvents are known to dissolve cellulose completely, and solid cellulose decomposes before melting. Therefore, it is difficult to study the mesophase behavior of cellulose. Chanzy et al. [32] reported lyotropic mesophases of cellulose in a mixture of jV-methyl-morpholine-Af-oxide and water (20-50%), but were unable to determine the nature of the mesophase. Lyotropic cholesteric mesophase formation in highly concentrated mixtures of cellulose in trifluoroa-cetic acid + chlorinated-alkane solvent [33] and in ammonia/ammonium thiocyanate solutions [34] has been studied, and although poor textures were obtained in the polarizing microscope, high optical rotatory power has been measured in an optical rotation (ORD) experiment, which could be fitted to the de Vries equation [Eq. (3)] for selective reflection beyond the visible wavelength region and was taken as proof of a lyotropic chiral nematic phase. 

Ethoxypropyl cellulose [59], an ethyl ether of HPC forms excellent thermotropic and lyotropic mesophases, the lyotropic ones with acetonitrile, dioxane, and methanol. Both thermotropic and lyotropic systems exhibit cholesteric phases with a right-handed helicoidal supermolecular structure,... 

A basic understanding of the structure and behavior of liquid-crystalline cellulosics has yet to evolve. From a conceptual point of view, the chirality of the cellulosic chain is most sensitively expressed in the super-molecular structure of the cholesteric phase, which may be described by the twisting power or the pitch. At present, no information is available about domains or domain sizes (correlation lengths) of supermo-lecular structures. The chirality in the columnar phases has not been addressed at all. The principal problem, i.e., how does chirality on a molecular or conformational level promote chirality on the supermolecular level, has not been solved. If this correlation were known, it would enable the determination of the conformation of cellulosic chains in the mesomorphic phase and the development of models for the polymer-solvent interactions for lyotropic systems. On the other hand, direct probing of this interaction would provide a big leap towards an understanding of lyotropic phases. 

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