Cholesteric mesophases

The other thermotropic mesophases (cholesteric and smectic) also result from an ordering of solvent molecules. The cholesteric occurring in solvents possessing a centre of asymmetry, so that the structure is characterised by a helical twist. The smectic mesophase consists of molecules parallel to each other with the extra order being in layers. They are generally viscous and are not so suitable for high resolution NMR spectroscopy. 

Many cellulose derivatives form Hquid crystalline phases, both in solution (lyotropic mesophases) and in the melt (thermotropic mesophases). The first report (96) showed that aqueous solutions of 30% hydroxypropylceUulose [9004-64-2] (HPC) form lyotropic mesophases that display iridescent colors characteristic of the chiral nematic (cholesteric) state. The field has grown rapidly and has been reviewed from different perspectives (97—101). 

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. 

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 6.1 Schematic representation for some different mesophases formed by calamitic meso-gens. P/2 is half of cholesteric pitch.

Induction of chirality in No phases was also shown to be possible using charge transfer interactions, via jt-jt stacking. The binary mixture of mesogen 8, which is electron rich, with chiral electron-deficient molecule 9 was shown to induce a twist in the mesophase.15 Furthermore, nonmesogenic 8 gave rise to a cholesteric mesophase, denoted as being of the columnar type (N ), when present in a ternary system with TNF (trinitrofluorenone, an electron acceptor)... 

Thermotropic cholesterics were officially discovered2 in 1888 by the Austrian botanist Friederich Reinitzer, while studying the melting of cholesterol esters cholesteryl benzoate first melted to give a cloudy liquid that, at higher temperature, turned into an ordinary clear liquid. The cloudy liquid (the mesophase) was a thermotropic cholesteric liquid crystal. These phases... 

The study of the cholesteric mesophases obtained by doping thermotropic nematics with chiral, nonracemic compounds, has lead to relevant information about the stereochemistry of the dopants. Chiral interactions change the structure of the phase and therefore molecular chirality can be mapped onto an achiral (nematic) phase to yield a superstructural phase chirality. In 1984 Sol-ladie and Zimmermann published the first review summarizing the state of the art at that time.52 Later on, several review articles updated this subject.53-55... 

From a cholesteric induction experiment, one can obtain chiral information on the induced cholesteric (namely, pitch and handedness) and therefore the helical twisting power of the dopant in that solvent (at a certain temperature). If a model or molecular theory relating molecular chirality to mesophase chirality is available, one can infer stereochemical information about the dopant (absolute configuration, preferred conformation). 

Discotic liquid crystals arise from disk-shaped molecules as nematic or cholesteric mesophases. Their structural characteristics are similar to the respective ealamitie mesophases, that is, the normals of the disks are oriented parallel. Instead of the smectic mesophases, diseotie columnar liquid crystals arise from eonnecting the disks to each other. The columns of the discotic columnar mesophase form a two-dimensional lattice whieh is in a hexagonal or rectangular modification. In addition, the columns may be tilted (Fig. 2f,g). 

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. 

Solvent viscosity vs, concentration plots for cellulose dissolved in TFA-CH2CI2 (70/30, v/v) do not exhibit a maximum (1I,S1) in contrast to the typicid behavior of polymer liquid crystal solutions. This same behavior is exhibited by other cellulose-solvent systems (52,fiQ). Conio et al. (59) si gest that due to the close proximity of the cholesteric mesophase to its solubility limit, it is only observed in a metastable condition. 

Cholesteric lyotropic mesophases of cellulose in LiCl-DMAC solutions at 1(>-15% (w/w) concentration have been observed by Ciferri and coworkers (19.59.61.62) and McCormick et al. (63). LiCl/DMAC ratios between 3/97 and 11/89 (w/w) were used. LiCl-DMAC does not degrade cellulose and does not react with the polymer (59). It does form a complex with the OH CToups on cellulose which is believed to result in dissolution (62). Optical rotary dispersions are negative, indicating the superhelicoidal structure has a left-handed twist. 

Solutions of cellulose in NH3/NH4SCN (27 73 w/w) are liquid crystalline at concentrations from 10-16% (w/w) depending on the cellulose molecular weight (64). Optical rotations of the solutions indicate the mesophase is cholesteric with a left-handed twist. The solvent does not react with cellulose. Recently, Yang (60) foimd that cellulose (D.P. 210) formed a mesophase at 3.5% (w/w) concentration at a NH3/NH4SCN of 30 70 (w/w). 

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). 

Bheda et al. ( ) showed that cellulose triacetate forms a mesophase in dichloroacetic acid. Navard and Haudin (18) examined the thermal behavior of liquid crystalline solutions of CTA in TFA. Navard et al. (23) studied the isotropic to anisotropic transitions of solutions of cellulose triacetate in TFA using differential scanning calorimetry. Navard and Haudin (S2) studied the mesophases of cellulose and cellulose triacetate calorimetrically. Navard et al. (83) report similar studies. Meeten and Navard (97) showed the twist of the cholesteric helicoidal structure of CTA and secondary cellulose in TFA is left-handed. 

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