Thermotropic cholesterics

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... 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

In the held of thermotropic cholesterics, the most promising approach seems to be that reported by Nordio and Ferrarini22 23 for calculating helical twisting powers. It allows one to tackle real molecules with rather complex structures and to describe them in detail. The model is currently being extended to include a better description of nematic solvents and specific solute-solvent interactions. Once tested also for conformationally mobile molecules, this model could allow the prediction of the handedness of single-component cholesterics, and, in the held of induced cholesterics, very interesting information on solute molecules could be obtained. 

Under certain conditions, stiff rod-like helical polymers can spontaneously form lyotropic or thermotropic cholesteric liquid crystal (TChLC) phases. 

Liquid crystals are mainly used for decorative purposes in cosmetics. Cholesteric liquid crystals are particularly suitable because of their iridescent color effects, and find applications in nail varnish, eye shadow, and lipsticks. The structure of these thermotropic liquid crystals changes as a result of body temperature, resulting in the desired color effect. In recent times, such thermotropic cholesteric liquid crystals have been included in body care cosmetics, where they are dispersed in a hydrogel. Depending whether this dispersion requires stirring or a special spraying process, the iridescent liquid crystalline particles are distributed statistically in the gel (Estee Lau-... 

Stiff rod-like helical polymers are expected to spontaneously form a thermotropic cholesteric liquid crystalline (TChLC) phase under specific conditions as well as a lyotropic liquid crystal phase. A certain rod-like poly(f-glutamate) with long alkyl side chains was recently reported to form a TChLC phase in addition to hexagonal columnar and/or smectic phases [97,98]. These properties have already been observed in other organic polymers such as cellulose and aromatic polymers. 

Gray et al. have reported that (acetoxypropyl)cellulose behaves as a thermotropic cholesteric liquid crystal below 164 °C. It has been also observed that some (hydroxy-propyljcellulose forms a thermotropic liquid crystalline phase at temperatures above 160 °C From these results together with our finding, we presume that rigid rod-like... 

Figure 6.6 (Left ) Stained section of a crab cuticle Carcinus maenas), showing the single twist of protein fibres (some indicated by red lines), characteristic of the chiral thermotropic cholesteric mesophase. (Right ) Schematic relation between nested arc texture in the micrograph and the cholesteric mesostructure. Micrograph and drawing adapted from (9. ...

It is observed from experiments that HPC exhibits thermotropic cholesteric liquid crystal characteristics as well (Tsvetkov, 1989). 

In addition to the above mentioned lyotropic cholesteric liquid crystalline polymers composed of rigid polymers, there is a diversity of thermotropic cholesteric liquid crystalline polymers which consist of a flexible chain incorporated with a mesogenic and chiral units. The thermotropic cholesteric liquid crystalline polymers are classified into two categories main chain and side chain. 

Finkelmann et al. described the synthesis of what appears to be the first thermotropic cholesteric copolymers ... 

Many cellulose derivatives form lyotropic liquid crystals in suitable solvents and several thermotropic cellulose derivatives have been reported (1-3) Cellulosic liquid crystalline systems reported prior to early 1982 have been tabulated (1). Since then, some new substituted cellulosic derivatives which form thermotropic cholesteric phases have been prepared (4), and much effort has been devoted to investigating the previously-reported systems. Anisotropic solutions of cellulose acetate and triacetate in tri-fluoroacetic acid have attracted the attention of several groups. Chiroptical properties (5,6), refractive index (7), phase boundaries (8), nuclear magnetic resonance spectra (9,10) and differential scanning calorimetry (11,12) have been reported for this system. However, trifluoroacetic acid causes degradation of cellulosic polymers this calls into question some of the physical measurements on these mesophases, because time is required for the mesophase solutions to achieve their equilibrium order. Mixtures of trifluoroacetic acid with chlorinated solvents have been employed to minimize this problem (13), and anisotropic solutions of cellulose acetate and triacetate in other solvents have been examined (14,15). The mesophase formed by (hydroxypropyl)cellulose (HPC) in water (16) is stable and easy to handle, and has thus attracted further attention (10,11,17-19), as has the thermotropic mesophase of HPC (20). Detailed studies of mesophase formation and chain rigidity for HPC in dimethyl acetamide (21) and for the benzoic acid ester of HPC in acetone and benzene (22) have been published. Anisotropic solutions of methylol cellulose in dimethyl sulfoxide (23) and of cellulose in dimethyl acetamide/ LiCl (24) were reported. Cellulose tricarbanilate in methyl ethyl ketone forms a liquid crystalline solution (25) with optical properties which are quite distinct from those of previously reported cholesteric cellulosic mesophases (26). 

Shown in the table below, the earliest recognition of liquid crystalline behavior, observed on a thermotropic cholesteric small molecule, is attributed to Reinitzer in 1888. In the Forties and Fifties, polymeric liquid crystallinity was noted and widely researched. The systems studied were mostly natural and synthetic lyotropic biopolymers such as... 

Many other cellulose derivatives were studied and, among them, acetoxyproylcellu-lose (APC) was found to develop a thermotropic cholesteric phase as well as a lyotropic phase, in several organic solvents, at room temperature. Gray et at [18] prepared this cellulose derivative by the acetylation of hydroxypropylcellulose (a schematic of the chemical reaction is shown in Figure 8.3). 

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... 

Just as for thermotropic cholesterics, the twist increases with increasing enantiomeric excess of a chiral surfactant. The same holds for the dopant concentration. The capacity of solubilization within the micelles depends on the physico-chemical nature of the dopant and host phase especially the size and amphiphilicity of the dopant are essential. The course of the twist versus dopant concentration x is linear for small x the initial slope /cIx) q) is defined as helical twisting power (HTP) as for thermotropics. Figures 14.10 and 14.11 show typical experimental data for a system consisting of chiral surfactants and for a guest/host system, respectively. For the chemical structures of the dopants of Figure 14.11, see Figure 14.9. 

However, there is an essential difference to thermotropic cholesterics the size and shape of the building blocks of the phase, which are the micelles, are subjected to substantial variations, see Section 14.2.6. 

Reeves et al. argued that a dispersion type of interaction between single chiral molecules in adjacent micelles is rather imlikely to be essential for the phase chirality [35], Dispersion interactions between colloidal particles, as well as between single molecules, decrease with a strong power of the interparticle distance d (by d to d ). In thermotropic cholesteric or chiral smectic phases the interparticle distance is of the order of 100 pm which is small compared to the typical values of 1 to 10 nm between micelles in lyotropic chiral nematic phases. 

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