Cholesteric gels

Then the liquid crystalline monomers were polymerized under an ultraviolet irradiation. The system became a cholesteric gel. After heating the sample and removing the chiral molecules CB15, the helical structure remained. In Table 6.10, the cholesteric pitch P and reflective peak wavelength Amax are listed as a function of temperature at which polymerization occurs. The CB15 weighed 28% in the original mixture before the removal. 

Plate 1. Photographs of a patterned cholesteric gel at various applied voltages. (See Page 6 of color insert.)... 

These broad band cholesteric gels could be switched reversibly between silver colored reflecting and non-reflecting transparent states. Upon application of die electric field, die cholesteric structure disappears and the cell becomes transparent as shown in Figure 11. Upon removal of the voltage, the cell reverts to the silver colored reflecting state very rapidly. 

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-... 

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. 

Selective reflection of light Thermochromic cholesteric devices, gel displays... 

There have been a lot of studies of cholesteric films and gels in order to exploit their potential as specific optical media and as other functional materials. Most of the preparations were achieved by modification or improvement of previous attempts to immobilize the cholesteric structure of cellulose derivatives into the bulky networks either by crosslinking of cellulosic molecules with functional side-chains in the liquid-crystalline state [203], or by polymerization of monomers as lyotropic solvents for cellulose derivatives [204-206],... 

Before and after the works described above, contributions to the design and fabrication of similar multicomponent films or gels of cholesteric character, mainly based on HPC, EC, or their derivatives were also made [202, 219-224], Some of these [219,220,224] dealt with shear-deformed network systems preserving a unique banded structure, so that the disappearance and recovery of the optical anisotropy could be controlled thermo-reversibly. Special mention should be made of the successful preparation of two novel classes of solid materials maintaining cholesteric liquid-crystalline order. One consists of essentially pure cellulose only, and the other is a ceramic silica with an imprint of cellulosic chiral mesomorphy. 

Water and isobutyric acid (IBA) have been chosen as solvents because they form a mesophase with HPC and they have a critical temperature, approximately 26 C for an IBA proportion of approximately 0.4. The phase diagram shows different regions one- or two-phase, mesomorphic, or not. The two-phase region can be segregated. The mesomorphic phase can form a gel or show cholesteric colors (25). The most important Information given by this phase diagram is that the behavior of phase separation of IBA/H2O/HPC system is governed by the extreme preference of HPC for IBA as compared with water. This results in the formation of the ordered phase in dilute HPC solutions as the IBA content in the solvent is reduced. Mesophases can be obtained for low polymer concentrations of a few percent. 

A similar thermally-induced inversion of the cholesteric sense was observed for the PBLG liquid crystal in benzyl alcohol. In this solution, a gel-like opaque phase coexists with the cholesteric phase at lower temperatures. The opaque phase disappears around 70 °C, where endothermic peaks are observed in the differential scanning calorimetry curve. The value of S below 70 °C remains constant, and then changes with temperature above 70 °C. The compensation occurs at about 103 °C, and the transition from biphasic phase to the isotropic phase is observed above 150 °C in this case. The results are summarized in Fig. 12, where the reciprocal of the half-pitch is plotted against temperature. The sign of 1/S is taken as positive when the cholesteric sense is the right-handed. 

R. Kishi and M. Sisido, Liquid-crystalline polymer gels, 4. Photocontrol of dye orientation in a polymer having a cholesteric liquid-crystalline order, Mackromol. Chem. 192, 2723-2732 (1991). 

Nematic gels are very interesting systems, thus deserving further study. Actually, these systems are being studied experimentally for applications. Examples are polymer dispersed liquid crystal displays are sometimes dispersed not in a polymer, but in a polymer network. Displays by means of the polymer stabilized cholesteric texture change, are also achieved in crosslinked systems. In addition, the chiral smectic phase has been obtained in such systems as well. Other types of liquid crystal gels have been applied or are expected to be applied in such devices. 

The liquid crystalline polymer has since developed far beyond imagination that a decade ago. The liquid crystalline polymer family has so far included the main chain-, side chain-, and crosslinked- (i.e. network or elastomer) types, and their solutions and gels. The liquid crystal phases cover nematic, cholesteric and smectics. Although the science of the liquid crystalline polymer is not fully mature, it has attracted significant research interests and has already made tremendous progress. As investments and human resources continue, the liquid crystalline polymer is expected to have an even brighter future. 

Fig. 18, Cholesteric liquid crystalline structure formed in a 50 wt % HPC gel at room temperature (Magnification 25x)...

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