Cholesteric group

Furthermore, Kunieda and coworkers were interested in replacing the traditional surfactants with environmentally friendly molecules to overcome biodegradation processes and aquatic toxicity [25]. The main environmentally friendly surfactants that were explored were poly(oxyethylene) cholesteryl ethers (ChEOn, where n is the number of oxyethylene, EO, units) with a bulky hydrophobic cholesteric group of natural origin [25-27]. Due to the distinct segregation tendency between the hydrophilic and hydrophobic groups, compared to the conventional alkyl ethoxylated surfactants, their phase behavior as a function of ethylene oxide... 

Ritchey et al. (113) showed the introduction of trifluoroacetate groups at the unsubstituted hydroxyls of cellulose acetate causes a reversal in handedness of the cholesteric structure. Likewise the introduction of an aceto group in acetox3rpropyl cellulose changes the twist (116). 

Guo and Gray (114) foimd that acetylation of the imsubstituted groups in ethyl cellulose changes the sense of the helicoidal cholesteric twist from leff-handed to right-handed in either CHCI3 or m-cresol. 

The article covers synthesis, structure and properties of thermotropic liquid-crystalline (LC) polymers with mesogenic side groups. Approaches towards the synthesis of such systems and the conditions for their realization in the LC state are presented, as well as the data revealing the relationship between the molecular structure of an LC polymer and the type of mesophase formed. Specific features of thermotropic LC polymers and copolymers of nematic, smectic and cholesteric types are considered. 

This review deals with LC polymers containing mesogenic groups in the side chains of macromolecules. Having no pretence to cover the abundant literature related to thermotropic LC polymers, it seemed reasonable to deal with the most important topics associated with synthesis of nematic, smectic and cholesteric liquid crystals, the peculiarities of their structure and properties, and to discuss structural-optical transformations induced in these systems by electric and magnetic fields. Some aspects of this topic are also discussed in the reviews by Rehage and Finkelmann 27), and Hardy 28). Here we shall pay relatively more attention to the results of Soviet researchers working in the field. 

A different phenomenon is the appearance of the so-called blue phases in case of chiral LC s, first observed at cholesteric LC s. Sometimes, one or more blue phases occur close below the clearing point with a very small phase width and bright blue or green colours. Figure 15 shows some typical representatives containing one or more X=Y groups in... 

Our spectroscopic studies of BN in mixture B and in hexane support our contention that ground state conformers are forced by cholesteric mesophases toward extremes of 0 (i.e., closer to 0° or 180° than in hexane solvent). As the two naphthyl groups become more coplanar, their u-overlap increases. Consequently, the 0-0 transitions in absorption (and excitation) occur at longer wavelengths (lower energies) (43). For the same reasons, the cholesteric solvent compresses excited singlets of BN, causing their fluorescence spectra to be red-shifted with respect to those in hexane. 

Similarly, the reaction field, R (88-90), associated with a group of solvent molecules with cholesteric phase order is much larger when operating on a triplet of BN R increases with increasing a. Hie limitations of the Onsager model to the very anisotropic environment experienced by 2BN preclude a reasonable quantitative discussion. The solute cavity Is not spherical BN may be described better for the purposes of elucidating its interactions with neighboring solvent molecules as a quadrupole... 

As an additional remark, the same research group designed a cholesteric network that acted as a humidity sensor, using a derivative of HPC esterified with acryloyl chloride to a low degree of substitution (0.22). The HPC acrylate hydrogel can change the cholesteric pitch, and thereby the reflective color changes with water uptake. 

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