Side groups cholesteric

Liquid Crystalline Order in Polymers and Copolymers with Cholesteric Side Groups... 

In our work with copolymers of cholesteryl methacrylate with a number alkyImethacrylates we have shown (21) that these considerations are qualitatively valid also for copolymers in their condensed state. It was found that for a random copolymer of a fixed composition the degree of organization dramatically decreased when side groups of the non-mesogenic comonomer unit interfered through steric hindrance with the formation of ordered arrangements of cholesteric side groups,... 

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. 

A large number of azobenzene-based amorphous and liquid crystalline polymers, particularly polyacrylates and polymethacrylates with chiral azobenzene pendants, have been prepared for the development of data storage and photonic devices [1-3,11-14]. For instance, the introduction of optically active mesogenic azobenzene residues into the side groups of the polymers produces chiral nematic and cholesteric phases, which are regulated by photoisomerization of the azobenzene units [10,14]. In most cases, however, the optical activity and chiroptical... 

In addition to the rigidity, steric effects and flexibility of side groups seems to Influence the formation and properties of cellulosic mesophases they allow (or not) the existence of a mesophase before crystallization, influence the temperature for onset of a mesophase, and contribute to the value of the cholesteric pitch. 

In the side chain liquid crystalline polymers the nematogenic side groups and chiral side groups are attached respectively to a polymer backbone to form a copolymer. They exhibit the cholesteric phase. A typical example is shown below. One is the homopolymer (shown in Figure 6.28) and the other is the copolymer (Figure 6.29). 

Introduce the chiral center into one end of the side groups and one may obtain a cholesteric liquid crystalline polymer. If both mesogenic units... 

The first report in the open literature concerning cellulose derivatives is that of Werbowyj and Gray [100], They observed that 20-50% aqueous solutions of HPC were birefringent and highly iridescent due to the cholesteric nature of the liquid crystalline phase. The formation took place at a higher polymer volume fraction than predicted by the Flory theory, because of some chain flexibility and of side group interaction. 

Chemical modification of the polymer structure allows the obtention of nematic and smectic phases [4, 5]. If the side group and/or the chain are chiral, then cholesteric or chiral smectic C (SmC) phases can be obtained. These can also be obtained by mixing a chiral compound with the SCLCP. SmC SCLCPs are of particular interest and their behavior is described in Sec. 2 of this Chapter. 

Cellulose and oligocellulose derivatives have been studied and a variety of thermotropic LC phases established, i.e., cholesteric and colunmar structures for cellulose derivatives, discotic columnar and smectic-type for the oligomers, depending on the side group and the main-chain lengths. A list of compounds exhibiting thermotropic me-sophases is presented in Table 5. 

C.-H. Lin and C.-S. Hsu. Synthesis and characterization of cholesteric liquid crystalline copolysiloxanes containing 4-biphenyl 4-allyloxybenzoate and [S]-l-(2-naphthyl)ethyl 6-[4-(10-undecen-l-yloxy) biphenyl-4 -carbon-yloxy]-2-naphthoate side groups. J. Polym. Res., 7 167-173, 2000. 

A recent publication (13) which appeared while this work was in progress described the spontaneous development of layered order in polymers with the cholesteric moiety attached to the backbone via a long, flexible molecular spacer constituted by an Co-amino-carboxylic acid chain. Layered structures are developed with the long side groups roughly perpendicular to the main chain. The authors attributed the development of these structures to the flexibility of the bridge connecting the cholesterol moiety. 

CHOLESTERIC POLYMERS WITH MESOGENIC SIDE GROUPS... 

The results of the present work demonstrate that the cholesteric mesophase of polymers having mesogenic side groups is, in many aspects, similar to that of low molecular cholesterics. The presence of the polymeric chain results, however, in the appearance of some specific features in the behavior of such systems. The possibility to quench the cholesteric structure by vitrification of polymers is worth emphasizing explicitly. The properties of polymeric cholesterics discussed above are far from covering the array of specific features particularly intrinsic to polymeric liquid crystals. The essential effect of molecular parameters on their structure and optical properties constitute one of these features. This problem will be treated in subsequent publications. 

Lin CH, Hsu CS. Synthesis and characterization of cholesteric liquid crystalline copolysilox-anes containing 4-biphenyl 4-allyloxybenzoate and -2-naphthoate side groups. J Polym Res 2000 7 167-73. 

Another method to obtain a uniformly-ordered cholesteric network was applied by Vallerian et al. [15]. They started with a linear cholesteric side-chain polymer which was prepared between glass slides. By surface effects, a Grandjean texture becomes stable. Because the linear polymer additionally contains polymerizable groups, a thermally-induced cross-linking reaction forms the network. A disadvantage of this method is the limited size of the sample and problems in analyzing the mechanical properties of the network. 

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