Cholesteric liquid crystalline polymers

Figure 1.68 The structure of liquid crystalline polymers (a) nematic, (b) smectic and (c) cholesteric. Reprinted, by permission, from J. L. Fergason, Scientific American, 211(2), pp. 78, 80. Copyright 1964 by Scientific American, Inc.

Finkelmann, F., Ringsdorf, H., Siol, W. and Wendorff, H. Synthesis of cholesteric liquid crystalline polymers. Makromol. Chem. 179, 829 (1978)... 

With these properties a wide field of application is revealed As the l.c. side chain polymers can be orientated in the l.c. state by an electric or magnetic field, it is possible to store any information obtained in the l.c. state by cooling the liquid crystalline polymer down to the glassy state. Obvious applications are e.g. optical filters or reflectors, prepared for linearly or circularly polarized light by cholesteric polymers. Furthermore the glassy polymers can serve as anisotropic matrices for dissolved molecules. 

The Principles of Formation and Some Properties of Smectic, Nematic and Cholesteric Mesophases of Liquid-Crystalline Polymers... 

Coatings derived from cholesteric liquid crystalline polymers are used commercially as reflective sheets and polarisers. The liquid crystal is cooled below the vitrification temperature resulting in a solid polymer that is amorphous but contains large regions of frozen liquid crystalline order. Such structures are also found in nature in the iridescent, almost metallic colours of beetles and other insects, which result from helical cholesteric structures in the outer layer of the carapace. 

Finkelmann, H., Koldehoff, J., Ringsdorf, H. 1979. Synthesis and Characterization of Liquid-Crystalline Polymers with Cholesteric Phase. Angew. Chem. Int. Ed. Engl. 17, 935... 

Fig. 12.25. Structure of liquid crystalline polymers showing (a) nematic, (b) smectic, and (c) cholesteric orders.

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... 

There have been many papers concerning liquid-crystalline polymers having azobenzenes in their side chains (Figure 14) because of the mesogenic nature of the azo chromophore. Liquid-crystalline polymers incorporating azobenzene moieties in their main chains (30) have also been prepared.102 A family of poly(acrylates) substituted with azobenzenes having chiral alkyl groups (31) showed smectic A as well as cholesteric phases.103 The photochromic behavior... 

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). 

Because of the additional translational order, the dislocations can exist in the cholesteric and smectic liquid crystals, which makes the texture of these liquid crystals even more complicated. Each liquid crystal phase shows characteristic textures and thus the optical texture becomes an important means to differentiate the phase of the liquid crystals. Liquid crystalline polymers have the same topologically stable defects as small molecular mass liquid crystals do, but the textures may be different due to the difference in the energetic stability of the same topological defects in both low molecular mass and polymeric liquid crystals (Kleman, 1991). In Chapter 3 we will discuss the textures in detail. 

In the above applications, cholesteric liquid crystals need to be sealed between two glass plates or in the form of micro encapsulates. However, cholesteric liquid crystalline polymers can easily form thin films or be coated on substrates. 

To form cholesteric liquid crystalline polymers, one either polymerizes cholesteric monomers or mixes low molecular mass cholesteric liquid crystals with polymers. In the latter case, two components may be mixed homogeneously or in such a way that the polymers act as a matrix while the small molecular mass cholesteric liquid crystals are in droplets, known as the polymer-dispersed liquid crystals (PDLC) (Doane et al., 1988) or the nematic curvilinear aligned phase (NCAP) (Fergason, 1985). In addition, there are many polymers in nature exhibiting the cholesteric phase such as PBLG, cellulose, DNA, etc. 

Other important lyotropic cholesteric liquid crystalline polymer are the cellulose esters. Cellulose is a natural polymer. Their chemical formula is shown in Figure 6.24. 

It is worth pointing out that the existence of cholesteric liquid crystalline polymers are far more common than one can imagine. 

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. 

Main chain cholesteric liquid crystalline polymers... 

In main chain cholesteric liquid crystalline polymers, the mesogenic groups and flexible spacers are linked alternatively. The flexible units contain asymmetrical carbon atoms which enable the polymers to possess chirality and thus form cholesteric liquid crystals. By varying the ratio of chiral to non-chiral parts, the cholesteric temperature range and pitch can be changed. The cholesteric range depends on the mol fraction of the polymers. A typical main chain cholesteric liquid crystalline polymer is shown in Figure 6.27. 

D-3-methyl-glyoxal is a common chiral spacer. In addition, D-butane-1, 3-diphenol, L-propane-1, 2-diphenol, and their dimers and trimers are used as the chiral spacers in cholesteric liquid crystalline polymers as well. 

Figure 6.27. A typical main chain cholesteric liquid crystalline polymer. (From Watanabe and Krigbaum, 1985.)...

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). 

High viscosity is the main drawback for cholesteric liquid crystalline polymers in applications. However, cholesteric polymers have their advantages. They may exhibit the memory effect. These polymers make processing and handling easily. Therefore, cholesteric liquid crystalline polymers have become the new materials for optical filters, temperature indicators, etc. The Merck product with the trade name Transmax is actually associated with the cholesteric liquid crystalline polymers. 

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

On the other hand, liquid crystalline polymers applied to optical information storage has attracted great attention. The liquid crystalline polymer is applied mainly in terms of the thermo-optical effect. The backbone of liquid crystalline polymer can be polysiloxane, polyacrylate, or polyesters. In order to enhance the absorption coefficient for the writing laser beam, the dyes may be either dissolved into the liquid crystalline polymer in the guest-host model or attached to the backbone of the liquid crystalline polymer to form a copolymer. The nematic, cholesteric and smectic liquid crystalline polymers are all be able to be utilized in optical information storage. 

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