Cholesteric copolymers

The main feature identifying a cholesteric mesophase in polymers is the presence of optical texture with selective circularly-polarized light reflection. This indicates the formation of 1-helical cholesteric structure in LC copolymers. The X-ray patterns of actually all cholesteric copolymers described (with the exclusion of polymers 3.1 and 4.1, Table 13) correspond to those of nematic and cholesteric low-molecular liquid crystals, which is manifested in a single diffuse reflex at wide scattering angles. At the same time, for copolymers 3.1 and 4.1 (Table 13) small angle reflexes were observed 123), that are usually missing in low-molecular cholesterics. 

As is seen from Table 13, cholesteric copolymers display a maximum of selective light reflection ( w) in an IR- or a visible part of the spectrum. By varying the composition of a copolymer, it is possible to vary Xmax, in accordance with the stipulation max = nP, is proportional to the pitch P of the helical structure of a LC polymer (n — is the refractive index). The pitch of the helix in cholesteric copolymers is usually decreased, when the temperature is raised 105) (at temperatures above Tg), which is equally common for low-molecular cholesterics142) (Fig. 23a). The observed fact that the helix pitch for LC copolymers 2.1-2.3 (Table 13, Fig. 23b) is increased, is rather unusual but explicable within the theoretical views regarding vibrational movement of macromolecular fragments and their conformational mobility 60). 

Figure 6.29. The side chain cholesteric copolymers. (From Freidzon et al., 1980.)...

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

The experimental results presented demonstrate that the optical properties of cholesteric copolymers are determined mainly by the nature of a nematogenic monomer. 

Copolymerization of chiral mesogenic monomers with nematogenic monomers makes it possible to obtain cholesteric copolymers in a very broad, almost unlimited range of concentrations of the different units. The greatest attention in the literature has been focused on the study of the structure and optical properties of these copolymers. 

Effect of the composition of cholesteric copolymers on the selective reflection of light. The concept of the twisting force of the chiral additive, or helix-induction force (A), expressed by the following ratio, is usually used in studying the optical properties of the induced cholesteric mesophase in low-molecular-weight nematic-cholesteric mixtures ... 

Let us examine what the helix induction force is equal to in cholesteric copolymers. For all of the copolymers listed in Table 7.5, the pitch of the cholesteric helix decreases with an increase in the concentration of the cholesterol-containing units, and selective reflection of visible light which has left-handed circular polarization is observed for a given concentration of these units. The transmission spectra of some copolymers at the temperature of T = 0.99T l are shown in Fig. 7.13. The optical properties of the different copolymers were compared at this temperature, since it is believed that the orientational order parameter, which fects the angle of twisting of the molecules in the system, has approximately the same value as for low-molecular-weight liquid crystals at a temperature close to the transition into the isotropic state. 

Anisotropy of the dielectric constant is the basic cause of the structural transformations which take place in an electric field in cholesteric copolymers. The copolymers... 

Fig. 8.5. Change in the selective reflection wavelength (1) and optical transparency (2) of cholesteric copolymer DC in an electric field (the direction of the axis of the cholesteric helix is indicated by the arrow) [13].

For the synthesis of the biaxial cholesteric copolymer, the side-on 6 polymer from the NMR study (see above) and cholesterylcarbonate in a concentration range from 1 to 10% were used. 

Conoscopy provides an extremely sensitive method with which to determine the degree of biaxiality. By the early 1990 s, conoscopic measurements had already indicated the presence of phase biaxiality in a nematic side-on liquid crystalline side-chain polymer [9]. However, the method s sensitivity is also its weak point because surface effects may induce optical biaxiality in an actual uniaxial system. For this reason, deuterium NMR was used to confirm phase biaxiality in a liquid crystalline polymer system similar to the one investigated with conoscopy by Leube [11-13]. Due to the fairly high viscosity of the polymeric samples, the tilt experiment, employed by Yu and Saupe to show phase biaxiality in a lyotropic liquid crystal [4], was used. The results obtained in this way are in good agreement with observations of optical textures in a biaxial cholesteric copolymer [16], where phase biaxiality disturbs the smooth optical periodicity of the cholesteric phase structure. 

The procedure based on the copolymerization of a cholesterogenic monomer with a mixture of nematogenic comonomers was, for the first time, introduced by Strzelecki and Liebert in the synthesis of crosslinked polymers that retained frozen cholesteric phases with rather extended helical pitches (1500-4400 nm). Five years later, Ringsdorf and co-workers reported the first preparation of an enantiotropic cholesteric copolymer. This... 

Most of the reported cholesteric copolymers spontaneously develop planar textures which reflect visible light. As a general remark, it is important to note, from an applicative point of view, that amorphous polymeric materials, that do not possess a smectic phase at low temperatures, can preserve their cholesteric structure in the glassy state. 

Another possibility is offered by introducing chirality into the molecular structure. Copolymerization or copolycondensation of a monomer, capable of forming a thermotropic nematic homopolymer, with a chiral compound yields cholesteric copolymers. 5,25,32,41 -47 optical properties of these cholesteric copolymers resemble those of conventional cholesteric compounds. ... 

---