Helix induction force

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. 

The helix induction force for the series of copolymers examined coincides with the corresponding value for a mixture of low-molecular-weight liquid crystals—p-butoxybenzylidene-p -butylaniline with cholesteryl pnq)ionate—where there are no specific interactions in the nematic matrix, and the chirality is caused by the cholesterol fragment... 

The helix induction force also changes in substitution of the nematic matrix. The helix induction force is lower in matrices which exhibit smectogenic properties the copolymers in series 10 and 11 with phenylbenzoate groups and... 

These data show that the strong side interactions between mesogenic groups, the cause of the aj earance of smectogenic properties, impede helical twisting in these systems. The dependences of on shown in Fig. 7.14 for mixtures of cholesteryl propionate with a cyano-containing nematic liquid crystal, p-butoxybenzylidene-p -aminobenzonitrile [6], indicate that the values of the helix induction force for Ais system and the copolymers in series 10 and 12 coincide. 

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