Electrooptical properties of mesogenic chain molecules in solution and in nematic state

Electrooptical characteristics of mesogenic chain molecules in solution and in mesophase can be described in terms of intra‐ and intermolecular orientational orders. The value and sign of electric birefringence δn in a solution of kinetically rigid chain molecules are determined by the combination of two factors: intramolecular orientational order which depends on the dipolar and anisotropic architecture of the molecule and intermolecular orientational order caused by the action of the external electric field E. The value and sign of the dielectric anisotropy δε of the polymer nematic phase are also determined by the combination of intra‐ and intermolecular orders. However, in this case the latter is not maintained by the external field but by the nematic potential of the mesophase. Therefore, comparative investigations of electrooptical properties of polymers in solutions and in nematic melts make it possible to obtain information about the intra‐ and intermolecular orientational orders of the molecules under investigation in these two states. These investigations were carried out using the method of electric birefringence in solutions and the method of orientational deformations of nematic textures in an electric field. The objects being investigated were nematogenic dimers and trimers. Experimental data obtained for these compounds showed the presence of intramolecular order in their molecules, which is manifested in the odd‐even oscillations of the value and sign of Kerr constant K≈δn/E 2 in solution and δε in the nematic phase when the number of C‐C bonds in the methylene spacers of these molecules is varied. This effect is particularly dramatic in the mesophase where it is enhanced by intermolecular nematic potential.