THE ORIENTATION-DEPENDENT INTERACTION ENERGY AS STUDIED BY THE POLARIZABILITY ANISOTROPY ANALYSIS

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Molecular asymmetry is a feature common to all liquid crystalline substances. Soft-tails or spacers joined with a mesogenic core tend to take extended conformations along the director of the field. The polarizability anisotropy of molecules causes the intermolecular London attractions to be anisotropic, with the results the parallel intermolecular alignment is favored. For regid rods, we can elucidate the anisotropic part of the polarizability tensor from the depolarization Rayleigh scattering (DRS) measurements. The same technique cannot be used for the flexible chains which orient only under the anisotropic environment. The polarizability anisotropy of these flexible chains may be calculated by assuming the bond-additivity rule along the chain, provided that the conformations permitted in the anisotropic state are known. As we have demonstrated in a separate work, the latter information can be obtained by using the deuterium NMR technique.In fiscal 1986, a commercial DRS apparatus, modified slightly in the filtering system, was installed in our laboratory. In the following year, the optical anisotropy measurements were carried out for a series of dipeptides. The limit of thereliability of the apparatus was thus tested. Measurements were also performed for some typical mesogenic compounds such as PAA and MBBA. Finally, the polarizability anisotropy of nCB, a compound comprising cyanobiphenyl core and n-alkyl tail, was estimated according to the procedure described above. Contribution from the flexible tail was found to be substantial. The results obtained for the system with n=5 and 8 are consistent with the odd-even effect observed in various thermodynamic properties at the nematic-isotropic transition temperature. The mothodology we have abopted in this work was thus proved to be useful.