NMR-STUDY OF MOLECULAR-DYNAMICS IN A MIXTURE OF 2 POLAR LIQUID-CRYSTALS (CBOOA AND DOBCA)

A mixture containing 70 per cent 4-cyanobenzylidene-4'-n-octylaniline (CBOOA) and 30 per cent 4-n-dodeciloxybenzylidene-4'-cyanoaniline (DOBCA) has been studied by optical microscopy, differential scanning calorimetry, X-ray diffraction and, primarily, by NMR spectroscopy. The smectic phase of the mixture is a partial bilayer smectic A phase (S(Ad)), in which about one half of the molecules are associated in pairs through their polar cyano end groups. The molecular dynamics in the smectic A(d) phase of the mixture was studied by proton spin-lattice relaxation. Self-diffusion and rotations/reorientations were found to be essential relaxation mechanisms while the contribution of the order director fluctuations seems to be very small in all frequency regions. The correlation times associated with the molecular rotational motion around the short molecular axis, tau(S) are of the order 10(-10)s and the ratio tau(S)/tau(L) almost-equal-to 6.2. The translational diffusion coefficients D(perpendicular-to) are of the order 10(-11) m2 s-1. The possible contribution to the relaxation rate of an additional relaxation mechanism associated with the dissociation and recombination of molecules in dimers within the smectic layers is also analysed.