2H-NMR Study of Orientational Order in Binary Mixtures: A Nematic Phase of Biaxial Molecules

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Abstract
Deuteron Magnetic Resonance (2H-NMR) spectra have been obtained from a binary mixture of 2-fluorenyl-4′-tetradecyloxy benzoate-d 9 (FLOC14) and para-Xylene-d 10 (p-Xy), as a function of temperature. The liquid crystal, FLOC14, is selectively deuterated on the rigid fluorene moiety while the p-Xy is perdeuterated. The quadrupolar splittings, v k Q allow the unambiguous determination of the temperature dependence of the orientational order parameters, S izz and (S ixx — S iyy), of both species, i = 1,2. These results are the first to provide information on the degree and asymmetry of orientational order, of both biaxial molecules, in the uniaxial nematic phase of a binary mixture. A new approach was adopted for analyzing the experimental data which uses a least squares fit of the quadrupolar spslittings, v k Q, to provide the four component order parameters, S 1zz, (S 1xx - S 1yy), S 2zz and (S 2xx - S 2yy). The experimental results are interpreted using a recently proposed mean field theory of binary mixtures of biaxial molecules.1 In the geometric mean approximation all particles couple to an identical mean field and the parameters of the theory are the ratios of intermolecular interaction strengths of the pure materials. There is good agreement between the predictions of theory and the four component order parameters determined from 2H-NMR experiments, over the entire temperature range studied. Thus this description is capable of predicting the temperature dependence and interrelation of the nematic orientational order parameters for both components of the mixture. Analogous calculations on data from pure FLOC14 2 gives a ratio of interaction strengths consistent with that obtained from the binary mixture.