A DEUTERIUM MAGNETIC RESONANCE STUDY OF BIAXIAL ORDERING AND SELF-DIFFUSION IN CHIRAL NEMATIC AND SMECTIC PHASES (NMR).

摘要

Deuterium NMR spectral patterns from cholesteric liquid crystals (twisted nematics) for the case where the pitch axis aligns perpendicular to the direction of the magnetic field are evaluated in terms of three quantities or effects: (1) An asymmetry parameter in the quadrupole interaction which has been motionally induced by biaxial ordering of the molecules; (2) a diffusion constant for migration of the molecules along the pitch axis; and (3) distortion of the cholesteric helix in the presence of an applied magnetic field. Calculated spectral patterns which include the above effects are fitted to patterns obtained experimentally from selectively deuterated nematics twisted by the addition of chiral compounds. Values of the asymmetry parameter and the diffusion coefficient constant are obtained directly from the experimental spectral patterns or from the calculated fittings. Because of the suitability of the experimental configuration (pitch axis perpendicular to the applied magnetic field), several new features of the cholesteric phase were obtained and studied: (1) The molecules are biaxially ordered in a cholesteric (twisted nematic) phase. This ordering is expressed by an asymmetry parameter, (eta)('lc), in ('2)H-NMR experiments. (2) The asymmetry parameter, (eta)('lc), increases with decreasing pitch and near the clearing point. (3) The diffusion coefficient along the pitch axis in an undistorted cholesteric decreases with decreasing pitch. (4) A space modulation of the diffusion coefficient along the pitch axis in a cholesteric phase distorted under the action of a magnetic field is observed. (5) The mechanism of biaxial ordering in cholesterics is primarily caused by anisotropic fluctuations of the long molecular axis with respect to the transverse directors. The less ordered axis is the pitch axis in that the fluctuations are stronger with respect to this axis. (6) In all the Blue Phase mixtures investigated, the BP appears at the same value of the asymmetry parameter, (eta)('lc) (DBLTURN) 0.35, suggesting that the biaxial ordering in cholesterics can play an important role in the formation of the mysterious BP.;Finally, unique NMR spectral patterns were also observed from biaxially ordered smectic phases. The asymmetry parameter and the distribution of the director were studied in the smectic H and smectic E phases.