NMR Studies of Self-Diffusion in Some Homologous Nematic Liquid Crystals

Abstract

After briefly reviewing the alternatives to measure diffusion in liquid crystals the paper sketches the principles and some recent refinements of the NMR field gradient (FG) technique, which allows to study mass transport very directly without the necessity to use tracer or dye molecules. In NMR FG experiments on mesophases difficulties arise from the fact that for anisotropic liquids the dipolar nuclear interaction is much less averaged than in isotropic liquids, so that the standard method is generally not applicable. As an alternative to external multi-pulse averaging we have developed a fast field cycling procedure that considerably reduces the effective dipolar coupling by a magic-angle orientation of the magnetic field relative to the director axis of the sample material. Combined with familiar pulsed field gradient (PFG) sequences this cycling made possible extensive measurements of the self-diffusion constants in nematic liquid crystals with values as low as 10⁻¹cm² s⁻¹. Results are presented for some homologues of the PAA and MBBA series. They show the expected odd-even effects as a function of chain carbon number, whereas most other findings (Arrhenius-type temperature dependence, a-nomal discontinuity at the nematic-isotropic phase transition, chain length independence of the anisotropy ratio) are not in accordance with existing theories.