Translational self diffusion in 4-n-octyloxy-4′-cyanobiphenyl (8OCB) exploited with a static field gradient 1H NMR diffusometry approach

Self-diffusion measurements along the phase director of the nematic and smectic A phase of the thermotropic liquid crystal 4-n-octyloxy-4'-cyanobiphenyl (8OCB) have been performed by 1 H fringe field NMR diffusometry in the static fringe field of a superconducting magnet. The method used proves well suited to a precise determination of the diffusion coefficients in partially oriented systems. The data obtained for DJ (self-diffusion along the phase director) are used to analyse the nematic to smectic A phase transition in terms of the theoretical model proposed by Volino and Dianoux. The results show that a weak nematic-smectic A phase transition has to be considered, with a progressive build up of the positional order (due to the increase of the potential barrier imposed by the smectic layers consolidation) as the temperature decreases.