Molecular Dynamics in a Partial Bilayer Smectic A Phase by Proton NMR Relaxation

Abstract

The dynamics of a partial bilayer smectic A phase is studied for the p-|(p-dodecyloxybenzylidene)–amino| benzonitrile by the NMR technique. The proton spin relaxation rate 1/T₁ has been measured as a function of the Larmor frequency between 5.6 and 84 MHz and as a function of the temperature for the whole domain of the smectic A phase. The relaxation measurements are discussed in terms of potential contributions of different molecular motions, namely translational self diffusion, rotations/reorientations and order director fluctuations. In the high frequency limit the relaxation is dominated by rotations/reorientations while translational self diffusion is dominant in the low frequency region. For all the frequencies of work it seems that order director fluctuations have a small contribution to the relaxation. This unsual result for a S_A phase must reflect the particular distribution of the polar asymmetric molecules in the smectic layers and also the molecular structure.