Molecular dynamics in the liquid crystal 40.6 A nuclear magnetic relaxation study

Abstract

A nuclear spin lattice relaxation study of the liquid crystal butoxybenzylidene hexylaniline (40.6) has been carried out as a function of Larmor frequency (4 to 30 MHz) and temperature covering all its mesophases (N, S_A and S_B), with a view to obtain information on different dynamic processes at the molecular level. These studies are also supplemented by a nuclear dipolar relaxation study as well as anisotropy of these relaxation rates with respect to the static magnetic field. The analysis of this data in nematic and smectic A phases based on a composite model, including the contributions from self diffusion (SD), order director fluctuations (ODF), and molecular reorientations (R), indicate that the nuclear spin relaxation is effectively mediated by diffusion and reorientation processes at frequencies above 10 MHz while at lower frequencies ODF also seem to contribute appreciably. The relative contribution from these mechanisms at different temperatures covering nematic and S_A phases are evaluated, and relevant dynamic parameters are calculated. The onset of the S_B phase is marked by sudden increase in T ₁ and subsequent frequency independence of this data at lower temperatures. The dipolar relaxation data show a dramatic decrease at this transition temperature. These observations are attributed to a sudden slowing down of diffusion in the S_B phase.