Probing director fluctuations in liquid crystals. 2H relaxation dispersion of deuterated toluene and p-xylene

Abstract

Deuterium spin-lattice relaxation rates in toluene-d8 and p-xylene-d10, dissolved in nematic Phase V, have been measured at five different field strengths. The frequency-dependent relaxation behavior can be satisfactorily described by a model in which the molecules undergo small step rotational diffusion in an axially symmetric orienting potential, with contributions from slow director fluctuations. The parameters defining the model are fitted to the relaxation rates of para and methyl deuterons, using a least-squares parameter estimation program. Parameter values obtained are consistent with a physically acceptable set of values of the associated hydrodynamic properties of the solutions, and are in agreement with results obtained previously on 14N relaxation in methylisocyanide. Relaxation rates of ortho and meta deuterons in toluene, and of the ring deuterons in p-xylene lead to a J1/J2 ratio smaller than unity. Moreover, both J1 and J2 are weakly frequency dependent. The former effect may turn up as a consequence of the assumption of axially symmetric diffusion; the frequency dependence of J2 is probably due to second order effects of the director fluctuations.