Time scale of short range order fluctuation in the isotropic phase of butylcyano-phenylcyclohexane liquid crystal

The NMR spin-lattice relaxation time at three Larmor frequencies - v(L) = 28 MHz, 1.235 MHz and 48 kHz - in the isotropic phase of butylcyano-phenylcyclohexane has been measured in the temperature range close to the isotropic-nematic phase transition. Spin-lattice relaxation mechanisms provided by both individual reorientational fluctuations and fluctuations of the local director can be distinguished at the different Larmor frequency scales. We conclude that the Landau-De Gennes theory for the weak first order nematic-isotropic phase transition, together the low frequency approximation can be applied if the experiment is carried out at Larmor frequencies in the kHz range. (C) 2000 Elsevier Science B.V. All rights reserved.