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) = 2 8 MHz, 1.235 MHz and 48 kHz - in the isotropic phase of butylcyano-phenylcy clohexane has been measured in the temperature range close to the isotropic -nematic phase transition. Spin-lattice relaxation mechanisms provided by b oth individual reorientational fluctuations and fluctuations of the local d irector can be distinguished at the different Larmor frequency scales. We c onclude that the Landau-De Gennes theory for the weak first order nematic-i sotropic phase transition, together the low frequency approximation can be applied if the experiment is carried out at Larmor frequencies in the kHz r ange. (C) 2000 Elsevier Science B.V. All rights reserved.