Proton magnetic relaxation study of pretransitional phenomena in the isotropic phase of a nematic liquid crystal I. Dynamics of local order fluctuations

Nuclear magnetic relaxation was investigated in a broad temperature region above the clearing point (T-c) of a nematic liquid crystal. Dependence of s pin-spin relaxation time on the pulse interval observed in the Curr-Purcell -Meiboom-Gill (CPMG) experiment indicates an exchange of nuclei between the states differing in local magnetic fields. By fitting of the Luz-Meiboom e quation to the CPMG results, the mean lifetime of sites and modulation freq uency delta omega were determined. The rather slow exchange (lifetimes chan ging with temperature in the range 20-90 ms) is suggested as manifesting th e local order fluctuations in the pretransitional zone of the LC. A simple two-site model of a pretransitional zone was considered (clusters double le ft right arrow isotropic surrounding). Dipole-dipole interactions in cluste rs are unaveraged due to the local ordering, whereas in the isotropic subph ase local magnetic fields are motionally averaged. Therefore, local order f luctuations are accompanied by the exchange observed in the CPMG sequence. Correspondence of the temperature dependence of delta omega to the Curie-We iss law was established: delta omega(-2) proportional to T - T*, (T* = Tc - 1), thus providing proof of our interpretation.