INFLUENCE OF LOCAL LIQUID STRUCTURE ON ORIENTATIONAL DYNAMICS - ISOTROPIC-PHASE OF LIQUID-CRYSTALS

Measurements of orientational relaxation over 6 decades in time (femto seconds to nanoseconds) and 10 decades of amplitude are reported for t he liquid crystal N-(methoxybenzylidene)butylaniline (MBBA) in its iso tropic phase. The measurements were made using the transient grating o ptical Kerr effect method. The faster dynamics (picoseconds to nanosec onds) display a power law decay that is temperature independent up to 43 deg above the nematic-isotropic phase transition. The slower dynami cs (tens of nanoseconds) obey the Landau-de Gennes (LdG) modified Deby e-Stokes-Einstein hydrodynamic equation. The faster dynamics become te mperature dependent at the same temperature that the slower dynamics b egin to deviate from LdG behavior. The onset of temperature dependence and the deviation from LdG behavior occur when the size of the pseudo nematic domain, defined in terms of the correlation length xi, falls b elow 3xi0. The temperature-independent dynamics is attributed to the l ocal nematic structure, which exists on short time and distance scales . The temperature-independent orientational dynamics occur by relaxati on of the local nematic structure back to a momentary minimum of the l ocal free energy surface, rather than by diffusive orientational rando mization. Two types of processes are discussed that can give rise to t he power law decay: a parallel process (Forster direct transfer model) and a serial process (hierarchically constrained dynamics model). The MBBA experimental results are compared to earlier work on the liquid crystal pentylcyanobiphenyl. Remarkably, the same power law decay expo nent (-0.63) and the same correlation length for the onset of deviatio n from LdG behavior (is-approximately-equal-to 3xi0) are seen in both liquid crystals.