Influence of Local Liquid Structure On Orientational Dynamics - Isotropic-phase of Liquid-crystals

Measurements of orientational relaxation over 6 decades in time (femtoseconds to nanoseconds) and 10 decades of amplitude are reported for the liquid crystal N-(methoxybenzylidene)butylaniline (MBBA) in its isotropic phase. The measurements were made using the transient grating optical Kerr effect method. The faster dynamics (picoseconds to nanoseconds) display a power law decay that is temperature independent up to 43 deg above the nematic-isotropic phase transition. The slower dynamics (tens of nanoseconds) obey the Landau-de Gennes (LdG) modified Debye-Stokes-Einstein hydrodynamic equation. The faster dynamics become temperature dependent at the same temperature that the slower dynamics begin to deviate from LdG behavior. The onset of temperature dependence and the deviation from LdG behavior occur when the size of the pseudonematic domain, defined in terms of the correlation length xi, falls below 3xi0. The temperature-independent dynamics is attributed to the local nematic structure, which exists on short time and distance scales. The temperature-independent orientational dynamics occur by relaxation of the local nematic structure back to a momentary minimum of the local free energy surface, rather than by diffusive orientational randomization. Two types of processes are discussed that can give rise to the 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 exponent (-0.63) and the same correlation length for the onset of deviation from LdG behavior (is-approximately-equal-to 3xi0) are seen in both liquid crystals.