Complex dynamics of isotropic 4-cyano-4-n-pentylbiphenyl (5CB) in linear and nonlinear dielectric relaxation studies.

Abstract

A comprehensive presentation of the temperature evolution of "linear" and "nonlinear" dielectric relaxation in the isotropic phase of nematic liquid crystalline compound 5CB (4-cyano-4-n-pentylbiphenyl) is given. The "nonlinear" relaxation is related to the strong pretransitional rise in the lifetime of prenematic fluctuations. The "linear" relaxation has a clear non-Debye and non-Arrhenius form. In the immediate vicinity of the nematic clearing point it shows a weak pretransitional anomaly. Results obtained coincide with the complex liquid relaxation pattern found in transient grating optical Kerr effect studies [A. Sengupta and M. D. Fayer, J. Chem. Phys. 102, 4193 (1995); R. Torre et al., Philos. Mag. A 77, 645 (1997)]. The striking similarity to the behavior found in critical, binary mixtures suggests the extension of the "fluidlike" hypothesis for the isotropic-nematic transition to dynamic phenomena in the isotropic phase [P. K. Mukherjee. J. Phys.: Condens. Matter 10, 9191 (1998)]. The presence of both glassy and fluidlike features in isotropic 5CB coincides with the recent results of simulation analysis for the system of hard ellipsoids by Latz et al. [Phys. Rev. E 62, 5173 (2000)] and with the novel general picture for liquid-liquid transitions proposed by Tanaka [Phys. Rev. E 62, 6968 (2000)].