Anomalous Decoupling of the dc Conductivity and the Structural Relaxation Time in the Isotropic Phase of a Rod-Like Liquid Crystalline Compound

Abstract

Recently, the isotropic phase of rod-like liquid crystalline compounds is advised as an experimental model system for studying complex glassy dynamics. One of unique phenomena occuring close to the glass temperature, for the time scale 10−7±1 s <τ < τ (T g ) ≈ 100s, is the fractional Debye-Stokes-Einstein (FDSE) behaviour στ = const with S<1, i.e. the coupling of dc conductivity (σ, translational processes) and dielectric (structural) relaxation time (τ, orientational processes). It is shown that this relation may be found also in the isotropic phase of nematic liquid crystalline compounds n-pentylcyanobiphenyl (5CB), although surprisingly for time-scales τ<10−8 s. The application of the derivative based analysis revealed a change of S on cooling towards the isotropic — nematic transition. The optimal description of the evolution of relaxation time and conductivity by the mode-coupling theory (MCT) dependence in the isotropic phase is shown: τ(t) ∞ (T−T Mct ) and σ (T) ∞ (T-T MCT ), where T MCT ≈T I−N −33K and T > T I−N is shown. The link between this behavior and the FDSE is suggested, namely: S = ϕ′/ϕ. Finally the call for further pressure studies is formulated.