Glassy dynamics of liquid crystalline 4′-n-pentyl-4-cyanobiphenyl in the isotropic and supercooled nematic phases

Abstract

The temperature evolution of dielectric relaxation times (tau(T)) in the isotropic (I) and supercooled nematic (N) phases of 4(')-n-pentyl-4-cyanobiphenyl is discussed. This compound may be considered as the classical model-material for the physics of liquid crystals and recently also for the glass transition physics. Experimental results extends from approximately T(I-N)+100 K down to T(g) approximately T(I-N)-100 K, where T(I-N) denotes the I-N phase transition temperature. The supporting derivative-based analysis enabled the distortion-sensitive tests of the validity of tau(T) parametrization using the Vogel-Fulcher-Tammann, Avramov model, and "critical-like" equations. Only the latter yielded the possibility of portraying tau(T) data using a single equation for the isotropic phase within the mode coupling theory and for the supercooled nematic phase within the dynamical scaling model. Issues related to the fragility of the system are also discussed. The existence of the hidden N-SmA transition, damped due to the earlier vitrification at the glass temperature (T(g)), is speculatively suggested. For the isotropic phase the supplementary discussion of dynamics in 4(')-n-butyl-4-cyanobiphenyl is presented.