Dielectric relaxation studies of 4-n-Alkyloxy-4′-Cyanobiphenyls (nOCB, n = 5 ÷ 8)

Abstract

The results of dielectric relaxation studies of series of four n-alkyloxy cyanobiphenyl (nOCB) compounds with n ranging from 5 to 8 are presented. The complex dielectric permittivity, ε*(ω)=ε′(ω)-i ε″(ω), was measured in the frequency range 1 kHz-3 GHz, with the use of an impedance analyzer and a time domain spectroscopy (TDS) set-up. The relaxation times: longitudinal τ∥ in the nematic and smectic A phase (8 OCB), transverse τ⊥ in the nematic phase, and τis in the isotropic phase, were obtained as functions of the temperature. This allowed the calculation of activation barriers for different molecular relaxation processes occurring in particular phases. Moreover, the retardation factors g∥ and g⊥ and the nematic potential q according to two theoretical models were calculated. The analysis of q(T) and its relation to the order parameter S is done taking into account the results of NMR studies of the compounds under consideration. Finally, the dependencies of the obtained quantities on the number of carbon atoms in the alkyloxy chains are analyzed.