Comparison of the dielectric properties of 4-(2-methylbutyl)-4′-cyanobiphenyl (5*CB) and 4-pentyl-4′-cyanobiphenyl (5CB) in the liquid state

Abstract

Dielectric relaxation studies of chiral 4-(2-methylbutyl)-4′-cyanobiphenyl (5*CB) in the liquid state at temperatures up to ca. 90 K above the melting point have been performed. Two experimental techniques were used: time domain spectroscopy (TDS) and an impedance analyser, which together cover the frequency range from 10 kHz to ca. 3 GHz. The obtained relaxation times τ_is as a function of the temperature were compared with those obtained recently for the isotropic phase of 4-pentyl-4′-cyanobiphenyl (5CB), being a typical liquid crystalline substance. It was found that for 5*CB, unlike 5CB, the relaxation time τ_is(T) shows a non-Arrhenius behaviour in the range of ca. 70 K above the melting point. The τ_is(T) behaviour was analysed using the Vogel–Fulcher–Tammann (VFT) formula. The VFT curve fits very well the experimental points as well as those obtained recently by Massalska-Arodż etal. in the supercooled state of 5*CB. For both substances the static permittivity ε_s was measured up to 365 K and the Kirkwood correlation factor g=µ_app²/µ² was established. The temperature dependence of g-factors indicates that the antiparallel dipole–dipole associations are stronger for the chiral compound than for the non-chiral one.