Dielectric studies of a 5-n-alkyl-2-(4'-isothiocyanatophenyl)- 1,3-dioxane (nDBT) homologous series (n=4-10)

Abstract

The results of dielectric studies of seven members of a 5-n-alkyl-2-(4'-isothiocyanatophenyl)- 1,3-dioxane (nDBT) homologous series (n=4-10) in the isotropic and smectic A phases are presented. The complex dielectric permittivity, epsilon* (nu) = epsilon'(nu) - iepsilon'' (nu), was measured with the aid of two experimental set-ups: an impedance analyser (10kHz-13 MHz) and a dielectric time domain spectrometer (TDS, 10 MHz-4 GHz). This allowed two main relaxation processes in both the phases studied to be separated: the low frequency (l.f.) process connected with molecular reorientations around the short axes, and the high frequency (h.f.) process connected with the rotations around the long axes. The measured dielectric increments enabled us to estimate the value and direction of the dipole moment of the nDBT molecules. The l.f. relaxation process in the isotropic and smectic A phases of the nDBT compounds exhibits some peculiar features which distinguish the materials from other similar substances. The observed decrease of the relaxation times and activation enthalpy with increasing n is discussed in relation to the molecular arrangements in the smectic layers.