Dipole moment and mobility of molecules in nematic liquid crystals of the 4-n-butyl ester of [4′-n-hexyloxyphenyl] benzoic acid in the absence of external orienting fields

Abstract

Dielectric polarization was studied in a nematic liquid crystal of the 4-n-butyl ester of [4′-n-hexyloxyphenyl] benzoic acid (BE[HOP]BA) in the absence of external orienting fields in the isotropic and mesophase states in a frequency range of 10³–10⁷ Hz. In the isotropic melt, three regions of dielectric absorption of a relaxation origin were revealed. It is shown that two of them are related to a reorientation motion of individual molecules about the longitudinal axes (process I) and about the short axes (process II). Processes I and II have relaxation times τ ∼ 10⁻⁹ and 10⁻⁸ s and activation energies ΔU ∼ 16 and 23 kcal/mol, respectively; the energy of activation of process II in the liquid-crystal phase increases to ΔU ∼ 38 kcal/mol. In the isotropic melt, in addition to processes I and II, process III occurs in the low-frequency range, which is characterized by greater relaxation times τ ∼ 10⁻⁷ s and an activation energy ΔU ∼ 28 kcal/mol. In order to establish the nature of process III, temperature dependences of the dipole moments and Kirkwood correlation factor g were studied in both BE[HOP]BA phases. The magnitude of the Kirkwood factor in the isotropic phase of the BE[HOP]BA near the transition temperature from the mesomorphic state (g ∼ 0.88) indicates the retention of the orientational ordering of molecules of the low-molecular liquid crystal and the compensation of their dipole moments intrinsic to the liquid-crystal state. This circumstance suggests that the third process of the relaxation of dipole polarization is due to a cooperative mode of motion of molecules in mesophase nuclei in the isotropic melt.