Influence of Molecular Conformations on the Dielectric Relaxation Spectra of Liquid Crystals

Abstract

Dielectric relaxation spectra observed for liquid crystals cannot be satisfactorily explained in terms of models which assume rod like molecules. Our investigations show that the low-frequency relaxation is in most cases a Debye-type process. On the other hand, the microwave relaxation region exhibits a distribution of the relaxation times, especially, for temperatures close to the clearing point. One can explain this effect by assuming intramolecular transitions between the conformational states of molecules. It is seen from the dielectric investigations that the relaxation rate for the intramolecular process (τ⁻¹ _intra) fulfills the condition τ⁻¹ _⊥≥τ⁻¹ _intra ≧ where τ′⁻¹ _‖ and τ′_‖ are the relaxation times connected with the reorientations of the long and short axes, respectively. Our results also indicate that the molecules of such compounds as PAP, HOAB, and 7S5 have well defined conformations in the low temperature region of the nematic phases. For such conformations the moment of inertia tensors as well as the principal axes frames (p.a.f.) were found. These calculations enable us to analyze the dielectric data in terms of the Bauer model for molecular reorientations. The values of the energies and entropies connected with the two molecular motions were also obtained.