Dielectric study of SA1, SA2 and SÃ phases in liquid crystals of cyano compounds

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Abstract
In this paper, the authors present a dielectric study of the monolayer (SA1) and bilayer (SA2) smectic phases and of the smectic antiphase (SÃ) for a mixture of 4-cyanobenzoyloxy-4'-pentyl stilbene (C5 stilbene for short) and 4-n-hexylphenyl-4'-cyanobenzoyloxy benzoate (DB6 for short) and for the pure compound DB6. For the measurement with the direction of the electric field E parallel to the director n, in the S A2 phase, the dielectric spectra observed reveal : — a decrease of ε' ||, — an increase of the activation energy of the molecular reorientation mechanism for DB6, — the appearance of a second absorption domain for the mixture, connected either to a cooperative phenomenon or to the existence of molecules of different types. For E ⟩ n, the study shows mainly an increase of ε'⟩ at the transitions N-S Ã and N-SA2. These results suggest that there are strong correlations between the molecular motions, due to the presence of strong interactions, particularly in the SA2 phase. These correlations are probably due, for the compounds studied, to the existence in the molecule of groups with dipolar moments whose transverse components are important