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Free keywords:
Smectic liquid crystal; 8CB; Deuterium NMR; Nematic liquid crystal; Director dynamics; Rotational viscosity
Abstract:
Deuterium NMR spectroscopy has been used to investigate the director dynamics in the smectic A phase of perdeuteriated 4-octyl-4′-cyanobiphenyl-d25 (8CB-d25), where the director was aligned by an electric field. The electric field direction made an angle of 44.7° with the magnetic field in order to provide a unique alignment pathway; accordingly the director is expected to rotate as a monodomain. When the electric field is applied to the nematic film, the director moves from being parallel to the magnetic field to being at an angle with respect to it. After the electric field is switched off, the director relaxes back to being parallel to the magnetic field. Deuterium NMR spectra were recorded during the turn-on and the turn-off alignment processes as a function of time. This particular technique was chosen because the spectral peaks associated with each rigid group in the molecule are clearly resolved and of comparable intensity. For all of the experiments at different temperatures in the smectic A phase of 8CB-d25 we found that the relaxation times are independent of the group used to determine the director orientation during the alignment process as same as the results for the nematic phase. It also was found that the relaxation time in the smectic A phase is about 1,000,000 times as large as that in the nematic phase. This results in a strong pretransitional growth of the relaxation time as the transition to the smectic A phase is approached.
