Molecular dynamics of the liquid crystal 6O2OCB in nanopores

Abstract

The complex dielectric permittivity of 4-(2-hexyloxyethoxy)4′-cyanobiphenyl(6O2OCB) enclosed in porous samples of ZrO2 has been studied in the frequencyrange 10−1–106 Hz and over a wide temperature range. For the substance in thenearly spherical pores of ZrO2 no phase transition has been found, while for thesubstance in the bulk a monotropic system of phases, with smectic and crystallinephases, was determined. The relaxation spectra observed for the liquid phase of6O2OCB in porous material are rather complex and they have been separatedinto three parts using sums of Havriliak–Negami equations. Two lower-frequencyrelaxations have been ascribed to collective and ionic motions. Our interest isin the high-frequency relaxation connected with the reorientations ofmolecules around the short axes. That process was the only one found inthe pure 6O2OCB substance. In both cases the temperature changes ofthat relaxation were found to conform with the Vogel–Fulcher–Tammannformula for the average relaxation time. The relaxation times of molecules inpores are smaller than in the bulk. In pores the substance survives in theisotropic liquid phase to lower temperatures before the glass transitionoccurs. For the substance enclosed in pores the thermal hystereses of thedielectric permittivity and the relaxation times were found and discussed.