Azimuthal and zenithal anchoring of nematic liquid crystals

Temperature dependence of azimuthal and zenithal anchoring energy coefficients of the nematic liquid crystal 4-n-pentyl-4(')-cyanobiphenyl on rubbed nylon is measured using dynamic light scattering. The method is based on observations of director fluctuations in a planarly aligned wedge cell, where the anchoring energy coefficients can be obtained without any external torques acting on the liquid crystal during the measurement. We found that both anchoring coefficients decrease steadily on approaching the nematic-isotropic transition. Moreover, in the whole temperature range of the nematic phase, the ratio between the zenithal and the azimuthal anchoring coefficients is almost equal to the ratio between the splay and the twist Frank elastic constants. The same result is obtained also for the nematic phase of 4-n-octyl-4(')-cyanobiphenyl. This indicates that the aligning nylon layer directly affects only the monomolecular layer at the surface whereas the observed anchoring is governed by the elastic properties of the alkyl-cyanobiphenyl.