Azimuthal anchoring energy at theinterface between a nematic liquid crystal and a PTFEsubstrate

The azimuthal anchoring energy of the nematic liquidcrystal 4-pentyl-4'-cyanobiphenil (5CB) on apoly(tetrafluoroethylene) (teflon, PTFE) film is measured forthe first time. The PTFE film is deposed using the Wittmann andSmith technique which consists on rubbing a bar of this polymeragainst a glass substrate at a controlled temperature andpressure. Measurements of the azimuthal anchoring energy aremade with a reflectometric technique which provides highaccuracy and sensitivity. The dependence of the azimuthalanchoring energy on temperature and on the rubbing pressure isinvestigated. The extrapolation length remains virtuallyconstant in the whole temperature range of the nematic phaseexcept for an increase of 25% close to the clearing temperature.The azimuthal anchoring energy is somewhat strong and increaseswith increasing the deposition pressure of PTFE. The observationof a relevant pre-transitional anisotropy of the reflectioncoefficients in the isotropic phase proves that the surfaceinteractions favor an excess of orientational order. Ageing ofthe anchoring energy and gliding of the easy axis areexperimentally observed. Both these phenomena suggest thepresence of an anisotropic adsorbed layer of nematic moleculeson the PTFE film.