Ultraweak azimuthal anchoring of a nematic liquid crystal on a planar orienting photopolymer

The search of weak anchoring is an important issue for a whole class of liquid crystal displays. In this paper we present an orienting layer showing unreached weak planar azimuthal anchoring for 4-$n$-pentyl-$4′$-cyanobiphenyl nematic liquid crystal (5CB). Azimuthal extrapolation lengths as large as $80μm$ are easily obtained. Our layers are made with the commercial photocurable polymer Norland optical adhesive 60. The anisotropy of the film is induced by the adsorption of oriented liquid crystal molecules under a $2T$ magnetic field applied parallel to the surfaces. We use the width of surface $π$-walls and a high-field electro-optical method to measure, respectively, the azimuthal and the zenithal anchorings. The azimuthal anchoring is extremely sensitive to the ultraviolet (UV) dose and it also depends on the magnetic field application duration. On the opposite, the zenithal anchoring is only slightly sensitive to the preparation parameters. All these results are discussed in terms of the adsorption/desorption mechanisms of the liquid crystal molecules on the polymer layer and of the flexibility of the polymer network.