A model describing the rubbing energy dependence of the pretilt angle in nematic liquid crystals

The dependence of the nematic liquid-crystal (LC) pretilt angle on the applied rubbing energy was measured for two commercially available polyimide alignment materials. The rubbed polyimide is described in terms of an oriented top layer and an isotropic bottom layer. The rubbing energy was quantified by the thickness of the top layer oriented by rubbing. This thickness was revealed by optical retardation measurements. A close correlation was found between the pretilt angle of the LC and the rubbing-oriented top layer thickness. The resulting anchoring properties are explained in terms of van-der-Waals interactions between the LC molecules and both the oriented polyimide top layer and the isotropic polyimide bottom layer. A prediction for the rubbing-energy dependence of the polar anchoring energy follows from the theory.