Alignment Properties of Rubbed Polymer Surfaces

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Abstract
The orientation of liquid crystal (LC) molecules on rubbed polymer surfaces, characterized by the tilt bias angle, is found to be determined by the physical and chemical properties of the polymer and liquid crystal material as well as by the coating and rubbing technique used. We compare different rubbing configurations with respect to their rubbing work, the resulting plastic deformation of the surface and the homogeneity of the LC alignment. The coefficient of friction is found to be a function of the rubbing speed and rubbing load. The effect of the rubbing work applied to polymercoated substrates on the tilt bias angle is shown for Polyimide, Polyamide and Polyamide-imide materials. Extremely small tilt bias angles could be realized with bidirectional rubbing or a teflon coated rubbing material, whereas high tilt bias angles (30[ddot]) could be achieved by adequate combination of rubbed polymer substrates and surfactants used for homeotropic orientation. On identically treated polymer coated substrates the tilt bias angle increases with the dielectric anisotropy of the liquid crystal material. The experimentally observed increase of the relaxation times with increasing rubbing work is compared to numerical calculations in order to study the effect of tilt bias angle, surface coupling elasticity and surface viscosity on the relaxation.