Orientation of thin liquid crystal films on buffed polyimide alignment layers: A near-edge x-ray absorption fine structure investigation

Near-edge x-ray absorption fine structure investigations were performed on thin films of the liquid crystal n-octyl-cyanobiphenyl (8CB) evaporated onto buffed PMDA-ODA polyimide alignment layers and onto highly oriented pyrolytic graphite. The tensor nematic order parameter was derived from the x-ray dichroism as a function of layer thickness and thermal history. The liquid crystal orientation on buffed polyimide is markedly different from that observed on graphite. While on graphite the aromatic rings are oriented parallel to the substrate, there is—on average—a cylindrical symmetry around the molecular main axes for the orientational distribution function of 8CB on polyimide. The main axes are oriented parallel to the substrate in both cases. A brief heat treatment induces desorption of most of the LC material, leaving only the most tightly bound molecules behind. On graphite the remaining 8CB layer is highly oriented, whereas annealing does not significantly change the orientational parameters on polyimide. LC films adsorbed on rubbed polyimide reveal an alignment of the molecular main axes with the rubbing direction.