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 ont o buffed PMDA-ODA polyimide alignment layers and onto highly oriented pyrol ytic graphite. The tensor nematic order parameter was derived from the x-ra y dichroism as a function of layer thickness and thermal history. The liqui d crystal orientation on buffed polyimide is markedly different from that o bserved on graphite. While on graphite the aromatic rings are oriented para llel to the substrate, there is-on average-a cylindrical symmetry around th e molecular main axes for the orientational distribution function of 8CB on polyimide. The main axes are oriented parallel to the substrate in both ca ses. A brief heat treatment induces desorption of most of the LC material, leaving only the most tightly bound molecules behind. On graphite the remai ning 8CB layer is highly oriented, whereas annealing does not significantly change the orientational parameters on polyimide. LC films adsorbed on rub bed polyimide reveal an alignment of the molecular main axes with the rubbi ng direction. (C) 2000 American Institute of Physics. [S0021-9606(00)71733- 8].