Near surface molecular orientation in polymeric alignment layers:: a NEXAFS investigation

Near-edge x-ray fine structure (NEXAFS) investigations were performed on buffed polyimide alignment layers for liquid crystal (LC) displays as well as on thin n-octyl-cyanobiphenyl (8CB) LC films evaporated in-situ onto the alignment layers. The full tensorial liquid crystal order parameter can be derived from the x-ray dichroism as a function of the substrate conditions, the layer thickness, and the thermal history. We compare a data set taken on poly-pyromellitimido-oxydianiline (PMDA-ODA) polyimide to a data set where highly oriented pyrolithic graphite (HOPG) was used as a substrate. While on graphite the aromatic rings are aligned parallel to the substrate, this is to a much lesser extent the case on polyimide. On polyimide, the LC molecular main axes are parallel to the substrate but the aromatic rings are about as often facing the substrate with the ring plane, as they do with the edge. During a brief heat treatment, most of the LC-material evaporates. On graphite, the remaining, rather tightly physisorbed layer is very well ordered, whereas on polyimide substrates the remaining layer is less ordered than thicker films.