In-plane order of liquid crystal molecules adsorbed on photoaligned polyimide films: Coverage dependence in submonolayer range

We have examined the in-plane orientation of liquid crystal (LC) molecules adsorbed on polyimide films as a function of coverage over a submonolayer range. In this study we used 12-nm -thick photoaligned films of polyimide containing azobenzene in the backbone structure (Azo-PI). The in-plane molecular order of the LC layer and the Azo-PI film was determined from the polarization angle dependence of infrared absorption. Even at ∼0.2 ML (monolayer) the orientational distribution of the LC molecules was anisotropic with an in-plane order parameter almost equal to that of the Azo-PI backbone structure in the underlying film. The in-plane order parameter of the LC molecules was constant up to 1 ML. This result indicates that the orientation of the LC molecules directly adsorbed on the polyimide film is determined by the interaction between the polyimide and LC molecules. The interaction among the LC molecules in the first monolayer was found to be negligibly weak compared with that between the polyimide and LC molecules.