Relationship between pretilt angles of liquid crystals and surface energies of polyimides containing fluorine atoms

We have synthesized the novel organic-solvent-soluble polyimides having helix backbone structure and containing four different types of diamine with trifluoromethyl groups as alignment films for liquid crystal displays. These alignment films show the pretilt angles of liquid crystal from 2 degrees up to 40 degrees. The surface energies of these polyimide films show a monotonic decrease with the increase of concentration of fluorine atoms in polyimide films. This tendency commonly occurs for four different fluorine containing diamines. A clear relationship between the surface energy of polyimide films and the pretilt angles of liquid crystal on polyimide films was not obtained. The dependence of the obtained pretilt angles on the chemical structure of the polyimides is as follows: the polyimides with fluorine atoms in the lateral benzene rings, which are capable of generating a high pretilt angle of up to 40 degrees, show the linear increasing of pretilt angles with the decreasing of the surface energies. On the other hand, the polyimides with fluorine atoms in polymer backbones show a maximum value of pretilt angles in the medium ranges. These results suggest that the microscopic surface structure of polyimides largely affect the generation of the pretilt angle at the boundary surface between liquid crystals and alignment films.