Surfaces and interfaces of liquid crystal alignment layer polyimides

Upon rubbing a thin layer (≃60 nm) of polyimide (PI) film which is employed to align nematic liquid crystals in modern TFT-LCDs, polar functional groups of a polymer alignment layer are reoriented out-of-the-plane of the surface while non-polar groups fold inward, toward the bulk of the polymer film. The driving force is an electronic attraction between the polar cotton rubbing cloth and the polar groups of an alignment layer polymer, and the repulsion between the polar cotton cloth and the non-polar groups of an alignment layer polyimide. The surface area on which the PI molecules are re-oriented increases as the rubbing force or density increases.