Investigation of orientation behavior of nematic liquid crystals on UV-irradiated polyimide films

Abstract The orientation mechanism of liquid crystals (LCs) on surfaces remains unclear, despite several methods for controlling pretilt angles. This study investigates the relationship between the surface condition of polyimide films, whose pretilt angles can be controlled by UV dose, and LC orientation behavior. Absorbance at wavelengths of 200 nm and 260 nm significantly decreases, while thickness reduces by approximately 4 nm. A rubbing treatment further decreases the thickness by approximately 2 nm. Atomic force microscopy confirmed the change in molecular conformation by UV-irradiation and rubbing treatment. The dispersive and polar components of the surface free energy of UV-irradiated polyimide films are evaluated, and it’s found that only the polar component changes with UV dose. Additionally, we confirm that the alignment of LCs transitions from homeotropic to planar with increased UV irradiation, demonstrating that pretilt angle distribution can be spatially controlled. These results contribute to establishing a photoalignment method for pretilt angle control.