UV-photoinduced surface anisotropy of polyimide studied by near-edge X-ray absorption fine structure spectroscopy

Anisotropic alignment of polyimide chains induced by linearly polarized deep UV (LPDUV) irradiation was studied by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the total electron yield (TEY) and partial electron yield (PEY) acquisition modes. The degree of the alignment manifested by the intensity of O K-edge pi* resonance of a C=O group is noticeably higher in the PEY acquisition mode than in the TEY one. The maximum anisotropy (linear dichroic ratio similar to10%) in the former case is observed at a lower exposure (similar to0.6 J/cm(2)) than in the latter case (similar to2 J/cm(2)). Considering the difference in electron escape depth between PEY (similar to2 nm) and TEY (similar to3.5 nm), we can conclude that the LPDUV-irradiated polyimide film exhibits depth dependence of anisotropy. The irradiation dependence of the PEY-derived anisotropy and the order parameter of liquid crystals (LC) showed good agreement, indicating that the anisotropy in the surface region is of major importance for LC alignment.