UV-Photoinduced Surface Anisotropy of Polyimide Studied by Near-Edge X-Ray Absorption Fine Structure Spectroscopy

Abstract

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 π* 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 ∼10%) in the former case is observed at a lower exposure (∼0.6 J/cm2) than in the latter case (∼2 J/cm2). Considering the difference in electron escape depth between PEY (∼2 nm) and TEY (∼3.5 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.