Molecular Orientation at the Near-Surface of Photoaligned Films Determined by NEXAFS

Because of the potential for use in display applications, photoalignment using photosensitive materials has received considerable attention. Herein, the influence of film thickness on thermally stimulated photoinduced molecular reorientation structures of photoreactive liquid crystalline polymer films at the near-surface and in bulk is investigated using near-edge X-ray absorption fine structure (NEXAFS) and polarization UV absorption spectroscopies, respectively. In films less than 90-nm thick, the uniaxial in-plane orientation order in bulk gradually decreases. In contrast, films thicker than 40 nm show a sufficient molecular orientation at the near-surface. The three-dimensional orientation structures are investigated using obliquely reoriented films fabricated by slantwise p-polarized UV light exposure. Films thicker than 40 nm display an effective slantwise orientation in bulk, but the slantwise angle at the near-surface is much smaller than that in bulk. Both the substrate film interface and film thickness play important roles in the reorientation behavior not only in bulk but also at the near-surface of the photoalignment film.