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

Abstract

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.

Comparison of the Photoinduced Orientation Structure in the Bulk and at the Near-Surface of a Photoalignable Liquid Crystalline Polymer Film

“…Annealing at 240°C produces similar angular dependences ( Figure S5a− d). These observations differ from those of oriented PMCB6M film, 17 where a significant in-plane orientation is achieved both in the bulk and at the inner-surface, and the angular dependences of the TEY and AEY intensities are similar. Thus, the different molecular orientation structure between the near-surface and inner-surface of the film, which can be controlled by the annealing temperature, is clarified.…”

“…This indicates a more significant in-plane orientation at the near-surface for a reoriented PMCB10 M film than at the innersurface, but little difference has been observed between at the near-surface and at the inner-surface for a reoriented PMCB6M film. 17 Annealing at 180 or 240°C results in a larger N z value for the TEY intensity and the difference between N x and N z is small, indicating biaxial orientation characteristics at the innersurafce (∼10 nm) of the film. However, the clear in-plane orientation (N x ≫ N y ) and the decrease in the N z are observed for AEY.…”

“…The molecular distribution factors ( f x , f y , and f z ) of the phenyl rings and fractional number of the mesogenic side groups (N x , N y , and N z ) were evaluated by a technique similar to the one previously reported. 17 Figure 3 shows the absorption spectra of a 150 nm thick PMCB10 M film as the incident angle of the probe beam is varied, while the inset plots the absorbance at 292 nm as a function of the incident angle of the p-polarized probe light. An absorption maxima (λ max ) appears at 292 nm, which differs from that in solution (λ max = 314 nm, Figure S2a).…”

“…The slight increase in absorbance at a longer wavelength suggests the formation of photo-Fries products, which is similar to the photoreaction property of the PMCB6M film. 17 The small photoinduced optical anisotropy thermally changes to show the in-plane and/or out-of-plane reorientation when the irradiated films are annealed in the initial LC temperature range of the material. Similar to that of a PMCB6M film, photoirradiation generates a small negative ΔA but a different thermally stimulated self-organization behavior depending on the annealing temperature.…”

“…The annealing process stimulates self-organization of the MCB groups parallel to the polarization (E) of LPUV light, which originate from an adequate amount of photo-cross-linked fragments parallel to E. 4,14 Furthermore, the near-edge X-ray absorption fine structure (NEXAFS) spectroscopy clarified a significant uniaxial in-plane molecular orientation at the nearand inner-surface (<10 nm) of the reoriented PMCB6M films. 17 In the meantime, the photoinduced out-of-plane molecular orientation using photoresponsive polymeric films has been examined to fabricate an optical film with mesogens in its liquid crystalline polymer possessing a homeotropic internal structure (a positive C optical film) and homeotropic LC alignment. 18−20 Furthermore, controlling the in-plane and out-of-plane orientations can adjust the three-dimensional (3D) birefringence characteristics of birefringent films.…”