Article pubs.acs.org/Macromolecules
Comparison of the Photoinduced Orientation Structure in the Bulk and at the Near-Surface of a Photoalignable Liquid Crystalline Polymer Film Nobuhiro Kawatsuki,*,† Yusuke Taniguchi,† Mizuho Kondo,† Yuichi Haruyama,‡ and Shinji Matsui‡ †
Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan ‡ Laboratory of Advanced Science and Technology for Industry, University of Hyogo, 3-1-2 Koto, Kamigori, Ako, Hyogo 678-1205 Japan S Supporting Information *
ABSTRACT: The thermally stimulated photoinduced inplane and out-of-plane molecular reorientation behaviors of a polymethacrylate film comprised of 4-methoxycinnnamoylbiphenyl (MCB) side groups connected with a decylene spacer (PMCB10M) are compared in the bulk (>10 nm), at the inner-surface (∼10 nm), and at the near-surface (10 nm) although the molecular orientation characteristics at the near-surface ( 30%). 3.3. Nematic LC Alignment on the PMCB10 M Films. The LC alignment behavior on the PMCB10 M films was investigated using a parallel LC cell. Three types of photoaligned PMCB10 M films were subjected to the LC alignment layer: type A (B) prepared by exposing to LPUV light for 500 mJ/cm2 and subsequent annealing at 120 °C (240 °C), and type C prepared by exposing to NPUV light for 500 mJ/cm2 and subsequent annealing at 240 °C. Figure 9a (b) shows POM photograph of the parallel LC cells using a type-A (-B) film, Both the type-A and -B films show homogeneous in-plane LC alignment parallel to E. However, the type-C film shows neither homogeneous in-plane nor out-of-plane LC alignment (Figure 9c). Because the type-A film reveals an in-plane molecular orientation, LC molecules can align parallel to the orientation direction due to the interaction with the aligned MCB side
Figure 9. POM photograph of parallel nematic LC cells using (a) Afilms, (b) B-films, and (c) C-films. White arrows indicate the polarizer and analyzer direction.
groups. A similar LC alignment is observed when using the type-B film despite its out-of-plane orientation characteristics in the bulk. It is assumed that the film’s surface exhibits an inplane optical anisotropy parallel to E. Furthermore, the lack of LC alignment on the C-type film suggests that the MCB groups at the film’s surface do not form an out-of-plane orientation structure although the bulk has an out-of-plane orientation characteristics. 3.4. NEXAFS Spectroscopy of the Oriented Films. The difference in the orientation structures at the inner-surface and the near-surface is evaluated by the angular dependence of TEY and AEY NEXAFS spectroscopy, respectively. Part a and b of Figure 10 plot the angular dependences of the normalized TEY intensity of the reoriented PMCB10 M film annealed at 120 °C when ϕ (Figure 2) is 0° and 90°, respectively. The π* intensity at 285 eV (1s to π* transition of aromatic ring) in the parallel E
DOI: 10.1021/ma5025957 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 10. Angular dependent NEXAFS spectra of a reoriented 150 nm thick PMCB10 M film recorded by (a and b) TEY and (c and d) AEY with the electric field vector oriented (a and c) parallel (∥, ϕ = 0°) and (b and d) perpendicular (⊥, ϕ = 90°) to the orientation direction. Exposure energy and annealing temperature are 500 mJ/cm2 and 120 °C, respectively.
Figure 11. Angular dependent NEXAFS spectra of a reoriented 150 nm thick PMCB10 M film recorded by (a and b) TEY and (c and d) AEY, with the electric field vector oriented (a and c) parallel (∥, ϕ = 0°) and (b and d) perpendicular (⊥, ϕ = 90°) to the orientation direction. Exposure energy and annealing temperature are 500 mJ/cm2 and 180 °C, respectively.
larger than those of the inner-surface (∼10 nm, TEY) although the orientation characteristics of the PMCB10 M film in bulk are biaxial. In contrast, the TEY intensity of the reoriented PMCB10 M film has a different dependence when the annealing temperature is 180 °C. Figure 11a (b) plots the angular dependences of the normalized TEY intensity for the reoriented PMCB10 M film annealed at 180 °C when ϕ is 0° (90°). The angular dependence of the intensity at 285 eV in the parallel configuration is much smaller than the reoriented film annealed at 120 °C, while the intensity in the perpendicular configuration (ϕ = 90°) is similar to the reoriented film annealed at 120 °C,
configuration (ϕ = 0°) increases as the X-ray incident angle decreases, which is an opposite tendency of the as-coated film, whereas that in the perpendicular configuration has a small and opposite angular dependence. Additionally, the AEY intensity at 285 eV has a larger angular dependence than the TEY intensity in the parallel configuration, and the AEY intensity in the perpendicular configuration shows the opposite tendency as the TEY (Figures 10c, d). These results suggest that the aromatic rings at the near-surface are preferentially oriented along the xaxis similar to a reoriented PMCB6M film which shows inplane orientation at the near-surface.17 However, the in-plane orientation characteristics at the near-surface (
