Article pubs.acs.org/Macromolecules
Molecular Orientation at the Near-Surface of Photoaligned Films Determined by NEXAFS Nobuhiro Kawatsuki,*,† Yonosuke Inada,† 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: 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. LC molecules and the alignment layer.19 Additionally, the surface structure of the film plays an important role in the L-LC alignment. We have conducted systematic studies on the photoinduced reorientation of photoreactive liquid crystalline polymers (PLCPs) that generate a sufficient molecular orientation.20 Axis-selective photocross-linking followed by a thermal treatment generates a self-organization to reveal molecular reorientation in a polymethacrylate comprised of cinnamatederivative mesogenic side groups (PMCB6M; Figure 1a), where adequate amount of axis-selective photocross-linked mesogenic side groups thermally reorients whole mesogenic side groups in a direction parallel to them.21 These reoriented films are applicable birefringent films due to their highly molecular oriented structure.20,22 Additionally, using oriented thin PMCB6M films as the L-LC photoalignment layers improves azimuthal anchoring compared to conventional poly(vinyl cinnamate) (PVCi) photoalignment layers due to the molecularly oriented structure of the film.19 However, the molecularly oriented structure at the near-surface of the oriented PMCB6M films has yet to be investigated.
1. INTRODUCTION Photoalignment based on an axis-selective photoreaction of photosensitive polymeric films has received much attention because it can fabricate oriented functional structures.1−5 An axis-selective photoreaction generates a small optical anisotropy in photosensitive films using linearly polarized (LP) light irradiation.3 For photoalignment of low-molecular-weight liquid crystals (L-LCs), uniaxial orientation is either parallel or perpendicular to the polarization (E) of LP light depending on the interaction between the L-LC and the photoalignment layers.4,5 Photoalignment of L-LC resolves a number of shortcomings of the traditional rubbing method utilized to fabricate LC displays, such as charging and dusting of the films. Many types of photosensitive materials have been investigated to explore their photoalignment ability (e.g., cinnamates,6−9 coumarins,10−12 cinnamic acids,13 and azobenzene-containing polymers14−18). In many cases of L-LC photoalignment, molecular orientation is unnecessary in a photosensitive polymeric film because the small chemical, optical, or mechanical anisotropy of the alignment layer controls the uniaxial L-LC orientation. However, the azimuthal anchoring of the L-LC on the photoalignment layer is small compared to a mechanically rubbed alignment film. To improve azimuthal anchoring, the LLC alignment layer should show a molecularly oriented structure to increase the anisotropic interaction between L© 2014 American Chemical Society
Received: January 9, 2014 Revised: February 21, 2014 Published: March 5, 2014 2080
dx.doi.org/10.1021/ma5000738 | Macromolecules 2014, 47, 2080−2087
Macromolecules
Article
Because NEXAFS spectroscopy is a more suitable method to evaluate the near-surface orientation structure of photoalignment films, herein both NEXAFS and polarization UV absorption spectroscopies are employed to compare the photoinduced molecular orientation behavior of photoalignment PMCB6M films at the near-surface and in bulk, and the influence of film thickness on the orientation behavior is clarified. The molecular orientation structure at the near-surface should be different from that in bulk since the molecular orientation is accompanied by the thermally stimulated molecular motion. Two types of NEXAFS spectroscopies, total electron yield (TEY) and auger electron yield (AEY), are used to evaluate the orientation structure of PMCB6M films, where the sampling depths in TEY (AEY) measurements are typically ∼10 (∼a few) nm. Furthermore, three-dimensional molecular orientation of obliquely oriented films, which are fabricated via slantwise p-polarized UV light exposure, is investigated in detail. Figure 1. (a) Chemical structure of PMCB6M. (b) NEXAFS geometry with polar angle (θ), azimuthal angle (ϕ), and incident ppolarized X-ray.
2. EXPERIMNTAL SECTION 2.1. Materials. All starting materials were used as received from Tokyo Kasei Chemicals. PMCB6M was synthesized according to the literature.21 The number-average molecular weight and polydispersity were 39000 and 2.1, respectively. 2.2. Photoreaction and Characterization. Thin PMCB6M films (thickness: 15−190 nm) were prepared by spin-coating a methylene chloride solution of the polymers (0.5−2 w/w%) onto quartz or ITO/ glass substrates. The photoreaction was performed using a highpressure Hg lamp equipped with Glan−Taylor polarizing prisms and a cutoff filter ( 0.24) are achieved in films thicker than 90 nm, but Sbulk is less than 0.6 for films thinner than 65 nm. Seki et al. reported that an improved photoinduced orientation in azobenzene mesogens is accomplished in diblock copolymers when a low Tg flexible chain exists at the interface between the substrate and photoalignment block.40 The interaction between the substrate and PMCB6M film reduces the orientation in bulk, deteriorating self-organization of the mesogenic side groups at the interface upon annealing. 3.3. NEXAFS Spectroscopy of Oriented PMCB6M Films. Parts a and b of Figure 6 plot the angular dependence of the normalized TEY intensity of the reoriented 170-nm thick PMCB6M film when ϕ (Figure 1b) is 0° and 90°, respectively. The angular dependence of the 1s to π* transition in the parallel configuration (ϕ = 0°) is much larger than that for the as-coated film, whereas the intensity of the whole region in the perpendicular configuration (ϕ = 90°) has a smaller angular dependence (Figure 6b). These spectra indicate that the aromatic rings are preferentially oriented along the x-axis as illustrated in the inset of Figure 6a, which is due to the uniaxial in-plane orientation of the mesogenic side groups parallel to E of LPUV light. The angular dependence of the AEY intensity at 2084
dx.doi.org/10.1021/ma5000738 | Macromolecules 2014, 47, 2080−2087
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Figure 7. Polarization dependence of the TEY and AEY π* intensities at 285 eV for reoriented (a) 170, (b) 40, and (c) 15 nm thick films as functions of the X-ray incident angle (θ) with the electric field vector oriented parallel (∥, ϕ = 0°) and perpendicular (⊥, ϕ = 90°) to the orientation direction. Lines indicate curve fittings.
Figure 8. (a) Incident angular dependence of the absorbance at 315 nm of an oriented 170-nm thick PMCB6M films as a function of incident angle (α) of the probe beam with various exposure angles (ψ). (b) Incident angular dependence of the absorbance at p-polarized 315 nm in the xgeometry of PMCB6M films exposed at ψ = 60° as a function of α of the probe beam for various film thickness.
in-plane reorientation at the near-surface in the thinner film as well as reducing the orientation ability in bulk. 3.4. Three-Dimensional Orientation of PMCB6M Films. Slantwise exposure to photoalignment films can be used to fabricate three-dimensional oriented birefringent films and the tilt-angle formation of uniaxial orientation of L-LCs on the photoalignment layer.2,8,22 For PMCB6M, p-polarized UV light exposure and subsequent annealing procedure generate a three-dimensional molecular orientation because the mesogenic groups reorient parallel to E of LPUV light.22 Figure 8a plots the incident angular dependences of the absorbances at p- and s-polarized 315 nm in the x-geometry of reoriented 170 nm thick films as functions of the incident angle of the probe beam when films are exposed to p-polarized UV light at ψ = 30°, 45°, and 60°. The inset displays the experimental geometry. Because the mesogenic side groups orient parallel to E, the angular dependence for the s-polarized probe beam is negligible and the absorbance of the p-polarized probe beam shows an asymmetrical angular dependence. The incident angle at the maximum absorbance (αmax) increases when the exposure angle (ψ) increases, indicating a slantwise molecular orientation in bulk. In this case, the αmax values indicate the average inclined angle of the oriented molecules, which are smaller than the refraction angles (ψ′) in the film (Table 2). Film thickness affects the slantwise performances. Figure 8b plots the incident angle dependences of absorbance at ppolarized 315 nm in the x-geometry for PMCB6M films exposed at ψ = 60° as functions of the incident angle of the probe beam for different film thicknesses. The inclined angle decreases as the film thickness decreases. A similar tendency is
285 eV (1s to π* transition) shows similar tendencies although spectra are somewhat noisy (Figure 6c,d). Parts a−c of Figure 7 plot the polarization dependence of the π* intensity at 285 eV of oriented films as a function of the Xray incident angle. In all cases, the intensity in the parallel direction is maximized when θ is zero and the angular dependence of the perpendicular direction is small. Table 1 summarizes the calculated molecular distribution factors ( f x, f y, and fz) of the phenyl rings, fractional number of side groups (Nx, Ny, and Nz), S3D, and Sbulk of the reoriented films with different film thickness. The molecular distribution factors of TEY and AEY are similar, indicating that the molecular orientation characteristics at the near-surface between 1 and 10 nm are comparable. For a 170-nm thick film, the S3D values of the oriented 170nm thick film are 0.64 for both TEY and AEY, which are comparable to that in bulk, indicating a sufficient molecular reorientation at the near-surface. Interestingly, the NEXAFS spectra of a 40-nm thick film have similar angular dependences to those of 170-nm thick film (Figures S2,7b); the molecular distribution factors and the S3D values of TEY and AEY are similar to those of 170-nm thick film, although the Sbulk is much smaller (Table 1). This means that an effective in-plane molecular reorientation is achieved at the near-surface, and the orientation characteristics at the near-surface are not influenced by the interface between the film and the substrate, which reduces the orientation order in bulk for a 40 nm thick film. In contrast, the orientation characteristics at the near-surface of a 15 nm thick film differ from those of 170 and 40 nm thick films (Figures S3,7c, and Table 1). The interaction between the film and the substrate suppresses the thermally stimulated uniaxial 2085
dx.doi.org/10.1021/ma5000738 | Macromolecules 2014, 47, 2080−2087
Macromolecules
Article
Table 2. Inclination Angles in Bulk (αmax) and at Nearsurface (γ) of Reoriented Films Exposed at Various Exposure Angles (ψ) thickness (nm)
ψ
ψ′a
αmax
γ
170 170 170 40 40 40 15 15
60 45 30 60 45 30 60 45
33 26 18 33 26 18 33 26
30 20 5 10 10
