Polyimide photo-alignment layers for inclined homeotropic alignment of liquid crystal molecules Kiyoaki Usami a,b, ⁎ , Kenji Sakamoto a , Junichiro Yokota c , Yoichi Uehara d , Sukekatsu Ushioda e a Organic Nanomaterials Center, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan b Department of Information Systems Engineering, Osaka Sangyo University, 3-1-1 Nakagaito, Daito-Shi, Osaka 574-8530, Japan c Chisso Petrochemical Co., Ltd., 5-1 Goikaigan, Ichihara, Chiba 290-8551, Japan d Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan e Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Available online 27 April 2007 Abstract We have succeeded in realizing an inclined homeotropic alignment of liquid crystal (LC) molecules by using photo-aligned films of a polyimide containing azobenzene in the backbone structure. To induce such an LC alignment, a side chain structure was introduced into the backbone structure. The LC pretilt angle, measured from the surface normal, could be controlled up to 1.75° by varying the light exposure in oblique angle irradiation with unpolarized light. Its thermal stability was examined by annealing the LC cell at 100 °C. No change was observed in the pretilt angle even after annealing for 36 h, indicative of its excellent thermal stability. Since photo-alignment has patterning capability, the photo-aligned polyimide film is expected as a promising alignment film for multi-domain vertical alignment mode LC displays. © 2007 Elsevier B.V. All rights reserved. Keywords: Photo-induced alignment method; Polyimide containing azobenzene in the backbone structure; Pretilt angle; Inclined homeotropic alignment of liquid crystal molecules 1. Introduction The photo-induced alignment of liquid crystal (LC) mole- cules has received much attention as an alternative to the conventional rubbing technique [1–12] due to the following reasons. Since photo-alignment is contact-free, it has no serious problems that arise from mechanical rubbing, such as creation of dust particles and generation of electrostatic charge. The process condition can be specified by only quantitative parameters, such as polarization state, angle of incidence, exposure, and wavelength of light. Thus, photo-alignment has high reprodu- cibility and controllability. In addition, it has potential capability for alignment patterning of LC molecules, which is of great advantage to the fabrication of multi-domain LC displays with a wide viewing angle. Therefore, until now, various photo- alignment methods have been proposed and actively studied. We are focusing on a method using polyamic acid containing azobenzene in the backbone structure (Azo-PAA) [6,11,13–21]. In this method, angular selective photo-isomerization reaction of azobenzene is used to induce anisotropic orientation of the backbone structures. The photo-alignment treatment is per- formed on the Azo-PAA film, and then the film is thermally converted into polyimide (Azo-PI). Since the rigidity of the backbone structure is significantly increased by thermal imidization, the orientation of the Azo-PI backbone structure becomes stable against further light irradiation. Thus, the photo- aligned Azo-PI film has optical stability, in addition to the inherent thermal and chemical stability of polyimide [13–15]. This is the reason why we are focusing on this photo-alignment method. Recently, we demonstrated that homogeneous (uniform and parallel) and inclined homogeneous LC alignment can be realized by using photo-aligned films of main-chain-type Azo- PI [11,17–20]. The average tilt angle of LC molecules, which is called the “pretilt angle”, can be controlled up to ∼ 3° from the surface plane [18,19]. This pretilt angle is sufficient for practical application to conventional twisted-nematic (TN) LC displays. Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 2652 – 2655 www.elsevier.com/locate/tsf ⁎ Corresponding author. Department of Information Systems Engineering, Osaka Sangyo University, 3-1-1 Nakagaito, Daito-Shi, Osaka 574-8530, Japan. E-mail address: usami@ise.osaka-sandai.ac.jp (K. Usami). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.04.082