Laser induced periodic structure on lecithin-doped polyimide film surface and its ability to align liquid crystal molecules Xuemin Lu, Qinghua Lu * , Zikang Zhu, Jie Yin, Zongguang Wang School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China Received 16 January 2003; received in revised form 17 May 2003; accepted 27 May 2003 Abstract Doping effect of lecithin in polyimide (PI) films on the preparation of laser induced periodic structure (LIPS) was investigated using single-beam polarized pulse UV laser. We found that the regularity of LIPS on lecithin-doped PI film irradiated at 266 nm was better than that at 355 nm and the threshold energy for the preparation of LIPS decreased. The amplitude of LIPS increased with the increase of lecithin content. In addition, lecithin molecules tended to migrate to the film surface after laser irradiation according to FT-IR spectroscopic results. We found that nematic liquid crystal molecules were azimuthally aligned on the irradiated doped PI surface when lecithin content was below 20 wt%, and were vertically aligned when the lecithin content was 30 wt%. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polyimide; Lecithin; Doping effect 1. Introduction Laser induced periodic structures (LIPS) are a focused interest in recent decades and have been observed on condensed materials, i.e. semiconductors [1], metals [2] and recently polymers such as polystyrene, poly(ethylene terephthalate) and polyethersulfone irradiated by excimer lasers [3–7]. Hiraoka et al initially prepared LIPS in 1994 by using Nd:YAG laser as the light resource at an energy lower than the ablation threshold energy of a polymer [8]. On the mechanism of the LIPS formation, most researchers accepted that the property of a polymer film had important effect on the formation of LIPS, especially the absorption of the film to the incident laser [9–10]. So recent works mostly focused on the preparation of LIPS on different polymer films, i.e. polyimide (PI) and polycarbonate [11–12]. According to the literature on laser ablation of polymers, doping of small molecules, which showed absorbance at incident laser, was helpful to the laser ablation process [13]. From this viewpoint we considered that the doping of small molecules might be helpful to accelerate the LIPS formation at lower energy, because the LIPS formation was a thermal physical process according to Hiraoka’s reports [9]. Lecithin, as a typical surfactant, not only shows certain absorbance at the wavelength range of 226–274 nm, but also could decrease the glass transition temperature ðT g Þ of a doped polymer. In this report, we described the doping effects of lecithin molecules on the LIPS formation on a PI surface. The surface character of the lecithin-doped PI film was investigated by using atomic force microscopy (AFM) and different modes of FT-IR spectroscopic techniques. Film surface with LIPS could be used in several fields such as optical gratings, data storage and liquid crystal (LC) devices [14–17]. Bryan–Brown’s group reported that vertical alignment of LC molecules can be realized by depositing a kind of chromophore on the groove surface and so on voltage-controlled LC device can be fabricated [16]. In a previous study on LIPS [17], we found that LC molecules showed azimuthal alignment behavior on the LIPS surface. If a PI film doped with lecithin, which has the ability to align LC molecules vertically to the film surface [18], can be used to prepare groove surface (LIPS), it is possible to realize the vertical alignment of LC molecules on groove surface. So the alignment behavior of LC molecules on laser irradiated lecithin-doped PI surface was another part of this report. 0032-3861/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0032-3861(03)00433-6 Polymer 44 (2003) 4501–4507 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 86-21-54743268; fax: þ 86-21- 54741297. E-mail addresses: qhlu@sjtu.edu.cn (Q. Lu).