Polyimide-Free Planar Alignment of Nematic Liquid Crystals: Sequential Interfacial Modifications through Dual-Wavelength in Situ Photoalignment

Abstract

High-quality alignment control of liquid crystals (LCs) for ultrahigh-definition large-sized display is a challenging task. A conventional rubbing method has obvious limitations for fabricating large-sized displays with a small pixel size and an uneven inner surface. To comply with the current trend, we propose a simple and reliable polyimide-less in situ photoalignment. It was achieved using a visible-light-sensitive azo-dye and a mesogenic acrylate, both doped to host LCs. Without using a pretreated alignment layer, mono- and multidomain uniaxial alignments of LC molecules were induced by linearly polarized visible light (LPVL) and subsequently stabilized by unpolarized UV-light irradiation. The stepwise process was monitored by adopting a fluorescent indicator. By loading the mixture into a confined cell, azo-dyes were spontaneously adsorbed at inner surfaces of the cell, whereas reactive mesogens (RMs) were homogeneously dissolved in an LC host. The molecular orientational anisotropy of dyes at the surface, induced by LPVL, aligned the LC director perpendicular to the polarization direction. Upon the second step, UV-irradiation, the RMs in an LC host were photopolymerized into thin interfacial layers, stabilizing the aligned LC director. The overlaid cross-linked RM layers secured a thermal and a radiative stability of LC alignment. The RM layers completely screened the effect of azo-dyes, which can be easily randomized by heat and irradiation. The interfacial RM layer functioned as a permanently stable alignment layer. It provided sufficient azimuthal anchoring strength together with heat and light stabilities, which are essential for practical applications. Such sequential interfacial modifications through dual-wavelength processes can completely avoid interference between forming alignment and stabilization layers, inevitable if the same wavelength light is used. The proposed method provides a simple fabrication process and reliable alignment characteristics by employing effective in situ photoalignment and without using a traditional alignment layer. Therefore, it meets a current trend in the display market toward ultrahigh-resolution and large-area displays.

Keywords: liquid crystal displays; liquid crystals; photoalignment; photochromic dyes; polymer stabilization.