Structure-performance relation of liquid crystal photoalignment with in-situ formation of protection layers
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Structure-performance relation of liquid crystal photoalignment with in-situ formation of protection layers Kai-Han Chang and Liang-Chy Chien Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, OH 44242, U. S. A. ABSTRACT We demonstrate a stabilized liquid crystal photoalignment using a surface-localized polymer method. A protection layer is developed on a photoalignment layer (PAL) through phase separation of a mixture composed of reactive monomers (RMs) and liquid crystals (LCs). The RM is polymerized on the PAL which enhances its stability against heat and light with short wavelength. The effects of the concentration and molecular structure of RMs on the electrooptical response and surface anchoring of photoaligned LC device are studied. The concentration of RMs affects the effective cell gap of the LC device. The rigid core length of the RM structure modulates the surface anchoring strength of the alignment layer. Both effective cell gap and surface anchoring strength are key elements for the enhanced dynamic response of LC devices. INTRODUCTION Liquid crystal (LC) alignment is a critical element in device application. Traditionally, the alignment is achieved by depositing a layer of polyimide on a substrate and rubbing it unidirectionally with a velvet cloth to generate surface anisotropy1. However, this approach has several drawbacks such as induced static electricity on the surface and limitation of resolution for patterned alignment. Therefore, non-contact alignment methods are intensively studied these years. Several non-contact alignment methods have been proposed such as photoalignment, atomic-beam bombardment, electron-beam lithography, silicon oxide evaporation, and inkjet printing2-6. Among all of the methods, photoalignment of photosensitive materials shows the most potential for both large-scale mass production and patterned alignment with high resolution. Current issues associated with azobenzene-based photoalignment layers (PALs) are vulnerability to thermal stress and light exposure to short wavelength. Furthermore, weak surface anchoring of PAL is also an obstacle for practical applications. To resolve instability, several approaches are reported such as reactive mesogen coating, surface localized polymer, and polymerizable photoalignment materials7-9. The surface-localized polymer method is also applied on surface anchoring condition modulation for dynamic response enhancement and stabilization of specific textures10-12. In this work, an in-situ protection formation on PALs using surface localized polymer method is demonstrated. The protection layers enhance the stability of PAL and further modulate the electro-optical (EO) properties of LC cells. The experimental results show that the effective cell gap can be tuned by the concentration of RMs and the surface anchoring strength is related to molecular structure of RM. With both factors, enhanced dynamic responses are demonstrated. Experimental Cell fabrication We used PAL material (LIA-01, DIC C
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