Control of alignment and pre-tilt on photoexposed surfaces

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Liquid crystals have been shown to align on polarized ultra- violet (PUV) exposed polyimide films with the liquid crystal director oriented perpendicular to the polarization direction. Uniform pretilt is generated by a second oblique PUV exposure. Our results indicate that the liquid crystal has uniform pre-tilt with no preferred alignment direction on unexposed polyimide films. Normal exposure aligns the director but the pre-tilt is degenerate, pointing either to the plus or minus direction. Oblique exposure breaks the degeneracy producing a unique pre-tilt direction. The pre- tilt angle is related to the degree of imidization of the polyimide film. By adjusting the chemistry of polyimide, the pre-tilt angle can be varied continuously. Varying the structure of polyimide results in liquid crystal aligning either parallel or perpendicular to the PUV direction. Uniform alignment and pre-tilt can be produced with a single oblique exposure if the liquid crystal aligns parallel to the polarization direction. The photoalignment of liquid crystal on thin films of the azo dye methyl red spin coated on indium tin oxide glass surface is observed for the first time. Methyl red molecules are believed to first adsorb on the substrate with an isotropic orientation. After exposure with polarized visible light, the elongated structure of the molecule results in dichroic absorption of the light by molecules oriented parallel to the polarization direction. Our study shows that photo induced cis/trans isomerization process produces the preferred alignment of methyl red and therefore the liquid crystal perpendicular to the polarization direction.