Control of Liquid Crystal Alignment Based on 3-Dimensional Molecular Manipulation

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Uniform alignment of liquid crystals (LCs) is indispensable for their optical and optpelectronic applications, which is conventionally obtained by mechanically rubbing a polyimide (PI) layer. However, dust particles and static surface charges are produced by the rubbing process, which would cause electrical and mechanical damage to thin film transistors. In recent years, the technique for unidirectional alignment of LCs caused by PI films exposed to linearly polarized UV (LPUV) light has been developed. this method allows for easy control of the alignment direction and the fast preparation of an alignment layer, and causes little dust particles and electrostatic damage. In this case, the alignment of LC is perpendicular to the polarization of LPUV light, and exposure should be made two times in order to give precise pretilt of LCs. In this project , we developed a novel method to induce a parallel alignment of LC to polarization of LPUV light by irradiating during the thermal treatment process. Polyamic-acid-coated ITO glass substrate was irradiated with LPUV light at 8.5 mW/cm^2 during the thermal imidization process. The irradiation time was about 30 min and the thermal treatment was processed from 100 to 250 ℃ at a heating rate of 5 ℃/mm. After irradiation, the substrate was then imidized for 10 min at 250 ℃. LC cells with a gap of 5 μm were constructed using two pieces of the above substrates with parallel polarization axes. 4'-Pentyl-4-cyanobiphenyl (5CB) doped with 0.3 wt% dichroic dye M-137 (λmax = 639 nm) was filled into the cells in an isotropic phase at 75 ℃ and cooled down to room temperature. By polarizing microscopy and polarized UV spectra, it was found that the LC molecules were aligned uniaxially and uniformly.