A Soluble Photoreactive Polyimide Bearing the Coumarin Chromophore in the Side Group:  Photoreaction, Photoinduced Molecular Reorientation, and Liquid-Crystal Alignability in Thin Films

A soluble photoreactive polyimide (PSPI) bearing the coumarin (COU) chromophore in the side group, 6F-HAB-COU PSPI, was successfully synthesized with a high molecular weight. Good quality films of this PSPI were obtained using a conventional solution spin-casting and drying process. 6F-HAB-COU PSPI was determined to be positively birefringent and thermally stable up to 300 °C. The photochemical reactions and molecular reorientations of the PSPI in films were investigated in detail using ultraviolet−visible and infrared spectroscopy, dissolution testing, and optical retardation measurements. The COU chromophores of the PSPI were found to undergo only photodimerization. Irradiation of the PSPI films with linearly polarized ultraviolet light (LPUVL) induced anisotropic reorientations of the polymer main chains and anisotropic alignments of the unreacted COU chromophores. Moreover, the LPUVL-irradiated films homogeneously aligned nematic liquid crystal (LC) molecules at an angle of 100° with respect to the polarization of the LPUVL, which coincided with the reorientation direction of the PSPI polymer chains. This result indicates that the LC alignment process at the surface of irradiated PSPI films is principally governed by the reorientations of the polymer main chains and the unreacted COU side groups, whose directionally anisotropic interactions with the LC molecules contribute to the LC alignment. In particular, it was found that LPUVL exposure at ≥0.5 J/cm2 causes the preferential reorientation of the polymer chains at the film surface, which is enough to induce a stable alignment of LC molecules, and their interactions with the LC molecules override interactions with those created by a second LPUVL exposure with a different polarization direction. The LC pretilt angle was measured to be 0.05−0.10°. The 6F-HAB-COU PSPI films retained their LC alignment properties even after heating to 210 °C, which is 78 °C higher than the Tg of the PSPI. Overall, the LC alignment characteristics of the 6F-HAB-COU PSPI make it a promising candidate material for use as a LC alignment layer in advanced LC display devices, in particular those with an in-plane switching mode that require the lowest possible LC pretilt angle.