Photoreactions and photoinduced molecular orientations of films of a photoreactive polyimide and their alignment of liquid crystals

A soluble photoreactive polyimide (PSPI) with side-group cinnamate (CI) chromophores that forms good quality films through conventional solution spin-casting and drying processes was successfully synthesized with a reasonably high molecular weight: poly(3,3'-bis(cinnamoyloxy)-4,4'-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB-CI). This 6F-HAB-CI PSPI is thermally stable up to 340 degreesC, its glass transition temperature (T-g) is 181 degreesC, and it was determined by prism coupling to be positively birefringent. The photochemical reactions of the PSPI in solution and in films, as well as the molecular orientations of the PSPI that are induced by exposure of its films to linearly polarized ultraviolet light (LPUVL), were investigated in detail by nuclear magnetic resonance, ultraviolet-visible, infrared, and Raman spectroscopies and by dissolution testing and optical retardation measurements. It was confirmed that the Cl chromophores of the PSPI undergo both photoisomerization and photodimerization. LPUVL exposure was found to induce anisotropic orientations of the polymer main chains and of the CI side groups in the films. Moreover, the LPUVL-irradiated films homogeneously aligned nematic liquid crystal (LC) molecules along a direction at an angle of 107degrees with respect to the polarization of the LPUVL, which coincides with the orientation direction of the PSPI polymer chains. This result shows that the LC alignment process is principally governed in irradiated PSPI films by the orientations of the polymer main chains and the unreacted Cl side groups. Along the director of the LC alignment in the cell, the pretilt angle was measured to be 0-0.15degrees, depending on the exposure energy and the exposure method. The LC alignment was found to be thermally stable up to 200 degrees, which is 20 degreesC higher than the T-g of the film. In summary, the LC alignment characteristics of the 6F-HAB-CI PSPI make it a promising candidate material for use as the LC alignment layer in advanced LC display devices with in-plane switching modes that require as low as possible LC pretilt angles.