Synthesis and photoalignment of soluble polyimides containing photoreactive 2-styrylpyridine derivatives as side groups

We describe the synthesis and characterization of soluble photoreactive polyimides. The precursor of the polyimides was prepared from 4,4′-(2,2,2-trifluoro-1-phenylethylidene)diphthalic anhydride and 3,3′-hydroxy-4,4′-diaminobiphenyl. The photoreactive polyimides (PI-SP6 and PI-SP12) were then prepared by the polymer reaction of the hydroxyl groups in the precursor polymer with 2-[2-{4-(ω-hydroxyalkyloxy)phenyl} ethenyl]pyridines as photoreactive 2-styrylpyridine derivatives (alkyl: hexyl and dodecyl). The photoreactive polymers and the precursor polyimide showed their initial decompositions at temperatures of 350 and 480°C, respectively. These polymers were soluble in various polar organic solvents and their thin films were easily formed by solution casting. The normalized absorbances of the photoreactive polymers in the film state decreased to ca. 74 % at a 2.0 J/cm² exposure dose. Transmittances of their films were ca. 90 % at room temperature and ca. 85 % at 200°C. These results indicate that the polyimides are photoreactive polymers exhibiting good photosensitivity and high optical transparency. The dichroic ratios of PI-SP6 and PI-SP12 were observed to be 0.019 and 0.030 at a 0.1 J/cm² exposure dose; these values indicate that the polymers show the effect of alkylene spacer length on the photoalignment. The liquid crystal in the film cells of the polymers was oriented perpendicularly to the electric vector of the linear polarized UV light. These photoalignment results are discussed in light of the differences between the structures of the photoreactive polyimides obtained in this study and those in a previous report.