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We have investigated the pretilt angle of liquid crystal (LC) molecules induced by photoaligned films of a series of polyimides. The polyimides were random copolymers synthesized from pyromellitic dianhydride and a mixture of 4,4′-diaminoazobenzene and 4-[4′-propylbi(cyclohexan)-4-yl]phenyl 3,5-diaminobenzoate (PBCP-DABA). PBCP-DABA is a diamine to introduce a side-chain structure into polyimide. We found that the pretilt angle of LC molecules can be controlled from 0° to 90° by varying the molar fraction (x) of PBCP-DABA from 0 to 0.5. Defect-free uniform LC alignment was observed for x≤0.125 and x≥0.3, but threadlike textures appeared for 0.15≤x≤0.25. Since the interaction between the polyimide backbone structure and the LC molecule may be blocked by relatively dense side-chains, the appearance of threadlike texture is tentatively attributed to weak azimuthal anchoring strength of the photoaligned polyimide films with x≥0.15. [ABSTRACT FROM AUTHOR]

We have examined the in-plane orientation of liquid crystal (LC) molecules adsorbed on polyimide films as a function of coverage over a submonolayer range. In this study we used 12-nm-thick photoaligned films of polyimide containing azobenzene in the backbone structure (Azo-PI). The in-plane molecular order of the LC layer and the Azo-PI film was determined from the polarization angle dependence of infrared absorption. Even at ∼0.2 ML (monolayer) the orientational distribution of the LC molecules was anisotropic with an in-plane order parameter almost equal to that of the Azo-PI backbone structure in the underlying film. The in-plane order parameter of the LC molecules was constant up to 1 ML. This result indicates that the orientation of the LC molecules directly adsorbed on the polyimide film is determined by the interaction between the polyimide and LC molecules. The interaction among the LC molecules in the first monolayer was found to be negligibly weak compared with that between the polyimide and LC molecules. [ABSTRACT FROM AUTHOR]

We have determined the relationship between the in-plane molecular orientations of a polyimide film and the liquid crystal (LC) monolayer in contact with it. A photoaligned film of polyimide, containing azobenzene in the backbone structure, was used, because its in-plane molecular order can be varied over a wide range without change in the morphology and the chemical nature of the film surface. The in-plane order parameter of the LC molecule was found to be almost equal to that of the polyimide backbone structure. This result shows that the molecular alignment of the LC monolayer is mainly induced by a short-range interaction between the LC and polyimide molecules. [ABSTRACT FROM AUTHOR]

Abstract: We have succeeded in realizing an inclined homeotropic alignment of liquid crystal (LC) molecules by using photo-aligned films of a polyimide containing azobenzene in the backbone structure. To induce such an LC alignment, a side chain structure was introduced into the backbone structure. The LC pretilt angle, measured from the surface normal, could be controlled up to 1.75° by varying the light exposure in oblique angle irradiation with unpolarized light. Its thermal stability was examined by annealing the LC cell at 100 °C. No change was observed in the pretilt angle even after annealing for 36 h, indicative of its excellent thermal stability. Since photo-alignment has patterning capability, the photo-aligned polyimide film is expected as a promising alignment film for multi-domain vertical alignment mode LC displays. [Copyright &y& Elsevier]