Inclined Alignment of Polyimide Backbone Structures Induced by Single Exposure of Un-Polarized Light

ABSTRACT

We have investigated the alignment of polyimide backbone structures induced by single oblique angle irradiation with un-polarized light (UP-light) of wavelength 340 ∼ 500 nm. The polyimide (Azo-PI) used in this study contains azobenzene in the backbone structure. The alignment of the backbone structure occurs through random angular rotation of the azobenzene molecule accompanied by its photo-induced trans-cis-trans isomerization. The photo-alignment treatment was performed on 16 nm thick films of polyamic acid (Azo-PAA) that is the precursor of Azo-PI. Subsequently, the Azo-PAA film was thermally imidized. The UP-light exposure was varied up to 882 J/cm². The orientational distribution of the Azo-PI backbone structure was determined, using polarized infrared absorption spectroscopy. We found that the average inclination angle and the in-plane order parameter of the backbone structure increased with UP-light exposure, reaching 51° from the surface plane and 0.1, respectively, at 882 J/cm².

Keywords:

• azobenzene
• liquid crystal
• photo-induced alignment
• photo-isomerization
• polarized IR absorption
• polyimide

Acknowledgments

The authors are grateful to Professor J. Nishizawa for his valuable advice. We would like to thank S. Murata, N. Narita, and H. Ono of Chisso Co. Ltd. for supplying the Azo-PAA solution used in this work. We also thank T. Tadokoro of J. A. Woollam Japan Corp. for performing spectroscopic ellipsometry measurements to determine the refractive indices of Azo-PAA and Azo-PI films in the UV-visible region. This work was supported in part by a Grant-in-Aid for Scientific Research (B) from Japan Society for the Promotion of Science (No. 13450003).