Alignment of polyamic acid molecules containing azobenzene in the backbone structure: Effects of polarized ultraviolet light irradiation and subsequent thermal imidization

Abstract

We have investigated the molecular orientation in the films of polyamic acid (PAA) with azobenzene units in the backbone structure. Anisotropic molecular orientation was induced by irradiation of linearly polarized ultraviolet light (LPUVL). The change in the molecular orientation caused by subsequent thermal imidization was also investigated. The orientation of the PAA and polyimide backbone structures was determined by measuring the polarized infrared absorption spectra of the films. When the PAA film was exposed to LPUVL of wavelength 365 to 400 nm at normal incidence, permanent orientational change of the PAA backbone structure occurred through repeated photoisomerization reactions of the azobenzene unit. The PAA backbone structure rotated toward the plane perpendicular to the polarization direction of LPUVL. In subsequent thermal imidization the molecular order increased significantly around the direction perpendicular to both the polarization direction of LPUVL and the surface normal. This enhancement of the molecular order was tentatively attributed to the crystallization of the film caused by thermal imidization.