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.

Articles you may be interested inPhotoinduced magnetic uniaxial anisotropy and molecular orientation of polymer containing azobenzene side groups with free radical end J. Appl. Phys. 93, 4392 (2003); 10.1063/1.1559634 Alignment of polyamic acid molecules containing azobenzene in the backbone structure: Effects of polarized ultraviolet light irradiation and subsequent thermal imidizationWe have investigated the inclined alignment of polyimide molecules ͑containing azobenzene in the backbone structure͒ induced by oblique angle irradiation of ultraviolet ͑UV͒ light. The UV irradiation was performed for a polyamic acid film, which then was thermally converted into a polyimide film. The orientation of the polyimide backbone structure was determined by measuring the polarized infrared absorption spectra as a function of the angle of incidence. We confirmed that inclined alignment of the polyimide backbone structures is obtained by oblique angle UV irradiation, and that this also induces tilted homogeneous liquid crystal ͑LC͒ alignment. For the UV irradiation conditions used in the present study, the average inclination angle of the polyimide backbone structure was about 4°from the surface plane. The pretilt angle of the LC molecules was 0.9°.

We have determined the in-plane molecular order of a polyamic acid (PAA) film irradiated with linearly polarized ultraviolet light (LPUVL), as well as that of the polyimide film obtained by thermally imidizing it. The PAA examined in this study contains azobenzene units in the backbone structure. The in-plane molecular order of the PAA and polyimide films was determined from the anisotropy in the polarized IR absorption of the phenyl C-C stretching vibration polarized along the backbone structure. We found that the photoinduced anisotropy in the in-plane molecular orientation was small, but it increased significantly after thermal imidization; i.e. the in-plane molecular order of the polyimide film was much greater than that of the PAA film. The enhancement of the in-plane molecular order was tentatively attributed to the crystallization of the film caused by thermal imidization