Dichroism of a Polyimide Chain for Ultraviolet Light

We have determined the absorption coefficient ratio a‖/a⊥ of a poly [4, 4′ -oxydiphe-nylene-pyromellitimide] (PMDA-ODA) chain in the wavelength range from 240 nm to 380 nm, where a‖ and a⊥ are the absorption coefficients of the polyimide chain for ultraviolet (UV) light polarized parallel and perpendicular to the chain direction, respectively. To accomplish that, we have measured the polarized UV absorption spectra of a rubbed polyimide film whose molecular orientation was previously determined by polarized infrared absorption spectroscopy. The a‖/a⊥ ratio becomes maximum in the wavelength region from 290 nm to 330 nm : a‖/a⊥ ≃ 5. The absorption coefficient peaks at ∼290 nm. These results sugest that linearly polarized UV light at 290 nm is most suitable for causing anisotropic decomposition of the polyimide chain. At 266 nm the a‖/a⊥ ratio was compared with the ratio of the photoinduced decomposition rates of the polyimide chain oriented parallel and perpendicular to the polarization direction of UV light. The anisotropy of the photoinduced decomposition rates of the polyimide chain is much smaller than that of the UV absorption coefficients.