Optical anisotropy of polyimide

Abstract Optical anisotropy of polyimide films prepared from polyamic acid solutions was investigated by conoscopic observation and refractive index measurements. The conoscopic figure (crossed pattern) characteristic of a negative uniaxial crystal with its optic axis perpendicular to the film plane was observed for samples imidized on substrates. Uniaxial drawing of a polyamic acid cast film with 40 wt % solvent gave a highly oriented sample with an elongation of more than 80%. This sample had a conoscopic figure characteristic of a positive uniaxial crystal with its optic axis equivalent to the molecular c axis (parallel to the film plane). A large optical anisotropy (bire‐fringence 0.18) was observed in the film plane. The TE‐mode refractive index (parallel to the film plane) ranged from 1.64 to 1.82 at a wavelength of 633 nm according to the sample rotation. These values corresponded to the refractive indices perpendicular and parallel to the c axis, respectively. The optical system change of the polyimide film was explained in terms of the uniaxial orientation of the in‐plane oriented molecular c axis.