Chain orientation and anisotropies in optical and dielectric properties in thin films of stiff polyimides

Abstract

Thin films of poly(p‐phenylene biphenyltetracarboximide) (BPDA‐PDA), prepared by thermal imidization of the precursor poly(amic acid) on substrates, have been investigated by optical waveguide, ultraviolet‐visible (UV‐VIS), infrared (IR), and dielectric spectroscopies. The polyimide films exhibit an extraordinarily large anisotropy in the refractive indices with the in‐plane index n∥ = 1.806 and the out‐of‐plane index n⊥ = 1.589 at 1064 nm wavelength. No discernible effect of the film thickness on this optical anisotropy is found between films of ca. 2.1 and ca. 7.8 μm thickness. This large birefringence is attributed to the preferential orientation of the biphenyltetracarboximide moieties with their planes parallel to the film surface, coupled with the strong preference of BPDA‐PDA chains to align along the film plane. The frequency dispersion of the in‐plane refractive index n∥ is consistent with the results calculated by the Lorentz–Lorenz equation from the UV‐visible spectrum exhibiting several absorption bands in the 170–500 nm region. The contribution from the IR absorption in the range 7000–400 cm,−1 computed by the Spitzer‐Kleinmann dispersion relations from the measured spectra, adds ca. 0.046 to the in‐plane refractive index n∥. Tilt‐angle–dependent polarized IR results indicate nearly the same increase for the out‐of‐plane index n⊥. Application of the Maxwell relation then leads to the out‐of‐plane dielectric constant ε⊥ ≃ 2.7 at 1.2 × 1013 Hz, as compared with the measured value of ca. 3.0 at 106 Hz. Assuming this small difference to remain the same for the in‐plane dielectric constants ε∥, we obtain a very large anisotropy in the dielectric properties of these polyimide films with the estimated in‐plane dielectric constant ε∥ ≃ 3.4 at 1.2 × 1013 Hz, and ε∥ ≃ 3.7 at 106 Hz. © 1992 John Wiley & Sons, Inc. Copyright © 1992 John Wiley & Sons, Inc.