ABSTRACTThin films of poly(p-phenylene biphenyltetracarboximide), prepared by thermal imidization of the precursor poly(amic acid) on substrates, have been investigated by optical waveguide, UV-visible, infrared (IR), and dielectric spectroscopies. The polyimide films exhibit an extraordinarily large anisotropy in the refractive indices with the in-plane index n║ = 1.852 and the out-of-plane index n┴ = 1.612 at 632.8 nm wavelength, indicating a strong preference of polymer chains to orient along the film plane. No discernible effect of the film thickness on this optical anisotropy is found in the range of ca. 0.4 μm to 7.8 μm in thickness. The frequency dispersion of the in-plane refractive index to 1.06 μm wavelength is consistent with the results calculated by the Lorentz-Lorenz equation from the UV-visible spectrum. The contribution from the entire IR range from 7000 to 200 cm,−1 computed by the Spitzer-Kleinmann dispersion relations from the measured spectra, adds ca. 0.07 to the in-plane refractive index n║. Approximately the same increase is assumed for the out-of-plane index n┴, based on the tilt-angle dependent IR results. Application of the Maxwell relation leads to the out-of-plane dielectric constant ε┴≃2.8 at ca. 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 a very large anisotropy in the dielectric properties of these polyimide films with the estimated in-plane dielectric constant ε║≃3.5 at ca. 1013 Hz, and ε.≃3.7 at 106 Hz.
On the behavior of the Lorenz equation backward in time
