CHARACTERIZATION OF PMDA-ODA POLYIMIDE FILMS BY EXTERNAL REFLECTANCE INFRARED-SPECTROSCOPY

Reflectance infrared spectroscopy has been carried out for quantitativ e structural analysis of polyimide films thinner than 1000 Angstrom. R eflection spectra have been simulated for isotropic polyimide films of specific thickness on a metallic substrate. Structural analysis was b ased on a number of infrared active vibrations particularly sensitive to changes in chain conformation or packing. The structure of these fi lms was compared with that obtained by annealing experiments on free s tanding films and crystalline powders. For adsorbed films with thickne sses <150 Angstrom, very low packing order was found. Frequency shifts in films of varying thicknesses were observed to be sensitive to chan ges in chain conjugation which are dependent on the coplanarity of rin gs along the backbone. Utilizing vibrations of known transition moment direction allowed one to characterize the degree of segmental orienta tion relative to the surface normal of variously prepared films. Spect roscopic data also revealed changes in orientation of the rigid planar structures in PMDA-ODA polyimide with respect to the surface normal u pon variation of film thickness. The imide ring, preferentially aligne d perpendicular to the surface normal in thin films, changes toward an isotropic orientation distribution. The chain orientation reached a c onstant value of f = -0.28. These spectroscopic studies were corrobora ted by electron diffraction tilting experiments obtained from flat fil ms lifted from their respective gold substrates.