X-RAY-SCATTERING STUDIES OF THIN-FILMS OF PHOTOSENSITIVE POLYIMIDES

Using the techniques of wide-angle X-ray diffraction (WAXD) and small- angle X-ray scattering (SAXS), the morphology of polyimide thin films thermally imidized from several photosensitive polyimide (PSPI) precur sors has been investigated and compared with that of the thin films pr epared from the corresponding poly(amic acid) precursors: poly(4,4'-ox ydiphenylene pyromellitimide) (PMDA-ODA), poly(p-phenylene biphenyltet racarboximide) (BPDA-PDA) and poly(4,4'-oxydiphenylene benzophenonetet racarboximide) (BTDA-ODA). The WAXD results indicate that regardless o f the precursor origin, the BTDA-ODA polyimide is amorphous, whereas t he other polyimides exhibit a molecular order. For both the PMDA-ODA a nd the BPDA-PDA, the molecular order is relatively higher in the films prepared from the PSPI precursors than in those from the correspondin g poly(amic acid)s, indicating that during thermal imidization, the ph otosensitive groups play an important role to improve the mobility of the polymer chains, which may be critically needed to make better mole cular packing, in spite of their bulkiness having the potential to hin der the molecular packing. In Lorentz-corrected SAXS analyses, a long period (130-156 angstrom mean periodicity) was observed for all the po lyimides except the PMDA-ODA. In particular, the microstructure in the BPDA-PDA could be described by an extended chain-based two-phase (ord ered and less ordered phase) model with diffuse boundaries because of its high chain rigidity. In addition, the Guinier SAXS analyses indica te the presence of voids in all the polyimide films, regardless of the precursor origin. The size of voids was 251-349 angstrom in radius, d epending upon the type of polyimide.