Molecular aggregation of solid aromatic polymers. I. Small‐angle x‐ray scattering from aromatic polyimide film

Abstract

AbstractMolecular aggregation in a commercial polyimide film, Du Pont Kapton, was investigated by small‐angle x‐ray scattering (SAXS). From the analysis of the desmeared SAXS curve, it is concluded that aggregation in the Kapton film can be elucidated in terms of a two‐phase structure having electron density fluctuations within the phases. For comparison with the molecular aggregation in Kapton, molecular aggregation in polyimides synthesized in our laboratory was also investigated. It was found in this case that molecular aggregation is controlled by the initial imidization temperature. Molecular aggregation of polyamic acid and polyimide cyclized at a low temperature gives amorphous structures. On the other hand, molecular aggregation of polyimide cyclized at high temperatures gives two‐phase structures like that of Kapton film. The SAXS curve for a polyimide having the two‐phase structure shows a peak due to interference between ordered regions. The two‐phase structure of the polyimide can be explained in terms of a one‐dimensional model. The more ordered phase is produced at the higher initial imidization temperature. The relative density difference between two phases is only a few percent for polyimide films cyclized at high temperatures. This result shows that the two‐phase structure of aromatic polyimide differs essentially from that of ordinary crystalline polymers.