Chemical bonding at the polyimide surface

Abstract

Surface spectroscopy techniques (UPS and XPS) have been combined with theoretical calculations to investigate the chemical bonding characteristics of the high-temperature polymer polyimide. Both valence and core electronic structure of the intrinsic (‘‘clean’’) polyimide surface as observed in UPS and XPS are well described by the results of the self-consistent-field molecular orbital calculations. Even though the polyimide is already fully cured to complete the imidization reaction and form the final molecular structure of the polymer chains, the valence electronic structure of such as-prepared surfaces changes significantly upon in situ annealing at temperatures above ∼195 °C but well below those which decompose the polymer (≥400 °C). This annealing causes desorption of gas phase H2O and produces the intrinsic polyimide surface. The sorption/desorption of H2O is reversible and thermally activated, and it seems to play an important role in the chemical properties of the polyimide surface. These changes upon annealing and H2O exposure appear primarily in the valence electronic structure as observed by UPS, but corresponding alterations of the core level spectra seen by XPS are considerably smaller.