Chemistry at interfaces: Electropositive metals on polymer surfaces

This paper presents a study of chemical interactions between polymer surfaces and metal atoms deposited from the vapor phase. Such interactions may play an important role in interfacial metal-polymer adhesion. The chemical nature of the interface formed when an electropositive metal (chromium or cesium) is deposited onto the surface of PMDA-ODA polyimide has been investigated using chemical model studies coupled with photoelectron spectroscopic techniques. X-ray photoelectron spectroscopy, synchrotron-radiation-excited core-level photoemission, and near-edge X-ray absorption spectroscopy were used to analyze changes in polymer surfaces during deposition of chromium and cesium. Chemical model studies using cyclic voltammetry and UV-visible spectroscopy were performed using several simpler polymers or monomeric model compounds which contained structural subunits of the polyimide. Results of these experiments show that chromium (and other electropositive metals studied so far) initially reacts rapidly with the carbonyl groups of polyimide, causing reduction of the dianhydride portion of the polymer, with concomitant chromium oxidation. Continued deposition of chromium onto the reacted polymer surface results in the formation of chromium carbide, oxide, and nitride species, indicating a disruption of the polymer chemical structure.