XPS Evidence of Redox Chemistry between Cold Rolled Steel and Polyaniline

Abstract

The electrochemical response of polyaniline (PANI) in its undoped intermediate (emeraldine base, EB) oxidation state coated on cold rolled steel (CRS) or glass substrate has been evaluated by X-ray photoelectron spectroscopy (XPS). The oxidation state of the polymer was determined from the ratio of amine vs imine nitrogen chemical states observed in the N(1s) photoelectron spectrum. A room-temperature air-dried PANI coating cast from NMP solution onto CRS was found to be reduced relative to its as-synthesized, air-stable state. Heating the PANI coated on CRS in the absence of air (165 °C, in ultrahigh vacuum) produced an even deeper reduction, resulting from the transfer of electronic charge to the polymer from the metal substrate. Subsequent brief exposure of this reduced PANI coating to air at elevated temperature (165 °C) reoxidized the PANI to the EB state. Analogous treatment of a PANI coating on glass caused no such change in the oxidation state of the polymer. Unlike steel, which provides a redox couple with PANI, glass is electroinactive. These findings are consistent with the hypothesis that PANI-EB can catalyze or otherwise enable passivation of steel surfaces, through electrochemical interaction with steel substrate. Neat PANI-EB heated to 200 °C under N2 within an FTIR diffuse reflectance cell demonstrated loss of interchain H-bonding, presumably leading to softening. The FTIR spectrum of PANI-EB which had been exposed to air at 200 °C gave no indication of polymer oxidation.