An in situ spectro-electrochemical monitoring of aqueous effects on polymer/metal oxide interfaces

Abstract

A spectro-electrochemical setup of Fourier transform infrared spectroscopy (FTIR) in the Kretschmann geometry and odd random phase multisine electrochemical impedance spectroscopy (ORP EIS) is applied to in situ monitor the effect of an aqueous electrolyte on the polymer/metal oxide interface. The interfacial interactions of an ultrathin polyacrylic acid (PAA) film on an aluminum oxide surface are identified as carboxylate ionic bonds and changes induced by the effect of water diffusion at the interface are monitored in situ. Initially after electrolyte exposure, an increase in ionic bonding is observed. However, eventually the interfacial interactions are replaced by water molecules, leading to macroscopic delamination. By comparing a variation of oxide types, the stability of the interfacial bonds is linked to the amount of free hydroxyl groups on the aluminum oxide surface. An electric equivalent circuit is proposed to model the ORP EIS response of the PAA/aluminum oxide system and the fitted resistance values could be interpreted in a physically meaningful way. Finally, a poly(methyl methacrylate) (PMMA) deposition on aluminum oxide is investigated to explore the effect of a variation in functional groups present at the polymer/metal oxide interface. It is shown that PMMA forms a more stable interface than PAA on native aluminum oxide. This work demonstrates that IR spectroscopy in the Kretschmann geometry and ORP EIS are suited techniques to in situ probe interfacial bonds at polymer/metal oxide systems exposed to aqueous conditions. Moreover, a variation of the surface properties of the metal oxide as well as the functional groups of the polymer alter the stability of their mutual interface when exposed to aqueous conditions.