Electrochemical Reduction of PMDA-ODA and its Effect on Metal/Polyimide Interfacial Reliability

Abstract

Numerous polymers, including many polyimides, are known to exhibit reversible electrochemical redox behavior. This phenomenon has been observed for thermally imidized films in aqueous electrolytes, and for chemically and thermally cured polyimides in nonaqueous electrolytes. Here we report that even relatively thick chemically imidized polyimides exhibit reversible redox behavior in simple aqueous electrolytes. In addition, the effect of this electroactivity on metal to polyimide adhesion was investigated by forming metal electrodes on Kapton®’ polyimide film and applying a cathodic bias in aqueous electrolyte solutions. UV-visible electronic absorption spectroscopy, Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and Cyclic Voltammetry (CV) demonstrate that the species formed at the solvent saturated polyimide/metal interface is the singly reduced or “radical anion” form of Kapton®’ paired with a charge-balancing counterion. The rate of formation of the radical anion has been found to be consistent with calculated hydrated-cation radial size and measured equivalent conductance data, relative cation acidity, and solution pH. When alkali metal counterions are used, reversible redox behavior of PMDA-ODA is preserved with polyimide mediated electron transfer into the bulk polyimide, or to oxygen (in O2 containing solutions), possible. Electron transfer to oxygen is believed to result in a locally high concentration of hydroxide anions at the metal/polyimide interface. Temperature and humidity exposure accelerates the rate of polyimide hydrolysis by this nucleophile, resulting in substantially increased rates of metal to polyimide adhesion degradation. When counterion cations are used, which are more acidic in nature, the stability of the radical anion and cation complex is increased, effectively slowing the rate of polyimide mediated electron transfer. At pH below 4.5 a highly stable covalent radical-alcohol complex is suggested to result from direct protonation of the radical anion.