Behaviour of pHsensitive polymers on metal electrodes

Abstract

The behaviour of two different enteric coatings with pHdependent properties has been investigated as a model system for the degradation of polymer coated electrodes using impedance spectroscopy. Since the stability of these polymers depends strongly on pH, it was possible to halt the breakdown at well defined stages, which were investigated systematically. Although both polymers, Eudragit S-100 and a modified maleic anhydride copolymer, have similar chemical structures, they had different breakdown mechanisms. Eudragit S-100 films dissolved, initially forming electrolyte filled pores through which electrolyte penetrated to the gold and spread along the electrode surface. The electrolyte layer between coating and electrode was found to be typically 0.7μm thick for a 30μm thick coating with a porosity of 2×10−4. Layers of the modified maleic anhydride copolymer, unlike the Eudragit S-100, did not dissolve, but swelled in the course of the breakdown. It was shown that only positively charged ions can penetrate this polymer during breakdown. For the redox couple [Ru(NH3)6]2+/[Ru(NH3)6]3+ a mean diffusion coefficient of D=0.7×10−8cm−2s−1in the swollen polymer was determined.