Reversible 2D to 3D electrode transitions in polypyrrole films

Abstract

Biological membranes change reversibly from a closed structure to an open one formed by channels and pores. This transition is triggered by ionic concentration gradients, and controls the movement of both muscular fibres and nervous pulses. Conducting polymers, like polypyrrole, show a similar behaviour when controlled electrochemically in an adequate solvent/electrolyte system. A compact structure is attained by polarization at more cathodic potentials than about —900 mV vs. SCE, arriving at a neutral state. In this situation, the material behaves as a 2D electrode: only the surface in contact with the electrolyte is electrochemically active. The polymer bulk remains a semiconducter. Under anodic polarization, electrons are extracted from polymeric chains. Coulombic repulsions between generated positive charges induce conformational movements, resulting in a slow expansion of the polymer. The structure becomes permeable to ions, hence resulting in a 3D electrode: every polymeric chain actuates as an electrodic active interface. From an electrochemical point of view, the transition can be followed by means of variations in the charge vs. current transients during chronocoulometric experiments. Experimental results can be explained by means of the Electrochemically Stimulated Conformational Relaxation (ESCR) model.