REVERSIBLE 2D TO 3D ELECTRODE TRANSITIONS IN POLYPYRROLE FILMS

Biological membranes change reversibly from a closed structure to an o pen one formed by channels and pores. This transition is triggered by ionic concentration gradients, and controls the movement of both muscu lar fibres and nervous pulses. Conducting polymers, like polypyrrole, show a similar behaviour when controlled electrochemically in an adequ ate solvent/electrolyte system. A compact structure is attained by pol arization at more cathodic potentials than about -900 mV vs. SCE, arri ving at a neutral state. In this situation. the material behaves as a 2D electrode: only the surface in contact with the electrolyte is elec trochemically active. The polymer bulk remains a semiconductor. Under anodic polarization, electrons are extracted from polymeric chains. Co ulombic repulsions between generated positive charges induce conformat ional movements, resulting in a slow expansion of the polymer. The str ucture becomes permeable to ions, hence resulting in a 3D electrode: e very polymeric chain actuates as an electrodic active interface. From an electrochemical point of view, the transition can be followed by me ans of variations in the charge vs. current transients during chronoco ulometric experiments. Experimental results can be explained by means of the Electrochemically Stimulated Conformational Relaxation (ESCR) m odel. (C) 1998 Elsevier Science B.V.