Mechanism of electromechanical actuation in polypyrrole

Conductive polymers, such as polypyrrole (PPy), undergo unusual mechanical responses when subjected to applied electrical potentials. This study reports on investigations into the molecular mechanisms of such responses in PPy, with specific reference to the role of the counterion and electrolyte cation. It has been found that a four-stage oxidation/reduction process occurs in the general case: (1) oxidation by cation ejection; (2) oxidation by anion incorporation; (3) reduction by anion ejection; and (4) reduction by cation incorporation. When immobile counterions and/or electrolyte anions were used, steps 2 and 3 were suppressed. Also, when divalent cations were used steps 1 and 4 produced minor volume changes only. Reproducible electroactuation behaviour can be achieved beyond the initial reduction of the polymer when immobile counterions are used, and when divalent cations are present in the electrolyte.