Speed and strain of polypyrrole actuators: dependence on cation hydration number

Abstract

Polypyrrole films have been characterized by simultaneous cyclic voltammetry driven force-displacement measurements. The aim was to clarify the role of cations in the electrolyte on the speed of response and on the strain of the film. The strain as a function of actuation frequency was studied in alkali metal chloride aqueous electrolytes. The intention was to test the hypothesis of the division of the inserted H2O molecules into categories: a smaller number strongly bound to the cation (corresponding to the inner solvation shell) and a larger number entering the film after the insertion of the cations because of forces related to osmotic pressure difference. The two processes have very different time constants: The solvated H2O molecules are associated directly with the cations, and are therefore inserted in a faster process, whereas the second type enters the film much more slowly. At higher frequencies, the strain depends almost exclusively on insertion of strongly solvated cations and therefore depends on the hydration number of the cations: Li+ (hydration number ~5.4) gives more strain than Na+ (~4.4) and much more than Cs+ (~0) as predicted by the model.