Electrochemomechanical Behaviour of Bilayer and Trilayer Films with PEDOT and PPY Conducting Polymers

A detailed study on bilayer and trilayer films prepared with polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymers is reported. Both polymers are doped with dodecyl benzenesulfonate (DBS) anions. These multi layer films were prepared electrochemically so that the PEDOT layer is very thin compared to that of the PPy layer, and characterized using cyclic voltammetry, optical absorption spectroscopy and electrochemical quartz crystal microbalance (EQCM) techniques.

Actuators of bilayer and trilayer free standing films were characterized electromechanically under a constant load of 1.5 g. The cyclic voltammograms as well as the UV-visible spectra of PEDOT and PPy are very different, pointing towards the possibility of being able to separate the two layers experimentally – even when combined in a single film.

Bilayer results show combined characteristics of each individual polymer. In trilayer films, the reduction of inner and outer PPy layers takes place at two different potentials. The oxidation occurs at one potential only. The separation between the two reduction peaks depends on the thickness of PEDOT layer, the scan rate and the concentration of cycling electrolyte.

This separation becomes smaller with the number of cycles, indicating the enhancement of ion diffusion through the PEDOT layer. Electrochemomechanical measurements show that the strain generated in the polymer significantly decreases with increasing scan rate. Rapid increment in strain and cycling charge is observed during the first few cycles.

Bilayer film shows a significant increase in the strain measured at higher scan rate (> 100 mV s-1). The force generation between reduced and oxidized states is much higher for trilayer films and higher for bilayer films than that in a single layer of PPy. These differences are not linked to the Young’s modulus of these films.

The addition of a thin PEDOT does not change the Young’s modulus, but changes the force generation significantly.