Actuation of polypyrrole in aqueous sodium hexafluorophosphate solution has been shown to produce relatively large strains. However little has been published on appropriate potential range of actuation in this electrolyte. This information is clearly crucial for applications. Our particular interest is in disposable applications where a relatively small number of cycles are needed, and maximum strain is desired. The electrochemical degradation as a function of voltage is investigated by cycling the film between fixed voltages and measuring the charge transfer. The experiment was done on a glassy carbon substrate in order to reduce effects of change in resistance with oxidation state, preventing actuation. The dependence of charging on voltage and the rate of reduction in the extent of charging are measured. The voltage range for effective operation of the device was found to be -0.4 V to 0.8 V versus a Ag/AgCl reference electrode in order to achieve stable performance over at least 30 minutes. The mechanisms of degradation at potentials beyond 0.8 V appear to be the substitution of hydroxyl ions in the polymer backbone, as suggested in reports on degradation of polypyrrole in other electrolytes. An observed reduction in charge transfer rate at potentials lower than -0.4 V is consistent with a reduction in ionic conductivity at highly reduced states, as has also been suggested in the literature.

Polypyrrole actuators offer attractive possibilities due to their large electrochemical stress (>5MPa), moderate to large strain (>2 %) and low voltage operation (<2 V). However, little is known about their temperature dependence response, with nearly all previous tests performed at room temperature. To test the effects of increasing temperature, polypyrrole films in water and propylene carbonate were thermally cycled twice from 22°C to 80°C. Length and actuation were measured before, after and between cycling. Results show that polypyrrole in TBAPF6(propylene carbonate) underwent a 4.8% irrecoverable contraction during the first cycle and lost half it’s actuation each cycle. Polypyrrole in NaPF6(aq) showed a 2.1% initial expansion in length on the first cycle followed by a 2.1% contraction on the next cycle, while active strain amplitude dropped from 7.7% to 5.9% to 4.9%. Polypyrrole in NaCl(aq) has a net contraction of 6% over two cycles with no significant change in it’s original 3.5% actuation amplitude. This suggests that although films in NaPF6(aq) have the best initial strain, films in NaCl(aq) maintain the most consistent strain amplitude in response to temperature on the timescales observed. Strain to charge ratio was found to decrease slightly, but the majority of the loss in actuation for films is correlated with a reduction in charge transfer each cycle. This reduction may result from a reduction in the active volume of film.

We summarize and extend our previous study of multilayer film adsorption on planar surfaces. The density profile of the adsorbed film is studied by a mean-field approximation. This leads to a self-consistent potential and a correction in thickness prediction to the result of Lifshitz and co-workers. The correction is determined by the van der Waals interaction and film compressibility and is found to be important for He films. A simple model is presented also for the physisorption isotherm inside a cylindrical pore. Comparisons with experimental data are provided.