Contractile behavior of polypyrrole films in air under ac voltages

The polypyrrole films doped with tetrafluoroborate were electrochemically synthesized and contractile behavior was measured in air under ac voltages. It was found that the film did not respond at MHz frequencies but underwent gradual contraction where the changes in the strain, electric current, and temperature of the film were independent on the frequency in a range from dc to 1 MHz under the same effective value of the electric field. The resistance of the film was constant about 35 Ω but the capacitance decreased with the frequency, indicating that the electrically induced contraction of the film under ac voltage could be explained by the same mechanism under dc voltage where the dehydration of polymer chains and/or dopant ions due to the Joule heating.