Characterizing the Effect of Temperature Increase on Polypyrrole Active Strength and Stress Rate

Conducting polymers can be utilized as actuators due to the materials’ ability to undergo volumetric change caused by an electrochemical stimulus. Polypyrrole is an attractive electroactive polymer and capable of producing large active stress. Its mechanical performance can be improved by increasing temperature. This work describes a custom built device that is capable of performing dynamic mechanical analyses and electrochemistry simultaneously. In addition, there is a temperature control system that heats (or cools) with Peltier thermoelectric devices controlled by a PID controller. In this study, we characterized the effect of temperature increase on polypyrrole actuation strength and stress rate. For approximately each increment of 10°C from 27–83°C, stiffness measurements and isometric tests in 1-butyl-3-methylimidazolium hexafluorophosphate were done with sample preloaded to about 4 MPa. Results showed that the stiffness decreased by 21% as the temperature was elevated from 25–80°C. The maximum charge per polymer volume increased by 983% as the temperature was increased from 27–75°C and started to level off past 75°C. The peak stress changed with temperature in a similar trend as maximum charge per volume. Peak stress was 0.2 MPa at 27°C and increased to 7 MPa as temperature was increased to 75°C. Moreover, the stress rate increased until 85°C. The results suggest that peak stress depends upon the ionic mobility as opposed to stiffness since both peak stress and charge start to plateau past 75°C.