Analysis of high performance polypyrrole actuators

Electrochemical actuators based on polypyrrole (PPy) have been constructed and characterized. The actuation performance was analyzed in terms of several main parameters: thickness, scan rate, applied stress, cycle life and creep. Two different dopants and two different forms (tubes and free standing films) were produced to investigate their effects on the actuation performance and the underlying mechanisms of actuation. The actuation strain generated by the PPy materials when an external load was applied was found to be the sum of three individual time-dependent processes: strain generated directly from charge and solvent injection; strain generated from elastic modulus changes; and strain caused by creep. It was found that the strain generated from charge and solvent injection behaved like a diffusion-controlled process and the process were found to be relatively slow, whether during ramp or step voltage scanning. A semi-empirical model based on Fickian diffusion was built to predict the actuation strain under different loads at different scan rate in ramp voltage scanning and for different actuator thicknesses. By using the model, work and power of PPy actuators were analyzed. A consequence of the slow charging process is that the thicker PPy materials produce a larger work output but a lower power output. The effect of dopant used for the PPy was quite significant with bis-(trifluoromethanesulfonimide) (TFSI) producing higher strains for equivalent conditions than hexafluorophosphate (PF6) doped PPy. PPy/TFSI films were found to respond slowly to a step voltage change with a significant amount of strain occurring after the applied current had decayed to zero. This contribution to strain was assumed to be due to solvent ingress associated with osmotic pressure. It was also found that the large strains produced in PPy/TFSI degraded rapidly cycle by cycle. Several approaches were investigated to improve stability. Current stimulation rather than voltage stimulation ensures equal oxidation and reduction per cycle and resulted in a constant 4% strain for at least 100 cycles. Creep of PPy at different redox state was investigated and modeled for the first time using classical spring-dashpot models. The model could successfully fit and predicting the creep behavior of PPy at different redox states. The viscoelastic parameters are found to be voltage-dependent with the modulus (instantaneous and delayed) and viscosity decreasing when the PPy/TFSI was reduced. By using Boltzmann superposition principle, an attempt to combine all strains was conducted.