Effects of humidity on the performance of ionic polymer-metal composite actuators: Experimental study of the back-relaxation of actuators

This article focuses on the dependence of water uptake on the displacement, velocity, mechanical force, and charging profiles of perfluorinated ionomer-platinum/Li+-based actuators. Both the displacement and force generation were found to be strongly dependent on the humidity. The primary reason for this effect is a decrease in the stiffness as a result of the humidity. The actuators demonstrated a dramatic reverse motion and a negative force, and this subsequent relaxation was dramatically decreased by decreasing humidity. This relaxation process can be explained by the slow diffusion of water into the elastically softened anode and out of the stiffened cathode. There are no clear inflection points on the charging profile during the reverse relaxation, and this suggests that the relaxation process does not involve a major redistribution of counter cations. An increase in water uptake resulted in an enhancement of the velocity of the displacement. A continuous generation of force was also examined by scanning potential, and the force was proportional to the potential. Humidities near 50-60% (i.e., water uptakes of ca. 5 wt %) gave a better actuator bending performance.