Water content criterion for relaxation deformation of Nafion based ionic polymer metal composites doped with alkali cations

Water-based ionic polymer-metal composites (IPMCs) exhibit an anode deformation followed by a back-relaxation deformation under a direct current voltage. The latter is strongly related to the water content. This Letter proposes an evaluation method of the critical water content for relaxation deformation. Based on Nafion-IPMC doped with various alkali cations, continuous deformations are measured under a periodic pulse voltage in air. A transformation of the relaxation deformation is found to exist between two successive pulses. The critical water content is confined between the two pulses and evaluated gravimetrically. Removal of no more than 4 wt. % water or 1.4-2 water molecules per cation from the saturated material can completely eliminate the large relaxation deformation in Nafion-IPMC doped with alkali cations.