Effect of Equivalent Weight on Water Sorption, PTFE-Like Crystallinity, and Ionic Conductivity in Bis[(Perfluoroalkyl)Sulfonyl] Imide Perfluorinated Ionomers

Measurements of water absorption and ionic conductivity as a function of relative humidity (RH) were carried out on membranes comprised of bis[(perfluoroalkyl)sulfonyl] imide ionomers of equivalent weights 1470, 1200, and 1075 g equiv^-1, and on a sample of the perfluorosulfonic ionomer Nafion™ ionomer of equivalent weight 1100 g equiv^-1 for comparison. All of the ionomers exhibited decreased water absorption and ionic conductivity with decreasing RH. Within the sulfonyl imide series, the extents of the decreases correlated with ionomer equivalent weight, such that the highest equivalent weight ionomer (1470 g equiv^-1) exhibited the lowest conductivity and the sharpest drop in conductivity with decreasing RH compared with the lower equivalent weight ionomers (1200 g equiv^-1 and 1075 g equiv^-1). This observation also correlates with WAXD and DSC data which indicate an increase in PTFE-like crystallinity with increasing ionomer equivalent weight. The observed dependencies of water absorption and ionic conductivity on RH and ionomer equivalent weight for the bis[(perfluoroalkyl)sulfonyl] imide ionomers are similar to that which has been reported for Nafion™, which suggests that the phase-separated ionomer structures and the effect of water sorption on the phase-separated structure are qualitatively similar for the two ionomer classes.