Hydration and Ionic Conductivity of Model Cation and Anion-Conducting Ionomers in Buffer Solutions (Phosphate, Acetate, Citrate)

We studied the gravimetric and volumetric water uptake and ionic conductivity of two model ionomers, cation-conducting sulfonated poly(ether ether ketone) (SPEEK) and anion-conducting polysulfone-trimethylammonium chloride (PSU-TMA), after immersion in phosphate, acetate, and citrate buffer solutions. The equilibrium swelling of SPEEK and PSU-TMA ionomer networks was determined as a function of the pH and buffer composition. The hydration data can be interpreted using the osmotic swelling pressure dependence on the ion-exchange capacity of the ionomers and the concentration of the electrolyte solutions. In the case of SPEEK, anisotropic swelling is observed in diluted buffer solutions, where the swelling pressure is higher. The large water uptake observed for citrate ions is due to the large hydration of this bulky anion. The ionic conductivity is related to the conducting ions and, in the case of SPEEK, to sorbed excess electrolyte. The highest ionic conductivity is observed after immersion in phosphate buffers. Ionic cross-linking is, for the first time, observed in the case of an anion-conducting ionomer in the presence of divalent citrate ions, which limits the volumetric swelling and decreases the ionic conductivity of PSU-TMA.