Cation-conducting ionomers made by ion exchange of sulfonated poly-ether-ether-ketone: Hydration, mechanical and thermal properties and ionic conductivity

• Cation-conducting (K, NH4, Mg, Ca, Zn) membranes prepared by ion-exchange route.
• Hydration measured and described by physical model.
• Schröder paradox observed only in non-crosslinked membranes with IEC>2 eq/kg.
• Ionic conductivity ranges from 3 mS/cm for K, NH4 and Mg-exchanged membranes to 0.6 mS/cm for Ca and Zn.

The hydration in liquid water of various thermally treated SPEEK membranes was determined and confronted with data in saturated water vapor. Significant differences, which correspond to the so-called Schröder paradox, are observed only in un-crosslinked ionomers with IEC>2 eq/kg.After cation exchange in electrolyte solutions (2 and 0.1 mol/L), the hydration and density of the ionomers was determined and confronted with a physical model. There is no clear correlation with cation properties, such as ionic radius or Robinson–Stokes hydration numbers.The ionic conductivity ranges from 3 mS/cm for K, NH4 and Mg-exchanged membranes to 0.6 mS/cm for Ca and Zn. Although the latter might be related to ionic cross-linking by the divalent cations, the relatively high conductivity of Mg ions might be useful for electrochemical applications.