A contact angle investigation of the surface properties of selected proton-conducting radiation-grafted membranes

The surface and interfacial properties of proton-conducting ion-exchange membranes are quite important in their application as solid polymer electrolytes in fuel cells and other electrochemical devices. Radiation-grafted ion-exchange membranes based on poly(ethylene-alt-tetrafluoroethylene) (ETFE) and poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) films have been characterized by measuring the contact angles of several liquids on their surfaces at various stages in their preparation and as a function of parameters in their preparation and their composition. Contact angles were measured using the recently-advanced contact angle and advancing contact angle methods. Surface energies were calculated based on their separation into either (1) Lifshitz-van der Waals (LW) and acid and base components or (2) dispersive and polar (non-dispersive) components. The Lifshitz-van der Waals components of the surface energies obtained in the former method were similar in magnitude to the dispersive surface energy components obtained in the latter method; however, the acid–base components obtained using the acid–base method were smaller in magnitude than the polar components obtained in the dispersive-polar method. The total surface energies obtained using either method varied as: starting film ≈ irradiated film < grafted film < sulfonated membrane. Wetting and surface properties were found to vary systematically as a function of graft level and other preparation parameters. For comparison purposes, some FEP-based radiation-grafted films and membranes and commercial perfluorinated membranes were also analyzed using the acid–base method. Qualitatively their surface energies and surface energy components resemble those of the respective ETFE-based grafted films or membranes.