The state of water in styrene‐grafted and sulfonated poly(vinylidene fluoride) membranes

Abstract

AbstractThe state of water in proton exchange membranes prepared by pre‐irradiation (electron beam, 100 kGy) grafting of styrene onto poly(vinylidene fluoride) films (PVDF‐g‐PS), followed by sulfonation (PVDF‐g‐PSSA), has been studied with thermal analysis, Raman spectroscopy and small angle X‐ray diffraction (SAXS). Raman spectra show that, in addition to free liquid water in the membranes, single water molecules are weakly bound to the polymer backbone. Thermal analysis shows that there are three types of water molecules in the membrane; non‐freezable water associated with the ionic sites, freezable free water, and freezable bound water. The amount of non‐freezable water is around 10 H2O/SO3H (mol/mol), and is independent of the degree of grafting (d.o.g.). The amount of freezable water is strongly dependent on the d.o.g. as long as the grafting has not penetrated the whole of the film, and reaches a value of around 40 H2O/SO3H (mol/mol) above a d.o.g. of 50%. The conductivity of membranes containing only the non‐freezable water is low, i. e. the ionically bound water alone does not form the domains necessary for proton and water transport. SAXS measurements show that water/sulfonic acid clusters in hydrated PVDF‐g‐PSSA membranes with a Bragg distance of 25 Å are formed; these form the ion conducting channels in the membrane.