PROTON-EXCHANGE MEMBRANES BY RADIATION GRAFTING OF STYRENE ONTO FEP FILMS .4. EVALUATION OF THE STATES OF WATER

The nature of water in proton exchange membranes, obtained by radiatio n grafting of styrene onto poly(tetrafluoroethylene-co-hexafluoropropy lene) (FEP) films and subsequent sulfonation, was investigated by diff erential scanning calorimetry. It was found that the membranes contain ed three different types of water, which were categorized as freezing- free, freezing-bound, and nonfreezing water. The degree of grafting ha d a considerable influence on the relative ratios of these three types of water molecules. The nonfreezing water per ion-exchange site ratio was found to be independent of the degree of grafting. However, freez ing-free and freezing bound water per ionic site ratios increased with increasing graft levels. A correlation between the water uptake and t he structural changes occurring in the membrane as a function of the d egree of grafting was established. (C) 1995 John Wiley and Sons, Inc.