Proton exchange membranes by radiation grafting of styrene onto FEP films. IV. Evaluation of the states of water

The nature of water in proton exchange membranes, obtained by radiation grafting of styrene onto poly(tetrafluoroethylene‐ co ‐hexafluoropropylene) (FEP) films and subsequent sulfonation, was investigated by differential scanning calorimetry. It was found that the membranes contained three different types of water, which were categorized as freezing‐free, freezing‐bound, and nonfreezing water. The degree of grafting had 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, freezing‐free and freezing‐bound water per ionic site ratios increased with increasing graft levels. A correlation between the water uptake and the structural changes occurring in the membrane as a function of the degree of grafting was established.