B. Gupta et al., PROTON-EXCHANGE MEMBRANES BY RADIATION GRAFTING OF STYRENE ONTO FEP FILMS .4. EVALUATION OF THE STATES OF WATER, Journal of applied polymer science, 57(7), 1995, pp. 855-862
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.