Side-chain degradation of perfluorosulfonic acid membranes: An ab initio study

Chemical degradation of the side-chain of perfluorosulfonic acid (PFSA) membranes by hydroxyl radicals (•OH) is examined with electronic structure calculations. The energetics associated with homolytic bond cleavage and for the sequence of reactions involved in the degradation was determined. Results show that the degradation of side-chain begins with the cleavage of the C–S bond. The sequence of reactions of the side-chain with dOH indicates scission of the backbone yielding reactive end-groups. The kinetics of the C–S bond cleavage was studied via: (i) reaction of anionic fragment with a •OH; and (ii) decomposition of fragment radical. The activation energy for the second pathway was calculated to be;11 kcal/mol lower requiring a change in symmetry of the molecular geometry of the sulfonate group from trigonal pyramidal to trigonal planar. This suggests that although the C–S bond may be the weakest in the side chain of a PFSA ionomer, its cleavage is kinetically hindered.