Surface analysis of the proton exchange membranes prepared by pre-irradiation induced grafting of styrene/divinylbenzene into crosslinked thin PTFE membranes

The proton exchange membranes were obtained by pre-irradiation induced grafting of styrene with or without divinylbenzene (DVB) into thin crosslinked polytetrafluoroethylene (RX-PTFE) membranes with the thickness around 10 mm and successive sulfonation. The surface chemical structure and morphology of the non-grafted, the grafted, and the grafted and sulfonated RX-PTFE membranes were studied by means of X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analysis. From the F/C atom ratio result, it was known that there are mainly polystyrene grafts on the surfaces of the styrene grafted RX-PTFE membranes, while there are mainly PTFE chains on the surfaces of the RX-PTFE membranes grafted by styrene together with DVB. ‘‘Wave’’ shape morphology on the surfaces of the grafted RX-PTFE membranes was observed. The F/C atom ratios on the surfaces of the grafted and sulfonated membranes are little lower than the corresponding grafted membranes before sulfonation. On the surface of the sulfonated membranes, the ‘‘wave’’ shape morphology was also observed. The distributions of the polystyrene grafts in the RX-PTFE membranes matrixes were strongly affected by the concentration of the crosslinking co-monomer, DVB, used in the graft polymerization, and the sulfonation will not change the distribution of the grafts obviously.