Free-volume structure of fluoropolymer-based radiation-grafted electrolyte membranes investigated by positron annihilation lifetime spectroscopy

In the field of polymer-electrolyte-membrane (PEM) fuel cell technology, the structures of free-volume holes in the PEMs are very important because they are correlated to the supplied-gas crossover phenomenon, which sometimes deteriorates the cell performance. In this study, we investigated the size and location of free-volume holes in the crosslinked-polytetrafluoroethylene (cPTFE) based radiation-grafted PEMs by positron annihilation lifetime (PAL) spectroscopy. For comparison, the base cPTFE and polystyrene grafted films were also measured. From the analysis of PAL spectra, it was found that there were free-volume holes with different radius of 0.28-0.30 nm and 0.44-0.45 am. The smaller holes should be located in both PTFE crystallites and poly(styrene sulfonic acid) grafts, while the larger holes are considered to exist in amorphous PTFE phases.