Positron Annihilation Lifetime Study of Subnano Level Free Volume Features of Grafted Polymer Electrolyte Membranes for Hydrogen Fuel Cell Applications

The subnano level free-volume hole features of poly(styrene sulfonic acid) (PSSA) grafted poly(ethylene-co-tetrafluoroethylene) polymer electrolyte membranes (ETFE-PEMs) are investigated using positron annihilation lifetime spectroscopy (PALS). The hole sizes are formed mainly at the graft-polymerization step and are not altered dramatically at the sulfonation. With increasing grafting degree, tau(4), standing for the annihilation of ortho-positronium (o-Ps) in the larger free-volume holes of the mobile amorphous layers and the PSSA grafts outside of the lamellae, decreases monotonously, whereas tau(3), representing for that in the smaller free-volume holes of the lamellar amorphous regions, the PSSA grafts, and the interface zones inside the lamellae, only shows a slight decrease. From the viewpoint of free-volume hole sizes, gas molecules are predicted to pass dominantly through the mobile amorphous phases and the PSSA grafts outside of the lamellae. Specifically, the limited hole concentration induced by confining lamellae at the interfacial zones is observed. Thus, the obtained results are associated with a significant reduction of gas passing and the moderate or higher tensile strength for the ETFE-PEMs in comparison to Nafion-212.