Positron annihilation lifetime study of graft-type fluorinated polymer electrolyte membranes

Positron annihilation lifetime spectroscopy (PALS) measurements were performed on polymer electrolyte membranes (PEMS) synthesized by radiation-induced graft polymerization of styrene into crosslinked-polytetrafluoroethylene (cPTFE) films and subsequent sulfonation. The base cPTFE and polystyrene grafted films were also measured as references. The cPTFE based PEM was found to have free-volume holes with different radius of 0.28-0.30 nm and 0.44-0.45 nm. The larger holes may be located in PTFE amorphous regions, while the smaller ones are considered to exist in both PTFE crystallites and poly(styrene sulfonic acid) grafts containing no water. Introduction There has been increasing interest in polymer electrolyte membrane (PEM) fuel cells for vehicles and other applications [1]. As one of the problems during fuel cell operation, gas fuels such as hydrogen and oxygen permeate through a PEM, resulting in degradation of the fuel cell performance. In order to clarify this gas-leak mechanism, a few researchers have studied on free-volume holes in Nafion membrane being the most widely used PEM [2-3]. Recently, as Nafion-alternative low-cost materials, much attention focused on PEMs prepared by a radiation grafting method [4]. So far, however, little is known about free-volume holes in these radiation-grafted PEMs. In this study, we performed the positron annihilation lifetime spectroscopy (PALS) measurement on the graft-type PEMs based on crosslinked-polytetrafluoroethylene (PTFE) films [51 to obtain information about the size and location of microvoids in them.