MEASUREMENT OF FREE-VOLUME HOLE SIZE DISTRIBUTION IN NAFION-117 USINGPOSITRON-ANNIHILATION SPECTROSCOPY

We report a new result of free-volume hole size distribution in water and ethanol-swollen Nafion-117 polymer. With the increase in water con tent, free-volume hole size decreases, but overall the volume fraction increases. The hole size distribution in dry polymer is seen to be di stinctly different from hydrated membranes. The narrow and symmetric d istribution in hydrated membrane as compared to dry membrane is believ ed to be a consequence of crosslinking due to cluster formation. In al cohol-swollen membranes, on the other hand, not only are the free-volu me size and fraction seen to be higher, the hole size distribution is seen to be broader compared to dry or hydrated membranes, indicating t he effect of penetration of alcohol into the hydrophobic backbone regi on. We have also examined our results vis-a-vis reported gas diffusion studies in Nafion in the framework of existing free-volume model. (C) 1998 John Wiley & Sons, Inc.