Morphology of dry and swollen perfluorosulfonate ionomer by fluorine-19 MAS, NMR and xenon-129 NMR

Abstract

At magic angle spinning speeds above 5kHz, two major fluorine-19 resonances can be resolved in the acid form of the perfluorosulfonate ionomer Nafion (DuPont trademark). The larger signal at −120ppm versus CFCl3 is assigned to the backbone CF2 groups and the second signal at −80ppm is assigned to the OCF2 and CF3 groups in the pendant chain. Spin diffusion experiments between these two resonances are performed on dry Nafion, water swollen Nafion and ethanol swollen Nafion. The analysis assumes a morphology comprised of pendant group domains and backbone CF2 group domains. In dry Nafion, the thickness of the pendant group domain is found to be 3.8nm with an overall periodicity of about 10nm. Upon addition of water, on one hand, the pendant group domain increases to 6.5nm at a level of 20wt% water while the overall periodicity hardly changes. On the other hand, both overall periodicity and the pendant group domain increase significantly upon the addition of ethanol. At 20wt% ethanol, the pendant group domain is 11nm and the overall periodicity is 19nm. Thus ethanol appears to induce a larger morphological rearrangement, though it is still the pendant group domain that primarily increases in size with increasing ethanol concentration. Analysis of the static fluorine-19 line shapes indicates that ethanol is a selective plasticizer of the pendant group domain. Xenon-129 NMR is used as an alternative probe of morphologically based sorption environments in Nafion. The xenon-129 spectrum of xenon sorbed into Nafion under a pressure of 12atm shows two resonances corresponding to two sorption environments. The first resonance closer to the signal from free gas has a shift comparable to that of xenon-129 in amorphous poly(tetrafluoroethylene) and is assigned to that type of environment in Nafion. The second broader and larger intensity peak is assigned to the pendant group domain. The two domain view from xenon-129 NMR is consistent with the assumption of the presence of pendant group and backbone domains used to interpret the fluorine-19 spin diffusion data. Two-dimensional xenon-129 NMR shows the onset of exchange at a mix time of 1ms at room temperature leading to an estimate of the average effective diffusion constant for xenon in Nafion of 4×10−11cm2/s.