High Resolution X-Ray Scattering of Ionic Clusters in Perfluorinated Ionomers

Scattering maxima caused by ionic groups in perfluorinated ionomers such as XUS (from Dow) and Nafion® were obtained by finescan wide angle X-ray diffraction (WAXS), from which high resolution is achieved. Contrary to the conclusion that WAXS results of ionomers are similar for various cation forms, our diffraction results show systematic increase of scattering intensity near 2θ=9° and 22° with increase of cation mass. Analysis of the spectra shows that cations must be separated by an average spacing of 11.9 Å and these ions compress the hydrophobic backbones of the ionomer so that the original peak at 17° associated with interchain distance is dispersed. In order to get a picture of the ionic cluster within ionomers consistent with current understanding, a new concept of sub ionic cluster is introduced. The results support bulk distribution of ions instead of ionic groups arranged as shell.