Direct Analysis of the Ion-Hopping Process Associated with the α-Relaxation in Perfluorosulfonate Ionomers Using Quasielastic Neutron Scattering

Abstract

This work demonstrates the ability of quasielastic neutron scattering (QENS) to measure the dynamics associated with counterions in perfluorosulfonate ionomers (PFSIs). PFSI membranes were prepared by neutralizing with hydrogenated alkyl ammonium counterions. Counterion dynamics were measured using the High-Flux Backscattering Spectrometer at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). Long-range mobility of the counterions was closely linked with the α-relaxation in these materials measured by dynamic mechanical analysis (DMA). The counterion motions in the membrane were found to follow a mechanism of random jump-diffusion within a confined spatial region with diffusion coefficients on the order of 10−7 cm2 s−1. These data are presented along with variable temperature X-ray scattering investigations of the melting behavior of these materials. Altogether, the data presented here show the link between the onset of long-range counterion mobility and the mechanical properties of these materials. These data provide further fundamental understanding of the link between electrostatic interactions and dynamics in PFSI materials.