A unified morphological description of Nafion membranes from SAXS and mesoscale simulations

Abstract

Although many different structural models have been proposed for perfluorosulfonic acid (PFSA) ionomers such as Nafion, the very high degree of disorder in these materials makes it difficult, if not impossible, to deduce their morphology unambiguously from small angle X-ray and neutron scattering (SAXS/SANS) experiments alone. We present a combination of a model-independent procedure for obtaining structural information from SAXS patterns based on a Maximum Entropy (MaxEnt) approach coupled with mesoscale simulations of the morphology of Nafion using Dissipative Particle Dynamics (DPD) parameterized with atomistic calculations and density functional theory. These two methods show that the nanoscale ionic clustering in PFSAs is intimately linked to, but spatially separate from, the larger scale organization of the fluorocarbon backbone. Although we are unable to directly observe crystallization of the backbone in the DPD simulations, the high density regions of fluorocarbon segments correspond exactly to those regions where the density of ionic clusters is lowest, each forming an independent bicontinuous domain. We are thus able to demonstrate a unified morphological description of PFSAs based on both statistical (MaxEnt) and thermodynamic (DPD) descriptions, which broadly favours a bicontinuous network of ionic clusters embedded in a matrix of fluorocarbon chains.