Phase Behavior and Microdomain Structure in Perfluorosulfonated Ionomers via Self-Consistent Mean Field Theory

Abstract

This paper describes the use of lattice-based self-consistent mean field (SCMF) theory to predict the equilibrium microstructure in perfluorinated ionomers used as separators in polymer electrolyte membrane (PEM) fuel cells. The SCMF model predicts segregation of ionomer−water mixtures into ionomer-rich and water-rich microdomains as the water content or the fluorocarbon−water interaction parameter (χFW) increases. Phase diagrams and domain structures are reported for water-swollen Nafion, Dow Short Side Chain, a new perfluorosulfonimide, and two other hypothetical ionomers. In general, the phase boundary for melting of the microdomain structure moves to lower values of χFW as the branch density decreases or the branch hydrophilicity increases. For ionomers having the same branch density and branch hydrophilicity, the position of the phase boundary depends on the details of the internal arrangement of chemical groups within the branches. Various phase transitions occur due to segregation of particular ion...