Phase behavior and microdomain structure in perfluorosulfonated ionomers via self-consistent mean field theory

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 (chi(FW)) 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 chi(FW) 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 ionomer chemical groups and can be rationalized in terms of the dominant fluorocarbon-water repulsion and branch stretching to maximize ether-water and sulfonyl-water contacts. With increasing chi(FW), branch collapse minimizes the fluorocarbon-water interaction while increasing the size of water-rich domains and the degree of sulfonyl segregation.