Morphology and properties of Nafion membranes prepared by solution casting

In this study, dilute Nafion solutions consisting of solvents with various dielectric constants ɛ and solubility parameters δ, i.e. N,N′-dimethyl acetamide, N,N′-dimethyl formamide, N-methyl formamide, methanol–water mixture (4/1 g/g), ethanol–water mixture (4/1 g/g), and isopropanol–water mixture (4/1 g/g), were freeze dried and the conformations of Nafion molecules in dilute solutions were observed using transmission electron microscope. The membranes were prepared by solution casting from these solutions and evaporating the solvents at temperatures below TG of Nafion, then annealing the membranes at 150 °C which was ∼50 °C above TG of Nafion. We show Nafion molecular conformations in dilute solutions are strongly influenced by δ and ɛ of solvents. And, thus the morphology, water uptake, proton conductivity, and methanol permeability of membranes prepared by solution casting are also influenced by δ and ɛ of solvents.