Behavior of Channel Water and CF₂H Side-Chain Terminal Groups in Swollen Nafion Polymer Electrolyte Membranes after...

Polymer electrolyte membranes (PEMs) are representative systems for the study of the proton conduction mechanism and water dynamics in nanopores/channels. Our ¹H nuclear magnetic resonance data for Nafion PEMs, which are subjected to thermal degradation and then swollen in water, indicate that (1) water is present next to the side chains even after the removal of the SO₃H groups, (2) longer heat-treatment depletes more SO₃H groups and produces more CF₂H groups, (3) the water near the side chains allows for the liquid-like motion of the CF₂H groups, and (4) the motion correlates well with the content and dynamics of water in the channels. As the thermal degradation progresses, the Nafion membranes lose their ionic and hydrophilic nature due to the conversion of CF₂SO₃H groups to CF₂H groups. In addition, our results demonstrate that increasing channel hydrophobicity leads to increased water dynamics in the channels.