Confinement and proton transfer in nafion thin films

Steady-state fluorescence measurements of two different probe molecules were undertaken to explore the water solvation environment and molecular mobility in 70-600 nm NAFION thin films and 50 μm thick NAFION membranes. The influence of film thickness, hydration number, and polymer-substrate interaction on proton dissociation and transfer from photoexcited 8-hydroxypyrene-1,3,6-trisulfonic acid sodium salt (HPTS) to the surrounding solvation environment and the local mobility of 9-(2-carboxy-2-cyanovinyl) julolidine (CCVJ) in the samples were investigated. Deprotonation of the photoacidic HPTS was suppressed in thinner films in both H⁺ and Na⁺ counterion-form samples. This observation revealed the presence of a solvation environment that was suitable for accepting and transporting protons in thinner films compared to a better proton accepting environment in thick films in which higher deprotonation of HPTS was observed. The results from HPTS studies indicated that smaller ionic domains formed in thinner samples regardless of the substrate type. NAFION membrane exhibited a continuous hydration induced plasticization as monitored by a steady decrease in CCVJ fluorescence intensity which was a result of a more mobile local environment in the 50 μm thick membrane. The plasticization behavior of the membrane was in contrast to restricted mobility and antiplasticization observed in thin films. The thickness-normalized fluorescence intensity of CCVJ in the thin film samples implied lower polymer chain mobility in the dry state in thin films on native oxide silicon (n-SiO₂) substrates compared to thin films on Au substrates. At similar hydration number, higher CCVJ fluorescence values were observed for hydrated films on n-SiO₂ compared to samples on Au, which was attributed to strong polymer-SiO₂ interactions at the interface.