Antiplasticization and Water Uptake of Nafion Thin Films

Fluorescence of 9-(2-carboxy-2-cyanovinyl)julolidine (CCVJ) was measured to probe the local motion within 70-600 nm thick Nafion films at controlled levels of hydration. Thinner films showed increased confinement under dry conditions as evidenced by their high fluorescence intensity values per unit thickness. Antiplasticization, or a stiffening of the film, was observed as an increase in fluorescence intensity when the sample was exposed to low water activity (≤30% RH) and the extent of antiplasticization changed with film thickness. At higher relative humidities, the films became plasticized with water and the fluorescence of CCVJ declined. Quartz crystal microbalance (QCM) experiments revealed higher hydration numbers (λ = nH₂O/nSO₃H) in thinner films on SiO₂ surfaces. The higher water uptake in thin films was explained by considering the polymer order at the Nafion-SiO₂ interface and the influence of this interfacial layer on the swelling properties of thin films.