Confinement, entropic effects and hydrogen bond network fluctuations of water in Nafion membrane

Nafion is, to date, the most widely used polymeric material as Proton Exchange Membrane. The complex structural organization of this amphiphilic polymer leads to unique physical properties, especially the proton conductivity, which is related to the properties of water confined in the hydrophilic nanochannels. At subzero temperatures, the water remains mobile down to ~ 200 K, desorbing and re-sorbing reversibly in and out of the membrane depending on the temperature. We therefore investigated the structure of water/ice by neutron diffraction and Compton Spectroscopy in order to check if some particularity in the hydrogen bond network was at the origin of the sorption/desorption phenomenon. No anomaly of the ice Bragg diffraction pattern is detected as a function of the temperature, indicating that the melting of ice is entirely driven by the membrane. The Compton profiles of water confined in Nafion at room temperature, instead, exhibit tiny differences with respect to bulk water. These differences increase on lowering the membrane hydration. The Compton investigations therefore show fluctuations in the hydrogen bond geometry of water that could either be due to confinement effect or to the presence of hydronium ions in the liquid.