Perfluorosulfonated membrane (Nafion): FT-IR study of the state of water with increasing humidity

Abstract

A detailed study of the Nafion–water system has been performed using water sorption isotherm, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and FT-IR with the aim of investigating the state of water in the Nafion membrane. The last technique in particular can yield valuable information on molecular interactions, especially those involving water molecules. The first water molecules interact with sulfonic groups, forming hydronium H₃O⁺ and –SO₃^- ions, while the following water molecules aggregate with H₃O⁺ to form counter-ion clusters with five or six water molecules. Increasing the water content promotes cluster growth and cluster aggregation with the formation of a continuous water phase, where for every four molecules of water one has two hydrogen bonds, and each of the other three only one. All the sorption, calorimetric and infrared data agree with the proposed model of two non-interacting phase systems: the water-phase and the perfluoro-phase with sulfonic groups at the interphase. The only observed effects of water on Nafion polymer are sulfonic group dissociation and partial destruction of crystallinity associated in polymer swelling.