Coupling of mechanical and transport properties in Nafion®

Water sorption and permeation involve interfacial mass transport, diffusion and polymer swelling. The limiting transport resistance changes from interfacial transport at the membrane/vapor interface to diffusion when there is a membrane/liquid interface. Water absorption into Nafion is initially limited by interfacial mass transport; above critical water content water sorption is limited by polymer relaxation dynamics associated with swelling. The dynamics of water absorption show that at room temperature it takes more than 105 s for Nafion to equilibrate to changes in water activity. The elastic modulus and creep rate of Nafion show complex dependence on water activity and temperature. The elastic modulus goes through a maximum and creep rate goes through a minimum at intermediate water activity that depends on temperature. At room temperature water plasticizes Nafion while above 80°C water stiffens Nafion. We shall relate mechanical and transport properties to the structure of Nafion and their implications for PEM fuel cell operation.