Kinetics of water sorption in Nafionthin films - Quartz crystal microbalance study

Quartz crystal microbalance measurements are used to investigate the water sorption in the cast Nafion films exposed to water vapor. Equilibrium and k inetic sorption data are reported for H- and Na-form Nafion films of a thic kness ca. 0.02 mum. Water sorption isotherms obtained agree well with the l iterature data for Nafion 117. However, the water diffusion coefficients ev aluated from the initial sorption and desorption rates by using the theory of diffusion in a plane sheet are ca. 7 orders of magnitude lower than thos e obtained from sorption measurements in ca. 200 mum thick Nafion 117 membr anes. It is concluded that the kinetics of water sorption (desorption) in t he partially hydrated Nafion exposed to water vapor is controlled by the fi rst-order process of water transfer in and out of the nanoscopic hydrophili c regions (ion clusters) for membranes thinner than 10 mum. On the other ha nd, the linear diffusion is the rate-determining step of water transport in thick Nafion membranes and in fully hydrated thin Nafion films.