Thermal annealing on free volumes, crystallinity and proton conductivity of Nafion membranes

Free volumes, backbone mobility, crystallinity and proton conductivity of Nafion membranes upon thermal annealing were intensively studied. The development of size and distribution of free volumes in Nafion membranes as a function of environmental humidity was found to be closely associated with the adsorbed water molecules and the Nafion backbone mobility. Results suggested a gradual decrement in Nafion backbone mobility as a function of annealing temperature, due to the thermal induced recrystallization. This was further supported by a linear decrement in the o-Ps intensity as a function of membrane crystallinity, because o-Ps atoms mostly annihilated in the amorphous phase in Nafion. Particularly, a significant increment in crystallinity was found in Nafion membranes annealed at temperatures higher than 140 degrees C. The present results showed that annealing at similar to 140 degrees C of Nafion membranes is favourable for their proton conductivity because of the low crystallinity.