Kinetics of water vapor sorption in sulfonated polyimide membranes

Water vapour sorption kinetics in a H+ form naphtalenic sulfonated polyimide membrane at 25°C has been investigated by using an electronic microbalance (IGA, Hiden). An effective diffusion coefficient Dexp has been calculated from kinetic data by using the Fickian model for the final part of the sorption. The variation of Dexp with the equilibrium water activity (0<awequi < 1) has been analysed considering the different sorption modes involved. The increase of Dexp with increasing awequi at low activities is explained on the basis of a dual mode (Henry–Langmuir) sorption model whereas the decrease of Dexp for the higher activities calls for the reduced mobility of water involved in the dilution of multiplets and for the immobilization of sorbed water by clustering.

and (5) where J is the diffusion flux, C is the overall absorbed water concentration and Cm is the mobile species concentration (assumed here to be equal to CD, the Henry’s concentration). The effective coefficient Dexp can be expressed as a function of Dm by: (6) where dCm/dC can be calculated from the experimental isotherm data [8]. Finally, the partial immobilization of the Henry’s population and the hindrance effect due to clustering can be accounted for on the whole by a phenomenological model analog to that proposed by Stern [11], which yields a decreasing diffusion coefficient Dm of the mobile species and considers the activity awequi=0.3 as a threshold value: (7) with Cm the concentration of the mobile species, Dm0 the diffusion coefficient of mobile species for Cm = Cm0 and $ a coefficient which may be <0 (anti-plasticization).