Dynamic water uptake of flexible polymer networks ion-containing polymer networks

Dynamic gravimetric water sorption and desorption was measured while incrementing the relative humidity of the sample environment for a variety of ion-containing polymers. The sorption and desorption experiments as a function of time were performed from 20 to 95% relative humidity and temperatures from 30 to 70 °C. The water uptake characteristics of the membranes were observed to depend on the flexibility of the polymer backbone, the ion exchange capacity of the membrane, and in the case of triblock copolymers, the molecular weight of the sulphonated block. Additionally, the water uptake tended to occur more slowly at high relative humidity where the enthalpy of solvation is low due to already well-hydrated ions in the membrane. At low relative humidity, the water sorption occurred quickly due to the high enthalpy of solvation at low λ, but the polymer matrix constrained the magnitude of the total water swelling. A unique physical crosslinking effect was observed for triblock copolymers where less swelling of the network occurred at high temperatures than at low temperatures. This deswelling phenomenon with increased temperature was not observed for Nafion, which behaved like a classic elastic solid and swelled to a greater extent with increased temperature.