Linear Hygrothermal Viscoelastic Characterization of Nafion NRE 211 Proton Exchange Membrane

Abstract

AbstractThe tensile relaxation modulus of a commercially available proton exchange membrane, Nafion® NRE 211, was obtained over a range of humidity levels and temperatures using a commercial dynamic mechanical analyzer (DMA). Hygral stress relaxation master curves were first constructed, followed by a hygrothermal master curve using the time temperature moisture superposition principle. The hygrothermal master curve was fitted using a 10‐term Prony series and validated using longer term stress relaxation tests. To validate the results from the stress relaxation experiments, short and long‐term creep compliance was converted into stress relaxation modulus using a well‐known viscoelastic conversion formula, and compared with the relaxation modulus obtained under identical conditions. Good agreement was found between the two datasets. It was evident that relaxation data at 2% RH at the test temperatures was not superposable with the master curves obtained at higher relative humidity (10% < RH < 90%) at the temperature range 70 °C < T < 90 °C. It was observed that the longer term relaxation modulus under humid conditions matched well with the hygrothermal master curve; however, the longer term relaxation modulus under dry conditions was significantly higher than the relaxation master curve obtained under dry conditions, raising the possibility of a physical aging process in the ionomer and/or irreversible morphological changes in the membrane under dry conditions.