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Keywords:

  • Doubly Shifted Master Curve;
  • Dry-Humid Transition;
  • Hygrothermal Stresses;
  • Mechanical Durability;
  • Nafion NRE 211;
  • Physical Aging;
  • Proton Exchange Membranes;
  • Time Temperature Moisture Superposition Principle;
  • Viscoelastic characterization

Abstract

The 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.