Viscoelastic properties of Nafion at elevated temperature and humidity

Tensile stress-strain and stress relaxation properties of 1100 equivalent weight Nafion have been measured from 23 to 120 °C at 0-100% relative humidity. At room temperature, the elastic modulus of Nafion decreases with water activity. At 90 °C, the elastic modulus goes through a maximum at a water activity of ~ 0.3. At temperatures greater-than-or-equal90 °C, hydrated membranes are stiffer than dry membranes. Stress-relaxation was found to have two very different rates depending on strain, temperature, and water content. At high temperature, low water activity, and small strain, the stress relaxation displays a maximum relaxation time with stress approaching zero after 10³-10⁴ s. Water absorption slows down stress-relaxation rates. At high water activity, the maximum stress relaxation time was >10⁵ s at all temperatures. No maximum relaxation time was seen at T [less-than or equal to] 50 °C. Increasing the applied strain also resulted in no observed upper limit to the stress relaxation time. The results suggest that temperature, absorbed water, and imposed strain alter the microstructure of Nafion inducing ordering transitions; ordered microstructure increases the elastic modulus and results in a stress relaxation time of >10⁵ s. Loss of microphase order reduces the elastic modulus and results in a maximum stress relaxation time of 10³-10⁴ s.