Structure and mechanical properties of nafion membranes

Thermal and mechanical properties of Nafion membranes in the dry and swollen states have been examined. The results of differential scanning calorimetry (DSC) and dynamic viscoelasticity measurements give the glass transition temperature (Tg) of the membranes lying around 120°C, which is close to that of polytetrafluoroethylenes (PTFEs ; Tg ∼ 115°C). This shows that Tg of the Nafion membranes stems from the relaxation of PTFE main chains of the Nafion membranes. Stress-strain curves of the dry membranes below Tg exhibit the yielding that originates from the failure of the glassy matrix, i. e., the PTFE domains in the membranes. The Nafion membranes swollen by water also show the yielding of the stress-strain curves, i.e., no plasticization effect takes place in the PTFE domains. This suggests that water is localized in hydrophilic domains in the Nafion films. A finite water flow is observed in the water permeability measurement using the static pressures. These results clearly indicate a bicontinuous microphase separated structure composed of hydrophobic and hydrophilic domains. © 2007 The Society of Materials Science.