Mechanical behavior of a hydrated perfluorosulfonic acid membrane at meso and nano scales.

Perfluorosulfonic acid (PFSA) is widely used as the membrane material for proton-exchange membrane fuel cells, and its mechanical properties directly affect the stability and the life of the internal structure of the proton exchange membrane. In the present research, mechanical behaviors of hydrated membranes are investigated through macro-mechanical experiments and nano-simulation. The commercial products of Nafion® 117 and Nafion® 212 are used as tensile testing samples, and the latter manifests a larger swelling ratio, smaller linear-elastic region, and higher Young's modulus at the same humidity. It is found that in comparison to Nafion 117, humidity (especially at less than 9 wt%) yields profound effects on true stress-true strain curves of Nafion 212. Further, two types of PFSA ionomers, representing the nanoscale parts of Nafion and Aquivion membranes, respectively, are studied on the uniaxial tensile deformation through molecular dynamics simulation, and the effects of side chain, water content, backbone length, and strain rate are examined. It is noticed that long side chains decrease the winding degree of polymer chains for the PFSA ionomers with the same backbone, and pinhole failure causes the declining trend of stress-strain curves. For meso- and nano-scale PFSA polymers, the difference between the volume of uptake water and swelling volume can be used to roughly judge the aggregation degree of polymer chains and explain the elastic-plastic deformation mechanism. The results of this work will serve as a baseline for understanding the swelling and mechanical behavior of PFSA membranes.