Mechanics of proton exchange membranes : time, temperature, and hydration dependence of the stress-strain behavior of persulfonated polytetrafluorethylene

[摘要] (cont) In order to accomplish this understanding a comprehensive experimental study was undertaken in which Nafion was characterized in uniaxial tension under monotonic, cyclic, and stress relaxation loading profiles at strain rates from 0.001/s to 0.1/s, temperatures from 250C to 1000C, and from dry to fully hydrated conditions. The evolution of the microstructure with applied deformation was then investigated with diffraction techniques. Wide and small angle x-ray scattering data was collected during uniaxial tensile monotonic extension, cyclic, and stress relaxation loading profiles. The SAXS peaks and two WAXS peaks were seen to be isotropic in the initial state. Their evolution with strain was interpreted to indicate that the ionic clusters deform to an elliptical shape with major axis parallel to the tensile direction with an applied strain, whereas the backbone segments align themselves parallel to the tensile direction with an applied strain. Combining these results with those in literature we revise an existing conceptual model for how each of the micromechanical features evolves with strain and how that contributes to the stress response. From mechanical and microstructural data, a constitutive model was developed which is able to capture the key features of the mechanical behavior of Nafion as functions of time, temperature, and hydration. The model is then applied to a simulated fuel cell. The results from the fuel cell simulations indicate that the hypothesis that cyclic stress states and permanent membrane deformation result from hygro-thermal cycling and can lead to pinhole formation.

[发布机构] Massachusetts Institute of Technology