Effects of water freezing on the mechanical properties of nafion membranes

Most of the research efforts on Nafion have been devoted to the study of the perfluorinated ionomer membranes at optimal conditions for the desired applications, such as high temperature and low relative humidity for polymer electrolyte membrane fuel cells (PEM FC). In view of the possible changes induced by the freezing of water in the structure of Nafion and considering that in cold start conditions of a PEM FC device, Nafion needs to work also below 273 K, we measured the Young's modulus (Y) and the elastic energy dissipation (tan d) in the temperature range between 90 and 470 K and the stressstrain curves between 300 and 173 K. The measurements reported here indicate that the mechanical properties of wet Nafion membrane change dramatically with temperature, that is, from a rubber-like behavior at room temperature to a brittle behavior below 180 K. Moreover, we observed that the freezing of the nanoconfined water is complete only below 180 K, as indicated by a large increase of the Young's modulus. Between 180 and 300 K, the large values of tan d suggest the occurrence of friction between the liquid water bound to the walls of the hydrophilic domains and the solid ice residing in the center of channels.