Perfluorosulfonic Acid (PFSA) ionomer membranes are commonly used as electrolytes in electrochemical devices. The membranes must be able to remain intact and function under a wide range of operating temperatures and humidities for the durability and efficiency of the electrochemical devices, such as fuel cells. Thus, characterization of the structural and physical properties of the ionomer membranes is important for the optimization of the mechanical and sorption behavior of the electrolytes in fuel cells. The goal of this work is this characterization through multi-scale mechanistic models and to develop effective strategies to optimize durability.

A geometry-dependent, mechanics model is developed to predict the swelling pressure in, and sorption behavior of, PFSA ionomers during water uptake. The membrane is represented as a two-phase system, where the water-filled hydrophilic domains, in spherical or cylindrical form, are dispersed throughout the hydrophobic backbone, which maintains the structural integrity. The model starts with the non-affine swelling behavior of the membrane to characterize the relationship between macroscopic and microscopic deformation. The results suggest that with increasing temperature or decreasing backbone stiffness, the constraining pressure due to the deformation of the polymer region decreases and therefore, water uptake in the membrane increases. The model can also account for the effect of residual water in the membrane – which is associated with the membrane’s thermal history – on the sorption behavior. The proposed mechanics-model can serve as a tool for deeper understanding of the sorption behavior of PFSA by bridging the gap between the molecular level descriptions and the experimental observations of macroscopic swelling.

The large-deformation stress-strain behavior of the PFSA membranes is found to follow that of semicrystalline polymers at low water contents, whereas at high water contents, the membrane exhibits characteristic features of elastomers. Pursuing these similarities, measured stress-strain data are reproduced by extending the constitutive models for semicrystalline polymers and elastomers. Then, the structure-property relationship of PFSA membranes is investigated using a mechanics approach for several proposed representative geometries to predict Young’s modulus at a wide range of temperatures (from –20 to +85°C) and water fractions (0-0.45). The agreement between the calculated and measured moduli enables us to suggest a possible nanostructural transition for the water domains during sorption, from spherical to cylindrical. The modulus-polymer fraction relationship of the membrane is further discussed and compared with those seen in other porous structures and gels.

Lastly, the mechanical and swelling properties are incorporated into a finite element model to determine the evolution and distribution of the water and swelling-induced stresses in an operating fuel cell. Swelling of the constrained membrane induces compressive stresses, which can lead to plastic deformation and subsequent residual tension upon dehydration. The results suggest that it may be possible to optimize a membrane with respect to its swelling behavior and mechanical properties to achieve better fatigue resistance, potentially enhancing the durability of fuel cell membranes.