Characterizing Compressive Response of Hydrated Ionomer Membranes in Water.

Ionomers are functional polymers used in many emerging technologies such as fuel cells and electrolysis, where they commonly operate under compressive stressors. However, deformation of ionomer membranes is often characterized in tension and not always under device-relevant conditions, limiting fundamental understanding and applicability of these materials for many electrochemical systems. Here, we develop a methodology to assess the compressive stress-strain response of ionomer membranes under hydrated, elevated-temperature conditions by using PFSA (Nafion) and a hydrocarbon (PiperION) ionomer as canonical cation/anion-exchange materials. We characterize compression of ionomers in situ and demonstrate that their stress-strain response can be captured by hyperelastic theories, especially at higher hydration-temperature conditions that mimic those in electrochemical devices. This suggests the combined effects of hydration and compression alter ionomer membrane behavior to such an extent that they activate a deformation mechanism reminiscent of hydrogels.