Atomistic and Mesoscale Molecular Simulation of Sulfonated Copolymer Membranes

Abstract

Hydrocarbon-based sulfonated copolymer membranes via direct copolymerization of sulfonated monomers and other comonomers usually consist of hydrophilic and hydrophobic segments. Due to thermodynamic immiscibility of the two segments, spontaneous microphase separation occurs resulting in complicated morphologies, depending on the composition and block length of each segment. Perfluorosulfonic Acid (PFSA) membranes, particularly Nafion, are primarily used as proton exchange membranes (PEMs) in fuel cell applications. Nafion consists of a highly hydrophobic polytetrafluoroethylene (PTFE) backbone with a fully perfluorinated ether side chain terminated by a strongly hydrophilic sulfonic acid group, leading to spontaneous phase segregation at the nano-structural level in the hydrated structure. In contrast, hydrocarbon-based sulfonated copolymer membranes have different phase separated structures from Nafion due to their distinguishable molecular compositions and sequences. In this study, a mesoscale modeling approach using a commercial computation program was used to investigate the details of morphology development in hydrocarbon-based copolymer membranes, which captured the phase separation at scales consistent with those experimentally observed at increasing the content of hydrophilic segments (i.e., sulfonated block). In addition, water diffusion coefficients were estimated from the atomistic molecular simulation of the copolymer membranes in the different hydrated level.