Nanoscale modeling of the structure of perfluorosulfonated ionomer membranes at varying degrees of swelling

We present a comprehensive model for the structure of swollen ionic membranes using a stochastic simulation process at the nanoscale level. In our model, the membrane is viewed as an entangled network of chains interacting at their backbone and ionic groups. Equilibration of the network is obtained through local moves of the groups according to the Boltzmann exponents of the corresponding changes in internal energy. The approach allows one to consider in detail the importance of equivalent weight, polymer molecular weight, length of pendant ionic groups as well as crosslinks and non-ionic polymer additives. At infinite dilution, our simulation results reveal that the ionic chains segregate into platelet-like structures with the ionic sites exposed on each surface and a core (∼2 nm thick) made of two layers of the hydrophobic polytetrafluoro-backbone units. As concentration is increased above the critical value for network formation, the platelets are stretched into narrow ribbons connected at entanglement points. Our description is in line with recent experimental evidence suggesting that ionic membranes can be considered as a connected network of rods. Finally, model predictions of X-ray scattering data are found to be in good agreement with experimental values obtained over a wide range of water content spanning two orders of magnitude.