From soup to structure: Simulating hydrated semi-crystalline proton exchange membranes

A methodology is presented for the simulation of complex high molecular weight polymers, with a primary focus on reproducing the structure of Nafion® proton exchange membranes. Enhanced computational efficiency is achieved in comparison to commonly employed Monte Carlo techniques by implementing random insertions of long polymer chains followed by a ghost chain randomization process. Dissipative particle dynamics is applied to relax strongly overlapped configurations and to access the long time scales necessary to capture the crystallization process. The developed protocol first relaxes soft polymer chains in a stage analogous to thermal activation, which promotes crystallinity, and subsequently incorporates chain stiffening to reproduce crystal growth during cooling. The approach is validated against experimental SAXS measurements by computing intensity profiles from the electronic density of simulated Nafion. The results highlight the strong influence of side chain distribution on crystallinity, emphasizing its role in the formation of realistic semi-crystalline morphologies. Key processes, including chain ordering, local alignment, and molecular packing, are resolved, providing an improved understanding of structure-property relationships. The methodology accurately predicts crystallinity content as well as crystallite size and shape, thus demonstrating strong predictive capability for a wide range of PEMs and related polymer systems.