Influencing Molecular Dynamics Simulations of Ion-Exchange Membranes by Considering Comonomer Propagation

Abstract

Ionic cross-linked copolymers have been successfully used as ion-exchange membranes (IEMs) for many years, with charged functional groups encouraging ion transport through hydrated regions of materials that include cross-linking agents [Kreuer, K.-D. Annu. Rev. Mater. Res.2021, 51, 21−46]. Subsequently simulating IEM formation is useful, with software such as Polymatic [Abbott, L. J. Theor. Chem. Acc.2013, 132, 1334] able to dynamically form a polymer from a box of monomers through a series of molecular dynamics simulations. In this study, we offer an alternative polymer formation scheme that accounts for propagation ratios of two monomers calculated with density functional theory after a comprehensive conformational search. The scheme was implemented as an extension of the Polymatic code. CR61 and AR103 IEMs were constructed while influencing propagation, with increased cross-linking resulting in membranes of greater tensile strength. To investigate which polymeric forms perform best in terms of predicting bulk properties, diffusion coefficients of ions inside each membrane were obtained, with simulations using polymers created without controlling reaction kinetics predicting diffusion to within 10% of experimental measurements. Influencing the polymerization process does not necessarily improve the performance of these membranes. However, the propagation rate coefficients dictate whether intramolecular cross-linking is required, and we recommend that these quantum calculations be performed before molecular dynamics simulations are attempted.