Atomistic and Coarse-Grained Molecular Dynamics Simulation of a Cross-Linked Sulfonated Poly(1,3-cyclohexadiene)-Based Proton Exchange Membrane

Abstract

Atomistic and coarse-grained (CG) molecular dynamics (MD) simulations were conducted for a cross-linked and sulfonated poly­(1,3-cyclohexadiene) (xsPCHD) hydrated membrane with λ­(H₂O/HSO₃) = 10 and 20. From the atomistic level simulation results, nonbonded pair correlation functions (PCFs) of water–water, water–H₃O⁺ ion, H₃O⁺–H₃O⁺, polymer–water, and polymer–H₃O⁺ ion pairs were obtained and studied. The water self-diffusivity and H₃O⁺ vehicular self-diffusivity were also obtained. Membrane structure was further studied at CG level using a multiscale modeling procedure. Nonbonded PCFs of polymer–polymer pairs were obtained from atomistic simulation of hydrated membrane with λ = 10 and 20. Two sets of CG nonbonded potentials were then parametrized to the PCFs using the iterative Boltzmann inversion (IBI) method. The CGMD simulations of xsPCHD chains using potentials from above method satisfactorily reproduced the polymer–polymer PCFs from atomistic MD simulation of hydrated membrane system at each hydration level. The transferability of above two set of CG potentials was further tested through CGMD simulation of hydrated membrane at an intermediate hydration level (λ = 15). Limited transferability was observed, presumably due to the use of an implicit solvent. Using an analytical theory, proton conductivity was calculated and compared with that from experimental measurement under similar conditions. Good agreement was obtained using inputs from both atomistic and CG simulation. This study provides a molecular level understanding of relationship between membrane structure and water and H₃O⁺ ion transport in the xsPCHD membrane