Computational Study of Microhydration in Sulfonated Diels–Alder Poly(phenylene) Polymers

The nature of microhydration in sulfonated Diels–Alder poly­(phenylene) (SDAPP) polymer membranes is explored using <i>ab initio</i> and density functional theory (DFT) electronic structure calculations. The impact of the aromatic poly­(phenylene) structure, including cooperative effects between multiple spatially adjacent sulfonic groups, on the hydration environment is addressed using a series of DFT B3LYP/6-311**-optimized structures for different SDAPP·<i>n</i>H₂O clusters. In addition, larger SDAPP polymer fragments, along with selected hydrophilic domain structures extracted from molecular dynamic (MD) simulations, are also evaluated using ONIOM HF/PM6 semiempirical calculations. The SDAPP clusters reveal that spontaneous proton dissociation occurs at low levels of hydration to form sulfonic-acid-associated H₃O⁺ contact ion pairs (CIPs), which then evolve into solvated CIPs at higher hydration levels. For multiple sulfonic acid groups located on the poly­(phenylene) side chains, the hydration energies are a function of the relative acid location and backbone configuration. Variations in the phenylene backbone torsional angles allow remote sulfonic acids to adopt an optimal separation to produce an extended hydrogen bonded network of waters between the SDAPP acids groups. These calculations provide a baseline to help describe the proton transport and hydration behavior of SDAPP membranes