Hydration Phenomena in Sulfonated Poly(arylene ether sulfone) Membranes*

The hydration mechanism of ionic polysulfones remains poorly understood, which hinders their utility in water purification membranes. Differential scanning calorimetry (DSC), density measurements, and molecular dynamics simulations are combined to improve molecular-scale understanding of how water interacts with different parts of sulfonated polysulfones as water uptake is varied. The results reveal a threshold value of water uptake below which virtually all water molecules are located in the primary hydration shell of ions (Na⁺ and SO₃^-) and polar groups (SO₂ and ether linkages). At higher water uptake, a significant fraction of the absorbed water molecules become more loosely associated with the polymer. This transition is indicated by the gradual appearance of a melting endotherm in DSC analysis, a maximum in measured density of the hydrated polymer, and appearance of water molecules with a bulk-like state in simulations. The simulation results further indicate that interactions between ions (Na⁺ and SO₃^-) and polar groups, particularly SO₂, play an important role in the hydration behavior of sulfonated polysulfones.