(400w) Simulations Predict Water Uptake in Polysulfone Membranes

Water uptake in ion exchange membranes governs the transport of ions and water. Higher water content would lead to loss of selectivity, while lower water content leads to hindered transport. Simulating water uptake in membranes has been complicated and computationally tedious. As a result, contemporary works have relied on short annealing cycles at high temperatures to equilibrate a membrane slab alongside water. However, temperature strongly influences water uptake, and measuring this property at a steady temperature over longer timescales would give a much more realistic picture of swelling.

Combining virtual sites with united-atom force fields enables microsecond-timescale simulations, promoting complete equilibration in a membrane slab setup with excess water. We note that charged polysulfone membranes swell uncontrollably at temperatures beyond 100°C and dissolve in water. To prevent such uncontrolled swelling, we introduce harmonic bonds between chains to resemble a crosslinked state. The crosslinked membranes swell less compared to the uncrosslinked membranes, and this effect is prominent in swelling simulations at higher temperatures.