Effect of cross-linking on the diffusion of water, ions, and small molecules in hydrogels

The present study reports on molecular dynamics investigations of chemically cross-linked poly(ethylene glycol) hydrogels with the aim of exploring the diffusion properties of water, ions, and rhodamine within the polymer at the molecular level. The water structure and diffusion properties were studied at various cross-linking densities with molecular weights of the chains ranging from 572 to 3400. As the cross-linking density is increased, the water diffusion decreases and the slowdown in diffusion is more severe at the polymer-water interface. The water diffusion at various cross-linking densities is correlated with the water hydrogen bonding dynamics. The diffusion of ions and rhodamine also decreased as the cross-linking density is increased. The variation of diffusion coefficient with cross-linking density is related to the variation of water content at different cross-linking densities. Comparison of simulation results and obstruction scaling theory for hydrogels showed similar trends.