Effect of a monomeric sequence on the structure of hydrated Nafion in the sandwich model and solvent dynamics in nano-channels: a molecular dynamic study

Molecular dynamics simulations are performed to obtain insight into the structural properties of hydrated Nafion using the sandwich model of the polymer membrane. It is shown that a larger distance between the sulfonate groups of a chain leads to the polymer forming a better inverted micelles structure. Water– and hydronium–polymer interfaces are investigated. Comparing our results with others indicates that, from the perspective of distance, the formation of shells of water and hydronium ions is independent of the model and monomer type, but depends on both if the coordination number is considered. The behaviour of water molecules and hydronium ions is also studied dynamically. Our survey shows that there is an increasing jump in the diffusion coefficient of water at a certain distance between the sulfonate groups of a chain, which then tends to change slightly. Such behaviour is discussed on the basis of density, the available space, as well as the loss of one translational degree of freedom of the water molecules at larger distances. The diffusion coefficient for the hydronium ions was also determined to be much smaller than that for water (by 3.5–6.1 times). The diffusion coefficient of the hydronium ions shows a declining jump at a certain distance between the sulfonate groups of a chain, but the jump is not significant as that for the water molecules.