Influence of the Temperature on the Proton Transport in Poly(styrene-co-divinylbenzene) Membranes from Molecular Dynamics Simulations

The effect of the temperature on the properties of sulfonated poly(styrene-co-divinylbenzene), P(S-DVB), cation exchange membranes has been examined in the absence and presence of an external electric field of 0.7 V.nm(-1). Results indicate that the temperature provokes local structural deformations around the sulfonate groups, which are manifested by the distortions at the C S bonds (random and preferential in the absence and presence of the electric field, respectively). These distortions affect both the density and porosity of the membranes, even though the effects of the electric field decrease with increasing temperature. The electric field dominates over the thermal energy in the motion of the hydronium ions, while the external field does not affect the motion of water molecules, which follow a Brownian behavior similar to that observed for the two species in the absence of an electric field. The calculated electro-osmotic drag coefficient indicates that the hydronium flow is several times higher than the water flow in the presence of the electric field, and this effect increases the temperature. The diffusion of hydronium ions in the absence of the external field increases slowly and isotropically with the temperature, this behavior being preserved in the presence of the external electric field once the component associated with the drift velocity is discounted. Finally, the structure and dynamics of hydration shells at hydronium and sulfonate ions have been examined. Interestingly, the shells associated with two such ions as well as their response toward the temperature and electric field are significantly different, which has been attributed to the fact that sulfonate-water interactions are stronger than hydronium water interactions.