Nafion polyelectrolyte is widely used in polymer electrolyte membrane fuel cells (PEMFC) due to its high proton conductivity. The properties of hydrated Nafion are attributed to its nanophase-segregated structure in which hydrophilic clusters are embedded in a hydrophobic matrix. However, there has been little characterization of how the monomeric sequence of the Nafion chain affects the nanophase-segregation structure and transport in hydrated Nafion. To study such properties, we carried out molecular dynamics (MD) simulations of Nafion 117 using two extreme monomeric sequences: one very blocky and other very dispersed. Both produce a nanophase-segregated structure with hydrophilic and hydrophobic domains. However, the blocky Nafion leads to a characteristic dimension of phase-segregation that is ∼60% larger than for the dispersed system. We find that the water-polymer interface is heterogeneous, consisting of hydrophilic patches (water contacting sulfonate groups of Nafion) and hydrophobic patches (water contacting fluorocarbon group). The distribution of the hydrophilic and the hydrophobic patches at the interface (i.e., the heterogeneity of interface) is much more segregated for blocky Nafion. This leads to a water diffusion coefficient for the dispersed case that is ∼25% smaller than for the blocky case (0.46 × 10 -5 vs 0.59 × 10 -5 cm 2 /s at 300 K). The experimental value (0.50 × 10 -5 cm 2 /s) is within the calculated range. On the other hand, we find that the vehicular diffusion of hydronium is not affected significantly by the monomeric sequence. These results should be useful in optimizing the properties of Nafion and as targets for developing other membranes to replace Nafion in PEMFC and other applications.

We used molecular dynamics (MD) techniques to examine the encapsulation of Bengal Rose (BR) molecules in the Meijer dendrimer box (DBox) formed by the addition of tert-butyloxycarbonyl-l-Phe (tBOC-l-Phe) cap molecules to the 64 terminal primary amines of a fifth generation poly(propyleneimine) (PPI-5) dendrimer. Using a large periodic box (including DBox, four to six BR, and CH2Cl2 solvent, totaling ∼25 000 atoms), we examined the MD of these systems for ∼0.5 ns. Without the cap we find that BR molecules establish a concentration dependent equilibrium between the interior and surface regions of PPI-5 and the solvent region outside the dendrimer. The number of BR molecules calculated to associate with the interior of the PPI-5 dendrimer agrees exactly with experiment (at the same BR/PPI concentration). MD simulations on the DBox in CH2Cl2 show that the tBOC-l-Phe surface is completely impermeable to encapsulated BR molecules, even when an excess is forced inside the box. The close correspondence of the theory with experiment suggests that these methods can be used to design such systems in advance of experiment.