Nanophase-Segregation and Transport in Nafion 117 from Molecular Dynamics Simulations: Effect of Monomeric Sequence Seung Soon Jang, Valeria Molinero, Tahir C ¸ ag ˇ ın, and William A. Goddard III* Materials and Process Simulation Center MC 139-74, California Institute of Technology, Pasadena, California 91125 ReceiVed: September 23, 2003; In Final Form: January 15, 2004 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. 1. Introduction Nafion is a polyelectrolyte consisting of nonpolar tetrafluo- roethylene (TFE) segments, N ) (CF 2 -CF 2 ) and polar per- fluorosulfonic vinyl ether (PSVE) segments, P ) (CF 2 -CF(O- CF 2 -CF(CF 3 ))-CF 2 -CF 2 -SO 3 H) (Figure 1) used in many important applications. 1 Particularly of interest to us are the properties relevant to polymer electrolyte membrane fuel cells (PEMFC) where its high proton conductivity and mechanical, chemical, and thermal stabilities are crucial. 2-6 There is a general consensus supported by experiments 7-22 and simulation 23-25 that these favorable characteristics of hydrated Nafion result from its nanophase-segregated structure in which hydrophilic clusters are embedded in hydrophobic matrix. Various models have emerged to explain the properties of hydrated Nafion membrane 11-13,26-30 since Eisenberg 7 suggested the concept of cluster formation for ionomers, which was extended by Mauritz and Hopfinger 26 in an attempt to represent the structural incorporation of water and ions by introducing configurational dipole-dipole interactions. One widely accepted empirical model for hydrated Nafion is the cluster-network model proposed by Hsu and Gierke 11,12 on the basis of small-angle X-ray scattering (SAXS) experi- ments. In this model, spherical hydrophilic clusters (∼4 nm diameter) of water are surrounded by sulfonate groups connected through cylindrical channels with ∼1 nm diameter. They conjectured that the balance between elastic deformation energy and hydrophilic surface interactions leads to these characteristic dimensions. Tovbin and Vasyatkin 31,32 used spectroscopic data to suggest a three-dimensional model of the amorphous part of the Nafion membrane as consisting of an ensemble of pores formed by closely packed polymer chains. In this empirical model, the walls of the pores are postulated to consist of polymer bilayers, with the sulfonate groups pointing inward toward the water channels. To rationalize the observed transport phenomena, Yeager and Steck 13 proposed a “three-phase model” consisting of the fluorocarbon phase (some of which would be microcrystalline), an interfacial region rich in free volume that contains the pendant chains, and the cluster region containing water and ionic groups. The clusters in this model are assumed to be spherical. It is generally understood that proton transport is strongly coupled with the distribution and transport of water in the hydrated Nafion membrane: (i) Proton conductivity in PEMFCs is possible only in the presence of water and thus a water-polymer structure with percolation in three dimensions is essential. * To whom correspondence should be addressed. wag@wag.caltech.edu. Figure 1. Chemical structure of Nafion. Nafion 117 has an average composition of x ) 6.5, y ) 1, and z ) 1. N indicates for the nonpolar monomeric units while P indicates the polar monomeric units. 3149 J. Phys. Chem. B 2004, 108, 3149-3157 10.1021/jp036842c CCC: $27.50 © 2004 American Chemical Society Published on Web 02/17/2004