Probing Selected Morphological Models of Hydrated Nafion Using Large-Scale Molecular Dynamics Simulations

Abstract

Atomistic molecular dynamics simulations were performed to study hydrated Nafion systems large enough (approximately 2 million atoms, approximately 30 nm box length) to directly observe several hydrophilic domains at the molecular level. These systems consisted of six of the most significant and relevant morphological models of Nafion to-date: (1) the cluster-channel model, (2) the parallel cylinder model, (3) the local order model, (4) the lamellar model, (5) the rod network model, and (6) a "random" model that does not directly assume any particular geometry, distribution, or morphology. Each system was initially built to closely approximate the proposed hydrophilic cluster structure in a given model. Molecular dynamics simulations were then used to observe resulting changes from and behavior of the assumed initial configurations. These simulations revealed fast intercluster "bridge" formation and network percolation in all models. Sulfonate groups were found inside these bridges and played a significant role in percolation. Sulfonates also strongly aggregated around and inside clusters. Cluster surfaces were analyzed to study the hydrophilic-hydrophobic interface. Interfacial area and cluster volume significantly increased during the simulations, and radial distribution functions and structure factors were also calculated. All nonrandom models clearly exhibited the characteristic experimental scattering peak, underscoring the insensitivity of this measurement to hydrophilic domain structure and highlighting the need for future work to clearly distinguish morphological models of Nafion.

Variational Models of Network Formation and Ion Transport: Applications to Perfluorosulfonate Ionomer Membranes

“…Even at a course-grained level, molecular and dissipative particle dynamics simulations of solvated, hydrophilic-hydrophobic polymer systems cannot compute the morphological development to its equilibrium [15], even in state-of-the-art simulations resolved to milliseconds of real time [16]. Indeed experimental evidence suggests PFSAs can possess 10-20 h transients after changes in external environment [17].…”

Molecular Dynamic Study of Water-Cluster Structure in PFSA and PFIA Ionomers

“…49 The six most significant morphological models (i.e., the cluster-channel model, the parallel-cylinder model, the local-order model, the lamellar model, the rod-network model, and a "random" model) were tested by atomistic MD simulations in in Ref. 33; however, the MD simulations did not give preference to any morphological model because all non-random models exhibited scattering peak close to the experimental one.…”

Dependence of electroosmosis on polymer structure in proton exchange membranes

“…An increase in EW results in the larger sulfonate-sulfonate separation, and thus hinders proton transport through hydrophilic channels (Allahyarov and Taylor, 2011). Theoretical and computational analyses, such as molecular dynamics (MD) simulations and ab initio simulations, have been used extensively to investigate proton transport properties including electroosmosis in Nafion membranes and thin films (Feng et al, 2012, Feng and Voth, 2011, Jorn et al, 2012, Petersen and Voth, 2006, Petersen et al, 2005, Tse et al, 2013, Seeliger et al, 2005, Spohr et al, 2002, Cui et al, 2007, Jang et al, 2004, Wang et al, 2011a, Karo et al, 2010, Knox and Voth, 2010, Komarov et al, 2013, Spohr, 2007, Mabuchi and Tokumasu, 2014, Wescott et al, 2006, Tuckerman et al, 1995, Eikerling et al, 2008, Kornyshev and Spohr, 2009, Choe et al, 2008, Yan et al, 2008, Aochi et al, 2016, Kurihara et al, 2017. However, extensive simulations using ab initio methods for a larger number of atoms, where systems reproduce nanostructured Nafion morphology and water domains with reliable statistics, have been limited because of the high computational cost of such simulations.…”