Atomistic Simulations of PEM Fragments and Substructures

Boundaries between atomistic and coarse-grained simulation approaches are floating. Atomistic simulations of ionomer systems typically employ all-atom representations of water molecules, anionic head groups, and protons. For the remaining components, the use of a coarse-grained or united-atom representation for the CF , groups in both the fluorocarbon backbone and the sidechains could markedly improve the computational efficiency of atomistic simulations. United-atom force fields permit simulations of substantially larger systems compared to all-atom force fields. For instance, Urata et al. (2005) have employed a united-atom representation of CF ,  

In general, results of fully atomistic or mixed representations have confirmed the formation of a microphase-separated morphology in PEMs, albeit results on sizes, shapes, and distributions of phase domains have remained inconclusive. In comparison to experiment, molecular models were found to underestimate the sizes of ionic 

Jang et al. (2004), using an all-atom MD approach, explored the effect of sidechain distribution at the backbone on structure formation. They showed that blocky Nafion ionomers with highly nonuniform distributions of sidechains at polymer backbones, form larger phase-segregated domains compared to systems with uniform distributions of sidechains at backbones. 

Flliott and Paddison (2007) used the ONIOM method of QM/MM calculations (Vreven et al., 2003) to understand the effects of hydration on the local structure of PFSA membranes. Calculations were performed on fragments of a short sidechain (SSC) PFSA ionomer with three sidechains affixed to the backbone. Full optimizations of the oligomeric fragment was carried out with six to nine water molecules added. The lowest energetic state was found with six water molecules. Upon addition of further water molecules, the energetic preference for uniform hydration of sidechains via interconnected water clusters disappeared. The optimized structures of a system of two oligomeric fragments at X = 2.5 showed that the structure with 

PFSA PEMs at various hydration levels and lECs. They found that longer sidechains lead to the formation of larger aggregates of sulfonate groups and consequently to larger water clusters, with cluster sizes varying from 2 to 13 nm for 5 A, 16. 

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