Many morphological models have been proposed to describe the water swelling behavior and proton transport mechanism of perfluorosulfonic acid (PFSA) polymer membranes through experimental and modeling studies. However, the ongoing structural debate has not been completely resolved yet. We here conducted a series of all-atom molecular dynamics simulations of hydrated PFSA membranes to evaluate changes in the membrane morphology at different water contents. We found a similar dependence of the morphology on the water content between PFSA membranes with equivalent weight (EW) of 844 and 1144 g/equiv. That is, the morphology of the aqueous domain changes with increasing water content from a channel-network structure to a tortuous layered structure, and once attaining the tortuous layered structure, the water layer just thickened gradually by further increasing water content. Furthermore, we found more heterogeneous water domains in the higher-EW PFSA membrane, demonstrating the stronger aggregation behavior of the aqueous domains in the high-EW membranes. The variation of the PFSA membrane morphology observed here is useful to understand the proton transport mechanism and design new materials suitable for polymer electrolyte fuel cells in the near future.

Silica aerogel has a variety of excellent properties, but the mechanical brittleness inhibits the practical applications. Recently, many experimental efforts have been made to improve the compressibility and bendability of aerogels by hybridization with organic materials; however, the reason of the flexibility has not yet been well understood. To identify the intrinsic origins of the flexibility of organic− inorganic hybrid aerogels, polymerization and mechanical responses of tetramethoxysilane (TMOS), methyltrimethoxysilane (MTMS), and 1,2-bis(methyldiethoxysilyl)ethane (BMDEE) polymers were investigated by using reactive molecular dynamics simulations. As a result, cyclic compressive deformation simulations successfully reproduce the experimental results that TMOS is substantially fragile, whereas MTMS and BMDEE are easy to be reshaped. Detailed structure analyses showed that Si−O−Si−O rings in TMOS are collapsed by compressive deformation, whereas any kind of ring structure in BMDEE is maintained even after large compression. Tetrahedral SiO 4 -based network structure (Q 4 ) in TMOS is found to be the source of the brittleness. On the contrary, the absence of Q 4 silicones and the presence of ethylene units, which provide rotatable dihedrals, in BMDEE allow it to deform without disrupting the microscale network. The insightful information provided by the theoretical investigation in atomistic scale is essential to design new composite aerogels.

Perfluorosulfonic acid (PFSA) polymer membranes are widely used as proton exchange membranes. Because the structure of the aqueous domain within the PFSA membrane is expected to directly influence proton conductance, many coarse-grained (CG) simulation studies have been performed to investigate the membrane morphology; these studies mostly used phenomenological models, such as dissipative particle dynamics. However, a chemically accurate CG model is required to investigate the morphology in realistic membranes and to provide a concrete molecular design. Here, we attempt to construct a predictive CG model for the structure and morphology of PFSA membranes that is compatible with the Sinoda-DeVane-Klein (SDK) CG water model [Shinoda et al., Mol. Simul. 33, 27 (2007)]. First, we extended the parameter set for the SDK CG force field to examine a hydrated PFSA membrane based on thermodynamic and structural data from experiments and all-atom (AA) molecular dynamics (MD) simulations. However, a noticeable degradation of the morphology motivated us to improve the structural properties by using the iterative Boltzmann inversion (IBI) approach. Thus, we explored a possible combination of the SDK and IBI approaches to describe the nonbonded interaction. The hybrid SDK/IBI model improved the structural issues of SDK, showing a better agreement with AA-MD in the radial distribution functions. The hybrid SDK/IBI model was determined to reasonably reproduce both the thermodynamic and structural properties of the PFSA membrane for all examined water contents. In addition, the model demonstrated good transferability and has considerable potential for application to realistic long-chained PFSA membranes.

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