Sprecher
Beschreibung
Quantifying the hydration structure of ionomer membranes remains one of the central challenges in understanding the relationship between nanoscale morphology and proton transport in polymer electrolyte membranes (PEMs). Although extensive studies using calorimetry, FTIR spectroscopy, nuclear magnetic resonance (NMR), and molecular dynamics (MD) simulations have established that water in Nafion exists in distinct populations commonly described as bound and free water, these approaches define hydration from thermodynamic, spectroscopic, dynamical, or computational perspectives. Direct experimental quantification of bound water from a purely structural viewpoint, however, remains largely unavailable. Such structural information is essential for establishing microscopic hydration models and for correlating membrane nanostructure with the performance.
Contrast variation small-angle neutron scattering (CV-SANS), combined with partial scattering function analysis (PSFA), provides a unique opportunity to resolve component-specific structural correlations in multicomponent soft-matter systems. Building upon our previous PSFA framework for hydrated Nafion, an updated methodology with a modified model has been developed in which the conventional side-chain component is replaced by a "hydrated side chain" with a variable number of associated water molecules. By systematically varying the assumed number of associated water molecules and analyzing the corresponding cross-term partial scattering function between hydrated side chains and unassociated water, the ensemble-averaged number of structurally bound water molecules can be determined from the condition where the spatial correlation vanishes at the ionomer peak. This methodology provides, for the first time, an experimentally accessible structural criterion for quantifying bound water directly from neutron scattering measurements.
The methodology has been applied to Nafion membranes over a broad hydration range, from low relative humidity conditions to fully hydration. The analysis reveals a systematic evolution of bound water populations with increasing hydration, yielding quantitative structural information that is consistent with previous calorimetric, FTIR, and MD studies while providing a fundamentally different structural interpretation. The presentation will demonstrate how advanced CV-SANS data analysis can provide quantitative structural descriptors that are inaccessible by conventional scattering analysis and discuss its potential applications to a broad range of hydrated soft-matter systems.