NafionR is a perfluorosulfonic acid polymer membrane exhibiting excellent thermomechanical and proton conducting properties. It is widely utilized as a proton exchange membrane in various applications such as automobile fuel cells. Typically, hydrated Nafion is a relatively complex system comprises three components: inert tetrafluoroethylene-like main chain, fluorinated side chain and water. When hydrated, the ionic groups at the end of the side chains absorb water resulting in the formation of ion channels for transporting water and proton, phase-separated from the hydrophobic polymer matrix.
The phase-separated morphology in the hydrated Nafion has been extensively studied using the small-angle scattering technique. Generally, the analysis is mainly based on its scattering intensity profiles, I(q), which show three main features: a small-angle upturn, first broad peak and second strong peak. These features are related to large structures, intercrystalline separation distance between the polymer domains as well as hydrophilic water domains, respectively. However, the I(q) profile is derived from the scattering contributions from all the components in Nafion, which only identifies the structure as a whole but fails to provide the concrete structure of individual components. This can be solved by using partial scattering function (PSF) analysis through contrast variation small-angle neutron scattering (SANS) method.
To this note, Dr. Yue Zhao, Dr. Kimio Yoshimura, Dr. Toshinori Motegi, Dr. Akihiro Hiroki and Dr. Yasunari Maekawa from National Institutes for Quantum Science and Technology (QST), together with Dr. Aurel Radulescu from Jülich Centre for Neutron Science at MLZ utilized contrast variation SANS technique to decompose I(q)s into PSFs to accurately identify the exact structure of the individual components of hydrated Nafion membrane (main-chain, side-chain and water). Additionally, the authors performed cross-term analysis to explore the correlation between two components to establish their locations. Their research work is currently published in the journal, Macromolecules.
The validity of the three components was successfully confirmed and the structural characterization was divided into three regimes: small scale (< 5nm), middle scale (5 – 30 nm) and large scale (> 30 nm). PSF analysis revealed a detailed structure of the individual components. In contrast, cross-term analysis correlated two components, providing their exact location. The authors observed that the main-chain phase-separated from either water domains or side-chain. All the components exhibited bicontinuous-like local structures, indicating high membrane connectivity due to the well-connected water network. The templating effects of the main-chain semicrystalline structure dominated the middle scale, producing bicontinuous-like crystalline structure and amorphous phases with an average separation distance of 11 nm. In the large-scale regime, the structural heterogeneities were present in both main- and side-chain domains.
In summary, the research team studied the structures of the individual components making up a fully hydrated Nafion membrane through contrast variation SANS experiments which decomposed the scattering intensity data into PSFs of the individual components. The results showcased the critical role played by PSFs in the quantitative characterization and understanding of the critical role of all the Nafion components. In a statement to Advances in Engineering, Dr. Yue Zhao stated that the study findings provided deep insights into the structural characterization of Nafion membrane components that might expand their practical applications.
Zhao, Y., Yoshimura, K., Motegi, T., Hiroki, A., Radulescu, A., & Maekawa, Y. (2021). Three-Component Domains in the Fully Hydrated Nafion Membrane Characterized by Partial Scattering Function Analysis. Macromolecules, 54(9), 4128-4135.