Three-Component Domains in the Fully Hydrated Nafion Membrane Characterized by Partial Scattering Function Analysis

Significance 

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.

Three-Component Domains in the Fully Hydrated Nafion Membrane Characterized by Partial Scattering Function Analysis - Advances in Engineering
Comparison of the structural characterization of hydrated Nafion membranes by traditional scattering intensity analysis and PSF analysis.

About the author

Dr. Yue Zhao obtained her PhD from the University of Science and Technology of China (USTC) in 2003. Following a postdoctoral stay at Kyoto University with Prof. Takeji Hashimoto, she started her academic career at Japan Atomic Energy Agency (Current: QST) in 2006. She is currently a principal research scientist in the Department of Advanced Functional Materials Research at National Institutes for Quantum Science and Technology (QST). Her current research interests focus on the visualization of polymer electrolyte membranes structures by small-angle Neutron & X-ray scattering and imaging techniques, and the establishment of the improved design rules for high-performance functional polymer materials through structure-designing, simulations and materials informatics.

About the author

Dr. Yasunari Maekawa is a Deputy Director General of Takasaki Advanced Radiation Research Institute, QST. He obtained his PhD from the University of Tokyo in 1991. After postdoctoral researches at IBM, Almaden Research Center and University of Wisconsin, Madison, he joined Hitachi Co. Ltd, Hitachi Research Laboratory in 1994. He moved to Japan Atomic Energy Research Institute (Current: QST) in 1998. He is a Vice-Chairperson of Japanese Society of Radiation Chemistry (2018- ). At QST, he focused on R&D of polymer electrolyte membranes for fuel cells by radiation-induced graft polymerization and structural analysis using X-rays and neutron scattering techniques. His current research interests include materials informatics and statistical experimental design to establish highly efficient R&D processes for novel polymer functional materials.

Reference

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 AnalysisMacromolecules, 54(9), 4128-4135.

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