Systematic in situ hydration neutron reflectometry study on Nafion thin films.

Area of Science:

• Materials Science
• Polymer Science
• Surface Science

Background:

• Nafion is a crucial polymer in fuel cells and other electrochemical devices.
• Understanding Nafion's hydration behavior is essential for optimizing device performance.
• Thin film characterization is key to studying polymer behavior at interfaces.

Purpose of the Study:

• To systematically investigate the hydration morphology of Nafion thin films using neutron reflectometry.
• To quantify water adsorption and lamellae formation in Nafion films across the full relative humidity range.
• To demonstrate a neutron reflectometry methodology for studying hydrophilic polymer hydration.

Main Methods:

• Neutron reflectometry (NR) was employed to study hydrated Nafion thin films (∼30 nm) on silicon substrates.
• Comparative studies using both H2O and D2O vapors were performed across the entire relative humidity range.
• Analysis of scattering signals allowed for the determination of water adsorption isotherms and lamellae evolution.

Main Results:

• A complete Type II water adsorption isotherm for Nafion thin films was established using D2O's strong scattering signal.
• Hydration-dependent formation of Nafion/water lamellae near the substrate was quantified using H2O's scattering signal.
• Lamellae layer formation was observed to increase continuously with hydration, not abruptly, and film swelling was characterized.

Conclusions:

• The study provides a detailed understanding of Nafion thin film hydration dynamics.
• The findings are valuable for quantifying hydration-dependent properties like conductivity in Nafion films and for Nafion/semiconductor devices.
• The presented NR methodology is applicable to other hydrophilic polymers, advancing thin film characterization techniques.