Phase Segregation of Sulfonate Groups in Nafion Interface Lamellae, Quantified via Neutron Reflectometry Fitting Techniques for Multi-Layered Structures
Steven C. DeCaluwe, Paul A. Kienzle, Pavan Bhargava, Andrew M. Baker, Joseph Dura
Neutron reflectometry analysis methods are developed and used to determine the composition depth profile in cases where the structure is no known a priori. These methods, including statistical methods, sophisticated fitting routines, and coupling multiple data sets, are applied to hydrated and dehydrated Nafion thin films with thickness comparable to those found coating electrode particles in fuel cell catalyst layers. These results confirm the lamellar structure previously observed on hydrophilic substrates, and indicate that for hydrated films they can be accurately be described as a lamellar phase segregation oscillating between domains rich in water and sulfonate groups and water-poor domains containing more fluorocarbon groups. The thickness of these layers increases slightly and the oscillation amplitude of the water volume fraction exponentially decreases away from the hydrophilic interface. For dehydrated films the oscillations die out more rapidly. The Nafion-SiO2 substrate interface contains a partial monolayer of sulfonate groups bonded to the substrate and a large excess of water compared to that expected by the water-to-sulfonate ration, λ, observed throughout the rest of the film. Films that were made thin enough to truncate this lamellar region showed a depth profile nearly identical to thicker films, indicating that there are no confinement or surface effects altering the structuring. Comparing the SLD profile measured for dried films to modeled composition profiles derived by removing water from the hydrated lamellae suggests a partial re-distribution of the polymer grows upon dehydration, indicated limited polymer mobility in these confined Nafion thin-films.
, Kienzle, P.
, Bhargava, P.
, Baker, A.
and Dura, J.
Phase Segregation of Sulfonate Groups in Nafion Interface Lamellae, Quantified via Neutron Reflectometry Fitting Techniques for Multi-Layered Structures, Soft Matter, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=915744
(Accessed December 7, 2023)