GISAXS Analysis for Ionomer Thin Films

Abstract

Perfluorinated sulfonic-acid (PFSA) polymers are a class of ion-conducting polymers (ionomers) heavily utilized in fuel cells and similar electrochemical technologies. The hydrophobic backbone gives excellent mechanical properties, while the side-chain terminated with a hydrophilic acid group bestows superior proton conductivity. The crystallinity of the backbone is believed to be inversely proportional to conductivity and proportional to the mechanical modulus in bulk membranes. Demonstrated previously, the transport and mechanical properties of PFSA ionomers deviate from bulk membranes as the ionomers are confined into thin-film geometries.[1]Under such confinement, conductivity and water uptake decrease, while the modulus increases. Studying this deviation from bulk behavior is crucial as ionomer is present as thin films in the catalyst layer of these electrochemical devices. In this work, we correlate the relative crystallinity of PFSA thin-films, calculated from Grazing-incidence Wide-angle X-ray Scattering (GIWAXS), to their mechanical properties measured using a cantilever bending system.[2]Using a laser array, thin-film stress can be measured as a function of humidity from which mechanical properties are calculated. This system allows the study of thin films on hard impenetrable surfaces that induce confinement effects, and probe the effect of interfacial interactions on different substrates. Understanding crystallinity and mechanical properties as part of the structure-property relationship will allow optimization of ionomer performance within the catalyst layer. Acknowledgement This work was sponsored by the Army Research Office under MIPR0010754069. The work utilizes beamline 7.3.3 at the Advanced Light Source, which is funded by DOE Basic Energy Science under contract number DE-AC02-05CH11231.