Stepwise Crystallization and Mass Transport Limitations in Annealed SPEEK Thin Films

Pt-supported SPEEK (sulfonated poly ether ether ketone) thin films, mimicking the ionomer-electrode interface in polymer electrolyte fuel cells (PEFCs), were synthesized with thicknesses from 12 to 105 nm. Their glass transition temperature (Tg) was analyzed via in-situ thermal ellipsometry, revealing a thickness-dependent Tg decrease, indicative of confinement effects. Particularly, a 30 nm film, typical of practical ionomer films, exhibited surface crystallization preceding that at the buried interface, a phenomenon linked to higher mobility at the free surface, as con-firmed by grazing incidence wide-angle X-ray scattering (GI-WAXS) at both sub-critical (0.09º) and supercritical (0.14º) angles. This study elucidates the dynamic interplay between the high-mobility free surface and the interaction-intensive buried interface in confined SPEEK thin films. It was observed that below 30 nm thickness, interfacial interactions become the primary factor influencing transition temperature. Additionally, spatially heterogeneous crystallization, more pronounced in the out-of-plane direction, correlates with reduced proton conduction.