Interesting facets of surface, interfacial and bulk characteristics of perfluorinated ionomer films.

Ion-containing perfluorinated polymers possess unique viscoelastic properties, excellent proton conductivity, and nano-phase-segregated structure all arising from the clustering of hydrophilic sulphonic acid groups within a matrix of hydrophobic fluorocarbons. When these ionomers are confined to nanothin films, a broad swathe of structural organization imparting a rich variety of surface, interfacial and bulk characteristics can be expected. However, our understanding of perfluorinated ionomer thin film behavior is still in a rudimentary stage with much of the focus on the hydration-related structure/properties pertinent to electrochemical applications. Thus, many of the hidden gems - interesting surface and interfacial properties - have been overlooked. In this Feature Article, which is a summary of the key contributions by author's group and several collaborative work on ionomer thin films, we unravel many of these facets. For example, we first focus on the self-assembly of ionomers and show that dispersion media and hydrophobicity/hydrophilicity of the substrate can result in partial or even no coverage of substrates shedding light on the complexity of polymer-substrate, polymer-solvent, and polymer-polymer interactions - an insight completely obscured when spin-coating method is adopted for film creation. We demonstrate that the same ionomer can be used to create a variety of surfaces ranging from super-hydrophilic to highly hydrophobic by controlling the film thickness or through choice of substrate material. The ultra-thin, hydrophilic surfaces of self-assembled Nafion ionomer films exhibit wettability switching behavior which opens up the door for creating stimuli-responsive smart surfaces. Thermally induced changes to the surface wettability, bulk-averaged thermal expansion and molecular motions is discussed. The substrate- and film thickness-dependent thermal expansion coefficients reinforce the importance of interfacial interactions and confinement on these films. They also open up the potential of tuning ionomer bulk properties via substrate chemistry. The discovery of a vibrational mode that becomes thermally activated at high temperature has provided new insights on the origins of the molecular motions responsible for α-relaxation of Nafion ionomer as well as on the underlying reason for wettability switching. Our recent neutron reflectometry study of different ionomers on platinum substrate shows that the interfacial hydration levels is correlated to side-chain lengths opening up the possibility of the controlling the interfacial electrochemistry. Finally, a systematic analysis of factors affecting proton conduction is presented to elucidate the yet-unresolved origins of suppressed conduction of nanothin ionomer films compared to that of the bulk membrane. By revealing these interesting yet poorly understood facets of ionomer thin films, the article aims to stimulate further scientific pursuit on this topic.