Fluoropolymer-based nanostructured membranes created by swift-heavy-ion irradiation and their energy and environmental applications

We have realized the importance of developing micro/nanofabrication techniques for fluoropolymers in order to further pursue their potential for future applications. This paper is devoted to the following two topics, i.e., ion-track membranes and ion-track-grafted electrolyte membranes for fuel cell applications, both of which include the creation of fluoropolymer-based nanostructured membranes with swift heavy ions. Latent tracks of the MeV-GeV heavy ions in an organic polymer foil can sometimes be chemically etched out to form a membrane with micro- and nano-sized through-pores, the so-called ion-track membrane. Our focus is on ion-track membranes of poly(vinylidene fluoride) (PVDF), which have also been considered as a matrix of functionalized polymer membranes. Although the PVDF-based ion-track membranes have already been reported, their preparation methods have never been optimized. The etching behavior mainly depended on the energy deposition of the ion beams, and thus its depth distribution, estimated by a theoretical simulation, was successfully applied to control the shapes and diameters of the etched pores. The electrolyte membranes for fuel cell applications were prepared by the direct ion-track grafting method. The membrane preparation involves (i) irradiation of a fluoropolymer (mostly the poly(ethylene-co-tetrafluoroethylene)) base film to create reactive species, (ii) graft polymerization of styrene or its derivative monomer into latent tracks, and (iii) sulfonation of the graft polymers. Interestingly, the resulting membranes exhibited an anisotropic proton transport, i.e., higher conductivity in the thickness direction. Based on microscopic observations, this is probably because the nearly columnar electrolyte phase with a width of tens-to-hundreds of nanometers extended through the membrane. Other excellent membrane properties, e.g., a high dimensional stability, should also be due to such a controlled structure.