Abstract
This review paper is devoted to two topics, i.e., fluoropolymer-based porous and ion-exchange membranes, both of which include the creation of nanostructure-controlled functional membranes with high-energy ion beams. Latent tracks of the MeV−GeV heavy ions in a 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 been considered as a porous matrix for nanotechnological applications. 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 of the etched pores. The oxidation of the tracks by means of heating in air and ozone exposure was effective as a pre-etching treatment. The cation- and anion-exchange membranes（CEMs and AEMs, respectively）were prepared with two methods, which are based on the chemical etching and/or modification of each track, in other words, the radiation grafting into the track-etched pores in PVDF and ion-track direct grafting into poly（ethylene-co-tetrafluoroethylene）（with no etching process）. The resulting “nano-structure controlled” CEMs and AEMs were found to have one-dimensional ion-conductive pathways parallel to their thickness direction, while, in contrast, most of the existing membranes exhibited ion transport in the three-dimensional random media. This is probably because the nearly columnar ionic phase with a width of tens-to-hundreds of nanometers extended through the membrane. Other excellent membrane properties for applications to fuel cells and seawater electrodialysis, e.g., high dimensional stability, should also be due to such a controlled structure. Finally, worldwide and TIARA’s recent progress in irradiation technologies are presented to pursue the possibilities for industrial applications of our new functional membranes.
