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Nafion TM is a perfluorosulfonic polymer membrane having exceptional chemical, thermal, and mechanical stability. It can act as a cation exchange membrane and it has found application as a separator in chlor-alkali cells. More recently, and due to its high proton conductivity, Nafion has attracted much interest as a polymer electrolyte membrane in polymer electrolyte fuel cells (PEFCs). Recent reviews summarize the applications as well as the structure and properties of perfluorinated membranes. 1,2 To enhance the performance of the PEFC, Nafion in solution is painted onto the electrode surfaces. Results from wide-angle X-ray diffraction (WAXS) studies are rather inconsistent and imply that there is either no crystallinity in Nafion membranes, 3 that they are partly crystalline, 4 or that crystallinity can be introduced in recast amorphous films by thermal treatment. 5 An electron microscope study suggests that crystalline Nafion has a linear zigzag structure just as polyethylene. 6 This polyethylene-like structure is also supported by an investigation of thermally treated recast Nafion. 7 Our question was whether it would be possible to introduce crystallinity into Nafion with some sort of heavy ion irradiation.We decided on xenon ions. Cast Nafion films irradiated in vacuum by xenon ions were in fact observed to crystallize. Even though the films were exposed to different doses of ions, they crystallized in the same way. The size of the crystalline regions was 0.5-10 mm and thus easily observed. This attracted our attention and some further investigations on this material were made, as well as on films prepared without any irradiation. ExperimentalThin films (ca. 1 m thick) of Nafion (equiv wt 1100) were cast on single-crystal silicon windows from a solution of lower aliphatic alcohols and water, Fluka Chemika. For use in another experiment, in an ongoing project, the films were put in vacuum, 10 Ϫ3 mbar, for approximately 10 h while they were irradiated with xenon ions. The irradiation was made on the cyclotron facility at the The Svedberg Laboratory, Uppsala University. Each film was exposed to between 1 and 100 ions/m 2 . The films were stored at room temperature in the laboratory for future investigations.After some time, large crystalline regions, 0.5-10 mm, were formed in the films (Fig. 1). If a drop of water was added to the crystalline region of the Nafion film, it became amorphous immediately. Fourier transform infrared (FTIR) microspectroscopy (reflection mode) and X-ray diffraction (XRD) were used to investigate the amorphous and crystalline regions. Due to the size of the crystalline regions, the infrared beam could be focused on either crystalline or amorphous regions to record infrared spectra in open air.Furthermore, in another ongoing project, reference spectra at different relative humidities (RH) were recorded. 8 This experiment included a cell where the desired RH and temperature could be achieved. In the experimental arrangement it was possible to achieve a state where n ϭ 1 (n ϭ H 2 O/SO 3...

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