Crosslinked sulfonated poly(ether ether ketone) (SPEEK) membranes were prepared by the radiation crosslinking of SPEEK with various crosslinker contents. The prepared membranes were subjected to a comparative study of proton exchange membranes (PEM) for fuel cell applications. The crosslinked SPEEK membranes were characterized by 1 H nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopyattenuated total reflectance (FTIR-ATR), dynamic mechanical analysis (DMA), and Small angle X-ray scattering (SAXS). 1 H NMR and FTIR-ATR confirmed the sulfonation and crosslinking of the membranes. DMA indicated that the ionic modulus and cluster T g increased with decreasing crosslinker content. This suggests that the amount of clustering in the crosslinked membrane could be reduced by increasing the crosslinker content. SAXS data showed that the crosslinker did not affect the ionic domain morphology in the membranes. The proton conductivity of all the membranes was > 10 -2 S/cm. The overall findings suggest that the crosslinked SPEEK membranes offer the possibility for improving the performance of proton exchange membrane fuel cell (PEMFC), provided membranes with thermal and hydration stability.

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The surfaces of poly(styrene-co-acrylic acid) copolymers and their Na-and Cs-neutralized ionomers were modified by O 2 -plasma source ion implantation (PSII) treatment to improve the surface wettability. The changes in the surface wettability, composition, and structure upon the PSII treatment were examined with contact-angle measurements and X-ray photoelectron spectroscopy. The untreated surfaces of the acid copolymers and ionomers exhibited different surface energies; this implied clearly that the type of ion species affects the surface hydrophilicity. Also, the PSII treatment induced oxygen-containing groups to reside on the surface and ionic groups to come out toward the surface; this made the surfaces of the ionomers more hydrophilic as compared with that of the acid copolymers. The ionomers also showed slow hydrophobic recovery. Thus, it was suggested that the reduced mobility of the polymer chain because of the presence of ionic aggregates results in restricted reorientation of oxygen-containing groups.

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Perfluorosulfonic acid ionomers (PFSAs), such as Nafion®, have become the benchmark membrane material for proton exchange membrane fuel cells (PEMFCs). Despite their commercial success, little is known about the complex morphology-property relationships governing the chemical and physical properties. For example, the detailed structure of PFSA crystallites within the amorphous phase and the spatial arrangement of the crystallites around the ionic aggregates of PFSA materials are virtually unknown. In addition, the effect of processing on the membrane performance and durability in PEMFC applications is not yet fully understood. The first part of this work focuses on the significance of the crystalline component with respect to polymer-water interactions. The second part of this work addresses the impact on changes in morphology that could occur during long-term operation in a fuel cell including the effects of heat and humidity.