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Radiation grafted membranes with controlled monomer content were prepared to investigate the role of nitrile containing comonomers on the properties of proton conducting membranes. The membranes consist of a partially fluorinated backbone of ETFE with grafted chains containing styrene sulfonic acid and its comonomer, methacrylonitrile (MAN) or acrylonitrile (AN). Upon grafting, the comonomers show a tendency to form an alternating copolymer with styrene. The ex situ properties of both types of co-grafted membrane, i.e., proton conductivity, water uptake, and dimensional stability, are largely insensitive to the type of comonomer but mainly governed by the ion exchange capacity (IEC). Styrene/AN co-grafted membranes undergo substantial nitrile hydrolysis during the sulfonation procedure compared to styrene/MAN co-grafted membranes. The substituent at the alpha position of the comonomer determines the susceptibility of the nitrile to hydrolysis during membrane preparation as well as during fuel cell operation. Despite the change in the chemical properties as a result of hydrolysis, the mechanical integrity of the co-grafted membranes is retained. However, the styrene/AN co-grafted membrane showed considerably inferior performance in the fuel cell compared to the styrene/MAN co-grafted membrane. These findings bring about a design rationale for membrane materials with enhanced stability against hydrolysis.

The relationships between the nano-scale structure and the monomer composition of proton exchange membranes (PEMs) are reported. The PEMs are synthesized by preirradiation-induced grafting ETFE with styrene and cross-linker, 1,3-diisopropenylbenzene (DIPB), where the styrene moieties are sulfonated subsequently. The degree of grafting is constant, while the DIPB level is varied systematically. The SAXS spectra of the dry membranes are isotropic and dominated by a single correlation peak, which results from the phase separation of the ion-rich phase from the polymer matrix. By analyzing the correlation peak we find that the number density and the typical size of the ion-rich domains decrease when the level of cross-linking is increased. The proton conductivity in the fully hydrated state is proportional to the volume fraction of the ion-rich phase. This suggests that the structure of the ion-rich phase found in the dry state has fundamental impact on the conductivity of the hydrated membrane. The relationship between the proton conductivity and the water volume fraction follows a power law in good agreement with percolation theory.

Selective exposures of poly(ethylene-alt-tetrafluoroethylene) (ETFE) films with hard X-rays through high aspect ratio Ni-masks were performed at the LIGA3 beamline of the “Angstrom Quelle Karlsruhe” (ANKA) to create patterns of radicals used as initiators for the grafting of styrene into the bulk of the ETFE films. Grafted films were then sulfonated to obtain proton-conducting membranes. The structure definition, as investigated by scanning electron microscopy (SEM), showed a perfect discrimination between exposed and shaded areas through all the film thickness. Structuring results in a more homogeneous appearance of the membrane without affecting the degree of grafting and proton conductivity in the grafted areas. In fuel cell tests the structured membranes showed slightly lower performance due to 10% lower active area, but had a significantly higher lifetime.