Effect of electron beam radiation on the mechanical and thermomechanical properties of proton exchange membranes

Introduction. Proton exchange membranes (PEMs) like Nafion® and other membrane materials are commonly tested for fuel cell applications. PEMs can be operated under harsh environments such as nuclear facilities that may lead to deterioration of material endurance. For example, oxidation may play an important role on the radiation induced degradation of Nafion® and release of fluorine products of radiolysis (1). Sulfonated poly-ether-ether ketone (SPEEK) containing non-fluorinated aromatic backbone is promising candidate for the applications under harsh environments (2). The aim of this study was to evaluate the influences of electron beam irradiation on the mechanical and thermomechanical properties of Nafion® and SPEEK membranes. Methods. Nafion® and laboratory made SPEEK membranes with a high degree of sulfonation were irradiated by accelerated electrons at a dose range of 50-500 kGy. The tensile tests, dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) were used to characterize change of PEM material properties depending on the dose of irradiation. Results. Preliminary analysis of the irradiated PEM samples by infra-red spectroscopy and their water solutions by ion chromatography indicated high differences in susceptibility to radiation of two PEMs– the Nafion® indicated oxidation and release of fluoride and sulfate anions with increase of the dose comparing to less pronounced changes in the structure of SPEEK membrane. The tensile tests at room temperature and DMA analysis, a notable reduction of mechanical properties (tensile strength, strain, storage modulus) was determined in the case of Nafion® due to the radiation induced oxidation and chain scission. The increase of the radiation dose affected improvements of the DMA characteristics of SPEEK. The study of thermal transitions including glass transition of SPEEK also confirmed that radiation mainly possessed little effect on the thermomechanical behavior as compared to that of negative effect on Nafion®. From the DSC data it was determined that Nafion® exhibited an endothermic peak of the first heating assigned to molecular rearrangements inside the polar clusters of the polymer. A gradual decrease from 167 to 139°C was determined with increase of the radiation dose from 50 to 500 kGy associated with radiation induced chain scission. There was not determined notable effect of the radiation dose on the first heating transformations of the SPEEK with exception of determined shift of endothermic peak from 102 to 110°C with increase of dose associated with the cluster changes due to the moisture content. As shift of the SPEEK glass transition temperature from 209 to 215 °C was indicated at 500 kGy during the comparison of secondary heating cycle. That may be due to the induced transformation of macromolecular arrangements. Conclusion. This study provides evidence of radiation induced degradation of Nafion® structure at irradiation doses exceeding 100 kGy. For comparison, SPEEK membrane presented rather high stability and improvement of mechanical properties at irradiation doses up to 250 kGy. (author)