Radiation induced, raft mediated grafting of styrene onto poly(ethylene-alt-tetrafluoroethylene) (ETFE)

Complete text of publication follows. The development of cost-effective proton exchange membranes to replace the state-of-the-art and expensive perfluorinated membranes such as Nafion, Flemion, and Aciplex is one of the main challenges for commercialization of polymer electrolyte fuel cell technology. Recently, partially fluorinated poly(ethlyene-alt-tetrafluoroethylene) (ETFE) has been identified as a promising alternative base polymer film to prepare low-cost polymer electrolyte membranes because of its advantageous characters like superior mechanical properties and high resistance to radiation-induced damage. The radiation-induced grafting technique, based on the utilization of a polymer material such as ETFE in combination with further chemical modification steps (sulfonation) allows the functionalization of the base material and the introduction of the desired property (proton conductivity) for preparing a fuel cell membrane. However this simple conventional method suffers from one simple flaw: The molecular weight and the polydispersity of the grafted chains cannot be controlled. Predetermined molecular weights and low dispersities as well as homogeneous composition and desired architecture can be achieved by grafting of monomer onto base polymer under living/controlled free radical polymerization (CRP) conditions. Among the CRP methods, Reversible Addition Fragmentation-Chain Transfer (RAFT) is of particular interest as a very wide range of functional monomers can be polymerized in a controlled manner under non-demanding reaction conditions (e.g., tolerance to oxygen and low temperatures). The present study deals with the RAFT mediated radiation-induced (0.032 kGyh-1, 60Co) grafting of styrene on ETFE films followed by the sulfonation of the polystyrene grafts. The effect of monomer concentration, absorbed dose and RAFT agent concentration on the grafting were investigated. The synthesized films were characterized by ATR-FTIR, XPS, DSC and TGA methods. Cross-sections of these films were investigated by SEM-EDS and Confocal Raman Spectroscopy to evaluate the grafting profile. These films have potential use as a fuel cell membrane and we believe that the method applied will present and advance in the ability of preparing fuel cell membranes with predefined graft architecture.