Versatile synthetic route to tailor-made proton exchange membranes for fuel cell applications by combination of radiation chemistry of polymers with nitroxide-mediated living free radical graft polymerization

A versatile method for the preparation of proton exchange membranes (PEMs) by the combination of radiation chemistry of polymers with nitroxide-mediated living free radical graft polymerization with subsequent sulfonation is presented. Thus, poly(vinylidene fluoride) (PVDF) membranes were first irradiated with electron beam and then the free radicals formed were immediately quenched with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO). In the second step, the produced TEMPO-capped macroinitiator sites were utilized in nitroxide-mediated living free radical graft polymerization of styrene or for the controlled graft copolymerization of styrene and N-phenylmaleimide onto the PVDF membrane. In the final step, the membranes were either directly sulfonated, or in the former case, alternatively the alkoxyamine moieties at the polymer chain ends were substituted by a maleimide derivative prior to sulfonation. The introduction of the N-phenyl maleimide moieties into the grafted chains significantly increased the thermo-oxidative stability of the PEMs as determined by thermogravimetric analysis (TGA). In this work, also a comparison between these membranes prepared by the controlled nitroxide-mediated graft polymerization and those membranes prepared by the conventional preirradiation grafting method in terms of grafting yields, thermo-oxidative stability, water uptake, and ion exchange capacity, proton conductivity, etc. is presented. Noteworthy is the fact that the membranes using the controlled grafting technique are grafted through the membrane already at a degree of grafting of 14%, whereas the penetration limit for the membranes prepared by conventional radiation-induced grafting is approximately 30% as determined by SEM-EDX analysis. Furthermore, preliminary H-2/O-2 fuel cell tests showed promise for the development of this type of PEMs prepared by means of a nitroxide-mediated living free radical grafting process. Thus, already the PVDF membrane that had been grafted with styrene by means of controlled radical polymerization could after sulfonation be used in a fuel cell for 930 h at 70 degreesC without any drop in current density. In contrast, according to our previous studies, PVDF membranes prepared by conventional preirradiation grafting of styrene fail within 150 to 200 h under similar conditions.