Block selective grafting of poly(vinylphosphonic acid) from aromatic multiblock copolymers for nanostructured electrolyte membranes

Alternating aromatic multiblock copolymers have been structurally designed to enable selective lithiation and subsequent anionic graft polymerization from only one of the two block types. The multiblock copolymers were prepared by coupling polyfluoroether (PFE) and polysulfone (PSU) precursor blocks under mild conditions. The judicious combination of blocks allowed for block selective lithiation of the PSU blocks to obtain a macroinitiator for anionic polymerization of diethyl vinylphosphonate. The block selective grafting was confirmed by 1H and 19F NMR spectroscopy. After hydrolysis to obtain poly(vinylphosphonic acid) (PVPA) side chains, mechanically stable transparent electrolyte membranes were cast from 1-methyl-2-pyrrolidinone solutions. Analysis by atom force microscopy showed that the copolymers self-assembled to form nanostructured membranes with continuous proton conducting PVPA phase domains. Calorimetry showed separate glass transition temperatures from the PFE and PVPA phases, with the latter increasing with increasing annealing temperatures as a result of anhydride formation. Fully hydrated multiblock copolymer membranes reached proton conductivities above 80 mS cm-1 at 120 °C. The approach of block selective lithiation and modification of aromatic block copolymers can be used as a general strategy to prepare durable and functional nanostructured polymer membranes and materials.