Anhydrous proton-conducting properties of triazole–phosphonic acidcopolymers: a combined study with MAS NMR‡

The synthesis, thermal and proton conducting properties of copolymers based on vinylphosphonic acid (VPA) and 1-vinyl-1,2,4-triazole (VTri) were investigated. The copolymers were synthesized by free-radical copolymerization of the corresponding monomers at several monomer feed ratios to obtain poly(VPA-co-VTri) copolymer electrolytes. The final structures of the copolymers were confirmed by spectroscopic methods. The composition of the low molecular weight copolymers was varied with the feed ratio of the monomers. The presence of triazole units in the copolymers suppresses the formation of phosphonic acid anhydrides up to 150 °C, as verified by both 31P NMR and TGA. The observation of defined glass transition temperatures indicated that the ionic interactions do not prevent segmental relaxations of the polymer chains. In the absence of humidity, the copolymer electrolyte, poly(VPA-co-VTri), S2 (with 33% triazole content) showed proton conductivity of 10−3 S cm−1 at 120 °C, which is far higher than in imidazole based copolymers. Two different types of hydrogen-bonded protons were detected by 1H MAS NMR in the solid copolymer systems, due to different arrangements of triazole and phosphonic acid units.