Investigation of structural changes related to temperature: An understanding of H-bond based proton transfer in 4(5)-vinylimidazole and acrylic acid copolymer membrane

An attempt to understand how proton transfer proceeds in poly (acrylic acid-co-4(5)-vinylimidazole) has been carried out based on the temperature dependent characterization techniques, i.e., Fourier transform infrared spectroscopy (FTIR), wide angle X-ray diffraction (WAXD), Raman spectroscopy, including the atomic distance calculations. Systematical studies are achieved from a series of poly (acrylic acid-co-4(5)-vinylimidazole) with different acrylic acid content. When the copolymer is almost an ideal in equimolar ratio of an alternating structure, the hydrogen bond between carboxylic acid and imidazole is maintained and initiates the proton conductivity even at 120 °C. Whereas when the copolymer is carboxylic acid rich, the dehydration to form anhydride proceeds resulting in the decrease in proton conductivity at high temperature. The radial distribution function (RDF) calculated from the WAXD pattern shows that the inter-atomic distances reflect how the increase in temperature induces a favorable packing structure under the hydrogen bond network and the chain mobility to enhance the proton transfer at high temperature, especially in the case of the copolymer with an ideal alternating structure.