Proton-conductive acid–base complex consisting of κ-carrageenan and 2-mercaptoimidazole

Novel acid–base complexes were prepared from κ-carrageenan (Cg) and 2-mercaptoimidazole (MIm). FTIR absorption measurements revealed that Cg and MIm interact through proton exchange reactions between the sulfonic acid groups of Cg and the imino nitrogen atoms of MIm. DSC thermograms suggested that the extent of the Cg–MIm intermolecular interactions could be modulated by the doping amount of MIm. From the TG curves, the thermal stability of the complexes was found to improve with an increase in the doping amount. Furthermore, the degree of water absorbability of the complexes was reduced to half of that of Cg. The Cg–MIm complex at the smallest doping amount showed the highest level of proton conductivity, which exceeds the conductivity of Cg. The enhanced conductivity is thought to be derived from the softening effect of MIm based on the increased mobility of the polymer chains of Cg.