New supramolecular ionic networks based on citric acid and geminal dicationic ionic liquids

Abstract

A new family of supramolecular ionic networks was synthesized by a straightforward method using citric acid and germinal dicationic ionic liquids. The networks were formed by the ionic boding of multi carboxylate and cationic molecules. The resulting networks are solid soft material at room temperature, which exhibited a thermoreversible transition, becoming a viscoelastic liquid at high temperatures. Thus, each supramolecular ionic network was characterized by an elastic modulus, G_e, and a network-liquid transition temperature, T_nl. The glass transition temperature, T_g, of the systems was evaluated by differential scanning calorimetry (DSC) measurements, which, however, were not able to detect the T_nl. Considering the effect of the employed dicationic ionic liquids, the three characteristic parameters of the networks, G_e, T_nl and T_g, followed the same trend, as they increased following the sequence: citrate-1,3-propanedipyridinium > citrate-1,3-propanedimethylimidazolium > citrate/1,3-propanedimethylpyrrolidinium and citrate/1,3-propanedibenzylimidazolium. The nature of the dication and its substituents determined the basic parameters of the elastic network, G_e and T_nl. The observed effect of the chemical structure on T_g was assumed to be due to a reduction of the free volume as G_e augments.