Synthesis and Rheological Behavior of Supramolecular Ionic Networks Based on Citric Acid and Aliphatic Diamines

Novel supramolecular ionic networks were obtained by reacting citric acid and aliphatic diamines. A proton transfer reaction takes place between the carboxylic acid of citric acid and the amine group leading to the corresponding ionic carboxylate and quaternary ammonium groups. By this method, a series of supramolecular ionic networks were obtained due to the multiple ionic interactions between the corresponding citrate and diammonium molecules as observed by FTIR spectroscopy. Rheological analysis of the ionic networks was carried out considering frequency and temperature sweeps in small-amplitude oscillatory flow and viscous measurements in continuous flow. At low temperatures and/or high frequencies the ionic interactions brought about an elastic network or gel which vanished at high temperatures and/or low frequencies. The viscoelastic behavior was governed by a single relaxation time and a very high plateau modulus, G(p) = 5 x 10(6) Pa. The relaxation time showed an Arrhenius-like dependency with temperature, leading to draw diagrams of the physical states for each sample. The obtained supramolecular ionic networks based on different aliphatic diamine molecules did not show differences in their respective solid and liquid states. However, the frequency-dependent network liquid transition temperature, T-nl, varied with the chemical nature of the diamines. The higher T-nl (45 degrees C) was found for the system that contains 1,3-diaminopropane which is attributed to stronger ionic bonds involving primary amines, with respect to ionic bonds with tertiary amines (between -1 and 32 degrees C). Comparing ionic networks obtained from different tertiary diamines, such as tetramethyl-1,3-propanediamine and tetraethyl-1,3-propanediamine, the lower T-nl was observed in the latter, ascribed to a higher mobility of the aliphatic pendant groups.