Effect of Eu(III) and Tb(III) chloride on the gelification behavior of poly(sodium acrylate)

Abstract

Supramolecular gels are an important class of soft materials: they are basically formed by solvent molecules inside in a 3D network structure and can be considered from soft and weak to hard and tough, depending on the respective mechanical properties. The gelation process can be accomplished in different ways using, for example, the interaction between a charged polymer and a metal ion. In this work, the interaction between poly(acrylic acid) sodium salt (PSA) and the trivalent ion (Eu3+ and Tb3+), to form luminescent gels, has been studied. For that, a phase diagram for polymer-ion mixture was constructed showing that for high molar ratios Ln3+/PSA different phase transitions occur. We have focused our work in the region where the formation of weak and strong gels is occurring. The phase transitions have been assessed by rheological and luminescence measurements. From rheological measurements, we have found that the mechanical strength of gels increases by increasing the concentration of PSA, since the elastic module (G´) increases when the concentration of PSA increases. A similar trend is found in yield stress values, from which the gel begins to behave as a liquid. Thus, the higher the concentration of PSA the higher is the yield stress. The frequency sweeps shows that Ln3+/PSA gels and weak gels exhibit a rheological solid-type behavior, with a storage module (G´) predominating over loss modulus (G´´) in the studied frequency. The formation of the gel phase can also be followed by fluorescence spectroscopy. It can be observed that the emission of fluorescence increases by increasing the molar ratio Ln3+/PSA. Such behavior may be explained by the binding of Ln3+ to deprotonated PSA and a decrease in the number of coordinated water molecules. However, the formation of gel phase is reversible and dependent on the Ln3+ concentration. In fact, by increasing the molar ratio values above a certain critical point, disaggregation of the gel structure occurs, mainly due to electrostatic repulsions between ions Ln3+ and Ln3+/PSA aggregates; this is supported by the decrease in the emission of fluorescence of Ln3+ mixed solutions. The effect of Ln3+ in the PSA gel formation mechanism is complemented with the analysis of FTIR, SEM and EDS mapping data.