Associative and Thermoresponsive Aqueous Polymer Formulations Based on Imine Chemistry

Abstract

Controlling the flow properties of aqueous formulations is essential in fields as diverse as biotechnology, cosmetics, paints, or chemically enhanced oil recovery, to cite a few. This work presents a study of the association, in aqueous solutions, of complementary polyacrylamides carrying amino and aryl aldehyde side-groups. Through free radical polymerization, copolymers with tunable molar masses (Mn ranging from 60 to 370 kg·mol–1), functionality (2 to 17 mol %), and permanent charge degrees (0 to 98 mol %) were obtained. Imine formation was studied on model systems consisting of amino polyacrylamides and a benzaldehyde derivative. It was found that the presence of permanent charges on the amino polyacrylamide does not prevent imine formation but influences the association degree. In addition, imines were efficiently formed in the presence of NaCl and the dynamic cross-links proved to be thermoresponsive, with a reversible displacement of the equilibrium toward amine and aldehyde with increasing temperature. The associative formulations were subsequently studied by rheology. It was shown that the functionality, charge degree, and molar mass of the copolymers can be used to rationally manipulate interchain association, which proved to be a powerful tool to control the viscosity and rheological profile of the different formulations. Dynamic light scattering was utilized to further study the microstructural dynamics of one of the formulations, showing good agreement with rheology. Finally, taking advantage of the influence of temperature on the imine cross-links, a thermoresponsive hydrogel was prepared that undergoes reversible sol–gel transition over multiple heating–cooling cycles between 30 and 80 °C.