Copper–calcium poly(acrylic acid) composite hydrogels as studied by Electron Paramagnetic Resonance (EPR) spectroscopy

Abstract

Incorporating metal ions into synthetic polymer hydrogels results in toughening of these hydrogels. Herein, it is demonstrated that addition of small volumes of a 0.1 m aq. solution of Cu2+-salts to poly(acrylic acid)-based hydrogels (physically cross-linked by Ca2+) increases their mechanical/rheological properties by several order of magnitude. Continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) spectroscopic techniques reveal that the origin of the observed boost in mechanical properties is due to parts of the hydrogel network that interact with and through hydrated copper ions. With EPR spectroscopy, it is found that these complexes mainly have a sixfold (octahedral) H2O-coordination shell. Hence, in copper-containing hydrogels, in addition to direct ionic and dipolar (H-bonding) interactions between calcium and carboxylic anions of the side chains of polymer, a second interaction arises from the noncovalent, likely electrostatic interactions of the copper hexaaqua complex and PAA carboxylic acids that are screened through the lower charge density in the hydrated complexes. Also, short-ranged hydrogen-bonded networks of Cu2+ with aqueous solvent molecules inside the water-filled regions of the polymeric hydrogel may play a pivotal role. The synergy of these interactions explains the improved elastic properties of copper containing hydrogels.