Ionic effects on the mechanical and swelling properties of a poly(acrylic acid/acrylamide) double crosslinking hydrogel

Abstract

Understanding the structure–property relationships of tough double crosslinking hydrogels, especially at the molecular level, is of great importance for the rational design and wider applications of this kind of soft material. Herein, a mechanically strong poly(acrylic acid/acrylamide) (P(AA/AM)) hydrogel was fabricated, based on a double crosslinking architecture which comprised a chemically crosslinked network and a metal ion crosslinked physical network. The ionic effect, including metal ions and counter anions, on the mechanical and swelling properties of the hydrogels was investigated in detail. Importantly, it was discovered that the P(AA/AM) hydrogel immersed in 0.06 M ferric nitrate (FeCl₃) exhibited a tensile strength of 1377 kPa while in ferric chloride (Fe(NO₃)₃), the tensile strength was 1471 kPa. These values were vastly superior to those obtained for the other metal salts tested. Notably, the distinct mechanical strengths of the hydrogels crosslinked by the ferric salts (FeCl₃, Fe(NO₃)₃, Fe₂(SO₄)₃) were investigated by molecular simulation, which demonstrated that the differing mechanical strengths arose from the varying bonding strengths between the ferric salts and AA groups. Moreover, the tough hydrogel of P(AA/AM) soaked in 0.06 M Fe(NO₃)₃ displayed an equilibrium swelling ratio of only 1.14 g g⁻¹, which offers great promise in applications such as in cartilage and other tissues. This work gives a valuable molecular insight into the understanding and design of double crosslinking gels with high strength.