Robust, notch-insensitive and impact-resistance physical hydrogels with homogeneous topologic network enabled by partial hydrolysis and metal coordination

Abstract

Acrylamide-based hydrogels usually have a random distribution of characteristic functional groups on the molecular chain due to differences in monomer reactivity ratios, resulting in an inhomogeneous polymer network. This inhomogeneity leads to severely inadequate mechanical properties, limiting their application in load-bearing fields. This study delineates a novel approach to synthesize homogeneous hydrogels with enhanced mechanical strength, heightened energy dissipation capacity, and superior impact resistance, termed partially hydrolyzed polymer hydrogels (HP hydrogel). The methodology involves the partial hydrolysis of polyacrylamide in an alkaline environment to convert amide groups into carboxylate groups, followed by the introduction of Fe³⁺ ions to establish a ligand network through coordination bonding. This partial hydrolysis technique significantly augments the homogeneity of the coordination network. The carboxylate-Fe³⁺ coordination bonds introduce an efficient energy dissipation mechanism, which is pivotal in enhancing the mechanical robustness of the hydrogels. Comparative analysis reveals that HP hydrogels exhibit mechanical properties substantially superior to those of conventional poly(acrylamide-co-acrylic acid) copolymer hydrogels (CP hydrogel). Notably, the tensile strength of HP hydrogels is quadruple that of CP hydrogels, reaching up to 7.0 MPa, while maintaining the same carboxylic acid content. Furthermore, HP hydrogels demonstrate remarkable tear and impact resistance, evidenced by a fracture energy of 7.5 kJ/m² in notched specimens and an 85.7 % enhancement in impact resilience. The strategic partial hydrolysis not only improves the homogeneity of the gel structure but also instills a robust energy dissipation mechanism, thereby significantly fortifying the comprehensive mechanical properties of the hydrogels. This advancement potentially broadens the applicability of acrylamide hydrogels in load-bearing and other mechanically demanding environments.