Homogeneous and Real Super Tough Multi-Bond Network Hydrogels Created through a Controllable Metal Ion Permeation Strategy

Poly(acrylic acid) (PAA) hydrogels with a multi-bond network composed of sparse chemical cross-links and carboxyl-Fe3+ coordination are prepared through a controllable permeation strategy utilizing ferric citrate (FeCA). The existing strategies that directly soak PAA hydrogels in Fe3+ solutions usually induce an inhomogeneous network with densely cross-linked shells and uncertain water content of the hydrogels, which brings about ambiguity when investigating strengthening mechanisms because water content significantly affects the mechanical properties of hydrogels. Herein, the controllable permeation of Fe3+ into PAA networks based on the competition between citric acid (CA)-Fe3+ chelation and PAA-Fe3+ coordination guarantees sustained release of Fe3+, facilitating homogeneous distribution of ionic cross-links and a certain water content. The obtained hydrogels exhibit excellent and balanced mechanical properties (high tensile strength of 3.28 to 6.95 MPa with large elongations at break of 1400 to 780% when water content decreases from 80 to 50 wt %). The real robust tensile strength of this hydrogel originates from the effective energy dissipation of the homogeneous PAA-Fe3+ cross-links, and the high water content ensures a large elongation at break. Furthermore, the hydrogel also has pH-responsive and shape-memory properties.