Stiffened and toughened polyacrylamide/polyanionic cellulose physical hydrogels mediated by ferric ions

High-strength hydrogels have recently attracted many attentions owing to their potential applications in various fields. Yet, how to relive the contradiction between strength and ductility is still a challenge. In this work, a series of Fe(III)-crosslinked polyacrylamide/polyanionic cellulose composite physical hydrogels [PAM/PAC-Fe(III)] were firstly prepared via polymerizing acrylamide in PAC solution free of chemical crosslinkers, and followed by posttreatment in 0.1 M iron chloride solution. The obtained hydrogels were characterized by FTIR spectroscopy and scanning electron microscopy as well as tensile and compressive mechanics. Herein, the mechanics of PAM/PAC-Fe(III) hydrogels exhibits both stiffened and toughened properties, benefitting from the synergy between hydrogen bonding and Fe(III)-COO- coordination interactions within the networks. Subsequently, their properties were compared with those of Zr(IV)- and cellulose nanofiber (CNF)-based analogues in our previous studies. Fe(III) species afford the hydrogels more flexibility than the Zr(IV) ones do due to the lower valency and weaker affinity of Fe(III) than those of Zr(IV); PAC-based systems demonstrate broader and/or higher mechanical reinforcement effects than CNF-based ones ascribing to higher carboxylate content and higher dosage of PAC than CNF. In brief, the present research provides an effective approach to fabricate simultaneously stiffened and toughened hydrogels and provides a guidance to rationally design metal-ion mediated PAM-based composite hydrogels.Graphical abstract