Metal–ligand complexation and clustering in mussel-inspired side-chain functionalized supramolecular hydrogels

Abstract

Byssus threads of mussels have high resistance against abrasion in wave-swept habitats because of their outer cuticle, which is rich in amino acid dopa complexes with Fe³⁺ ions. This stems from the transient nature of metal–ligand complexes that creates extra relaxation mechanisms. Inspired by this concept, in this work, supramolecular hydrogels based on poly(acrylic acid) functionalized with nitrocatechol groups are synthesized. Polymer chains are physically crosslinked via nitrocatechol–Fe³⁺ complexes. The hydrogels have different polymer volume fractions as well as different nitrocatechol : Fe³⁺ molar ratios. The strength of the supramolecular crosslinks strongly depends on the pH of the medium. The dynamics of these hydrogels are studied by stress relaxation experiments followed by calculation of the relaxation time spectrum. Generally, samples have three relaxation modes, including dissociation of distinct metal–ligand complexes, reptation of sticky polymer chains, and disengagement of network segments from supramolecular aggregates and clusters. Such clusters hinder the terminal relaxation and potentially increase the stability of supramolecular hydrogels.