Moduli of Mussel‐Inspired, Metal‐Coordinated Hydrogels Comprising Four‐, Six‐, and Eight‐Arm Polymers

Abstract

AbstractMetal‐coordinated hydrogels can form a percolated network with transient bonds due to metal ions‐functional group coordination. Each metal ion can link with more than one ligand, leading to intricate speciation of bonding modes. While the mechanics of transient gels made with four‐arm polymers are often studied, less is known about how increasing the number of arms affects the modulus. Using shear rheology, the modulus of hydrogels prepared from four‐, six‐, and eight‐armed poly(ethylene glycols), functionalized with histidine ligands that complex with nickel (II) ions is measured. These gels have matched polymer wt.% and varied pH to compare their moduli. It is considered whether the modulus can be described by established polymer network models by calculating the speciation of metal‐coordinated cross‐links and then incorporating it into a phantom network prediction. This study finds that 1) increasing the number of polymer arms increases the modulus, 2) the phantom network allows reasonable modulus approximation for four‐arm and six‐arm gels, and 3) the modulus of eight‐arm gels exceeds the phantom network prediction. Since polymer cores act as chemical cross‐links and metal‐coordinated cross‐links form network strands, it is possible that increasing the number of metal‐coordinated linkages per molecule reinforces the chemical cross‐link at the polymer core.