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

Metal-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.