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

Abstract

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 an 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, we measure the modulus of hydrogels prepared from four‐, six‐, and eight‐armed poly(ethylene glycols), functionalized with histidine ligands that complex with nickel (II) ions. These gels have matched polymer wt% and varied pH to compare their moduli. We consider if the modulus can be described by established polymer network models by calculating the speciation of metal‐coordinated crosslinks, and then incorporating it into a phantom network prediction. We find 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 exceed our phantom network prediction. Since polymer cores act as chemical crosslinks and metal‐coordinated crosslinks form network strands, it is possible that increasing the number of metal‐coordinated linkages per molecule reinforces the chemical crosslink at the polymer core.This article is protected by copyright. All rights reserved

Coordination Geometry and Mineralization in Self-Healing Mussel-Inspired Hydrogels

“…Accordingly, when metal ions are engaged in the formation of transient bonds, a monomodal relaxation process appears, but when excess metal ions exist or the affinity of association decreases upon the protonation at lower pH values, the tendency of mineralization increases. This is less reflected and explored in the model-type telechelic systems based on tetraPEG precursors, probably since the density of the ligand in our system is considerably larger than those reports. ,,,,, This may be indicative of the possible control over the extent of mineralization versus complex formation; nevertheless, a complete understanding of the mineralization process clearly needs a deeper study, focused more on the characterization of the morphology at the nanoscale.…”

“…This is less reflected and explored in the model-type telechelic systems based on tetraPEG precursors, probably since the density of the ligand in our system is considerably larger than those re-ports. 7,11,34,35,68,75 This may be indicative of the possible control over the extent of mineralization versus complex formation; nevertheless, a complete understanding of the mineralization process clearly needs a deeper study, focused more on the characterization of the morphology at the nanoscale.…”

The network structure in transient telechelic polymer networks: extension of the Miller–Macosko model

“…Back to the discrepancies highlighted in the Introduction section, Darby et al have prepared hydrogels upon the complexation of histidine functionalized tetraPEG precursors. 25 They kept the ligand concentration constant and decreased the polymer concentration upon increasing the number of arms. Accordingly, their modulus at the stoichiometric Lig : M ratio follows an ascending trend with f , as also suggested in Fig.…”

“…Similarly, Darby et al investigated the mechanical properties of MSPN hydrogels made using Histidine (His)-functionalized four-, six-, and eight-arm PEG precursors and as such studied the influence of the polymer functionalities. 25 These His–Zn 2+ complexes can form mono-, bis-, and tris-complexes, upon the increase of pH and progressive deprotonation of the ligand. The outcome, in contrast to Nicolella's conclusion, was the increase of the modulus upon increasing the number of arms, so that, the modulus of the network formed by the octaHis system exceeded the predictions of the phantom network model.…”