Expanding the stoichiometric window for metal cross-linked gel assembly using competition

Abstract

Polymer networks with dynamic cross-links have generated widespread interest as tunable and responsive viscoelastic materials. However, narrow stoichiometric limits in cross-link compositions are typically imposed in the assembly of these materials to prevent excess free cross-linker from dissolving the resulting polymer networks. Here we demonstrate how the presence of molecular competition allows for vast expansion of the previously limited range of cross-linker concentrations that result in robust network assembly. Specifically, we use metal-coordinate cross-linked gels to verify that stoichiometric excessive metal ion cross-linker concentrations can still result in robust gelation when in the presence of free ion competing ligands, and we offer a theoretical framework to describe the coupled dynamic equilibria that result in this effect. We believe the insights presented here can be generally applied to advance engineering of the broadening class of polymer materials with dynamic cross-links.

Interfacial Adhesion of Fully Transient, Mussel‐Inspired Hydrogels with Different Network Crosslink Modalities

“…[ 1–4 ] In synthetic materials, reversible crosslinks mimicking those in mussels are often introduced into a covalently crosslinked polymer network, offering new handles to tune toughness, adhesion, and self‐healing. [ 5–13 ] For example, incorporation of Fe‐DOPA complexes into elastomers increases their bulk strength and toughness, [ 14 ] where enhancements in toughness arise from the breaking and reforming of reversible crosslinks. [ 5,6,14,15 ] However, the permanent structure of covalent hydrogels can limit their use for certain applications like injectable medical treatments, [ 16,17 ] (bio)adhesives, [ 18 ] or mechanically reversible materials.…”

“…To coordinate with metal ions, ligands must be deprotonated, and the concentration of available deprotonated ligands relative to the concentration of available metal ions determines the distribution of ligand coordination modalities at equilibrium within the network. [ 9,10 ] Hence, the distribution of crosslinking modalities can be tuned by changing pH (to control deprotonation) or changing the metal to ligand (M:L) ratio, which in turn dictates the viscoelastic properties of the bulk network. [ 9,19 ]…”

Shape retaining self-healing metal-coordinated hydrogels

“…To ensure scalability, we end-functionalize commercially available hydroxy-terminated linear PEG, analogous to nitrocatechol-functionalized linear PEGs. 51 We produce the hydrogels by dissolving 10 wt% 2gPEG in water and adding iron chloride (FeCl 3 ), selected because of the high affinity between pyrogallol and iron(III) (Fe 3+ ). For each pyrogallol end-group, we add twice the molar concentration of Fe 3+ to achieve gelation, a ratio higher than the stoichiometric ratio of 3 ligands per ion, known for catechol hydrogels, 20,23 resulting in a solution pH of 1.5.…”

Demonstration of Environmentally Stable, Broadband Energy Dissipation via Multiple Metal Cross‐Linked Glycerol Gels

“…Coupling of the histidine ligand to the polymer was performed by slightly modifying a procedure used elsewhere. [ 28,29 ] Briefly, 4‐arm star, 10 kDa PEG, terminated with NH 2 ⋅ HCl (four equivalence per polymer), was mixed with 1.5 equiv. Boc‐His(Trt)‐OH and 1.5 equiv.…”

“…A 0.04 M histidine hydrogel was formed by modifying the method described elsewhere. [ 8,29 ] 97.6 L of stock 4‐arm histidine was dispensed onto Parafilm along with a 1 M stock Ni, Co, Cu, or Zn, solution, a 4 M NaOH solution, and water so that the final hydrogel volume would be 200 L. The total salt ion concentration was fixed at 0.08 M , representing a value twice the ligand concentration.…”