Monovalent and Divalent Ions Impair Recovery of Strength when Self-Healing Is Facilitated by Hydrogen Bonding

Self-healing materials are those that can recover fromphysicalor chemical damage autonomously. To be applied in underwater applicationssuch as water treatment, self-healing materials need to demonstratesufficient healing ability in complex water matrices. Herein, we investigatedhow monovalent (NaCl) and divalent (MgSO4) ions at concentrationsrelevant to brackish and seawater salinity impact the self-healingefficiency of a model 2-acrylamido-2-methyl-1-propanesulfonic acid(AMPS) and N,N & PRIME;-methylenebis(acrylamide)(MBA) hydrogel. It has been assumed that divalent ions would formionic bonds and act as crosslinkers between viable functional groups(negatively charged oxygens, etc.). However, our results suggest thatthis assumption needs to be reconsidered. Under concentrations relevantto seawater (35 g/L), magnesium ions hindered self-healing efficiencyby & SIM;30% as measured by recovery of ultimate tensile (UT) strength.On the other hand, they improved self-healing efficiency by & SIM;100%as measured by recovery of UT strain. A similar trend was also observedfor sodium ions. The chemical crosslinker ratio when doubled did notimpact self-healing efficiency. These results challenge the assumptionthat divalent ions always form ionic bonds that enhance healing andthat chemical crosslinking alters the self-healing performance.

Automated summary

Self-healing materials have the ability to autonomously recover from physical or chemical damage. In this study, the impact of monovalent and divalent ions on the self-healing efficiency of a model hydrogel was investigated. Contrary to the assumption, the results showed that divalent ions hindered self-healing efficiency in terms of ultimate tensile strength, but improved it in terms of strain recovery. The ratio of chemical crosslinkers did not affect self-healing efficiency. These findings challenge the existing assumptions about the role of divalent ions and chemical crosslinking in self-healing materials.