Differentiating the Role of Osmotic Pressure and Ionic Interactions on Self-Healing Polymers

Self-healing polymers can recover from physical and chemical damage autonomously, which improves the durability and performance of systems that rely on these polymers. To design self-healing polymers that work well in practical applications, it is important to understand the impact that the presence of different ions has on self-healing mechanisms. In this paper, we investigate the role of monovalent (Na⁺) and divalent (Ca²⁺) ions in the self-healing efficiency of a model polymer, namely, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS), whose network is dominated by hydrogen bonding. By pre-embedding ions into the bulk gel, our method systematically eliminates the confounding ion concentration gradients and osmotic pressure differences that complicated previous studies. Through tensile testing, we find that at high concentrations, divalent ions improve the strength and modulus recovery of polymer samples and slightly reduce the strain recovery relative to samples without any ions in them. Monovalent ions did not result in a statistically significant change in strength recovery but increased strain recovery at high concentrations. Using additional rheological measurements, we find that both monovalent and divalent ions decrease the relaxation time of the PAMPS chains, with monovalent ions doing so to a much larger extent. This suggests that changes in chain mobility might be the key factor that controls any improvements in strain and strength recovery. Overall, our results deconvolute the competing roles of ionic cross-linking and chain mobility and highlight the importance of controlling for osmotic artifacts in ion-containing hydrogels.