A pH responsive system capable of forming a self-healing transient network across the pH 4.5 to 7.5 range is developed by increasing the ratio of phenylboronic acid (PBA) moieties to salicylhydroxamic acid (SHA) moieties incorporated in poly[N-2-hydroxypropyl]methacrylamide polymer backbones. We used particle tracking to assess network formation across the pH range of 4.5 to 7.5 with PBA:SHA stoichiometry of reactive groups of 1:1, 5:1 and 10:1. The 10:1 ratio of PBA:SHA forms a transient network across the entire pH range, while the 1:1 PBA:SHA materials demonstrate more liquid like behavior at pH 4.5 and forms a transient network only at neutral pH. At pH 7.5 increasing the ratio of PBA to SHA from 1:1 to either 5:1 or 10:1 increases the G′plateau sixty-fold, indicating that the higher probability of crosslink formation impacts the density of crosslinks compared to a 1:1 ratio of PBA to SHA. Above pH 5.5 the 5:1 and 10:1 PBA:SHA gels behaved similarly. Over the temperature range of 10 to 55 °C, the unequal stoichiometric ratios of 10:1 and 5:1 PBA:SHA transient networks behave similarly, with G′plateau dropping with a corresponding decrease in the characteristic relaxation time. Master curve analysis demonstrates superposition of the 1:1, 5:1 and 10:1 PBA:SHA stoichiometry except under the weakest network conditions (low pH, high temperature) suggesting that the same crosslinking mechanism dominates the network behaviour. The simple and complex viscosity is compared as a function of pH with shear thickening behaviour observed below pH 5.5. The 10:1 PBA:SHA crosslinked-network displays self-healing properties after repeated break cycles at both pH 4.5 and pH 7.5 demonstrating that increasing the ratio of PBA to SHA provides a material that can form a self-healing transient network at low pH, where the PBA-SHA crosslink association is weak, to neutral pH where the PBA-SHA complex is stabilized.
ASJC Scopus subject areas
- Condensed Matter Physics