Unequal stoichiometry between crosslinking moieties affects the properties of transient networks formed by dynamic covalent crosslinks

Abstract

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.