Dynamic Covalent Star Poly(ethylene glycol) Model Hydrogels: A New Platform for Mechanically Robust, Multifunctional Materials

We develop a new platform of dynamic covalent model polymer networks comprising two types of four-armed star poly(ethylene glycol)s (tetraPEG), one end-functionalized with benzaldehyde groups and the other with benzaacylhydrazides, resulting in hydrazone cross-links. These materials, henceforth to be called tetraPEG DYNAgels, display remarkable mechanical properties, much superior to those based on randomly cross-linked analogues. Fast aqueous gel formation takes place both at acidic and, unexpectedly, at alkaline conditions, with gel formation times covering 4 orders of magnitude in the pH range from 2.0 to 12.5 and a maximum gelation time appearing at pH 8.5. Frequency-dependent oscillatory rheology indicates a finite lifetime of the cross-links in tetraPEG DYNAgels at acidic conditions. Furthermore, these materials exhibit self-healing ability and reversibility under acidic and moderately acidic conditions; however, these properties can be canceled by chemical reduction of the cross-links. The system is highly modular, allowing the facile incorporation of other functionalities, e.g., hydrophobicity or amphiphilicity, introduced via polymers bearing terminal benzaldehyde groups.