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

Abstract

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.

Dynamic covalent polymer hydrogels and organogels crosslinked through acylhydrazone bonds: synthesis, characterization and applications

“…After this first study with model dynamic covalent polymer organogels, we proceeded to the development of model dynamic covalent polymer hydrogels . These hydrogels were based on water‐soluble, end‐functionalized, four‐arm star PEGs (tetraPEG stars) with M n = 10 000 g mol −1 .…”

“…Dependence of G ′ (filled symbols) and G ″ (open symbols) on the angular frequency of tetraPEG DYNAgels prepared at various pH values: (a) 1.5, (b) 2.0, (c) 4.5 and (d) 7.0. Reprinted with permission from Apostolides et al Copyright 2017, American Chemical Society.…”

“…Hoare's research group prepared a number of randomly crosslinked dynamic covalent polymer hydrogels, [30][31][32][33][34] by combining pairs of water-soluble random copolymer precursors mainly consisting of N-isopropylacrylamide (NIPAM) or oligo(ethylene glycol) methacrylate (OEGMA) of M n = 475 g mol −1 , but also a 50 minority component based on a monomer bearing carbonyl or acylhydrazide side-groups. The acylhydrazide-bearing monomer repeat units were, in all cases, prepared post-polymerization by the attachment of adipic acid dihydrazide (ADH) onto acrylic acid monomer repeat units already present in the copolymers, yielding 5-(2-acryloylhydrazinyl) pentanehydrazide (ADHA) units.…”

Double‐networks based on interconnected amphiphilic “in–out” star first polymer conetworks prepared by RAFT polymerization

“…Over the past 20 years, several successful approaches that enhance the mechanical properties of hydrogels have been reported. These include slide‐ring hydrogels, nanocomposite hydrogels, double‐network (DN) hydrogels, ion cross‐linked hydrogels, and well‐defined (model) hydrogels . Among them, the DN hydrogel approach stands out for the great enhancement of the hydrogel mechanical properties afforded, but also for its simplicity and generality …”

Tetra‐PEG‐Based Nano‐Enhanced Hydrogel with Excellent Mechanical Properties and Multi‐Functions

“…Thus, making the high‐strength hydrogels smart is another significant issue. Although there have been numerous reports about functionalizing other high‐strength hydrogels, researches concerning functionalizing the tetra‐PEG hydrogel are still few . Therefore, from the above, designing an approach that simultaneously strengthens the tetra‐PEG hydrogel further and endows it with functionality is a valuable project.…”