High Fracture Efficiency and Stress Concentration Phenomenon for Microgel-Reinforced Hydrogels Based on Double-Network Principle

Abstract

Double-network hydrogels (DN gels) have aroused considerable interest because of their excellent mechanical strength and toughness, low sliding friction, good biocompatibility, as well as wide tunability in components. By revisiting DN gels, we provide an ingenious way to fabricate a kind of strong and tough microgel-reinforced hydrogels (MR gels), that densely cross-linked polyelectrolyte microgels of poly(2-acrylamido-2-methylpropanesulfonic sodium) (PNaAMPS) (replacing the densely crosslinked PNaAMPS macro-network for conventional DN gels) are incorporated into sparsely cross-linked neutral polyacrylamide (PAAm) matrix. The structure of MR gels can be considered as a two-phase composite, where the disperse phase is the rigid DN microgels, and the continuous phase is the sof t PAAm matrix. Similar to DN gels, MR gels show the irreversible energy dissipation in the hysteresis measurement, demonstrating the permanent fracture of the brittle PNaAMPS phase. Thus, the discontinuous brittle phase also serves as sacrif icial bonds. Through quantitative comparison of the hysteresis curves with DN gels and monitoring the morphology change of the embedded microgels in MR gels during the real-time stretching process, we conclude that the DN microgels in MR gels show four times higher in fracture efficiency of the sacrificial bonds than bulk DN gels at the same strain, as a result of the stress concentration around the microgels. ■ INTRODUCTIONHydrogel science and engineering is a field of important research today where the quest for the Holey Grail is clearly to reinforce the mechanical strength and toughness of these soft materials. Various solutions have been proposed to solve the paradox of tough hydrogels since 2001, like double-network hydrogels (DN), 1 slide-ring hydrogels (SR), 2 nanocomposite hydrogels (NC), 3 etc. In the subsequent decade, a wide range of hydrogels with improved mechanical strength and toughness have been developed significantly based on or inspired from these robust hydrogels. 4−12 Among them, DN gels are the toughest synthetic hydrogels with a high modulus, even as tough as load-bearing cartilages and filled rubbers. 13−15 A universal molecular stent method has been developed recently to toughen any hydrogels based on this double network concept, which substantially broadens the applicability of this technology to various functional polymer systems. 16 Extensive studies on the toughening mechanism of DN gels, consisting of polyelectrolyte as the first network and neutral polymer as the second network, have shown that yielding and large hysteresis appear in tensile deformation, and a large damage zone is formed at the crack tip, which effectively relieves the stress concentration and increases the resistance against the crack propagation. 17−19 The hysteresis behavior of DN gels is associated with the fracture of the rigid and brittle polyelectrolyte network, which serves as sacrif icial bonds in the toughening of DN gels. 20 This sacrif icial bonds mechanism has some common features with th...

Control of thermal fabrication and size of β-lactoglobulin-based microgels and their potential applications

“…9b, left), wherein interactions between the microgel and hydrogel network provide stimuli-responsiveness (e.g. temperature-response) [27], improve resistance to bulk fracture [14], or even enhance release of entrapped drugs compared to traditional hydrogels [36]. At the interface of emulsions or foams, the particle-like properties of microgels gives them similar interfacial properties as Pickering emulsifiers (Fig.…”

Robust biopolymer based ionic–covalent entanglement hydrogels with reversible mechanical behaviour

“…Tough hydrogels have been prepared using methods such as slip-ring hydrogel synthesis, nano-composite hydrogels, and double network hydrogels 11,[13][14][15][16][17] . Of all the tough hydrogel synthetic strategies, the double network approach affords the highest versatility in terms of composition and resulting properties 17 .…”

“…A relatively recent innovation is the advent of ioniccovalent entanglement (ICE) network hydrogels which can be prepared in a "one-pot" synthetic approach 14,16,[18][19][20] . ICE hydrogels consist of a tough and self-recovering, interpenetrating network of an ionotropic polymer and a chemically cross-linkable polymer and have demonstrated some impressive mechanical properties 10,21,22 .…”

Multi‐scale modeling of double network hydrogels

“…The clusters further break into small and undeformable pieces in necking and hardening. Second network acts as submissive matrix component, if it is loaded as a single network [4]. Since the DN gel is produced in such a way that interactions between networks are restricted, second network in the DN gel is assumed to behave like a single second network at the beginning of the deformation.…”