Mesoscale bicontinuous networks in self-healing hydrogels delay fatigue fracture

Abstract

Load-bearing biological tissues, such as muscles, are highly fatigue-resistant, but how the exquisite hierarchical structures of biological tissues contribute to their excellent fatigue resistance is not well understood. In this work, we study antifatigue properties of soft materials with hierarchical structures using polyampholyte hydrogels (PA gels) as a simple model system. PA gels are tough and self-healing, consisting of reversible ionic bonds at the 1-nm scale, a cross-linked polymer network at the 10-nm scale, and bicontinuous hard/soft phase networks at the 100-nm scale. We find that the polymer network at the 10-nm scale determines the threshold of energy release rateG0above which the crack grows, while the bicontinuous phase networks at the 100-nm scale significantly decelerate the crack advance until a transitionGtranfar aboveG0. In situ small-angle X-ray scattering analysis reveals that the hard phase network suppresses the crack advance to show decelerated fatigue fracture, andGtrancorresponds to the rupture of the hard phase network.

Effect of mesoscale phase contrast on fatigue-delaying behavior of self-healing hydrogels

“…When  max >  tran , the crack length c increases rapidly with cycle number N and the whole sample ruptures quickly (Fig. 3B), resulting in a fast crack propagation rate c/N, as observed in our previous work (25). For the weakly phase-separated PA-2.5-0.5, the crack length c always increases linearly with N for  max >  0 , even at an extremely small  max (Fig.…”

“…At the fast stretch rate used here (~1 s −1 ), the deformation only partially recovers upon unloading, as seen by the large residual strain at zero stress (fig. S4C), and the phase networks in the bulk gradually adapt to form an oriented metastable structure under the cyclic loading, which is detected by SAXS (25). This orientation of the phase networks causes a blunting of the crack tip and suppresses crack advance.…”

“…As clarified above, the two regimes (slow and fast modes) of crack propagation only occur for the PA gels with a strong phase contrast, corresponding to a high modulus contrast between the hard and soft phase networks. We hypothesize that the delayed crack growth above the threshold  0 is due to the weak stress concentration at the crack tip (strong crack blunting) (25). Therefore, the following two questions arise: What determines the value of  tran and the transition to a fast crack growth rate?…”

Hierarchical Network-Augmented Hydroglasses for Broadband Light Management

“…Owing to the dynamic feature of physically crosslinked networks, a fractured HNAH autonomously self-heals within 48 h and almost recovers the mechanical strength and stretchability (the blue line in Figure 2(c) ). Moreover, compared to conventional glasses crosslinked by single layer of chemical bonds, which usually cause concentrated stresses in a notched region, the HANH crosslinked by locally polymer-rich domains requires more energy for fracture and helps to resist crack growth in a notched sample ( Figure 2(d) ) [ 23 – 25 ]. This is confirmed by digital image correlation (DIC) analysis on a notched HNAH under cyclic loads.…”

Ultrarobust, tough and highly stretchable self-healing materials based on cartilage-inspired noncovalent assembly nanostructure

“…The side chains of proteoglycan molecules in cartilage matrix are connected with collagen fibers through hydrogen bonds to form a network structure, and a large number of collagen fibers are interwoven into a network which can bear the force. Previously, a number of studies have combined woven fiber networks or hard plastic skeletons with soft matrix to achieve ultrahigh mechanical properties of composites [35][36][37][38] . This undoubtedly proves the effectiveness of the cartilage-like structure strategy of assembling a rigid "skeleton" into a flexible matrix by strong adhesion.…”