Lamellar Bilayers as Reversible Sacrificial Bonds To Toughen Hydrogel: Hysteresis, Self-Recovery, Fatigue Resistance, and Crack Blunting

Abstract

We report the extraordinary toughness, hysteresis, self-recovery, and persistent fatigue resistance of an anisotropic hydrogel with single-domain lamellar structure, consisting of periodical stacking of several thousands of rigid, hydrophobic bilayers in the ductile, hydrophilic polymer matrix. The stratified lamellar bilayers not only diffract light to exhibit magnificent structural color but also serve as reversible sacrificial bonds that dissociate upon deformation, exhibiting large hysteresis as an energy dissipation mechanism. Both the molecular dissociation and lipid-like mobile nature of bilayers dramatically enhance the resistance to crack propagation by suppressing the stress concentration at the crack tip with the formation of extraordinary crack blunting. This unique toughening phenomenon could allow deep insight into the toughening mechanism of the hydrogel-like soft materials such as biological soft tissues.

Fatigue fracture of tough hydrogels

“…Consequently, tough hydrogels are prone to degradation under cyclic loads. Examples include changes in elastic modulus [22], in hysteresis of stress-stretch curves [23,24], and in functional characteristics of devices [25]. Fatigue fracture of tough hydrogels, however, has remained unexplored.…”

Multi-functions of hydrogel with bilayer-based lamellar structure

“…However, this micro-domain lamellar superstructure in hydrogel plays negligible improvement in the functional hybrid gel material due to the poly-domains of the lamellar phase in macro-scale [24][25][26][27]. Very recently, Gong's group has developed a successful method for formation of macroscopic (integrated microscopic) single-domain lamellar bilayer structure with periodical stacking inside the polymer matrix of the hydrogel [28,29]. The hybrid hydrogel system 4 already attracted great attention in material science because two novel phenomena, excellent mechanical performances and tunable structural color, are emerged in one material.…”

Mechano-actuated ultrafast full-colour switching in layered photonic hydrogels

“…Since the bilayers are formed by hydrophobic association, they are strongly viscoelastic. As a result, the PDGI/PAAm hydrogel exhibits a relaxation time longer than 10 min under a loading-unloading deformation cycle 18 , which leads to very slow colour response ( Supplementary Fig. 1).…”

“…The PDGI/PAAm gel shows an appreciable hysteresis (Fig. 3a), indicating energy dissipation due to the breakage of the non-covalent hydrophobic bonds of the bilayers upon deformation 18 . Though the broken hydrophobic bonds can reform, it takes a long time (Supplementary Movie 1).…”

“…The other is based on the mechanical deformation. The existing mechanochromic photonic materials usually have slow colour switching ability due to their strong time-dependent mechanical behaviour 13,18 . So the strategy to develop fast colour switching photonic materials is to use elastic soft materials that can deform and recover instantly in response to stress on and off without any delay.…”