Sacrificial Bonds Research Articles

A soy protein-based film by mixed covalent cross-linking and flexibilizing networks

In order to develop a strong, tough, water-resistant, and mildew-resistant soy protein film replacing petroleum-based films, in this study, a covalent and hydrogen bond double network structure was constructed in soy protein film using self-synthesized crosslinker triglycidylamine (TGA), soy protein (SPI), and poly(vinyl alcohol) (PVA). The covalent bond network from the crosslinking reaction between TGA and SPI as the skeleton structure improved the tensile strength and water resistance of the film. The hydrogen bond network from interaction between PVA and SPI as a sacrificial bond network to improve the toughness of the film. Compared with the SPI film, the tensile strength and the insoluble matter content of the resultant film increased by 261.2% and 22.1%, respectively. The elongation at break of film increased by 106 times and the fracture toughness improved 183 times. Notably, the resultant film has anti-fungal property, which extends the anti-mold time of film to more than two weeks. This double network construction strategy effectively enhanced the soy protein film and can be applied to reinforce other composite materials and bio adhesives.

A molecular interpretation of the toughness of multiple network elastomers at high temperature

SignificanceSoft materials can be toughened by creating dissipative mechanisms in stretchy matrixes. Yet using them over a wide range of temperatures requires dissipative mechanisms independent of stretch rate or temperature. We show that sacrificial covalent bonds in multiple network elastomers are most useful in toughening elastomers at high temperature and act synergistically with viscoelasticity at lower temperature. We do not attribute this toughening mechanism only to the scission of bonds during crack propagation but propose that the highly stretched network diluted in a stretchy matrix acts by simultaneously stiffening the elastomer and delaying the localization of bond scission and the propagation of a crack. Such a toughening mechanism has never been proposed for elastomers and should guide network design.

Butyl rubber-based interpenetrating polymer networks with side chain crystallinity: Self-healing and shape-memory polymers with tunable thermal and mechanical properties

A self-healing and shape-memory interpenetrating polymer network (IPN) is produced by UV polymerization of n-octadecyl acrylate (C18A) monomer in a toluene solution of butyl rubber (isobutylene-isoprene rubber, IIR) using Irgacure 2959 photoinitiator at ambient temperature. IPNs containing 20–80 wt% IIR have crystalline domains formed by side-by-side packed octadecyl side chains aligned perpendicular to the poly(C18A) (PC18A) backbone. TEM images reveal that the morphology of IPNs consists of crystalline domains dispersed in a continuous amorphous matrix where the size of the dispersed phase could be adjusted between µm and nm level by changing the amount of IIR. Calculations indicate that the effective cross-link density of IPNs is mainly determined by their crystalline domains followed by hydrophobic associations while the chemical cross-links between IIR and PC18A components are negligible. We also show that the crystalline domains acting as sacrificial bonds dissipate energy under strain leading to a significant toughness improvement. IPNs exhibit tunable melting temperature (46–50 °C), crystallinity (1.5–25%), Young’s modulus (0.6–35 MPa), toughness (1.3–12 MPa), and a stretchability of up to 1200% by varying the amount of IIR component. What is more, they also exhibit temperature induced healing behavior with 59–77% efficiency, and an effective shape-memory function. The strategy presented here is applicable for the preparation of IPNs based on various rubbers and poly(n-alkyl(meth)acrylates) with long alkyl side chains.

Effect of sacrificial bond on molecular dynamics and rheological behavior of hybrid butadiene‐styrene‐vinylpyridine rubber vulcanizates with reversible sacrificial network

AbstractDue to the improvement of strength and toughness at the same time, construction of reversible sacrificial bond network has attracted extensive attention to realize the high performance and functionalization of rubber materials. However, the research on the viscoelastic behavior of vulcanized rubber with reversible sacrificial bond network lags behind seriously. In this work relaxation kinetics, linear rheological behavior and nonlinear rheological behavior of hybrid crosslinked butadiene‐styrene‐vinylpyridine rubber (VPR) with reversible Zn2+‐pyridine coordination bond were investigated in detail. It was found that, the tensile results under small strain were more conducive to reflect the contribution of sacrificial bond after sequential tensile‐recovery process than those under large strain. Although sacrificial bond could affect both G' and G" at low temperature, the contribution of it to G" was more prominent. When the temperature was higher than the dissociation temperature of Zn2+‐pyridine coordination bond, the sacrificial bond could only affect G", which was due to the fact that the pyridine group was equivalent to the graft on VPR backbone and acted as dangling chain. In addition, the destruction and reconstruction of coordination bond was the dominant factor in modulus recovery.

Stretchable Hydrogels with Low Hysteresis and High Fracture Toughness for Flexible Electronics.

Stretchable materials, especially hydrogels, are emerging in various fields recently. Many applications demand low hysteresis and high fracture toughness of the materials to be used under dynamic mechanical loads. Herein, the authors report a hydrogel with high fracture toughness and low hysteresis through using a strong metal coordination bond and relatively high crosslinking density. This design allows the sacrificial bond to remain intact under normal operation, while fracturing to dissipate mechanical energy in the fracture zone to prevent propagation of the cracks. The obtained hydrogel exhibits a low hysteresis (≈1.5%) and a high fracture toughness (≈2,164 J m-2 ). Moreover, the hydrogel possesses a high fatigue threshold of ≈141 J m-2 and a reasonable conductivity. This study provides a worth-adopted approach to synthesize hydrogels with low hysteresis and high fracture toughness.

Hybridly double-crosslinked carbon nanotube networks with combined strength and toughness via cooperative energy dissipation.

Although chemical crosslinking has been extensively explored to enhance the mechanical properties of network-type materials for structural and energy (electrochemical, thermal, etc.) applications, loading-induced energy dissipations usually occur through a single channel that either leads to network brittleness or low strength/stiffness. In this work, we apply coarse-grained molecular dynamics simulations to explore the potential of hybridly double-crosslinked carbon nanotube (CNT) networks as a light weight functional material with combined strength and toughness. While increasing the crosslinking density or strong crosslink composition may, in general, enhance the strength and toughness, further increasing the two parameters would surprisingly lead to deteriorated strength and toughness. We find that double-crosslinked networks can nicely achieve cooperative energy dissipation with minimal structural damage. In particular, the weak crosslinks serve as "sacrificial bonds" to dissipate elastic energies from external loading, while the strong crosslinks act as "structure holders" and break at a much later stage during the tensile test. Therefore, the combination of more than one type of crosslinking with hybrid potential energy landscapes and breaking time scales can prevent premature simultaneous breaking of multiple strong crosslinks. By deploying intermediate amounts of weak and strong crosslinks, we observe an outstanding density-normalized strength of 227-2130 kPa m3 kg-1 as compared to many structural materials and advanced nanocomposites. The crosslinking strategies developed here would pave new avenues for the rational design of functional network materials beyond CNTs, such as hydrogels, nanofibers, and nanocomposites.

Bio-fabricated nanocomposite hydrogel with ROS scavenging and local oxygenation accelerates diabetic wound healing.

Chronic wounds, especially diabetic wounds, involve abnormally long inflammatory periods due to their pathological microenvironment of high reactive oxygen species (ROS) levels and lack of blood vessels. Here, via a mild, simple and feasible fabrication approach, a sustained oxygenation system is proposed, consisting of MnO2 nanosheets and a dual-network hydrogel fabricated from natural biomaterials including silk fibroin (SF) and carboxymethyl cellulose (CMC). Compared with the initial value (61.09 kPa), the compression modulus of the dual-network hydrogel increased by 116.2% through the coordination of strong covalent bonds and sacrificial coordination bonds constructed by enzymatic crosslinking and UV-irradiation crosslinking; the intrinsic shear-thinning effect endows the dual-network hydrogel with satisfactory injectable properties to be customized as a predetermined shape to accommodate the irregular wounds of diabetes. The encapsulated MnO2 nanosheets can catalyze the excessive ROS into necessary O2in situ and, after co-incubating with the SF/CMC@MnO2 hydrogels, cells in oxidative stress show significantly lower ROS (3 times) and higher O2 (17 times) levels that are conductive to relieving oxidative stress, promoting angiogenesis and reducing inflammation in vivo. Meanwhile, these SF-based hydrogels can offset the overexpression of matrix metalloproteinases (MMPs) in diabetic wounds (more than 80%) and promote remodeling of the extracellular matrix. Eventually, wound healing rates >76% in 7 days and 100% in 14 days were achieved by the bio-fabricated nanocomposite hydrogel and are remarkably faster than the commercial dressing healing rates (<30% in 7 days and <80% in 14 days). These results indicate that this bio-fabricated hydrogel system with multiple and customizable functions has great promise in the personalized clinical care of chronic wounds.

Ru(II) Catalyst Enables Dynamic Dual‐Cross‐Linked Elastomers with Near‐Infrared Self‐Healing toward Flexible Electronics

AbstractPerusing elastomers capable of self‐healing offers new opportunities to enable next‐generation flexible electronics, but remains greatly challenging; because, few of them can simultaneously possess desirable mechanical strength, elasticity, and self‐healing efficiency. Herein, a dynamic dual‐cross‐linked networks strategy is employed to develop the elastomers via synthesis of ionic polymer catalyzed using the Grubbs's third‐generation catalyst (G3) and subsequent ureido‐pyrimidinone (UPy) grafting. Specifically, UPy exhibits a unique gradient distribution on ionic polymer chains. Large UPy clusters form in the dense UPy region with robust cross‐links that can stabilize the system, while UPy dimers in sparse regions with weak ionic interactions can act as sacrificial bonds for energy dissipation. Therefore, the obtained elastomers exhibit superb mechanical properties with high stretchability (1900%), high toughness (33.8 MJ m−3), and excellent elasticity (&gt;85%). Importantly, deactivated [Ru] = CHOEt complex (G3 derivative) is proved to have an inherently outstanding photothermal effect under near‐infrared (NIR) irradiation. Benefiting from this feature, the elastomers achieve nearly complete self‐healing within 4.5 min under NIR. Furthermore, the elastomers are employed as the base materials to construct flexible self‐healing conductors with stable conductivity even being stretched to 1200%. This work exercises a profound influence on the rational design of high‐performance self‐healing elastomers.

Mechanically Interlocked Vitrimers.

Mechanically interlocked networks (MINs) have emerged as an encouraging platform for the development of mechanically robust yet adaptive materials. However, the difficulty in reversibly breaking the mechanical bonds poses a real challenge to MINs as customizable and sustainable materials. Herein, we couple the vitrimer chemistry with mechanically interlocked structures to generate a new class of MINs─referred to as mechanically interlocked vitrimers (MIVs)─to address the challenge. Specifically, we have prepared the acetoacetate-decorated [2]rotaxane that undergoes catalyst-free condensation reaction with two commercially available multiamine monomers to furnish MIVs. Compared with the control whose wheels are nonslidable under applied force, our MIVs with slidable mechanically interlocked motifs showcase enhanced mechanical performance including Young's modulus (18.5 ± 0.9 vs 1.0 ± 0.1 MPa), toughness (3.7 ± 0.1 vs 0.9 ± 0.1 MJ/m3), and damping capacity (98% vs 72%). The structural basis behind unique property profiles is demonstrated to be the force-induced host-guest dissociation and consequential intramolecular sliding of the wheels along the axles. The peculiar behaviors represent a consecutive energy dissipation mechanism, which provides a complement to other pathways that mainly depend on the breaking of sacrificial bonds. Moreover, by virtue of the vitrimer chemistry of vinylogous urethanes, we impart reprocessability and chemical recyclability to the MINs, thereby empowering the reconfiguration of the networks without breaking of the mechanical bonds. Finally, it is disclosed that the intramolecular motions of [2]rotaxanes could accelerate the dynamic exchange of the vinylogous urethane bonds via loosening the network, suggestive of a synergistic effect between the dual dynamic entities.

Energy dissipation of osteopontin at a HAp mineral interface: Implications for bone biomechanics

Osteopontin (OPN) is a one of the most abundant non-collagenous proteins in the bone's organic matrix. OPN is responsible for mediating bonding at mineral interfaces in the extrafibrillar space and recent evidence shows that it is a major contributor to bone's fracture resistance. While several experimental studies have identified an important role for calcium ions in mediating energy dissipation in OPN protein networks, the underlying molecular mechanisms remain largely unknown. In the current study, the role of calcium ions on energy dissipation at OPN interface with hydroxyapatite (HAp) as the main bone mineral was investigated. For the first time, the three-dimensional structure of OPN proteins were predicted, and it was found that calcium ions greatly influenced the final protein configuration and energy dissipation performance. Under small deformation, the compact cOPN structure, resulting from calcium ions presence, facilitated greater energy dissipation through sacrificial bond breaking and mechanisms mediated by the surface-bound calcium. At larger deformation, the compact structure also enabled cOPN to dissipate higher energy. Moreover, it was found that phosphorylation of OPN played an important role in energy dissipation. While previous studies have shown that OPN dissipated energy by forming aggregate networks, this study also showed that network formation is not necessary and that individual OPN proteins can dissipate large amounts of energy at HAp interfaces.

Order-Tuned Deformability of Bismuth Telluride Semiconductors: An Energy-Dissipation Strategy for Large Fracture Strain.

In addition to thermoelectric (TE) performance tuning through defect or strain engineering, progress in mechanical research is of increasing importance to wearable applications of bismuth telluride (Bi2Te3) TE semiconductors, which are limited by poor deformability. For improving dislocation-controlled deformability, we clarify an order-tuned energy-dissipation strategy that facilitates large deformation through multilayer alternating slippage and stacking fault destabilization. Given that energy dissipation and dislocation motions are governed by van der Waals sacrificial bond (SB) behavior, molecular dynamics simulation is implemented to reveal the relation between the shear deformability and lattice order changes in Bi2Te3 crystals. Using the disorder parameter (D) that is defined according to the configurational energy distribution, the results of strain rates and initial crack effects show how the proper design of the initial structure and external conditions can suppress strain localization that would cause structural failure from the lack of energy dissipation, resulting in large homogeneous deformation of Bi2Te3 nanocrystals. This study uncovers the essence of the tuning mechanism in which highly deformable Bi2Te3 crystals should become disordered as slowly as possible until fracture. This highlights the role of the substructure evolution of SB-defect synergy that facilitates energy dissipation and performance stability during slipping. The disorder parameter D provides a bridge between micro/local mechanics and fracture strain, hinting at the possible mechanical improvement of Bi2Te3 semiconductors for designing flexible TE devices through order tuning and energy dissipation.

Dynamic Control of Sacrificial Bond Transformation in the Fe-N-C Single-Atom Catalyst for Molecular Oxygen Reduction.

Atomically dispersed metal-nitrogen sites show great prospect for the oxygen reduction reaction (ORR), whereas the unsatisfactory adsorption-desorption behaviors of oxygenated intermediates on the metal centers impede improvement of the ORR performance. We propose a new conceptual strategy of introducing sacrificial bonds to remold the local coordination of Fe-Nx sites, via controlling the dynamic transformation of the Fe-S bonds in the Fe-N-C single-atom catalyst. Spectroscopic and theoretical results reveal that the selective cleavage of the sacrificial Fe-S bonds induces the incorporation of the electron-withdrawing oxidized sulfur on the Fe centers. The newly functionalized moieties endow the catalyst with superior ORR activity and remarkable stability, owing to the reduced electron localization around the Fe centers facilitating the desorption of ORR intermediates. These findings provide a unique perspective for precisely controlling the coordination structure of single-atom materials to optimize their activity.

Dynamic Control of Sacrificial Bond Transformation in the Fe−N−C Single‐Atom Catalyst for Molecular Oxygen Reduction

AbstractAtomically dispersed metal‐nitrogen sites show great prospect for the oxygen reduction reaction (ORR), whereas the unsatisfactory adsorption‐desorption behaviors of oxygenated intermediates on the metal centers impede improvement of the ORR performance. We propose a new conceptual strategy of introducing sacrificial bonds to remold the local coordination of Fe−Nx sites, via controlling the dynamic transformation of the Fe−S bonds in the Fe−N−C single‐atom catalyst. Spectroscopic and theoretical results reveal that the selective cleavage of the sacrificial Fe−S bonds induces the incorporation of the electron‐withdrawing oxidized sulfur on the Fe centers. The newly functionalized moieties endow the catalyst with superior ORR activity and remarkable stability, owing to the reduced electron localization around the Fe centers facilitating the desorption of ORR intermediates. These findings provide a unique perspective for precisely controlling the coordination structure of single‐atom materials to optimize their activity.

Why is mechanical fatigue different from toughness in elastomers? The role of damage by polymer chain scission.

Although elastomers often experience 10 to 100 million cycles before failure, there is now a limited understanding of their resistance to fatigue crack propagation. We tagged soft and tough double-network elastomers with mechanofluorescent probes and quantified damage by sacrificial bond scission after crack propagation under cyclic and monotonic loading. Damage along fracture surfaces and its spatial localization depend on the elastomer design, as well as on the applied load (i.e., cyclic or monotonic). The key result is that reversible elasticity and strain hardening at low and intermediate strains dictates fatigue resistance, whereas energy dissipation at high strains controls toughness. This information serves to engineer fatigue-resistant elastomers, understand fracture mechanisms, and reduce the environmental footprint of the polymer industry.

Mechanically Robust Elastomers Enabled by a Facile Interfacial Interactions-Driven Sacrificial Network.

Strength and toughness are usually mutually exclusive for materials. The sacrificial bond strategy is used to address the trade-off between strength and toughness. However, the complex construction process of sacrificial network limits the application of sacrificial network. This work develops a facile strategy to construct an interfacial interactions-driven sacrificial network. The authors' group finds that there are the interfacial interactions between arginines (A) aggregates and molecular chains. Such interfacial interactions result in the mechanical properties of samples having a strong dependence on extension rates, which shows that A aggregates construct a network structure by interfacial interactions. The interfacial interactions between A aggregates and chains improve the strength of samples; while the A aggregate network driven by interfacial interactions preferentially ruptures to dissipate large energy for the improvement of fracture toughness, which can be considered as a sacrificial network. Therefore, their designed elastomers have both high strength and high toughness. This work provides an easier strategy for the construction of sacrificial networks, which can promote the industrial application of sacrificial networks in elastomer materials.

Bioinspired mineral–organic strategy for fabricating a high-strength, antibacterial, flame-retardant soy protein bioplastic via internal boron–nitrogen coordination

Plant-derived renewable and biodegradable bioplastics are attractive alternatives to petroleum-based materials, but their applicability is limited by insufficient interface bonding and the antibacterial properties of biopolymer. Inspired by the unique hierarchical structure of nacre, we report a facile and sustainable mineral–organic strategy that fabricates a soy protein (SP)-based film with excellent mechanical strength and toughness. The nitrogen-coordinating boronic diester (NB) is synthesized from boronic acid and diols, which form enhanced cross-linking networks through dynamic glue linkages. As a glue molecule, caffeic acid (CA) containing phenolic compounds can co-assemble with hydroxyapatite (HA) in the SP matrix to produce inorganic–organic CA-functionalized HA (CHA) hybrids. The strong affinity between the NB and CHA hybrids combined with dynamic covalent bonds and sacrificial hydrogen bonds substantially improves the bonding strength and cohesion of the SP-based composites. The tensile strength and toughness of the resultant film were 13.89 MPa and 14.72 MJ/m3, respectively, 361% and 489% higher than the pristine SP film. Owing to the biomineralization and phenolic components, the incorporated NB@CHA hybrids endow the film with desirable water resistance, flame retardancy, UV protection performance, and antibacterial activity. This biomimetic strategy provides a novel and versatile fabrication route to high-performance SP-based bioplastics for engineering and biological applications.

Tiny yet tough: Maximizing the toughness of fiber-reinforced soft composites in the absence of a fiber-fracture mechanism

Tiny yet tough: Maximizing the toughness of fiber-reinforced soft composites in the absence of a fiber-fracture mechanism

Toughening polyisoprene rubber with sacrificial bonds: The interplay between molecular mobility, energy dissipation and strain-induced crystallization

Sacrificial bonds have been utilized to improve the mechanical properties of self-reinforced rubbers with strain-induced crystallization (SIC) behavior. However, the influence of sacrificial bonds on the SIC behavior under quasistatic and dynamic processes is rarely reported. Herein, weak hydrogen bonds and strong metal coordination bonds are introduced as sacrificial bonds to toughen polyisoprene rubber (IR). Compared with the sample containing only hydrogen bonds, the further introduction of coordination bonds form denser network and impose stronger restriction on the molecular mobility, leading to higher energy dissipation. The hydrogen bonds suppress the crystallization rate and crystallinity of SIC, due to the suppressed molecular mobility and orientation of amorphous chains. By contrast, the coordination bonds lead to smaller onset crystallization strain and partially recovered crystallinity, as they are difficult to relax during stretching and thus can maintain a higher degree of chain orientation. However, under dynamic cyclic loading, both the hydrogen and metal coordination bonds obviously retard the SIC and reduce the crystallinity after the first loading cycle, which is unfavorable for the dynamic mechanical properties. Despite this fact, the synergistic effect of SIC and energy dissipation significantly improves the quasistatic mechanical properties of the modified IR containing both hydrogen and metal coordination bonds.

Structure and Unique Functions of Anisotropic Hydrogels Comprising Uniaxially Aligned Lamellar Bilayers

Abstract In this account, we provide an overview of unique anisotropic hydrogels comprising uniaxially aligned lamellar bilayers embedded in amorphous gel matrix. Owing to their unique structures, the lamellar gels exhibit many unique functions that cannot be obtained from conventional hydrogels featuring amorphous and isotropic structures. For example, the periodical multilayer structure having interlayer spacing of several hundreds of nanometers imparts beautiful structural colors to the gel that respond sensitively to mechanical and chemical stimuli, the water-impermeable nature of the bilayers imparts one-dimensional swelling and diffusion, and the hydrophobic association of the bilayers serves as sacrificial bonds to impart high mechanical strength and toughness during deformation. This work demonstrates the significant potential of hydrogel materials fabricated by introducing anisotropic ordered structures.

The Manufacture of Unbreakable Bionics via Multifunctional and Self-Healing Silk-Graphene Hydrogels.

Biomaterials capable of transmitting signals over longer distances than those in rigid electronics can open new opportunities for humanity by mimicking the way tissues propagate information. For seamless mirroring of the human body, they also have to display conformability to its curvilinear architecture, as well as, reproducing native-like mechanical and electrical properties combined with the ability to self-heal on demand like native organs and tissues. Along these lines, a multifunctional composite is developed by mixing silk fibroin and reduced graphene oxide. The material is coined "CareGum" and capitalizes on a phenolic glue to facilitate sacrificial and hierarchical hydrogen bonds. The hierarchal bonding scheme gives rise to high mechanical toughness, record-breaking elongation capacity of ≈25 000%, excellent conformability to arbitrary and complex surfaces, 3D printability, a tenfold increase in electrical conductivity, and a fourfold increase in Young's modulus compared to its pristine counterpart. By taking advantage of these unique properties, a durable and self-healing bionic glove is developed for hand gesture sensing and sign translation. Indeed, CareGum is a new advanced material with promising applications in fields like cyborganics, bionics, soft robotics, human-machine interfaces, 3D-printed electronics, and flexible bioelectronics.