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Vitrimers, possessing associative covalent adaptable networks, are cross‐linked polymers exhibiting malleable (glass‐like) feature and recyclable and reprocessable (thermoplastics‐like) properties. The dynamic behaviors of vitrimer are dependent on both chain/molecular mobility (glass transition temperature, Tg) and dynamic bond‐exchanging reaction rate (topology freezing transition temperature, Tv). This work aims on probing the effect of high Tg on the stress relaxation and physical recyclability of vitrimers, employing a polyimide cross‐linked with dynamic ester bonds (Tg: 310 °C) as the example. Due to its high Tg and chain rigidity, the cross‐linked polyimide does not exhibit a high extent of stress relaxation behavior at 320 °C (10 °C above its Tg), even though the temperature is much higher than the hypothetical Tv. While raising the processing temperature to 345 °C, the cross‐linked polyimide exhibits a stress relaxation time of about 3300 s and physical malleability. Nevertheless, side reactions may occur in the recycling and reprocessing process under the harsh condition (high temperature and high pressure) to alter the thermal properties of the recycled sample. The diffusion control plays a critical role on the topography transition of a vitrimer having a high Tg. The Tg ceiling is noticeable for developments of vitrimers. [ABSTRACT FROM AUTHOR]

Disulfide‐contained vitrimers that possess unprecedented functions, such as reprocessing and self‐healing, are considered to be one of the most promising materials since they also exhibit mechanochromic behaviors. A few reports have given the phenomenon of mechanochromism, but the relationship between their structure and mechanochromic behavior needed to further research. Herein, three aromatic disulfide‐contained epoxy vitrimers with different degree of unsaturation are prepared by changing the epoxy monomer. The effect of network structure on the dynamic and mechanochromic properties are investigated. With increasing the degree of unsaturation from the aliphatic to aromatic epoxy vitrimers, the Tg‐value increases and the chain mobility decreases, so the aliphatic epoxy vitrimers shows the lowest activation energy, the lowest temperature of malleability Tv, and the shortest relaxation times. In addition, these epoxy vitrimers all showes reversible mechanochromic behaviors due to the generation and coupling of sulfenyl radicals, which can be accelerated by the high temperature and solvent. Meanwhile, the epoxy structure has a great influence on the mechanochromic behavior, including the absorption wavelength and fading kinetics of grinding powders. These results can be utilized to design desired vitrimer materials for various applications that requires both the intrinsically self‐report damage and the dynamic properties such as recycling. [ABSTRACT FROM AUTHOR]

Dithioacetals are heavily used in organic, material and medical chemistries, and exhibit huge potential to synthesize degradable or recyclable polymers. However, the current synthetic approaches of dithioacetals and polydithioacetals are overwhelmingly dependent on external catalysts and organic solvents. Herein, we disclose a catalyst‐ and solvent‐free acetal‐thiol click‐like reaction for synthesizing dithioacetals and polydithioacetals. High conversion, higher than acid catalytic acetal‐thiol reaction, can be achieved. High universality was confirmed by monitoring the reactions of linear and cyclic acetals (including renewable bio‐sourced furan‐acetal) with aliphatic and aromatic thiols, and the reaction mechanism of monomolecular nucleophilic substitution (SN1) and auto‐protonation (activation) by thiol was clarified by combining experiments and density functional theory computation. Subsequently, we utilize this reaction to synthesize readily recyclable polydithioacetals. By simple heating and stirring, linear polydithioacetals with M‾ ${\bar M}$ w of ~110 kDa were synthesized from acetal and dithiol, and depolymerization into macrocyclic dithioacetal and repolymerization into polydithioacetal can be achieved; through reactive extrusion, a semi‐interpenetrating polymer dynamic network with excellent mechanical properties and continuous reprocessability was prepared from poly(vinyl butyral) and pentaerythritol tetrakis(3‐mercaptopropionate). This green and high‐efficient synthesis method for dithioacetals and polydithioacetals is beneficial to the sustainable development of chemistry. [ABSTRACT FROM AUTHOR]

Clusteroluminescence (CL) has recently gained significant attention due to its unique through‐space interactions associated with a high dependence on the aggregation of subgroups. These distinct features could easily transform the stimuli into visual fluorescence and monitor the fluctuation of the environment but have not received sufficient attention before. In this work, supramolecular films are designed based on the neutralization reaction of anhydride groups and the self‐assembly of dynamic covalent disulfide bonds in NaOH aqueous solution. The self‐assembly of hydrophilic carboxylate chromophores and hydrophobic disulfide‐containing five‐membered rings could be observed by the variation of the aggregation state of carboxylate in CL. Furthermore, the dynamic cross‐linking films obtained with water‐sensitive carboxylate chromophores could alter the aggregation distance stimulated by surrounding water vapor, causing the emission wavelength to change from 534 to 508 nm by varying the relative humidity. This work not only provides an approach to monitor the self‐assembly of clusteroluminogens but also offers new strategies for designing stimuli‐responsive materials that utilize the intrinsic features of CL. [ABSTRACT FROM AUTHOR]

Covalent adaptive networks (CANs) epoxy based on the Diels‐Alder (DA) reaction usually are commonly used for self‐healing materials. However, poor toughness greatly limits its application in innovative materials. Herein, based on retaining the excellent dynamic characteristics of DA reactive CANs, we introduced thermoplastic polyurethane (TPU) in situ during crosslinking, improving the composite material's strength and toughness. A multicomponent polymeric system with advanced performance can be produced by the individual components on the condition of the synergistic hybrid effects. Molecular‐level interlocking polymer networks between the chain entanglement of TPU and the reversible covalent crosslink of DFB were formed by the topological reorganization of these two immiscible polymers. As the gelation proceeds, a homogenous structure instead of conventional phase separation is formed. When the mixing ratio of DFB and TPU is 1:1, the tensile strength of the prepared sample reaches 35 MPa, and the elongation at break reaches 110%. At the same time, the composites possess good thermal stability and solvent resistance due to the presence of interlocking polymer networks. Furthermore, these DFB‐TPU composites show excellent shape memory, self‐healing, and closed‐loop recycling properties. [ABSTRACT FROM AUTHOR]

Abstract Liquid crystal polymers (LCPs) have gained tremendous attention in recent years due to their great potentials from fabrication of responsive actuators and sensors to construction of intelligent soft robotic and light modulators. However, conventional LCPs with permanent cross‐links present tedious and unmodifiable stimuli‐responsiveness. Recently, dynamic bonds capable to reversibly break and reform have been integrated into LCP, imparting intrinsic dynamic characteristics. The dynamic LCP possesses unprecedented diverse functionalities including reprogrammability, recyclability, and self‐healing ability, becoming much more adaptive to surrounding environmental changes compared with the conventional counterpart. In this review, recent progress of dynamic bond‐based LCPs is summarized. The mechanism, preparation, and functionalities of dynamic LCPs based on dynamic noncovalent bond (DNCB) and dynamic covalent bond (DCB) are poised to be discussed, followed by introducing emergent LCPs combining both of DNCB and DCB. Consequently, the unique functionalities of dynamic bond‐based LCPs will be given. Finally, outlooks of development of the dynamic bond‐based LCPs are presented.

Abstract Clusteroluminescence (CL) has recently gained significant attention due to its unique through‐space interactions associated with a high dependence on the aggregation of subgroups. These distinct features could easily transform the stimuli into visual fluorescence and monitor the fluctuation of the environment but have not received sufficient attention before. In this work, supramolecular films are designed based on the neutralization reaction of anhydride groups and the self‐assembly of dynamic covalent disulfide bonds in NaOH aqueous solution. The self‐assembly of hydrophilic carboxylate chromophores and hydrophobic disulfide‐containing five‐membered rings could be observed by the variation of the aggregation state of carboxylate in CL. Furthermore, the dynamic cross‐linking films obtained with water‐sensitive carboxylate chromophores could alter the aggregation distance stimulated by surrounding water vapor, causing the emission wavelength to change from 534 to 508 nm by varying the relative humidity. This work not only provides an approach to monitor the self‐assembly of clusteroluminogens but also offers new strategies for designing stimuli‐responsive materials that utilize the intrinsic features of CL.

Cellulose acetate (CA) is an important group of polymeric materials that have found broad applications. However, the narrow range of processing temperature causes difficulties to improve the mechanical properties of CA. In this work, poly(4‐pinacolatoborylstyrene) (PB) was synthesized and employed as a macromolecular cross‐linker for CA. Due to the dynamic coordination interaction between the boron center and hydroxyl groups on PB and CA, respectively, CA samples could be easily handled via the solution processing method. The addition of macromolecular PB additives (2.5 wt%) have successfully enhanced the mechanical properties, rheological properties of CA without losing the transparent appearance. Furthermore, the obtained CA materials could be recycled via solution reprocessing without apparent decrease of the mechanical properties. [ABSTRACT FROM AUTHOR]

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In this work, we report the fabrication and functional demonstration of a kind of dually responsive nanoparticles (NPs) as a potential drug delivery vector. The pH value, corresponding to the acidic microenvironment at the tumor site, and mannitol, to the extracellular trigger agent, were employed as the dually responsive factors. The function of dual responses was achieved by breaking the dynamic covalent bonds between phenylboronic acid (PBA) groups and diols at low pH value (pH 5.0) and/or under the administration of mannitol, which triggered the decomposition of the complex NPs and the concomitant release of anticancer drug of doxorubicin (DOX) loaded inside the NPs. The NPs were composed of modified chitosan (PQCS) with quaternary ammonium and PBA groups on the side chains, heparin (Hep), and poly(vinyl alcohol) (PVA), in which quaternary ammonium groups offer the positive charge for the cell-internalization of NPs, PBA groups serve for the formation of dynamic bonds in responding to pH change and mannitol addition, PVA furnishes the NPs with diol groups for the interaction with PBA groups and the formation of dynamic NPS, and Hep plays the roles of reducing the cytotoxicity of highly positively-charged chitosan and forming of complex NPs for DOX up-loading. A three-step fabrication process of drug-loaded NPs was described, and the characterization results were comprehensively demonstrated. The sustained drug release from the drug-loaded NPs displayed obvious pH and mannitol dependence. More specifically, the cumulative DOX release was increased more than 1.5-fold at pH 5.0 with 20 mg mL−1 mannitol. Furthermore, the nanoparticles were manifested with effective antitumor efficient and apparently enhanced cytotoxicity in response to the acidic pH value and/or mannitol. [ABSTRACT FROM AUTHOR]

Epoxy resins, as a typical class of thermosetting polymers, have widespread industrial applications such as adhesives, coatings, electronic encapsulants, and polymer matrices for advanced composites, owing to their excellent mechanical performance, adhesive capacity, dimensional stability, heat, and chemical resistance. However, conventional epoxy resins cannot be reprocessed, repaired, or recycled due to their permanent crosslinked structures, making it a long-standing challenge to process and recycle the ever-increasing epoxy and epoxy-based composite similar to thermoplastics. Therefore, both economic and environmental factors are driving the development of reformable and recyclable epoxy resins. In this paper, various such epoxy vitrimers containing dynamic covalent bonds are reviewed. Future potentials of these epoxy vitrimers are also discussed.

In this work, we report the fabrication and functional demonstration of a kind of dually responsive nanoparticles (NPs) as a potential drug delivery vector. The pH value, corresponding to the acidic microenvironment at the tumor site, and mannitol, to the extracellular trigger agent, were employed as the dually responsive factors. The function of dual responses was achieved by breaking the dynamic covalent bonds between phenylboronic acid (PBA) groups and diols at low pH value (pH 5.0) and/or under the administration of mannitol, which triggered the decomposition of the complex NPs and the concomitant release of anticancer drug of doxorubicin (DOX) loaded inside the NPs. The NPs were composed of modified chitosan (PQCS) with quaternary ammonium and PBA groups on the side chains, heparin (Hep), and poly(vinyl alcohol) (PVA), in which quaternary ammonium groups offer the positive charge for the cell-internalization of NPs, PBA groups serve for the formation of dynamic bonds in responding to pH change and mannitol addition, PVA furnishes the NPs with diol groups for the interaction with PBA groups and the formation of dynamic NPS, and Hep plays the roles of reducing the cytotoxicity of highly positively-charged chitosan and forming of complex NPs for DOX up-loading. A three-step fabrication process of drug-loaded NPs was described, and the characterization results were comprehensively demonstrated. The sustained drug release from the drug-loaded NPs displayed obvious pH and mannitol dependence. More specifically, the cumulative DOX release was increased more than 1.5-fold at pH 5.0 with 20 mg mL−1 mannitol. Furthermore, the nanoparticles were manifested with effective antitumor efficient and apparently enhanced cytotoxicity in response to the acidic pH value and/or mannitol.

The combination of three-dimensional (3D) printing and shape memory polymers (SMP) enables programmable shape morphing of complex 3D structures, which is commonly termed four-dimensional (4D) printing. The process requirements of vat photopolymerization-based 3D printing impose limitations on the molecular structure design of SMPs, making it challenging to achieve triple- or multiple-shaped memory effects. Herein, we printed SMPs with different T g values and obtained an SMP assembly through interfacial welding. The welding process is facilitated by the dynamic exchange of hindered urethane bonds at the interface. The resulting SMP assembly exhibits a quadruple shape memory effect, enabling programmable sequential deformation. The advantage of this approach is that the molecular design and the corresponding thermodynamic properties of different welding SMP components can be independently adjusted, enabling a greater range of shape and functional variations in the final 3D SMP assembly.

A cellulose-based multi-responsive hydrogel was prepared by the facile incorporation of enamine and disulfide bonds in the same system at physiological pH. The cellulose hydrogel was obtained by simply mixing aqueous solutions of cellulose acetoacetate (CAA) and cystamine dihydrochloride (CYS) at room temperature. The internal morphology, structure, and mechanical properties of the cellulose hydrogel were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) and Raman spectroscopies, nuclear magnetic resonance (NMR), and water retention, porosity, and rheology measurements. The cellulose hydrogel showed reversible sol–gel transitions in response to both pH and redox triggers. In addition, it displayed good stability under physiological conditions. Gels loaded with small molecules showed variable release properties in response to pH or redox stimuli. The preparation protocol presented here could be used to fabricate other multi-responsive polysaccharide hydrogels. [ABSTRACT FROM AUTHOR]

Transient polymer networks are ubiquitous in natural and engineered materials and contain cross-links that can reversibly break and re-form. The dynamic nature of these bonds allows for interesting mechanical behavior, some of which include nonlinear rheological phenomena such as shear thickening and shear thinning. Specifically, physically cross-linked networks with reversible bonds are typically observed to have viscosities that depend nonlinearly on shear rate and can be characterized by three flow regimes. In slow shear, they behave like Newtonian fluids with a constant viscosity. With further increase in shear rate, the viscosity increases nonlinearly to subsequently reach a maximum value at the critical shear rate. At this point, network fracture occurs followed by a reduction in viscosity (shear-thinning) with a further increase in shear rate. The underlying mechanism of shear thickening in this process is still unclear with debates between a conversion of intra-chain to inter-chain cross-linking and nonlinear chain stretch under high tension. In this paper, we provide a new framework to describe the nonlinear rheology of transient polymer networks with the so-called chain distribution tensor using recent advances from the transient network theory. This tensor contains quantitatively and statistical information of the chain alignment and possible anisotropy that affect network behavior and mechanics. We investigate shear thickening as a primary result of non-Gaussian chain behavior and derive a relationship for the nonlinear viscosity in terms of the non-dimensional Weissenberg number. We further address the criterion for network fracture at the critical shear rate by introducing a critical chain force when bond dissociation is suddenly accelerated. Finally, we discuss the role of cross-linker density on viscosity using a “sticky” reptation mechanism in the context of previous studies on metallo-supramolecular networks with reversible cross-linkers.

Supramolecular full silicone networks are built on the basis of siloxane polymers with low content of amino (6.4 or 9.8 mol%), and carboxyl (14.9 mo%) groups. Alternatively, the carboxyl groups involved are attached to a polyhedral silsesquioxane cage (i.e., octakis(2-carboxymethylene-thioethyl)silsesquioxane), POSS-COOH, which have also been found to play a filler role. Networks substantiated as free-standing films through the development of physical interactions between polar groups forming transverse bridges between chains were analyzed structurally by IR analysis, but also in a more original way by fluorimetric approach, and evaluated in terms of thermal, mechanical, electrical properties and their suitability as biointerfaces. It has been found that these properties are strongly dependent and can be adjusted by the content and the ratio between the two types of polar groups (-NH 2 and -COOH), as well as by the substrate to which the carboxyl groups are attached (polysiloxane or POSS). Being elastomers crosslinked by supramolecular interactions, it was necessary to evaluate their reversibility, for which, temperature-dependent IR and dielectric spectroscopy studies were performed. The reflection of this reversibility in the self-healing capacity of the material in the film and solvoplasticity in solvents with different polarities and media that mimic biological fluids was highlighted supporting the sustainability of these materials obtained in the absence of any catalyst. [Display omitted] • Biocompatible polysiloxanes with side-attached amino and carboxyl groups are prepared. • Silicone networks are assembled by cross-supramolecular interactions between chains. • Transparent, thermally and mechanically stable, and high dielectric films are obtained. • Polar connections dynamics make networks self-repairing, reprocessable, sensitive dielectric. • Swelling in like-biological fluids and mucoadhesivity to biological substrate are assessed. [ABSTRACT FROM AUTHOR]

Epoxy resins, as a typical class of thermosetting polymers, have widespread industrial applications such as adhesives, coatings, electronic encapsulants, and polymer matrices for advanced composites, owing to their excellent mechanical performance, adhesive capacity, dimensional stability, heat, and chemical resistance. However, conventional epoxy resins cannot be reprocessed, repaired, or recycled due to their permanent crosslinked structures, making it a long-standing challenge to process and recycle the ever-increasing epoxy and epoxy-based composite similar to thermoplastics. Therefore, both economic and environmental factors are driving the development of reformable and recyclable epoxy resins. In this paper, various such epoxy vitrimers containing dynamic covalent bonds are reviewed. Future potentials of these epoxy vitrimers are also discussed. • Summarizes recent development in the field of recyclable epoxy based on dynamic covalent bonds. • Suggests potential research subjects as well as potential future application. • Provides a critical discussion on the progress of recyclable and reformable epoxy. [ABSTRACT FROM AUTHOR]

SignificanceDynamic bonds have been found to enhance fracture toughness of hydrogels as sacrificial bonds, but the role of dynamic bonds to fatigue threshold of hydrogels is poorly understood because the wide dynamic range of viscoelastic response imposes a challenge on fatigue experiments. Here, by using polyampholyte hydrogels, we adopted a time-salt superposition principle to access a wide range of time scales that are otherwise difficult to access in fatigue tests. Relations between fatigue threshold and strain rate in elastic and viscoelastic regimes and the corresponding mechanism correlated to permanent/dynamic bonds were revealed. We believe that this work gives important insight into the design and development of fatigue-resistant soft materials composed of dynamic bonds.

Thermosetting polyethylene (PEG)-based solid-solid phase change materials networks (PCMNs) have high latent leat and form stability from the covalent crosslinking, yet they remain a great challenge in reprocessing and recycling of PCMNs without compromising a high latent heat above 100 J/g. Herein, the polyester-based solid-solid PCMNs with PEG as phase change component were synthesized via ionic crosslinking between the linear multiple carboxyl-functionalized phase change materials (CPCMs) polyester and zinc cations. The CPCMs were synthesized via polyesterification reaction between pyromellitic dianhydride (PMDA) and diol containing PEG and ethylene glycol, meanwhile forming multiple carboxyl along molecular chains. The dynamic ionic bonds and hydrolysable ester bonds endowed PCMNs with the reprocessability and biodegradability, respectively. PCMNs had the Δ H = 120 J/g with PEG weight fraction of ∼0.91 and concentration of carboxyl of ∼0.26 mmol/g. Also, PCMNs had good phase change ability, thermal stability and thermal reliability before and after thermal cycling and reprocessing. The robust approach enabled the combination of the reprocessing ability and high latent heat of PCMNs by ionic crosslinking. • The PCMNs were synthesized via ionic crosslinking of PEG-based polyester. • The PEG-based polyester had multiple carboxyl along molecular chains. • The maximum latent heat reached up to 120 J/g. • The PCMNs were reprocessed and recycled by both melt and solution methods. [ABSTRACT FROM AUTHOR]

• A polymer with low temperature self-healing ability was developed for smart coating. • The self-healing ability was determined by Zn-urethane bonds and hydrogen bonds. • The polymer film also displays outstanding anti-icing ability. • This polymer is a highly promising for self-healable anti-icing paint. Achieving the artificial supramolecular polymer with excellent low temperature self-healing capability has been a new topic of great significance in soft robotic actuators and smart coating. Nevertheless, the production of robust polymer with decent low temperature self-healing ability is severely impeded by its high crosslink density and low dynamic crosslink bond. Here, we report a design strategy that both low crosslink density and high dynamic crosslink bonds are introduced into polyurea networks to achieve a tough and low-temperature healable supramolecular system of polypropyleneglycol (PPG)-polydimethylsiloxane (PDMS)-Zn (abbreviated as PPG-PDMS-Zn). Incorporating the long PPG segments into linear PDMS chains endows the resulting polymer with relatively low crosslink density, allowing fast polymer chains movement to unite fractured surfaces. Moreover, the dynamic characteristics and self-healing capability of the supramolecular network were effectively enhanced by combining the unique dynamic exchange feature of weak Zn-urethane coordination bonds and low temperature inhibition-dissociation effect of hydrogen bonds. As a result, the representative sample of PPG-PDMS-Zn-0.5 polymer displays a robust tensile strength of 0.98 MPa and a high self-healing efficiency of 97% at −20 °C, and these features have rarely been achieved in the previously reported materials. With these promising characters, such kind of supramolecular polymers can find wide applications in fields of anti-icing and winter anti-frosting paints, and antarctic pole exploration as well. [ABSTRACT FROM AUTHOR]

Combining functional proteins with small molecular drugs into one entity may endow distinct synergistic advantages. However, on account of completely different physicochemical properties of such payloads, co-delivery through systemic administration for therapeutic purpose is challenging. Herein, we designed the protein-drug conjugate HSAP-DC-CAT (human serum albumin/Pt (IV)-dibenzocyclooctyne/chlorin e6-catalase) by modification of CAT and cisplatin pro-drug loaded HSA with pH-sensitive azide linker 3-(azidomethyl)-4-methyl-2,5-furandione (AzMMMan) followed by click chemistry assembly with DC. The dynamic covalent bonds between linker and proteins, on the one hand, can bridge proteins and small molecular drugs in the intermediate state for systemic delivery in the harsh in vivo environment; on the other hand, it can trigger traceless cleavage and release of drugs and proteins with full bioactivity in acidic microenvironment of tumor. The multifunctional HSAP-DC-CAT provides efficient cytosolic transduction in vitro, excellent blood half-lives after systemic administration, and significant antitumor outcome via integrated cisplatin-based chemotherapy and Ce6-based photodynamic therapy enhanced by catalase-induced manipulation of tumor hypoxia microenvironment. This study describes a universal formulation strategy for protein and small molecular drug by a bifunctional linker through amide reaction and click chemistry, with traceless in vivo release of therapeutic units. [ABSTRACT FROM AUTHOR]

The development of biobased polymers from renewable resources offers a solution to the growing environmental concerns and scarcity of fossil feedstock. The synthesized biobased polymers, especially for biobased elastomers, are mechanically weak, which greatly restricts their applications. In the present work, we demonstrate that the integration of transient and sacrificial quadruple H-bonding motifs into as-synthesized biobased elastomer can enhance the mechanical properties and bestow it with adaptive performance. Specially, the biobased elastomer was synthesized through melting poly-condensation using biobased di-acids and diols as the starting materials. The biobased elastomer is covalently crosslinked and grafted with ureido-pyrimidinone (UPy) containing acrylate through thiol-ene click reaction by using pentaerythritol tetra(3-mercaptopropionate) as the linkers. Under external load, UPy aggregations based on H-bonding can function as sacrificial units through reversible rupture and re-formation events, leading to significant improvements on the modulus and strength of the biobased elastomer while maintaining the extensibility. In addition, the dissociation and re-formation of H-bonding under thermal stimuli impart the elastomer with thermo-activated shape memory behavior. The incorporation of transient and sacrificial quadruple H-bonding motifs into bio-based elastomer that is synthesized from bio-based di-acids and diols can enhance the mechanical properties and bestow it with adaptive recovery. Image 1 • Quadruple H-bonding motifs are introduced into as-synthesized biobased elastomer through thiol-ene click reaction. • UPy aggregations based on H-bonding can act in sacrificial manner to improve modulus and tensile strength of the elastomer while maintaining the extensibility. • Dynamic nature of H-bonding allows the elastomer to access thermo-activated shape memory behavior. • The structure-property relationships of the resulted elastomer materials are studied. [ABSTRACT FROM AUTHOR]

The construction of a single polymer network with extreme stretchability, relatively high mechanical strength, and fast and facile autonomous room-temperature self-healing capability still remains a challenge. Herein, supramolecular polymer networks are fabricated by synergistically incorporating metal-ligand and hydrogen bonds in poly(propylene glycol) (PPG). The representative specimen, PPG-Im-MDA-1.5-0.25-Cu, shows a combination of notable mechanical properties involving an extreme stretching ratio of 346 ± 14× and a Young's modulus of 2.10 ± 0.14 MPa, which are superior to the previously reported extremely stretchable polymeric materials. Notably, the destroyed specimen can fully recover mechanical performances within 1 h. The tunability of mechanical properties and self-healing capability has been actualized by merely tailoring the content of a chain extender. The application of the as-prepared supramolecular PPG network in constructing a flexible and self-healable conductor has been demonstrated. This strategy provides some insights for preparing extremely stretchable and self-healable polymeric materials.

Hydrogel electrolytes are of particular interest in the fabrication of flexible supercapacitors that are able to withstand deformation and physical damage. Nevertheless, there still exists a huge space in the design of hydrogel electrolytes with comprehensive performances including high processability, conductivity, mechanical strength, and self-healability. Herein, a slidable polymer network is constructed through the cross-linking reaction among commercially available polyethyleneimine (PEI), polyvinyl alcohol (PVA), and 4-formylphenylboronic acid (Bn) to generate PEI-PVA-Bn hydrogels, which have high adaptability to various electrolytes such as LiCl, NaCl, KCl, and ionic liquids. The as formed hydrogel electrolytes not only show excellent mechanical property (elongation at break up to 1223%, strength of 34.6 kPa) and self-healability (highest strain self-healing efficiency reaches 94.3% within 2 min) but also exhibit high conductivity (up to 21.49 mS cm -1 ). Flexible supercapacitors constructed by sandwiching the PEI-PVA-Bn-LiCl hydrogel electrolyte between two multiwalled carbon nanotube electrodes demonstrate a broadened operating potential window of 1.4 V, specific capacitance of 16.7 mF cm -2 , high cycling stability up to 10 000 charge/discharge cycles, and excellent mechanical stability.

Fabricating a single polymer network with a combination of a multi-shape memory effect (multiple-SME), solid-state plasticity, recyclability and self-healing behavior remains a challenge. We designed imine bond and ionic hydrogen bond dual cross-linked polybutadiene (PB) networks. The resulting PB networks showed a triple-shape memory effect, where imine bonds could be used to fix the permanent shape and ionic hydrogen bonds and glass transition acted as the transition segments for fixing/releasing the temporary shapes. Additionally, the dual dynamic bonds offered PB networks outstanding solid-state plasticity, recyclability and self-healing behavior. This strategy provides some insights for preparing shape memory polymers integrating multiple-SME and multi-functionality.

Stretchable conductors have been achieved by stacking conductive nanomaterials onto the surfaces of elastomeric substrates. However, many of them show a dramatic decrease in conductivity under strain without an efficient way for the conductive layer to release strain. Here, we report a transparent, stretchable, and self-healing conductor with excellent mechanoelectrical stability by introducing dynamic bonding between conductive nanomaterials and an elastomeric substrate. We prepare the conductor by semiembedding Ag nanowires (AgNWs) into a self-healing polydimethylsiloxane (PDMS)-based elastomer, which is modified with bipyridine (Bpy) ligand and further cross-linked by adding Zn 2+ as coordinator (Zn-Bpy-PDMS). The dynamic Ag-N bonds not only improve the wettability of the substrate and facilitate the spreading of AgNWs but also reversibly break and reform to accommodate the deformation of AgNWs. As a result, the resistance increase of Zn-Bpy-PDMS/AgNWs is much smaller than that without the dynamic bonding (PDMS/AgNWs). Besides, this conductor exhibits excellent conductivity (76.2 Ω/sq) and transparency (86.6% @ 550 nm), as well as extraordinary self-healing property with a low resistance increase (Δ R / R 0 ∼ 1.4) after healing at room temperature for 1 day. This work provides insights into the future design of integrated electronic skin with transparency, stretchability, conductivity, and self-healing capability for applications in wearable optoelectronic devices.

Transient polymer networks are ubiquitous in natural and engineered materials and contain cross-links that can reversibly break and re-form. The dynamic nature of these bonds allows for interesting mechanical behavior, some of which include nonlinear rheological phenomena such as shear thickening and shear thinning. Specifically, physically cross-linked networks with reversible bonds are typically observed to have viscosities that depend nonlinearly on shear rate and can be characterized by three flow regimes. In slow shear, they behave like Newtonian fluids with a constant viscosity. With further increase in shear rate, the viscosity increases nonlinearly to subsequently reach a maximum value at the critical shear rate. At this point, network fracture occurs followed by a reduction in viscosity (shear-thinning) with a further increase in shear rate. The underlying mechanism of shear thickening in this process is still unclear with debates between a conversion of intra-chain to inter-chain cross-linking and nonlinear chain stretch under high tension. In this paper, we provide a new framework to describe the nonlinear rheology of transient polymer networks with the so-called chain distribution tensor using recent advances from the transient network theory. This tensor contains quantitatively and statistical information of the chain alignment and possible anisotropy that affect network behavior and mechanics. We investigate shear thickening as a primary result of non-Gaussian chain behavior and derive a relationship for the nonlinear viscosity in terms of the non-dimensional Weissenberg number. We further address the criterion for network fracture at the critical shear rate by introducing a critical chain force when bond dissociation is suddenly accelerated. Finally, we discuss the role of cross-linker density on viscosity using a "sticky" reptation mechanism in the context of previous studies on metallo-supramolecular networks with reversible cross-linkers. [ABSTRACT FROM AUTHOR]

Structurally dynamic polymers are recognized as a key potential to revolutionize technologies ranging from design of self-healing materials to numerous biomedical applications. Despite intense research in this area, optimizing reactivity and thereby improving self-healing ability at the most fundamental level pose urgent issue for wider applications of such emerging materials. Here, the authors report the first mechanistic investigation of the fundamental principle for the dependence of reactivity and self-healing capabilities on the properties inherent to dynamic polymers by combining large-scale computer simulation, theoretical analysis, and experimental discussion. The results allow to reveal how chain stiffness and spatial organization regulate reactivity of dynamic polymers grafted on Janus nanoparticles and mechanically mediated reaction in their reverse chemistry, and, particularly, identify that semiflexible dynamic polymers possess the optimal reactivity and self-healing ability. The authors also develop an analytical model of blob theory of polymer chains to complement the simulation results and reveal essential scaling laws for optimal reactivity. The findings offer new insights into the physical mechanism in various systems involving reverse/dynamic chemistry. These studies highlight molecular engineering of polymer architecture and intrinsic property as a versatile strategy in control over the structural responses and functionalities of emerging materials with optimized self-healing capabilities., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)