Bio-inspired high-performance and recyclable cross-linked polymers

Biomass-Based Functional Composite Resins with Recyclable and Shape Memory Properties

A key challenge in developing advanced functional thermosets lies in designing molecular architectures capable of integrating different specific performances into one material to meet diverse application demands. Here, a chitosan-derived trifunctional compound containing maleimide groups was used to directly cross-link tung oil-based polymer for fabricating multifunctional composite bioresins with reversible Diels-Alder bonds. The reversible cross-linking networks within resins were featured with stress relaxation, thermal reprocessability, and recyclability. The retro D-A reaction at relatively high temperatures provided the dynamic characteristics of the resins while ensuring their dimensional stability. Moreover, chitosan enhanced the mechanical properties of the resins while forming supramolecular hydrogen bonds via its abundant amino/hydroxyl groups, realizing shape memory of the resins. Furthermore, the synergistic interaction between chitosan functional groups and hydrogen bonding also imparted proton conductivity to the resins. This work provided a molecular design paradigm that harmonizes multifunctional integration in fully biomass resins, aiming for high-value applications.

Ambient Solvent Evaporation-Triggered Irreversible Covalent Crosslinking for Robust Adhesion in Extreme Conditions

Covalently crosslinked polymers, renowned for their stability and superior performance, play a pivotal role in materials science and technology. Yet, conventional crosslinking, reliant on external agents and energy-intensive processes, prompts a growing demand for sustainable and energy-efficient alternatives. Here, it is demonstrated that irreversible covalent crosslinking can be achieved simply through ambient solvent evaporation in a molecularly engineered polydimethylsiloxane system functionalized with dithiolane moieties. This process, validated using reduced density gradient analysis, forms a robust, irreversible covalent polymer network (CTP), fundamentally distinct from conventional dynamic disulfide-based reversible crosslinking systems or physically bonded polymers typically formed via solvent evaporation. The resultant CTP demonstrates strong adhesion to various substrates, as analyzed quantitatively through Density Functional Theory simulations. Furthermore, the CTP displays excellent waterproofing, high optical transparency, and notable resistance to extreme temperatures and highly corrosive solvents. The superior performance of CTP derives from its robust covalent network, enriched with disulfide and peptide bonds and liquid-like PDMS segments. Moreover, the CTP's preparation is straightforward, sustainable, and cost-effective. These advancements position CTP as a promising development in adhesive technology, suitable for a wide range of applications requiring mechanical robustness, chemothermal resilience, and optical clarity, particularly in scenarios sensitive to thermal or radiation exposure.

Self-healing, stretchable and recyclable polyurethane-PEDOT:PSS conductive blends

Future electronics call for materials with mechanical toughness, flexibility, and stretchability. Moreover, self-healing and recyclability are highly desirable to mitigate the escalating environmental threat of electronic waste (e-waste). Herein, we report a stretchable, self-healing, and recyclable material based on a mixture of the conductive polymer poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS) with a custom-designed polyurethane (PU) and polyethylene glycol (PEG). This material showed excellent elongation at brake (∼350%), high toughness (∼24.6 MJ m-3), moderate electrical conductivity (∼10 S cm-1), and outstanding mechanical and electrical healing efficiencies. In addition, it demonstrated exceptional recyclability with no significant loss in the mechanical and electrical properties after being recycled 20 times. Based on these properties, as a proof of principle for sustainable electronic devices, we demonstrated that electrocardiogram (ECG) electrodes and pressure sensors based on this material could be recycled without significant performance loss. The development of multifunctional electronic materials that are self-healing and fully recyclable is a promising step toward sustainable electronics, offering a potential solution to the e-waste challenge.

Reversible Crosslinking of Commodity Polymers via Photocontrolled Metal-Ligand Coordination for High-Performance and Recyclable Thermoset Plastics

Thermoset plastics, highly desired for their stability, durability, and chemical resistance, are currently consumed in over 60 million tons annually across the globe, but they are difficult to recycle due to their crosslinked structures. The development of recyclable thermoset plastics is an important but challenging task. In this work, recyclable thermoset plastics are prepared by crosslinking a commodity polymer, polyacrylonitrile (PAN), with a small percentage of a Ru complex via nitrile-Ru coordination. PAN is obtained from industry and the Ru complex is synthesized in one step, which enables the production of recyclable thermoset plastics in an efficient way. In addition, the thermoset plastics exhibit impressive mechanical performance, boasting a Young's modulus of 6.3 GPa and a tensile strength of 109.8 MPa. Moreover, they can be de-crosslinked when exposed to both light and a solvent and can then be re-crosslinked upon heating. This reversible crosslinking mechanism enables the recycling of thermosets from a mixture of plastic waste. The preparation of recyclable thermosets from other commodity polymers such as poly(styrene-coacrylonitrile) (SAN) resins and polymer composites through reversible crosslinking is also demonstrated. This study shows that reversible crosslinking via metal-ligand coordination is a new strategy for designing recyclable thermosets using commodity polymers.

Construction and mechanism study of lignin-based polyurethane with high strength and high self-healing properties

Lignin is a natural polymer with abundant functional groups with great application prospects in lignin-based polyurethane elastomers with self-healing abilities. In this study, a lignin self-healing polyurethane (PUDA-L) was specially designed using lignin as the raw material of polyurethane, combining lignin with Diels-Alder (DA) bond and hydrogen bonds. The experimental results showed that PUDA-L was prepared with good thermal stability, fatigue resistance, shape memory effect, excellent mechanical strength, and self-healing ability by partially replacing the crosslinking agents with bio-based lignin and hydroxylated modified lignin to increase the hydroxyl content. Polyurethane has a tensile strength of up to 29 MPa and an elongation at break of up to 500 %. The excellent self-healing ability of PUDA-L originates from the internal DA bonds and cross-linked hydrogen bonds. After the dumbbell sample was fused and heated at 130 °C for 4 h, the elastomer could be completely healed, the tensile strength was restored to 29 MPa, and the self-healing efficiency was up to 100 %. The developed PUDA-L elastomer has promising applications in sensors and smart skins.

Mechanical property-enhanced thermally conductive self-healing composites: preparation using designed self-healing matrix phase and hyBNNSs

Polymer composites with good thermal conductivity are gaining more and more attention in the current electronics sector, due to their superior heat management capabilities. However, conventional thermally conductive polymer composites are usually subject to interruptions in heat transfer because of physical damage. The present study prepared mechanical property-enhanced thermally-conductive self-healing composites through compositing a self-healing polyurethane matrix with hydroxylated boron nitride (hyBNNSs). The self-healing polyurethane was obtained by incorporating ligands and cerium(III) triflate [Ce(SO3CF3)3] as the metal center into the polyurethane elastomer. An optimal sample (PUp2C) with high tensile strength (6.8 MPa) and stretchability (1053%), ideal toughness (49.2 MJ m-3), and remarkable healing efficiency (97% healing after 48 h at 35 °C) was obtained. An increase in the content of hyBNNSs from 10% to 30% led to a significant increase in the mechanical performance of hyBNNSs20%/PUp2C, which manifested as the increase in the elongation at break (from 1053% to 1302.5%) and stress (from 6.8 MPa to 16.4 MPa). The XRD results revealed that combining PU with hyBNNSs through coordination bonds could significantly promote the crystallization of PUp2C, which was beneficial to enhancing the mechanical properties of the composites. The through-plane (λ⊥) and the in-plane (λ∥) values of the BNNSs30%/PUp2C composite reached 0.41 and 1.42 W mK-1, respectively, which were 195.2% and 507.1% higher than those of the original PUp2C, respectively.

Polyurethanes Modified by Ionic Liquids and Their Applications

Polyurethane (PU) refers to the polymer containing carbamate groups in its molecular structure, generally obtained by the reaction of isocyanate and alcohol. Because of its flexible formulation, diverse product forms, and excellent performance, it has been widely used in mechanical engineering, electronic equipment, biomedical applications, etc. Through physical or chemical methods, ionic groups are introduced into PU, which gives PU electrical conductivity, flame-retardant, and antistatic properties, thus expanding the application fields of PU, especially in flexible devices such as sensors, actuators, and functional membranes for batteries and gas absorption. In this review, we firstly introduced the characteristics of PU in chemical and microphase structures and their related physical and chemical performance. To improve the performance of PU, ionic liquids (ILs) were applied in the processing or synthesis of PU, resulting in a new type of PU called ionic PU. In the following part of this review, we mainly summarized the fabrication methods of IL-modified PUs via physical blending and the chemical copolymerization method. Then, we summarized the research progress of the applications for IL-modified PUs in different fields, including sensors, actuators, transistors, antistatic films, etc. Finally, we discussed the future development trends and challenges faced by IL-modified PUs.

New Building Blocks for Self-Healing Polymers

The healing efficiency in self-healing materials is bound by the ability to form blends between the prepolymer and curing agent. One of the problems in the development of self-healing polymers is the reduced affinity of the bismaleimide curing agent for the elastomeric furan-containing matrix. Even when stoichiometric amounts of both components are applied, incompatibility of components can significantly reduce the effectiveness of self-healing, and lead to undesirable side effects, such as crystallization of the curing agent, in the thickness and on the surface. This is exactly what we have seen in the development of linear and cross-linked PUs using BMI as a hardener. In this work, we present a new series of the di- and tetrafuranic isocyanate-related ureas-promising curing agents for the development of polyurethanes-like self-healing materials via the Diels-Alder reaction. The commonly used isocyanates (4,4'-Methylene diphenyl diisocyanate, MDI; 2,4-Tolylene diisocyanate, TDI; and Hexamethylene diisocyanate, HDI) and furfurylamine, difurfurylamine, and furfuryl alcohol (derived from biorenewables) as furanic compounds were utilized for synthesis. The remendable polyurethane for testing was synthesized from a maleimide-terminated prepolymer and one of the T-series urea. Self-healing properties were investigated by thermal analysis. Molecular mass was determined by gel permeation chromatography. The properties of the new polymer were compared with polyurethane from a furan-terminated analog. Visual tests showed that the obtained material has thermally induced self-healing abilities. Resulting polyurethane (PU) has a rather low fusing point and thus may be used as potential material for Fused Deposition Modeling (FDM) 3D printing.

Recycling of Thermoset Materials and Thermoset-Based Composites: Challenge and Opportunity

Thermoset materials and their composites are characterized by a long life cycle with their main applications in aircrafts, wind turbines and constructions as insulating materials. Considering the importance of recovery and valorization of these materials at their end-of-life, avoiding landfilling, the interest concerning their recycling grows continuously. The thermoset materials and their composites, to be successfully recovered and valorized, must degrade their three-dimensional structures and recover the mono-oligomers and/or fillers. The thermoset materials could successfully degrade through thermal treatment at different temperatures (for example, above 1000 °C for incineration, ca. 500 °C for oxidation/combustion of organic constituents, etc.), chemical degradation by catalyst, irradiation with or without the presence of water, alcohol, etc., and mechanical recycling, obtaining fine particles that are useful as filler and/or reinforcement additives. Among these recycling methods, this mini-review focuses on the formulation and recovery method of innovative thermoset with in-build recyclability, i.e., materials having chemical links that could be degraded on-demand or containing dynamic covalent bonds to have re-processable and/or recyclable thermoset. This issue could be considered the future perspective in developing novel thermoset materials. The aim of this review is to get an overview of the state of the art in thermoset recycling and of the most commonly used thermoset composites, recovering valuable reinforcing fibers. Additionally, in this work, we also report not only known recycling routes for thermoset and thermoset-based composites, but also new and novel formulating strategies for producing thermosets with built-in recyclability, i.e., containing chemical-triggered on-demand links. This mini-review is also a valuable guide for educational purposes for students and specialized technicians in polymer production and recycling.

Synthesis, characterization and post-modification of aromatic (Co)polyesters possessing pendant maleimide groups

A new series of (co)polyesters possessing pendent maleimide groups was synthesized by low temperature solution polycondensation of 4, 4’-(5-maleimidopentane-2, 2-diyl) diphenol (BPA-MA) with isophthalic acid chloride (IPC), terephthalic acid chloride (TPC) and a mixture of TPC and IPC (50:50 mol %). Copolyesters were also synthesized by polycondensation of varying compositions of BPA-MA and bisphenol-A (BPA) with IPC. The chemical structures and compositions of (co)polyesters were confirmed by NMR spectroscopy. Inherent viscosity values and number-average molecular weights of (co)polyesters were in the range 0.50–0.76 dL/g and 17,700-32,100 g/mol, respectively, indicating the formation of reasonably high molecular weight polymers. (Co)polyesters were readily soluble in common organic solvents and could be cast into tough, transparent and flexible films from chloroform solutions. (Co)polyesters exhibited 10% weight loss and glass transition temperatures in the range 464–468 and 142–178°C, respectively. A representative copolyester possessing pendant maleimide groups was chemically modified via metal-free azide-maleimide 1,3-dipolar cycloaddition click reaction with two azido compounds, namely, (azidomethyl)benzene (Bz-N3) and 1-(azidomethyl)-pyrene (Py-N3) to yield corresponding modified copolyesters in a quantitative manner.

Ancient fibrous biomaterials from silkworm protein fibroin and spider silk blends: Biomechanical patterns

Silkworm silk protein fibroin and spider silk spidroin are known biocompatible and natural biodegradable polymers in biomedical applications. The presence of β-sheets in silk fibroin and spider spidroin conformation improves their mechanical properties. The strength and toughness of pure recombinant silkworm fibroin and spidroin are relatively low due to reduced molecular weight. Hence, blending is the foremost approach of recent studies to optimize silk fibroin and spidroin's mechanical properties. As summarised in the present review, numerous research investigations evaluate the blending of natural and synthetic polymers. The effects of blending silk fibroin and spidroin with natural and synthetic polymers on the mechanical properties are discussed in this review article. Indeed, combining natural and synthetic polymers with silk fibroin and spidroin changes their conformation and structure, fine-tuning the blends' mechanical properties. STATEMENT OF SIGNIFICANCE: Silkworm and spider silk proteins (silk fibroin and spidroin) are biocompatible and biodegradable natural polymers having different types of biomedical applications. Their mechanical and biological properties may be tuned through various strategies such as blending, conjugating and cross-linking. Blending is the most common method to modify fibroin and spidroin properties on demand, this review article aims to categorize and evaluate the effects of blending fibroin and spidroin with different natural and synthetic polymers. Increased polarity and hydrophilicity end to hydrogen bonding triggered conformational change in fibroin and spidroin blends. The effect of polarity and hydrophilicity of the blending compound is discussed and categorized to a combinatorial, synergistic and indirect impacts. This outlook guides us to choose the blending compounds mindfully as this mixing affects the biochemical and biophysical characteristics of the biomaterials.

Healable Strain Sensor Based on Tough and Eco-Friendly Biomimetic Supramolecular Waterborne Polyurethane

Stretchable sensors are essential for flexible electronics, which can be made with polymer elastomers as the matrix. The main challenge in producing practical devices is to obtain polymers with mechanical stability, eco-friendliness, and self-healing properties. Herein, we introduce urea bonds and 2-ureido-4[1H]-pyrimidinone (UPy) to synthesize tailored waterborne polyurethanes (WPU-UPy-x) with a hierarchical hydrogen bond (H-bond). Accordingly, sound tensile performance (strength: 53.33 MPa, toughness: 128.97 MJ m^-3), satisfying deformation recovery, and good self-healing capability of the WPU-UPy-x film are demonstrated. With atomic force microscope characterization, we find that UPy groups contribute to the highly improved microphase separation of WPU-UPy-x, responsible for good mechanical properties. As a proof of concept, a strain sensor is successfully configured, thanks to the good interfacial interactions between the polyurethane matrix and the Ti₃C₂T_x MXene conductive filler, which features sensitive and stable performance for monitoring diverse human and mechanical motions. Intriguingly, this sensor is capable of self-healing after cutting and displays well-retained sensitivity to detect the stretched signal. The as-proposed design concept for healable and sensitive strain sensors can shed light on future wearable electronics.

Fast-reprocessing, Post-adjustable, Self-healing Covalent Adaptable Networks with Schiff base and Diels-Alder Adduct

Covalent adaptable networks (CANs) are a new type of polymers which possess excellent performance of thermosets and reprocessability of thermoplastics. Nevertheless, it is still a challenge to realize rapid reprocessing and post-adjusting (adjust properties after preparation). Herein, for the first time, a method of combining Schiff base and Diels-Alder adduct in one network was developed to achieve rapid reprocessing and post-adjusting. Through the dissociation of the Diels-Alder adduct at high temperatures, the cross-link densities of the networks were reduced, thereby accelerating the rearrangement of the networks and realizing the rapid reprocessing and self-healing. Moreover, the reconnecting degree of network after dissociation of Diels-Alder adduct could be easily controlled by annealing, as a result, the properties of the obtained CANs were post-adjustable. This work provides a simple and promising approach of achieving excellent reprocessing and post-adjusting for CANs via the synergism of an associative dynamic chemistry with a dissociative dynamic chemistry. This article is protected by copyright. All rights reserved.

Dynamic covalent polymers enabled by reversible isocyanate chemistry

Reversible isocyanate chemistry containing urethane, thiourethane, and urea bonds is valuable for designing dynamic covalent polymers to achieve promising applications in recycling, self-healing, shape morphing, 3D printing, and composites.

Progress in Utilizing Dynamic Bonds to Fabricate Structurally Adaptive Self-Healing, Shape Memory, and Liquid Crystal Polymers

Stimuli-responsive structurally dynamic polymers are capable of mimicking the biological systems to adapt themselves to the surrounding environmental changes and subsequently exhibiting a wide range of responses ranging from self-healing to complex shape-morphing. Dynamic self-healing polymers (SHPs), shape-memory polymers (SMPs) and liquid crystal elastomers (LCEs), which are three representative examples of stimuli-responsive structurally dynamic polymers, have been attracting broad and growing interest in recent years because of their potential applications in the fields of electronic skin, sensors, soft robots, artificial muscles, and so on. We review recent advances and challenges in the developments towards dynamic SHPs, SMPs and LCEs, focusing on the chemistry strategies and the dynamic reaction mechanisms that enhance the performances of the materials including self-healing, reprocessing and reprogramming. We compare and discuss the different dynamic chemistries and their mechanisms on the enhanced functions of the materials, where three summary tables are presented: a library of dynamic bonds and the resulting characteristics of the materials. Finally, we provide a critical outline of the unresolved issues and future perspectives on the emerging developments. This article is protected by copyright. All rights reserved.

Preparation and Characterization of Self-Healing Polyurethane Powder Coating Using Diels-Alder Reaction

Although powder coating systems offer many environmental, ecological and energy related benefits over liquid based coatings, in the case of uretdione based polyurethane systems, high curing temperature is still an issue. On the other hand, powder coating systems make it possible to reduce the costs and enhance the process of forming complex 3D structures using the deep drawing method by pre-coated metal substrates. During this processing method, there is a probability of micro crack formation in the coated film due to strain impact on the coating layer. A powder coating with self-healing ability is an ultimate solution to face not only this kind of fraction but also any other possible ones (such as defects caused by any impact on film surface during processing, transporting or even service). Here, a single molecule that is prepared via Diels–Alder cycloaddition reaction and retro Diels–Alder cleavage reaction was utilized as a self-healing additive to achieve self-healing ability in the powder coating system that is based on a commercially available uretdione cross-linker and OH-polyester resin. Coatings were prepared through melt mixing of components in a lab mixer, milling, sieving, and then application on the metal substrate through the electrostatic spraying method. To illustrate the role of self-healing additive, various concentrations (4 and 9% wt.) in combination with different curing temperatures (80 °C to 200 °C) were investigated. Both samples containing HA showed self-healing ability at elevated temperature around 120 °C for about 30 min with acceptable roughness and surface properties. Hardness measurement of cured film as well as thermal investigation indicate the chemical reaction of HA in a cross-linked network of cross-linker and resin. In addition, using HA leads to a 40 K drop in curing temperature of the system without using any catalyst. A 2.58% improvement in hardness values at a lower curing temperature and healing time of around 12.5 min at 120 °C to recover 100% of initial scratch (more than 10 cycles) in the sample containing 9% wt. HA was observed.

Fluorescent, electrically responsive and ultratough self-healing hydrogels via bioinspired all-in-one hierarchical micelles

Intelligent hydrogels that simultaneously exhibit excellent toughness, self-healing ability and photoelectronic responsiveness are in high demand but are greatly challenging to prepare. Inspired by the hierarchical structure of fluorescent proteins in jellyfish and biomembranes in nature, herein, a facile and universal all-in-one strategy is demonstrated to construct fluorescent, electrically responsive and ultratough self-healing hydrogels via aqueous self-assembly of polyelectrolyte-surfactant micelles with hierarchical structures and functionality. The self-assembled 2-ureido-4-[1H]-pyrimidone (UPy) hydrophobic core containing reversible physical crosslinks embedded in micelles leads to a durable network structure with excellent toughness and self-healing ability. Moreover, dramatically enhanced fluorescence emission is obtained due to the formation of nanoclusters with electron-rich moieties that show restricted intramolecular motion induced by hydrogen bonding networks from UPy dimer aggregation. The micelle-incorporated sulfonic acid groups mimic the function of biological membrane proteins that deftly control the micelle size, leading to electro-responsiveness, enhanced toughness and fluorescence emission.

Trends in the Diels-Alder reaction in polymer chemistry

The Diels-Alder (DA) reaction is regarded as quite a useful strategy in organic and macromolecular syntheses. The reversibility of this reaction and the advent of self-repair technology, as well as other applications in controlled macromolecular architectures and crosslinking, have strongly boosted the research activity, which is still attracting a huge interest in both academic and industrial research. The DA reaction is a simple and scalable toolbox. Though it is well-established that furan/maleimide is the most studied diene/dienophile couple, this perspective article reports strategies using other reversible systems with deeper features on other types of diene/dienophile pairs being either petro-sourced (cyclopentadiene, anthracene) or bio-sourced (muconic and sorbic acids, myrcene and farnesene derivatives, eugenol, cardanol). This review is composed of four sections. The first one briefly recalls the background on the DA reactions involving cyclodimerizations, dienes, and dienophiles, parameters affecting the reaction, while the second part deals with the furan/maleimide reaction. The third one deals with petro-sourced and bio-sourced (or products becoming bio-sourced) reactants involved in DA reactions are also listed and discussed. Finally, the authors' opinion is given on the potential future of the crosslinking-decrosslinking reaction, especially regarding the process (e.g., key temperatures of decrosslinking) or possibly monocomponents. It presents both fundamental and applied research on the DA reaction and its applications.

Diels-Alder Cross-Linked, Washing-Free Hydrogel Films with Ordered Wrinkling Patterns for Multicellular Spheroid Generation

Three-dimensional (3D) multicellular spheroids are a new generation in vitro cell model, however, their applications are severely limited by difficulties in their generation. Here patterned poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel films were synthesized for their generation. Instead of polymerization of HEMA monomers in the presence of a cross-linker, here the PHEMA films were synthesized by cross-linking furan-functionalized linear PHEMA, PHEMA-furan, and maleimide-functionalized linear PHEMA, PHEMA-mal, via Diels-Alder (DA) reaction between furan and maleimide groups. A thermal treatment temperature of 75 °C was chosen for the cross-linking reaction. The occurrence of DA reaction was confirmed by IR spectra. Using this method, cross-linked PHEMA films with smooth surface were successfully synthesized in situ in the well of cell culture plates. The films were then patterned by simply adding water to swell them. Highly ordered, honeycomb-like wrinkling patterns were successfully obtained by adjusting the furan and maleimide contents in the precursor linear polymers. The patterned hydrogel films were used to generate multicellular spheroids. Guided by the patterns, 3D spheroids with narrow size distribution, tunable size, and high cell viability were successfully obtained. The patterned PHEMA films reported here exhibited a lot of advantages. The patterning method was quite simple and required no template or special equipment. They were synthesized in situ in commercial cell culture plates. Particularly, thanks to the clean nature of the DA reaction, no low molecular weight monomer, cross-linker, initiator, or catalyst, which were potentially cytotoxic, was involved in the film synthesis, and no byproduct was produced and left in the film. The resulting films presented a high biocompatibility, allowing the avoidance of the tedious washing step. The films synthesized here were expected to have high potential for massive production of well-defined multicellular spheroids.

Thermally Remendable Polyurethane Network Cross-Linked via Reversible Diels-Alder Reaction

We prepared a series of thermally remendable and recyclable polyurethanes crosslinked via reversible furan-maleimide Diels-Alder reaction based on TDI end-caped branched Voranol 3138 terminated with difurfurylamine and 4,4'-bis(maleimido)diphenylmethane (BMI). We showed that Young modulus strongly depends on BMI content (from 8 to 250 MPa) that allows us to obtain materials of different elasticity as simple as varying BMI content. The ability of DA and retro-DA reactions between furan and maleimide to reversibly bind material components was investigated by NMR spectroscopy, differential scanning calorimetry, and recycle testing. All polymers obtained demonstrated high strengths and could be recovering without significant loss in mechanical properties for at least five reprocessing cycles.

Dually cross-linked single networks: structures and applications

Cross-linked polymers have attracted an immense attention over the years, however, there are many flaws of these systems, e.g. softness and brittleness; such materials possess non-adjustable properties and cannot recover from damage and thus are limited in their practical applications. Supramolecular chemistry offers a variety of dynamic interactions that when integrated into polymeric gels endow the systems with reversibility and responsiveness to external stimuli. A combination of different cross-links in a single gel could be the key to tackle these drawbacks, since covalent or chemical cross-linking serve to maintain the permanent shape of the material and to improve overall mechanical performance, whereas non-covalent cross-links impart dynamicity, reversibility, stimuli-responsiveness and often toughness to the material. In the present review we sought to give a comprehensive overview of the progress in design strategies of different types of dually cross-linked single gels made by researchers over the past decade as well as the successful implementations of these advances in many demanding fields where versatile multifunctional materials are required, such as tissue engineering, drug delivery, self-healing and adhesive systems, sensors as well as shape memory materials and actuators.

Dynamic Semi IPNs with Duple Dynamic Linkers: Self-Healing, Reprocessing, Welding, and Shape Memory Behaviors

Thermoset polymers show favorable material properties, while bringing about environmental pollution due to non-reprocessing and unrecyclable. Diels-Alder (DA) chemistry or reversible exchange boronic ester bonds have been employed to fabricate recycled polymers with covalent adaptable networks (CANs). Herein, a novel type of CANs with multiple dynamic linkers (DA chemistry and boronic ester bonds) was firstly constructed based on a linear copolymer of styrene and furfuryl methacrylate and boronic ester crosslinker. Thermoplastic polyurethane is introduced into the CANs to give a semi Interpenetrating Polymer Networks (semi IPNs) to enhance the properties of the CANs. We describe the synthesis and dynamic properties of semi IPNs. Because of the DA reaction and transesterification of boronic ester bonds, the topologies of semi IPNs can be altered, contributing to the reprocessing, self-healing, welding, and shape memory behaviors of the produced polymer. Through a microinjection technique, the cut samples of the semi IPNs can be reshaped and mechanical properties of the recycled samples can be well-restored after being remolded at 190 °C for 5 min.

Bioinspired Polyurethane Using Multifunctional Block Modules with Synergistic Dynamic Bonds

Nature embraces an intriguing strategy to create high-performance biomaterials, such as spider silk which presents an unparalleled combination of stiffness, tensile strength, and toughness via hierarchical structures. However, to fabricate synthetic polymers with such excellent properties remains a challenging task. Inspired by the integration of multiblock backbone and densely H-bonding assemblies in spider silk as well as the delicate iron-catecholate complexes in mussel byssus, we proposed a novel molecular design with multifunctional block modules to obtain polymer materials that exhibit excellent mechanical property, self-healing ability, and reprocessability. It was achieved by introducing reversible iron-catechol (DOPA-Fe³⁺) cross-links and quadruple H-bonds bearing 2-ureido-4-[1H]-pyrimidinone (UPy) dimers as multifunctional blocks into a segmented polyurethane backbone with urethane blocks and semicrystalline polycaprolactone (PCL) blocks. These two types of dynamic cross-linking knots served as the sacrificial bonds to dissipate energy efficiently under external stress burden, endowing the dual physical cross-linked networks with increased toughness and breaking elongation. Moreover, the DOPA-Fe³⁺ complexes could increase the crystallization of PCL, leading to remarkably enhanced Young's modulus and tensile strength. Solid-state NMR revealed the formation of quadruple H-bonds in UPy dimers and the presence of DOPA-Fe³⁺ complexes, which restricted the mobility of the mobile phase and enhanced the crystallinity of the PCL domain. This work provides a feasible way to develop bioinspired materials with self-healable and reprocessable features, in addition to balanced enhancement of both stiffness and toughness.

Polypyrrole-Doped Conductive Self-Healing Composite Hydrogels with High Toughness and Stretchability

In recent years, hydrogels with self-healing capability and conductivity have become ideal materials for the design of electrodes, soft robotics, electronic skin, and flexible wearable devices. However, it is still a critical challenge to achieve the synergistic characteristics of high conductivity, excellent self-healing efficiency without any stimulations, and decent mechanical properties. Herein, we developed a ferric-ion (Fe³⁺) crosslinked acrylic acid and chitosan polymer hydrogel using embedded polypyrrole particles with features of high conductivity (2.61S·m^-1) and good mechanical performances (a tensile strength of 628%, a stress of 0.33 MPa, an elastic modulus of 0.146 MPa, and a toughness of 1.14 MJ·m^-3). In addition, the self-healing efficiency achieved 93% in tensile strength after healing in the air for 9 h without any external stimuli. Therefore, with these outstanding mechanical, self-healing, and conductive abilities all in one, it is possible to fabricate a new kind of soft material with wide applications.

Dually Crosslinked Polymer Networks Incorporating Dynamic Covalent Bonds

Covalent adaptable networks (CANs) are polymeric networks containing covalent crosslinks that are dynamic under specific conditions. In addition to possessing the malleability of thermoplastics and the dimensional stability of thermosets, CANs exhibit a unique combination of physical properties, including adaptability, self-healing, shape-memory, stimuli-responsiveness, and enhanced recyclability. The physical properties and the service conditions (such as temperature, pH, and humidity) of CANs are defined by the nature of their constituent dynamic covalent bonds (DCBs). In response to the increasing demand for more sophisticated and adaptable materials, the scientific community has identified dual dynamic networks (DDNs) as a promising new class of polymeric materials. By combining two (or more) distinct crosslinkers in one system, a material with tailored thermal, rheological, and mechanical properties can be designed. One remarkable ability of DDNs is their capacity to combine dimensional stability, bond dynamicity, and multi-responsiveness. This review aims to give an overview of the advances in the emerging field of DDNs with a special emphasis on their design, structure-property relationships, and applications. This review illustrates how DDNs offer many prospects that single (dynamic) networks cannot provide and highlights the challenges associated with their synthesis and characterization.

A biomass-based Schiff base vitrimer with both excellent performance and multiple degradability

Vitrimers with both excellent performance and multiple degradability were obtained by curing vanillin dialdehyde monomer with triamino T403.