A Thermoreversible Supramolecular Polyurethane with Excellent Healing Ability at 45 °C

Dynamic Non-Covalent Exchange Intrinsic Self-Healing at 20 °C Mechanism of Polyurethane Induced by Interactions among Polycarbonate Soft Segments

Two polyurethanes (PUs) were similarly synthesized by reacting a cycloaliphatic isocyanate with 1,4-butanediol and two polyols of different nature (polyester, polycarbonate diol) with molecular weights of 1000 Da. Only the PU synthesized with polycarbonate diol polyol (YCD) showed intrinsic self-healing at 20 °C. For assessing the mechanism of intrinsic self-healing of YCD, a structural characterization by molecular weights determination, infrared and X-ray photoelectronic spectroscopies, differential scanning calorimetry, X-ray diffraction, thermal gravimetric analysis, and dynamic mechanical thermal analysis was carried out. The experimental evidence concluded that the self-healing at 20 °C of YCD was due to dynamic non-covalent exchange interactions among the polycarbonate soft segments. Therefore, the chemical nature of the polyol played a key role in developing PUs with intrinsic self-healing at 20 °C.

Simple and Efficient Synthesis of Oligoetherdiamines: Hardeners of Epoxyurethane Oligomers for Obtaining Coatings with Shape Memory Effect

In this work, new polymers with a shape memory effect for self-healing coatings based on oligomers with terminal epoxy groups, synthesized from oligotetramethylene oxide dioles of various molecular weights, were developed. For this purpose, a simple and efficient method for the synthesis of oligoetherdiamines with a high yield of the product, close to 94%, was developed. Oligodiol was treated with acrylic acid in the presence of a catalyst, followed by the reaction of the reaction product with aminoethylpiperazine. This synthetic route can easily be upscaled. The resulting products can be used as hardeners for oligomers with terminal epoxy groups synthesized from cyclic and cycloaliphatic diisocyanates. The effect of the molecular weight of newly synthesized diamines on the thermal and mechanical properties of urethane-containing polymers has been studied. Elastomers synthesized from isophorone diisocyanate showed excellent shape fixity and shape recovery ratios of >95% and >94%, respectively.

Bio-Based Healable 2K PU Textile Coating for Durable Applications

A biobased healable 2K polyurethane (PU) coating incorporating a Schiff base was synthesized and applied as a thin coating on textiles. The Schiff base, made out of cystine and vanillin, contained reversible imine and disulfide bonds and was used as a chain extender in PU synthesis. The FT-IR analysis indicated the successful incorporation of the Schiff base in the PU backbone. Compared with control PU coatings, the healable bio-based PU coating with the Schiff base showed very good healing properties using heat as external stimuli: a healing recovery of 75% was obtained after applying a 2 N scratch and complete recovery of the resistance to hydrostatic pressure. SEM analysis revealed complete closure of the scratch after healing for 30 min at 90 °C. The healing properties are attributed to the synergy of the dual-dynamic metatheses of the imine and disulfide bonds.

An unparalleled H-bonding and ion-bonding crosslinked waterborne polyurethane with super toughness and unprecedented fracture energy

Self-healing polyurethane elastomers have been extensively studied; however, developing an eco-friendly self-healable waterborne polyurethane (WPU) with exceptional mechanical properties remains a great challenge. Herein, we report healable, and highly tough WPU elastomers with unprecedented crack tolerance by introducing the concerted interactions of strong multiple H-bonds and ionic bonds in the network. The WPU elastomer demonstrated that the microphase separation structure contributes to an ultrahigh tensile strength (≈58 MPa), super toughness (≈456 MJ m^-3), and unprecedented record fracture energy (≈320 kJ m^-2). Due to the dynamic reconstruction of reversible H-bonds and ionic bonds, the WPU elastomer demonstrates a robust self-healability at 50 °C, allowing complete recovery of mechanical properties. Importantly, the thermoplasticity and reprocessability of WPUs enable direct 3D printing of different objects and electrospinning of tubes, showing great potential for expanding their application scope in soft robots and artificial stents.

Main-Side Chain Hydrogen Bonding-Based Self-Healable Polyurethane with Highly Stretchable, Excellent Mechanical Properties for Self-Healing Acid-Base Resistant Coating

Developing an autonomous self-healing polyurethane (PU) elastomer with excellent mechanical properties and high ductility has attracted increasing attention. Nowadays, the synthesis of elastomers with excellent mechanical properties and rapid self-healing at room temperature faces a huge challenge. Herein, This work reports a new supramolecular PU with excellent mechanical properties and rapid self-healing at room temperature through the introduction of T-type chain extender into the supramolecular polymer chain. The introduction of T-chain extender can be used to enhance the mechanical strength of PU, and the multiple hydrogen bonds on the side-chain provide theoretical support for the rapid self-healing ability of PU. Maximum stress of the synthesized PU can reach 3.4 ± 0.15 Mpa, and maximum elongation at break can reach 3200% ± 160%. Due to flexibility and re-constructability of side-chain hydrogen bonds, PU stress repair efficiency can reach 96.7%, and can be self-healing scratches rapidly and effectively at room temperature. The mechanical properties and self-healing properties of PU can be adjusted by the content of T-type chain extender. The PU is applied to the metal surface coating, which has excellent acid-base resistance, bond strength up to 2.9 ± 0.1 Mpa, and the ability to eliminate local damage on the coating surface quickly at room temperature.

From Scratch Closure to Electrolyte Barrier Restoration in Self-Healing Polyurethane Coatings

The effects of the soft block fraction and H-bond state in thermoplastic polyurethanes on autonomous entropy-driven scratch closure and barrier restoration are studied. To this aim, comparable polyurethanes with different segmentation states are applied as organic coatings on plain carbon steel plates, scratched under very well-controlled conditions, and the scratch closure and sealing kinetics are studied in detail. The scratch closure is measured optically, while the barrier restoration is probed by the accelerated cyclic electrochemical technique (ACET). Scratch closure, attributed to entropic elastic recovery (EER), is followed in a marked two-step process by barrier restoration governed by local viscous flow and the state of the interfacial hydrogen bonding. Polyurethanes with a lower soft phase fraction lead to a higher urea/urethane ratio, which in turn influences the healing efficiency of each healing step. Interestingly, softer polyurethanes leading to efficient crack closure were unable to sufficiently restore barrier properties. The present work highlights the critical role of the soft/hard block and urea/urethane H-bond state content on crack closure and barrier restoration of anticorrosive organic coatings and points at design rules for the design of more efficient corrosion-protective self-healing polyurethanes.

Catalyst-free room-temperature self-healing polymer networks based on dynamic covalent quinone methide-secondary amine chemistry

A novel type of dynamic covalent polymer network with a catalyst-free room-temperature self-healing ability was developed on a new dynamic covalent chemistry of aza-Michael addition between para-quinone methide and secondary amine.

The effect of chiral end groups on the assembly of supramolecular polyurethanes

We describe the generation of supramolecular polyurethanes and the positive effect that chirality has upon the physical properties of these materials.

Eco-Friendly Sustainable Poly(benzoxazine-co-urethane) with Room-Temperature-Assisted Self-Healing Based on Supramolecular Interactions

This work is an attempt to develop bio-based eco-friendly poly(benzoxazine-co-urethane) [poly(U-co-CDL-aee)] materials using cardanol-based benzoxazines (CDL) and hexamethylene diisocyanate (HMDI) to check their self-healing ability and thermal properties. CDL monomers were synthesized using cardanol, amino ethoxyethanol (aee) or 3-aminopropanol (3-ap), and paraformaldehyde through the Mannich reaction. Later, CDL-aee or CDL-3-ap monomers were copolymerized with a urethane precursor (HMDI), followed by ring-opening polymerization through thermal curing. The thermal properties of poly(U-co-CDL) were evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The self-healing behavior of the bio-based poly(U-co-CDL) was checked by applying a mild external pressure. The results revealed that the developed poly(U-co-CDL) showed repeatable self-healing ability due to supramolecular hydrogen-bonding interactions. Further, the self-healing ability of poly(U-co-CDL) was studied using density functional theory (DFT). From the above results, the developed material with superior self-healing ability can be used in the form of self-healing coatings and composites for various applications with extended shelf-life and reliability.

Flexible Self-Repairing Materials for Wearable Sensing Applications: Elastomers and Hydrogels

Flexible pressure and strain sensors have great potential for applications in wearable and implantable devices, soft robots, and artificial skin. The introduction of self-healing performance has made a positive contribution to the lifetime and stability of flexible sensors. At present, many self-healing flexible sensors with high sensitivity have been developed to detect the signal of organism activity. The sensitivity, reliability, and stability of self-healing flexible sensors depend on the conductive network and mechanical properties of flexible materials. This review focuses on the latest research progress of self-healing flexible sensors. First, various repair mechanisms of self-healing flexible materials are reviewed because these mechanisms contribute to the development of self-healing flexible materials. Then, self-healing elastomer flexible sensor and self-healing hydrogel flexible sensor are introduced and discussed respectively. The research status and problems to be solved of these two types of flexible sensors are discussed in detail. Finally, this rapidly developing and promising field of self-healing flexible sensors and devices is prospected.

Achievement of Both Mechanical Properties and Intrinsic Self-Healing under Body Temperature in Polyurethane Elastomers: A Synthesis Strategy from Waterborne Polymers

Inspired by the growing demand for smart and environmentally friendly polymer materials, we employed 2,2'-disulfanediyldianiline (22DTDA) as a chain extender to synthesize a waterborne polyurethane (WPUR). Due to the ortho-substituted structure of the aromatic disulfide, the urea moieties formed a unique microphase structure in the WPUR, its mechanical strength was enhanced more 180 times relative to that of the material prepared without 22DTDA, and excellent self-healing abilities at body temperature in air or under ultrasound in water were obtained. If the self-healing process was carried out at 37 °C, 50 °C or under ultrasound, the ultimate tensile strength and elongation at break of the healed film could reach 13.8 MPa and 1150%, 15.4 MPa and 1215%, or 16 MPa and 1056%, respectively. Moreover, the WPUR films could be re-healed at the same fracture location over three cutting-healing cycles, and the recovery rates of the tensile strength and elongation at break remained almost constant throughout these cycles.

Advances in intrinsic self-healing polyurethanes and related composites

Fascinating and challenging, the development of repairable materials with long-lasting, sustainable and high-performance properties is a key-parameter to provide new advanced materials. To date, the concept of self-healing includes capsule-based healing systems, vascular healing systems, and intrinsic healing systems. Polyurethanes have emerged as a promising class of polymeric materials in this context due to their ease of synthesis and their outstanding properties. This review thereby focuses on the current research and developments in intrinsic self-healing polyurethanes and related composites. The chronological development of such advanced materials as well as the different strategies employed to confer living-like healing properties are discussed. Particular attention will be paid on chemical reactions utilized for self-healing purposes. Potential applications, challenges and future prospects in self-healing polyurethane fields are also provided.

Preparation and Properties of Self-Healing Polyurethane Elastomer Derived from Tung-Oil-Based Polyphenol

A tung-oil-based polyphenol (ATOM), containing the phenolic hydroxyl group, was synthesized from tung oil and 4-maleimidophenol by the Diels-Alder addition reaction. Then self-healing thermosetting polyurethanes were prepared from ATOM and the polyurethane prepolymer. The chemical structure and cross-link network were confirmed by Fourier transform infrared spectroscopy (FTIR) and swelling tests. The products partially dissolved in trichlorobenzene when the temperature rose to 110 °C. Temperature-variable FTIR confirmed that the phenolic urethane starts to partially dissolve at 100 °C, which can be explained by the experimental phenomenon in swelling tests. Tensile property analysis showed that the broken and healed thermosets maintained about 46-64% of their original tensile strengths and 81-88% of their original elongations at break, respectively.

A 3D printed drug delivery implant formed from a dynamic supramolecular polyurethane formulation

Prototype drug eluting implants have been 3D printed using a supramolecular polyurethane-PEG formulation. The implants are capable of releasing a pharmaceutical active with effective drug release over a period of up to 8.5 months.

Self-healing and shape memory metallopolymers: state-of-the-art and future perspectives

Recent achievements and problems associated with the use of metallopolymers as self-healing and shape memory materials are presented and evaluated.

Imidazolium-based ionic polyurethanes with high toughness, tunable healing efficiency and antibacterial activities

Ionic polyurethanes (PUs) with high toughness, fast self-healing ability, antibacterial activity and shape memory behaviors are synthesized.

Toughening of Glassy Supramolecular Polymer Networks

A modular approach for the design of two-component supramolecular polymer (SMP) networks is reported. A series of materials was prepared by blending two (macro)monomers based on trifunctional poly(propylene oxide) (PPO) cores that were end-functionalized with hydrogen-bonding 2-ureido-4[1H]pyrimidinone (UPy) groups. One monomer was based on a PPO core with a number-average molecular weight (M_n) of 440 g mol^-1. The SMP formed by this building block is a glassy, brittle material with a glass transition temperature (T_g) of about 86 °C. The second monomer featured a PPO core with an M_n of 3000 g mol^-1. The SMP formed by this building block adopts a microphase-segregated morphology that features a rubbery phase with a T_g of -58 °C and crystalline domains formed by the UPy assemblies, which act as physical cross-links and melt around 90-130 °C. Combining the two components allows access to microphase-segregated blends comprised of a rubbery phase constituted by the high-M_n cores, a glassy phase formed by the low-M_n component, and a crystalline phase formed by UPy groups. This allowed tailoring of the mechanical properties and afforded materials with storage moduli of 37-609 MPa, tensile strengths of 2.0-5.4 MPa, and melt viscosities of as low as 11 Pa s at 140 °C. The materials can be used as reversible adhesives.

Improvement of Mechanical Properties and Self-Healing Efficiency by Ex-Situ Incorporation of TiO2 Nanoparticles to a Waterborne Poly(Urethane-Urea)

This research work was focused on the incorporation of TiO₂ nanoparticles into synthesized solvent-free waterborne poly(urethane-urea) (WPUU) based on hydrophilic poly(ethylene oxide) (PU0) in order to improve both the mechanical properties and self-healing effectiveness of a polymer matrix. The incorporation of TiO₂ nanoparticles resulted in a successful enhancement of the mechanical properties of nanocomposite films when compared to PU0. Simultaneously, the obtained nanocomposite films did not only maintain the self-healing ability of the PU0 film, measured by means of mechanical properties after successive cutting/recovery cycles, but they also showed a higher self-healing efficiency than the PU0 film. Moreover, the well-dispersed TiO₂ nanoparticles, visualized by atomic force microscopy (AFM), kept their conductive properties when embedded in the PU0 matrix, as was confirmed by electrostatic force microscopy (EFM). This research work described a simple and industrially appealing way to control the dispersion of commercially available TiO₂ nanoparticles in waterborne poly(urethane-urea) for the designing of inorganic/organic hybrid nanocomposites with enhanced mechanical properties and self-healing efficiency, in which TiO₂ nanoparticles preserved their conductive properties within the polymer matrix.

Scalable Manufactured Self-Healing Strain Sensors Based on Ion-Intercalated Graphene Nanosheets and Interfacial Coordination

Desirable mechanical strength and self-healing performance are very important to highly sensitive and stretchable sensors to meet their practical applications. However, balancing these two key performance parameters is still a great challenge. Herein, we present a simple, large-scale, and cost-efficient route to fabricate autonomously self-healing strain sensors with satisfactory mechanical properties. Specifically, ion-intercalated mechanical milling was utilized to realize the large-scale preparation of graphene nanosheets (GNs). Then, a well-organized GN-nanostructured network was constructed in a rubber matrix based on interfacial metal-ligand coordination. The resultant nanocomposites show desirable mechanical properties (∼5 times higher than that of control sample without interfacial coordination), excellent self-healing performance (even healable in various harsh conditions, for example, underwater, at subzero temperature or exposed in acidic and alkaline conditions), and ultrahigh sensitivity (gauge factor ≈ 45 573.1). The elaborately designed strain sensors offer a feasible approach for the scalable production of self-healing strain-sensing devices, making it promising for further applications, including artificial skin, smart robotics, and other electrical devices.

The journey of self-healing and shape memory polyurethanes from bench to translational research

In this critical review, we have enlisted a comprehensive summary of different approaches that have been used over the past decade to synthesize self-healing polyurethanes including “close then heal” and “shape memory assisted self-healing” concept.

An Elastic Autonomous Self-Healing Capacitive Sensor Based on a Dynamic Dual Crosslinked Chemical System

Adopting self-healing, robust, and stretchable materials is a promising method to enable next-generation wearable electronic devices, touch screens, and soft robotics. Both elasticity and self-healing are important qualities for substrate materials as they comprise the majority of device components. However, most autonomous self-healing materials reported to date have poor elastic properties, i.e., they possess only modest mechanical strength and recoverability. Here, a substrate material designed is reported based on a combination of dynamic metal-coordinated bonds (β-diketone-europium interaction) and hydrogen bonds together in a multiphase separated network. Importantly, this material is able to undergo self-healing and exhibits excellent elasticity. The polymer network forms a microphase-separated structure and exhibits a high stress at break (≈1.8 MPa) and high fracture strain (≈900%). Additionally, it is observed that the substrate can achieve up to 98% self-healing efficiency after 48 h at 25 °C, without the need of any external stimuli. A stretchable and self-healable dielectric layer is fabricated with a dual-dynamic bonding polymer system and self-healable conductive layers are created using polymer as a matrix for a silver composite. These materials are employed to prepare capacitive sensors to demonstrate a stretchable and self-healable touch pad.

Bioinspired Underwater Adhesives by Using the Supramolecular Toolbox

Nature has developed protein-based adhesives whose underwater performance has attracted much research attention over the last few decades. The adhesive proteins are rich in catechols combined with amphiphilic and ionic features. This combination of features constitutes a supramolecular toolbox, to provide stimuli-responsive processing of the adhesive, to secure strong adhesion to a variety of surfaces, and to control the cohesive properties of the material. Here, the versatile interactions used in adhesives secreted by sandcastle worms and mussels are explored. These biological principles are then put in a broader perspective, and synthetic adhesive systems that are based on different types of supramolecular interactions are summarized. The variety and combinations of interactions that can be used in the design of new adhesive systems are highlighted.

Enhancement of microphase ordering and mechanical properties of supramolecular hydrogen-bonded polyurethane networks

Enhanced thermoreversible and mechanical properties in supramolecular polyurethanes have been realised by the incorporation of flexible DBDI derived hard segments.

Superior Toughness and Fast Self-Healing at Room Temperature Engineered by Transparent Elastomers

The most important properties of self-healing polymers are efficient recovery at room temperature and prolonged durability. However, these two characteristics are contradictory, making it difficult to optimize them simultaneously. Herein, a transparent and easily processable thermoplastic polyurethane (TPU) with the highest reported tensile strength and toughness (6.8 MPa and 26.9 MJ m^-3 , respectively) is prepared. This TPU is superior to reported contemporary room-temperature self-healable materials and conveniently heals within 2 h through facile aromatic disulfide metathesis engineered by hard segment embedded aromatic disulfides. After the TPU film is cut in half and respliced, the mechanical properties recover to more than 75% of those of the virgin sample within 2 h. Hard segments with an asymmetric alicyclic structure are more effective than those with symmetric alicyclic, linear aliphatic, and aromatic structures. An asymmetric structure provides the optimal metathesis efficiency for the embedded aromatic disulfide while preserving the remarkable mechanical properties of TPU, as indicated by rheological and surface investigations. The demonstration of a scratch-detecting electrical sensor coated on a tough TPU film capable of auto-repair at room temperature suggests that this film has potential applications in the wearable electronics industry.

Environmentally-Friendly Synthesis of Carbonate-Type Macrodiols and Preparation of Transparent Self-Healable Thermoplastic Polyurethanes

Carbonate-type macrodiols synthesized by base-catalyzed polycondensation of co-diols and dimethyl carbonate as an environmentally-friendly route were subsequently utilized for the preparation of transparent and self-healable thermoplastic polyurethanes (TPUs) containing a carbonate-type soft segment. Three types of macrodiols, obtained from mono, dual and triple diol-monomers for target molecular weights of 1 and 1.5 kg mol-1, were analyzed by ¹H NMR integration and the OH titration value. Colorless transparent macrodiols in a liquid state at a room temperature of 20 °C were obtained, except the macrodiol from mono 1,6-hexanediol. Before TPU synthesis, macrodiols require pH neutralization to prevent gelation. TPUs synthesized by a solution pre-polymer method with 4,4'-methylene(bisphenyl isocyanate) and 1,4-butanediol as a chain extender exhibited moderate molecular weights, good transparencies and robust mechanical properties. Especially, the incorporation of 3-methyl-1,5-pentanediol within carbonate-type macrodiols enhanced the transparency of the resultant TPUs by decreasing the degree of microphase separation evidenced by ATR-FTIR and DSC. Interestingly, packing density of hard segments and the degree of microphase separation determined the self-healing efficiency of TPUs, which showed good performances in the case of sourced macrodiols from triple diol-monomers.