Self-healing Polymers Research Articles

Cover Image, Volume 139, Issue 16

The cover image by Yufeng Ma and Puyou Jia shows the recent progress of biomass-based self-healing polymers. The self-healing polymer materials derived from biomass resources can reduce the dependence on traditional non-renewable resources such as petroleum. The structures and properties of biomass-based self-healing polymer materials are of great significance for their processing, recycling, and self-healing. The research progress of bio-based self-healing polymer materials based on dynamic non-covalent bonds and dynamic covalent bonds including hydroxyl ester, Schiff base, disulfide bond, and hydroxyl urethane derived from vegetable oil, lignin, cellulose, vanillin, and natural rubber. DOI: 10.1002/app.51977

Highly Flexibility, Powder Self-Healing, and Recyclable Natural Polymer Hydrogels.

Based on the good self-healing ability to repair mechanical damage, self-healing hydrogels have aroused great interest and been extensively applied as functional materials. However, when partial failure of hydrogels caused by breaking or dryness occurs, leading to recycling problems, self-healing hydrogels cannot solve the mentioned defects and have to be abandoned. In this work, a novel recyclable and self-healing natural polymer hydrogel (Chitosan/polymethylacrylic acid-: CMA) was prepared. The CMA hydrogel not only exhibited controlled mechanical properties from 26 kPa to 125 kPa with tensile strain from 1357% to 3012%, but also had good water retaining property, stability and fast self-healing properties in 1 min. More importantly, the CMA hydrogel displayed attractive powder self-healing performance. After drying–powdering treatment, the mentioned abandoned hydrogels could easily rebuild their frame structure to recover their original state and performance in 1 min only by adding a small amount of water, which could significantly prolong their service life. These advantages guarantee the hydrogel can effectively defend against reversible mechanical damage, water loss and partial hydrogel failure, suggesting great potential applications as a recyclable functional hydrogel for biomaterials and electronic materials.

Design of self-healing biodegradable polymers

A biodegradable thermoplastic polymer has been formulated by solubilizing Murexide (M) salts in a commercial biodegradable vinyl alcohol copolymer (HVA). The Murexide has been employed as a self-healing filler with the aim to impart the auto-repair ability to the formulated material. Three different percentages (1, 3, and 5 mass%) of filler have been solubilized in HVA to evaluate the effect of the filler concentration on the thermal and self-healing properties of the resulting polymeric materials. The samples have been thermally characterized by Differential Scanning Calorimetry (DSC) and Thermogravimetric Analyses (TGA), while their self-healing ability has been evaluated through the estimation of the storage modulus recovery, measured by Dynamic Mechanical Analysis (DMA). The results of DSC analysis have highlighted that the increase of the amount of Murexide anticipates the thermal events such as glass transition, crystallization and melting. TGA measurements have evidenced that, although there is a reduction of thermal stability of the materials in the presence of a high concentration of M, the polymer still remains stable up to 270 °C. Healing efficiency higher than 80%, at a temperature beyond 60 °C, has been detected for the samples loaded with 3 and 5 mass% of Murexide, thus confirming the efficacy of this compound as an auto-repair agent and the relationship between the self-healing efficiency and its amount. For a temperature lower than 70 °C, the healing tests, carried out at different values of tensile deformation frequency, have highlighted a frequency-dependent healing efficiency. This dependence becomes negligible at higher temperatures for which the healing efficiency approaches the value of 100%.

Intrinsically self-healing, reprocessable and recyclable epoxy thermosets based on dynamic reversible urea bonds

Developing self-healable, recyclable and reprocessable polymer materials is one of the effective ways to relieve plastic pollution and save resources. Using dynamic reversible hindered urea bonds (HUBs) to construct self-healing materials have become a hot topic to investigate renewable materials. HUBs have a large substituent on its nitrogen atom, which can be dynamically dissociated into related amines and isocyanates without the need for catalysts. Here, integrating HUBs into the epoxy cross-linking network would obtain dynamic covalent self-healing epoxy resin based on large hindered urea bonds (HUBs-EP), which could endow the crosslinked networks with fast reconfigurability under relatively low temperature. In this work, HUBs-EP has optional recyclability (In N, N-dimethylformamide and tetrahydrofuran), good reprocessability (under simple hot-pressing procedure) and rapid self-healing property under 110 °C within 10 min all due to reversible dissociation and recombination of dynamic covalent urea bonds. The extremely simple manufacturing method of dynamically reversible polymers obtained by integrating dynamic covalent bonds into thermoset will give a major hint for the development of sustainable materials.

Self-healing metallo-supramolecular polymers showing luminescence off/on switching based on lanthanide ions and terpyridine moieties

Multi-color luminescence stimulus-responsive materials based on lanthanide (Ln3+) complexes have attracted increasing interest because of their considerable potential applications in various fields. However, enabling materials with rapidly stimulus-responsive property remains a challenge. Herein, luminescent elastomers were prepared via the coordination of Ln3+ ions to nitrogen atoms from terpyridine moieties tethered to the polymer chains composed of polydimethylsiloxane (PDMS) and hexamethylene diisocyanate (HDI) segments. The emitting color of the luminescent elastomers can be simply modulated by changing the excitation wavelengths and altering the Eu3+/Tb3+ molar ratio. The break/recovery of “antenna effect” between Ln3+ and Terpyridyl upon acid-base vapor endow the materials with fast luminescence off/on switching ability. Furthermore, the luminescent elastomers display fast self-healing capability at room temperature due to the attendance of the dynamic Ln-ligand coordination and sufficient mobility of the flexible polymer chains. This work opens up new possibilities for future information encryption to carry more codes by combining the multi-color ability as well as luminescence off/on switching property, and showing the potential application as a powerful tool in the base-acid detector fields.

Ultrasound controlled mechanophore activation in hydrogels for cancer therapy

Mechanophores are molecular motifs that respond to mechanical perturbance with targeted chemical reactions toward desirable changes in material properties. A large variety of mechanophores have been investigated, with applications focusing on functional materials, such as strain/stress sensors, nanolithography, and self-healing polymers, among others. The responses of engineered mechanophores, such as light emittance, change in fluorescence, and generation of free radicals (FRs), have potential for bioimaging and therapy. However, the biomedical applications of mechanophores are not well explored. Herein, we report an invitro demonstration of an FR-generating mechanophore embedded in biocompatible hydrogels for noninvasive cancer therapy. Controlled by high-intensity focused ultrasound (HIFU), a clinically proven therapeutic technique, mechanophores were activated with spatiotemporal precision to generate FRs that converted to reactive oxygen species (ROS) to effectively kill tumor cells. The mechanophore hydrogels exhibited no cytotoxicity under physiological conditions. Upon activation with HIFU sonication, the therapeutic efficacies in killing invitro murine melanoma and breast cancer tumor cells were comparable with lethal doses of H2O2 This process demonstrated the potential for mechanophore-integrated HIFU combination as a noninvasive cancer treatment platform, named "mechanochemical dynamic therapy" (MDT). MDT has two distinct advantages over other noninvasive cancer treatments, such as photodynamic therapy (PDT) and sonodynamic therapy (SDT). 1) MDT is ultrasound based, with larger penetration depth than PDT. 2) MDT does not rely on sonosensitizers or the acoustic cavitation effect, both of which are necessary for SDT. Taking advantage of the strengths of mechanophores and HIFU, MDT can provide noninvasive treatments for diverse cancer types.

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. Recent advances and challenges in the developments toward dynamic SHPs, SMPs, and LCEs are reviewed, focusing on the chemistry strategies and the dynamic reaction mechanisms that enhance the performances of the materials including self-healing, reprocessing, and reprogramming. The different dynamic chemistries and their mechanisms on the enhanced functions of the materials are compared and discussed, where three summary tables are presented: A library of dynamic bonds and the resulting characteristics of the materials. Finally, a critical outline of the unresolved issues and future perspectives on the emerging developments is provided.

Ultra-robust, self-healable and recyclable polyurethane elastomer via a combination of hydrogen bonds, dynamic chemistry, and microphase separation

Flexibility, robustness, transparency, and recyclability are critical to the application of self-healing polymer materials in the field of flexible electronics. However, integrating all the above properties remains a huge challenge to date. In this work, we put forward a facile strategy to prepare polyurethane (PU) elastomer with ultra-high strength and self-healing performance based on hydrogen bonds, disulfide dynamic chemistry, and microphase separation at the same time. Three different self-healing PUs were obtained by introducing disulfide bonds and different types of hydrogen bonds. A robust, transparent, and recyclable PU with amino-terminated chain extender (PUA) with fast and efficient self-healing performance was prepared. The mechanical and self-healing properties of the PUA were effectively balanced by the synergistic effect of reversible interaction of disulfide bonds and the formation of microphase separated structure. The results indicated that the PUA exhibited high transparency up to 90% and excellent mechanical property, e.g. the tensile strength and elongation at break can reach 37.10 MPa and 1080%, respectively. Meanwhile, it can achieve a high self-healing efficiency of 96.8% at 80 °C for 4 h and maintain 84% of the initial mechanical strength even after four times of recycling. Moreover, the colloid graphite/PUA flexible strain sensor was prepared by the combination of colloid graphite and PUA, which can accurately detect both large and tiny scale deformations.

Study of the interfacial adhesion properties of a novel Self-healable siloxane polymer material via molecular dynamics simulation

The reported outstanding adhesive and self-healing properties of a novel siloxane polymer – DOSS, have made it a promising candidate as polymer matrix of thermal interface materials (TIM). To explore its potential use in TIM, here we conduct systematic investigations on the interfacial properties of the DOSS/NiO interfaces at microscopic-scale using molecular dynamics (MD) simulations. MD can provide atomic-scale information which can hardly be obtained using experimental techniques. NiO presents at the surface of the plating layer of copper heat spreader in electronic packaging. DOSS is found to form rich hydrogen bonds with NiO, resulting in stronger adhesion to NiO compared to polydimethylsiloxane (PDMS), a conventional TIM matrix. The greater strength and ductility of DOSS compared to PDMS is ascribed to its rich hydrogen bonds and star-like topology. The strength changes with tensile rate, and is attributed to different strength and relaxation time of the bonded and non-bonded interactions. For ductility, an optimal temperature is found at 375 K. The DOSS has good self-healing ability, which is characterized by an 85% recovery of hydrogen bonds at 450 K. The microscopic insights provided by current work may shed lights on the design of DOSS as future potential polymer matrix to be used in TIM.

Maltotriose-based star polymers as self-healing materials

Self-healing polymers with complementary functional groups can spontaneously repair mechanical damage. We developed star-like block copolymers from a naturally-derived maltotriose core with acrylate arms consisting of soft poly(n-butyl acrylate) (PnBA) and hard poly(tert-butyl acrylate) (PtBA) segments, exhibiting self-repair behavior. A facile and efficient two-step synthesis involving a “core-first” approach employed a simplified electrochemically mediated ATRP (seATRP) with only 25 ppm of a copper catalyst. Precise control was maintained during the polymerization of both acrylates, yielding soft and hard segments. Macromolecules with maltotriose core and polymer arms composed of PnBA homopolymer or poly(n-butyl acrylate)-block-poly(tert-butyl acrylate) (PnBA-b-PtBA) block copolymer had a narrow molecular weight distribution (Mw/Mn = 1.13 and 1.17, respectively). Thermomechanical properties of maltotriose-based star-like copolymers and their linear counterpart were studied by thermogravimetry (TGA), differential scanning calorimetry (DCS) and tensile analysis. The star-like polymers exhibited self-healing behavior. A complete repair of the damaged material occurred after 30 min without external intervention under ambient conditions.

Self-healable fiber-reinforced vitrimer composites: overview and future prospects.

To achieve sustainable development goals, approaches towards the preparation of recyclable and healable polymeric materials is highly attractive. Self-healing polymers and thermosets based on bond-exchangeable dynamic covalent bonds, so called "vitrimers" could be a great effort in this direction. In order to match the industrial importance, enhancement of mechanical strength without sacrificing the bond exchange capability is a challenging issue, however, such concerns can be overcome through the developments of fiber-reinforced vitrimer composites. This article covers the outstanding features of fiber-reinforced vitrimer composites, including their reprocessing, recycling and self-healing properties, together with practical applications and future perspectives of this unique class of materials.

Vapor-phase synthesis of a reagent-free self-healing polymer film with rapid recovery of toughness at room temperature and under ambient conditions

A rapidly self-healable polymer is highly desirable but challenging to achieve. Herein, we developed an elastomeric film with instant self-healing ability within 10 s at room temperature. For this purpose, a series of copolymers of poly(glycidyl methacrylate-co-2-hydroxyethyl acrylate) (poly(GMA-co-HEA), or pGH) were synthesized in the vapor phase via an initiated chemical vapor deposition (iCVD) process. The elastomer includes a large amount of hydroxyl groups in the 2-hydroxyethyl acrylate (HEA) moiety capable of forming rapid, reversible hydrogen bonding at room temperature, while glycidyl methacrylate (GMA) with a rigid methacrylic backbone chain in the copolymer provides mechanical robustness to the elastic copolymer. With the optimized copolymer composition, pGH indeed showed instant recovery of the toughness within a minute; a completely divided specimen could be welded within a minute at room temperature and under ambient conditions simply by placing the pieces in close contact, which showed the outstanding recovery performance of elastic modulus (93.2%) and toughness (15.6 MJ m-3). The rapid toughness recovery without supplementing any external energy or reagents (e.g. light, temperature, or catalyst) at room temperature and under ambient conditions will be useful in future wearable electronics and soft robotics applications.

Insights on surface characterization of 3D printed polymeric parts

For polymeric materials, FDM provides limitations in accuracy and surface finish. Polymeric 3D printed parts have a rough surface because of the filament deposition, texture of the slices laid and staircase effect. Application of surface coatings to these parts may improve mechanical properties, including surface texture and dimensions accuracy. Recent studies indicate development of surface-coated 3D printed polymeric parts primarily for use in automobiles, aviation, ships, food packaging, electrical, and textile applications. Researchers have studied the impact of surface coatings on surface finish and dimensional accuracy. Various processes, such as icing-delay, anti-frosting, and self-cleaning, benefit from super hydrophobic surface. Super hydrophobic surfaces are characterized by important characteristics, like durability and storage, corrosion resistance. Self-healing polymers are emerging as a novel smart material as they possess the ability to heal upon damage without any external mediation. These can appear in a variety of forms, intrinsic or extrinsic, and can be made to take different forms. This paper presents insights into the state of research being carried out in the field of surface characterization especially on 3D printed polymeric parts.

Mechanically robust, shape memory, self-healing and 3D printable thermoreversible cross-linked polymer composites toward conductive and biomimetic skin devices applications

The current work demonstrates excellent thermal, mechanical, shape memory, and self-healing properties of 3D printable, thermoreversible, cross-linked network composites. The thermoreversible cross-linked networks are composed of Diels-Alder (DA) reversible covalent and transient cross-links, which are formed by thermoplastic polyurethane (TPU) and poly(ε-caprolactone) (PCL). Also, cellulose nanocrystalline (CNC) is utilized as a cross-linking agent. The incorporated CNC, modified with furan (CNC-FA) and maleic anhydride groups (CNC-MAH), exhibits excellent compatibility with TPU/PCL matrix, forming physical and chemical cross-linking structures in the composites. Also, the mechanical properties, thermal stability, and self-healing properties are significantly improved after the addition of modified CNC. In addition, the mechanism of performance enhancement is further analyzed by molecular dynamics simulations. One should note that the mechanically robust and 3D printable self-healing composites enable the fabrication of durable conductive devices and biomimetic skin devices with high mechanical integrity, excellent electrical healing and superior strain-induced fluorescence properties, showing promise in next-generation flexible electronics, encryption devices and electronic skins, and may broaden the variety and applications of 3D-printable materials. • The thermoreversible composites are composed of DA reversible covalent and transient crosslinks. • The CNC modified with furan and maleic anhydride groups, exhibit compatibility with polymer matrix. • The mechanism of performance enhancement is further analyzed by molecular dynamics simulations. • The damage could be healed due to the good self-healing capability, thanks to the shape memory effect.

Fast self-healing solid polymer electrolyte with high ionic conductivity for lithium metal batteries

By introducing multiple molecule/intermolecular hydrogen bonds into the polydimethylsiloxane elastomer system, the solid polymeric electrolyte with high ion conductivity and high electrochemical stability obtains the fast self-healing speed.

3D-printing self-healing composite polymer reinforced with carbon nanotubes

EMAA copolymer neutralized with sodium salt is a type of ionomer with self-healing abilities. This polymer has been used as raw material for the manufacturing of nanocomposite material by the addition of multiwall carbon nanotubes (MWCNTs). The optimization of the extrusion process, as well as the mixing and dispersion of the nano-reinforcement in the polymeric matrix during extrusion, deals with the manufacturing of a homogeneous reinforced filament. The obtained filaments were used afterwards as feeding materials of a 3D polymer printing machine and tensile test samples were manufactured. Mechanical properties of both unreinforced and reinforced samples were tested. The MWCNTs-reinforced samples exhibited higher mechanical properties without affecting its self-healing ability.

Self-healing dynamic bond-based robust polyurethane acrylate hybrid polymers

Herein, a self-healing hybrid polyurethane acrylate was prepared by solution polymerization of acrylic monomers in the presence of preformed polyurethane chains containing aliphatic disulfide bonds with terminal isocyanate groups.

Highly stretchable and stretch-induced fluorescence chromism self-healing materials based on boroxine and dynamic imine bond

A new self-healing luminescent polymer obtained by combining double covalent bond of boroxine with imine bond was designed. It demonstrated excellent self-healing, high stretchability and stretch-induced fluorescence chromism from green to cyan.

Highly ionic conductive, self-healable solid polymer electrolyte based on reversibly interlocked macromolecule networks for lithium metal batteries workable at room temperature

A self-healing solid polymer electrolyte based on reversibly interlocking polymer networks (RILNs) with enhanced mechanical properties and ionic conductivity is developed for lithium metal batteries workable at room temperature.

Laboratory Evaluation of a Novel Self-Healable Polymer Gel for Co2 Leakage Remediation During Co2 Storage and Co2 Flooding

Laboratory Evaluation of a Novel Self-Healable Polymer Gel for Co2 Leakage Remediation During Co2 Storage and Co2 Flooding