Self-healing Polymer Networks Research Articles

Additive manufacturing of Diels-Alder self-healing polymers: Separate heating system to enhance mechanical, healing properties and assembly-free smart structures

Over the past decades, self-healing polymers have become increasingly popular due to their unique ability to recover mechanical and functional properties after sustaining structural damage, which significantly extends their lifespan compared to traditional polymers. Material Extrusion (MEX) 3D printing has recently emerged as a possible manufacturing approach for processing self-healing polymers; however, commercial MEX 3D printers lack of the flexibility to fabricate complex and functional structures based on such materials. In this work, an innovative MEX setup for extruding self-healing polymer networks based on a thermo-reversible reaction is presented. The proposed approach is based on the leverage of a separate heating system (SHS), enabling the degelation of the self-healing polymer network into a printable ink. This SHS regulates both the syringe-barrel, and nozzle temperatures during the processing (degelation and extrusion) of self-healing inks, leading to enhanced mechanical performance (Young modulus, tensile strength), and extrusion accuracy of 3D printed structures. The effectiveness of the SHS-based approach is demonstrated by an improved geometrical accuracy (filament deviation reduced by 26 %), which is directly correlated to the mitigation of the extrusion force (variability reduced by 77 %). Moreover, the SHS approach also improved both the mechanical properties and the self-healing performance of the printed parts. Finally, two different self-healing polymers a dielectric and an electrically conductive were extruded in a single manufacturing cycle to fabricate a self-sensing structure. This structure is capable of detecting bending with a sensitivity of 3.10 Ω/degree, even after healing. This paper aims to advance the role of MEX beyond its current limitations by enabling processing of high-quality self-healing structures with embedded sensors.

Cross-scale interface engineering strategy to achieve self-healing antifouling material strength and toughness symbiosis

The incorporation of self-healing technology into antifouling coatings is crucial for the extension of their service life and durability. However, the challenge lies in the insufficient coexistence of strength and toughness hinders their long-term serviceability. Here, this study proposes a cross-scale interfacial engineering strategy for self-healing antifouling coatings inspired by the multiscale microstructure of natural biomaterials The strategy involves the in-situ growth of antifouling zinc-based metal–organic frameworks (Zn-MOF) within a self-healing polymer network constructed by multiple hydrogen bonds. The synergistic cross-scale interfacial interactions between Zn-MOF and the polymer molecules, combined with the intrinsic antifouling properties of Zn-MOF, significantly enhance the strength, toughness to 27.48 MPa, 277.81 MJ/m3, and benefit improving antifouling performance. Moreover, the molecular integration of Zn2+-pyridine coordination optimizes the microphase structure of the polymer matrix and embeds the advantage of its fast damage response, thereby further enhancing the robustness, and reaching self-healing efficiency to 91.74 % at 40 °C for 24 h. Overall, the proposed strategy represents a viable approach for the fabrication of highly resilient and tough self-healing antifouling coatings.

The addition reaction of alcohols and aromatic carbodiimides and its application to produce thermo-reversible polymer networks

The addition reaction of primary alcohols onto aromatic carbodiimides giving iso-ureas was studied in bulk at various temperatures. A side reaction at 60 °C was found to occur between carbodiimide and iso-urea yielding N,N′-substituted polyguanidine oligomers. The occurrence of this side reaction coincides with a reduced conversion of the alcohol; the carbodiimide was always fully converted to either iso-urea or polyguanidine. The formation of the polyguanidine was reversible at higher temperatures to reform iso-urea and carbodiimide. The latter reaction was complete at about 130 °C. The thermo-reversibility of the guanidine bond was utilized to produce thermo-reversible self-healing polymer networks by reaction of polyether diol and a 1.5 to 2-fold molar excess of aromatic biscarbodiimide. The materials were soft and elastic and showed a glass transition at −60 °C. The guanidine linkages in the polymers started to unzip above 60 °C as was indicated by a drop in the E-modulus, at about 130 °C sufficient guanidine crosslinks had unzipped to provide the polymer liquid-like properties. The guanidine crosslinks were reformed after cooling to ambient temperature giving back the polymer network.

Structural Design Strategy of a Biobased Allyl Compound with Dynamic Boronic Ester Bonds to Form Rigid-Soft Self-Healing Polymer Networks via Thiol–Ene “Click” Photopolymerization for Integrated Preparation of a Flexible Device

Structural Design Strategy of a Biobased Allyl Compound with Dynamic Boronic Ester Bonds to Form Rigid-Soft Self-Healing Polymer Networks via Thiol–Ene “Click” Photopolymerization for Integrated Preparation of a Flexible Device

Endowing rubber with intrinsic self-healing properties using thiourea-based polymer.

Self-healing polymers are extensively researched for the sustainability of materials. The introduction of dynamic networks instead of traditional cross-linkers for an autonomous healing mechanism in elastomers is a promising strategy for improving rubber properties. However, exchangeable covalent bonds in a dynamic network generally rely on external stimulants and fillers, which can compromise the material's performance. Herein, we introduce a mechanically strong yet resilient and independent self-healing polymeric network by dual cross-linking of bonds based on covalent and non-covalent dual interaction. The thiourea-based polymer polyether thiourea ethylene glycol (PTUEG) was blended with natural rubber (NR) and epoxidized natural rubber (ENR) to strengthen the mechanical characteristics of the material NR-ENR-PTUEG3. In the material, the thermoplastic polymer PTUEG3 applied the thiourea linkage as a hydrogen bonding and dynamic covalent motif together to enhance mechanical adaptability in a self-healing polymer network exhibiting stiffness, toughness, and resilience, thereby extending its longevity. The resulting mechanical characteristics of the NR-ENR-PTUEG3 with 25 phr PTUEG3 exhibited tensile stress 4.8 ± 0.3 MPa and high elongation at break 833 ± 0.1%, demonstrating far better performance than that of pristine NR, and 85% recovery of its original strength at ambient temperature. The healing behaviour is strongly influenced by thiourea-based polymer contents, enabling autonomous self-healing at ambient temperature, exhibiting in situ load-bearing efficiency in the repaired material, and maintaining their mechanical characteristics.

Room-temperature self-healing polysiloxane networks for highly sensitive piezoresistive pressure sensor with microdome structures

Soft piezoresistive pressure sensor is an important component for next generation flexible electronics. Micropatterning of such sensors significantly improves the pressure sensitivity, however, the short device lifetime due to scratches and motions limits the translation to industrial applications. Herein, we report a novel poly(ODMS-co-MAA) for self-healing piezoresistive sensor to effectively overcoming such shortcoming. Self-healing capability was achieved by incorporating dynamic Fe(III)-carboxylate linkages in the polymer networks. These linkages work as sacrificial bonds and dissipate energy caused by mechanical damage. The bonds subsequently reform and functionality of the sensor can be restored. The poly(ODMS-co-MAA)-based self-healing polymer networks (SPN) were effectively healed from damages showing the recovery of electrical resistance (>95%) within 10 hrs. SPN was fabricated with microdome structures and exhibited a superior pressure sensitivity (2 kPa−1) as well as stability compared to planar features (9.13 × 10−3 kPa−1), demonstrating its excellent potential as pressure sensor. Combined results demonstrate the potential of the self-healing networks for next generation piezoresistive pressure sensors.

Structure–Property Relationships of Self-Healing Polymer Networks Based on Reversible Diels–Alder Chemistry

Structure–Property Relationships of Self-Healing Polymer Networks Based on Reversible Diels–Alder Chemistry

Self-Healing Polymer Network with High Strength, Tunable Properties, and Biocompatibility

Self-Healing Polymer Network with High Strength, Tunable Properties, and Biocompatibility

Ion-conductive self-healing polymer network based on reversible imine bonding for Si electrodes

Si electrodes have attracted considerable attention as materials for next-generation lithium-ion batteries owing to their high theoretical capacity and natural abundance. However, many limitations must be overcome before Si electrodes can be commercialized, particularly the steady degradation in battery performance caused by large volume changes in the active material. Here, an ion-conductive self-healing polymer binder is developed for high-performance silicon electrodes. Glycol chitosan with dialdehyde-terminated polyethylene glycol as a macro-crosslinker is used in a simple process to form the crosslinked polymer network based on imine double bonds, which further improves ion conductivity. Strong and reversible imine double bonds in the crosslinked polymer provide self-healing ability. Si electrodes using the developed polymer network results in an initial Coulombic efficiency of 82.2%, a discharge capacity of 2141 mAh g−1 after 150 cycles, and a reversible capacity of 2700 mAh g−1 at a current density of 3C. These outstanding electrochemical performances demonstrate that the self-healable network and ion-conductive functionality of the developed polymeric binder significantly improves the operation of Si electrodes.

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.

Ion-Conductive Self-Healing Polymer Network Based on Reversible Imine Bonding for Si Electrodes

Lithium-ion battery (LIB), one of the most promising energy storage devices due to higher energy density than other secondary batteries, have continuously developed, and their market has been growing steadily. Currently, the efforts have been produced for using LIB in electric vehicles (EVs) and energy storage systems (ESS) storage beyond small portable devices. Higher energy density and faster charge/discharge rate are required for the practical application of LIB to these large-scale energy devices, and new materials to meet these requirements are hence necessary. Silicon has been reported as an anode material for the next-generation LIB due to its outstanding theoretical capacity and natural abundance. However, there are many limitations in practical application due to the continuous deterioration of the battery performances by enormous volume change of this active material. These problems are mainly originated from the structural deterioration of the electrode causing loss of conductive pathway and the active area for the electrode. Hence, many approaches to securing the structural stability of the Si electrode through various methods, including polymeric binders with good adhesion with Si were developed.In this study, self-healable polymeric binder was designed through a crosslinking polymer network using a reversible imine chemistry. Additionally, the ion conductive polymer was introduced to improve ion conductivities of this self-healable binder. The synthesis, characterization and the electrical properties of the proposed polymeric binder for high-performance silicon electrodes will be discussed in detail.

Diffusion- and Mobility-Controlled Self-Healing Polymer Networks with Dynamic Covalent Bonding

A systematic study of diffusion-controlled reversible Diels–Alder (DA) network formation is performed under both isothermal and nonisothermal reaction conditions based on two amorphous furan–maleim...

Mechanics of light-activated self-healing polymer networks

Optically healable polymers represent an interesting stimuli-responsive self-healing material as the healing process can be controlled on-demand and remotely. The fundamental mechanism of the light-activated interfacial self-healing process has not been theoretically understood. Here, we present a theoretical framework to understand the light-activated interfacial self-healing of soft polymers with light-responsive photophores. We consider that the light propagation through the material matrix triggers the production of free radicals that facilitate the interfacial self-healing process. The self-healing process is considered as a coupled behavior that polymer chains diffuse across the interface and re-form the dynamic bonds assisted by the free radicals. We theoretically relate the light property to the interfacial self-healing strength of the polymers. We predict that the interfacial self-healing strength of the polymer increases with the light illumination time until reaching a plateau. We theoretically explain the effects of the light intensity, light wavelength, and photoinitiator concentration on the self-healing performance. We then apply the theory to two types of soft polymers with inorganic and organic photophores, respectively. The experimentally measured stress–strain behaviors of the original and self-healed samples can be consistently explained by the theory. The experimentally measured relationships between the healing strength and the healing time also agree well with the theoretical results.

Thermally reversible polymer networks for scratch resistance and scratch healing in automotive clear coats

This study investigated the application of a thermally reversible polymer network fabricated using Diels-Alder (DA)/retro-DA (rDA) reactions for use as a scratch-healing automotive clear coat. For this purpose, a new scratch-healing poly(urethane acrylate) network containing a DA adduct unit (DA-CL) was prepared, and its material properties and scratch-healing performance were compared to the properties of a commercial clear coat system (C-CL). The thermally reversible crosslinking and de-crosslinking reactions among the DA-CL, via DA and rDA reactions, were systematically evaluated using in-situ oscillatory rheology coupled with FT-IR spectroscopy. The material properties of the DA-CL and C-CL materials, including the thermal stability, thermal transitions, hardness, and scratch resistance, were measured using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), nanoindentation, and nanoscratch test methods. The scratch-healing performance of the DA-CL was quantitatively characterized and compared to the performance of the C-CL using a nanoscratch tester equipped with an optical microscope (OM) and an atomic force microscope (AFM). The DA-CL polymer network exhibited superior scratch healing and scratch resistance compared to the C-CL polymer network. These data indicated that the DA self-healing polymer network is potentially useful as a scratch-healing clear coat for the automotive industry.

Mechanics of self-healing polymer networks crosslinked by dynamic bonds

Dynamic polymer networks (DPNs) crosslinked by dynamic bonds have received intensive attention because of their special crack-healing capability. Diverse DPNs have been synthesized using a number of dynamic bonds, including dynamic covalent bond, hydrogen bond, ionic bond, metal-ligand coordination, hydrophobic interaction, and others. Despite the promising success in the polymer synthesis, the fundamental understanding of their self-healing mechanics is still at the very beginning. Especially, a general analytical model to understand the interfacial self-healing behaviors of DPNs has not been established. Here, we develop polymer-network based analytical theories that can mechanistically model the constitutive behaviors and interfacial self-healing behaviors of DPNs. We consider that the DPN is composed of interpenetrating networks crosslinked by dynamic bonds. The network chains follow inhomogeneous chain-length distributions and the dynamic bonds obey a force-dependent chemical kinetics. During the self-healing process, we consider the polymer chains diffuse across the interface to reform the dynamic bonds, being modeled by a diffusion-reaction theory. The theories can predict the stress-stretch behaviors of original and self-healed DPNs, as well as the healing strength in a function of healing time. We show that the theoretically predicted healing behaviors can consistently match the documented experimental results of DPNs with various dynamic bonds, including dynamic covalent bonds (diarylbibenzofuranone and olefin metathesis), hydrogen bonds, and ionic bonds. We expect our model to be a powerful tool for the self-healing community to invent, design, understand, and optimize self-healing DPNs with various dynamic bonds.

Polysiloxane/Polystyrene Thermo-Responsive and Self-Healing Polymer Network via Lewis Acid-Lewis Base Pair Formation.

The use of thermo-reversible Lewis Pair (LP) interactions in the formation of transient polymer networks is still greatly underexplored. In this work, we describe the synthesis and characterization of polydimethylsiloxane/polystyrene (PDMS/PS) blends that form dynamic Lewis acid-Lewis base adducts resulting in reversible crosslinks. Linear PS containing 10 mol % of di-2-thienylboryl pendant groups randomly distributed was obtained in a two-step one-pot functionalization reaction from silyl-functionalized PS, while ditelechelic PDMS with pyridyl groups at the chain-termini was directly obtained via thiol-ene “click” chemistry from commercially available vinyl-terminated PDMS. The resulting soft gels, formed after mixing solutions containing the PDMS and PS polymers, behave at room temperature as elastomeric solid-like materials with very high viscosity (47,300 Pa·s). We applied rheological measurements to study the thermal and time dependence of the viscoelastic moduli, and also assessed the reprocessability and self-healing behavior of the dry gel.

Do You Get What You See? Understanding Molecular Self-Healing.

The self-healing ability of self-healing materials is often analyzed using morphologic microscopy images. Here it was possible to show that morphologic information alone is not sufficient to judge the status of a self-healing process and molecular information is required as well. When comparing molecular coherent anti-Stokes Raman scattering (CARS) and morphological laser reflection images during a standard scratch healing test of an intrinsic self-healing polymer network, it was found that the morphologic closing of the scratch and the molecular crosslinking of the material do not take place simultaneously. This important observation can be explained by the fact that the self-healing process of the thiol-ene based polymer network is limited by the mobility of alkene-containing compounds, which can only be monitored by molecular CARS microscopy and not by standard morphological imaging. Additionally, the recorded CARS images indicate a mechanochemical activation of the self-healing material by the scratching/damaging process, which leads to an enhanced self-healing behavior in the vicinity of the scratch.

Eigenvector Centrality is a Metric of Elastomer Modulus, Heterogeneity, and Damage

We present an application of eigenvector centrality to encode the connectivity of polymer networks resolved at the micro- and meso-scopic length scales. This method captures the relative importance of different nodes within the network structure and provides a route toward the development of a statistical mechanics model that correlates connectivity with mechanical response. This scheme may be informed by analytical and semi-analytical models for the network structure, or through direct experimental examination. It may be used to predict the reduction in mechanical performance for heterogeneous materials subjected to specific modes of damage. Here, we develop the method and demonstrate that it leads to the prediction of established trends in elastomers. We also apply the model to the case of a self-healing polymer network reported in the literature, extracting insight about the fraction of bonds broken and re-formed during strain and recovery.

Dynamic sulfur chemistry as a key tool in the design of self-healing polymers

The rich variety of reversible or dynamic covalent chemistries based on sulfur offers a unique opportunity for the design of self-healing polymer networks. The reversibility of such chemical bonds can be used to create soft systems which can self-mend at ambient conditions. Here we focus on the mechanism of three different dynamic sulfur chemistries which have been used for the development of self-healing elastomers and hydrogels: thiolate/nanoparticle exchange, aromatic disulfide exchange and gold(I)-thiolate/disulfide exchange.

Systematic Binder Design in High Capacity Silicon Anodes

Polymeric binder has turned out to be very critical for stable operation of high capacity silicon (Si) anodes, as the binder could stabilize the electrode films even during the large volume change of Si. In this talk, I will present novel approaches for functional binder design. They include 1) the use of mussel-inspired catechol functional group, 2) multi-dimensional cross-linkable hydrogen bonding network, 3) self-healing polymer network, and 4) host-guest interaction network. The series of these investigations suggest the usefulness of noncovalent polymer interactions and the future role of supramolecular chemistry in the binder design.