Self-healing polymers with nanomaterials and nanostructures

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Review

Self-healing polymers with nanomaterials and nanostructures Lei Zhai a,b,c,∗ , Ameya Narkar b , Kollbe Ahn b,∗ a

NanoScience Technology Center, University of Central Florida, Orlando, FL, 32826, USA Department of Chemistry, University of Central Florida, Orlando, FL, 32826, USA c Department of Materials Science and Engineering, University of Central Florida, Orlando, FL, 32826, USA b

a r t i c l e

i n f o

Article history: Received 26 June 2019 Received in revised form 8 October 2019 Accepted 9 December 2019 Available online xxx Keywords: Self-healing polymers Nanomaterials Nanostructure Flexible electronics Energy storage 3D printing

a b s t r a c t Self-healing polymers have attracted a lot of attentions in the past two decades, driven by their intriguing applications, new synthetic approaches and understanding of nanoscale mechanism and discovery of nanomaterials. Nanomaterials and nanostructures in polymers provide large surface area, rich functional groups and unique properties that facilitate the healing process. This review provides an introduction of the key studies from a historical standpoint and the chronological advancement of the design philosophy behind self-healing phenomena of polymers. Recent advance in utilizing nanomaterials and nanostructures to facilitate the healing and introduce novel functionalities in self-healing polymers is extensively reviewed. In addition, innovative characterization methods are employed to analyze and understand the underlying polymer chain interactions occurring at the interface at micro- and nanoscale. The knowledge of the healing process at the nanoscopic level and the contribution of nanomaterials and nanostructures in self-healing has greatly advanced the design, fabrication and application of self-healing polymers. © 2019 Elsevier Ltd. All rights reserved.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 History of self-healing polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Recent progress in self-healing polymers with nanomaterials and nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Nanomaterials improve the efficiency of external stimuli in self-healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Nanomaterials improve the intermolecular interactions in self-healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Nanostructures improve the mechanical properties and healing rate self-healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Characterization methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Atomic force microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Transmission electron microscopy and small angle X-ray scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Stress/strain test and in situ scanning electron microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Surface force apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Applications of self-healing polymers with nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Conclusion and perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Introduction Self-healing in materials science is the ability of a material to repair its damages autonomously and retain its structural integrity [1–12]. The design, investigation and application of self-healing polymers blossomed in 21st century driven by the understanding

∗ Corresponding author. E-mail addresses: [email protected] (L. Zhai), [email protected] (K. Ahn).

of interfacial properties at nanoscale and the advancement of flexible devices. A quick search at Web of Science shows that about 360 papers were published from 1965 to 2000, and more than 5100 papers have been published since 2000 with the subject of “self-healing”. Breaking a polymer involves the rupture of chemical and physical bonds, and the reformation of these bonds results in selfhealing. The concept of self-crack-healing of polymeric materials was reported in 1981 [13] in a study of re-healed and welded glassy polymers under compact tension tests. Self-healing through

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Fig. 1. A summary of dynamic covalent bonds and noncovalent interactions used in self-healing polymers. [18] Printed with permission from Frontiers Media S.A.

chemical approaches [14,15] utilizes dynamic covalent bonds from reactions such as Diels–Alder reaction [16] and disulfide bond formation [17], while self-healing through physical approaches encompasses intermolecular interactions like hydrogen bonds and metal to ligands interactions [4,7,9,10,18–23] (Fig. 1). The selfhealing through chemical and physical routes usually is achieved through an autonomic or nonautonomic path. In the autonomic path, the damaged polymer uses chemical potentials (e.g., chemical reactions and reformation of intermolecular interactions) spontaneously to facilitate the repair, while an external stimulus (e.g., heat, light and magnetic field) is used to initiate chemical reactions or physical interactions for self-healing processes in the nonautomonic path. Since the healing of polymers occurs from molecular scale damages to macroscopic damages, it is important to recognize the multiple length scale of heterogeneities. The formation of chemical bonds and physical interactions and the movement of polymer chains and nanomaterials (e.g., nanofillers and fibers) happen at molecular or nanoscopic scale. The molecular scale interface is one of the most important factors in a healing process. Understanding the interfacial properties (e.g., surface area and molecule movement) at nanoscale and using nanomaterials to facilitate the healing process have significantly contributed to the advancement of selfhealing polymeric materials. Recently, nanomaterials [24–31] and nanostructures (e.g. nanodomains in block copolymers) [32–37] have demonstrated great potential in facilitating self-healing attributed to their novel functionalities and large surface area. Theoretical simulations have demonstrated that nanoparticles imbedded in compatible polymer matrices migrate autonomously to the crack (air/polymer interface) driven by entropy [38,39]. The simulation was later experimentally supported by the same research group showing the migration and clustering of the embedded nanoparticles around the cracks in a multilayered composite structure [39]. Incorporating nanomaterials in self-healing sys-

tems grants large interfacial surface area, enhanced electrical and mechanical properties, improved the response to external stimuli, and increase the conversion efficiency of electromagnetic energy to heat. Self-healing polymers with nanostructures have been used in electronic devices such as “skin-like” sensors [12,40–42], energy harvest and storage devices [43–49], advanced coatings [50,51] among others. The intrinsic self-healing is usually evaluated by comparing the mechanical properties of a self-healed specimen (or an area of the self-healed specimen) to that of a pristine specimen (or an area of the pristine specimen). Recently, more advanced characterization methods and instruments have been developed to monitor and understand the self-healing process. In this review article, we briefly overview the history of self-healing polymers research, followed by an extensive review of recent progress in nanoscopic approaches in preparation and characterization of selfhealing polymers. The applications of nanostructured self-healing polymers, e.g., applying nanomaterials and nanostructures, are also discussed. History of self-healing polymers The concept of welding polymers through interfacial molecular diffusion was established in the early 1950s. Early studies laid the foundation not only for self-healing polymers but also for modern polymer science. Bueche and coworkers studied the relationship between self-diffusion constant and bulk viscosity of polystyrene and pure poly(n-butyl acrylate) in 1952 [52]. They established a mathematical relationship between the diffusion constant (D) and bulk viscosity (␩), and predicted   the diffusion constant from the viscosity [D

 

=

 AkT  36

R2 M

], where A is Avagodro’s number, k

is Boltzmann’s constant, ␳ is polymer density, T is the absolute temperature, M is the molecular weight, and R2 is the average square end to end distance of a single polymer chain. The study

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of the self-healing properties of poly(vinyl acetate) (PVA) in the 1970s revealed that the increase of strength during the relaxation time was directly proportional to the healing temperature [53–55]. In the 1980’s, the self-healing efficiency of polymethylmethacrylate (PMMA) was greatly improved when the temperature was close to the glass transition temperature (Tg ) due to the accelerated diffusion occurring at the interface [13]. The fracture mechanics involved crack formation and crack-healing was more pronounced slightly above its Tg associated with a diffusional interpenetration of chain segments [13,56]. Soon after the concept of self-healing was proposed, Wool and O’connor hypothesized that micron-level crack healing phenomenon in polymers involved five stages, rearrangement of polymer chains on the two bisected surfaces, alignment of the two surfaces (orientation of polymer chains) at contact, wetting of two surfaces after contact, diffusion and interpenetration of the cleaved chains and eventual randomization to eliminate the initial configuration memory of the crack interface [57]. This model has been widely accepted although the model has not been experimentally or theoretically demonstrated. From 1970’s to1990’s, majority of the intrinsic self-healing occurred due to the diffusion of the polymer chains caused by providing heat as the stimulus [58–62]. Diffusion leads to the formation of new entanglements, eventually healing the defect. Two studies involving methanol [63] and ethanol [64] induced crack-healing of PMMA demonstrated the impact of swelling on the self-healing. Extensive swelling of ethanol interfered with the healing process and the fracture morphology of the healed polymer was different from that of the virgin PMMA whereas the methanol treated PMMA (less swelling) showed a comparable tensile fracture stress to virgin PMMA. The first autonomic healing of polymer was reported by Carolyn Dry in a study of healing a polymer matrix triggered by a healing reagent released from embedded fibers when ruptured [65]. White and coworkers followed the concept and designed a extrinsic self-healing polymer using liquid healing agents (monomer and catalyst) embedded in microcapsules (Fig. 2a) [1]. The self-healing process started when the crack ruptured the embedded microcapsules to release the healing agent. The liquid healing agent filled the crack by a capillary force and polymerized in the crack to re-heal. This approach using fillers/containers of size in microns and nanometers has been used extensively to prepare self-healing coatings [66–74]. Li and coworkers have developed interesting nanocontainers that can release a curing agent upon external stimuli [73]. In this study, porous silica nanotubes were coated with stimuli responsive polymers such as poly(methyl acylic acid), poly(N-isopropylacrylamide), and poly(ethylene glycol methacrylate), leading to pH-, temperature-, and redox-dependent release of the monomers encapsulated in silica nanotubes, respectively. The major limitation of the rupture-induced-release self-healing approach is the capacity to rebuild the polymer after further damage occurs repeatedly because the stored healing agents cannot be refilled [1,5,6,75,76]. In addition, the copper(I)- catalyzed azide/alkyne cycloaddition (CuAAC) “click” reaction [77,78], was utilized to self-heal polymers based on the concept of re-healing through the rupture of microcapsules where healing agents (multivalent alkynes and azides) were encapsulated separately in a polymer matrix with embedded copper (I) catalyst [79,80]. To overcome the limitation of one-time only healing, bio-inspired microchannels have been developed for a precise delivery of healing agents [2,3,81–83]. Reversible bonds (or dynamic covalent bonds) provide an approach to achieve multiple self-healing or repeated damage recoveries (Fig. 2b). Otto Diels and Kurt Alder were awarded the Nobel prize in 1950 in Chemistry for the discovery of DielsAlder (DA) reaction. The DA reaction involves the coupling of a

3

“diene” with a “dienophile”. Stevens and Jenkins pioneered the concept of a thermally-reversible polymer network employing a DA reaction [84] where an addition of diene and dienophile happened upon heating. While the DA reaction has been utilized extensively in self-healing [85–89], other reversible covalent bond formation reactions such as disulfide bonds [17,90,91] and alkoxyamine units as reversible covalent bonds [92] have also been studied. The formation of covalent bonds usually requires catalysts or external stimuli (e.g., heat and light) to overcome the activation energy. Utilizing dynamic molecular interactions (e.g., hydrogen bonds, van der Waals forces, electrostatic interactions and ligands/metal interactions) leads to a spontaneous and inherent self-healing [34,93–96]. The research on the dynamic interactions has been greatly advanced by the design and synthesis of supramolecular polymers [31,96–103]. One example of using the dynamic molecular interactions is marine mussels that use unique chemistry in their byssus to cling to mineral surfaces at intertidal zone [104–106]. The chemistry has inspired the study of underwater adhesion and self-healing of polymeric materials. One of the popular chemical features among many key elements that mussels use in their byssus is catechol chemistry. High amount of L-3,4-dihydroxyphenylalanine (DOPA), found in the proteins in mussel byssus, contributes to the variety of unique adhesion, self-assembly, self-healing and toughening mechanism of mussel byssus in wet conditions [107]. Catechol chemistry has been widely exploited to design underwater self-healing of polymers [69,104,108–111]. Ahn and coworkers introduced underwater selfhealing of rigid acrylate/methacrylate polymers (non-hydrogel) triggered by intermolecular hydrogen bonds of catechols. Catechol groups initiate self-healing in wet conditions overcoming the limitation posed by hydration layer at the damaged surfaces [108]. It was shown that the strong adhesion between two surfaces of catechol-containing poly(methylacrylates) could be engineered to improve self-healing performance in aqueous media. To confirm the significant contribution of the interfacial H-bonds, one of the two polymer surfaces was intentionally oxidized to quinone, and the other left in its reduced state; maximum adhesion to trigger the self-healing was observed in a given short contact time (5 s) as a result of hydrogen bonding between catechol (donor) and quinone (acceptor). Particular emphasis was placed on the molecular mobility associated with the rearrangements of the rather compliant polymer chains after and the time required for these rearrangements. Catechol-metal coordination chemistry has also studied extensively for self-healing of hydrogels using a four-arms PEG [106] and self-recovery of toughness of epoxy polymers [112]. In this study, tris-iron-catechol complexes were prepared at a basic pH as a dynamic crosslinker, which broke at strain and re-formed at relaxation. Self-healing process includes surface molecule rearrangement, surface contact and wetting, chain inter-diffusion and randomization or/and chemical interactions according to Wool and O’Connor model [57]. Recently, nanomaterials and nanoscale structures have attracted much attention in facilitating the proposed healing steps such as surface contact and chain diffusion. Nanomaterials and nanostructures have demonstrated exceptional ability to facilitate the self-healing process, improve the properties and introduce novel functionality to polymers. Generally, self-healing of polymers with nanomaterials and nanostructures is accomplished through three routes: nanomaterials facilitating self-healing through improved response to external stimuli, functionalized nanomaterials enabling self-healing through the formation of covalent bonds and intermolecular interactions, and nanostructures assisting self-healing through intermolecular interactions.

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Fig. 2. Schematic illustration of intrinsically self-healing polymer systems (a) through exhaustion of healing agents (encapsulated microcontainers) and (b) with reversible chemical bonds. [47] Printed with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Recent progress in self-healing polymers with nanomaterials and nanostructures Nanomaterials improve the efficiency of external stimuli in self-healing Non-autonomous self-healing relies on external stimuli such as heat, light, magnetic fields to provide activation energy for the formation of covalent bonds or intermolecular interactions [90,113–119]. The unique properties of nanomaterials such as surface plasmon resonance and infrared (IR) absorption can greatly improve the efficiency of external stimuli in a self-healing process by converting laser irradiation to heat. For example, graphene nanosheets and silver nanowires were used in polyurethane to initiate the Diels-Alder healing chemistry upon an IR irradiation [120,121]. It was demonstrated that graphene nanoribbons could effectively convert the IR irradiation to thermal energy. 1 % of graphene in polyurethane increased the temperature from 30 ◦ C to 225 ◦ C in 20 s upon an IR laser irradiation (980 nm, 200 mW, 3 mm spot diameter). The DA healing mechanism was that DA products underwent a retro-Diels − Alder (rDA) reaction at higher temperature, the disconnected furan and maleimide moieties migrated to the fractured area, reacted and rebuilt the network via a DA reaction during cooling down to room temperature. The initial mechanical properties were restored with more than 96 % healing efficiency after one-minute irradiation time. Metallic nanomaterials like gold nanoparticles and silver nanowires have demonstrated excellent heating capability upon laser irradiation because of surface plasmon resonance [122,123]. Incorporating metallic nanomaterials as “nanoheaters” is another effective approach to heal polymers through laser irradiation [124–126]. A silver nanowire (AgNW) aerogel framework was used to build a high-performance stretchable conductor with selfhealing capability by filling the aerogel with N-isopropylacrylamide (NIPAM) and N,N’-bis(acryloyl)cystamine (BACA) followed by a polymerization that generated a hydrogel conformal coatings on AgNWs [124]. The authors demonstrated that, putting two separated pieces of hydrogel in a close contact, they were able to self-heal naturally in less than 1 min upon an exposed to a NIR laser (Fig. 3a). AgNWs had a photothermal effect upon NIR irradiation because of the surface plasmon resonance, resulting in a fast temperature increase in the hydrogel. The temperature of the fractured joints of the hydrogel increased from 23.4–59.3 ◦ C in one minute (Fig. 3b). The heat triggered the Ag/sulfide bond forma-

tion between AgNWs and BACA molecules, and healed the hydrogel with a healing efficiency of nearly 93 % (Fig. 3d). Besides generating heat through laser irradiation using nanomaterials, magnetic or metallic nanoparticles can also act as a heat source under electromagnetic fields through Joule heating [127]. Magnetic Fe3 O4 cubic nanoparticles were mixed with ionomeric elastomers containing poly (n-bultylacylate-b-acylic acid) and zinc ions [128]. The ionomeric elastomer can self-healing through the reformation of zinc/carboxylic acid interactions where the kinetic energy of the ions and the polymers have a significant effect on the healing efficiency. Adding magnetic nanoparticles to ionomeric elastomers greatly improved the healing efficiency by increasing the kinetic energy around the nanoparticles under a magnetic field. The polymer with 0.05 vol percent nanoparticles restore 90 % of original strength after 10 min under the employed field conditions (250 kHz, 31.5 kAm−1 ) treatment. In contrast, it took 30 min for a pure ionomeric elastomer to restore 90 % of its original strength [129]. Nanomaterials improve the intermolecular interactions in self-healing Nanomaterials provide large surface area to attach functional groups that facilitate the healing through the formation of covalent bonds or intermolecular interactions. Carbon nanotubes (CNTs), graphene and graphene derivative such as graphene oxide (GO) have been extensively investigated in self-healing because of their good mechanical properties and functional groups (e.g. carboxylate and hydroxyl) on the surface. The functionalization of CNTs and GO usually starts with converting carboxylic acid groups on their surfaces to more reactive acylchloride groups using thionylchloride. The targeted functional groups including furfuryl groups for DA reactions [130] and amine groups for hydrogen bonding [131] are attached to CNT and GO surfaces through the formation of amide bonds with acylchloride groups. A covalently bonded and reversibly cross-linked multiwalled carbon nanotube/ styrene-butadiene rubber (MWCNT/SBR) composite were produced through a DA reaction of furfuryl modified styrene-butadiene rubber (SBR-FS) and furfuryl functionalized MWCNT with bismaleimide. The furfuryl functionalized MWCNT demonstrated dual function of reinforcing and improving healing in the rubber composites [130]. Bao and coworkers produced a self-healing elastic nanocomposite by combining the unique features of hydrogen-bonded polymer and graphene oxide (GO) with highly branched amine groups as a macro-

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Fig. 3. (a) Photographs showing NIR laser-induced healing process of a separated AgNW aerogel-based ternary network (AATN) hydrogel. The healed sample presented high stretchability. (b) Temperature changes of AATN hydrogel with 60 s of NIR laser irradiation. Scale bars, 1 cm. (c) Schematic illustrations of healing mechanism of the AATN hydrogel through the dynamic Ag–S interaction induced surface reconstruction under the laser. (d) Tensile stress–strain curves of the original and healed AATN hydrogels. The inset showing the healed AATN hydrogel lightening up the lamp under stretched deformation. [124] Printed with permission from Springer Nature Limited.

crosslinker. Adding less than 2 wt% of GO to the polymer produced an elastic material (elastomer) with similar mechanical property to that of conventional rubbers. This composite could restore 80 % of its initial extensibility in 10 min while the pure polymer could only restore 50 % [131]. Hydrogen bonds and ligands/metal ion interactions have been extensively used to fabricate hydrogel based self-healing polymeric systems with nanomaterials. A self-healing hydrogel system with a combined synergistic “soft and hard” hierarchical network structure demonstrated high mechanical strength, toughness, and stretchability [132–134]. Nanomaterials in self-healing polymers not only improve the healing efficiency but also introduce interesting responsive properties that lead to intriguing self-healing flexible electronics [21,41–43,45,47,48,120,132,133,135–140]. For example, Wan and coworkers have designed a self-healing hydrogel containing a “soft” homogeneous polymer network via covalent cross-linking of poly(vinyl alcohol) (PVA) and poly(vinylpyrrolidone) (PVP), and embedded “hard” Fe3+ crosslinked cellulose nanocrystals (CNCs) network as nanoreinforcing domains. The autonomous self-healing ability of the hydrogel was attributed to the reorganization of CNCs and Fe3+ via ionic coordination on the notch surface [132]. The hydrogel also changed its ionic conductivity upon stretching because the deformation of the hierarchically porous network inside hydrogels altered the pathway of ionic transport. Zhang and coworkers have synthesized self-healing MXene hydrogels through a simple mixing MXene nanosheets with a commercially available low-cost hydrogel namely “crystal clay,” which is composed of PVA, water, and antidehydration additives [133]. MXenes are a new family of 2D materials recently developed with the ternary layered carbides and nitrides. MXenes combine the

properties of ceramics and metals and exhibit high hardness and melting points, high stability, and large surface area [141]. Titanium carbide MXene (Ti3 C2 Tx ) nanosheets had serval significant contribution to the functional hydrogel. First, the addition of MXene nanosheet greatly improved the polymer stretchability from 2200 % (no MXene) to 3400 % (Fig. 4b). Negatively charged MXene formed a homogenous mixture with PVA, creating a densely entangled polymer network structure which increased the elastic modulus and toughness of the composite hydrogel. Secondly, MXene facilitated the self-healing property of PVA hydrogel derived from the hydrogen bonding between the −OH groups. The abundant surface functional groups on MXene provided additional hydrogen bonding sites (Fig. 4c). Thirdly, the electrical conductivity of MXene granted an interesting sensing capability of the composite hydrogel that could conveniently detect motion direction and speed on its surface. The 3D transmission electron microscopy (TEM) images show that randomly oriented MXene nanosheets interconnected with each other through a surface-edge contact in the hydrogel matrix (Fig. 4d-f). The system has demonstrated advanced sensing applications in detecting subtle changes in motion speed and traces, such as handwriting, facial expressions, and vocal signals with high accuracy and sensitivity. Carbon nanotube sheets have been used with self-healing elastomers [96] to make sandwich-structure lithium ion battery electrodes [142]. The nanotube sheets offered high electrical conductivity and mechanical stability, and facilitated the self-healing of the electrodes. It was observed that the aligned CNT composite electrode showed a much higher self-healing performance than the random CNT composite electrode due to a better reconnection for the broken aligned CNTs. The self-healing was demonstrated

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Fig. 4. Characterizations of M-hydrogel. (a) TEM of MXene nanosheets; shown in the upper right is the HRTEM image of a typical nanosheet, and shown in the lower right is the corresponding FFT. (b) Photographs demonstrating the stretchability of M-hydrogel. (c) Photographs depicting the self-healing capability of the M-hydrogel: two cut pieces (top), once gently touched (middle), and showing retention of the original stretchability of M-hydrogel (bottom). (d) TEM image of M-hydrogel. (e) 3D tomography image based on the entire area in (d) and (f) zoomed-in 3D tomography image of a selected volume marked with red box in (e). [133] Printed with permission from American Association for the Advancement of Science.

Fig. 5. Schematic illustration of the preparation of CNC/CNT/XNBR composites and proposed structures. [140] Printed with permission from Royal Society of Chemistry.

by restoring electrical conductivity over ten cutting-healing cycles. The tensile strengths of these hydrogel electrodes were ∼ 92 and 94 % of the original value after the 3rd and 5th healing cycles, respectively. Zhang and coworkers have produced self-healing strain sensors from commercially available XNBR latex (containing 33 wt% acrylonitrile and 7 wt% carboxylic groups) with cellulose nanocrystals (CNCs) and CNTs through a simple mixing and hot-

pressing method [140]. Fig. 5 shows the schematic fabrication procedure of NSCE composites where CNCs were functionalized with polyethylenimine (PEI) and mixed with CNTs. The XNBR latex was added to a CNC/CNT aqueous suspension followed by filtration and hot-pressing. It was found that CNCs stabilized CNTs and formed a segregated nanostructured conductive network in rubber latex, generating a hierarchical nanostructure and supramolecular

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network. The conductivity of NSCE composites with a nanostructured conductive network was 7 orders of magnitude higher than that of the homogeneously dispersed CNTs/XNBR composites. The dynamic hydrogen bonds in CNC/CNT network also granted higher tensile strength (3.7 MPa) than CNTs/XNBR composites (0.9 MPa) and crosslinked CNTs/XNBR composites (1.4 MPa). The dynamic hydrogen bond in CNC/CNT led to high healing efficiency. The healed sample recovered about 83 % of its original strength, 80 % of its original elongation at break and retained the original electrical conductivity. Nanostructures improve the mechanical properties and healing rate self-healing A trade-off between mechanical properties and dynamic healing is a dilemma in self-healing systems. Strong interactions provide tough polymers but limit molecular movement, impeding autonomous healing. On the other hand, weak interactions afford fast healing, but yield relatively weak materials. The introduction of a mobile phase in an elastomer, which is essential for self-recovery, usually decreases its mechanical strength. A novel concept of “phase-locked dynamic bonds” where the highly dynamic chemical bonds are protected by the viscoelastic hard domains has been proposed to solve such dilemma. Naoko Yoshie’s group has designed an ABA triblock copolymer to form hard domains with dense crosslinks in a softer matrix with sparse dynamic crosslinks [143]. A-block was a short homopolymer (PU) of a 2ureido-4[1H]-pyrimidinone (UPy)-functionalized norbornene and the B-block was a long random copolymer consisting of UPyfunctionalized norbornene and a flexible dodecanyl norbornene. The hard A-blocks prevented macroscopic deformation and preserved recoverability, while the B-blocks formed a softer matrix with sparse and dynamic crosslinks that served as sacrificial bonds to dissipate energy. It was demonstrated that the tough elastomer was able to recover around 83 % area of the hysteresis loop during the second loading-unloading regime. This was attributed to the mobility provided by the end polyurethane functionalities which acted as sacrificial crosslinks and dissipated the energy during deformation. A scratch on the surface of the elastomer healed in 3 h at 50 ◦ C. A post-healing mechanical test revealed that the elastomer did not show significant recovery of mechanical properties. Since the authors could not quantify the depth of the crack made, the mechanical properties were not further analyzed. Through controlled living polymerization, Guan and coworkers have designed block copolymers that can generate nanoscale hard domains and soft domains to achieve fast selfhealing and maintain good mechanical properties [34,36,144,145]. For example, a block copolymer containing polystyrene (PS) backbone with polyacrylate amide (PA-amide) brushes that phase separated into nanostructured hard (PS) and soft (PA-amide) phase could self-heal through hydrogen bonding. When the rupture of the polymer resulted in the breakage hydrogen bonds in the soft phase, spontaneous self-healing occurred with the reformation of hydrogen bonds [34]. Such dynamic hydrogen bonding offered spontaneously self-heal over time under ambient conditions without any treatment. 90 % percent of the original extensibility could be recovered after 24 h. Similarly, a self-healing polyurethane containing stable and inert soft segments and active hard segments with dynamic aliphatic disulfides and strong hydrogen bonding interactions was reported [146]. Poly(tetramethylene ether glycol) (PTMEG) with a moderate molecular weight (Mn = 1000) was used as the soft segment in the system, polyurethanes were grown from both ends of PTMEG using hydrogenated 4,4’-methylenediphenyl diisocyanate (HMDI) and aliphatic disulfide bis(2-hydroxyethyl) disulfide (HEDS) as the hard segments (Fig. 6a). The atomic force microscope (AFM) phase images of the samples with 8 and

7

9 wt percent of sulfur (i.e., PUDS8 and PUDS9) show nanoscale phase separation of soft and hard domains (Fig. 6b). Four samples (PUDS4, PUDS6, PUDS8, and PUDS9) were used to evaluate self-healing capability. The PUDS6 sample displayed the fastest self-healing, while the PUDS8 sample reached a higher level selfhealing efficiency of up to 86.4 % at 70 ◦ C for 6 h. The self-healing ability of the polymer mainly depended on the disulfide content and the interaction of hydrogen bonds between different polymer chains. Polymers with more hard reactive domains had lower healing rate but higher recovery of the mechanical properties. Block copolymers containing different compositions of hard block (polystyrene) and soft block with self-healing capability (poly (n-butylacrylate) with terpyridine units) were used to study the respective contribution of the hard and soft blocks of a metal-ligand containing block copolymer to the self-healing behavior [32]. As shown in Table 1, four types of polymer systems with controlled ratio of polystyrene, poly(n-butylacrylate), terpyrindine and metal ion (Mn vs. Ni) were synthesized. The block copolymers complexed with manganese(II) chloride to introduce transient crosslinks, but formed permanent crosslinks with nickel nitrate. The first observation was that the inclusion of soft blocks and reversible units in the polymer structure allowed a higher global mobility (lower Tg), which is a primary requirement for an efficient healing system. The low Tg of MP3 was due to the phase separation that led to nm-thick PS layers. The rheology study showed that MP1, MP2 and MP3 had a net change in the elastic and viscous behaviors with a relatively slower/delayed terminal relaxation process, while MP4 had no terminal relaxation because of irreversible crosslinked nature of the nickel(II) nitrate containing network. MP1 showed complete healing within 20 min. MP2 showed complete scratch healing in 45 min, while it took MP3 only 5 min to heal. The difference in the healing ability was attributed to the increased fraction of the soft block. MP4 and P4 did not self-heal because of irreversible crosslink and the lack of metal/ion crosslink, respectively. Very recently, Urban and coworkers reported a “key-and-lock” commodity self-healing copolymers completely based on van der Waals (vdW) interactions [93]. Copolymers with different ratio of methyl methacrylate (M) and butyl acrylate (B) were investigated to understand the impact of polymer structures on self-healing efficiency. Both experimental data and theoretical modeling indicated that copolymers with alternative A and B units were more effective in self-healing than block copolymers. It was proposed that “key-and-lock” interactions of interdigitating alkyl pendant groups between chains generated a viscoelastic response that promoted self-recovery of neighboring chains upon separation. This discovery suggests that fundamental features of macromolecules (e.g., chain conformation and nanostructures) are crucial in designing self-healing polymers utilizing different intermolecular interactions. Revealing the molecular movement and phase separation at nanoscale is important to understand new self-healing mechanism.

Characterization methods Typically, macroscopic characterization methods like rheometry, and tensile testing are used to evaluate the bulk mechanical properties while FT-IR spectroscopy and Raman spectroscopy,are used to monitor the molecular interaction and bonds alternation before and after self-healing [147]. Optical and electron imaging technology including laser reflection imaging, optical microscopy and scanning electron microscopy provides the information of healing process of surface scratches [148]. However, some of these methods yield only qualitative results, other tech-

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Fig. 6. The multiphase design and mechanism of tougher and more robust self-healing thermoplastic elastomers. B. AFM phase images of PUDS8 and PUDS9 show nanoscale hard domains and soft domains. [146] Printed with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Table 1 List of polymer structures and Tg. Hard vs. soft ratio

Metal salt

Tg [◦ C]

P1

–

MnCl2

11

MP2

P2

1:1

MnCl2

25

MP3

P3

1:2

MnCl2

−5

MP4

P3

1:2

Ni(NO3 )2

16

–

P4

–

–

72

Metallo-polymer

Polymerused

MP1

Polymer structure

niques cannot be used independently to monitor the changes in the micro/nanostructure. Recent advance in nanoscience and characterization techniques have significant contribution to the understanding of nanostructures in self-healing polymers and the healing process at the interfaces. For example, transmission electron microscopy (TEM) has been used to reveal three-dimensional nanostructures in polymers [133]. Surface forces apparatus (SFA) and atomic force microscopy (AFM) have demonstrated the ability to probe the pristine [146] as well as the damaged material at the rupture [149] and allowed for a dynamic monitoring of the healing process.

Atomic force microscopy Generally, the AFM tip can be used to induce damage, as well as continuously monitor the healing process down to the nanometer scale [74,148–150]. For example, the self-healing of poly(n-butyl acrylate) grafted star polymers with disulfide functional groups was continuously monitored using AFM by Matyjaszewski’s group [149]. A self-healing mechanism was proposed based on studying the healing process of films with different thickness and cut sizes. Molecules flew towards the center of a cut due to the surface tension driven viscoelastic restoration. Such movement was

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Fig. 7. IRIRI (upper) images of OXE-CHI-PUR networks recorded at different UV exposure time. (a–c) image captured at 1542 cm−1 . (d–f). image captured at 1348 cm−1 . [115] Printed with permission from the American Association for the Advancement of Science

determined by cross-linked nature of the polymer (i.e. molecule mobility) and exposed surface area of the cut (i.e. molecule availability). Healing reactions such as thiol–disulfide exchange reaction, disulfide–disulfide exchange reaction, and reduction of thiols to disulfides occur when the molecules at two sides of the cut contact each other. A thick film can self-heal wider cuts because of larger exposed area. Although AFM can provide topological information in nanoscale, it cannot obtain any information about chemical composition of the location. Urban’s group have used localized micro-attenuated total reflectance (ATR) Fourier transform IR (FTIR) spectroscopy and internal reflection IR imaging (IRIRI) to monitor the local chemical reactions [93,151–153]. A time-based self-healing of a scratched surface of a polyurethane network based on an oxetane-substituted chitosan (OXE-CHI-PUR) was performed using IRIRI with one micron spatial resolution [115]. The scratched samples were exposed to UV-light and IR absorption change at 1542 cm−1 (N H bending of PUR) (Fig. 7a-c) and 1348 cm−1 (CH2 wagging of ether linkages of the OXE ring caused by ring-opening reaction of the oxetane ring) (Fig. 7d-f) were monitored. The study suggested that the cut broke the PUR amide bonds and opened the rings of oxetane while the UV exposure regenerated these bonds and healed the polymer. It is obvious that AFM combined with IR can provide local information of chemical composition at nanoscale resolution, and has significant contribution in monitoring self-healing process. AFM-IR systems have been developed and used to survey the chemical composition at nanoscale [154,155]. In an AFM-IR system, an attenuated total reflectance (ATR) accessory is combined with an AFM tip such that the sample is in simultaneous contact with the tip as well as the IR laser. A brief excitation of the tip by the thermal expansion of the sample caused impulse of the laser creates oscillations in the cantilever beam which are recorded in conjunction with the IR spectra. Raman spectroscopy Raman spectroscopy can identify the chemical bonds of the materials and weak interactions. Spatial distribution of vibrational frequencies obtained using Raman spectroscopy after inducing

mechanical damage can be used to monitor the self-healing process via the assessment of the formation of new bonds or intermolecular interactions, thereby reveal the interfacial chemistry and the rate of self-healing. In addition to traditional Raman spectroscopy, several advanced technologies are used in studying the healing process. For example, two-dimensional Raman correlation spectroscopy like Fourier Transform temperature dependent Raman spectroscopy was used to study chemical reactions initiated by temperatures [156,157], where the spectra were collected at different time after heat was applied to the sample. The temperature-dependent spectral dataset was analyzed by the correlation procedure either based on the Fourier- or the Hilbert–Noda-transformation [158]. The correlation analysis compared the deviation of spectra from a predefined reference state and produced synchronous and asynchronous correlation spectra. The synchronous spectrum provides the information about which spectral features change “in-phase”, while the asynchronous spectrum shows which changes take place “out-of-phase”. This analytical method provides an in situ monitoring of chemical bond evolutions like DA reaction [156] and halogen bonds formation [157] after the broken polymer samples were heated. Although morphological imaging is the most common method to track the healing process, it can only provide morphology information on the surface, but cannot show the chemical bond formation. Coherent anti-Stokes Raman scattering (CARS) is a powerful tool to locate the position of a specific compound and functional groups in situ [159]. CARS was used side-by-side with morphological laser reflection imaging to monitor a healing process of an intrinsic self-healing polymer network through thiol-ene bonds formation [160]. It was observed that the morphologic closing of the scratch and the molecular crosslinking of the material did not take place simultaneously. Fig. 8a and b clearly show that the crosslink is not completed in the red square region but Fig. 8c shows a complete healing on the surface. It was obvious that the reformation of the thiol-ene based polymer network took time to complete after the molecule made contact in the scratch. A standard morphological imaging observed the physical contact of the polymers, but the reaction could only be monitored by molecular CARS microscopy.

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Fig. 8. Raman-resonant CARS (a) and laser reflection (c) images of a scratch within a thiol-ene crosslinked polymer network after healing 6 min. (b) is false color CARS images of the self-healing scratch after healing for 6 min. The red square in (a) and (c) highlights a region where it appears that the scratch is healing faster in the reflection. [160] Printed with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Transmission electron microscopy and small angle X-ray scattering TEM and small angle X-ray scattering (SAXS) have been combined to study the nanostructures in self-healing polymers, especially block copolymers that involve phase separation. For example, the phase separation of poly(styrene-b-polyacrylate amide) block copolymers were clear revealed by the diffraction peak in SAXS and TEM images of the polymer film [34]. In another study, SAXS was used to study the phase separation of poly(styrene-b-n-butylacrylate) and provided the evidence that the phase-separation can affect the Tg-value of the block copolymer with different block size [32]. In a study of the heat-induced self-healing of crosslinked metallosupramolecular copolymers of terpyridine and alkyl methacyrlates [161]. molecular-level properties were probed using SAXS, which was used to verify the hypothesis that ionic clusters (iron-bisterpyridine) played an important role in self-healing. The self-healable copolymers based on butyl methacrylate and lauryl methacrylate exhibited SAXS signal at 2 theta = 1.44◦ , corresponding to the correlation distance between of 6.4 nm between iron crosslinked features. In contrast, the copolymer based on methyl methacylate could not heal upon heating and did not have SAXS signal, suggesting the absence of the ionic clusters.

Stress/strain test and in situ scanning electron microscopy Stress/strain test is a general method to evaluate the healing efficiency by comparing the mechanical performance between pristine samples and healed samples [41,120,124,131,133,162]. For example, Bao, Son and coworkers have produced an self-healing electrical conductive polymer composites using PDMS4,4 -methylenebis(phenyl urea) (MPU)0.4 -isophorone bisurea units (IU)0.6 ) and two-dimensional silver nanoflakes [42]. Stress/strain tested were performed on pristine samples, the samples self-healed under different conditions and the self-healed samples with and without encapsulation of the pure self-healing polymer. Fig. 9a and b clearly show that the sample healing under elevated temperature can recover about 90 % of the mechanical properties of the pristine sample and the encapsulated sample is more flexible than the bare composite sample. In addition, in situ scanning electron microscopy (in situ SEM) has been used to understand the function of silver nanoflakes in restoring electrical properties of the nanocomposite conductor after stretching (Fig. 9c). It was discovered that after silver nanoflakes were separated upon stretching, the stress-relaxation of the dynamically cross-linked self-healing

polymer caused silver nanoflakes to regain the contact. This example demonstrates that in situ SEM has great potential in revealing the contribution of nanomaterials in self-healing process. Surface force apparatus Surface force apparatus (SFA), pioneered by Israelachvili’s group, has been used to measure the intermolecular forces between two interfaces in a dry or wet environment [163]. To study the interfacial forces between two surfaces upon contact, polymer films of interest are deposited on two mica substrates and two films are put together, separated and brought back to recontact. In this process, the distance between the surfaces, the shape of the interface, and the refractive index of the media between the surfaces are determined accurately and simultaneously by Multiple Beam Interferometry (MBI) in the SFA [164]. By monitoring the motion of “fringes of equal chromatic order” (FECO) when the sample surfaces are separated and brought back together symmetrically and unsymmetrically, the adhesive and cohesive force can be distinguished. On the other hand, the adhesive and cohesive force are undistinguishable on surfaces with Root Mean Square (RMS) roughness larger than 10 nm. Therefore, mica is generally used as a substrate in SFA. SFA can control normal (compressive or tensile) load and measure transverse (frictional) forces, allowing a dynamic examination of the interaction between the functional groups at the contact interface, and monitoring adhesion/cohesion changes related to self-healing at the interface at a nanoscopic (or molecular scale) level. Data obtained from SFA at different contact time scales accurately determine reflects the molecular mobility (also measured with dynamic contact angle measurement) at the interface [108]. Combining SFA data with tensile testing data enables the understanding to the self-healing process at the interfaces. In addition, the near-edge X-ray absorption fine structure (NEXAFS) spectroscopy confirmed the presence of H-bond at the interface, which corroborates SFA data showing H-bond formation between catecholic functional groups at the interface, which trigger the selfhealing of the polymer [108]. Applications of self-healing polymers with nanomaterials Self-healing polymer nanocomposites have demonstrated interesting properties has new sustainable, safer and longer lasting materials for applications including energy storage devices, [44,135,139,165,166] flexible electronics [41,167–169], actuators [170], structural materials and corrosion protecting coatings [19,171]. Incorporating conductive nanomaterials in flexible

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Fig. 9. Stress/strain characteristics of (a) pristine samples and the samples healed under different conditions and (b) self-healed samples with and without encapsulation with pure self-healing polymer. (c) in situ SEM images of the nanocomposite conductor before, just stretched to 1000 % strain, and 20 h of rearrangement after 1000 % stretched. Magnified images in green-and orange-dashed regions show rearrangement of Ag flakes. [42] Printed with permission from American Chemical Society.

self-healing polymers has realized various types interesting applications such as electric-skin [118,133,172–182]. Taking the previously discussed self-healing MXene hydrogels as an example [133], the electrical conductivity of the hydrogel increased upon stretching and compression because of the reduction of the distance between MXene sheets. The MXene hydrogel was used to fabricate motion sensors based on the change of electrical conductivity with compression. Energy storage devices such as supercapacitors and lithium ion batteries often use polymeric electrolytes, which makes self-healing polymers intriguing candidates for the fabrication of self-healing energy storage devices [44,135,139,165,166]. For example, Yu, Cong and coworkers have built highly stretchable and self-healable supercapacitors by sandwiching an electrolyte hydrogel of polypyrrole-incorporated gold nanoparticles and poly(acrylamide) between two hydrogel electrodes consisting of polypyrrole-incorporated gold nanoparticle, CNTs and poly(acrylamide) (GCP@PPy). Ag nanowire films were deposited on the hydrogel electrodes as the current collector. The self-healing capability of the device was attributed to the NIR laser or elec-

tricity induced dynamic interactions between the sulfide groups on N,N-bis(acryloyl)cystamine (crosslinked with poly(acrylamide) in the hydrogel) and gold nanoparticles (Fig. 10a). It was reported that cut samples had more than 90 % healing efficiency through a NIR irradiation or heating by passing electricity. The energy storage recovery efficiency was evaluated by the cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) curves. The electrochemical performances of the healed supercapacitor before and after the cutting/healing process under NIR irradiation (Fig. 10bd) and electrical heating (Fig. 10e-g). Electrochemical impedance spectroscopy (EIS) of cut-contact and healed samples (Fig. 10d) shows that electrical conductivity was almost fully recovered after first heal and increased a little after 10th heal. The CV and GCD profiles had minor shift before and after different cutting/healing cycles, suggesting no obvious degradation in specific capacitance. The supercapacitor also demonstrated outstanding performance under bending and stretching, maintaining more than 80 % of original specific capacitance after being bent 180 degrees for 1000 times or under 800 % strain. Gao and coworkers have used reduced graphene oxide (rGO) in fabricating a self-healing all-fiber-

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Fig. 10. Multiresponsive healing performance of the supercapacitor device. a) Schematic illustration of the real-time healing mechanism of the device during the chargedischarge process. Notably, the M-SR triggered interface reconstruction takes place at the crack region when stimulated by the large-electrical-resistance-induced high temperature. b–d) NIR-laser-irradiation-induced healing process over ten cutting–healing cycles: b) CV curves at a scan rate of 50 mV s−1 , c) GCD curves at a current density of 10 mA cm−2 , and d) EIS spectra. e–g) Electrically induced healing process over ten cutting–healing cycles: e) CV curves at a scan rate of 50 mV s−1 , f) GCD curves at a current density of 10 mA cm−2 , and g) areal capacitance and healing efficiency during GCD cycles at 10 mA cm−2 . h) Optical images show good electrical conductivity of the healed device. i) Optical images show two pieces of supercapacitors connected in series to power a lamp. [166] Printed with permission from WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

based quasi-solid-state lithium-ion battery [139]. As illustrated in Fig. 11, the cathode fiber was produced by freeze drying a mixture of LiCoO2 nanoparticles and rGO in a tube and the anode fiber was produced by drying a mixture of SnO2 quantum dots and rGO in a tube. The rod-like gel electrolyte was produced using poly (vinylidenefluoride-co-hexafluoropropylene) (PVDF-co-HFP) soaked in LiClO4 with ethylene carbonate and diethyl carbonate and a self-healable carboxylated polyurethane (PU) as a package layer. The self-healing of the broken battery was realized by the formation of hydrogen bonds on rGO. The intersheet stacking and binding of rGO nanosheets granted intrinsic plasticity and flexibility that enhanced the robust architecture for the binder-free electrode and kept the integrity of the electrode during deformation such as bending and twisting. The self-healing electrodes showed 82.6 mA h g−1

after a complex deformation and retained 50.1 mA h g−1 after 5th healing process at a current density of 0.1 A g−1 , suggesting a capacity retention of 82.2 % and 50.3 %, respectively. These two examples among many others clearly demonstrate that high flexibility and self-healing properties have grant these energy storage devices great potential in wearable electronics. A self-healing, stretchable and transparent energy-harnessing triboelectric nanogenerator was produced by sandwiching a buckled silver nanowires/poly(3,4-ethylenedioxythiophene) composite electrode with self-healable (Ag-PEDOT) poly(dimethylsiloxane) (PDMS) elastomers. The self-healing PDMS was produced through a reaction between bis(amine)-terminated PDMS and 1,3,5-triformylbenzene (molar ratio is about 1:1). The imine bonds cleaved when the polymer was cut, and hydrolyzed

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Fig. 11. (a) Schematic illustration of the fabrication procedure of an all-fiber-based LIB. (b) Cycling performance of the quasi-solid-state LIB at straight, bending and twisting states. (c) Cyclic performance of self-healing LIB with different healing times at a current density of 0.1 A g−1 . [139] Printed with permission from Elsevier.

back to aldehyde and amine, which regenerated imine bonds and cured the damage in the healing process. More interestingly, the composite film was used to produce electricity by pressing the film which caused the electrification at the Ag-PEDOT/PDMS interface, generating electrostatic charges with opposite signs at the two surfaces. It was reported that the composite film recovered 94 % of original mechanical properties and 100 % electricity generation capability after healing from a cut [183]. Self-healing polymers have been extensively investigated in corrosion protection applications [66,69,184–188]. Generally, reactive healing agents are encapsulated separately in micro- or nanocontainers and released to react and heal the coatings when the capsules are erupted by fractures [66,187,189]. Shchukin and coworkers have developed a pH trigged self-healing corrosion protection coatings using polydopamine coated silica nanoparticles as nanocontainers of benzotriazole, a steel corrosion inhibitor [69]. Polydopamine has played two roles in this system. First, polydopamine was responsive to a pH change and enabled the release of benzotriazole when the local pH decreased because of corrosion. Second, polydopamin formed a solid complex with ferric ions produced by the corrosion and healed the cut. Gao and coworkers encapsulated photo-responsive healing agents (isphenol, an epoxy acrylate resin, trimethylolpropane-triacrylate, and Irgacure 184) in poly(urea-formaldehyde)/TiO2 nanoparticle hybrid microcapsules [190]. TiO2 nanoparticles functioned as UV blockers to keep heal-

ing agents from reacting. These microcapsules were then mixed into EPON 828 epoxy polymer and coated onto a steel plate. When the cut of the polymer coating broke the microcapsules, the healing agents were released into crack, exposed to UV light and solidify to heal the cut. Anti-corrosion testing indicated that the coatings were able to self-heal in 30 s upon exposing the damaged polymercoated substrates to UV light. Conclusion and perspective The investigation of self-healing properties of polymers has been mostly limited to testing of mechanical properties before and after the damage. Moreover, this evaluation is primarily performed at the macro and very rarely at the micro or nanoscale. There are enormous opportunities to explore the mechanism of self-healing at the smaller scales and apply the knowledge to make advanced materials, and we have only discovered the tip of the iceberg with the aid of recent advances in nanotechnology. A better understanding of the underlying chemical interactions at the interface of the crack (damage site) would provide an insight into the possible means that could be used to restore the materials’ pristine properties. Nanoscale characterization instruments such as AFMIR and AFM-Raman that can monitor the healing process in situ will have significant contribution to the understanding of healing mechanism at interfaces.

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The ideal self-healing polymer has excellent mechanical properties and is able to restore its original mechanical properties quickly after breakage. Strong intermolecular interactions provide tough polymers but limit molecule movement and slow down the healing process. On the other hand, weak interactions afford fast healing, but yield relatively weak materials. Traditional one-component self-healing polymers always face the dilemma of strength and healing speed. Enabled by controlled polymerization and nanoscale characterization, copolymer systems that contain “soft and hard” hierarchical network structure have greatly improved the healing efficiency while still maintained the mechanical properties. In these polymers, the hard domains contribute to the strength while the soft domains contribute to healing through the reformation of directional interactions such as hydrogen bond and ion-metal interactions. Various types of nanomaterial such as nanoparticles, nanowires, graphene, CNTs, and 2D materials have been produced and studied in the past two decades. When incorporated with self-healing polymers, nanomaterials provide large surface area, intriguing optical, electrical and mechanical properties, and surface functionalities. Nanomaterials facilitate

self-healing through improving the response to external stimuli and providing surface functional groups for the formation of covalent bonds and intermolecular interactions. In addition, nanomaterials have demonstrated the capability of improving the strength of polymer matrix and giving exciting properties to the composites, leading to a variety of potential applications in selfhealing flexible electronics, energy harvesting and storage devices, and advanced coatings. Three-dimensional printing (3DP) has greatly advanced the manufacturing capability of producing hierarchically complex architectures, which are not always achievable by conventional manufacturing technologies. 3DP is an additive manufacturing method of printing solid objects layer-by-layer from a virtual computer model, which offers operational diversity and simplicity. 3DP has been used to produce various components for the applications in education, medicine, electronics, robotics, construction, aerospace, and etc. [191–203]. Fig. 12 summarizes common 3DP methods used in producing polymer samples [201], where crosslinking is required in some 3DP technologies to convert liquid inks into solid samples. For example, one or more laser beams are

Fig. 12. Schematic diagrams showing different 3DP processes: (a) laser assisted Stereolithography (SLA), (b) laser assisted Selective Laser Sintering (SLS), (c) inkjet printing, (d) Fused Deposition Modeling (FDM), and (e) Liquid Deposition Modeling (LDM). [201] Printed with permission from American Chemical Society.

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Lei Zhai is a professor at the Department of Chemistry and NanoScience Technology Center at the University of Central Florida (UCF). He received his Ph.D. in Chemistry from Carnegie Mellon University in 2002, and worked as a postdoctoral researcher at Massachusetts Institute of Technology before he joined UCF in 2005. His research focuses on the self-assembly of polymers, producing polyelectrolyte nanostructures, and polymer derived ceramic composites. He is a recipient of NSF CAREER Award, is a Scialog Fellow (Research Corporation for Science Advancement), and has received the Outstanding Chemist Award at American Chemical Society (ACS) Orlando Section in 2013. Prof. Zhai is the associate editor of Materials, and was the Chair of ACS Orlando Section. Ameya Narkar received his PhD degree from Michigan Technological University, USA, in 2018. He is currently a Postdoctoral Associate in the Department of Chemistry at the University of Central Florida, under the supervision of Dr. Kollbe Ahn. His research interests involve the understanding and application of bioinspired adhesive materials for biomedical applications.

Kollbe Ahn has received his PhD from Kansas State University. During his postdoctoral appointment at the University of California, Santa Barbara, he first documented the self-healing of rigid polymers in aqueous media mediated by catechol-dependent hydrogen bonding at damaged interfaces in addition to many reports about mussel-inspired self-assemblies and underwater adhesives. Currently, Kollbe Ahn is an assistant professor at the University of Central Florida. His research group continues replicating unique properties of marine sessile organisms in synthetic materials including dynamic molecular interaction for self-healing and selfrecoverable polymer networks.

Please cite this article as: L. Zhai, A. Narkar and K. Ahn, Self-healing polymers with nanomaterials and nanostructures, Nano Today, https://doi.org/10.1016/j.nantod.2019.100826