Notch-Insensitive, Ultrastretchable, Efficient Self-Healing

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Notch-Insensitive, Ultrastretchable, Efficient Self-Healing Supramolecular Polymers Constructed from Multiphase Active Hydrogen Bonds for Electronic Applications JianHua Xu, Peng Chen, JiaWen Wu, Po Hu, YongSheng Fu, Wei Jiang, and JiaJun Fu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02136 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

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Chemistry of Materials

Notch-Insensitive, Ultrastretchable, Efficient Self-Healing Supramolecular Polymers Constructed from Multiphase Active Hydrogen Bonds for Electronic Applications JianHua Xu,† Peng Chen,‡ JiaWen Wu,† Po Hu,† YongSheng Fu,‡ Wei Jiang,§ and JiaJun Fu*,† †

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, People’s Republic of China ‡

Key Laboratory for Soft Chemistry and Functional Materials of Ministry Education, Nanjing University of Science and Technology, Nanjing, 210094, People’s Republic of China §

National Special Superfine Powder Engineering Technology Research Center, Nanjing University of Science and Technology, 210094, People’s Republic of China ABSTRACT: Self-healing polymers with microphase-separated structure are plagued with inferior self-healing efficiency at room temperature due to lack of dynamic interactions in hard domains. Herein, we describe a novel strategy of multiphase active hydrogen bonds (H-Bonds), toward realizing fast and efficient self-healing at room temperature, even under harsh conditions. The core conception is to incorporate thiourea moieties into microphase-separated polyurea network to form multistrength H-bonds, which destroy the crystallization of hard domains, and at the same time, inserted the dynamic reversible H-bonds in both hard and soft segments, accounting for the surprisingly self-healing performances. The synthesized polymeric material, PDMS-MPI-TM completely recovers all the mechanical properties within 4 h at room temperature after rupture. Significantly, self-healing process can also take place at low temperature (restoration with 85% efficiency in 48 h at -20℃) or in the water (restoration with 95% efficiency in 4 h). Depending on the cleavage/re-formation of multiphase H-bonds, the material exhibits the unprecedented ultrastrechability and notch-insensitiveness. It can be stretched up to 31500% without fracture, and reach notch-insensitive stretching up to 18000%. These exceptional characteristics inspired us to fabricate highly stretchable self-healable underwater conductor and protective self-healing film for suppressing shuttling of polysulfides and preventing crack propagation in S cathode, which provide the pathway for applications in underwater electronic devices or advanced Li-S batteries.

INTRODUCTION Man-made self-healing polymeric materials (SHPMs) capable of recovery of mechanical properties and restoration of original functionalities when wounded, are of great significance to prolong service life, enhance durability and reduce maintenance costs.1-6 With the development of biomimetics and materials science, a variety of well-designed SHPMs have emerged in the past decades and fascinated the interdisciplinary attentions because of their wide potential applications, spanning from traditional protective products to latest fashionable consumer electronics.7-12 In contrast to the chemo-mechanical self-healing SHPMs based on microcapsules or vascular networks,5 the SHPMs constructed via incorporating dynamic covalent bonds or supramolecular noncovalent bonds realize theoretically infinite repairing, which are more similar to the behavior of biosystems.13-15 More importantly, benefiting from the regeneration of surface topography and chemical integrity, they have successfully restored some important functionalities, such as, conductivity,16 superhydrophobicity17 antibiofouling,18 sensing,19 and energy storage.20 Self-healing

efficiency of mechanical properties is the basis of functional restoration. The incomplete recovery of mechanical properties is likely to raise concerns about reliability of functionalities in practical use. Furthermore, if microcracks cannot be remedied in time, healing components on fracture surface maybe undergo undesired chemical/physical transformation (such as, annihilation of free radicals,21 saturation of hydrogen bonds,22 reduction of disulfide bonds,23 etc.), preventing further repair and making self-healing meaningless. Therefore, more attentions should be paid to enhance self-healing efficiency and accelerate healing process, especially under working conditions, such as at room temperature, in water or other solvent, etc., ensuring safety of products. Many design strategies have been proposed and demonstrated to prepare versatile SHPMs with diverse functions.24-26 Taking the balance of contradiction between mechanical properties and dynamic healing into account, microphase-separated structure is frequently used.27-32 Guan and co-colleagues pioneered a thermoplastic elastomer based on multiphase design, in which polystyrene as hard segment provided mechanical strength and polyacrylateamide as soft segment containing hydrogen bonds

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Figure 1. Design strategy and structure of notch-insensitive, ultrastretchable, efficient self-healing PDMS-MPI-TM supramolecular polymer. (a) Molecular design and chemical structure of PDMS-MPI, PDMS-MPI-TM elastomer. (b) Schematic illustration of the proposed mechanism for healing efficient and superior stretchable PDMS-MPI-TM. (H-bonds) was responsible for self-healing tasks.27 This material combines high strength and room-temperature self-healing ability. However, its healing efficiency calculated based on toughness is relatively low. When scientists are aware that the reason for inferior healing efficiency is lack of supramolecular interactions in hard segments, microphase-separated polyurethane or polyurea (PU) have been put forward and developed, where both dispersed hard phase and continuous macromolecular soft phase contain sufficient H-bonds.28 According to this strategy, Bao and co-colleagues placed multistrength H-bonds into PU network, and thus yielding supertough, healable materials.30 Noticeably, a healing efficiency of only 78% was reported after 48 h at room temperature, which can be ascribed to crystallization of hard segment, impeding the activities of hydrogen bonds. Dong and co-colleagues introduced phase-locked dynamic bonds design, where the dynamic disulfide bonds were embedded in hard segments to prevent crystallization by steric hindrance.31 Unfortunately, the exchange of aliphatic disulfide cannot occur at room temperature, needing high temperature (70 °C) for fast elimination of scratches. Taken together, how to endow microphase-separated structure with autonomous, rapid and efficient self-healing characteristics via reasonable design and make them efficiently work under harsh conditions is highly desired and still full of challenge. In this work, we raised the novel design concept to convert crystalline and non-healable polyurea (PU) matrix to fast and efficient self-healing supramolecular polymer at room temperature (Figure 1a and 1b). Upon damage, all the mechanical properties can be restored with ～100% healing efficiency for only 4 h at room temperature. Further more,

the healing process can also take place at -20 ℃ or in water. Significantly, relying on cleavage/reformation of multiphase H-bonds, supramolecular polymer also exhibits ultrastretchability and extreme notch-insensitiveness. Such unique features make our self-healing supramolecular polymer applicable in various situations, especially where it can be used to construct highly stretchable and flexible conductor that can restore its mechanical properties and electrical conductivity underwater, or used as a functional interlayer for Li-S batteries to suppress the notable “shuttle effect” and overcome the short cycle-life.

RESULTS AND DISCUSSION Material design and characterization. In the present study, bis(3-aminopropyl)-terminated poly(dimethylsiloxane) (NH2-PDMS-NH2, Mn=5000) and 4,4’-methylenebis(phenyl isocyanate) (MPI) at a 1:1 molar ratio were used to fabricate translucent PU elastomer (PDMS-MPI, Figure 1a, Figure S1). In Figure 2a, atomic force microscopy (AFM) phase images of PDMS-MPI display the distinct microphase-separated structure with the average size of hard domains of 9.19±1.58 μm. The formation of hard domains is ascribed to the close packing of 4,4’-methylenebis(phenyl urea) (MPU) moieties during the process of gradual evaporation of chloroform at room temperature, in which H-bonds (urea units) and π-π interactions (4,4’-methylenebis phenyl units) co-exist.34 Indeed, within MPU moieties, the π-π stacking between aromatic rings greatly increase the intermolecular interactions within hard domains, as evidenced by the quantum chemical calculation results (Figure S2). It is believed that the strong intermolecular interactions is beneficial to the formation of distinct microphase-separated structure. The microphase


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Figure 2. Characterization of PDMS-MPI-TM supramolecular polymer. AFM images of (a) PDMS-MPI and (b) PDMS-MPITM. (c) SAXS diffraction and the corresponding 2D-GISAXS pattern of PDMS-MPI, PDMS-TM and PDMS-MPI-TM. (d) DSC curves of PDMS-MPI and PDMS-MPI-TM. (e) DFT-optimized structure of different hydrogen bonds constructed by MPU and TU units. All DFT calculations were performed at the ωB97X-D/6-31 G(d,p) level of theory. (f) Synchronous 2D correlation spectra of PDMS-MPI-TM calculated from 20-60 °C. The different boxes represent correlation peaks of MPU and TU. structure is also verified by small-angle X-ray diffraction (SAXS, Figure 2c). A single, broad scattering peak at around q～0.98 nm-1 manifests the characteristic of weakly ordered microphase separation in nanoscale, which is assigned to the molecular arrangements.33 The presence of endothermic melting peak at 106.3 °C and exothermic crystallization peak at 63.6 °C in differential scanning calorimetry (DSC) curve (Figure 2d) suggests the crystallization of PDMS-MPI polymer, which derives from regular packing of MPU units.9 PDMS-MPI could not self-heal in either scratch recovery tests or resplicing tests at room temperature (Figure S3). It is reasonable to hypothesize that the tightly packed hard segments reduce the activities of Hbonds, depriving PDMS-MPI of self-healing1. Herein, using PDMS-MPI as a template, we propose a novel self-healing design strategy of ‘multiphase active hydrogen bonds’. Briefly, the appropriate amount of 1,1’-thiocarbonyldiimidazole (TM) (molar ratio PDMS:MPI:TM=10:4:6) was added to participate in condensation process of PDMS-MPI and thereby inserting thiourea (TU) moieties into PDMS-MPI chains through nucleophilic substitution reaction. PDMS-MPI-TM film was obtained via solvent casting method (Figure S4). Detailed polymerization procedure, Fourier transform infrared spectroscopy (FT-IR), 1H nuclear magnetic resonance (1HNMR), gel permeation chromatography (GPC), thermal gravity (TG) and gas-chromatograph-mass spectroscopy (GC-MS) data are included in Supplementary Information (Figure S5-S9). The conception of this design is to implant

dynamic reversible H-bonds in soft segments and active originally inert hydrogen bonds in hard segments simultaneously. As expected, the PDMS-MPI-TM film demonstrates outstanding self-healing performances (Figure 1b). To confirm our conception, quantum chemical calculations and two-dimensional infrared (2D-IR) correlation spectroscopy technique were employed to investigate the influence of thiourea moieties on molecular interactions. The optimized complex structures between TU and MPU calculated by density functional density are illustrated in Figure 2e and the interaction energies are shown below the complexes. Judging from the bond distances between electron-rich atoms and the hydrogen atom, which are shorter than the sum of the van der Waals atomic radii, we can infer that thiourea-induced multistrength H-bonds are formed. The 2D-IR spectra further support the theoretical calculation results. Figure 2f shows the synchronous 2D-IR spectrum based on autocorrelation calculations of temperature-dependent FT-IR spectra in the regions of 1200-1700 cm-1 and 3000-3800 cm-1, where the absorption peak at 3371, 3300, 1652, and 1445 cm-1 are assigned to vN-Hurea, vN-Hthiourea, vC=O, and vC=S bonds, respectively. Four correlation cross peaks, Φ(1652, 3371), Φ(1652, 3300), Φ(1445, 3371) and Φ(1445, 3300) are apparent in synchronous 2D-IR spectrum, whereas disappear in asynchronous 2D-IR spectrum (Figure S10), demonstrating the existence of the thioureabased intramolecular/intermolecular H-bonds. 35 The newly generated molecular interactions change network structure. Although PDMS-MPI-TM film held microphase



separated morphology, the hard domain size dramatically reduced to nanometer (Figure 2b). It is not easy to recognize the peak in SAXS profile, meanings the long-length order is vanished. The absence of endothermic/exothermic peaks in DSC curve establishes the destruction of crystallization (Figure 2d)1. The comparative analyses clearly point out the fundamental causes for the reversing changes in self-healing: (i) constructing a zigzag thiourea H-bonded arrays in soft segments,36 which readily break and reform; (ii) destroying the regular crystallized hard phase via formation intermolecular H-bonds between thiourea and urea groups, embedding dynamic interactions in hard segments. Mechanical properties. The mechanical properties of PDMS-MPI-TM film were measured under ambient temperature(20 °C and ~35% relative humidity) using a tensile testing at a deformation rate of 100 mm min-1. The stressstrain curve contains an initial stiffen region corresponding Young’s modulus of 53.4 kPa, after emerging of a yield point at a strain of 136%, a gradual decrease of stress with

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the increase of strain to limitation of tensile machine (Figure. 3a). Impressively, PDMS-MPI-TM film exhibits extraordinary stretchability that can be stretched up to >315 its initial length without rupture. The stretched film at a strain of 31500% could still withstand violently transversal fluctuation, manifesting the potential for further extension (Figure 3b, Movie S1).16 Moreover, it can well resist finger compression or knife cutting (Figure S11) and sustain substantial load in creep experiments (Figure S12), demonstrating its tough nature.25 To the best of our knowledge, PDMS-MPI-TM film possesses the highest strain-at-break so far among self-healable polymers(Figure 3c).8,19,25,37-39 To clarify the reasons for ultrastretchability, its parent materials were first studied. As shown in Figure 3a, PDMS-MPI film crosslinked by strong hydrogen bonds andπ-π interactions owns a high Young’s modulus of 1.45 MPa and low strain-at-break of 954%, whereas PDMS-TM film assembled via weak hydrogen bonds is a viscous fluid with no appreciable mechanical properties. Similar mechanisms in terms of mixing strong/weak supramolecular bonds for fabrication of highly stretchable elastomers have been utilized in previous literatures.37,38 In view of this, we attribute

Figure 3. Mechanical properties of PDMS-MPI-TM supramolecular polymer. (a) Tensile curves of PDMS-MPI-TM and PDMS-MPI; insert pictures showing the basic properties of PDMS-TM. (b) Photographs of a PDMS-MPI-TM film before stretching, showing superior stretchability to >31500%. (c) Comparison of the break-at-strain of PDMS-MPI-TM with reported highly stretchable and self-healable polymer. (d) Tensile curves of PDMS-MPI-TM and PDMS-MPI-CM; insert picture summarizing their mechanical properties (e) Stress relaxation curves of PDMS-MPI-TM and PDMS-MPI-CM films to 100% strain at 25 °C. (f) A notched PDMS-MPI-TM film showing notch insensitive ability that can be stretched to ～18000%. (g) Tensile curves of PDMS-MPI-TM under different deformation rate from 100 to 1000 mm min-1.


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the unprecedented stretchability to the synergistic combination of fast inter-exchange of multiphase H-bonded pairs and persistent slippage of folded polymeric chains. We designed control experiment and subsequently employed stress-relaxation and DFT calculations to protrude the special role of thiourea in ultrastretchability. 1,1’-carbonyldiimidazole (CM) was chosen to replace TM, and the strain-at-break of resultant PDMS-MPI-CM film is about 17821%, far inferior to PDMS-MPI-TM (Figure 3d, Movie S2). The stress relaxation data show that the rate of stress relaxation of PDMS-MPI-TM is faster than that of PDMSMPI-CM, providing strong evidence that thiourea moiety makes exchange of multiphase H-bonds more easily than urea moiety when stretching H-bonded network (Figure 3e).16,24 Moreover, DFT calculations show that the bonding strengths of H-bonds within hard domains of PDMS-MPICM are larger than that of PDMS-MPI-TM (Figure S13). The crosslinkages of PDMS-MPI-CM owning the relatively higher bonding strength result in the more robust mechanical strength, as confirmed by the stress-strain curves (Figure 3d). It is generally known there is often a trade-off between mechanical strength and break-at-strain.40 Therefore, it is not difficult to understand that PDMS-MPI-CM film is more susceptible to breakage than PDMS-MPI-TM film during stretching process. In addition to ultrastrechability, notch-insensitive exceptional stretchability is also attractive. For a typical experiment (Figure 3f), PDMS-MPI-TM film was notched in one side and a maximum fracture strain was up to about

18000% (Figure S14 and Movie S3). The blunting of the notch demonstrates that fracture-tolerant PDMS-MPI-TM film is able to effectively dissipate or spread the stress concentrated at the crack tip via dissociation of relatively weak H-bonds or rearrangement of entangled polymeric chains, preventing damage propagation under loading.19,25,30 Figure 3g shows the stretching-speed-dependent tensile curve of PDMS-MPI-TM film. As shown, all the mechanical properties are deformation rate-dependent. This phenomenon was also found in other supramolecular elastomers.37,41 The dramatic decrease of strain-at-break at high deformation rate can be attributed to that the multistrength H-bonds do not have sufficient time to recombine after rupture at timescale.41 Cyclic tensile tests were performed to investigate the self-recovery performance of PDMS-MPI-TM. As shown in Figure 4a, this supramolecular polymer exhibited ~100% self-recovery performance and possessed high fatigue resistance at small tensile strain (ε≤400%). Pronounced hysteresis and distinct yield point were observed during the first loading-unloading cycle with 400% strain, indicating that the multistrength H-bonds within polymer network can be served as sacrificial bonds and then effectively dissipate the energy. The residual strain and hysteresis ratio (estimated from hysteresis area change) gradually recovered with time (Figure 4b). After resting for 300 min, the cyclic tensile curve basically overlapped with its original profile. Besides, even suffering from five consecutive loading-unloading cycles at a strain of 400% without delay,

Figure 4. Cyclic tensile properties of PDMS-MPI-TM supramolecular polymer. (a) Recovery of the sample for different waiting times performed by cyclic tensile test. (b) Waiting time dependence of the corresponding hysteresis ratio and residual strain. (c) Fatigue resistance by five successful loading-unloading cycles at 400% strain without delay and the corresponding recovered curve after relaxing for 300 and 600 min. (d) Consecutive cyclic loading-unloading curves for PDMSMPI-TM from 200% to 6400% without intervals. (e) Schematic illustrations of the proposed mechanism for strain recovery.



PDMS-MPI-TM polymer could also recover its residual strain and hysteresis ratio after relaxing for 600 min (Figure 4c). These results suggested that the partially broken weak H-bonds at small applied strain can reform after rupture, endowing PDMS-MPI-TM with well self-recovery capability. For comparison, at high applied strain of 1000%, the stretched film could not return back to its initial state even prolonging waiting time to 24 h (Figure S15). This phenomenon can be ascribed to the severe damage of hard phase during large stretching process, in which the strong H-bonds are continuously cleaved and pulled out from hard domains, leading to chain sliding and then producing large plastic deformation (Figure 4e). Unfortunately, the damaged hard domains cannot be fully restored, resulting in residual strain. Consecutive cyclic tensile test at different strains without any delay were also carried out to investigate the elastic resilience. As shown in Figure 4d, it is worthy to note that the yield point appears in every loop, indicating that PDMS-MPI-TM film can provide necessary elasticity during repeated loading-unloading process through rapid yet incomplete reformation of multistrength H-bonds.25 Self-healing properties. As a result of abundant multiphase dynamic H-bonds, PDMS-MPI-TM film exhibits outstanding self-healing capability at room temperature. It is observed from Figure 5a that an artificial scratch on film surface almost totally disappeared after healing for 4 h (Figure S16). Fragment splicing test was performed to further evaluate its superior self-healing properties (Figure 5b). Typically, a cuboid PDMS-MPI-TM film (30 × 10 × 1.0 mm) was completely cut in halves and gently reattached. Notably, after healing for only 1 h, the healable film could be readily stretched up to 70 times of its original length. Figure 5c depicts the representative stress-strain curves for the rejoined films at different healing times, and the healing efficiency, which is calculated according to the recovery of toughness, are listed in Table S1 (Supplementary Information). The healing proceeded with time, and the complete recovery of all mechanical properties took only 4 h. The Boltzmann time-temperature superposition (TTS) technique was employed to create a master rheological curve to calculate the apparent activation energies (Ea) for assessment of the slippage of polymer chains (Figure S17). The calculated Ea value for PDMS-MPI-TM is only 35.4 kJ mol-1, revealing that the multistrength H-bonds within polymer network can enable the chains to slip past each other easier, which is account for the fast and efficient self-healing behavior.16 Certainly, based on our design conception, the healing efficiency depends heavily on dosage of TM. When we decrease the amount of TM, at a molar ratio of of PDMS:MPI:TM as 10:7:3, PDMS-MPI-TM-ref film recovered only 40% within 24 h (Figure S18). The poor self-healing performance maybe associated with the incompletely destroyed crystallization phase (Figure S19). Indeed, except for PDMS-MPI-TM, PDMS-MPI-CM film also possesses satisfactory self-healing performance at room temperature (Figure S20), which is ascribed to the destroy of crystallization hard phase by the formation of ‘competing’ H-bonds

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between urea and 4’4-methylenebis(phenyl urea) moieties (Figure S21). In addition, we also designed the other control experiment by replacing aromatic rings in hard domains with aliphatic chain to probe the general applicability of our core conception. Hexamethylene diisocyanate (HDI) was chosen to supersede MPI, and the resultant PDMS-HDI also cannot restore itself at room temperature due to crystallized hard phase (Figure S22). However, upon inserting thiourea moieties into PDMS-HDI chains, the asprepared PDMS-HDI-TM owns fast and efficient self-healing performance (Figure S23), suggesting that our core conception is suitable for isocyanate chain extenders with regular and symmetrical structure, such as, MPI and HDI. Interestingly, we further observed that self-healing of PDMS-MPI-TM can also take place below zero degree centigrade or underwater. Healing at -20 °C for 48 h gave a tensile strain up to 26411% (Figure 5d), which is due to an ultralow Tg value (Figure S24), enabling sufficient re-entanglement of polymer chains at damaged surface even at -20 °C.8,16 When two separated pieces were closely contacted underwater for 4 h and then withdrawn for tensile test (Figure S25), the strain-at-break reached 29107% and the healing efficiency was 95% (Figure 5e). Such a high underwater healing efficiency considerably surpasses the previously reported self-healable polymers,30,42-44 which motivates us to investigate underwater working mechanisms. As observed from swelling experiments, PDMS-MPI-TM film did not exhibit obvious swelling behavior after soaking (Figure S26). The tensile curve of swollen film is general consistant to the dry one (Figure 5e), suggesting an excellent water insensitive capability. Although the PDMS-MPITM films are hydrophobic and hardly swollen by water, quantitative dynamic vapour sorption (DVS) analysis shows the film is still a little water absorption (Figure S27). It is well-known that water is a strong competitor for Hbonds, which can cause dissociation of H-bonds.27 In our case, the permeation of water molecules may bind with MPU and TU units, and then destroy the original H-bonds arrays. However, continuous PDMS phase in PDMS-MPITM matrix provide apolar and hydrophobic environment which cannot be influenced by water, protecting H-bonds from uncontrolled dissociation. Thus, a small amount of retained water molecules are most likely to act as strong competitor to dissociate the original H-bonds and reconstruct the new dynamic H-bonds system in multiphase, accounting for underwater healing (Figure S28). This hypothesis is also confirmed by ATR-FT-IR, in which a hydrogen/deuterium (H/D) exchange method of using active protons in NH is used (Figure 5f).32,45 Upon soaking in D2O, two characteristic peaks of N-H at 3301 and 1598 cm-1 shift accordingly to form N-D bonds. Meanwhile, the H/D exchange between N-H and D2O also influence the vibration of adjacent C=O and C=S bond; the C=O bond originates in MPU group shift from 1652 to 1636 cm-1, whereas the intensity of C=S bond (1445 cm-1) from TU group significantly decreases with a new peak generating at 1470 cm-1 (Figure 5f). These indicate the D2O water assisted dissociation of original H-bond arrays and then reconstruct the new dynamic H-bonds system in polymer matrix (Figure S28).


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Figure 5. Self-healing properties of PDMS-MPI-TM supramolecular polymer. (a) Optical microscope images demonstrating self-healing of PDMS-MPI-TM. (b) Photographs of PDMS-MPI-TM film healed for 1 h before and after stretching. (c-d) Tensile curves of PDMS-MPI-TM at various healing times at room temperature and -20 °C. (e) Tensile curves of PDMSMPI-TM immersed in water and healed under water for 4 h. (f) ATR-IR spectra of PDMS-MPI-TM film before D2O immersion (black line), after D2O immersion (pink line) and then removing the D2O (orange line); the variation of the -NH, -C=O and C=S groups demonstrating H/D exchange (interactions) between urea/thiourea H-bonds and D2O. to >3000% while maintaining the high conductivity, as evHowever, when the wet films was dried by dehumidifier in idenced by almost invariable brightness. As far as we know, air at room temperature, all the changed peaks returned to it is the first report of self-healable conductor capable of their original states within 5 min because of the reversible being stretched to 3000% in water and keeping conductivinteractions between water and H-bond arrays constructed ity after underwater healing.46-48 High recovery efficiency by MPU and TU groups (Figure S29). Notably, simultaneof mechanical properties in water of PDMS-MPI-TM maously achieving high efficient self-healing under low temtrix guarantees the high stretchability of PDMS-MPIperature or in water offers material the possibility to be TM/EGaInPs conductor. Moreover, the deformation stress used under harsh conditions. is easy to rupture thin G2O3 shell of EGaInPs and release Application in flexible underwater self-healing conEGaIn liquid metal to connect the conductive layer via isoductor. Taking advantage of the unique features of PDMStropic transportation,49 which is the critical factor to reMPI-TM, we proceeded to fabricate the highly stretchable, main stable conductivity upon stretching and pave the way self-healable conductor in a sandwich configuration, for applications of flexible electronics or underwater rowhere PDMS-MPI-TM-EGaInPs (eutectic gallium indium bots (Figure 6c). particles labelled as EGaInPs) as composite conductive layer is encapsulated by two PDMS-MPI-TM layers (Figure 6a). To provide detailed insights into the electrical and mechanical recovery of PDMS-MPI-TM/EGaInPs, the pristine conductor immersed in water was connected to a lightemitting diode (LED) and a power source (3V) to compose a circuit. As shown in Figure 6b, the red LED was lit, showing the good underwater electrical conductivity. On rupture, the lighted LED immediately went off. However, the LED instantaneously relit when two separated pieces were contacted in water. Surprisingly, even after 10th cuttinghealing cycles (healing time: 30 s), the electrical conductivity remained stable (Figure S30). After healing 4 h under water, the healed conductor can be manually stretched

Application in advanced Li-S batteries. Lithium-sulfur (Li-S) batteries are one of the most attractive energy storage systems due to its high-theoretical energy density, low cost and nontoxicity.50 However, the commercialization of Li-S batteries face with many challenges and obstacles, among which volume expansion and shuttling of soluble polysulfides between electrodes are annoying problems (Figure 7a).51 Recently, reports of SHPMs applied in Li-ion batteries are increasing.20,52,53 From the point of view of mechanisms, SHPMs meeting the following criterions are promising attempt to resolve abovementioned problems: (1) modest conductivity; (2) permeation-resistant to polysulfides; (3) notch-insensitiveness for microcracks derived from volume expansion; (4) self-healing under working



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Figure 6. Application in flexible self-healing conductor. (a) Schematic illustration of the components and structure for PDMS-MPI-TM-EGaInPs conductor. (b) Photographs demonstrating good and healable electrical conductivity of PDMSMPI-TM-EGaInPs conductor when healed under water and stretched >30× its original length. (c) Schematic mechanism of electrically stretchable and underwater self-healing PDMS-MPI-TM-EGaInPs electrical conductor. of the PDMS-MPI-TM/MWCNTs-C/S electrode with tradiconditions for preserving all the functionalities. tional C/S electrode. The galvanostatic charge/discharge Taking electronic conductivity into account, we proprofiles for different cycles are illustrated in Figure 7f, 7g ceeded to add 20wt% multi-walled carbon nanotubes and Figure S31. PDMS-MPI-TM/MWCNTs-C/S cell deliv(MWCNTs) into PDMS-MPI-TM to fabricate PDMS-MPIered discharge capacities of 910 and 660.9 mAh g-1 for the TM/MWCNTs composite, and form conductive network first and 500th cycles at 1C (capacity retention of 72.5%), re(Figure S31). Such a high content of inorganic fillers usually spectively, much higher than those of a traditional C/S cell dramatically changes mechanical properties of poly(772.1 and 398.5 mAh g-1, respectively, Figure S32), and pre20,22 mers. However, thanks to the unique characteristics of sented tremendously enhanced cycling stability. The CouPDMS-MPI-TM matrix, the fabricated composite still exlombic efficiency of PDMS-MPI-TM/MWCNTs-C/S cell hibits notch-insensitiveness upon stretching (Figure 7c). became stable to 98% in the 50th cycles. Moreover, the speTo simulate the magnitude of volume expansion for S cathcific capacity of PDMS-MPI-TM/MWCNTs-C/S cell deode(about 70%), the predamaged film stood 20 cycles of creases with increasing current rates, and high reversible loading-unloading under a stain of 100% and no crack capacity was achieved (Figure 7h). To search the reasons propagation was founded (Movie S4). Meanwhile, as exfor superior electrochemical performance of PDMS-MPIpected, PDMS-MPI-TM/MWCNTs composite not only TM/MWCNTs-C/S cell, the electrolyte extracts from cathquickly restored conductivity at room temperature (Figure odes/anodes after cycling and the morphologies of cath7b), but also repaired artificial microcracks in electrolyte odes/anodes were analyzed. There exists obvious differsolution (Figure 7d). Afterward, we set up equipment to asence in color of electrolyte extracts from cathodes/anodes sess the permeation-resistant capability. As a control polyof two cells (Figure 7i). Quantitative detection from UVpropylene (PP) separator sealed U-shape tube, polysulfide Vis absorption spectra show that the concentration of poltotally passed through separator within 1 h. When the ysulfides from C/S cell is higher than that of PDMS-MPIhealed PDMS-MPI-TM/MWCNTs composite was emTM/MWCNTs-C/S cell (Figure 7j). These mean a good efployed as separator, the migration of polysulfides stopped, fect of suppressing polysulfides diffusion after coating the revealing restoration of nature barrier performances (Figself-healing PDMS-MPI-TM/MWCNTs layer, which is also ure 7e). corroborated by the findings in Li anodes (Figure S33). The Based on these distinct advantages, we fabricated scanning electronic microscopy images of two cathodes afPDMS-MPI-TM/MWCNTs-C/S electrode by coating ter cycling are depicted in Figure 7k and 7l. PDMS-MPIPDMS-MPI-TM/MWCNTs on the surface of C/S cathode TM/MWCNTs-C/S cathode kept a smooth surface morfor Li-S batteries. We compared the cycling performance phology even after 500 cycles at 1 C, whereas the C/S


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Figure 7. Application in advanced Li-S battery. (a) Schematic illustrations of the comparisons between traditional and designed functional Li-S battery. (b) Demonstration of the electrical healing process with a blue LED in series with PDMSMPI-TM/MWCNTs composite. (c) Optical images of a notched PDMS-MPI-TM/MWCNTs composite before and after stretching. (d) Optical microscope images of PDMS-MPI-TM/MWCNTs with an about 130 μm hole before and after be healed in electrolyte under room temperature. (e) Photographs of the polysulfides diffusion experiment of PP and PDMSMPI-TM/MWCNTs separators. (f-g) Cycling performance of C/S and PDMS-MPI-TM/MWCNTs-C/S electrodes at 0.2 C, and the corresponding galvanostatic charge/discharge profiles at the first, 10th, 50th and 100th cycles. (h) Rate performance of C/S and PDMS-MPI-TM/MWCNTs-C/S electrodes.( i-j) Photos and UV-vis spectra of the extracts washed from 500th cycled C/S and PDMS-MPI-TM/MWCNTs-C/S battery. (k-l) SEM images of C/S and PDMS-MPI-TM/MWCNTs-C/S electrodes after cycling for 500 cycles at 1 C. cathode displayed large numbers of severe cracks.52,53 On the whole, the stable electrochemical performances of PDMS-MPI-TM/MWCNTs-C/S cell is attributed to the fact that PDMS-MPI-TM/MWCNTs as self-healing protective film facilitates the transportation of Li+, suppress the polysulfide shuttling, and maintain the integrate of electrodes. It is expected to be used in advanced Li-S batteries to improve its lifespan.

CONCLUSION

In summary, we employed multiphase active hydrogen bonds strategy to render microphase-separated structures fast and efficient self-healing capabilities. Remarkably, autonomous self-healing process can occur at low temperature or in the water. Besides superior self-healing properties, the synthesized PDMS-MPI-TM film also exhibits unprecedented ultrastretchability and notch-insensitiveness. Based on these unique characteristics, using PDMS-MPITM as substrate material, we fabricated highly stretchable self-healable conductor, which can remain conductivity



even at a strain of 3000% after underwater healing for 4 h. In addition, PDMS-MPI-TM-MWCNTs as protective selfhealing film considerably enhanced the cycling stability and improved electrochemical performance of Li-S batteries. Our research work opens up a new design principle of high-efficient and multifunctional self-healable materials.

EXPERIMENTAL SECTION Materials. Bis(3-aminopropyl) terminated poly(dimethylsiloxane) (H2N-PDMS-NH2, Mn=5000) was purchased from Gelest. 4,4’-methylenebis(phenyl isocyanate) (MPI), 1,1’-thiocarbonyldiimidazole (TM) and 1,1’-carbonyldiimidazole (CM) were supplied by Sigma-Aldrich. EGaIn was purchased from Fanyada Electronics Technology Co., Ltd. In China. Multi-walled carbon nanotubes (MWCNTs, diameter 10-20 nm, length 5-15 um) was purchased from TCI. Chloroform (CHCl3) was dried with the molecular sieves for 48 h before use. Water used in all the experiments was obtained via a Milli-Q water system with a resistivity of 18.0 MΩ cm. Synthesis of PDMS-MPI-TM. 5 g of H2N-PDMS-NH2 (Mn=5000, 1 eq.) was dissolved in 40 mL anhydrous CHCl3 under nitrogen atmosphere at 0 ℃. After stirring for 30 min, a mixture of MPI (0.1 g, 0.4 eq.) and TM (0.11 g, 0.6 eq.) was dissolved in 10 mL anhydrous CHCl3, and then the obtained solution was injected dropwise into the above H2NPDMS-NH2 solution. Subsequently, the resulting solution was stirring for 1 h while the temperature was kept at 0 ℃ with ice water. Afterwards, the reaction temperature was gradually increase up to room temperature and the mixed solution was further stirred under nitrogen atmosphere for 4 days, resulting in a viscous solution. After reaction, the solution was concentrated to 1/3 of its volume and then 50 mL MeOH was poured into it to precipitate. upon settling for 2 h, the upper clear solution was decanted to obtain the white precipitate-like viscous liquid. This dissolution-precipitation-decantation process was repeated for three times to obtain the purified product. Finally, the purified product was re-dissolved in CHCl3, poured into Teflon mold and then dried at room temperature for 24 h and 60 °C in a vacuum oven for 24 h to completely remove chloroform. Clean and dry films with a thickness of ~1 mm were peeled off from the PTFE mold for further testing. The controlled PDMS-MPI-CM, PDMS-MPI, PDMSTM, PDMS-HDI and PDMS-HDI-TM samples were synthesized by the same procedure that used for PDMS-MPITM. Either replace 1,1’-thiocarbonyldiimidazole with 1,1’carbonyldiimidazole, or just use 1,1’-thiocarbonyldiimidazole or 4,4’-methylenebis(phenyl isocyanate) to react with H2N-PDMS-NH2. Fabrication of self-healing PDMS-MPI-TM/EGaInPs conductor. Typically, PDMS-MPI-TM/EGaInPs composite was fabricated by combining PDMS-MPI-TM and EGaInPs at a mass fraction of 1:2. Firstly, EGaInPs were prepared by the sonication of EGaIn in 20 mL of CHCl3 for 1 h using a bath sonicator.[48] Meanwhile, PDMS-MPI-TM material was totally dissolved in 10 mL CHCl3 for further use. Subsequently, the PDMS-MPI-TM solution and the

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EGaInPs were mixed thoroughly with a vortex mixer, forming a PDMS-MPI-TM suspension of EGaInPs dispersed in polymer matrix. After mixing, the PDMS-MPITM/EGaInPs composite could be cast on PTFE mould to obtain conductive film. Afterwards, the conductive film was cut into small pieces and then sealed between two insulating PDMS-MPI-TM layers by pressing to fabricate PDMS-MPI-TM/EGaInPs conductor.[42] Construction of C/S electrode. The C/S electrode was constructed by mixing 70 wt% of S, 20 wt% of activated carbon, 5 wt% of super P, and 5 wt% of polyacrylonitrile (PAN) in H2O solvent. The suspension was then spread uniformly on an Al foil current collector. Finally, the C/S electrode was obtained after drying in a vacuum oven at 60 °C for 48 h. Construction of functional PDMS-MPI-TM/MWCNTsC/S electrode. PDMS-MPI-TM was dissolved in CHCl3 and then mixed with 20 wt% multi-walled carbon nanotubes (MWCNTs) using centrifugal mixer. Then, the PDMS-MPI-TM/MWCNTs-contained CHCl3 solution was dropped cast onto the C/S electrode and then dried to form a uniform coating. The thickness of the PDMS-MPITM/MWCNTs coating was about 200-500 nm. Preparation of L2S8 electrolyteThe Li2S8 electrolyte (0.2 M) was prepared by dissolving 97.5mg Li2S, 472.5 mg sulfur, 0.01 mol LiTFSI and 2 wt% LiNO3 in 10 mL DOL/DME (volume ration=1:1) solution. Then, this prepared electrolyte was employed to carry out polysulfides permeation experiments.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Materials and Methods, characterization data, mechanical and self-healing studies. (PDF) Movie of a PDMS-MPI-TM film showing stretching up to 315× its original length without rupture at a deformation rate of 100 mm min-1. (AVI) Movie of a control PDMS-MPI-CM film only showing stretching up to 178× its original length at a deformation rate of 100 mm min-1. (AVI) Movie showing that a notched PDMS-MPI-TM film being stretching up to 180× its original length at a deformation rate of 100 mm min-1. (AVI) Movie showing that the predamaged PDMS-MPITM/MWCNTs film with a 5 mm notch standing 20 cycles of loading-unloading under a stain of 100% without crack propagation.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ORCID: JiaJun Fu: 0000-0002-8542-9556 Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the National Nature Science Foundation of China (Grant No. 51672133; Grant No. U1737105); the National Science Foundation of Jiangsu Province (Grant No. BK20161496); the Fundamental Research Funds for the Central Universities (Grant No. 30918012201); the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX19_0287); A project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Notch-Insensitive, Ultrastretchable, Efficient Self-Healing

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