Extremely Stretchable, Self-Healable Elastomers with Tunable

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Extremely Stretchable, Self-Healable Elastomers with Tunable Mechanical Properties: Synthesis and Applications JianHua Xu, Wei Chen, Cheng Wang, Ming Zheng, ChenDi Ding, Wei Jiang, LingHua Tan, and JiaJun Fu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02320 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Chemistry of Materials

Extremely Stretchable, Self-Healable Elastomers with Tunable Mechanical Properties: Synthesis and Applications JianHua Xu,† Wei Chen,† Cheng Wang,† Ming Zheng,† ChenDi Ding,† Wei Jiang,‡ LingHua Tan‡ and JiaJun Fu*,† †

School of Chemical Engineering, 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: Stretchable and autonomously self-healable elastomers with wide-range tunable mechanical properties have attracted increasing attentions in various industries. Up to now, it is still a huge challenge to synthesize self-healing elastomers integrating extreme stretchability, relatively high mechanical modulus, autonomous and rapid self-healing capability. Herein, we propose a novel covalent/supramolecular hybrid construction strategy, in which the covalent crosslinks are responsible for providing high modulus and elasticity; while supramolecular crosslinks realize extreme stretchability and rapid self-healing under room temperature depending on the ultrafast exchange kinetics of metal-ligand motifs and multi-coordination modes. The representative polyurea hybrid elastomer, CSH-PPG-Zn-0.25, can be stretched more than 180× its original length with the highest Young’s modulus (1.78±0.08 MPa) among reported ultrastretchable materials. CSH-PPG-Zn-0.25 can fully restore mechanical properties of completely cut samples within 3 h. Note that the healing process is able to take place under a low temperature of -20 °C and unaffected by surface ageing and atmospheric moisture. Merely tailoring molar ratio of metal/ligand actualizes wide-range tunability of mechanical and dynamic properties, such as Young’s modulus (from 1.71±0.08 MPa to 5.56±0.22 MPa), maximum tensile strength (from 0.32±0.03 MPa to 4.42±0.23 MPa), strain at break (from >18000% to 630±27%) and storage modulus, ascribing to the increase of crosslinking density and formation of stiff ionic clusters. Based on the different material characteristics, two typical elastomers are respectively employed as flexible and self-healable conductor and self-healable automotive paint. Benefiting from the fantastic anti-ageing and low temperature healing features of CSH-PPG-Zn-0.25, the prepared Ag-NWs/CSH-PPG-Zn-0.25 conductor can even regain its conductive function below zero. CSH-PPG-Zn-0.50 material, meeting mechanical restrict requirements of automotive paints is able to thoroughly eliminate the surface scratches and recover anticorrosion function in local damaged region under atmospheric environment.

INTRODUCTION Synthetic self-healing polymeric materials, inspired by “accidental injury-spontaneous healing” for natural living systems, which can autonomously repair themselves and regain mechanical and functional properties upon local damage, have attracted considerable attentions over the past decades due to their widely potential applications, including: wearable electronics, energy storage, protective coatings, healthcare materials, sensors, and so on.[1-6] Incorporation of self-healing units into synthetic materials is to prolong their service life, reduce maintenance costs and ultimately avoid failure of materials stemming from internal microcracks.[7-10] Hitherto, successful construction strategies to self-healing polymer that have been reported can be divided into the following two categories, chemomechanical self-healing and self-healing reactions.[11,12]Typically chemo-mechanical self-healing strategy is encapsulation technology, in which the reactive microcapsules or hollow fibers containing reactive agents are embedded into polymer matrix. These self-healing polymers, containing microcapsules or microvascular networks,

are capable of closing cuts or cracks at the microscale via transporting or circulating encapsulated healants to damaged area driven by capillary forces.[13,14] With the advance of microvascular technique, self-healing polymers based on encapsulation technology have strength on repairing large-scale damage and address the weakness of limited healing times.[14] However, these self-healing polymers focus more on the restoration of mechanical or barrier attributes,[13-15] due to the inherent differences between healants and polymeric substrates, it is difficult to restore their original functionalities, which are diminished after structural damage and are highly expected to be reacquired. Besides encapsulation technology, remote selfhealing technology via electromagnetic radiation, magnetic fields or NIR light is an another chemo-mechanical selfhealing strategy that can simultaneously restore the mechanical strength and functionalities.[11,16] Self-healing polymers based ondifferent self-healing reactionsin polymer matrix have been engineered through the incorporation dynamic covalent bonds (such as, Diels-Alder reaction,[17] disulfide[18], dynamic boroxine bonds[19]) or supramolecu-

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lar non-covalent bonds (such as, hydrogen bonding[20], host-guest molecular recognition[21], π-π stacking[22], electrostatic interactions[23], and metal-ligand interactions[24]) into polymeric networks, and mend fracture viaexchangeable dynamic reactions or reversible bond scissionreformation. Intriguingly, some innate functionalities, including: superhydrophobicity, corrosion resistance, antifouling performance, electronic conductivity, etc. have been reported to be completely or partially resumed following structural recovery, showing competitive advantages over encapsulation self-healing strategies.[25-29] However, most dynamic covalent bonds require input of external energy (heat, light or catalyst) to facilitate association/dissociation reversion.[17-19,30,31] Considering the nondetectability of internal microcracks in some cases and the possibility of subsequent disastrous consequence, supramolecular polymers easily realize autonomous self-healing process under mild conditions according to the two fundamental principles: (i) assurance of mobility of polymeric chains; (ii) selection of suitable supramolecular motifs, which are easy to meet the practical environmental needs and gain particular interest.[7,9,20,24] Although great progresses have been made in the development of supramolecular polymers with roomtemperature self-healing feature, the resulting poor mechanical strength limits their application.[32] Great efforts have been devoted to tackle the contradiction between mechanical robustness and chain mobility for dynamic healing,[20,33] which also reminder us to pay close attention to mechanical properties before being subjected to practical applications. Guan and coworkers proposed the hardsoft multiphase design, in which hard phases (polystyrene, poly methyl methacrylate, etc.) with high Tg values provide mechanical stiffness/strength, while soft phases (poly methacrylate, etc.) with low Tg values and elegant supramolecular motifs, are responsible for room-temperature self-healing behavior.[20] Recently, another promising approach is to assemble covalent/supramolecular hybrid polymeric networks, where the incorporation of appropriate covalent crosslinkers for enhancing overall mechanical properties and do not compromise self-healing capability.[34,35] Nowadays, the flourishing exploitation of supramolecular self-healing polymers greatly broadens scopes available to engineering applications.[25-29] Selfhealing capability, particularly recovery ability of special functionalities as the crucial factor is needed to be guaranteed, at the same time, it is of significance to adjust mechanical properties of polymers for meeting the criteria for various industrial uses. For example, the utilization of flexible self-healable conductor not only demands a quick recovery of electrical conductivity after fracture, but also emphasize on ability to accommodate large strain and stable electrical conductivity during deformation.[36] Another example of self-healable automotive paint in different working environments, a simultaneous combination of superior mechanical properties, satisfactory anticorrosion performance as well as self-diagnosis/selfhealing functions is urgently demanded.[37] Diverse appli-

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cations have different requirements for material characteristics, therefore, the design and facile preparation of autonomous self-healing polymers with tunable mechanical properties for multipurpose uses are innovative work and still full of challenge. The investigations of mussel-inspired iron-catechol materials offer the optional route to build self-healing polymers by making use of dynamic and reversible coordination bonds.[38,39] The adjustable metal-ligand binding constants and kinetical volatility are convenient to equilibrate mechanical properties and macroscopic dynamic nature of the resultant self-healing polymers.[24,25,33,40] Herein, we report a series of polyurea hybrid elastomers, containing covalent crosslinks and supramolecular metalligand crosslinks, by simple regulating metal/ligand ratio. Through reasonable control ratio between covalent and supramolecular crosslinks, an extremely stretchable, autonomous and rapid self-healing polyurea hybrid elastomer was readily synthesized. The polyurea hybrid elastomer can be stretched more than 180× its original length with the highest reported Young’s modulus (1.71±0.08 MPa) among ultrastretchable materials. The covalent crosslinks provide robustness and elasticity, while the ultrafast exchange kinetics of supramolecular crosslinks areresponsible for the extreme stretchability. Furthermore, this material exhibits a fast self-healing capability. After the material is cut in half, all the mechanical properties can be fully restored at room temperature for only 3 h. Combined with anti-ageing and low-temperature healing characteristics, the polyurea hybrid elastomer demonstrates bright application prospect in flexible and selfhealable conductors. On the basis of this structure, only varying metal/ligands ratio actualizes wide-range tunability of mechanical properties. Another hybrid polyurea elastomer, integrating balanced stiffness, strength, toughness and autonomous self-healing capability, was fabricated and used as self-healable automotive paint. The mechanical properties of this kind of paint are all compliant with the industrial standard. More importantly, it can thoroughly eliminate surface scratches under atmospheric environment and recover anticorrosion function without input of external energy.

RESULTS AND DISCUSSION Design, synthesis and characterization of polyurea hybrid elastomer. Our design strategy to fabricate covalent/supramolecular hybrid polyurea elastomers primarily comprises two fundamental principles: (i) The incorporation of covalent crosslinks into polymeric networks is to improve mechanical robustness; (ii) The utilization of dynamic supramolecular metal-ligand crosslinks is to impart polymeric networks with self-healing capability under room temperature. The whole synthetic route is illustrated in Figure 1a. Firstly, tolylene 2,4-diisocynate terminated polypropylene glycol (PPG-NCO), 1-(3aminopropyl) imidazole (IMD-NH2) and trimethylolpropane tris[poly(propylene glycol), amine termi-


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Figure 1. (a) Schematic illustration of the synthetic process of CSH-PPG-Zn sample. (The inserted logo reproduced with permission of Nanjing University of Science and Technology) (b) 1H NMR of CSH-PPG sample in CDCl3 at various concentrations. (c) FT-IR spectra of CSH-PPG and CSH-PPG-Zn samples. (d) SEM image and corresponding EDX (Zn K) line scan along the dashed line of a cross sectional view of CSH-PPG-Zn sample. (e) Elemental mapping images of the selected region in section (d) confirming uniformity of the added Zn 2+. (f) HAADF-STEM image of CSH-PPG-Zn sample. (g) DSC curves of CSH-PPG-Zn sample, the enlarged image inserted in section (g) clearly showing the Tg value. nated] ether (PPG-NH2) were gently mixed and reacted at 60 °C with a fixed 3:3:1 stoichiometric molar ratio (PPGNCO: IMD-NH2 : PPG-NH2) for 6 h to afford a viscous liquid. The reaction was monitored by Fourier transform infrared spectroscopy (FTIR), where the disappearance of stretching bands at 2319 cm-1 corresponding to isocyanate groups and the appearance of two new peaks at 1690 cm-1 and 1650 cm-1, assigned to stretching vibrations for C=O in urea groups and N=C-N in imidazole rings,[26,41] respectively, establish the complete polycondensation reaction and formation the permanent covalent network, carrying imidazole ligands. It should be noted that the atmospheric water vapor is likely to act as the extra chain extender during the reaction process (CSH-PPG, Figure S1, Supporting Information).[42]In addition, with increasing concentration in solution, the more and more apparent downfield shift of the NMR signal of -NH-C-NH- atoms confirms the existence of intermolecular hydrogen bonding from urea groups (Figure 1b).[3,43] In a second step, a

zinc (II) salt (zinc trifluoromethanesulfonate, Zinc(CF3SO3)2) chosen as metal source due to its good solubility as well as high mobility of counter ions,[33] CF3SO3- in solid states, was added to introduce supramolecular metal-ligand crosslinks, keeping the molar ratio between Zn2+ and imidazole group at 1:4. After curing under vacuum to remove solvent, the resultant polyurea hybrid elastomer appearing as transparent, colorless, freestanding film was obtained (CSH-PPG-Zn, Figure S2). Both covalent crosslinks and supramolecular noncovalent crosslinks play the pivotal roles in polymer molding process. As comparison, in the absence of PPG-NH2 as covalent crosslinkers, only resulted in viscous fluid with no appreciable mechanical properties, as evidenced by the dynamic mechanical tests in which the loss modulus is higher than the storage modulus over the tested frequency (25 °C, Figure S3 and Movie S1). After addition of Zn(CF3SO3)2, the storage modulus (25 °C, 1Hz) substantially increased from 1383 Pa (CSH-PPG) to 494632 Pa



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Figure 2. (a) Photographs of a CSH-PPG-Zn sample before stretching (left) and showing high stretchability at >18000% stretching (right) without fracture. (b) Raman spectra of CSH-PPG and CSH-PPG-Zn samples. XPS spectra of the CSHPPG-Zn sample: (c) high resolution N1s spectra; (d) high resolution O1s spectra. (e) 1H NMR spectra of CSH-PPG-Zn sample with increasing of Zn2+ content. (f) Tensile curves of CSH-PPG-Zn sample at a deformation rate of 100 mm min-1. (g) Enlarged tensile curves of CSH-PPG-Zn sample collected from section (f). (CSH-PPG-Zn), preliminarily suggesting the formation of supramolecular crosslinks (Figure S4, SI). In order to further examine the interactions between Zn2+ and imidazole, Attenuated total reflectance-FTIR (ATR-FTIR) experiments were conducted and the results are shown in Figure 1c. An absorption peak at 1650 cm-1,assigned to the characteristic vibration of N=C-N in imidazole ring was observed on a spectrum of CSH-PPG sample.[41] Upon coordination with Zn2+, this characteristic peak shifted to a high wavenumber, 1668 cm-1, revealing the Zn2+/imidazole coordination complexes. Cross-sectional line-scan and mapping images shown in Figure1d and 1e manifest the uniform distribution of Zn2+ within polymeric network. No Zn2+ aggregates were detected in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Figure 1f) even at the nanoscale level (sensitive for high atomic number). Differential scanning calorimetry (DSC) curve shows that the glasstransition temperature (Tg) of the CSH-PPG-Zn film is about -50.1 °C (Figure 1g), far below room temperature and no melting point is found in DSC curve (-80 °C to

150 °C), demonstrating the amorphous structure, which is further corroborated by X-ray diffraction (Figure S5). The thermal stability of film was assessed by thermal gravimetric analysis (TGA). As shown in Figure S6, the initial decomposition temperature is up to 250 °C, exhibiting good thermal stability. Investigation of mechanical properties of polyurea hybrid elastomer. In order to characterize mechanical properties of CSH-PPG-Zn, we cut the CSH-PPG-Zn film, fabricated by solvent evaporation method, into rectangular shape with a tensile size of 5 mm (L) × 10 mm (W) × 1 mm (T) and quantify its stress-strain behavior using uniaxial tensile testing machine with a 100-N load cell, which was performed in air, at room temperature (20 °C). In a typical test, CSH-PPG-Zn sample was stretched at a deformation rate of 100 mm min-1. It is of great interest to note that CSH-PPG-Zn sample exhibits extreme stretchability, and can be stretched to >180 times its original length without fracture (limited by the measurementrange of the machine, Figure 2a, Movie S2). Curiously, at a strain of 18000%, CSH-PPG-Zn sample could also be vio-


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Figure 3. (a) CSH-PPG-Zn sample was subjected to cyclic tensile test of varying maximum stretching. (b) Consecutive cyclic loading-unloading curves of CSH-PPG-Zn sample with 5 min intervals and a maximum strain of 50%. (c) After the first loading-unloading cycle up to 1000% (gray curve), the sample being allowed to recover at different condition and reloaded again. (d) Consecutive cyclic loading-unloading curves with gradual strain increase up to 4000% for CSH-PPGZn sample; the dashed lines are the loading−unloading curve with 4000% strain for the same film. (e) Schematic illustration of the proposed mechanism for the strain recovery; coordinate bonds reform at their original position at small strain, whereas they migrate and reconnect to adjacently accessible sites at large strain, causing residual strain. lently shaken (Movie S3), implying the potential for further stretching. Recently, Bao and coworkers reported synthesis of a highly stretchable elastomer using Fe(Ⅲ)2,6-pyridinedicarboxamide complex with different bond strengths, which could be stretched over 100× its initial length (note: tensile rate was 10 mm min-1) and the Young’s modulus and maximal strength were 0.09±0.01 MPa and 0.03±0.01 MPa, respectively.[24] For comparison, CSH-PPG-Zn sample possesses not only higher stretchability, but also larger Young’s modulus and maximal strength, up to 1.71±0.08 MPa and 0.32±0.03 MPa, respectively. To the best of our knowledge, the stretchability and Young’s modulus are evaluated to be the highest reported so far for polymeric materials owning extremely stretchable property.[24,28,44-49] Attempting to clarify the reasons for combination of extreme stretchability and superior mechanical strength, we firstly carried out the systematic study on the designed supramolecular motifs, which have been believed to account for the high toughness or stretchability via bond breakage and reformation in many previous literatures.[24] According to our original design, Zn2+/imidazole coordination complex, which has fast exchange kinetics and relatively small binding constant,[33,40] was employed as key component for spontaneous self-healing, and the molar ratio of Zn2+ to imidazole ligand was controlled as 1:4 to consciously assemble tetradentate bridged theoretically stable complexes.[40] However, UV-Vis titrations of two synthesized compounds, 1-butyl imidazole (BIMD, only imidazole moiety is in BIMD) and urea, N-butyl-N'-[3-(1H-imidazol-1-yl)

propyl] (UBIMD, imidazole and urea moieties are present in UBIMD) with Zn(CF3SO3)2 show when imidazole and urea ligands coexist, Zn2+ simultaneously coordinate with two ligands, and thus alter the original 1:4 Zn2+ to imidazole stoichiometry (Figure S7-S10). Considering the proven urea-induced hydrogen bonds in CSH-PPG networks, it is necessary to make out the influence of Zn(CF3SO3)2 on structures of supramolecular crosslinks. As a result, not only imidazole moiety, but also urea moiety participates in the coordination process, as evidenced by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) spectra of CSH-PPG-Zn sample. Compared with CSH-PPG, the new characteristic peaks at 256 cm-1 and 316 cm-1, respectively assigned to stretching vibration of Zn2+-N and Zn2+-O bonds,[50,51] appeared in Raman spectrum of CSHPPG-Zn (Figure 2b). Meanwhile, as shown in Figure 2c and 2d, the Zn2+-N (399.6 eV, high resolution N1s spectrum) and Zn2+-O (532.2 eV, high resolution O1s spectrum) could be easily distinguished from the deconvoluted peaks in XPS spectra of CSH-PPG-Zn, in agreement with Raman results, strongly evidencing the multicoordination modes[52]. 1H NMR titrations were performed to further elucidate the coordination modes for Zn(CF3SO3)2 and CSH-PPG in CDCl3 (Figure 2e, Figure S11). The clear downfield shifts of H1,2,3protons of imidazole moiety were observed and can be explained by a paramagnetic shielding effect induced by the Zn2+ bound imidazole ligands.[53] At the same time, the relatively small shifts of H4,5protons were also found particularly for urea signals (Figure 2e). It is worth noting that the proton sig-



nals of imidazole and urea moieties continued to shift toward downshift even when the molar ratio of Zn2+ to imidazole exceeded 1:4. Obviously, as a result of existence of urea moiety, substantial uncoordinated free imidazole ligands are present in CSH-PPG-Zn network, which increase the exchange kinetics of Zn2+/imidazole complex on the basis of the metal center preferred exchange mechanism proposed by Guan and coworkers.[40] It is believed that the accelerated exchange kinetics of Zn2+/imidazole complex and the enormous additional urea coordination sites caused by the multi-coordination modes are the two crucial factors for the unprecedented stretching behavior. Afterward, we carefully analyzed the stress-strain curve, which can be divided into four distinct regions as shown in Figure 2f and 2g. Region I (purple line): at small strain (ε

Extremely Stretchable, Self-Healable Elastomers with Tunable

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