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Covalently Cross-Linked and Mechanochemiluminescent Polyolefins Capable of Self-Healing and Self-Reporting

2020; Chinese Chemical Society; Volume: 3; Issue: 5 Linguagem: Inglês

10.31635/ccschem.020.202000303

2096-5745

Yakui Deng, Yuan Yuan, Yulan Chen,

Luminescence and Fluorescent Materials

Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2021Covalently Cross-Linked and Mechanochemiluminescent Polyolefins Capable of Self-Healing and Self-Reporting Yakui Deng, Yuan Yuan and Yulan Chen Yakui Deng Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, Tianjin University, Tianjin 300354. , Yuan Yuan Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, Tianjin University, Tianjin 300354. and Yulan Chen *Corresponding author: E-mail Address: [email protected] Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, Tianjin University, Tianjin 300354. https://doi.org/10.31635/ccschem.020.202000303 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Compared with non-cross-linked and dynamically covalent polymers, covalently cross-linked networks are irreplaceable in many areas; however, they are difficult to repair once fractured, due to limited polymer chain diffusion after cross-linking. Herein, the authors have reported a new kind of permanently cross-linked polyolefin, which when attached with amide side groups, yield mechanically robust yet readily repairable materials. A key is to use low cross-linking density, which enables satisfactory elasticity and chain mobility for thermodynamically favored healing. Another key is to incorporate dense hydrogen bonds that can undergo reversible associations. These factors jointly promise polyolefin networks with good mechanical properties and self-healing performance (recovered spontaneously up to 96% of its original tensile strength). More importantly, by means of mechanochemiluminescence from 1,2-dioxetane, which serves as the cross-linker and built-in self-reporting stress probe, a microscopic evaluation of how the chain entanglement proceeds upon healing and how failure occurs in the network can be obtained. Download figure Download PowerPoint Introduction Permanently cross-linked polyolefins represent an important class of engineering materials in many demanding applications, such as the aircraft industry, high-performance coatings, and so on.1,2 They have superior mechanical properties, dimensional stability, and solvent resistance relative to their linear counterparts.3,4 Besides, polyolefin-based elastomers are unique concerning their energy savings during production in comparison with other materials, their low cost and easily available raw materials, as well as their versatility in physical and mechanical properties.5 These advantages highlight their aggressive growth and tremendous potential. However, these polymeric materials are inevitably and irreversibly damaged due to the formation and propagation of cracks. To improve their safety features and lifetime, it is of great significance to develop intrinsic self-healing cross-linked polyolefins that can fully or partially heal spontaneously following mechanical damage. Moreover, revealing the mechanical events correlated to the healing and failure process is of fundamental interest and a priori useful in designing durable networks. Soft and ductile polymers often exhibit good healing performance, based on reversible associations of dynamically covalent bonds6–8 or supramolecular interactions,9–11 such as hydrogen bonds.12–14 However, with such reversible associations, they are relatively weak compared with their covalent analogs, or mostly, to reorganize the dynamically cross-linked networks for healing, external stimuli or catalyst loading is required.15,16 Introducing permanently covalent cross-links into a reversible network improves its mechanical properties, but usually at the expense of poor self-healing efficiency. This is because entropy changes of polymer networks are small due to limited mobility in the surroundings of polymer matrix confinements. In this regard, promoting segmental flexibility and introducing efficiently reversible associations in permanently cross-linked polyolefins become critical, yet a great challenge, for excellent self-healing performance with minimum intervention in robust polyolefin networks. Recent research efforts on polymer mechanochemistry have witnessed the development of self-reporting stress sensors that enable in situ monitoring of mechanical events with a clearly perceptible optical signal.17–22 One outstanding example was developed by Sijbesma et al. by using 1,2-dioxetane as the mechanophore.23 They found this molecular unit could be broken mechanically with concomitant light emission when incorporated in cross-links of the network.23–25 Such a mechanochemiluminescent process constitutes a sensitive way to probe the mechanochemical bond scission by translating the invisible mechanical signal into visible light in case of sufficient chain entanglement. As for self-healing polymers, it is recognized that the outcome of the healing process is to regain mechanical properties via chain entanglement.26–28 In turn, this knowledge highlights further utility of the mechanoluminescent probe: we thereby envision that if 1,2-dioxetane is integrated as the covalent cross-linker into healable systems, a microscopic view of self-healing behaviors in covalent networks, in particular when and how the interpenetrating network being recovered for efficient mechanical force transduction, would be obtained. Ring-opening metathesis polymerization (ROMP) is versatile in preparing polyolefin-based elastomers with readily tunable network structures and mechanical properties.29 Herein, with the availability of a bifunctional dioxetane precursor for ROMP, a new kind of mechanochemiluminescent, permanently cross-linked cyclooctene-based polyolefin was reported (Figure 1a). By exploring the chain flexibility and hydrogen-bonding functionality of polyolefin networks, autonomously self-healing behavior was realized from these robust nondynamically cross-linked elastomers. Importantly, optomechanical studies of the healing samples provide detailed information regarding the buildup of chain entanglement at the fracture surface during the healing process, which is beyond the capability of conventional macroscopic techniques. Figure 1 | (a) Schematic representation of self-healable and mechanoluminescent polyolefin network HBM-Ad-COEU. (b) Synthesis of HBM-Ad-COEU and the controls (N1, M1) in this study. Download figure Download PowerPoint Experimental Methods General All work involving air- and/or moisture-sensitive compounds was carried out in an Etelux glovebox or under a nitrogen atmosphere using standard Schlenk techniques unless otherwise noted. All materials were obtained from Sigma-Aldrich (St. Louis, Missouri, and USA) or Tokyo Chemical Industry (TCI) and used without further purification if not mentioned otherwise. The bulky cyclic olefin monomer exo-1,4,4a,9,9a,10-hexahydro-9,10(1′,2′)-benzeno-l,4-methanoanthracene (HBM) and 5-norbornene-2-yl methyl 11-(2-ethylhexanamido)undecanoate (NMEU) were prepared according to literature procedures.12 Grubbs' third-generation catalyst (G3) was stored and weighed in the glovebox. Dichloromethane (DCM) was distilled under argon over CaH2 prior to use. All reactions were performed under nitrogen atmosphere unless otherwise specified, and all glassware was oven dried before use. Detailed monomer synthesis [cyclooct-4-en-1-yl-11-(2-ethylhexanamido)undecanoate (COEU), bis(adamantyl)-1,2-dioxetane (Ad), and cyclooct-4-en-1-yl-11-(2-ethyl-N-methylhexanamido)undecanoate (COMU)], and additional characterization data for polymers can be found in the Supporting Information. Representative cross-linking procedure Taking HBM-Ad-COEU-1 for example, COEU (1188 mg), Ad (12 mg), and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole23 (6 mg) were mixed with 10 mL of anhydrous DCM in a Schlenk flask. Then the removal of oxygen was accomplished by applying at least three freeze–pump–thaw cycles. The G3 catalyst (3.8 mg) was weighed in a glovebox and dissolved in 3 mL of DCM. The catalyst solution was transferred into another Schlenk flask and cooled to −10 °C. After being stirred at −10 °C for 5 min, 5 mL of DCM solution of HBM (64 mg) was added and stirred for another 30 min. Then the solutions in the two Schlenk flasks were simultaneously poured into a Teflon mold for mixing and cross-linking in a glovebox for 72 h, followed by removing the solvent at 40 °C for 24 h under vacuum. Optomechanical testing Tensile experiments were carried out on a TA Rheometrics, Discovery Hybrid Rheometer-2 (DHR-2) equipped with an Xpansion Instrument, Sentmanat Extensional Rheometer 3 (SER3), and extensional fixture. The drums of the fixture were colored black by permanent marker to eliminate reflecting light. The transparent double-sided sticky tape was put on the drums to tightly fix the polymer film without slippage. The pco.edge (version 5.5; Kelheim, Free State of Bayern, Germany) camera equipped with a Nikon AF NIKKOR 50 mm 1∶1.4 D lens was used to record videos in darkness. All of the videos were recorded in the rolling shutter color mode with a shooting rate of 200 fps and exposure time of 5.0 ms. The frames of the resulting video were exported as separate monochrome TIF files, and light intensity was analyzed with a home-made program in MATLAB as literature.23 Results and Discussion As shown in Figure 1b, first, the steric bulk monomer, HBM, was subjected to ROMP with the Grubbs' third-generation catalyst (G3), affording the preoligomer of HBM (PHBM, Mn = 5.3 kDa, Supporting Information Figure S1).12 To construct a "soft" block with high segment mobility and introduce sufficient H-bonding association entities, the second monomer of COEU with pendant amide groups was prepared. Specifically, norbornene-modified Ad ( Supporting Information Scheme S1) was successfully developed, which can serve as both the cross-linker and mechanoluminescent stress probe in the polymer skeleton. Subsequently, the target loose networks HBM-Ad-COEUs were obtained through one-pot ROMP, using COEU as the monomer and Ad (0.60 mol % for HBM-Ad-COEU-1 and 0.74 mol % for HBM-Ad-COEU-2) as the cross-linker ( Supporting Information Table S1). To reveal the role of COEU as the soft segment to promote self-healing, a relatively rigid monomer NMEU was selected to offer the control sample N1.12 To investigate the effect of hydrogen-bonding, we synthesized another control sample M1 with blocked amide hydrogen bonds ( Supporting Information Table S1). Fourier transform infrared (FT-IR) spectra indicated that the cross-linked polymers were successfully obtained with characteristic peaks assignable to the amide groups in HBM-Ad-COEU-1, HBM-Ad-COEU-2, and N1. As for the control network M1, the disappearance of N–H stretching and in-plane bending bands proved its amide hydrogen bonds were blocked ( Supporting Information Figure S2).8,11 The cross-linking densities of HBM-Ad-COEUs were evaluated by swelling experiments.30,31 With cross-linker Ad content increasing, the swelling ratio in CHCl3 decreased, while the gel content increased instead ( Supporting Information Figure S3). In addition, the good thermal stability of HBM-Ad-COEUs was supported by thermogravimetric analysis (TGA; Supporting Information Figure S4). Differential scanning calorimetry (DSC) analyses demonstrated that all polymers were amorphous with a Tg below room temperature ( Supporting Information Figure S5); therefore, these materials are rubbery cross-linked elastomers at room temperature. Notably, compared with control N1, both HBM-Ad-COEU samples showed a rather low Tg (e.g., ca. −20 and −30 °C), which is essential to achieve excellent dynamics and reversibility at room temperature (Table 1).11,12 Table 1 | Mechanical Propertiesa and Tg of All Polymers Sample Tensile Strength (MPa) ɛb (%) TgDSC (°C) HBM-Ad-COEU-1 42 ± 4 243 ± 1 −30.7 HBM-Ad-COEU-2 116 ± 2 249 ± 15 −20.9 N1 123 ± 3 228 ± 13 3.1 M1 17 ± 2 212 ± 3 −18.1 HBM-Ad-COEU-1(9 h)c 38 ± 6 225 ± 37 — HBM-Ad-COEU-1(11 h)c 41 ± 1 230 ± 17 — aAll tensile tests were conducted at a Hencky strain rate of 8 s−1. bMaximum Hencky strain at break. cSelf-healed samples after the designated time. The mechanical properties of the as-synthesized networks were characterized via static tensile tests, which were carried out on a rheometer equipped with an Expansion Instruments SER Universal Testing Platform ( Supporting Information Figure S6).32 This geometry ensured a uniform extensional deformation of the sample at a high Hencky strain rate and facilitated the stress–strain relationship and in situ luminescence (see below) measurements by maintaining the center of the stretch zone in a fixed location. First, the incorporation of labile Ad did not weaken the mechanical properties of the network, as can be seen from the tensile results of the Ad-free material HBM-EG-COEU ( Supporting Information Figure S7). Since the mechanical properties of our networks were comparable with that of the documented secondary amide-containing cyclooctene-based polyolefins8,12,33 ( Supporting Information Figure S8 and Supporting Information Table S2), and HBM-Ad-COEU-2 with more cross-linkers was much tougher than HBM-Ad-COEU-1, we therefore concluded that such mechanical robustness can be mainly derived from their covalent cross-linking nature. Besides, the reinforcement effect brought by effective hydrogen-bonding interactions played a joint role, as can be deduced from the study of the control M1: going from M1 to HBM-Ad-COEU-1, an enhancement in maximum stress and Hencky strain at breakage (ca. 2.5- and 1.15-fold, respectively) was observed. The HBM-Ad-COEU samples exhibited excellent self-healing capability under ambient conditions. Typically, when the HBM-Ad-COEU-1 film (20 × 5.3 × 0.34 mm) was cut into two pieces, following by healing under ambient conditions for 11 h, the self-repaired sample could bear manual stretching without tearing (Figure 2a). Also, a scratch recovery test illustrated that the scratch on the HBM-Ad-COEU-1 film became obviously shallow after 11 h of healing (Figure 2b). To evaluate the mechanical properties and self-healing efficiencies of these samples more quantitatively, polymer films were first damaged (cut through 70–90% of width), which was then followed by the interfaces being brought together at room temperature without any pressure, and then mechanical tensile tests were conducted on the healable films over different time periods. The healing efficiency was quantified according to the recovery of tensile stress (%stress) relative to that of pristine sample, showing that healing was time dependent (Figures 2c and 2d; Supporting Information Figures S9–S11 and Supporting Information Table S3). HBM-Ad-COEU-1 exhibited excellent self-healing capability over other samples, that is, 96.2% stress recovery was achieved after 11 h. This contrasts with HBM-Ad-COEU-2, which recovered a tensile stress about 80.4% of its original value under the same conditions. In contrast, the control networks N1 and M1 exhibited only partial recovery of its mechanical properties (42.3% and 25.2%). Their poor self-healing performance could be ascribed to either the increased rigidity and stiffness of polymer backbone, or the blocked hydrogen bonds, and in turn, suggested that the superior self-healing capability of HBM-Ad-COEU-1 was dependent on both the thermodynamic and kinetic control.8,34 Figure 2 | (a) Images of self-healing process of full-cut HBM-Ad-COEU-1 films and (b) optical microscopic images of the scratched surface healed for 11 h at room temperature. Optomechanical tests of the self-healed HBM-Ad-COEU-1 films at a Hencky strain rate of 8 s−1: comparison of the (c) mechanical properties, (d) healable efficiency of tensile stress (%stress) and light intensity (%light), and (e) total light intensity emitted upon straining films. %stress is the recovery of the rupture stress of the healing sample relative to that of the pristine sample. %light is the recovery of mechanochemiluminescent light intensity of the healing sample relative to that of the pristine sample. Download figure Download PowerPoint The self-healing capability in covalently cross-linked elastomers is rare. Examples include olefin-containing network healing via Ru catalyst-mediated dynamic olefin cross metathesis,8,33 and a hybrid elastomer showing 30% healing of its original tensile strength driven by reversible hydrogen bonds.35 Herein, the exceptionally excellent self-healing behaviors from HBM-Ad-COEU samples are supposed to follow different mechanisms. First, it should be noted that despite the similar cyclooctene-based polyolefin skeleton to that of the polymers developed by Guan et al.,8 our networks did not self-repair in a dynamic metathetic way. The adaptive properties of the potentially Ru-activated self-healing behaviors in our networks were excluded, since the sample can fully return to its original shape after step cyclic tensile deformation ( Supporting Information Figure S12).12,33,36 Next, the relaxation behaviors of our elastomers were surveyed through a series of rheological tests, which can be good indicators of different self-healing properties. First, the frequency sweep tests with varying temperature from 30 to 70 °C demonstrated that these networks ( HBM-Ad-COEU-1 and HBM-Ad-COEU-2) were elastic solids over a wide range of shear frequencies and temperatures (storage modulus G′ > loss modulus G″; Supporting Information Figure S13).37,38 Given the loosely cross-linked characteristics of the networks and high loading of hydrogen bonds, the upward transition of both modulus on the tested frequency and temperature regions is probably due to the ease of polymer chain rearrangement and energy dissipation by sacrificial hydrogen bonds.8 Based on the data from dynamic shear measurements (see Supporting Information), the calculated relaxation times (λ) for the flow transitions of these networks are small (on the order of a second).39 In particular, HBM-Ad-COEU-1 exhibited the best self-healing performance at a shorter relaxation time and lower modulus than those of HBM-Ad-COEU-2 and N1, reflective of the flow of polymer chains greatly contributing to the excellent healing behavior ( Supporting Information Figure S14).6 The intercept of the plots of the square root of G″ against the square root of angular frequency can be regarded as a measure of the yield stress.39 As shown in Supporting Information Figure S15, HBM-Ad-COEU-1 has a lower yield stress than HBM-Ad-COEU-2 and the value decreases with increasing temperature, which indicates that HBM-Ad-COEU-1 has the highest polymer chain mobility amongst all obtained networks. Then, the healing and energy dissipation properties of HBM-Ad-COEU-1 sample were probed through a set of cyclic loading tests.8 Unlike other hydrogen bond-rich polymers, the hysteresis in the first loading–unloading cycle was unremarkable (Figure 3a), revealing satisfactory elastomeric properties of the network with little energy dissipation.35 Moreover, about 40% of the energy dissipation observed in cycle 1 was lost in cycle 2, followed by continuing slight decreases in the subsequent cycles (cycles 3–10), reflecting the additional elastic deformation that cannot relax to equilibrium on the time scales of individual cycles. After 10 cycles, the sample was allowed to relax under ambient conditions for 30 min to regain most of its original mechanical performance. The recovery of most dissipated energy over the short period is mainly due to the survival of covalent cross-links, which prevent permanent deformation and assist in the reformation of hydrogen bonds. Altogether, we can deduce that the permanent cross-linking nature dominates the overall mechanical strength featured with notable elastomeric properties, which simultaneously offers favorable thermodynamics for self-healing by fine-tuning the cross-link density. Furthermore, hydrogen-bonding interactions play an additional role in reinforcing the mechanical properties and self-healing performance of our networks (Figure 3b). Figure 3 | (a) Recovery and cyclic loading of HBM-Ad-COEU-1. HBM-Ad-COEU-1 specimens were loaded (150%), unloaded, and immediately reloaded 10 times (strain rate: 100 mm/min, 25 °C). Cycles 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11 are plotted for clarity. Cycle 11 took place after no stress for 30 min at room temperature. Bar graphs summarize the percent energy dissipation for each sample. (b) Proposed mechanism illustrating the high chain mobility and dense reversible hydrogen bonds accounting for excellent self-healing capacity of HBM-Ad-COEU network. Download figure Download PowerPoint Another distinct feature of our networks is their mechanochemiluminescent behavior upon deformation of these 1,2-dioxetane cross-linked films. Control networks cross-linked with bis(adamantanylidene) instead of Ad, or with physically mixed 1,2-dioxetane, cannot give light, confirming the mechanical nature of luminescence.23 Analysis of a representative video taken from the failure process of a HBM-Ad-COEU film demonstrates good temporal and spatial resolutions of mechanoluminescence (Figure 4 and Supporting Information Figure S16). That is, the sample starts to emit light uniformly throughout the film at low stress, followed by a strongly increased luminescent intensity with a pronounced localization of scission at the location of the fracture. After fracture, no further luminescence is observed. Similar transient and stress-dependent light emission were also recorded from N1 ( Supporting Information Figure S11d). The results highlight the ability of dioxetane to visualize the stress distribution and covalent bond scission information within the network. Figure 4 | Optomechanical test of HBM-Ad-COEU-2 in time and space. (a) Light intensity analysis of HBM-Ad-COEU-2 film during stretching at a Hencky strain rate of 8 s−1, displayed with analyzed images at a Hencky strain >2.1. (b) Evolution of stress and light intensity versus Hencky strain of HBM-Ad-COEU-2. The analyzed intensity is based on the same region within the sample. (Inset is the picture showing light emission at fracture.) Download figure Download PowerPoint The question about how the chemical and physical events recover during the repair process is difficult to understand because the resolution of most developed characterization methods for self-healing elastomers cannot reach the microscale.27 Again, benefiting enormously from the 1,2-dioxetane probe, recovery of chain entanglement can be characterized at the microscopic level. As previously mentioned, the transduction and accumulation of force in the fractured surface can proceed smoothly with 1,2-dioxetanes being activated only when sufficient chain entanglement has accumulated. Therefore, the self-healing efficiency regarding the recovery of chain entanglement can be also evaluated in terms of the recovery of light intensity (%light as the recovery of the rupture stress of the healing sample relative to that of the pristine sample), instead of the macroscopic evaluation by %stress.40 Similar to the findings of recoverable tensile stress, HBM-Ad-COEU-1 exhibited the highest self-healing efficiency with %light reaching 60.5% and 92.1% after healing for 9 h and 11 h, respectively (Figures 2d and 2e), while the mechanoluminesent intensity of HBM-Ad-COEU-2 was recovered only up to 79.6% after 11 h ( Supporting Information Figure S10). Notably, at the early stage of healing, for instance 9 h, %light appeared much lower than %stress ( Supporting Information Table S3), and the deviation value of %stress–%light gradually became smaller as healing proceeded. Therefore, the recovery of mechanoluminesent intensity lags behind that of the tensile stress. This is reasonable that in the earlier stage of healing, chain entanglement contributes to the experienced stress, but is not effective enough to pull other network strands in high enough stress to activate the mechanophore. At later stages, %stress and %light coincide with each other, indicating the reconstruction of effective chain entanglement for efficient mechanotransduction, or, in other words, self-healing of our elastomer was mostly accomplished after 7 or 9 h, but microscopically, the process was far from completed with the most efficient self-healing and accumulation of interpenetrating network occurring at later stages. Such microscopic descriptions of healing also work for control networks N1 and M1, whose recovery of mechanoluminescence was maintained at a rather low level due to their self-healing abilities, and similarly, lagged behind the mechanical property recovery ( Supporting Information Table S3). Thus, the 1,2-dioxetane-based mechanoluminescent probe is able to yield more fundamental and precise insights into the microscopic origins of self-healing, relative to macroscopic techniques. Conclusions A new kind of covalently cross-linked polyolefin that is mechanically robust and capable of self-healing and self-reporting was synthesized. We have demonstrated that the key structural elements for their prominent mechanical and self-healing properties are the low covalent cross-linking density and dense hydrogen bonds. By combining actions from two effects—the high chain mobility and large number of noncovalent associations, the obtained networks, although permanently cross-linked, exhibited satisfactory elasticity and excellent self-healing performance, with a maximum 96% recovery of its original tensile strength. Moreover, taking advantage of the mechanoluminescent stress probe that allows the study of bond scission events with high sensitivity and resolution, microscopic pictures of the failure events and the reconstruction of effective chain entanglement during healing are offered. Altogether, the present findings greatly extend our knowledge of the structures and functions of self-healing networks. Our design strategy by molecular enforced integration of self-healing and self-reporting moieties is expected to be versatile and simple for access to many other permanently cross-linked networks. Supporting Information Supporting Information is available. Conflict of Interest There is no conflict of interest to report. Acknowledgments This research was made possible as a result of a generous grant from the National Key Research and Development Program of China (grant nos. 2017YFA0207800 and 2017YFA0204503) and the National Natural Science Foundation of China (grant nos. 21975178 and 21734006). The authors appreciate Prof. Z. Ma (Tianjin University) for his helpful discussions. References 1. Röttger M.; Domenech T.; Weegen R.; Breuillac A.; Nicolaÿ R.; Leibler L.High-Performance Vitrimers from Commodity Thermoplastics Through Dioxaborolane Metathesis.Science2017, 356, 62–65. Google Scholar 2. Sauter D. W.; Taoufik M.; Boisson C.Polyolefins, a Success Story.Polymers2017, 9, 185. Google Scholar 3. Doremaele G.; Duin M.; Valla M.; Berthoud A.On the Development of Titanium κ1-Amidinate Complexes, Commercialized as Keltan ACE™ Technology, Enabling the Production of an Unprecedented Large Variety of EPDM Polymer Structures.J. Polym. Sci. 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Yan C.; Yang F.; Wu M.; Yuan Y.; Chen F.; Chen Y.Phase-Locked Dynamic and Mechanoresponsive Bonds Design toward Robust and Mechanoluminescent Self-Healing Polyurethanes: A Microscopic View of Self-Healing Behaviors.Macromolecules2019, 52, 9376–9382. Google Scholar Previous articleNext article FiguresReferencesRelatedDetails Issue AssignmentVolume 3Issue 5Page: 1316-1324Supporting Information Copyright & Permissions© 2020 Chinese Chemical SocietyKeywordsself-reportingmechanochemiluminescencepolyolefinsself-healingcovalently cross-linkedAcknowledgmentsThis research was made possible as a result of a generous grant from the National Key Research and Development Program of China (grant nos. 2017YFA0207800 and 2017YFA0204503) and the National Natural Science Foundation of China (grant nos. 21975178 and 21734006). The authors appreciate Prof. Z. Ma (Tianjin University) for his helpful discussions. Downloaded 1,491 times Loading ...