Self-Healing of a Cross-Linked Polymer with Dynamic Covalent

Article pubs.acs.org/Macromolecules

Self-Healing of a Cross-Linked Polymer with Dynamic Covalent Linkages at Mild Temperature and Evaluation at Macroscopic and Molecular Levels Keiichi Imato,†,‡ Atsushi Takahara,*,‡,§ and Hideyuki Otsuka*,† †

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan ‡ Graduate School of Engineering and §Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: A lot of self-healing materials using dynamic bonding systems have been reported, while the focus is mainly on the macroscopic self-healing behavior such as visually recognizable healing. Because the healing originates from microscopic chemical reactions of the dynamic bonds, evaluation of the reactions in the materials is necessary for elucidation of the healing mechanisms and development of the healing ability. Herein, we demonstrated self-healing of a cross-linked polymer with diarylbibenzofuranone (DABBF)-based dynamic covalent linkages at mild temperature and investigated the healing behavior from both macroscopic and microscopic viewpoints. The macroscopic behavior was inspected by mechanical tests, and the linkage reaction (equilibrium) was evaluated by electron paramagnetic resonance measurements. These assessments revealed that the healing is strongly dependent on temperature, which is attributable to synergism between changes in the chain mobility and in the equilibrium of the incorporated linkages. These findings would be applicable to other dynamic bonding systems.

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INTRODUCTION Materials with self-healing ability are particularly attractive in the field of materials science because these materials have the intrinsic capacity to repair damage done to them without intervention. Although small external and internal damages in materials generally result in irreparable damage, self-healing can obviate such risks and thereby lead to reduced waste and improved lifetime, durability, and reliability of materials. In addition, the application of self-healing materials to artificial organ and space development purposes, where there is little scope for intervention and assisted repair, is anticipated. Selfhealing of polymeric materials has been achieved by various approaches such as monomer release and subsequent polymerization,1−3 unique entropic elasticity of slide-ring networks,4 topological interaction of dangling chains in polymer networks,5,6 and irreversible chemical reactions (re-cross-linking).7−10 Similarly, reversible dynamic bonds (or interactions) have been utilized for polymer self-healing with the advantage of an unlimited number of healing times.11 Dynamic bonding systems can be categorized into two classes: healing in the gel state with solvent and in the bulk state without solvent. Although the polymer chains in gels have higher mobility than in the bulk state and are therefore expected to exhibit better healing ability, gels generally suffer from volatilization of © XXXX American Chemical Society

solvents and show lower mechanical properties, with some exceptions.12−16 Therefore, self-healing in the bulk state is more desirable, except in cases where these materials are intended for biomaterials application. 17 Various dynamic bonds and reactions are prospectively exploitable for healing bulk polymers, including hydrogen bonding,18,19 coordination bonds,20 π−π stacking interactions,21,22 Diels−Alder reactions,23−26 disulfide bonds,27−31 trithiocarbonate linkages,32 alkoxyamine linkages,33−39 siloxane chemistry,40 transesterification,41 olefin metathesis reactions,42 and urea bonds.43 There has been a recent shift of the focus to healing with high mechanical performance that has been realized by exploiting phase separation,44−46 nanocomposites,47−53 and crystalline components54,55 to enable practical applications.56−59 However, previous reports on healing in dynamic bonding systems have mainly focused on the macroscopic healing behavior such as visually recognizable healing and mechanical properties. Because the healing originates from microscopic chemical reactions and equilibria of the dynamic bonds, detailed investigation of the healing process from the microscopic viewpoint is necessary for elucidation of the mechanisms and Received: April 18, 2015 Revised: July 30, 2015

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DOI: 10.1021/acs.macromol.5b00809 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. (a) Equilibrium between diarylbibenzofuranone (DABBF) and the corresponding radicals, (b) synthesis scheme for cross-linked polymer with DABBF linkages, and (c) schematic illustration of the healing of two segments of the cross-linked polymer by bond exchange between the DABBF linkages.

improvement in the healing ability but has been rarely undertaken except for the Diels−Alder and alkoxyamine systems.23,33−39 Therefore, there are still gaps in the comprehension of the modus operandi of these dynamic bonding systems for self-healing. Here, we demonstrate self-healing of a cross-linked polymer with exchangeable dynamic covalent bonds at mild temperature and evaluate the polymer from the macroscopic perspective as well as in terms of the microscopic chemical reaction to elucidate the healing mechanism. The strategy utilizes diarylbibenzofuranone (DABBF), in which the central C−C bond is in a state of equilibrium between homolytic bond cleavage and recombination at room temperature (Figure 1).60,61 The radical species formed by cleavage of DABBF are stable in air, although carbon-centered radicals generally react readily with oxygen.62−66 This unique radical process enables electron paramagnetic resonance (EPR) spectroscopic assessment of the equilibrium in the polymer, where the radicals can be detected in situ and the radical concentration can be quantitatively estimated. We have previously reported network rearrangement and self-healing of gels with DABBF linkages, but applications and detailed analysis of the healing were limited due to the

issues of solvent volatilization and low mechanical properties.67−70 Therefore, in this study, we designed a DABBFcontaining cross-linked polymer that is healable at mild temperature in the bulk state without solvent, which enables detailed evaluation of the healing mechanism by means of mechanical tests, rheological tests, and EPR measurements.

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RESULTS AND DISCUSSION Preparation of Cross-Linked Polymer. A cross-linked polymer with DABBF linkages was prepared by polyaddition of poly(propylene glycol) (PPG) (Mn = 2700), hexamethylene diisocyanate (HDI), dihydric DABBF, and triethanolamine (TEA) as a cross-linker in the presence of di-n-butyltin dilaurate (DBTDL, catalyst) in N,N-dimethylformamide (DMF) in a manner similar to that previously reported (Figure 1).67,69,70 Because the cross-linking points are not highlybut triplybranched and the theoretical molecular weight between the cross-linking points is approximately 7000, this network would be flexible. The fluidity of the reaction mixture slowly decreased during the course of the reaction. The reaction mixture completely lost fluidity after 48 h, indicating that polymerization was successful. After polymerization, the crude B


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Figure 2. (a) Photographs of cross-linked polymer film with DABBF linkages illustrating self-healing behavior at 50 °C: original, cut, healed after 12 h, and stretched. (b) Typical stress−strain curves for the polymer before and after healing for different times (6, 12, 24, and 72 h) at 50 °C. Tensile tests were performed by using a strain rate of 833%/min at 25 °C. (c) Dependence of fracture strain and maximum stress on healing times illustrating degree of recovery of the healed polymer. Error bars show maximum and minimum values for six specimens. (d) Typical stress−strain curves for the polymer before and after healing for 24 h at different temperatures (30, 40, and 50 °C). Tensile tests were performed by using a strain rate of 833%/min at 25 °C. (e) Dependence of fracture strain and maximum stress on healing temperatures illustrating degree of recovery of the healed polymer. Error bars show maximum and minimum values for six specimens.

gel was immersed in chloroform at −15 °C to remove the catalyst and unreacted monomers, then immersed in hexane at room temperature to shrink the gel, and subsequently dried. Differential scanning calorimetric (DSC) analysis of the obtained polymer indicated that the glass transition temperature (Tg) was −58 °C (Figure S1, Supporting Information). The polymer was swollen with excess chloroform, which is a good solvent for the polymer, and formed a gel at 0 °C, confirming the sufficient cross-linking (Figure S2). The polymer could also be swollen with a small amount of chloroform at room temperature, but the polymer dissolved completely in an excess of chloroform at room temperature. This may be due to network expansion and elution of cyclic oligomers formed through bond exchange between the DABBF

linkages.69 We utilized this property to investigate the polymer structure via 1H NMR in CDCl3 solution (Figure S3). The composition ratio was determined to be TEA/DABBF/HDI/ PPG = 1/1.40/5.20/2.30, which was largely similar to that of the reaction mixture (1/1.5/6/3). The polymer could also be processed into films at room temperature by the solution casting method, and the cast films were punched into dumbbell-shaped specimens for tensile tests (Figure S4). Self-Healing at Mild Temperature. The macroscopic selfhealing behavior of the obtained polymer at mild temperature could be demonstrated by using a lizard-shaped film, as shown in Figure 2a. The polymer film was cut with a razor blade, and the cut pieces were immediately pressed together tightly. After healing for 12 h at 50 °C, the polymer did not fracture at the C


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Figure 3. Optical microscope images of self-healing behavior of scratched cross-linked polymer films with DABBF linkages healed at 50 °C for 12 h (top), at 40 °C for 72 h (middle), and at 30 °C for 72 h (bottom). Scale bars are 0.1 mm.

cut and resealed position even under manual stretching stress. On the other hand, a chemically cross-linked polymer with a similar chemical structure to the healable polymer but without DABBF linkages could not heal damage after placing it at 50 °C for 24 h (Figure S5). These results indicate that the healing ability originates from the dynamic DABBF linkages. To quantitatively evaluate this healing behavior, tensile tests were performed using the dumbbell-shaped specimens, and the effects of the healing time and temperature on the healing process were systematically evaluated. Figure 2b shows typical stress−strain curves for the polymer before and after healing for 6, 12, 24, and 72 h at 50 °C. The stress−strain curves of the damaged specimens gradually approached that of the original intact specimen with increasing healing time. Recovery of more than 90% of the original fracture strain and maximum stress was possible over a period of 12 h, as shown in Figure 2c. Notably, the specimens healed for more than 12 h fractured at random positions, similar to the intact specimens, although those healed for 6 h fractured only at the cut and resealed positions (Figure S6). This observation also indicates that the original mechanical properties were regained in the damaged specimens, and a state of equilibrium was achieved after healing for 12 h. Figure 2d shows typical stress−strain curves for the polymer before and after healing for 24 h at 30, 40, and 50 °C. Healing at higher temperature led to more efficient healing. The specimens that were healed at 30 and 40 °C fractured at the cut and resealed positions, while those healed at 50 °C fractured at random positions for the same reason as discussed in the preceding paragraph (Figure S7); that is, temperature strongly affected the healing behavior. However, recovery of approximately 50% of the original fracture strain and maximum stress could be achieved after healing at a temperature as low as 30 °C for 24 h, indicating that the polymer was self-healable at mild temperature (Figure 2e). In addition to the relatively high self-healing ability, the

Young’s modulus of the polymer (0.35 MPa) was 7-fold that of a gel cross-linked by DABBF linkages (0.05 MPa) that we previously reported.67 For detailed analysis of the healing behavior, healing of scratches on the film surface was evaluated. The polymer films were scratched with a needle (about 0.1 mm wide cuts), and the healing processes at 30, 40, and 50 °C were observed by optical microscopy, as shown in Figure 3. At 50 °C, the scar gradually disappeared with time and was almost invisible after 12 h. Similarly, the scar was almost undetectable after healing for 72 h at 40 °C. On the other hand, the scratch remained evident even after healing for 72 h at 30 °C. Early in the healing process, the scratch closed to a certain extent, probably due to elastic recovery, whereas no subsequent notable change was observed. These results are consistent with those of the tensile tests presented above, confirming the significant influence of temperature on the healing. Evaluation of Self-Healing from Microscopic Viewpoint. The influence of the thermal mobility of the polymer chains on the temperature effect was evaluated via dynamic mechanical analysis (DMA). As shown in Figure 4a, large αarelaxation was observed around −50 °C, and the polymer was rubbery at 30, 40, and 50 °C. However, the storage elastic modulus decreased slightly with increasing temperature subsequent to αa-relaxation, although the storage elastic modulus of chemically cross-linked polymers usually increases with temperature due to entropic elasticity. This probably originates from a decrease in the cross-linking density derived from a shift in the equilibrium of the DABBF linkages to the dissociated state with increasing temperature, which makes the chain motion and bond exchange more rapid and leads to better healing. To straightforwardly investigate actual changes in the equilibrium, that is, the ratio of dissociated DABBF linkages with respect to temperature, EPR measurements of the polymer D


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Figure 4. (a) Temperature dependence of dynamic viscoelasticity (1 °C/min) of cross-linked polymer with DABBF linkages at constant frequency (1 Hz). (b) EPR spectra of the polymer in the temperature range varying from −100 to 80 °C. (c) Percentage of dissociated DABBF linkages at different temperatures in the range varying from −100 to 80 °C. (d) van’t Hoff plot for the DABBF linkages in the polymer showing high linearity.

were performed. Figure 4b shows the EPR spectra acquired in the temperature range varying from −100 to 80 °C. The g value determined from these spectra was 2.0034, suggesting the presence of oxygen radicals and carbon radicals. Therefore, the spectra originated from the radicals formed from cleavage of the DABBF linkages in the polymer (Figure S9).69,70 Below 0 °C, the peak intensity was negligible but increased as the temperature increased, indicating that the equilibrium position of the linkages shifted to the dissociated state. Additionally, slight splittings were observed in the spectra with increasing temperature due to the increased mobility of the detected radicals. Based on the assumption that the equilibrium simply involves the associated DABBF and the dissociated radicals without irreversible side reactions, the ratio of dissociated DABBF linkages was calculated from the area of observed integral spectra by using 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) as a standard (Figure 4c). The ratio was extremely low below 0 °C, i.e., approximately 0.001%, and was about 0.002% at room temperature, indicating that the DABBF linkages in the polymer were in equilibrium and a minor amount of these linkages dissociated at such low temperatures. Above 0 °C, the ratio of dissociated linkages increased exponentially with increasing temperature and was approximately 0.018% at 80 °C. The equilibrium was found to be highly sensitive to temperature above 0 °C. Consequently, higher temperature led to more dissociation of the linkages,

higher chain mobility, more rapid bond exchange, and better healing in the polymer. Stated another way, because polymer chain motion generally becomes more rapid with increasing temperature, the temperature effect on the above-mentioned healing process was affected by the synergy between the changes in the thermal chain mobility and in the equilibrium state in response to variation of the temperature. It is also worth noting that the ratio of dissociated linkages was quite small while the polymer showed self-healing at mild temperature, which is due to the dynamic nature of DABBF. The bond dissociation energy (ΔH) and dissociation entropy (ΔS) of the central C−C bond in the DABBF linkages in the polymer were estimated from a van’t Hoff plot; ΔH and ΔS can be determined from the slope and intercept, respectively. Figure 4d shows the high linearity of the van’t Hoff plot for the linkages in the polymer. The ΔH and ΔS values were 16.2 kcal mol−1 and 10.7 cal K−1 mol−1, respectively, which were smaller than those of the dihydric DABBF monomer in certain organic solvents (ΔH = 20.4−24.3 kcal mol−1 and ΔS = 22.2−37.5 cal K−1 mol−1).70 This result may suggest that the thermal mobility and large entropy of the polymer chains linked to DABBF units caused the cleavage to be enthalpically easier and entropically more difficult. Therefore, it was confirmed that the equilibrium state of the DABBF linkages in the polymer was definitively different from that of the DABBF monomer in solution. E


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Macromolecules It is presumed that self-healing via dynamic bonding systems in polymeric materials involves preferential cleavage of the dynamic bonds in response to external force and that healing proceeds by recombination of the dynamic bonds, subsequent bond exchange, and network rearrangement. However, the preferential cleavage of the dynamic bonds in self-healing materials has not been conclusively demonstrated. Herein, EPR data for the DABBF-containing cross-linked polymer were acquired under tensile deformation (100% strain) or after cut damage in order to demonstrate dissociation of the DABBF linkages and radical generation by the external forces. However, no pronounced increase in detected radicals in response to the forces was observed (Figures S10 and S11). This is probably because the strain and extent of the damage were too small for cleavage of a detectable amount of DABBF linkages and/or because the recombination was too quick for the cleavage to be observed. A greater extent of deformation and damage for quantitative evaluation of the radicals was not possible. Thus, preferential cleavage of the DABBF linkages in the polymer by external force could not be conclusively demonstrated. Network Rearrangement. Evaluation of network rearrangement is important for elucidation of healing mechanisms in the dynamic bonding systems because such healing involves the rearrangement as mentioned above. We investigated the network rearrangement of the polymer through the creep and strain recovery. The strain increased with time under constant stresses of 0.05 and 0.1 MPa; the rates were 13%/h at 0.05 MPa and 43%/h at 0.1 MPa (Figure 5a). After removal of the stress, the strain recovered to some extent, but residual strain was observed in both cases. Chemically cross-linked polymers generally show a constant strain under a constant stress and recover to almost the original shapes after removal of the stress. Therefore, this unique behavior of the DABBF-containing cross-linked polymer was attributed to bond exchange of the DABBF linkages and resulting network rearrangement, as previously reported in addition−fragmentation chain transfer,71 transesterification,72 olefin metathesis,73 and hydrogen-bonding systems.74,75 After removal of the constant stress of 0.1 MPa, the residual strain increased as the holding time increased (Figure 5a). The residual strain at the respective times was 17% for 0.5 h, 23% for 1 h, and 34% for 2 h. This trend also indicates that the network underwent rearrangement under the constant stress via bond exchange of the DABBF linkages. Additionally, the polymer showed slow stress relaxation under constant strain (Figure S12). The relaxation time (τ) values were 5 h under 50% strain and 8 h under 100% strain. The long relaxation times are due to origination of the relaxation from topological rearrangement of the network. These results also indicate that the polymer is dynamic at mild temperature; that is, the DABBF linkages in the polymer are exchanging their bonds, in spite of the small ratio of dissociated (Figure 4c). We also investigated the network rearrangement of the polymer through the hysteresis of the stress−strain curve. The polymer showed large hysteresis in the stress−strain curve and showed residual strain, even after small elongation (Figure 5b and Figure S14), indicating that the network structure changed just after the start of elongation. However, this residual strain decreased with time after the loading−unloading test (Figure 5c), and the curve almost completely recovered to the original curve after a short waiting time (10 min). Because almost all of the DABBF linkages in the polymer were bound at 25 °C, as shown in Figure 4c, the network was recovered by entropic elasticity. On the other hand, the bond exchange and

Figure 5. (a) Creep measurements of cross-linked polymer with DABBF linkages under stresses of 0.05 MPa for 1 h and 0.1 MPa for 0.5, 1, and 2 h at 25 °C and subsequent strain recovery measurements. (b) Recovery of the polymer for different waiting times performed by cyclic tensile tests at a strain rate of 83%/min at 25 °C. (c) Waiting time dependence of the residual strain. The residual strains were calculated based on the strains at 0.01 MPa.

subsequent network rearrangement delayed the recovery.76 For these reasons, the delayed but almost complete recovery of the network was observed, which is considered self-healing at the molecular level. Finally, the recyclability of the polymer was evaluated for a demonstration of the dynamic network. Generally, chemically cross-linked polymers cannot be processed and reshaped after synthesis. Because the DABBF-containing cross-linked polymer F


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is operative under mild conditions. Furthermore, the DABBF system enables in situ evaluation of the dynamic bonds in materials unlike other dynamic bonding systems, and the findings of this study would be applicable to the systems because they have similar healing mechanisms. Thus, the present system would facilitate deeper understanding of the healing mechanism of the dynamic bonding systems and improvement of the healing ability.

dissolved in an excess of chloroform at room temperature, used films that had previously been subjected to tensile tests and fractured could be recast (Figure 6a). The mechanical properties of the recast films were largely similar to those of the initially cast films, indicating the good recyclability of the polymer (Figure 6b).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00809. Experimental details, synthetic procedures, characterization data, and additional figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (H.O.). *E-mail [email protected] (A.T). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Funding Program for Next Generation World-Leading Researchers (No. GR077) from Japan Society of the Promotion of Science (JSPS) and JSPS KAKENHI (No. 26288057 and No. 26620175). This work was also funded by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

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REFERENCES

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Figure 6. (a) Photograph of a dumbbell specimen punched out from a recast film of cross-linked polymer with DABBF linkages. (b) Young’s moduli, fracture strains, and maximum stresses of the first and second cast films of the polymer measured using a strain rate of 833%/min at 25 °C.

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CONCLUSIONS In this study, a cross-linked polymer with DABBF linkages showed good self-healing at mild temperature, and the healing behavior was evaluated from the macroscopic and microscopic viewpoints in detail. Almost complete recovery of the mechanical properties was achieved after healing at 50 °C for 12 h, and the healing was strongly dependent on temperature. DMA and EPR measurements revealed that the significant temperature effect was attributable to synergism between the changes in the thermal chain mobility and in the equilibrium state of the DABBF linkages. Additionally, we found that the equilibrium state of the linkages in the polymer, including the bond dissociation energy and dissociation entropy, was notably different from that of the DABBF monomer in solution. The healable polymer also exhibited unique mechanical properties and good recyclability due to bond exchange of the linkages and subsequent network rearrangement. We believe that this healing system employing DABBF linkages is potentially exploitable for sustainable materials because the healing process G


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