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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Distinct Mechanical and Self-Healing Properties in Two Polydimethylsiloxane Coordination Polymers with Fine-Tuned Bond Strength Da-Peng Wang,† Jian-Cheng Lai,† Hui-Ying Lai, Sheng-Ran Mo, Ke-Yu Zeng, Cheng-Hui Li,* and Jing-Lin Zuo* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China S Supporting Information *

ABSTRACT: Coordination bonds are effective for constructing highly efficient self-healing materials as their strength is highly tunable. To design self-healing polymers with better performance, it is important to get a profound understanding of the structure−property relationships. However, this is challenging for self-healing polymers based on coordination bonds, because many parameters, such as bond energy, bond dynamics, and coordination number will have an essential effect on the mechanical and self-healing properties of the polymer. In this work, we synthesized two poly(dimethylsiloxane) (PDMS) polymers cross-linked by different Zn(II)-diiminopyridine coordination complexes (denoted as PDMS-NNN-Zn, PDMS-MeNNN-Zn respectively). The two cross-linking Zn(II)-diiminopyridine complexes are similar in coordination modes, but differ in coordination dynamics. As manifested by ITC, rheology, and tensile experiments, we confirm that the coordination bond in PDMSMeNNN-Zn polymer films is weaker but more dynamic. Consequently, the PDMS-MeNNN-Zn polymer has poorer mechanical strength but higher stretchability and better self-healing properties. The inflicted cracks on PDMS-MeNNN-Zn polymer films can be completely healed after healing at room temperature for only 30 min with healing efficiencies higher than 90%. Such fast self-healing properties have never been achieved in self-healing polymers based on coordination bonds. Our results also demonstrate the important impact of the thermodynamic stability and kinetic lability of coordination complexes on the mechanical and self-healing properties of polymers. Such a comprehensive understanding is helpful for further design of novel synthetic polymers, which can achieve an optimal balance between the mechanical strength and self-healing performance.

1. INTRODUCTION

Coordination bonds are a unique type of noncovalent interactions which form between a metal ion (known as coordination center) and its surrounding array of organic molecules (known as ligands). Such interactions are highly tunable in strength. With different combinations of metal ions and ligands, the bond strength could be readily adjusted in a broad range from approximately 2595% of a covalent C−C bond (with a bond energy of about 350 kJ/mol). As a result, we can take advantage of the abundant well-studied metal−ligand combinations to construct self-healing materials. Rowan et al. reported a [Zn(Mebip) 2 ] 2 + (Mebip = 2,6-bis(1′methylbenzimidazolyl)pyridine) complex cross-linked metallosupramolecular polymers that can be mended through exposure to light.34,35 Holten-Andersen et al. demonstrated pH-induced metal−ligand cross-links that yield self-healing polymer networks with near-covalent elastic moduli.36 Our group also have been engaged in developing highly stretchable

Self-healing ability is an important survival feature in nature, with which living beings can repair damage when wounded. To mimic the self-healing properties of natural biomaterials and extend the lifetime of the material in numerous applications, many synthetic self-healing polymers which can repair the internal and/or external damages have been developed.1−4 The initial polymeric self-healing materials relied on encapsulating healing agents (in microcapsules5−9 or microvascular10−12 networks) into bulk polymer. Because of the consumption of the encapsulated agents, the repair of such systems is not repeatable. To solve this problem, reversible bonds (such as alkoxyamine,13 disulfide14−16), cross-linking reaction (such as diels-alder reaction,17−19 radical dimerization reaction,20 cycloaddition reaction21,22) or weak dynamic bonds (such as hydrogen bonds,23−28 π−π stacking interactions,29−31 host− guest interactions32,33) are typically incorporated into polymers as cross-linkages so that they will break first upon damaging and reform after external stimuli, leading to repeatable self-healing. © XXXX American Chemical Society

Received: December 27, 2017

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DOI: 10.1021/acs.inorgchem.7b03260 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

distilled from Na prior to use. 2,6-pyridinedicarboxaldehyde, 2,6diacetylpyridine and the remaining reagents were purchased from Sigma-Aldrich and used without purification. 1H NMR (500 MHz) spectra were recorded on a Bruker DRX 500 NMR spectrometer in deuterated solvents at room temperature (25 °C). Chemical shifts are reported in ppm relative to tetramethylsilane as an internal standard (1H). ESI-mass spectra (ESI-MS) were recorded by direct injection on an Agilent mass spectrometer with the acetonitrile as solvent and mobile phase. The ionization temperature was set as 290 °C and measured under the positive ion mode. UV−vis absorption spectra were measured on a UV-3600 spectrophotometer at room temperature. Thermal gravimetric analysis (TGA) data were obtained on a STA 449C thermal analysis system with a heating rate of 10 °C/min under N2 atmosphere. Differential scanning calorimetry (DSC) experiments were performed using a Mettler DSC 1 analyzer under dry nitrogen atmosphere (50 mL/min). Temperature and enthalpy calibrations were performed before the experiments using zinc and indium standards. The temperature range is from −120 to 50 °C, at a heating and cooling speed of 10 °C/min under a nitrogen atmosphere. For each sample, two cooling−heating runs were performed and the data were obtained from the second cooling−heating curves. 2.2. Synthesis of Model Ligands and Their Corresponding Zn(II) Complexes. Ligand L1. 2,6-Ppyridinedicarboxaldehyde (135.2 mg, 1 mmol) was dissolved in 20 mL of anhydrous methanol and methylamine in methanol (33 wt %, 345 μL, 2.2 mmol) was slowly added to the solution with stirring. Four Å molecular sieves (2 g) was put into the solution as water absorbent. The reaction was sealed and stirred at room temperature in the fume hood for 6 h. The reaction mixture was allowed to filter to remove 4 Å molecular sieves and the filtrate was evaporated by rotary evaporation, yielding the yellow oily liquid product (137.6 mg, yield 90.1%). 1H NMR (500 MHz, CDCl3) δ 8.37 (s, 2H), 7.90 (d, J = 7.7 Hz, 2H), 7.74 (t, J = 7.7 Hz, 1H), 3.53 (s, 6H). ESI-MS m/z (%): calcd for C9H11N3 162.10 [M + H] +; found 162.17 (100) [M + H] +. Ligands L2, L3, and L4 were synthesized in the similar procedure as ligand L1 (see details in the Supporting Information). The 1H NMR spectra for all these ligands are shown in Figures S1−S4. Complex [Zn(L1)2](OTf)2. Ligand L1 (54.6 mg, 0.34 mmol) was dissolved in 20 mL of anhydrous methanol and Zn(OTf)2 in methanol solution (0.2 g/mL) (309 μL, 0.17 mmol) was slowly added to the solution with stirring. The reaction was sealed and stirred at room temperature for 12 h. The reaction mixture was evaporated by rotary evaporation, yielding red solid powder (114.3 mg, yield 99.5%). 1H NMR (500 MHz, CD3CN): δ 8.69 (s, 2H), 8.60 (t, J = 7.8 Hz, 1H), 8.25 (d, J = 7.8 Hz, 2H), 3.11 (s, 6H). ESI-MS m/z (%): found 193.17(100) [Zn(L1)2]2+. Complexes [Zn(L2)2](OTf)2, [Zn(L3)2](OTf)2 and [Zn(L4)2](OTf)2 were prepared in the same procedure (see detailed description in the Supporting Information). 2.3. Synthesis of PDMS Ligands and PDMS-Zn Polymer Films. PDMS-NNN Ligand. 2,6-Pyridinedicarboxaldehyde (1.35 g, 10 mmol) was added to a solution of H2N-PDMS-NH2 (53 g, accurate molecular weight was determined by 1H NMR as shown in Figure S8) in redistilled toluene (100 mL) with stirring under argon atmosphere. The solution of TsOH·H2O (19.0 mg, 0.1 mmol) in redistilled toluene (20 mL) was added dropwise as a reaction catalyst. Dean−Stark trap was used to remove the water generated during the reaction process. The resulting mixture was stirred for 6 h at 120 °C in oil bath. The solution was then allowed to cool down to room temperature and concentrated to 1/4 of its volume. 60 mL anhydrous methanol was then poured into it for purification. White precipitate-like viscous liquid appeared and the mixture was settled for half an hour. The upper clear solution was then decanted and 20 mL of CH2Cl2 was added to dissolve the product. The dissolution−precipitation-decantation process was repeated for three times and the final product was subjected to vacuum evaporation to remove the solvent, yielding transparent orange viscous liquid (46.5 g, 84.7%). The molecular weight was estimated by 1H NMR according to the integration ratio of methyl groups (-Si-CH3, denoted as ‘b’, δ 0.12 ppm) and methylene group near terminated amino (H2N−CH2−, denoted as ‘a’, δ 2.73

and functional self-healing polymer based on coordination bonds.37−40 To design self-healing polymers with better performance, it is important to get a profound understanding of the structure− property relationships. However, this is challenging for selfhealing polymers based on coordination bonds, because many parameters, such as bond energy, bond dynamics, and coordination number will have an essential effect on the mechanical and self-healing properties of the polymer. It is difficult to change one parameter while keeping the other parameters constant to investigate the effect of this parameter. For example, Holten-Andersen and Guan et al. studied a series of imidazole-containing copolymer system cross-linked by various divalent cations of zinc, copper, and cobalt. Through rheology and tensile experiments, they have correlated the emergent mechanical properties to the stoichiometric ratio of ligand to metal as well as the coordination number and ligand exchange mechanism of the imidazole-metal cross-links.41,42 However, both coordination number and coordination bond energy are varied when different metal ions are used. It is therefore difficult to correlate directly the bond energy to mechanical and self-healing properties of the polymer. In this work, we synthesized two linear poly(dimethylsiloxane) (PDMS) polymers containing different 2,6-diiminopyridine ligands (denoted as PDMS-NNN and PDMS-MeNNN, respectively), which were cross-linked into three-dimensional networks (denoted as PDMS-NNN-Zn and PDMS-MeNNN-Zn, respectively) upon reacting with ZnCl2. Both polymer ligands have the similar molecular weight and coordinate to Zn(II) ion in the same 2:1 fashion. Therefore, the only difference between PDMS-NNN-Zn and PDMSMeNNN-Zn is the coordination bond energy. As manifested by isothermal calorimetric titrations (ITC), electrospray mass spectrometry (ESI-MS), and 1H NMR studies, all results suggest that the coordination bond in PDMS-MeNNN-Zn polymer is weaker but more dynamic. Consequently, PDMSMeNNN-Zn polymer has poorer mechanical strength (lower Young’s modulus) but higher stretchability and better selfhealing properties (shorter healing time or higher healing efficiencies). Therefore, it is concluded that the mechanical robustness of a polymer is determined by the thermodynamic stability (bond strength) of the cross-linking sites. The more stable of the cross-linking sites, the more robust of the polymer. In contrast, the self-healing properties of a polymer are determined by the kinetic lability (bond dynamics) of the crosslinking sites as labile cross-linking sites are favorable for selfhealing. The inflicted cracks on PDMS-MeNNN-Zn polymer films can be completely healed after healing at room temperature for only 30 min with healing efficiencies higher than 90%. Such fast self-healing properties have never been achieved in self-healing polymers based on coordination bonds. The results presented herein also indicate that a subtle modification on the coordination complexes can cause significant changes in the mechanical and self-healing properties of the resulting polymer. Such understanding is helpful for further design of novel synthetic polymers which can achieve an optimal balance between the mechanical strength and selfhealing performance.

2. EXPERIMENTAL SECTION 2.1. Materials and General Measurements. Poly(dimethylsiloxane) bis(3-aminopropyl) terminated (H2N-PDMSNH2, Mn = 5000−7000) were purchased from Gelest. Toluene was B


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Inorganic Chemistry

Table 1. Thermodynamic Parameters for the Coordination of Zn(II) Binding to PDMS-NNN/PDMS-MeNNN Ligands Determined by ITCa host (in cell) PDMS-NNN (0.4 mM) PDMS-MeNNN (0.3 mM) a

K

guest (in syringe) 2+

Zn (3 mM) Zn2+ (3 mM)

1.05 (± 0.10) × 10 2.51 (± 0.41) × 104 5

ΔH (kcal/mol)

ΔS (cal/(mol K))

N

−7.66 (± 0.11) −4.11 (± 0.29)

−2.75 6.30

0.48 0.47

All titration experiments were performed in THF at 298.15 K.

Figure 1. Model small molecule studies. (a) Structures of the model ligands. (b) Structures of the model complexes. (c) Ligand exchange process in the model complexes. (d) ESI-MS spectrum for the mixture of [Zn(L1)2](OTf)2 and [Zn(L2)2](OTf)2 in 1:1 molar ratio for 3600s. (e) ESI-MS spectrum for the mixture of [Zn(L3)2](OTf)2 and [Zn(L4)2](OTf)2 in 1:1 molar ratio for 180s. (f) Relative abundance of dynamic exchange products Zn(L1L2)2+(black plot) and Zn(L3L4)2+ (red plot) changing with time monitored by ESI-MS. (d, J = 7.7 Hz, 2H), 7.79 (t, J = 7.7 Hz, 1H), 3.68 (t, J = 6.7 Hz, 4H), 1.82−1.77 (m, 4H), 0.63−0.58 (m, 4H).

ppm, triplet). The estimated molecular weight of PDMS-NNN is 44600 (Figure S9). 1H NMR (500 MHz, CDCl3): δ 8.41 (s, 2H), 8.01 C


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Inorganic Chemistry PDMS-MeNNN Ligand. 2,6-diacetylpyridine (1.63 g, 10 mmol) was added to a solution of H2N-PDMS-NH2 (53 g) in redistilled toluene (100 mL) with stirring under argon atmosphere. The subsequent processes are the same to PDMS-NNN as described above. As shown in Figure S10, the estimated molecular weight of PDMS-MeNNN is 42300, which is comparable to that of PDMS-NNN. 1H NMR (500 MHz, CDCl3): δ 8.07 (d, J = 7.8 Hz, 2H), 7.72 (t, J = 7.8 Hz, 1H), 3.52 (t, 6.7 Hz, 4H), 2.41 (s, 6H), 1.82−1.76 (m, 4H), 0.68−0.62 (m, 4H). PDMS-NNN-Zn/PDMS-MeNNN-Zn Polymer Films. Typical procedure for the preparation of PDMS-NNN-Zn/PDMS-MeNNN-Zn polymer films is: A certain amount of ZnCl2 (0.2 g/mL) solution in methanol (the molar ratio of PDMS-NNN/PDMS-MeNNN to Zn(II) is 2:1) was slowly added to the solution of PDMS-NNN/PDMSMeNNN ligand (1 g) in CH2Cl2 (10 mL) with stirring. The mixed solution was stirred for 24 h at room temperature and then concentrated to about 2 mL. The concentrated solution was poured into a polytetrafluoroethylene (PTFE) mold and dried at room temperature for 1 day followed by drying at 60 °C for 12 h. The polymer films were then peeled off from the PTFE mold for further testing. 2.4. Isothermal Titration Calorimetry. All titrations were performed using a MicroCal ITC200 isothermal titration calorimeter at 298.15 K.43,44 The solutions were prepared in dry THF using vacuum-dried hosts and guests. The concentrations of the ligands (in cell) and the metal source (in syringe) are given in Table 1. Twenty aliquots (2 μL each) of ZnCl2 in dry THF were added into the ligand solution with 150 s injection interval to measure the heat of complexation. Blank titrations in dry THF were performed and subtracted from the corresponding titrations to remove the effect of dilution. The titration curve thus obtained was analyzed using ORIGIN software, giving parameters concerning the complex stoichiometry (N), the binding affinity (K) and thermodynamic parameters (ΔH and ΔS) of different titrations (Table 1). 2.5. Rheological Measurement. The rheological behaviors were carried out on a TA DHR-2 Rheometer. All sweeps were performed with 20 mm parallel plates on circular samples with a 20 mm diameter at room temperature (25 °C). Strain sweeps were run from 0.1 to 1000% strain at 1 Hz. Frequency sweeps were performed from 0.0006 to 628 rad/s with a constant 1% strain. 2.6. Mechanical and Self-Healing Tests. Uniaxial tensile measurements were performed on an Instron 3343 with the strain rate of 10 mm/min at 25 °C, equipped with a 500 N load cell, using a rectangular film with an effective gauge dimensions: 5 mm (L) × 4 mm (W), thickness of 0.8−1 mm. The Stress-relaxation analysis (SRA) experiments were performed in a strain control (100% strain) mode with the strain rate of 10 mm/min. After equilibrating at room temperature for about 30 s, the strain was applied and stress decay was monitored. For self-healing test, the polymer film was cut into two pieces and then put together. The polymer film was then healed for different durations. The healed polymer films were then stretched following the same procedure to obtain the stress−strain curves. The mechanical healing efficiency (η), was defined as the ratio between the fracture strain restored relative to the original fracture strain. Values of Young’s modulus, maximal strengths, breaking strains, and toughness were presented as the mean ± standard deviation according to data of at least five trials.

fore, various molecular- to nano(micro)-scale systems were obtained by controlling the constituents and the interactions between them, leading to constitutional dynamic libraries and networks. Compared to -H substituted 2,6-diiminopyridine ligands, the electron donating methyl group in 2,6-diiminopyridine reduces the coordination ability and therefore makes the resulting coordination complexes more dynamic. However, both the H-substituted 2,6-diiminopyridine ligands and 2,6diiminopyridine ligands have the same coordination configuration when coordinate with metal ions. Such features enable us to investigate the effect of coordination bond energy and bond dynamics on the mechanical and self-healing properties of polymers while keeping the coordination number and coordination configurations the same. We first studied the structures and dynamic exchange properties of model complexes. Model ligands L1, L2, L3, and L4 were prepared from the aldimine condensation reaction between methylamine/n-butylamine and 2,6-pyridinedicarboxaldehyde/2,6-diacetylpyridine (Figure 1a). These ligands were then complexed with Zn(II) salts to afford the model complexes (Figure 1b). ESI-MS spectra showed that the metal-to-ligand molar ration in the model complexes is 1:2, which is typical for Zn(II) complexes with tridentate ligands.47,48 To directly observe the dynamic ligand exchange behavior in solution, we mixed complexes [Zn(L1)2](OTf)2 and [Zn(L2)2](OTf)2 in acetonitrile, and the formation of [Zn(L1L2)](OTf)2 was monitored by ESI-MS spectra over time at 25 °C (Figure 1c).49−51 Figure S6 shows the time-dependent ESI-MS spectrometry of the mixture solution. The ESI-MS signals of [Zn(L1)2](OTf)2 and [Zn(L2)2](OTf)2 were found at m/z 193.17 and 277.25, respectively, which correspond to the ion peaks of Zn(L1)22+ and Zn(L2)22+. After mixing for 30 s, a new signal occurred at m/z 235.25 with 15.6% relative abundance, which corresponds to the ion peak of Zn(L1L2)2+, suggesting the formation of mixed ligand product [Zn(L1L2)](OTf)2 through dynamic exchange. The relative abundance of Zn(L1L2)2+ at m/z 235.25 increased slowly and finally reached constant 100% abundance within 3600 s (Figure 1d), indicating the mixture achieved an equilibrium after 1 h. When the methyl groups were introduced into L1 and L2, the resulting complexes [Zn(L3)2](OTf)2 and [Zn(L4)2](OTf)2 exhibit similar but much faster ligand exchange properties. As shown in Figure 1e and Figure S7, the relative abundance of dynamic exchange product [Zn(L3L4)]2+ occurred at m/z 263.25, increased rapidly and achieved the balance after 180 s. Figure 1f depicts the relative abundance of dynamic exchange products Zn(L1L2) 2+ and Zn(L3L4)2+ changing with time as monitored by ESI-MS. To know if these ligand exchange processes were due to imine exchange reaction, we measured the 1H NMR spectrum of the mixture of L1 and L2, L3, and L4 respectively. Figure S5a shows the time-dependent 1H NMR spectrum of the mixture of L1 and L2 over time in acetonitrile-d6 at 25 °C.52 The 1H NMR signals for the methyl group in L1 (labeled as ‘a’) and methylene group in L2 (labeled as ‘b’) appear at 3.52 and 3.65 ppm, respectively. The methyl and methylene signals of L1L2 (labeled as ‘c’ and ‘d’) will appear beside 3.52 and 3.65 ppm if the imine exchange reaction proceeds successfully. However, no such new signals were found in 1H NMR spectrum even after mixing for 72 h. As for the mixture of L3 and L4 (Figure S5b), new methyl and methylene signals of L3L4 appeared at 3.39 and 3.57 ppm (labeled as C and D, respectively) after 2 h. However, the integration of the new signal was small and almost had no

3. RESULTS AND DISCUSSION 3.1. Synthesis and Dynamic Exchange Properties of Model Complex. Zn(II) complexes with tridentate terpyridine or terpyridine-type ligands have been extensively studied in the literature. Mihail Barboiu et al. reported a series of Zn(II) complexes with H-substituted 2,6-diiminopyridine ligands or 2,6-diiminopyridine ligands and observed the intriguing ligand exchange behavior in solutions.45−47 The ligand exchange process produces heteroleptic complexes, which convert into homoleptic complexes via constitutional crystallization. ThereD


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Inorganic Chemistry

Figure 2. Structure and fundamental properties of polymer films. (a) Structure of PDMS-NNN-Zn and PDMS-MeNNN-Zn polymer. (b) Optical image of the PDMS-NNN-Zn (upside) and PDMS-MeNNN-Zn (bottom) polymer. (c) UV−vis spectra of H2N-PDMS-NH2, PDMS-NNN, PDMSMeNNN, PDMS-NNN-Zn, and PDMS-MeNNN-Zn with the same concentration 0.3 mM in THF solutions at 25 °C. (d) DSC curves of PDMSNNN-Zn and PDMS-MeNNN-Zn polymer films.

Figure 3. Isothermal titration calorimetry data. (a) Zn (II) binding to PDMS-NNN ligand in THF at 298.15 K. (b) Zn (II) binding to PDMSMeNNN ligand in THF at 298.15 K.

growth even after mixing for 72 h. These results suggest that the imine exchange reaction in ligands L1−L4 is negligible at 25 °C. 3.2. Synthesis and General Characterization of Polymer. The coordination complexes [Zn(L1)2]2+/[Zn(L2)2]2+ and [Zn(L3)2]2+/[Zn(L4)2]2+ described above were introduced into a linear poly(dimethylsiloxane) (PDMS) polymer backbone, and served as a cross-linking unit (Figure 2a). Briefly, the PDMS oligomer that contained NNN and MeNNN groups (denoted as PDMS-NNN and PDMSMeNNN) was prepared by aldimine condensation reactions

between bis(3-aminopropyl)-terminated poly(dimethylsiloxane) (H2N-PDMS-NH2, Mn = 5000−7000) and 2,6-pyridinedicarboxaldehyde/2,6-diacetylpyridine to give a pale orange viscous liquid. The PDMS-NNN and PDMSMeNNN oligomers with similar molecular weight (Mn = 40 000−45 000) were subsequently cross-linked into transparent red/orange polymer films (denoted as PDMS-NNN-Zn and PDMS-MeNNN-Zn) by Zn(II) chloride (Figure 2b), with a molar ratio of Zn(II) ion to PDMS-NNN and PDMSMeNNN ligand of 1:2. As illustrated in Figure 2c, the UV−vis spectrum of the PDMS-NNN-Zn and PDMS-MeNNN-Zn E


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Inorganic Chemistry

Figure 4. Rheological studies. (a) Dynamic oscillatory strain sweep of PDMS-NNN-Zn at f = 1 Hz. (b) Dynamic oscillatory strain sweep of PDMSMeNNN-Zn at f = 1 Hz. (c) Frequency sweep of PDMS-NNN-Zn with 1% strain amplitude. (d) Frequency sweep of PDMS-MeNNN-Zn with 1% strain amplitude. All measurements were performed at room temperature.

Figure 5. Mechanical properties. (a) Typical tensile stress−strain curves of PDMS-NNN-Zn and PDMS-MeNNN-Zn polymer films with the strain rate of 10 mm/min at 25 °C. (b) Stress relaxation curves: the films were primarily stretched to 100% strain and then allowed to relax for 60 min at room temperature.

polymer film shows a new band at 296.1 and 290.6 nm respectively, which is similar to those of the model complexes and indicates the dominant presence of such complexes. According to the DSC measurement, the Tg for the resulting polymer network was measured to be about −69 °C (Figure 2d), indicating an excellent flexibility of the polymer main chain under ambient conditions. Furthermore, as shown in Figure 3a, b, the tendency of Zn (II) binding to PDMS-NNN/PDMSMeNNN ligand to form a 1:2 complex is confirmed by ITC titrations (the complex stoichiometry N = 0.48/N = 0.47). The binding affinity (K) of PDMS-NNN-Zn (1.05 × 105) is about four times larger than that of PDMS-MeNNN-Zn (2.51 × 104), suggesting the stronger coordination bonds in PDMS-NNN-Zn polymer (Table 1). 3.3. Rheological and Mechanical Properties. The rheological property of PDMS-NNN-Zn and PDMSMeNNN-Zn polymer was studied. According to the strain sweep curve of PDMS-NNN-Zn (Figure 4a), the storage modulus (G′) is higher than the loss modulus (G″) as the strain

is less than 100%, indicating that the sample is predominantly elastic at low strain. At larger strain (>100%) G′ falls behind G″, manifesting that the sample turns into viscous due to partial broken of the network. As for PDMS-MeNNN-Zn polymer films (Figure 4b), similar trend was observed. However, in the linear viscoelastic region, the storage modulus of PDMS-NNNZn (36 kPa) is twice higher than that of PDMS-MeNNN-Zn (14 kPa), suggesting the stronger cross-linking interactions in PDMS-NNN-Zn polymer. The result presented here is in highly consistent with the conclusions inferred by ITC measurements as mentioned above. To study the time-dependent mechanical properties of PDMS-NNN-Zn and PDMS-MeNNN-Zn polymer films, we performed oscillatory frequency sweeps ranging from 0.0006 to 628 rad/s at 25 °C with a constant 1% strain. Storage (G′) and loss (G″) moduli were measured as a function of angular frequency (ω). In metal−ligand cross-linked networks, the reciprocal of the crossover angular frequency (ωc), where G′(ω) = G″(ω), can be taken as a characteristic relaxation time F


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Inorganic Chemistry Table 2. Key Mechanical Properties of PDMS-NNN-Zn and PDMS-MeNNN-Zn Filmsa sample

Young’s modulus (MPa)b

elongation at break

tensile strength (MPa)

toughness (MJ/m3)c

PDMS-NNN-Zn PDMS-MeNNN-Zn

0.12 (± 0.01) 0.067 (± 0.012)

230 (± 26)% 456 (± 33)%

0.091 (± 0.007) 0.066 (± 0.010)

0.15 (± 0.01) 0.23 (± 0.03)

a Strain rate = 10 mm/min. The data shown here are the average values with standard deviation derived from five measurements on different samples at 25 °C. bYoung’s modulus, calculated from the initial slope of stress−strain curves. cToughness, calculated by manually integrating the area under the stress−strain curve.

Figure 6. Rheological recovery tests at 25 °C. (a) Continuous step strain measurements of PDMS-NNN-Zn at f = 1 Hz. (b) Continuous step strain measurements of PDMS-MeNNN-Zn at f = 1 Hz. (under a large strain 500% or a small strain 1%).

of the network (denoted as τc).53−55 Thus, at ω > 1/τc, the material behaves more elastic-like, storing more energy than it dissipates, while at ω < 1/τc the material exhibits viscous-like behavior, dissipating more energy than it can store. That is to say, the material exhibits smaller ωc shows more widely elasticlike behavior region. As shown in Figure 4c, d, the ωc of PDMS-NNN-Zn and PDMS-MeNNN-Zn is equal to 7.7 × 10−4 and 4.3 × 10−2 rad/s respectively, demonstrating the fact that PDMS-NNN-Zn is more elastic than PDMS-MeNNN-Zn. According to the observed ωc, the calculated relaxation time τc of the metal−ligand cross-linked network in PDMS-MeNNNZn is 23.3 s, which is far less than that of PDMS-NNN-Zn (τc = 1298.7 s), indicating much faster dynamic exchange of coordination bonds network in PDMS-MeNNN-Zn polymer films. In addition, temperature-dependent mechanical properties of PDMS-NNN-Zn and PDMS-MeNNN-Zn have also been investigated by performing oscillatory temperature sweeps ranging from −10 to 120 °C at 1 Hz with a constant 1% strain. As illustrated in Figure S11a, b, both storage modulus G′ and loss modulus G″ of PDMS-NNN-Zn and PDMS-MeNNN-Zn decrease when the temperature rises. Furthermore, storage moduli G′ of both PDMS-NNN-Zn and PDMS-MeNNN-Zn are higher than the loss moduli G″ at the whole temperature range, indicating the polymer films exhibit more elastic-like behavior in a wide temperature range. The mechanical properties of PDMS-NNN-Zn and PDMSMeNNN-Zn polymer films were investigated by strain−stress measurements. As presented in Figure 5a, due to the existence of methyl groups in CH3−CN−, the mechanical properties of PDMS-MeNNN-Zn polymer films are significantly different from that of PDMS-NNN-Zn. In detail, PDMS-MeNNN-Zn has a relatively low Young’s modulus (0.067 ± 0.012 MPa), which is 44.2% less than that of PDMS-NNN-Zn polymer (0.12 ± 0.01 MPa). However, the sacrifice in strength endows PDMS-MeNNN-Zn polymer films with much more stretchability (456 ± 33%) and higher toughness (0.23 ± 0.03 MJ/ m3). Table 2 shows the key parameters for mechanical

properties of PDMS-NNN-Zn and PDMS-MeNNN-Zn polymer films. Figure 5b depicts the stress relaxation curves of PDMSNNN-Zn and PDMS-MeNNN-Zn polymer films at 25 °C. Obviously, owing to much faster ligands exchange reaction, the normalized relaxation modulus of PDMS-MeNNN-Zn rapidly decreased from 1 to 0.01 within 20 min, indicating a 99% release of the internal stress within the polymer. In contrast, although taking much longer relaxation time (60 min), the normalized relaxation modulus of PDMS-NNN-Zn only decreased from 1 to 0.29, suggesting much slower release of the internal stress within PDMS-NNN-Zn. Not surprisingly, such results are consistent with the relaxation time (τc) determined by oscillatory frequency sweeps. 3.4. Self-Healing Property. The distinct self-healing properties of PDMS-NNN-Zn and PDMS-MeNNN-Zn were first observed in oscillation step strain tests.56−58 The recovery of all samples after mechanical breakdown was studied by subjecting to high-amplitude oscillatory strain (500%) and then monitoring the recovery of elasticity over time by measuring G′ at a much smaller strain amplitude (1%) (Figure 6a, b). Initially, all samples underwent the typical oscillation time sweep for 60 s at a low strain amplitude (1%) in order to determine the original value of G′. During the period of the high strain amplitude (500%), G′ dropped by at least 2 orders of magnitude to below the level of G″, indicating the elastic samples had been broken partially and the coordination bonds were subjected to a rearrangement. Upon switching to a low strain amplitude (1%) again, the sample PDMS-MeNNN-Zn began recovering its elastic properties immediately (G′ became higher than G″ again), with 83.7% of the original G′ regained within 10 s. In contrast, the sample PDMS-NNN-Zn could only recover 0.9% of its original stiffness and G′ still fell behind G″. However, when taking more time for recovering (60 s), all broken samples achieved at least 90% recovery of the original G′ upon switching to a low strain amplitude. For these metallopolymers networks, the self-healing mechanism is inferred to be due to reversible metal−ligand interactions. When the sample was broken, the metal−ligand coordination G


Article

Inorganic Chemistry

Figure 7. Self-healing properties. (a) Optical microscope images of damaged and healed PDMS-MeNNN-Zn 25 °C. (b) Photos giving the cutting/ healing/stretching process of PDMS-MeNNN-Zn films at 25 °C. (c) Typical stress−strain curves and (d) self-healing efficiencies of PDMS-NNNZn healing for described times at room temperature. (e) Typical stress−strain curves and (f) self-healing efficiencies of PDMS-MeNNN-Zn healing for described times at room temperature.

Table 3. Summary of the Self-Healing Performance of Previous Metallopolymers and This Study sample

stimulus

healing temperature (°C)

healing time

Zn2+-Mebip Zn2+/Eu3+-BTP Co2+-Terpyridine Fe3+-Hpdca-PDMS Zn-bpy-PDMS Fe3+/PDA@ENR PDMS-MeNNN-Zn PDMS-NNN-Zn

ultraviolet chloroform heat no no no no no

220 25 120 25 25 25 25 25

30 s 3h 16 h 48 h 48 h 24 h 0.5 h 12 h

bonds disassemble. At rest, the coordination bonds reform again, leading to self-healing behavior. Therefore, both PDMSNNN-Zn and PDMS-MeNNN-Zn samples exhibit the ability for recovery and self-healing at room temperature without the addition of any stimulus, but PDMS-MeNNN-Zn shows much better self-healing performance than PDMS-NNN-Zn because of the much faster dynamic exchange reaction of coordination bonds within the polymer films.

healing efficiency (%)

ref

± ± ± ± ± ± ± ±

35 60 55 38 59 61 this study this study

100.0 99.1 86.1 90.0 76.0 89.3 94.7 91.3

36.0 4.3 2.7 3.0 22.0 3.6 2.8 2.2

To directly observe the self-healing capability of PDMSNNN-Zn and PDMS-MeNNN-Zn polymer films at room temperature, we cut the samples into two pieces and subsequently brought into contact at the fracture surfaces. The cutting and healing process was recorded by photomicroscope. As shown in Figure 7a, the cut on PDMSMeNNN-Zn became narrow slowly and finally disappeared after healing for 30 min. The healed PDMS-MeNNN-Zn polymer films can be stretched to a strain more than three H


Article

Inorganic Chemistry

thermodynamic stability of the cross-linking sites. The more stable of the cross-linking sites, the more robust of the polymer. In contrast, the self-healing properties of a polymer are determined by the kinetic lability of the cross-linking sites as labile cross-linking sites are favorable for self-healing. The results presented herein also demonstrate that a subtle modification on the coordination complexes can cause significant changes in the mechanical and self-healing properties of the resulting polymer. Such a comprehensive understanding is helpful for further design of novel synthetic polymers, which can achieve an optimal balance between the mechanical strength and self-healing performance.

times of the original length. After loosening the hands, the stretched sample quickly returns to the original length owing to its natural elasticity, resulting in recyclable cutting-healing procedures (Figure 7b). However, the scratch on PDMS-NNNZn would never disappear until healing for 24 h (Figure S12). Typical stress−strain tests of PDMS-NNN-Zn and PDMSMeNNN-Zn healing for described times at room temperature were also performed. As illustrated in Figure 7c, d, after healing at 25 °C for just 1 min, the sample could withstand hand stretching and bending deformations with 38.4 ± 7.1% healing efficiency (Movie S1). Healing for 30 min led to a recovered fracture strain of 407 ± 6% and achieved 94.7 ± 2.8% healing efficiency. Such fast self-healing properties have never been achieved in self-healing elastomers based on coordination bonds (Table 3). In contrast, the healing process of PDMSNNN-Zn polymer film was much slower. The cracked PDMSNNN-Zn polymer film could not sustain hand stretching and bending deformations until healing for 1h. The cut on the sample almost disappeared after healing for 24 h, and the healed film ultimately achieved 100.3 ± 1.2% recovery of the elongation as compared to the original film (Figure 7e, f). PDMS-NNN-Zn and PDMS-MeNNN-Zn polymer films not only have the excellent ability to self-heal at 25 °C but also exhibit the self-healing capability at low temperatures. As shown in Figure S14a, b, after healing at 0 °C and −30 °C for 24 h, the PDMS-NNN-Zn polymer film regained 92.9 ± 2.2 and 68.0 ± 3.9% elongation recovery, respectively. As for the PDMSMeNNN-Zn polymer film, 100.5 ± 1.2 and 76.4 ± 3.5% selfhealing efficiencies were achieved, indicating even better performance than that of PDMS-NNN-Zn (Figure S14c, d). The excellent self-healing properties of all samples are attributed two factors:(1) the presence of highly dynamic metal−ligand coordination bonds, and (2) the low Tg (below −68 °C) of the polymer films which enables a high mobility of polymer chains. The above analysis suggests that introducing side methyl groups to −CN− have led to a significant perturbation to metal−ligand bonding interactions, and thus weakens the coordination bonding interactions and increases the dynamic exchange rate. The flexible chains locating at the broken interface are prone to interpenetrate each other allowing more rapid and autonomic self-healing. Upon bringing the separate pieces of PDMS-MeNNN-Zn polymer films into close proximity at room temperature, the new coordination bonds are formed at interface immediately, contributing to the rapid mending, which is remarkably different from the healing behavior of PDMS-NNN-Zn.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03260. Synthesis of model ligands L2, L3, and L4 and their corresponding Zn(II) complexes; 1H NMR spectrum of all model ligands (Figure S1−S4); imine exchange reaction of ligands L1 and L2, L3, and L4 monitored by 1 H NMR at 25 °C (Figure S5); dynamic coordination bonds exchange of model complexes monitored by ESIMS at 25 °C (Figure S6 and S7);1H NMR spectrum of H2N-PDMS-NH2, PDMS-NNN, and PDMS-MeNNN (Figures S8−S10); oscillatory temperature sweeps of PDMS-NNN-Zn and PDMS-MeNNN-Zn (Figure S11); optical microscope images of damaged and healed PDMS-NNNN-Zn polymer films at 25 °C (Figure S12); thermal gravimetric analysis (TGA) tests of asprepared PDMS-based films before and after self-healing (Figure S13); tensile stress−strain curves and self-healing efficiency of PDMS-NNN-Zn and PDMS-MeNNN-Zn polymer films healing at low temperatures for 24 h (Figure S14) (PDF) Movie S1, cutting and healing procedures (healing at room temperature for 30 s) of PDMS-MeNNN-Zn polymer film (AVI) Movie S2, cutting and healing procedures (healing at room temperature for 30 s) of PDMS-NNN-Zn polymer film (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

4. CONCLUSIONS In summary, two linear PDMS cross-linked by similar Zn(II)diiminopyridine complexes were prepared, and the mechanical and self-healing properties of these polymers were studied. Due to the presence of side methyl groups to -CN- moiety, weaker but more dynamic coordination bonds were generated. Consequently, the resulting PDMS-MeNNN-Zn polymer has poorer mechanical strength but higher stretchability and better self-healing properties. The surface scratch on PDMSMeNNN-Zn polymer films can be completely healed after healing at room temperature for only 30 min with healing efficiencies higher than 90%. Such fast self-healing properties have never been achieved in self-healing elastomers based on coordination bonds. Therefore, it is concluded that the mechanical robustness of a polymer is determined by the

ORCID

Cheng-Hui Li: 0000-0001-8982-5938 Jing-Lin Zuo: 0000-0003-1219-8926 Author Contributions †

D.-P.W. and J.-C.L. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21631006 and 21771100), the I


Article

Inorganic Chemistry

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Natural Science Foundation of Jiangsu Province (Grants BK20170016 and BK20151377), the project of the Scientific and Technological Support Program in Jiangsu Province (Grant BE2014147-2), the Fundamental Research Funds for the Central Universities (020514380121). We thank Prof. XiaoLiang Wang for valuable discussion on the rheological measurement.

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