Using Dynamic Bonds to Enhance the Mechanical Performance: From

Article Cite This: Macromolecules 2019, 52, 5014−5025

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Using Dynamic Bonds to Enhance the Mechanical Performance: From Microscopic Molecular Interactions to Macroscopic Properties Chi Zhang,†,○ Zhijun Yang,†,○ Nghia Tuan Duong,‡ Xiaohui Li,§ Yusuke Nishiyama,‡,∥,⊥ Qiang Wu,† Rongchun Zhang,*,# and Pingchuan Sun*,†,∇

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†

Key Laboratory of Functional Polymer Materials of the Ministry of Education and College of Chemistry, Nankai University, Tianjin 300071, P. R. China ‡ RIKEN-JEOL Collaboration Center, RIKEN, Yokohama, Kanagawa 230-0045, Japan § School of Materials Science and Engineering, and Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, P. R. China ∥ JEOL Resonance Inc., Musashino, Akishima, Tokyo 196-8558, Japan ⊥ NMR Science and Development Division, RIKEN SPring-8 Center, Yokohama, Kanagawa 230-0045, Japan # South China Advanced Institute for Soft Matter Science and Technology (AISMST), School of Molecular Science and Engineering, South China University of Technology, Guangzhou 510640, P. R. China ∇ State Key Laboratory of Medicinal Chemical Biology, Nankai University, Tianjin 300071, P. R. China S Supporting Information *

ABSTRACT: Polymeric materials combining good mechanical performances with self-healing ability and malleability have attracted dramatic attention, but it presently remains a challenge for the facile fabrication of such high-performance materials, not to mention the atomic-level characterization for understanding the molecular origin of the macroscopic properties. Herein, we proposed a facile strategy to fabricate a dual-crosslinked poly(n-butyl acrylate) polymer material, in which the selfcomplementary quadruple hydrogen bonding interactions between 2ureido-4[1H]-pyrimidinone (UPy) dimers were utilized as the dynamic sacrificial cross-linkages, and thus to enhance the mechanical strength and toughness. The hydrogen bonding interactions between UPy dimers in such synthetic cross-linked polymer material were revealed in detail by selective saturation double-quantum (DQ) solid-state NMR spectroscopy under ultrafast magic-anglespinning beyond 60 kHz. In the meantime, the self-healing capability and recyclability were achieved by utilizing dynamic fast boronic ester transesterification at an elevated temperature. A novel symmetrical diboronic ester cross-linker was developed and employed to enhance the probability of bornoic ester transesterification at an elevated temperature. The boronic ester transesterification was verified on a small molecular model and polymer materials by solution 1H NMR spectroscopy and swelling experiments, respectively, and the cross-linking structure of polymer materials was addressed by low-field proton multiple-quantum NMR spectroscopy and T2 relaxometry. Overall, it is well demonstrated that a combination of diboronic ester bonds and UPy dimers as the chemical and physical cross-linkage, respectively, can impart the rubbery materials with enhanced mechanical stiffness and toughness, good healing and recycling efficiency, and elucidation of the structure−property relationship here can further provide piercing insights into the development of high-performance polymeric materials.

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requires the approach of polymer chains and interdiffusion,19 moderate/high chain mobility is required for fulfilling selfhealing at room temperature, and thus in general will inevitably lead to the compromise in mechanical strength and stiffness.20−23 On the other hand, chain mobility can also be enhanced by heating the sample above the glass transition temperature (Tg), and thus to trigger self-healing behaviors. However, the enhancement of mechanical strength usually

INTRODUCTION

Over billions of years of evolution by natural selection, biological systems like animals and plants have possessed unique hierarchical and highly sophisticated structures as well as a broad variety of unparalleled functions, and thus have attracted dramatic attention from materials scientists.1−7 Although rapid advances have been accomplished in the fabrication of multifunctional polymeric materials through biomimetic designs,8−16 it still remains a challenge to impart the materials with excellent mechanical stiffness, toughness, and self-healing/recycling efficiency simultaneously.17,18 On the one hand, because the healing of cracks/scratches generally © 2019 American Chemical Society

Received: March 12, 2019 Revised: June 10, 2019 Published: June 21, 2019 5014

DOI: 10.1021/acs.macromol.9b00503 Macromolecules 2019, 52, 5014−5025

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Scheme 1. (a) Molecular Design of the Dual-Crosslinked Polymer Network Containing Dynamic Covalent Diboronic Ester Bonds and Self-Complementary Quadruple Hydrogen Bonds, and (b) the Schematic Synthetic Pathway for the Preparation of Dual-Crosslinked Polymersa

a

Details about the synthesis of material can be found in the SI.

applications of dynamic boronic ester exchange in bulk solid materials are rarely reported35,58−60 and still remain to be explored. Polymers with a glass transition temperature far below the room temperature are generally polymerized based on some liquid monomers, and such liquid-monomer-based polymers are widely used in adhesive, rubbers, and coating due to their relatively higher chain mobility. In order to further expand the application of these soft polymers,48,61 we proposed a simple strategy here to significantly enhance the mechanical properties through the incorporation of orthogonal dynamic bonds, taking poly(n-butyl acrylate) (PBA) as a model system. The schematic molecular design is shown in Scheme 1a. Both the dynamic boronic ester bonds and self-complementary quadruple hydrogen bonds between UPy dimers not only act as the chemical and physical cross-linkages to enhance the mechanical properties, respectively, but also imparts the cross-linked PBA material recyclable and healable at an elevated temperature. In order to enhance the density of boronic ester bonds and thus the probability of boronic ester transesterification in the system, a novel symmetric diboronic ester (DE) crosslinker was developed and synthesized. The UPy motifs and 1,2diol moieties were embedded into the polymer chains as attachment sites, where the 1,2-diol moieties can undergo boronic ester transesterification reaction with the DE crosslinker (i.e., first dynamic chemical cross-linking), and the UPy motifs will form self-complementary quadruple hydrogen bonds between each other, leading to the second physical cross-linking. As will be shown below, the boronic ester transesterification reaction can be well demonstrated by solution 1H NMR spectroscopy and swelling experiments. Particularly, the quadruple hydrogen bonding interactions between UPy dimers in synthetic cross-linked polymer materials were revealed by selective saturation double-quantum (DQ) solid-state NMR spectroscopy under ultrafast magicangle-spinning (MAS) beyond 60 kHz. In addition, the boronic ester transesterification reactions can be triggered at an elevated temperature for topology rearrangement, while the densely hydrogen bonds can be used as sacrificial bonds to efficiently dissipate stress energies under tensile stretching and

results in reduced toughness and tensile extensibility for the polymeric materials, leading to a trade-off between stiffness and toughness as often observed in the nanofilled polymer nanocomposites.24,25 Herein, addressing the above issues is still crucial for the development of high-performance materials. In recent years, dynamic bonds are widely incorporated into polymer networks and endow materials with functionality and adaptability, such as shape memory,26−29 self-healing,30−33 recycling,34−36 stimuli-responses,37−39 and so on. In particular, dual-cross-linked polymeric systems involving orthogonal dynamic interactions are of broad interests, and generally possess enhanced mechanical properties and multifunctional stimuli-responsive behaviors,40−45 in contrast to the system with single dynamic molecular interactions. However, the incorporation of dual dynamic interactions to achieve both enhanced mechanical property and self-healing capability into a single network is not trivial and remains to be explored. Among a wide range of dynamic covalent/physical bonds,46 hydrogen bonds are widely present in biological tissues or natural adhesives, which can dissociate and reassociate rapidly and reversibly during tensile stretching, and thus lead to efficient stress energy dissipation and improved mechanical toughness. In particular, the self-complementary quadruple hydrogen bonding interactions between UPy (2-ureido-4pyrimidone) dimers, with a dimerization constant above 108 M−1 in toluene,47 have been widely incorporated into synthetic polymeric materials for directed formation of ordered nanostructures and enhancement of mechanical properties.48−56 However, the direct atomic evidence for the hydrogen bonding interactions between UPy dimers in synthetic cross-linked polymers is still missing as far as we know. Furthermore, exchangeable dynamic covalent bonds, as widely introduced into the thermoset polymers, render them behave like permanently cross-linked materials with good solvent resistance at service temperature, and healable and recyclable at an elevated temperature due to the fast bond exchange reactions. Such kind of thermoset materials are named as vitrimers,36,57 and have showed great potential in industrial applications due to the possibility of extending the service life and sustainability of products. Notably, the 5015


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Macromolecules Table 1. Raw Materials Used for the Preparation of Polymer Samples samples

n-BA [mmol]

UPy-MA [mmol]

MA-diol [mmol]

DE [mmol]

DVB [mmol]

PBA-4.0U PBA-4.0U-0.5DE PBA-4.0U-1.0DE PBA-4.0U-1.5DE PBA-1.0DE PBA-2.0U-1.0DE PBA-6.0U-1.0DE PBA-4.0U-1.0DVB PBA-1.0DVB

72 72 72 72 72 72 72 72 72

4 4 4 4 0 2 6 4 0

8 8 8 8 8 8 8 8 8

0 0.5 1 1.5 1 1 1 0 0

0 0 0 0 0 0 0 1 1

thus enhance the mechanical properties. Besides, the assynthesized PBA material with orthogonal dynamic bonds exhibits superior mechanical properties than the similar kind of soft materials. The cross-linking structures of PBA materials were further addressed by low-field proton multiple-quantum NMR spectroscopy and T2 relaxometry. Moreover, the polymer presents very good healing and recycling efficiency as confirmed from the tensile experiments on the healed and recycled samples, respectively. We believe that such an elegant and straightforward approach to incorporate orthogonal dynamic bonds into a single network can be employed to improve the mechanical properties of a broad variety of selfhealing and recyclable polymers. All experimental details about the synthesis and characterization can be found in the Supporting Information (SI), and a simple schematic synthetic pathway is shown in Scheme 1b. Samples were denoted as PBA-xU-yDE, where x and y represent the content of UPy motifs and DE cross-linker, respectively. Details about the raw materials used for the sample preparation are summarized in Table 1, and the corresponding molar fractions of the used monomers are included in Table S1 in the SI. All the samples were prepared with the same content of n-BA monomers, 72 mmol. A series of samples with variable content of DE cross-linker were synthesized using a constant content of UPy motifs (4 mmol), and they are denoted as PBA-4.0U (without using any DE cross-linker), PBA-4.0U-0.5DE, PBA-4.0U-1.0DE, and PBA4.0U-1.5DE, respectively. Similarly, samples with variable content of UPy motifs were also prepared using a constant content of DE cross-linker (1 mmol) to understand the role of quadruple hydrogen bonding interactions. These samples were denoted as PBA-1.0DE (without any UPy motifs), PBA-2.0U1.0DE, PBA-4.0U-1.0DE, and PBA-6.0U-1.0DE, respectively. Two permanently cross-linked PBA samples were also prepared for control using DVB (p-divinylbenzene) instead of DE as the chemical cross-linker, which were denoted as PBA-4.0U-1.0DVB and PBA-1.0DVB (without any UPy motifs). Chemical structures of those polymers were characterized by solution 1H NMR (PBA-4.0U, Figure S11) and FTIR (Figure S12) spectroscopy. The IR peaks ranging from 1485 to 1683 cm−1 are ascribed to UPy motifs, while the peak at around 654 cm−1 is attributed to the vibration of B−C bonds.

Figure 1. Demonstration of boronic ester transesterification reactions using small molecules as the model system. (a) Scheme of the boronic ester transesterification. DE (0.207 g, 0.5 mmol) and 3-allyloxy-1,2propanediol (0.132 g, 1 mmol) were dissolved in 2 mL acetone-d6 and stirred in a vial at 60 °C for 60 min. (b) 1H NMR spectra of the mixed solution and other reactants in deuterated acetone. “*” denotes the proton signals of acetone-d6 (δiso= 2.05 ppm).

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Figure 2. Boronic ester transesterification reaction in polymers. (a) The bulk PBA-4.0U-1.0DE sample was put into the DMAc solvent in a glass bottle. (b) The PBA-4.0U-1.0DE sample was swollen in DMAc at 100 °C for several hours but did not get dissolved. (c) Samples were completely dissolved with addition of excess diols in an hour due to the fast transesterification reaction between DE and 3-allyloxy-1,2propanediol.

RESULTS AND DISCUSSION Boronic Ester Transesterification in Small Molecules and Bulk Materials. To confirm that the ester transesterification could take place between DE and 1,2-diol on the side-chains of PBA samples, we further employed a small molecular model as shown in Figure 1a. DE and 3-allyloxy-1,25016


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Figure 3. Stress relaxation curves of PBA-4.0U-1.0DE, PBA-4.0U1.0DVB, PBA-1.0DE, and PBA-1.0DVB at 100 °C. A 5% strain step was first applied after the temperature equilibrium in 5 min.

propanediol were mixed in acetone-d6 and kept at 60 °C for 60 min. The resulting mixture was then taken for measurement of solution NMR and HRMS (high-resolution mass spectroscopy). As shown in Figure 1b, signals at 5.6 ppm (peak b′), and 6.1 ppm (peak a′) for the mixed solution reveal the existence of MA-diol, which resulted from the exchange reaction between DE and 3-allyloxy-1,2-propanediol. Moreover, signals appeared at 4.75 ppm (peak c) indicates the existence of a second type of boronic esters (in compound C) due to the transesterification reaction shown in Figure 1a. The presence of DE (compound A) and the second type of boronic ester (compound C) were also confirmed by HRMS as shown in Figure S14. We envisioned that the reversible exchange reactions would enable the occurrence of topology rearrangement in bulk materials, leading to self-healing and recycling as will be shown below. To further provide direct insights into the boronic ester transesterification in the bulk polymer materials, we further examine the swelling behaviors of PBA-4.0U-1.0DE sample in DMAc solvent as shown in Figure 2. It is clearly observed that the PBA-4.0U-1.0DE sample was not dissolved even at 100 °C (Figure 2b), indicating the presence of boronic ester transesterification instead of complete dissociation of boronic ester bonds. However, when excess 3-allyloxy-1,2-propanediol was added, all the compounds were dissolved, and the solution became transparent (Figure 2c), which was ascribed to the fast exchange reaction between diboronic ester and 3-allyloxy-1,2propanediol. The topology rearrangement of the polymer network can occur through the exchangeable transesterification reaction as well as the dissociation of hydrogen bonds, which can be characterized by the stress relaxation experiment as shown in Figure 3. It can be observed that PBA-4.0U-1.0DVB sample shows higher relaxation rate than that of PBA-1.0DVB due to the dissociation of hydrogen bonds (PBA-1.0DVB shows higher relaxation rate for the first 3 min due to the much higher chain mobility than PBA-4.0U-1.0DVB), but even at a temperature of 100 °C, the relaxation rate of PBA-4.0U1.0DVB is still very slow, since there are only dynamic hydrogen bonds in the sample. However, with the incorporation of diboronic ester bonds in the cross-linkages, the stress relaxation rate of PBA-4.0U-1.0DE was significantly enhanced. Actually, after 5 min under the same temperature at 100 °C, the stress (σ) only decayed to ca. 85% of the initial stress (σ0) for PBA-4.0U-1.0DVB sample, while it is around 37% for PBA-

Figure 4. (a) Two different hydrogen bonding arrays for the UPy motifs in the polymers. (b) Proton spectra of PBA-4.0U-1.0DE sample at variable spinning speed from 30 kHz to 70 kHz under 600 MHz magnetic field. (c) The 1D DQ filtered proton spectrum at 70 kHz MAS, where the aliphatic protons signals are greatly suppressed by using selective saturation, and thus the relative signal intensities of UPy protons are enhanced in comparison to the 1D single-pulse proton spectrum without presaturation. (d) Selective saturation proton double-quantum/single-quantum (DQ/SQ) chemical shift correlation spectrum obtained under 600 MHz magnetic field and 70 kHz MAS. The DQ correlations between UPy motifs are indicated with red lines. The pulse sequence for this experiment is shown in Figure S15. BABA-xy16 sequence64 was used to excite the DQ coherence, and the DQ excitation and reconversion time was set as 8 rotor periods, i.e., 114.3 μs. More experimental details can be found in the SI.

4.0U-1.0DE sample with dynamic boronic ester bonds. With further increasing waiting time to 20 min, the stress of PBA4.0U-1.0DVB has decayed to around 67% of the initial stress, while it is 27% for that of PBA-4.0U-1.0DE. Notably, PBA1.0DE shows much higher stress relaxation rate than PBA4.0U-1.0DE. On one hand, the incorporation of UPy motifs renders the material enhanced stiffness but also slightly compromised chain mobility; on the other hand, quadruple hydrogen bonds cannot be completely dissociated in a short time scale even at 100 °C. Therefore, the relaxation rate induced by dissociated hydrogen bonds is much slower than that induced by boronic ester transesterification. Herein, the topology rearrangement of network within a short time range (e.g., 5 min) should be attributed to the boronic ester transesterification instead of dissociation of hydrogen bonds, while the slow relaxation relies on the disassociation of 5017


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Figure 5. Stress−strain curves of PBA materials obtained from tensile test at a stretching rate of 20 mm min−1. (a) Stress−strain curves of samples. (b) Partial enlargement of Figure 5a for clarity. (c−f) Mechanical parameters of samples. The toughness was calculated from the area under stress− strain curve up to the strain at break.

hydrogen bonds at the current temperature of 100 °C. Note that there is an abundance of free hydroxyl groups (from MAdiol) in the side chains, leading to the formation of hydrogen bonds, which may impose restriction on the chain motions to some extent and thus leads to incomplete stress relaxation in short experimental time. Similar results were also observed in previous work of Guan et al.58 Quadruple Hydrogen Bonding Interactions between UPy Dimers. Although the self-complementary quadruple hydrogen bonding interactions between UPy dimers have been widely adapted and incorporated into polymers for mechanical property enhancement, atomic evidence for the formation of hydrogen bonds between UPy dimers is still missing for the synthetic cross-linked polymer materials. In fact, the quadruple hydrogen bonds of UPy motifs and tautomeric rearrangement in the small molecule-based supramolecular solid and solution has been confirmed by proton double-quantum (DQ) magicangle-spinning (MAS) solid-state NMR and solution NMR spectroscopy, respectively.62,63 However, it becomes much more challenging for synthetic polymer materials with a much lower content of UPy motifs, around 4.7 mol % in the current investigated sample, PBA-4.0U-1.0DE. Particularly, it is still not clear whether the hydrogen bonding sites of UPy dimers are arranged in a donor−donor−acceptor−acceptor (DDAA) or DADA array (Figure 4a) in the polymer materials. Herein, these challenges can be clearly addressed using the highresolution proton DQ solid-state NMR spectroscopy under ultrafast MAS beyond 60 kHz. As shown in Figure 4b, with increasing the spinning speed, the proton spectral resolution is significantly enhanced due to the better suppression of 1H−1H homonuclear dipolar couplings at fast MAS. Particularly, in the chemical shift range of 8−14 ppm, all the signals of amide

protons between 10 and 14 ppm are clearly resolved. However, due to the extremely low abundance of UPy motifs, the signals of amide protons even look noisy in the single-pulse proton spectra, which could render it challenging for the observation of DQ correlations in 2D DQ/SQ spectrum due to the presence of large t1 noise. Fortunately, with the application of DQ filter (Figure S15), the aliphatic signals can be greatly reduced due to their relatively high mobility, and amide proton signals become visible (Figure 4c). In order to further suppress the aliphatic signals, selective saturation pulses were employed to minimize the aliphatic signals with little changes on the signals of amide protons as shown in Figure 4c. Thus, clear 2D DQ/SQ NMR spectrum can be obtained as shown in Figure 4d. The DQ correlations between Ha and Hd can be clearly observed in the selective saturation 2D DQ/SQ spectrum, whereas such correlation was hardly observed in the regular 2D DQ/SQ NMR spectrum (Figure S16). Such correlation clearly indicates the intermolecular proximity of UPy motifs. Moreover, strong DQ correlations between Hb and Hc, as well as Hb and Hb, are observed, indicating that UPy motifs form hydrogen bonds in a DDAA array. Similarly, strong DQ correlations between Ha and Hb also suggest the presence of hydrogen bonds between UPy motifs in a DADA array. However, the chemical shift value of amide protons (with a chemical shift of 13.0, 11.7, and 10.4 ppm for Ha, Hb, and Hc, respectively) is closer to the values of the keto form (i.e., DDAA array) in the supramolecular system as previously reported,62 indicating that the DDAA array make take a dominating role. In short, the 2D DQ NMR spectrum clearly demonstrates the presence of hydrogen bonds between UPy motifs. 5018


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Figure 6. (a) Cyclic tensile test curves of PBA-4.0U-1.0DE under a maximum strain of 25%, 50%, 75%, 100%, 125%, and 150%, respectively. (b) Dissipated energies (i.e., hysteresis area) of four samples under different strains. (c) Cyclic tensile test curves of PBA-4.0U-1.0DE loaded to a strain of 50% with delay time ranging from 0 to 240 min. (d) The hysteresis ratio and residual strain of PBA-4.0U-1.0DE sample loaded to a strain of 50% with delay time ranging from 0 to 240 min.

Mechanical Properties and Thermomechanical Behaviors. The mechanical performance was examined by tensile tests as shown in Figure 5. In contrast to PBA-1.0DE and PBA-1.0DVB, all the other samples containing UPy motifs display outstanding mechanical performance (both in strength and toughness) due to the formation of densely quadruple hydrogen bonds in the system. Meanwhile, with a fixed content of DE cross-linker, the Young’s modulus and the tensile strength at break of samples exhibit obvious enhancement with increasing content of UPy motifs as shown in Figure 5c,e, respectively. In fact, the hydrogen bonds can be broken and reformed during the tensile stretching, and thus serve as sacrificial bonds to dissipate the stress energies and enhance the mechanical properties. With the content of UPy motifs fixed, the dynamic covalent cross-linker, DE, also plays a key role in the dual-cross-linked polymer as revealed in Figure 5. As expected, with increasing the DE content, the extensibility was a bit compromised due to the increase in chemical crosslinking density, while the Young’s modulus and toughness were both enhanced. It is observed that PBA-4.0U-1.0DE has comprehensively improved mechanical performance (highest tensile strength and toughness, and moderate Young’s modulus and strain at break among the dual-cross-linked samples). Notably, PBA-4.0U-1.0DE has a larger Young’s modulus than that of PBA-4.0U-1.0DVB, which may be ascribed to the stronger B−O bonds in DE than C−C bonds in DVB. To study the energy dissipation caused by the hydrogen bonding interactions between UPy dimers, cyclic tensile tests were further conducted with variable maximum strain as shown in Figures 6 and S17. It is clearly shown that all samples display

substantially enhanced hysteresis loops with increasing the strain except PBA-1.0DE sample (Figures 6a and S17). Little hysteresis energy increases with the strain for PBA-1.0DE sample (Figure 6b), while for the PBA samples containing UPy motifs, the hysteresis energy increases rapidly with the strain, and the raising rate is enhanced with increasing the content of UPy motifs in the samples, demonstrating that the quadruple hydrogen bonding interactions between UPy dimers are essential for the efficient stress energy dissipation. To further elucidate the recovery properties of the material, PBA-4.0U1.0DE was repeatedly loaded to a 50% strain and unloaded with different waiting time ranging from 0 to 240 min, as shown in Figure 6c,d. Compared to the virgin PBA-4.0U1.0DE sample, the second circle with a delay time of 0 min shows a smallest hysteresis loop, due to the breakage of hydrogen bonds during the first loading cycle. With increasing the waiting time, the hydrogen bonds can reassociate with each other gradually, leading to the increment of hysteresis area and recovery of the loading curves. The hysteresis ratio increases quickly for the first 30 min, then slows down and reaches up to more than 90% of the original hysteresis area in 240 min (Figure 6d). However, even after a waiting time of 240 min, there is still a residual strain in the loading−unloading curve (ca. 2%, Figure 6d), indicating that a complete reconstruction of the hydrogen bonding network may take much longer time, since the chain motions will become more restricted by the enhanced association of hydrogen bonds with increasing waiting time. In short, all the results have demonstrated the efficient energy dissipation induced by hydrogen bonding 5019


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lowest storage modulus among them. In addition, in the tan δ curves (Figure 7c), the first peak indicates the glass transition of PBA, while the second peak corresponds to the dissociation of hydrogen bonds between UPy dimers, which is not observable in samples without containing UPy motifs (PBA1.0DE and PBA-1.0DVB). These two peaks are also in accordance with the two regions of decrease observed in the E′ curve, occurring at the temperature around −25 and 85 °C, respectively. It is clearly shown from the tan δ curves that the dual-cross-linked samples (PBA-4.0U-1.0DE and PBA-4.0U1.0DVB) have a slightly higher glass transition temperature than the one with single physical cross-linking (PBA-4.0U), while the one with single chemical cross-linking (PBA-1.0DE and PBA-1.0DVB) are the lowest, which should be ascribed to the enhanced restriction of segmental dynamics imposed by the quadruple hydrogen bonds between UPy dimers. Notably, the dual-cross-linked samples (PBA-4.0U-1.0DE and PBA4.0U-1.0DVB) also have a higher temperature for the dissociation of UPy dimers than that of PBA-4.0U as observed in tan δ curves, demonstrating that the covalent cross-linking may impose restrictions on polymer dynamics and facilitate the formation of quadruple hydrogen bonds between UPy dimers by bringing the backbone chains closer to each other. Cross-Linking Structures. In order to further investigate the cross-linking structures in the PBA materials, we further implemented proton magic-sandwich echo (MSE, Figure S19) and multiple-quantum NMR (Figure S20) experiments on a benchtop low-field (20 MHz) NMR spectrometer, as shown in Figure 8. In principle, MSE pulse sequence is similar to but has higher efficiency than solid-echo in refocusing the 1H−1H dipolar coupling evolution.65 Therefore, it can be well utilized to recover the lost free-induction-decay (FID) signals due to the presence of long dead time (generally above 10 μs) of the receiver, and thus can be used for quantitative/qualitative determination of the fraction of different components in the system, such as polyurethane,66 semicrystalline polymers67,68 and so on. The decay of FID signals is dominated by the widespread 1H−1H dipolar couplings, which is closely related to the mobility of polymer segments. Faster segmental dynamics can significantly reduce the 1H−1H dipolar couplings, and thus lead to slower decay of FID signals. On the contrary, restricted segments or rigid components will have a larger dipolar coupling in average and thus results in faster decay of FID signals. Here we only use the MSE-FID results for qualitative analysis since the fraction of rigid components, corresponding to the cross-linkages, is very small (∼ 5%), which is nearly on the level of uncertainty as obtained from brute-force least-squares fitting. As is clearly shown in Figure 8a, with increasing DE content in the PBA materials, the decay of FID signals becomes faster, indicating an enhanced restriction on the PBA segments due to the increased density of chemical cross-linking. Notably, it is found that the FID signal decay of PBA-1.0DE sample is much slower than that of PBA-4.0U sample, demonstrating that the physical crosslinking due to hydrogen bonding interactions between UPy dimers has a more significant effect on the segmental dynamics than the chemical cross-linking, which may be ascribed to the higher molar content of UPy motifs than DE. This is in a good agreement with the DSC results shown in Figure 7a, where the glass transition is less obvious for PBA-4.0U compared to PBA1.0DE. The restricted segmental dynamics in PBA materials can be further characterized by proton multiple-quantum NMR spectroscopy as shown in Figure 8b-8d. Proton multiple-

Figure 7. Dynamic mechanical analysis data of PBA materials. (a) DSC curves of PBA materials. The glass transition temperature of each sample was indicated by the arrow. The heating rate was 20 °C/ min. (b) Storage modulus (E′) and (c) tan δ curves as a function of temperature obtained from the DMA measurements. The heating rate and frequency was set as 5 °C min−1 and 1 Hz for the DMA measurement, respectively.

interactions between UPy dimers, which endow the PBA materials with improved mechanical properties. Subsequently, the influence of the DE and UPy motifs on the thermal-mechanical behaviors of PBA materials were also investigated utilizing a combination of TGA (thermalgravimetric analysis), DSC (differential scanning calorimetry), and DMA (dynamic mechanical analysis) experiments. TGA curves (Figure S18) clearly reveal quite good sample stability within 200 °C, while DSC curves (Figure 7a) indicate an increased glass transition temperature (Tg) with increasing the content of DE and UPy motifs. Besides, the glass transition of samples containing UPy motifs looks less obvious than that of PBA1.0DE and PBA-1.0DVB, due to heterogeneous restrictions on molecular motions imposed by the quadruple hydrogen bonds between UPy dimers. DMA results of a series of specific samples are shown in Figure 7b,c. As expected, physically and chemically cross-linked PBA-4.0U-1.0DE and PBA-4.0U1.0DVB samples have a relatively higher storage modulus at room temperature than PBA-4.0U, while samples cross-linked only by DE or DVB (PBA-1.0DE and PBA-1.0DVB) show the 5020


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Figure 8. (a) MSE-FID curves of PBA materials. (b) DQ signals as a function of DQ excitation time as obtained from the MQ NMR experiment, adapting a three-pulse sequence (90x-τ-180y-τ-90x) for the excitation of DQ signals. (c) Normalized DQ (nDQ) signals obtained through a pointby-point normalization procedure as explained in the SI. (d) Comparison of the nDQ curves plotted in a log scale (x-axis) for PBA-1.0DE and PBA-4.0U-1.0DE samples.

correspond to the signals of protons at the cross-linkages or the restricted segments nearby, while the second peak can be ascribed to the mobile amorphous PBA segments. Notably, with increasing the content of DE, the intensity of the peak at 22 μs increases significantly, while the peak at 0.28 ms is substantially suppressed. This is completely the result of the enhanced restriction on the segmental dynamics due to the enhanced chemical cross-linking. Since the as-obtained DQ signals suffer from severe relaxation at the long DQ excitation time, the DQ curves were further processed through a pointby-point normalization procedure (as explained in the SI) to obtain a normalized DQ (nDQ) buildup curves as shown in Figure 8c. In the nDQ buildup curves, the signal buildup rate is proportional to Dres. As is clearly shown, with increasing the DE content, the nDQ signals increase faster, where the PBA1.0DE sample has the slowest signal buildup rate, indicating least restricted segmental dynamics in PBA-1.0DE sample. In principle, the nDQ buildup curve can be fitted with a numeric integration method to obtain the average dipolar coupling (Davg) and its distribution;71 however, the presence of multiple components with mobility contrast has rendered the quantification difficult due to the diverse relaxation behaviors.72 But still, a two-step buildup of nDQ signal can still be observed if the DQ excitation time axis was plotted in a log scale as shown in Figure 8d, taking PBA-4.0U-1.0DE and PBA1.0DE nDQ buildup curves as the examples. In the nDQ buildup curves, there is a plateau of 0.5 at a long DQ excitation time. Therefore, the turning point in the two-step growth of the nDQ buildup curve can be used to qualitatively compare the fraction of rigid components. The intensity of the turning point on the PBA-4.0U-1.0DE nDQ curves is much larger than that of PBA-1.0DE, indicating a higher content of restricted segments. Also, the DQ excitation time of the turning point is

Figure 9. Optical microscopy images of self-healing. (a) PBA-1.0DE, (b) PBA-4.0U, and (c) PBA-4.0U-1.0DE. All the samples were kept at each temperature for 5 min and then photos were taken.

quantum NMR spectroscopy is a very robust approach for providing quantitative information on the structure and dynamics of polymer network, using the residual proton dipolar coupling (Dres) as the probe.69,70 Herein, we adapted three-pulse segment (90x-τ-180y-τ-90x) pulse sequence to excite the DQ signals due to the very restricted segmental dynamics around the cross-linkages, as reflected from the fast signal decay in the initial 50 μs in the MSE-FID. In Figure 8b, it is clearly observed that there are two peaks in the DQ curves of PBA-1.0DE sample, one at the DQ excitation time around 22 μs and the other at about 0.28 ms. The first peak should 5021


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Figure 10. Tensile curves of as synthesized and healed (a) PBA-4.0U-1.0DE and (b) PBA-4.0U-1.0DVB samples for 4 h, 8 h, 16 h, 24 h, and 48 h, respectively. (c) Healing efficiency of PBA-4.0U-1.0DE and PBA-4.0U-1.0DVB for 48 h. The healing efficiency was defined as the ratio between the mechanical parameter of healed sample and that of as-synthesized sample. (d) Tensile curves of PBA-4.0U-1.0DE before and after reprocessing by hot compression. The sample was cut into small pieces and reprocessed as shown in the inset images.

time, leading to insufficient polymer chain diffusion. However, with healing time prolonged to 48 h, the healing efficiency of samples were significantly enhanced due to sufficient hydrogen bond exchanges and polymer chain diffusions. As shown in Figure 10c, PBA-4.0U-1.0DE exhibits much higher healing efficiency both in tensile strength and the strain at break, which indicates that the boronic ester transesterification greatly facilitated and accelerated the healing of polymer networks, while the healing induced by only hydrogen bonding interactions will take a relatively long time. It is worth noting that the Young’s modulus displays almost full recovery for PBA-4.0U-1.0DE sample, while the recovery of the tensile strength and strain at break is only around 50%. In fact, once the sample is partially healed through dynamic bond exchange and chain diffusion, we will get the 100% recovery of Young’s modulus (Figure 10a), because we are using the same effective cross section area when calculating the stress, and thus the crack will not affect the Young’s modulus. However, the full recovery of tensile strength and strain at break will require the full reconstruction of polymer network, i.e., the complete healing of the bulk material, which is particularly difficult for the broken PBA-4.0U-1.0DVB sample with irreversible broken chemical bonds. In addition, the boronic ester transesterification can facilitate and accelerate the healing of polymer network at 100 °C, whereas the permanent crosslinking in PBA-4.0U-1.0DVB may impose restriction on the chain mobility and thus the dynamic exchange of hydrogen bonds, leading to unsatisfactory healing efficiency compared to PBA-4.0U-1.0DE sample. The efficient recycling of PBA-4.0U-1.0DE sample was also demonstrated by hot compression. The as-synthesized sample was first cut into tiny granules before hot compression molding

shorter for PBA-4.0U-1.0DE, again demonstrating more restricted segmental dynamics at the cross-linkages due to the increased cross-linking density. Self-Healing and Recycling Properties. Given that the boronic ester bonds and densely hydrogen bonds between UPy dimers can endow the polymer networks with superior dynamic reversibility, it is expected that the dual-cross-linked PBA material can be efficiently healed and recycled. Herein, optical microscopy was adapted to observe the healing of scratches on the sample surface, as shown in Figure 9. Both PBA-1.0DE and PBA-4.0U-1.0DE samples exhibited efficient healing within 5 min at 100 °C due to the fast boronic ester transesterification, while the healing of PBA-4.0U sample was relatively poor since the quadruple hydrogen bonds cannot be completely dissociated at 100 °C in such a short time, and the reassociation rate of hydrogen bonds is relatively slow. It should be mentioned here that the quadruple hydrogen bonds between UPy dimers can also render the material healable at room temperature. However, the exchange of hydrogen bonds is very slow at room temperature, and thus will require much longer time to heal the scratches, maybe even more than 12 h.53 To further examine the healing efficiency of polymers, the tensile test on a healed sample was also performed as shown in Figure 10a−c. A dog-bone-shaped test stripe was cut into two pieces with a blade, then the cut faces were pressed into each other gently for 1 min and allowed to heal at 100 °C for 4, 8, 16, 24, and 48 h before the tensile test, respectively. As shown in Figure 10a,b, both PBA-4.0U-1.0DE and PBA-4.0U-1.0DVB samples displayed lower healing efficiency when the samples were allowed to heal for only 4 h, as the quadruple hydrogen bonds could not dissociate and reform efficiently in a short 5022


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Macromolecules at 110 °C for 30 min. The as-synthesized and reprocessed samples were both examined by standard tensile experiment as shown in Figure 10d, which clearly indicated the excellent malleability of the dual dynamic cross-linked polymers. Within 30 min of hot compression, the strain at break and tensile strength of recycled sample has reached 81.0% and 75.5% of that of as-synthesized sample, respectively, demonstrating a high recycling efficiency. Notably, the Young’s modulus for the recycled sample is a bit higher, which can be ascribed to the increased network density (packing) in the recycled samples. Similar results were also reported by DuPrez et al, where the Young’s modulus of poly(thioether) vitrimers increase with increasing numbers of recycling.73

Author Contributions

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ACKNOWLEDGMENTS

CONCLUSIONS In summary, a simple strategy was proposed to fabricate a mechanical enhanced healable and recyclable polymeric material by incorporating orthogonal dynamic interactions into a single polymer network. A novel small molecule, containing dynamic diboronic ester bonds, was developed and adapted here as the dynamic covalent cross-linker, rendering the thermoset material recyclable and healable. The dynamic nature of the boronic ester cross-linker was well demonstrated with solution 1H NMR and mass spectroscopy on small molecules and swelling experiments on bulk polymer materials as well. In addition, the self-complementary quadruple hydrogen bonding interactions between UPy dimers were incorporated as the physical cross-linkages, which greatly enhanced the mechanical strength and toughness. Particularly, the quadruple hydrogen bonding interactions between UPy motifs (∼4.7 mol %) in synthetic cross-linked polymer materials were clearly demonstrated by the high-resolution selective saturation proton DQ/SQ spectrum under ultrafast MAS beyond 60 kHz. The cross-linking structures were further addressed in detail by low-field proton multiple-quantum NMR spectroscopy and T2 relaxometry, which were in a good agreement with the DSC results. Tunable mechanical performance can be realized by simply adjusting the content of UPy motifs and the dynamic covalent cross-linker, which makes the materials be potential for a wide range of applications. We believe that such robust healable and recyclable materials are promising for industrial applications, and the design concept here provides a new insight into the development of highperformance stimuli-responsive polymers.

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REFERENCES

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Notes

The authors declare no competing financial interest.

This work was supported by the National Natural Science Foundation of China (21534005 and 21704046), PCSIRT (IRT1257), and the 111 Project (B2015). R.Z. also gratefully acknowledges the financial support of Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06C322) and the startup support from South China University of Technology.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00503.

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These authors have made equal contributions to this study.

Experimental details about the sample preparation and characterization (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.Z.). *E-mail: [email protected] (P.S.). ORCID

Nghia Tuan Duong: 0000-0001-5761-3666 Yusuke Nishiyama: 0000-0001-7136-1127 Rongchun Zhang: 0000-0002-2480-2652 5023


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Using Dynamic Bonds to Enhance the Mechanical Performance: From

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