A Self-Healing Cellulose Nanocrystal-Poly(ethylene glycol

Research Article pubs.acs.org/journal/ascecg

A Self-Healing Cellulose Nanocrystal-Poly(ethylene glycol) Nanocomposite Hydrogel via Diels−Alder Click Reaction Changyou Shao, Meng Wang, Huanliang Chang, Feng Xu, and Jun Yang* Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, No. 35, Tsinghua East Road, Haidian District, Beijing 100083, China S Supporting Information *

ABSTRACT: Self-healing hydrogels are particularly desirable for increased safety and functional lifetimes because of stress-induced deformation and propagation of cracks. In this paper, we report a tough, highly resilient, fast self-recoverable, and self-healing nanocomposite hydrogel, which builds an interpenetrated network encapsulating rod-like cellulose nanocrystals (CNCs) by flexible polymer chains of poly(ethylene glycol) (PEG). A thermally reversible covalent Diels−Alder click reaction between furyl-modified CNCs and maleimideend-functionalized PEG was confirmed by Fourier transform infrared spectroscopy. Uniaxial tensile tests and unconfined compression tests displayed outstanding mechanical properties of the hydrogels with a high fracture elongation up to 690% and a fracture strength up to 0.3 MPa at a strain of 90%. Cyclic loading−unloading tests showed excellent self-recovery and antifatigue properties of the nanocomposite hydrogels. The self-healing capability of nanocomposite hydrogels assessed by tension tests was found to be as high as 78%. The self-healing CNC-PEG nanocomposite hydrogels would shed insight into designing reusable and renewable polymeric hydrogels. KEYWORDS: Self-healing hydrogels, Cellulose nanocrystals (CNCs), Diels−Alder (DA) reaction

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bonds(imine,25−27 acylhydrazone28,29), carbon−carbon/carbon−sulfur bonds (reversible radical reaction30), and cyclohexenes (reversible Diels−Alder cycloaddition31,32). Among them, the Diels−Alder (DA) reaction has been developed as one of the most well-known chemical reactions for designing self-healing hydrogels, which belongs to the family of “click chemistry” characterized by high efficiency, high selectivity, high yields without byproducts, and the involvement of catalysts and coupling reagents.33 Moreover, thermally reversible DA linkages formed by a conjugated diene and a dienophile can be cleaved on heating and reach a new equilibrium. Therefore, the DA click reaction is considered as an efficient cross-linking strategy to synthesize robust and selfhealing hydrogels which are widely applied to tissue engineering.4 In this study, we utilize a furyl/maleimide pair to design a novel self-healing and self-recoverable hydrogel with cellulose nanocrystals (CNCs) acting as both a reinforcing phase and chemical cross-linker via a reversible DA reaction, and the general modification and cross-linking reaction are shown in Scheme 1. Due to the outstanding renewability, biocompatibility, and ultrahigh tensile stiffness, cellulose nanocrystals, which are the main naturally occurring polysaccharides on Earth, have been widely investigated as environmentally friendly reinforcing particles.34 Poly(ethylene glycol) (PEG)

INTRODUCTION Polymeric hydrogels are a unique form of soft and wet elastomers with a three-dimensional hydrophilic network entrapping a large amount of water while maintaining their network structure integrity.1,2 In the past few decades, hydrogels have been prominently used in biological and biomedical fields such as controlled drug release,3 tissue engineering,4 biosensors,5 and hygiene products.6 The densely cross-linked structures are the basis of superior mechanical properties such as high modulus, high fracture strength, and high toughness. However, traditional hydrogels are vulnerable to stress-induced deformation and propagation of cracks, which may lead to a dangerous loss in load-carrying capacity and limit the service lifespan.7 Consequently, self-healing hydrogels that can restore pristine structures and functions after damage have been pioneered to address this challenge,8 which is beneficial to enhance durability and reliability by avoiding failures caused by the accumulation of cracks in certain applications.9 According to the healing mechanisms, two major strategies concerning the synthesis of hydrogels with self-healing performance have been proposed.10,11 One is based on dynamic cross-links of attractive noncovalent interactions between molecules, oligomers, or polymer chains, such as hydrophobic interactions,12,13 host−guest interactions, 14,15 hydrogen bonds,16,17 crystallization,18 and electrostatic interactions.19,20 Another strategy of self-healing is to re-establish networks through reversible chemical reactions with dynamic covalent bonds, such as boron−oxygen bonds (phenylboronate ester),21−23 sulfur−sulfur bonds (disulfide),24 carbon−nitrogen © 2017 American Chemical Society

Received: April 7, 2017 Revised: May 11, 2017 Published: May 31, 2017 6167

DOI: 10.1021/acssuschemeng.7b01060 ACS Sustainable Chem. Eng. 2017, 5, 6167−6174

Research Article

ACS Sustainable Chemistry & Engineering

mechanical stirring (300 rpm) in an ice−water bath. Then, the suspension was heated to 55 °C under stirring for 2 h. The obtained suspension was washed with deionized water until pH neutrality to remove excess aqueous acid. Finally, the CNC powder was collected by freeze-drying and was cryopreserved (4 °C) before usage. The obtained CNCs are rod-like nanofibers with lateral diameters of 20−50 nm and lengths ranging from 400 to 600 nm (TEM image, Scheme 1). Synthesis of Furyl-Modified CNC (CNC-F). CNCs (0.50 g) and anhydrous DMF (200 mL) were added into a three-necked 250 mL round-bottomed flask equipped with a magnetic stirring bar, reflux condenser, and nitrogen inlet, where the mixture was stirred for 30 min and sonicated for 2 h under nitrogen atmosphere. After that, furfural (220 μL, 3 mmol (DS1); 110 μL, 1.5 mmol (DS2)) and solid acid catalyst Amberlyst 15 (2.4 g) were added into the CNC suspension under the protection of nitrogen. The mixture solution was allowed to stir at 110 °C for 24 h, then cooled to room temperature. The furfural-modified CNC (CNC-F) was collected by centrifugation at 8000 rpm for 15 min after removing the solid acid catalyst, then thoroughly washed by Soxhlet extraction with methanol and dried under vacuum at 60 °C for 12 h. Preparation of CNC-PEG Hydrogels. CNC-PEG nanocomposite hydrogels were prepared based on the DA reaction between Mal-PEGMal and CNC-F. Typically, CNC-F was dispersed in deionized water with at a concentration of 1.5% w/v using sonication (20 min). Subsequently, a certain amount of Mal-PEG-Mal (the mole ratio of furyl to maleimide was controlled at 1:1, 3:1, and 9:1) was added into the dispersed suspension; then, the mixed solution was stirred vigorously for 30 min to obtain the uniform composite solution. Finally, the composite solution was degassed under vacuum and poured into a PETF mold to form the gel at different temperatures (37−77 °C). The detail components are summarized in Table 1.

Scheme 1. Synthesis Route to Self-Healing CNC-PEG Nanocomposite Hydrogelsa

a

Inserted images are the CNC aqueous solution and its TEM images, bar = 500 nm). (i) CNCs were modified with furfural by applying a well-known acetalization reaction in the presence of a solid acid catalyst. (ii) Reversible DA reaction between individual furyl and maleimide groups.

Table 1. Compositions of CNC-PEG Nanocomposite Hydrogels code DS1 DS1 DS1 DS2 DS2 DS2

is selected as the backbone chain due to its nontoxicity, excellent water solubility, and biocompatibility, which results in widespread application in biomedical fields.35 Thus, the components of furyl-modified CNCs acting as multifunctional cross-links and PEG with maleimide terminal groups as a polymer matrix are chosen to prepare self-healing nanocomposite hydrogels by a reversible DA reaction in this work. The gelation time, swelling ratio of the gels, is measured as a function of different components, and the results show that the mechanical properties and self-healing properties can be tailored by tuning the substitution degree of furyl functional groups and the mole ratio of furyl to maleimide. Importantly, this hydrogel preparation procedure where nanoparticles easily incorporate into the polymeric matrix by suspending in the cross-linked network is favorable for deepening the understanding of the relation between macroscopic-functional and microscopic-structural properties and shedding insight into designing reusable and renewable polymeric hydrogels.

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9:1 3:1 1:1 9:1 3:1 1:1

mH2O (mL)

mCNC‑F (mg)

mMal‑PEG‑Mal (mg)

10 10 10 10 10 10

150 150 150 150 150 150

40 116 263 30 91 201

Characterization. The gelation time of the hydrogels was determined using a test tube inverting method at different temperatures (37, 47, 57, 67, 77 °C). The swelling experiments were performed by immersing as-prepared cylindrical samples (30 mm height and 20 mm diameter) in a phosphatic buffer solution (PBS, 100 mM, pH 6.8) at 37 °C for 72 h until the equilibrium of swelling was attained. The swollen hydrogels were weighed after the excess of water on the surface was absorbed with filter paper. The swelling ratio (Qe) was defined as follows:

Q e = (Wt − Wd)/Wd

(1)

where Wt and Wd represent the weight of swollen hydrogel and dry hydrogel, respectively. The microscopic morphology was observed using a Hitachi S-3500 scanning electron microscope (SEM). The cross-sections of the hydrogels were stacked on an aluminum stub using carbon tape and coated with a thin gold layer. The observation was operated at an accelerating voltage of 5 kV and a working distance of 6.5 mm. Fourier transform infrared spectroscopy (FTIR) was conducted on an infrared spectrophotometer (Nicolet iN10-MX, Thermo Scientific). 1H NMR spectra measurements were performed on a (AVANCEIII 400, Bruker) NMR spectrometer operating at 400 MHz, using deuteroxide (D2O-d) as solvent and tetramethylsilane (TMS) as an internal standard. The degree of substitution (DS) of CNC-F can be calculated according to the following equation:

EXPERIMENTAL SECTION

Materials. A never-dried bleached wood pulp was purchased from Donghua Pulp Factory, China. Solid acid catalyst Amberlyst 15 and N,N-dimethylformamide (DMF) were purchased from Sigma-Aldrich. Dimaleimide poly(ethylene glycol) (Mal-PEG-Mal) (Mw = 2 × 103 Da) was purchased from Shanghai Sunway Pharmaceutical Technology Co., Ltd. (Shanghai, China). All other reagents and solvents were analytical grade and used as received without further purification. Preparation of CNCs. Cellulose nanocrystals (CNCs) were prepared by sulfuric acid hydrolysis according to the previous literature.36 In brief, wood pulp (5 g) was cut into small pieces and added to the 60 wt % sulfuric acid solution (200 mL) under 6168


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Figure 1. (a) 1H NMR spectrum of CNC-F and (b) FTIR spectra of pristine CNC, CNC-F, Mal-PEG-Mal, and DA adduct.

Figure 2. (a) Gelation time as a function of temperature (inserted images show the sol−gel transition using a test tube inverting method) and (b) swelling ratio for CNF-PEG gels with different DS and mole ratio of furyl to maleimide at 37 °C.

DS = (S1 × 7)/(S2 × 3)

NMR. As shown in Figure 1(a), CNC-F was successfully obtained following the synthesis routes in Scheme 1 with evidence of peaks at 6.27, 6.46, and 7.41 ppm, which were ascribed to furyl groups and assigned to the protons 1, 2, and 3, respectively.37 The degree of substitution (DS) of CNC-F varied in additive amount of furfural, which was calculated to be 0.67 (DS1) and 0.33 (DS2) according to eq 2, where S1 and S2 were labeled integral areas of the furyl ring (6.27, 6.46, and 7.41 ppm) and anhydroglucose ring (2.8−4.5 ppm), respectively. Also, FTIR spectra verified the grafting modification of CNCs (Figure 1(b)). CNC-F and Mal-PEG-Mal spectra indicates C C adsorption at 1420 and 1450 cm−1 corresponding to the furyl on CNC-F and the maleimide on Mal-PEG-Mal, respectively. The spectrum shows decreased absorption of CC peaks and increased absorption at 1460 cm−1 (CC in DA adduct), suggesting the disappearance of individual furyl and maleimide groups and the formation of a maleimide−furyl adduct during the DA reaction. Gelation Time. Since the Diels−Alder reaction belongs to a thermally induced process and the higher temperature leads to a faster reaction rate,38,39 there should have been a correlation between the gelation time and the temperature. In this work, the gelation time was recorded using a test tube inverting method at different temperatures (Figure 2(a)), and the results showed that the gelation time decreases and gel/sol ratio increases when the temperature increased from 37 to 77 °C (Figure S1), which can be explained by the accelerated thermal motion of the molecules with the elevation in temperature. Moreover, the gelation time was affected by the substitution degree of furyl and the mole ratio of furyl to maleimide. Since no additional chemical cross-linking agent was applied in the gel preparation, the gelation of CNC-PEG nanocomposite

(2)

where S1 and S2 are labeled integral areas of the furyl ring and anhydroglucose ring, respectively. Mechanical properties of hydrogels were examined using an electronic universal testing machine (UTM6530, Shenzhen Suns Technology Co., Ltd.) with a load cell of 100 N at room temperature. The uniaxial tensile tests of the rod-like sample (100 mm length and 5 mm diameter) were uniaxially stretched at a crosshead speed of 60 mm min−1. The unconfined compression tests of the cylindrical sample (30 mm height and 20 mm diameter) were conducted at a crosshead speed of 10 mm min−1. For stress relaxation, the specimens were stretched to a strain of 200% at a crosshead speed of 60 mm min−1. Then, the strain was held constant, and the time-dependent relaxation of stress was recorded. Self-recovery properties were investigated by typical tensile (60 mm min−1) and compressive (10 mm min−1) loading− unloading tests. In the tensile mode, the specimens were first stretched to 400% strain and then unloaded with different interval resting time (0, 1, and 5 min) between consecutive cycles. In the compressive mode, successive loading−unloading compressive tests were measured 10 times under different strains (85%, 90%). Also, the specimens were initially compressed (80%, 85%, 90%) by loading−unloading and tested again with a resting time (1 min). Macroscopic self-healing tests were performed to demonstrate the self-healing ability of the CNC-PEG nanocomposite hydrogels. In detail, the rod-shaped specimens (100 mm length and 5 mm diameter) were cut into halves, and then, each piece was merged by simply putting the halves together in a sealed container for a specified time under 90 °C without any outside intervention (the specimens were stained by rhodamine B for better visual inspection). The healing efficiency (HE) is defined as the tensile strength ratio between the healed gel and original gel.

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RESULTS AND DISCUSSION Structure Analysis of Furyl-Modified CNCs and DA Reaction. The furyl-modified CNCs were characterized by 1H 6169


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Figure 3. Cross-section SEM pictures of CNC-PEG hydrogels (bar = 20 μm).

Figure 4. Mechanical behaviors of nanocomposite hydrogels. (a) Images of hydrogels (DS1 1:1) during a uniaxial tensile test and (b) uniaxial tensile stress−strain curves. (c) Stress relaxation curves (200% elongation), the percentage of the residual stress after 10 min being indicated for each sample. (d) Unconfined compression stress−strain curves. (e) Unconfined compressive test.

hydrogels was mainly attributed to the covalent network crosslinked between furyl groups and maleimide groups via the DA click reaction. The substitution degree of furyl is constant at a given temperature, and the acceleration of the mole ratio of furyl to maleimide on gelation time is shown in Figure 2(a), in which a pronounced decrease in the gelation time associated with increasing mole ratio of furyl to maleimide (the concentration of maleimide functional groups decreases

correspondingly) is exhibited. However, the mole ratio of furan to maleimide is constant, but the substitution degree of furan functional groups decreases (the concentration of furyl functional groups decreases correspondingly) at a given temperature, resulting in a increase in gelation time. Using hydrogels DS1 1:1 and DS2 1:1 as examples, it is obvious that hydrogel DS1 1:1 possesses more of a chance than hydrogel DS2 1:1 to interact between furan and maleimide groups, 6170


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Figure 5. Cyclic tensile and compressive properties of hydrogels (DS1 1:1). (a) Loading−unloading tensile tests under different strains (200%, 300%, 400%, and 500%). (b) Typical successive loading−unloading tensile tests for five cycles. (c) Change in stress as a function a time during a cyclic loading−unloading process at 400% strain. (d) Antifatigue properties of hydrogels with different resting time by cyclic tensile tests (25 °C). Typical successive loading−unloading compression tests for 10 times at 85% strain (e) and 90% strain (f). Self-recovery behavior at 80% (g), 85% (h), and 90% (i) compression strains, respectively.

leading to significant acceleration of gelation. Therefore, a higher substitution degree of the furyl group and higher concentration of Mal-PEG-Mal resulted in a shorter gelling time at a given temperature. Swelling Properties. The swelling properties of CNC-PEG nanocomposite hydrogels were probed gravimetrically by being immersed into the PBS until they reached equilibrium at 37 °C for 72 h. The equilibrium swelling ratios of hydrogels with different components are shown in Figure 2(b). One can note that a lower furyl substitution degree led to a higher equilibrium swelling ratio when the mole ratio of furyl to maleimide was constant (e.g., equilibrium swelling ratio DS2 1:1 > DS1 1:1; DS2 3:1 > DS1 3:1; DS2 9:1 > DS1 9:1). Besides, the swelling ratio of hydrogels decreased as the mole ratio of furyl to maleimide increased (e.g., equilibrium swelling ratio DS1 1:1 < DS1 3:1 < DS1 9:1). According to the report by Tamura,40 swelling properties of hydrogels are related to the effective cross-link density of the hydrogels, which can be tailored by simply tuning the substitution degree of furyl functional groups and the mole ratio of furyl to maleimide in this work. Morphology Observation (SEM). The cross-section morphologies of freeze-dried hydrogels with different components are shown in Figure 3. Showing hydrogels with different sizes of pore structures in the SEM images indicates that the gel with a higher substitution degree of furyl and a lower mole ratio of furyl to maleimid is prone to form the more homogeneous structure with tighter networks and smaller pores, which may lead to outstanding mechanical properties of CNC-PEG hydrogels. It is reasonable to assume that the homogeneously

distributed small pore size distributes stress more evenly in the hydrogel network when resisting stress concentration, and small pores as well as increased pore number can act as a barrier against crack propagation.41 These results indicate that the pore size dimension of CNC-PEG hydrogels with covalently crosslinked DA networks can be tailored by simply varying the different reagent ratios, which would determine the swelling property and mechanical property of a nanocomposite hydrogel. Mechanical Properties. Uniaxial tensile tests (Figure 4(a)) were conducted for nanocomposite hydrogels with different components to examine the strength and extensibility of CNCPEG nanocomposite hydrogels. Figure 4(b) shows the tensile stress−strain curves of hydrogels with varied substitution degree of furyl groups and the mole ratio of furyl to maleimide. One can see that the CNC-PEG hydrogels demonstrated good mechanical performances with tensile strength and ultimate elongation up to 75−160 kPa and 370−690%, respectively. We noted that the tensile strength and ultimate elongation of the gels enhanced with increasing substitution degree of furyl or decreasing mole ratio of furyl to maleimide. Thus, it was evident that the improved mechanical properties ascribed to the increasing covalent DA cross-linking points, which helps the gels sustain stress and improve the failure elongation. In addition, stress relaxation curves are shown in Figure 4(c), where the specimens were stretched to a strain of 200%, and the time-dependent stress relaxation was recorded. The result indicates that a rapid drop of stress occurs immediately once the elongation is terminated at 200% strain, and then, the stress 6171


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Figure 6. (a) Self-healing performance of hydrogel DS1 1:1 by direct visual inspection. (b) Stress−strain curves of the original and self-healed hydrogel DS1 1:1 at various healing times. (c, d) Self-healing efficiency of the hydrogel measured from tensile tests at room temperature.

rest at ambient temperature after the first cycle, and the stress− strain curves gradually recover to the initial loading path (∼5 min) and exhibit decreased residual strain over time, which demonstrates the recovery feature of hydrogels with the DAformed covalent network. In the consecutive compressive mode, the hydrogel shows excellent self-recovery and antifatigue properties (Figure 5(e) and (f)), which confirms that the hydrogels could almost recover the original strength and exhibit overlapping hysteresis loops under the consecutive compressive loading−unloading tests after 10 cycles. The recovery properties of the CNC-PEG hydrogels are further evaluated with different compression strains (Figure 5(g−i)), which contributes to understanding the recoverable energy dissipating mechanism ascribed to inner interactions of hydrogels. When the compressive loading− unloading cycling test is conducted at 80% strain (Figure 5(g)), the hysteresis loop is small and could recover completely after resting for 1 min, which can be explained as the flexible polymer chains tend to curl under the external compression force and recover the intrinsic state once the stress is released. Therefore, the hydrogel could sustain the stability of an internal structure under large deformations and resist structural fatigue during the low amplitude strain deformation. The hysteresis loop increased with an increase in strain (85%), which is because of the rupture of noncovalent interactions between CNCs and the polymer matrix to dissipate more energy. However, the noncovalent interactions (such as hydrogen bonds) could recombine once the stress is removed, which causes the hysteresis loop to recover almost entirely after resting for 1 min (Figure 5(h)). This full recovery feature demonstrates that the covalent networks have survived by reversibly breaking and reforming sacrificial bonds. When compressive strain increases to 90%, it exhibits an expanding

decreases slowly afterward. The percentages of the remaining stress ranges from 85.0% to 93.9% in comparison to the initial stress, which is consistent with the results based on the uniaxial tensile tests. It should be pointed out that the low stress relaxation corresponded excellently with restrictions on polymer chain mobility under deformation, implying a good shape fixity and recovery ability for the hydrogels. Typical compressive stress−strain curves of hydrogels are shown in Figure 4(d). Analogous to the extension tests described above, CNC-PEG hydrogels display an increased compressive strength with a higher furyl substitution degree and lower mole ratio of furyl to maleimide because the enhanced stiffness is related to the increased DA-bonded covalent interaction density. Figure 4(e) shows the process of an unconfined compression test of hydrogel DS1 1:1, which exhibited outstanding mechanical properties with strain up to 90% without fracture or collapse. Self-Recovery Properties. The cyclic tensile and compressive loading−unloading tests are implemented to further investigate fatigue resistance, resilience, and self-recovery capabilities of nanocomposite hydrogels. When the hydrogels (DS1 1:1) are subjected to tensile loading−unloading cycles to examine self-recovery capabilities, no pronounced hysteresis is observed with the increasing strain from 200% to 500% (Figure 5(a)). The typical successive loading−unloading tensile tests with five cycles exhibits good resilience and fatigue resistance because each cycle nearly coincides with others as shown in Figure 5(b) and (c), which indicates that a cross-linked covalent network ascribed to the interconnected CNCs and PEG in a three-dimensional configuration and could retain structural integrity. Cyclic tensile tests with different resting times are conducted for the same hydrogel samples, and the results are shown in Figure 5(d). The specimens are allowed to 6172


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renewable high performance biocompatible polymer hydrogels, expanding the potential applications of cellulose-based selfhealing hydrogels in the biomedical field.

hysteresis loop that could not recover completely after 1 min of rest (Figure 5(i)) because the rupture of covalent DA crosslinks leads to further dissipation of energy, but these covalent bonds could not be recombined at ambient condition. Self-Healing Properties. To prove the self-healing capability of the nanocomposite hydrogel, we performed a macroscopic self-healing test by direct visual inspection and mechanical investigation. As shown in Figure 6(a), the selfhealing properties of the hydrogel were evaluated by using a well-established tensile test protocol, which consists of splitting a test sample of hydrogel DS1 1:1 into two separate pieces to expose fresh surfaces, and then, the two fresh fracture surfaces were brought into contact immediately. After incubating at 90 °C for a prescribed contact time under nitrogen without any external intervention and then cooled to room temperature, the healed hydrogel was stretched again, and no splitting was observed, verifying that the two pieces merged into a complete hydrogel and the interphase layer was strong enough to sustain tensile stress. The healing process can be explained by the dynamic cleavage and regeneration of the reversible cross-links with the presence of a thermal reversible DA bond, which may further accelerate the formation of new phases across the fractured surfaces of the self-healed hydrogels.42 Consequently, self-healing hydrogels can fuse into their original shape after damage and further recover the initial properties. In order to further quantitatively examine the self-healing behaviors of nanocomposite hydrogels, we investigated the tensile properties for various fracture surface-contacting times. Figure 6(b) displays the tensile stress−strain curves of the healed hydrogel and original hydrogel. On the basis of stress− strain curves shown in Figure 6(b), it is obvious that the ultimate elongation and the tensile strength increase with increasing healing time. For instance, the fracture stress reached 52 kPa after 1 h, corresponding to 35% HE, while the recovered stress is up to 116 kPa after 24 h, which is about 78% HE compared with the original hydrogel. Furthermore, in order to get more insight into the selfhealing behavior, the healing efficiency (HE) of the hydrogels with different components was measured for various fracture surface-contacting times. The results in Figure 6(c) and (d) indicate that HE of hydrogels increased with increase in healing time. Besides, the higher DS of furyl and the lower molar ratio of furyl-to-maleimide resulted in an increasing HE over the entire time period. Since the self-healing property of nanocomposite hydrogels is contributed to the reversible DA bonds via furyl groups grafted on a CNC and malaimide group terminated in PEG, the higher content of DA bonds within nanocomposite hydrogels would accelerate the healing process and hence lead to improvement in HE.

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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/acssuschemeng.7b01060. Representative gel/sol curve of hydrogel DS2 9:1 as a function of time at different temperatures. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-10-62337223. ORCID

Changyou Shao: 0000-0003-2464-7792 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by Fundamental Research Funds for the Central Universities (2017ZY35), and National Natural Science Foundation of China (21404011, 21674013).

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REFERENCES

(1) Lu, Z.; Yin, Y. Cheminform abstract: colloidal nanoparticle clusters: functional materials by design. Chem. Soc. Rev. 2012, 41 (21), 6874−6887. (2) Han, D.; Yan, L. Supramolecular hydrogel of chitosan in the presence of graphene oxide nanosheets as 2d cross-linkers. ACS Sustainable Chem. Eng. 2014, 2 (2), 296−300. (3) Chen, C.; Zhang, T.; Dai, B.; Zhang, H.; Chen, X.; Yang, J.; Liu, J.; Sun, D. Rapid Fabrication of Composite Hydrogel Microfibers for Weavable and Sustainable Antibacterial Applications. ACS Sustainable Chem. Eng. 2016, 4 (12), 6534−6542. (4) Lee, K. Y.; Mooney, D. J. Hydrogels for Tissue Engineering. Chem. Rev. 2001, 101 (7), 1869−1879. (5) Appel, E. A.; del Barrio, J.; Loh, X. J.; Scherman, O. A. Supramolecular polymeric hydrogels. Chem. Soc. Rev. 2012, 41 (18), 6195−6214. (6) Caló, E.; Khutoryanskiy, V. V. Biomedical applications of hydrogels: A review of patents and commercial products. Eur. Polym. J. 2015, 65, 252−267. (7) Taylor, D. L.; in het Panhuis, M. Self-healing hydrogels. Adv. Mater. 2016, 28 (41), 9060−9093. (8) Wei, Z.; Yang, J. H.; Zhou, J.; Xu, F.; Zrínyi, M.; Dussault, P. H.; Osada, Y.; Chen, Y. M. Self-healing gels based on constitutional dynamic chemistry and their potential applications. Chem. Soc. Rev. 2014, 43 (23), 8114−8131. (9) Pratama, P. A.; Sharifi, M.; Peterson, A. M.; Palmese, G. R. Room temperature self-healing thermoset based on the Diels-Alder reaction. ACS Appl. Mater. Interfaces 2013, 5 (23), 12425−12431. (10) Amamoto, Y.; Otsuka, H.; Takahara, A.; Matyjaszewski, K. Selfhealing of covalently cross-linked polymers by reshuffling thiuram disulfide moieties in air under visible light. Adv. Mater. 2012, 24 (29), 3975−3980. (11) Herbst, F.; Döhler, D.; Michael, P.; Binder, W. H. Self-healing polymers via supramolecular forces. Macromol. Rapid Commun. 2013, 34 (3), 203−220. (12) Tuncaboylu, D. C.; Sahin, M.; Argun, A.; Oppermann, W.; Okay, O. Dynamics and large strain behavior of self-healing hydrogels with and without surfactants. Macromolecules 2012, 45 (4), 1991− 2000.

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CONCLUSIONS In this work, we developed a facile and efficient strategy to fabricate a CNC-PEG nanocomposite hydrogel via a reversible Diels−Alder reaction (DA). Interestingly, the nanocomposite hydrogel not only endowed good mechanical properties but also exhibited excellent recovery performance and a self-healing property. The gelation time and swelling ratio of the gels are measured as a function of different components, and the results show that the mechanical properties and self-healing properties can be tailored by simply tuning the substitution degree of furyl functional groups and the mole ratio of furyl to maleimide. The self-healing CNC-PEG nanocomposite hydrogels will provide new insights and platforms for designing reusable and 6173


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ACS Sustainable Chemistry & Engineering (13) Akay, G.; Hassanraeisi, A.; Tuncaboylu, D. C.; Orakdogen, N.; Abdurrahmanoglu, S.; Oppermann, W.; Okay, O. Self-healing hydrogels formed in catanionic surfactant solutions. Soft Matter 2013, 9 (7), 2254−2261. (14) Nakahata, M.; Takashima, Y.; Yamaguchi, H.; Harada, A. Redoxresponsive self-healing materials formed from host-guest polymers. Nat. Commun. 2011, 2 (1), 487−502. (15) Kakuta, T.; Takashima, Y.; Nakahata, M.; Otsubo, M.; Yamaguchi, H.; Harada, A. Preorganized hydrogel: self-healing properties of supramolecular hydrogels formed by polymerization of host-guest-monomers that contain cyclodextrins and hydrophobic guest groups. Adv. Mater. 2013, 25 (20), 2849−2853. (16) Phadke, A.; Zhang, C.; Arman, B.; Hsu, C. C.; Mashelkar, R. A.; Lele, A. K.; Tauber, M. J.; Arya, G.; Varghese, S. Rapid self-healing hydrogels. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (12), 4383−4388. (17) Cui, J.; del Campo, A. Multivalent H-bonds for self-healing hydrogels. Chem. Commun. 2012, 48 (74), 9302−9304. (18) Nowak, A. P.; Breedveld, V.; Pakstis, L.; Ozbas, B.; Pine, D. J.; Pochan, D.; Deming, T. J. Rapidly recovering hydrogel scaffolds from self-assembling diblock copolypeptide amphiphiles. Nature 2002, 417 (6887), 424−428. (19) Haraguchi, K.; Uyama, K.; Tanimoto, H. Self-healing in nanocomposite hydrogels. Macromol. Rapid Commun. 2011, 32 (16), 1253−1258. (20) Wang, Q.; Mynar, J. L.; Yoshida, M.; Lee, E.; Lee, M.; Okuro, K.; Kinbara, K.; Aida, T. High-water-content mouldable hydrogels by mixing clay and a dendritic molecular binder. Nature 2010, 463 (7279), 339−343. (21) Cromwell, O. R.; Chung, J.; Guan, Z. Malleable and Self-healing covalent polymer networks through tunable dynamic boronic ester bonds. J. Am. Chem. Soc. 2015, 137 (20), 6492−6495. (22) Cash, J. J.; Kubo, T.; Bapat, A. P.; Sumerlin, B. S. Roomtemperature self-healing polymers based on dynamic-covalent boronic esters. Macromolecules 2015, 48 (7), 2098−2106. (23) Bapat, A. P.; Roy, D.; Ray, J. G.; Savin, D. A.; Sumerlin, B. S. Dynamic-covalent macromolecular stars with boronic ester linkages. J. Am. Chem. Soc. 2011, 133 (49), 19832−19838. (24) Pepels, M.; Filot, I.; Klumperman, B.; Goossens, H. Self-healing systems based on disulfide−thiol exchange reactions. Polym. Chem. 2013, 4 (18), 4955−4965. (25) Yang, J.; Ma, M.; Zhang, X.; Xu, F. Elucidating dynamics of precoordinated ionic bridges as sacrificial bonds in interpenetrating network hydrogels. Macromolecules 2016, 49 (11), 4340−4348. (26) Yang, B.; Zhang, Y.; Zhang, X.; Tao, L.; Li, S.; Wei, Y. Facilely prepared inexpensive and biocompatible self-healing hydrogel: a new injectable cell therapy carrier. Polym. Chem. 2012, 3 (12), 3235−3238. (27) Zhang, Y.; Tao, L.; Li, S.; Wei, Y. Synthesis of multiresponsive and dynamic chitosan-based hydrogels for controlled release of bioactive molecules. Biomacromolecules 2011, 12 (8), 2894−2901. (28) Wei, Z.; Yang, J. H.; Liu, Z. Q.; Xu, F.; Zhou, J. X.; Zrínyi, M.; Osada, Y.; Chen, Y. M. Novel biocompatible polysaccharide-based selfhealing hydrogel. Adv. Funct. Mater. 2015, 25 (9), 1352−1359. (29) Yang, X.; Bakaic, E.; Hoare, T.; Cranston, E. D. Injectable polysaccharide hydrogels reinforced with cellulose nanocrystals: morphology, rheology, degradation, and cytotoxicity. Biomacromolecules 2013, 14 (12), 4447−4455. (30) Amamoto, Y.; Kamada, J.; Otsuka, H.; Takahara, A.; Matyjaszewski, K. Repeatable photoinduced self-healing of covalently cross-linked polymers through reshuffling of trithiocarbonate units. Angew. Chem. 2011, 123 (7), 1698−1701. (31) (a) Wei, H. L.; Yang, Z.; Zheng, L. M.; Shen, Y. M. Thermosensitive hydrogels synthesized by fast Diels−Alder reaction in water. Polymer 2009, 50 (13), 2836−2840. (b) Tang, L.; Li, T.; Zhuang, S.; Lu, Q.; Li, P.; Huang, B. Synthesis of ph-sensitive fluorescein grafted cellulose nanocrystals with an amino acid spacer. ACS Sustainable Chem. Eng. 2016, 4 (9), 4842−4849. (c) Yu, F.; Cao, X.; Li, Y.; Chen, X. Diels−Alder click-based hydrogels for direct spatiotemporal postpatterning via photoclick chemistry. ACS Macro Lett. 2015, 4 (3), 289−292.

(32) (a) Reutenauer, P.; Buhler, E.; Boul, P. J.; Candau, S. J.; Lehn, J. M. Room temperature dynamic polymers based on Diels-Alder chemistry. Chem. - Eur. J. 2009, 15 (8), 1893−1900. (b) GarcíaAstrain, C.; González, K.; Gurrea, T.; Guaresti, O.; Algar, I.; Eceiza, A.; Gabilondo, N. Maleimide-grafted cellulose nanocrystals as crosslinkers for bionanocomposite hydrogels. Carbohydr. Polym. 2016, 149, 94−101. (33) (a) Bai, J.; Li, H.; Shi, Z.; Yin, J. An eco-friendly scheme for the cross-linked polybutadiene elastomer via thiol−ene and Diels−Alder click chemistry. Macromolecules 2015, 48 (11), 3539−3546. (b) Altintas, O.; Tunca, U. Synthesis of terpolymers by click reactions. Chem. - Asian J. 2011, 6 (10), 2584−2591. (c) Pressly, E. D.; Amir, R. J.; Hawker, C. J. Rapid synthesis of block and cyclic copolymers via click chemistry in the presence of copper nanoparticles. J. Polym. Sci., Part A: Polym. Chem. 2011, 49 (3), 814−819. (34) (a) Zhao, D.; Huang, J.; Zhong, Y.; Li, K.; Zhang, L.; Cai, J. High-strength and high-toughness double-cross-linked cellulose hydrogels: a new strategy using sequential chemical and physical crosslinking. Adv. Funct. Mater. 2016, 26 (34), 6279−6287. (b) Li, K.; Huang, J.; Gao, H.; Zhong, Yi.; Cao, X.; Chen, Y.; Zhang, L.; Cai, J. Reinforced mechanical properties and tunable biodegradability in nanoporous cellulose gels: poly(l-lactide-co-caprolactone) nanocomposites. Biomacromolecules 2016, 17 (4), 1506−1515. (c) De France, K. J.; Hoare, T.; Cranston, E. D. Review of hydrogels and aerogels containing nanocellulose. Chem. Mater. 2017, DOI: 10.1021/ acs.chemmater.7b00531. (35) Phelps, E. A.; Enemchukwu, N. O.; Fiore, V. F.; Sy, J. C.; Murthy, N.; Sulchek, T. A.; Barker, T. H.; García, A. J. Maleimide cross-linked bioactive PEG hydrogel exhibits improved reaction kinetics and cross-linking for cell encapsulation and in situ delivery. Adv. Mater. 2012, 24 (24), 64−70. (36) Lin, N.; Bruzzese, C.; Dufresne, A. TEMPO-oxidized nanocellulose participating as crosslinking aid for alginate-based sponges. ACS Appl. Mater. Interfaces 2012, 4 (9), 4948−4859. (37) Vilela, C.; Cruciani, L.; Silvestre, A. D.; Gandini, A. Reversible polymerization of novel monomers bearing furyl and plant oil moieties: a double click exploitation of renewable resources. RSC Adv. 2012, 2 (7), 2966−2974. (38) Polaske, N. W.; Mcgrath, D. V.; Mcelhanon, J. R. Thermally reversible dendronized linear AB step-polymers via “click” chemistry. Macromolecules 2011, 44 (9), 3203−3210. (39) Polaske, N. W.; McGrath, D. V.; McElhanon, J. R. Thermally reversible dendronized step-polymers based on sequential huisgen 1,3dipolar cycloaddition and Diels−Alder “click” reactions. Macromolecules 2010, 43 (43), 1270−1276. (40) Tamura, G.; Shinohara, Y.; Tamura, A.; Sanada, Y.; Oishi, M.; Akiba, I.; Nagasaki, Y.; Sakurai, K.; Amemiya, Y. Dependence of the swelling behavior of a pH-responsive PEG-modified nanogel on the cross-link density. Polym. J. 2012, 44 (3), 240−244. (41) Li, Z.; Zheng, Z.; Yang, Y.; Fang, G.; Yao, J.; Shao, Z.; Chen, X. Robust Protein Hydrogels from Silkworm Silk. ACS Sustainable Chem. Eng. 2016, 4 (3), 1500−1506. (42) Yang, Y.; Ding, X.; Urban, M. W. Chemical and physical aspects of self-healing materials. Prog. Polym. Sci. 2015, 49-50, 34−59.

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A Self-Healing Cellulose Nanocrystal-Poly(ethylene glycol

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