Tough, Swelling-Resistant, Self-Healing, and Adhesive Dual-Cross

Feb 20, 2018 - Leveraging hydrogen bonding to construct the second cross-link can be an alternative path to form DC polymer network, and it would be s...
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Tough, Swelling-Resistant, Self-Healing, and Adhesive Dual-CrossLinked Hydrogels Based on Polymer−Tannic Acid Multiple Hydrogen Bonds Hailong Fan, Jiahui Wang, and Zhaoxia Jin* Department of Chemistry, Renmin University of China, Beijing 100872, P. R. China S Supporting Information *

ABSTRACT: We demonstrate a facile and universal strategy in the fabrication of dual-cross-linked (DC) single network hydrogels with high toughness, “nonswellability”, rapid self-healing, and versatile adhesiveness based on polymer−tannic acid (TA) multiple hydrogen bonds. Two widely used hydrogels, physically cross-linked poly(vinyl alcohol) and chemically crosslinked polyacrylamide, have been transformed to TA-based DC hydrogels by dipping the corresponding aerogels into TA solution. The second cross-link via multiple polymer−TA hydrogen bonds effectively suppresses the crack propagation, resulting in both DC gels with high mechanical strength. But these two TA-based DC hydrogels go through different deformation mechanisms during the stretching based on analyzing their stress−strain curves using the Mooney−Rivlin equation. Moreover, these DC hydrogels are swelling-resistant, with strong toughness, good self-recoverability, rapid self-healing, and versatile adhesiveness. This work provides a simple route to fabricate multifunctional DC hydrogels, hopefully promoting their applications as biomedical materials. based on the carboxylic−Fe3+ coordinate bond.14−18 Although these equilibrium swollen DC hydrogels exhibited superior mechanical strength, the acrylic acid segment is the essential requirement for second cross-linker. It is highly desirable to develop a universal approach to build the second physical crosslinks. Hydrogels have strong ability to absorb water and form hydrogen bonds. Leveraging hydrogen bonding to construct the second cross-link can be an alternative path to form DC polymer network, and it would be suitable for a broad range of hydrogels. Multiple hydrogen bond motifs, such as selfcomplementary 2-ureido-4[1H]-pyrimidinone (UPy),19 are highly efficient cross-linkers for hydrogels. However, multiple hydrogen bond motifs in general do not exist in most of the water-soluble polymers. A concerted effort is therefore required to graft multiple hydroxyl groups, such as UPy, N-acryloyl glycinamide, and pyrogallol, onto polymer chains.19−21

1. INTRODUCTION Hydrogels, cross-linked polymer networks containing a large amount of water, have been extensively explored in decades due to their wide application in biomedicine.1−3 To date, based on the dissipation-induced toughening theory,4 the construction of complex networks such as interpenetrating, hybrid, and double network (DN) significantly enhances the mechanical strength and toughness of hydrogels.5−11 However, most of tough hydrogels still have been limited in a narrow field for application because of their susceptibility to water swelling, which weakens the mechanical strength and toughness of hydrogels.12 Therefore, developing an equilibrium swollen hydrogel with superior mechanical strength and versatile functions becomes an attractive but highly challenging proposition.12,13 Recently, it has been achieved in doublecross-linked (DC) hydrogels via introducing strong noncovalent interactions, for example, coordinate interaction as the second cross-link. Lin et al. have introduced the carboxylic− Fe3+ coordinate bond into chemically cross-linked network to form a DC network hydrogel.14 After that, a series of DC hydrogels with strong mechanical strength have been developed © XXXX American Chemical Society

Received: December 15, 2017 Revised: February 6, 2018

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DOI: 10.1021/acs.macromol.7b02653 Macromolecules XXXX, XXX, XXX−XXX

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This new strategy provides a universal approach to transform normal hydrogels to DC hydrogels with superior mechanical properties and versatile functions.

Tannic acid (TA), a natural polyphenol compound, is rich of pyrogallol and catechol groups that possess diverse interactions. Our previous study has shown that TA can be a universal gelation binder to cross-link polymers into hydrogels.22,23 Chen et al. reported that the poly(vinyl alcohol) (PVA)/TA hot solution can be gelled while it has been cooled down to room temperature due to their strong hydrogen bonds.24 However, leveraging hydrogen bonding provided by polyphenol (TA) as the second cross-linker in ultrastrong DC hydrogels has not been investigated.25 Using TA to construct double-cross-linked hydrogels faces a major obstacle brought by the chemical nature of TA. The freeradical polymerization would be inhibited and retarded in the presence of high amount of TA due to its strong free-radical scavenging ability.22,26 Thus, the trial through building one network via free radical polymerization and using TA as crosslinker for the second network failed in generating strong DC hydrogels. Hu et al. fabricated PAAm/TA hydrogel by free radical polymerization through cryogelation techniques.27 The mechanical strength of their hydrogel was relatively low (compressive modulus ∼9 kPa). On the other hand, the strong hydrogen bond between polymer and TA prevents the formation of polymer crystallites,24 thus building double cross-links through polymer crystallization and the cross-linking of TA and polymers simultaneously has hardly succeeded. Moreover, the interaction between TA and polymers usually leads to the formation of a condensed layer on the surface of hydrogels that significantly hampers the continued diffusion of TA inside hydrogels, resulting in nonhomogeneity of the obtained hydrogels and failure of the step-by-step-fabrication of strong DC hydrogels.28 Therefore, an alternative approach is needed to construct TA-based DC hydrogels. Aiming to solve these problems in constructing strong DC hydrogels based on TA, we demonstrated a simple but universal approach for generating highly tough DC hydrogels with versatile properties. In our strategy, the hydrogels were fabricated first, followed by a freeze-drying treatment. After that, the porous aerogels were immersed into TA solution to form the second cross-link. The porous architecture allows fast and thorough contact between TA and polymers of original hydrogels, avoiding inhomogeneity induced by the skin layer of the TA/polymer complex; thus, the second cross-link based on TA can be successfully constructed. Two widely used polymeric hydrogel networks have been tested, physically cross-linked PVA hydrogel and chemically cross-linked polyacrylamide (PAAm) hydrogel, in which PVA and PAAm can interact with TA through a hydrogen bond.24,27,28 Because of the successful formation of second cross-links, the mechanical strength of DC hydrogels enhanced significantly. For the asprepared polymer-TA DC hydrogels, the tensile strength can reach to megapascals with large elongation (>600%). For example, the tensile strength of PVA-TA300 gel is ∼9.5 MPa at elongation around 1000% with elastic modulus of 1.7 MPa, which is the top value in reported PVA gels systems.24,28 For the PAAm-TA300 gel, the tensile strength is ∼2.4 MPa with elongation around 670%, which is comparable with most of DN gels. The equilibrium swollen DC hydrogels still present strong tensile strength at the megapascal level and can dissipate large amount of energy during the deformation (∼ MJ m−2), their fracture energies are up to the kJ m−2 level, and they exhibit good self-recovery and self-healing abilities. In addition, TA endows these DC hydrogels high adhesiveness (>70 kPa) to diverse substrates including metal, glass, plastic, and tissues.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(vinyl alcohol) (Mowiol PVA-117, Mw 145 000 g/mol), acrylamide (AAm, 99%), N,N′-methylenebisacrylamide (MBAA, 97%), N,N,N′,N′-tetramethyldiamine (TMEDA, 99%), and ammonium persulfate (APS, AR) were purchased from Aladdin Company (Shanghai, China). Tannic acid (TA, ACS reagent) was purchased from Sigma-Aldrich Inc. All these reagents were used as received. All aqueous solutions were prepared by using deionized water. 2.2. Synthesis of Polymeric Hydrogels. PVA hydrogel: PVA (10 g) was dissolved in distilled water (40 g) at ∼90 °C under vigorous stirring to form a 20 wt % PVA aqueous solution. The homogeneous solution was then cast into a mold, which was covered with two pieces of glasses and the interval was 1.0 mm, and cooled at −18 °C for 8 h, followed by thawing at room temperature for 4 h. The freezing/ thawing circle was repeated five times. PAAm hydrogel: AAm (13.8 g), MBAA (30.6 mg) (0.1 mol % to AAm), and APS (25.8 mg) were dissolved in deionized water (the total mass was fixed at 60 g) and degassed for 10 min by bubbling N2 gas. After that, 44.4 μL of TMEDA was added in solution. Then the solution was cast into a mold. The radical polymerization was conducted at room temperature for 4 h to form a covalently crosslinked hydrogel. 2.3. Synthesis of Polymer−TA DC Hydrogels. The prepared polymeric hydrogels were freeze-dried, and then the aerogel (0.5 g) was immersed into TA aqueous solutions (30 mL) of different concentrations (i.e., 5, 10, 20, 50, 80, 100, 200, and 300 mg mL−1) for 20 h. For PAAm hydrogel, the soaking temperature was 65 °C, in order to accelerate the diffusion rate. For PVA hydrogel, the soaking temperature was room temperature (the crystalline region will be melt at high temperature). The hydrogels were named as polymer−TAx, where x is the concentration (mg mL−1) of TA soaking solution. For example, PVA−TA100 designates the PVA gel soaked in a TA solution of 100 mg mL−1. Then the as-prepared polymer−TA hydrogels were immersed in deionized water for 3 days to reach the swelling equilibrium state. These hydrogels were named as polymer−TAx(S). 2.4. General Characterization of Hydrogels. Scanning electron microscopy (SEM, JEOL 7401) was performed at an accelerating voltage of 5 kV. The freeze-dried hydrogel samples were coated with a thin layer of gold before SEM characterization. The PVA and PVA-TA gels before and after freeze-drying were characterized by wide-angle Xray diffraction (XRD-7000 diffractometer, Shimadzu) using Cu Kα radiation (λ = 0.154 18 nm) in the 2θ range 10°−80°. 2.5. Measurement of Mechanical Properties. All mechanical properties of these hydrogels were tested by a universal testing machine (UTM, INSTRON 5583) at room temperature (∼25 °C, humidity 45%). For the tensile test, hydrogels were cut into rectangle shape (40 mm × 10 mm × 1 mm). The rate of extension was fixed at 100 mm min−1 for tensile test and loading−unloading test. The elastic modulus (E) was calculated from the slope over 10−20% of strain ratio of the stress−strain curve. The tensile stress σ was calculated by the force divided by the initial cross-sectional area of hydrogel sample. The dissipated energy was calculated from the area between the loading−unloading profiles. For tearing test, the hydrogel samples were cut into trouser shape according to the GBT 529-2008 A standard. The rate of extension was fixed at 100 mm min−1. The fracture energy (Γ) was calculated from the average loading force (Fave) according to the equation Γ = Fave/d, where d is the thickness of trouser-shaped samples.20 The gel samples were coated with a thin layer of silicon oil to prevent evaporation of water during the test, and each test was repeated at least five times. 2.6. Measurement of Adhesiveness. Tensile-adhesion measurements of polymer−TA hydrogels were performed by a universal testing machine (UTM, INSTRON 5583). The substrates were commercially available polycarbonate (PC), poly(methyl methacryB

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Figure 1. (a) Illustration of the preparation of polymer−TA DC hydrogels. (b) Chemical structure of TA. (c) The hydrogen bond between PVA and TA. (d) The hydrogen bond between PAAm and TA.

Figure 2. (a−d) The water content and TA content of polymer−TA hydrogels. (a, b) PVA−TA hydrogels, (c, d) PAAm−TA hydrogels. The numbers in abscissa are the concentrations of TA solution in the preparation of polymer−TA hydrogels. Error bars indicate standard deviation; N = 5. (e, f) Photos of polymer and polymer−TA DC hydrogels before and after swelling in water. (a) PVA system; (b) PAAm system; the scale bar is 10 mm. late) (PMMA), glass, titanium (Ti), stainless steel (SS), and pork skin tissue. The engineered solid specimens (25 mm × 100 mm × 1 mm) were washed with deionized water and ethanol and then dried before use. The pork skin tissue were cut into 25 mm × 25 mm squares and attached to aluminum fixtures by using cyanoacrylate glue before test.29 To adhere the substrate, a hydrogel with 20 mm × 20 mm × 1 mm was placed between two specimens and compressed with a 100 g weight for 2 min. The adhered plates were clamped to the UTM and then separated at a crosshead speed of 10 mm min−1. The adhesion strength was calculated by the measured maximum load divided by the bonded area. Each sample was tested five times in parallel.

through cyclic freezing−thawing method (Figure S1). For PAAm hydrogel, acrylamide monomers were polymerized to form covalent cross-linking. After freeze-drying, the obtained PVA and PAAm aerogels were immersed in TA aqueous solutions with different concentrations for 20 h to generate new intermolecular hydrogen bonds between polymers and TA, resulting in polymer−TA DC hydrogels (Figure S2). The asprepared hydrogels were immersed in water for 3 days to release the unbonded TA molecules and, meanwhile, to advance the rearrangement of hydrogen bonds in DC hydrogels, resulting in equilibrium swollen hydrogels (Figure S3). The as-prepared and equilibrium swollen gels are named as polymer−TA-x and polymer−TA-x(S), where x is the concentration of TA soaking solution. For the as-prepared DC hydrogels, water contents of these hydrogels were significantly affected by the second cross-linking based on TA. With the increase of soaking TA concentration (x), the TA content in DC hydrogels increased, while their

3. RESULTS AND DISCUSSION 3.1. Formation and Swelling Behavior of Polymer−TA DC Hydrogels. The schematic diagram of the preparation of polymer−TA DC hydrogels is shown in Figure 1. In our strategy, polymer hydrogels with single cross-linked network were prepared first. For PVA hydrogel, the physical crosslinking was first formed by the formation of crystalline regions C

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Figure 3. Tensile stress−strain curves of polymer−TA DC hydrogels at as-prepared state and at swelling equilibrium state: (a, c) PVA−TAx hydrogels; (b, d) PAAm−TAx hydrogels.

Figure 2a−d. In the case of PVA−TA systems, the water contents of PVA-TA200 and PVA-TA300 gels increased due to their relatively low water contents, but the water content of PVA−TA10, PVA−TA20, PVA−TA50, and PVA−TA100 gels decreased after soaking in water, which may be caused by the formation of more densely cross-linked coagulates that expelled water.24 Because of the variation of water content and the release of TA molecules, the portion of TA in hydrogels at swelling equilibrium states changed as well. TA contents of PVA−TA10(S), PVA−TA20(S) and PVA−TA50(S) slightly increased, while that of PVA−TA100(S), PVA−TA200(S) and PVA−TA300(S) decreased. In the case of PAAm−TA systems, TA contents of all PAAm−TA gels had marginal change after soaking that was distinct from PVA−TA gels. PAAm−TA20 slightly swelled while other gels deswelled due to the strong polymer−TA interactions. 3.2. Mechanical Properties of Polymer−TA DC Hydrogels. The construction of second cross-link based on TA− polymer interaction in hydrogels enhances their mechanical properties significantly. Figure 3 shows the tensile stress−strain curves of TA-based DC hydrogels at as-prepared and at swelling equilibrium states. Compared with neat polymer hydrogels, introducing TA in polymer hydrogels significantly enhanced their mechanical properties. For as-prepared PVA− TA300 DC hydrogel, the tensile strength can reach up to 9.5 MPa with elongation of 1000%, and the elastic modulus is ∼1.7 MPa, which are top values in ever reported PVA gels systems.24,28,33−36 For as-prepared PAAm−TA DC hydrogels, the tensile strength of PAAm−TA300 is about 2.3 MPa with elongation of 670%, and the elastic modulus is ∼0.2 MPa. The polymer−TA DC hydrogels still exhibit high mechanical strength even at swelling equilibrium state due to the formation of polymer−TA hydrogen bonds. For PVA−TA300(S) DC hydrogel, the tensile strength can reach to 5.6 MPa with elongation of 630%, and the elastic modulus is ∼1.6 MPa. The tensile strength of PAAm−TA300(S) is about 1.8 MPa with elongation of 880%, and the elastic modulus is ∼0.18 MPa.

water content decreased (Figure 2a−d). PVA or PAAm prefers to form a hydrogen bond with dendritic polyphenol than with water molecules. Therefore, during the soaking process, more and more hydrogen bond acceptors on polymer chains were occupied by TA molecules with increasing concentration of TA solution. Compared to the mix method (TA content less than 10 wt %),24 the PVA−TA DC hydrogels contained higher amount of TA (up to ∼25 wt %) after soaking. Regarding the PAAm−TA system, the polymer network contained a high amount of TA around 30−45 wt %. SEM images of the inner morphologies of freeze-dried neat hydrogels and polymer−TA DC hydrogels are shown in Figure S4. For neat polymer hydrogels, the gels had loose microstructures. After introducing TA, polymer−TA hydrogels became slightly dense in pore wall, which may be caused by the formation of densely cross-linked polymer−TA coagulates. For the PVA−TA system, the advantage of the aerogelintermediate process has been clearly demonstrated while comparing these polymer−TA DC hydrogels with those obtained through directly soaking process. That PVA−TA hydrogel, obtained by directly soaking PVA gel into TA solution, has shown compact structures at the boundary (Figure S5), which dramatically influenced the further diffusion of TA molecules into inner part of hydrogel. As a consequence, those PVA−TA hydrogels need much longer time (7 days) to get equilibrium states (Figure S6). So the intermediate PVA aerogel is the key step for the fabrication of tough PVA−TA DC hydrogels in our strategy. Porous architecture of aerogel effectively guarantees fast and thorough contact between TA and polymers of original hydrogels, avoiding inhomogeneity induced by skin layer of the TA/polymer complex. These strong interactions between polymer and TA endow the obtained DC hydrogel good swelling resistance. The asprepared hydrogels were immersed into water for 3 days to reach the equilibrium state. The volumes of hydrogels were almost unchanged (Figure 2e,f), indicating the swelling of polymer−TA DC hydrogels was suppressed. The water and TA content of hydrogels before and after swelling are shown in D

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Figure 4. Mechanical strength values of neat hydrogels and DC hydrogels. (a−c) PVA−TAx hydrogels, (d−f) PAAm−TAx hydrogels. (a, d) Tensile strength, (b, e) breaking strain, and (c, f) elastic modulus. The numbers in abscissa are the concentrations of TA solution in the preparation of polymer−TA hydrogels. Error bars indicate standard deviation; N = 5.

Figure 5. Mooney−Rivlin curves of neat polymer hydrogels and polymer−TA DC hydrogels at as-prepared and swelling equilibrium states: (a) PVA−TAx hydrogels, (b) PVA−TAx(S) hydrogels, (c) PAAm−TAx hydrogels, and (d) PAAm−TAx(S) hydrogels.

property with TA content. However, as for breaking strain, physical cross-linked PVA gel and chemical cross-linked PAAm gel present a totally different variation tendency. With the increase of TA content, the breaking strain increased in the PVA system but decreased in the PAAm system. To better understand the deformation mechanism of DC hydrogels in different systems, we further analyzed the stress− strain curves by the phenomenological Mooney−Rivlin equation37,38

The tensile strength, breaking strain, and elastic modulus of the as-prepared and equilibrium swollen polymer−TA DC hydrogels are summarized in Figure 4. With increasing concentration of TA solutions (x) (namely hydrogen bond cross-linker density), the tensile strength and elastic modulus of polymer−TA DC hydrogels enhanced dramatically. However, previously reported PVA−TA gels fabricated by mixing method have shown a complex change due to structural inhomogeneity of their hydrogels: the tensile strength and elastic modulus of their PVA−TA hydrogels first increased and then decreased with increasing TA content.24 The good homogeneity of DC hydrogels keeps the continuous increase of their mechanical

σred = E

σ 1 = 2C1 + 2C2 λ λ − λ −2 DOI: 10.1021/acs.macromol.7b02653 Macromolecules XXXX, XXX, XXX−XXX

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Figure 6. Fracture energy of polymer−TA DC hydrogels fabricated in different TA solutions at as-prepared and swelling equilibrium states compared with neat hydrogel: (a) PVA−TAx hydrogels; (b) PAAm−TAx hydrogels. The numbers in the abscissa are the concentrations of TA solution in the preparation of polymer−TA hydrogels. Error bars indicate standard deviation; N = 5.

Figure 7. (a) Loading−unloading tests of PVA and PVA−TAx(S) hydrogels under 200% strain. (b) The calculated dissipated energy and energy dissipation coefficient of PVA and PVA−TAx(S) hydrogels under 200% strain. (c) Loading−unloading tests of PAAm and PAAm−TAx(S) hydrogels under 100% strain. (d) The calculated dissipated energy and energy dissipation coefficient of PAAm and PAAm−TAx(S) hydrogels under 100% strain. The numbers in abscissa are the concentrations of TA solution in the preparation of polymer−TA hydrogels. Error bars indicate standard deviation; N = 3.

∼0.55), strain hardening was observed due to the finite extensibility of polymer chains, similar to the neat PVA gel. While at large deformation, obvious strain softening has been observed, indicating that large amount of interchain hydrogen bonds were broken during the tensile deformation. Then, the PVA−TA DC gels exhibited slightly strain hardening at further deformation before fracture. In the case of the PAAm system, σred is almost independent of λ−1 for pure PAAm gel, suggesting a pure rubber-like elasticity of the PAAm gel (Figure 5c,d). For PAAm−TA DC gels, at λ−1 > ∼0.22, distinct strain softening was observed, indicating that large amount of interchain hydrogen bonds have been broken during the tensile deformation. At large deformation (λ−1 < ∼0.22), the PAAm−TA DC gels have shown distinct strain hardening due to the finite extensibility of polymer chains. 3.3. Toughness of Polymer−TA DC Hydrogels. The formation of hydrogen-bond-based second cross-link in hydrogels not only enhances their mechanical strength but also dramatically toughens hydrogels. Either as-prepared or equilibrium swollen TA-based DC hydrogels exhibited high

where σred is the reduced stress, λ is the extension ratio that is related to the strain ε as λ = ε + 1, and C1 and C2 are material constants. 2C1 is equal to the shear modulus (= E/3), and C2 is related to the strain softening (C2 > 0) or hardening (C2 < 0) beyond the Gaussian elasticity region. Figure 5 shows the σred against λ−1 for neat polymer gels and polymer−TA DC gels at as-prepared and equilibrium swollen states. The curves of hydrogels demonstrated a system-dependent feature, whereas the swelling treatment has limited influence. In the case of the PVA system, the neat PVA gel showed distinct strain hardening during the deformation (Figure 5a,b). This phenomenon may be caused by the fact that the PVA gel was cross-linked by crystalline domains, the polymer chains between crystalline domains have been prestretched, and they showed finite extensibility to afford further deformation, so that strain hardening was observed. At large deformation, the crystalline slippage or/and chain breakage introduced crack propagation, leading to the rapid fracture of the PVA gel. When the second cross-linker was introduced into the network, the gels showed different curves (Figure 5a,b). At small deformation (λ−1 > F

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Figure 8. Cyclic tensile loading−unloading curves of TA-based DC hydrogels for different soaking times in water: (a) PAAm−TA300(S) gel at a strain of 300%, (b) PAAm−TA300(S) gel at a strain of 700%, (c) PVA−TA300(S) gel at a strain of 100%, (d) PVA−TA300(S) gel at strain of 300%.

toughness with increasing x as well (Figure 6). The fracture energy (Γ) of PVA−TA300 and PAAm−TA300 hydrogels is about 4.1 and 3.4 kJ m−2, respectively, and at the swelling equilibrium state, these gels still have high fracture energy of 3.8 kJ m−2 for PVA−TA300(S) and 3.1 kJ m−2 for PAAm− TA300(S), which are almost 30 times compared with their neat polymer gels in both systems. Loading−unloading tests were performed to evaluate the energy dissipation and energy dissipation coefficient of hydrogels (Figure 7). It can be seen that at same strain the area of hysteresis loop increased with the increase of the TA content (hydrogen bond cross-linker density). For the PVA system, the ability to dissipate energy of single PVA networks was dominated by its physical cross-linked structure: the deformation of crystalline domains and dissociation of interchain hydrogen bonds dissipated about 104 kJ m−3 of energy (Uhys) with a corresponding coefficient 29.9% (Figure 7b). At same strain, the PVA−TAx(S) gels exhibited larger hysteresis loops than PVA gels. For example, the Uhys of PVA− TA300(S) gel was 1.13 MJ m−3 with a coefficient 62.4% under 200% strain. Compared with the PVA gel, the Uhys of PVA− TA300(S) was elevated around 10 times, which was caused by the large amount of PVA−TA hydrogen bonds. For the PAAm system, the dissipate energy of PAAm networks was about 9 kJ m−3 with a corresponding coefficient 26.5%, but the Uhys of PAAm−TA300(S) gel was 74 kJ m−3 with a coefficient 78.7% under 100% strain (Figure 7d), which is 8 times increased as compared with neat PAAm hydrogels. 3.4. Self-Recovery and Self-Healing of Polymer−TA(S) Hydrogel. The dynamic feature of hydrogen bond provides polymer−TA(S) DC hydrogels with good self-recovery ability. Cyclic tensile tests were performed to investigate their selfrecovery behavior of the polymer−TA(S) DC hydrogels. Figures 8a and 8b show the cyclic loading−unloading tests of PAAm−TA300(S) hydrogels at a strain of 300% or 700%, respectively. The hydrogel presented outstanding self-recovery behavior. After the first loading−unloading circle, the hydrogel

exhibited notable residual strain because of the structural destruction in dissipating energy. However, the hydrogel returned to its original properties immediately when it was reimmersed in water. After soaking for 5 min, the second test was conducted. For strain of 300%, the loading−unloading loop was fully overlapped with the first one. While for a strain of 700%, the loop recovery efficiency was as high as 96%. For PVA−TA300(S) gel, the damaged internal networks in hydrogels can self-recover gradually (Figures 8c and 8d). They recovered to nearly 68% after 12 h when the strain was 100% (Figure S7). The good self-recovery behavior of polymer−TA(S) DC hydrogel relies on dynamic cross-links via the hydrogen bond between TA and polymer. However, the variation of different hydrogels in self-recovery represented the influence from original hydrogel networks as we observed in their deformation mechanism. For the PVA system, the first network is physically cross-linked by the PVA crystalline. When the external load was applied, the deformation of crystalline dissipated energy first, and then the hydrogen bonds did. When the external load was removed, the temporarily fractured hydrogen bonds can rapidly re-form without any external stimuli at room temperature, but the reconstruction of crystalline needs a much longer time. Therefore, the PVA−TA(S) hydrogels cannot fully recover at room temperature; their recovery efficiency depends on the deformation degree and recovering time. For the PAAm system, the first network was cross-linked by chemical crosslinker which is much stronger than noncovalent cross-link. When the deformation is small, the energy dissipation is mainly caused by the breakage of hydrogen bonds, which can be quickly recovered. However, if the deformation is large, the irreversible breakage of polymer chains will induce the gel partially self-recovering. Because the equilibrium association constant of single hydrogen bond is much lower than that of coordinate bond,40 the self-recover rate of polymer−TA DC hydrogels is much faster than that of carboxylic−Fe3+ based DC G

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Figure 9. Tensile stress−strain curves of original and self-healed polymer−TA(S) hydrogels: (a) PVA−TA300(S) gels; (b) PAAm−TA300(S) gels.

“nonswellable” and exhibit strong mechanical strength, large elongation, excellent toughness, good self-recovery, and selfhealing abilities at either as-prepared or swelling equilibrium states. By analyzing the stress−strain curves using the phenomenological Mooney−Rivlin equation, we have revealed that DC hydrogels in different systems went through different deformation mechanisms during the stretching. Besides, the chemical nature of TA endows these hydrogels high adhesiveness to diverse substrates. This work demonstrated a new method for developing novel dual cross-linked hydrogels with superior mechanical strength and versatile functions. More importantly, this new strategy is simple, and TA is a cheap natural product, hence suitable for a large-scale preparation. In further studies, it can be easily extended to generate tough hydrogels by combining these double network hydrogels with hybrid multi-cross-linked hydrogels and nanocomposite hydrogels.

hydrogels, which needed more than 4 h but still were not fully recovered in general.14−17,41,42 The dynamic nature of cross-linkers also endows the polymer−TA(S) DC hydrogels self-healing abilities. Take PVA−TA300(S) and PAAm−TA300(S) as examples, we first cut the gels into half and then put the fracture surfaces together at room temperature under water for 1 h. Figure 9 shows the tensile stress−strain curves of original and self-healed hydrogels. It can be seen that the cut gels were partially self-healed; for PVA−TA300(S) gel, the healed gel can sustain a tensile stress of 1.46 MPa with elongation of 200% before fracture. The healing efficiency is 39.8% based on the stress or 28.5% based on the strain. For PAAm−TA300(S) gel, the tensile stress of healed gel is of 0.27 MPa with a strain over 500%. The healing efficiency is 16.5% based on the stress or 57.9% based on the strain. Thanks to the multiple hydrogen bonds between polymer chains and TA molecules, the fractured gels can be healed spontaneously and rapidly without external treatment, which is much convenient compared with other self-healable tough hydrogels.15,42−44 3.5. Adhesiveness of Polymer−TA Hydrogels. Combining high mechanical strength and high adhesiveness into hydrogels is a great challenge.45,46 However, in TA-based DC hydrogels, natural polyphenol TA forms hydrogen bonds with polymer as the second cross-link to toughen the polymer network and supplies a large amount of free pyrogallol/catechol groups as well. Catechol can bind to both organic and inorganic surfaces through the formation of reversible noncovalent or irreversible covalent interactions; thus, hydrogels with catechol groups demonstrate good adhesiveness.23,26 Figure S8 shows the adhesive strengths of PVA−TA300(S) and PAAm− TA300(S) hydrogels to diverse substrates, including metal, glass, plastic, and tissue materials. For PVA−TA300(S) hydrogel, the adhesive strengths are in a range of 70−110 kPa, while the adhesive strengths of PAAm−TA300(S) gel are between 60 and 80 kPa. Both TA-based DC hydrogels have versatile adhesive ability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02653.

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Experimental section; XRD data; SEM images; mass ratio data; self-recovery efficiency data; adhesive strength data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.J.). ORCID

Zhaoxia Jin: 0000-0002-6108-0636 Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS In summary, a facile and universal strategy to fabricate dual cross-linked (DC) hydrogels has been demonstrated. The original polymer networks (PVA or PAAm) were first constructed by either physical cross-linking or chemical crosslinking, and then TA was introduced into the above networks to further cross-link hydrogels via hydrogen bonds, forming double-cross-linked hydrogels. In DC hydrogels, the hydrogen bond is a dynamic, reversible, and sacrificial cross-link which dissipates more energy through bond’s breakage and assists quick reconstruction in the recovering of hydrogels. In the cooperation of double cross-links, these DC hydrogels are

ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grants 21374132 and 51673210) for financial support. Prof. Decheng Wu at Institute of Chemistry, Chinese Academy of Sciences, Prof. Taolin Sun, Dr. Kunpeng Cui, and Dr. Junchao Huang at Faculty of Advanced Life Science, Hokkaido University, and Dr. Xunda Feng at Department of Chemical and Environmental Engineering, Yale University, are highly acknowledged for useful suggestions and discussions. H

DOI: 10.1021/acs.macromol.7b02653 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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DOI: 10.1021/acs.macromol.7b02653 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b02653 Macromolecules XXXX, XXX, XXX−XXX