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Tough, Stretchable, Compressive Novel Polymer/Graphene Oxide Nanocomposite Hydrogels with Excellent Self-Healing Performance Chenguang Pan, Libin Liu, Qiang Chen, Qiang Zhang, and Gailan Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12932 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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Tough, Stretchable, Compressive Novel Polymer/Graphene Oxide Nanocomposite Hydrogels with Excellent Self-Healing Performance Chenguang Pan1, Libin Liu1*, Qiang Chen2, Qiang Zhang1, Gailan Guo1

1

C. Pan, Prof. L. Liu, Q. Zhang, G. Guo

Shandong Provincial Key Laboratory of Fine Chemicals, Key Laboratory of Fine Chemicals in Universities of Shandong, Qilu University of Technology, Jinan 250353, China, E-mail: [email protected] 2

Dr. Q. Chen

School of Material Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China

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Abstract Designing hydrogels with high mechanical properties without sacrificing their self-healing efficiencies remains great challenges. We have fabricated cationic polyacrylamide/graphene oxide (GO) hydrogels by free-radical polymerization of acrylamide (AM) and 2-(dimethylamino)ethylacrylatemethochloride (DAC) in the presence of GO. The mechanical properties and self-healing ability can be tuned by GO content and mass ratio of AM and DAC. The ionic bonds between DAC and GO and hydrogen bonds between AM and GO can efficiently dissipate energy and rebuild the networks. The resulting composite hydrogels possess high stiffness (Young’s modulus: ~1.1 MPa), high toughness (~ 9.3 MJ m-3), and high fatigue resistance, as well as high self-healing efficiency (>92% of tensile strength, >99% of tensile strain and >93% of toughness). In addition, the completely dried hydrogels can recover their original mechanical values by spraying water and still possess outstanding self-healing efficiency. Our design can provide better fundamental understanding of physicals properties of hydrogels and should enable the development of tough, self-healing hydrogels for practical applications.

Keywords: hydrogel, graphene oxide, cationic polymer, toughness, compression, self-healing

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Introduction Recently, there has been a growing interest in self-healing materials due to their ability to repair themselves after damage, as this property can increase life-time of materials, reduce maintenance costs and improve total safety of systems1, 2. There are three strategies to achieve self-healing materials including the storage of healing agents3, dynamic covalent bond formation4, 5, and utilization of non-covalent bonds interaction (e.g. hydrogen bonding6,

7

, hydrophobic association8, host–guest

interactions9-11, π − π stacking12, and electrostatic interactions13, 14). Hydrogels, as a soft three-dimensional reticular material, not only can be applied in bioengineering due to their biocompatibility, but also have potential applications in waste treatment, superabsorbents, and electronic devices. However, the relatively poor mechanical behaviors limit their applications. In addition, incorporation of self-healing properties into hydrogels increases the difficulties in the preparation of high mechanical hydrogels. Most of the reported tough or stiff hydrogels usually possess no or weak self-healing property15, 16 and the self-healing hydrogels are generally mechanically very weak13,

17-20

. Therefore, developing hydrogels with both high mechanical

strength and good self-healing ability is highly demanded. To achieve this end, one strategy is to design double network (DN) hydrogels which refer to the step-wised synthesized hydrogel composed of the firstly, highly crosslinked and secondly, loosely crosslinked polymer networks21, 22. For example, Zheng et al.23 proposed a new design strategy to improve both fatigue resistance and self-healing property of DN gels by introducing hydrophobically associated polyacrylamide into fully physically cross-linked agar. Gong et al.24 reported a new class of tough and viscoelastic polyampholyte hydrogels synthesized by random copolymerization of oppositely charged ionic monomers. The hydrogels possess both enhanced toughness and self-healing ability due to the multiple ionic bonds. Although DN hydrogels could achieve the enhanced mechanical strength and self-healing ability simultaneously, the complicated synthesis process and multiple influencing factors of the first network on the second network limit their practical applications25. An alternative way to obtain tough hydrogels with self-healing ability, is forming 3

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nanocomposite hydrogels, which is involved in simple synthesis procedure and only by introducing nanofillers, such as clays26, 27, carbon nanotubes28, graphene29-32 into hydrogels. Among these nanofillers, graphene-based materials, due to their ultrahigh mechanical strength and chemical stability, excellent electrical and thermal conductivity, have been widely introduced into hydrogels and endowed them with special properties33-39. For example, Tong et al.40 prepared GO-hectorite clay-poly(N,N-dimethylacrylamide) hybrid hydrogels with enhanced mechanical properties and fast self-healing capability. Fu et al.41 also prepared poly(vinyl alcohol) /GO hydrogels with super mechanical and chondrocyte cell-adhesion properties by using 2,2’-(ethylenedioxy)-diethanethiol as chemical cross-linker. Although great progresses have been achieved, the magnitude of the tensile strength recovered after healing for most healable hydrogels is below 0.5 MPa and self-healing efficiency is below 90%. Therefore, construction of novel self-healing materials, which possess enhanced mechanical properties without sacrificing their healing efficiencies, still remains great challenges. Herein,

we

have

fabricated

cationic

poly(acrylamide-co-2-(dimethylamino)ethylacrylatemethochloride) (P(AM-co-DAC)) hydrogels containing GO as macro-crosslinker, which exhibit both high self-healing efficiency and outstanding mechanical properties. The self-healing is achieved by water assistance without any other stimuli. Deionized water is widely used in self-healing of hydrogels and polymer films42-44 . In our system, water is sprayed on the fracture surface, then leaving the material in air at room temperature for hours completes the self-healing process. The self-healing ability and mechanical strength are enhanced through multiple interactions between P(AM-co-DAC) and GO. Distinct from other nanocomposite hydrogels, in basic environment the ionic bonds between N(CH3)+ of DAC and COO- of GO function as strong crosslinking, providing elasticity. The hydrogen bonds between NH2 of AM and oxygen-containing group of GO server as reversible crosslinking that break and reform at deformation to dissipate energy (Figure 1). As a result, the density of available ionic bonds and hydrogen bonds can be controlled by GO content and mass ratio of AM and DAC. The resulting 4

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P(AM-co-DAC)/GO composite hydrogels exhibit outstanding mechanical properties, such as high stiffness (Young’s modulus: ~1.1 MPa), high tensile strengths up to 2.1 MPa, high elongation of 800-1700% and excellent fatigue-resistance (20 tensile cycles and 30 compression cycles allowing for >90% recovery of elasticity), as well as extreme self-healing efficiency (>92% of tensile strength, >99% of tensile strain and >93% of toughness).

Figure 1. Schematics of preparation of hydrogels copolymerization of AM, DAC and N,N’-methylene-bisacrylamide (MBA) in the presence of graphene oxide along with a proposed molecular mechanism of the loading and recovery process. The ionic bonds and hydrogen bonds contribute to the high mechanical properties and self-healing properties. Experimental Section Materials Graphite powder (8000 mesh, purity 99.95%), sodium hydroxymethanesulfinate dihydrate (ALD), N,N′-methylene-bisacrylamide (MBA), acrylamide (AM) and 2,2′-azobis(2-methylpropionamidine) dihydrochloride (AIBA) were supplied by Aladdin. Ammonium persulphate (APS), concentrated sulfuric acid (95–98%), 5

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concentrated hydrochloric acid (36–38%), ammonium hydroxide, potassium permanganate, were all analytically pure and purchased from Beijing Chemical Factory (China). Hydrogen peroxide (H2O2) and sodium nitrate were supplied by LaiYang Shi Kant chemical company, 2-(dimethylamino)ethylacrylatemethochloride, 80 wt% in water (DAC) was purchased from J&K Scientific Co.

Preparation of P(AM-co-DAC)/GO composite hydrogels Graphene oxide (GO) was prepared from natural graphite by the Hummers method (see supporting information). P(AM-co-DAC)/GO composite hydrogels were synthesized by in situ free radical polymerization of DAC and AM in the presence of GO. Firstly, the GO (40 mg) was dispersed in water (11 mL) by ultrasonic radiation for about 1 h. The pH of the GO aqueous dispersion was adjusted to 10 by dropping ammonium hydroxide, and the mixture was stirred for 10 minutes in an ice bath. Successively, the monomer DAC (2.67 g) and AM (1.34 g) were added into the GO suspension under stirring, 1 mL of MBA (3 mg/mL) aqueous solution was added in the mixtures. The mixtures were stirred for another 2 h to make a uniform solution under the ice bath. Finally, the initiator KPS (10 mg), AIBA (15 mg) and ALD (15 mg) were added to the solution and stirring in ice bath for 1 h. Then after ultrasonication of 5 min to remove the bubble, the mixtures were transferred to the glass tube with inner diameter of 5.5 mm. Polymerization was carried out in an oven at 35 oC for 12 h. The resulting hydrogel was removed from the glass tubes and dehydrated in air at room temperature for about 20 h to reduce the water content to 30 wt%.

Self-Healing process The as-prepared cylindrical hydrogels were cut into halves. Subsequently, the two halves were slightly put together with the fracture surfaces contacting with each other. Because the fresh fracture surfaces are relatively adhesive when the hydrogel was cut, no additional external force is required for connecting the broken parts. Then the connected hydrogel was placed in a plastic petri dish and a drop of water was dropped 6

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on the fracture surfaces. The dish containing the hydrogel was placed in air at room temperature. After standing in air for several hours, the healed samples were tested at different healing time. The cutting and healing process can be found in Movie S3.

Mechanical testing The tensile test is done with a universal test instrument (Hensgrand, WDW-02, China). As-prepared hydrogel samples with 3 mm diameter, 50 mm length, and 10 mm gauge length were tested at a crosshead speed of 100 mm min−1 and at 25 oC. The tensile stress (σ) was calculated as σ = F/πR2, where F is the load and R is the original radius of the specimen. The tensile strain (ε) was defined as the change in length (l) relative to the initial gauge length (l0) of the specimen, ε = (l − l0)/l0×100%. The Young’s modulus was calculated as the slope of the stress−strain curve within the range ε = 0% - 100%. The fracture toughness was characterized by the fracture energy (U, MJ m−3), which is calculated by integrating the area under the stress−strain curve:

U = ∫ σdε . Cyclic tensile tests with a maximum strain of 800% were conducted on specimens with the same gauge length and at the same crosshead speed. The dissipated energy for each cycle, ∆U, is defined as the area of hysteresis loop encompassed by the loading−unloading curve: ∆U= ∫loading σdε -

∫unloading σdε .

Compression cycle test was carried out using a diameter of 6 mm, height of 10 mm cylindrical sample with the maximum compression strain of 80%, compression speed of 10 mm min-1. The calculation method of the dissipation energy of the compression cycle is the same as that of the stretching cycle. Rheology measurements of the hydrogels were conducted with a TA DHR-2 rheometer using a parallel plate of diameter 20 mm. (1) The dynamic strain sweep from 0.1% to 10% was firstly carried out at an angular frequency of 10 rad s-1 to determine the linear viscoelasticity region. (2) The frequency sweep was performed over the frequency range of 0.1–100 rad s-1 at a fixed strain of 0.5 %. (3) The alternate step strain (1%, 50%, 100%, 200%) sweep of hydrogel was measured at a fixed 7

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angular frequency (10 rad s−1). All measurements were performed at 25 oC controlled by a Peltier plate.

Characterizations FTIR spectra were obtained on a FTIR spectrometer IR Prestige-21 (Shimadzu, Japan). X-ray diffraction (XRD) was carried out on a D8 ADVANCE X-ray diffractometer (Bruker AXS, Germany). Raman spectra were obtained using a LabRAM tHR800 Raman spectrometer (HORIBA JY,France). Zeta potentials were obtained on a Zetasizer Nano ZS90 particle size analyze (Malvern, England).

Results and Discussion The cationic PAM hydrogels were synthesized copolymerization

of

acrylamide

2-(dimethylamino)ethylacrylatemethochloride

(DAC)

by one-pot free-radical (AM) using

and

N,N’-methylene

bisacrylamide (MBA) as chemical cross-linker. Before polymerization, GO suspension was added into the solution of the monomers at different pH values (Figure 2a). When the pH values are lower than 9, the mixtures become inhomogeneous due to flocculation behavior of DAC. With increasing in pH value, the solution gradually turns darker and appears almost opaque at pH 12, indicating a better dispersion of GO nanosheets45. The zeta potential, which represents the degree of electrostatic repulsion between particles, is directly related to the colloidal dispersion stability. As the pH values increase, the absolute values of zeta potential for GO and DAC dispersions are higher, indicating a more stable dispersion system with higher resistance to aggregation 45 (Figure 2b). In addition, GO nanosheets have a negative zeta potential in solution, whereas DAC solutions have a positive zeta potential. Therefore, an optimal pH value of 10.0 was selected to polymerize the AM and DAC monomer in the presence of GO, which would be more favorable for electrostatic interactions of DAC and GO (Figure 2b). The polymerization was conducted at 35 oC for 12h, then leaving hydrogels in air to lose water, the resulting P(AM-co-DAC)/GO composite hydrogels with about 30 wt% of water content were 8

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obtained. X-ray diffraction (XRD) reveals a sharp peak of GO at the 2θ position of 11.3o, whereas P(AM-co-DAC) and P(AM-co-DAC)/GO hydrogels show similar XRD patterns without any sharp peaks, indicating that GO nanosheets are uniformly dispersed in P(AM-co-DAC) hydrogel systems (Figure S1).

Figure 2. (a) Photograph of GO and DAC mixed dispersion solution at different pH values. (b) pH values versus zeta potential for GO and DAC solutions, respectively. (c) FTIR spectra of GO powder, P(AM-co-DAC) and P(AM-co-DAC)/GO, respectively. (d) Raman spectra of P(AM-co-DAC) and P(AM-co-DAC)/GO, respectively.

The aim of our design is to obtain hydrogels with both high mechanical properties and high self-healing ability by controlling hydrogen bonding between amino of AM and oxygen-containing group of GO and electrostatic interaction between quaternary ammonium group of DAC and carboxyl of GO (Figure 1). To prove the presence of the two kinds of interactions, FTIR and Raman were performed. In P(AM-co-DAC)/GO composite hydrogels, the bands belonging to GO powder (e.g. 9

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3062, 1730, 1228, and 1055 cm-1 for OH stretching in carbonyl acid, C=O carbonyl stretching, and asymmetric and symmetric C–O stretching in the C–O–C group, respectively) are greatly decreased or even disappeared, possibly due to the low content of GO in the hydrogel34. However, the bands at 3413 and 3198 cm-1 attributed to the characteristic N–H stretching of AM are shifted to 3402, 3190 cm-1, respectively, in the P(AM-co-DAC)/GO hydrogels. Furthermore, the typical C=O stretching of the –CO–NH2 group is shifted from 1663 and 1610 cm-1 of AM to 1669 and 1619 cm-1 in P(AM-co-DAC)/GO hydrogels (Figure 2c). These shifts fully indicate the existence of hydrogen-bonding interactions between amino groups of PAM chains and oxygen-containing groups of GO sheets34, 39. The sharp peak at 950 cm-1 highlights the presence of quaternary ammonium group in P(AM-co-DAC) and P(AM-co-DAC)/GO hydrogels. The high basic environment (pH=10) makes nearly all carboxylic acid groups of GO exist in –COO– form. Therefore, the strong ionic bonds are formed between COO- of GO and N(CH3)3+ of DAC (Figure 1). The peaks at 2945, 1480, 1406 cm-1 assigned to symmetric CH3 stretching, asymmetric and symmetric CH3 deformation vibration of – N(CH3) in P(AM-co-DAC) are shifted to 2957, 1472, 1417 cm-1 after formation of ionic bonds in P(AAM-co-DAC)/GO hydrogel (Figure 2c). Furthermore, the Raman spectra clearly show that the symmetric and asymmetric CH3 stretching of N(CH3)3+ in DAC is shifted from 2927 and 2971 cm-1 to 2933 and 2960 cm-1 after incorporation of GO (Figure 2d), demonstrating the formation of ionic bond46. Again, the difference of binding energy of N1s peaks in P(AM-co-DAC) before and after addition of GO into the hydrogels also proves the formation of hydrogen bonds and ionic bonds between P(AM-co-DAC) and GO (Figure S2). The excellent mechanical performance of the P(AM-co-DAC)/GO hydrogels is attributed to a proper balance between the density of ionic bonds and hydrogen bonds. To make clear the effect of the two kinds of interactions, the content of chemical crosslinker of MBA was fixed at 0.075 wt% in all hydrogels during polymerization (see supporting information, Figure S3). Therefore, we have tuned this balance by controlling GO content and mass ratio of AM and DAC. Different hydrogels 10

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(abbreviated AxDyGz, where A, D and G represent AM, DAC and GO, x and y refer to mass ratio of AM and DAC, z is GO content, respectively) by using different mass ratio of AM and DAC (1:0.5, 1:1, 1:2, 1:4) with different GO contents (0.5, 1, 1.5, 2 wt%) were synthesized. For A1D4Gz samples, due to too much content of DAC, the hydrogels remain weak and can not hold their shape (Figure S4). Therefore, unless otherwise noted, for all hydrogels discussed in this paper, x:y is equal to 1:0.5, 1:1 and 1:2, z is equal to 0, 0.5, 1, 1.5, 2, respectively. Figure 3a shows tensile stress-strain curves of A1D2Gz samples. Without GO, A1D2G0 hydrogel reveals the fracture stress is 96.8 kPa. When addition of small amount of GO, the fracture stress of A1D2G0.5 reaches to 357.9 kPa, almost 4 times of that of A1D2G0. Correspondingly, the fracture strain also increases from ~1150 to ~1562%. This means that the ionic bond and hydrogen bonds formed between GO and P(AM-co-DAC) can enhance the mechanical strength distinctly. When the GO content reaches to 1 wt%, the fracture stress and strain increased to 564.1 kPa and 1608%. Further increase in the GO content results in the increase in the fracture stress and slight decrease in the fracture strain (Figure 3a). For A1D2Gz series, the effect of GO on the mechanical strength of hydrogels were also measured by rheology. The oscillation strain dependence of the storage modulus G’ and loss modulus G’’ were tested at oscillation strain ranged from 1% to 100% and at 1 rad s-1 for A1D2Gz samples to determine the linear viscoelasticity region (Figure S5). Thereby all the viscoelasticity tests were carried at strain of 0.5% to ensure the availability of the liner viscoelasticity and enough sensitivity. Figure 3b illustrates the angular frequency dependence of G’ and G’’ for A1D2Gz hydrogels over a broad angular frequency range from 1% to 100%. Distinctly, G’ is always higher than G’’ over the observed frequency range for all hydrogels, which indicates that cross-linked networks had been formed in these hydrogels. The higher GO content gives rise to higher G’, which is well in line with the results of tensile tests. In order to understand the effect of GO on mechanical properties of hydrogel, we determined the crosslinking density from the equilibrium shear modulus based on the rubber elasticity at small deformation of 0.5% strain to ensure availability of the assumptions 11

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in the theory. The crosslinking density N in hydrogels is related to the equilibrium shear modulus Ge 47, 48. Ge = NRT Here, Ge was taken from the plateau modulus G’ in frequency dependence of storage and loss modulus curves where the plateau appeared and loss modulus G’’ was much smaller than G’. R and T are the gas constant and absolute temperature, respectively. The cross-linking density can be calculated from G’ obtained in Figure 3b. The estimated N of A1D2Gz hydrogels increased from 6.43, 14.61, 16.13, 37.69, to 66.74 mol/m3 as GO increased from 0, 0.5, 1, 1.5, to 2 wt%, suggesting that the crosslinking density in P(AM-co-DAC)/GO composite hydrogels increases as GO content increases. These results reveal the cross-linking effect supplied by GO indeed emerges in hydrogels.

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Figure 3. Mechanical properties of hydrogels. (a) Tensile stress–strain curves of A1D2Gz hydrogels with different GO content. (b) Frequency dependence of storage (G’, filled symbols) and loss (G’’, open symbols) modulus of A1D2Gz hydrogels at strain of 0.5%. (c) The change of Young’s modulus of hydrogels with different GO content. (d) Toughness of hydrogels with different GO content. A1D2G1 hydrogels can be (e) bent, (f) knotted, (g) loaded and (h, i) stretched, respectively. 13

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By tuning the chemical composition, we have achieved to balance the aforementioned interactions and design materials with superb mechanical properties. The ionic bond and hydrogen bond interaction in the hydrogel were controlled by different mass ratio of AM and DAC. The tensile stress-strain curves of A1D0.5Gz and A1D1Gz hydrogels are shown in Figure S6. For all A1D0.5Gz and A1D1Gz samples, the fracture stresses are increased as GO contents increase, similar to the results of A1D2Gz hydrogel. For example, the tensile strength is ~1.9 MPa for A1D0.5G1.5, which is more than three times of magnitude increase from A1D0.5G0 (~0.58 MPa) and further increases to ~2.1 MPa for A1D0.5G2. The detailed mechanical properties of all hydrogels are shown in Table S1. Young’s modulus of hydrogels (reflecting stiffness) is summarized in Figure 3c. The hydrogels can be ranged from very soft to stiff by tuning GO content and ratio of AM and DAC. At the same GO content, the stiffness of the hydrogels increases with increasing AM content. Higher AM content leads to higher stiffness. For instance, without addition of GO, A1D0.5G0 sample displays a Young’s modulus of ~304 kPa, higher than A1D1G0 and A1D2G0 (Figure 3c). This composition (A1D0.5Gz series) with higher AM content allows for a higher total number of hydrogen bonds, which results in a higher crosslinking density. The stiffness could be further improved by incorporation of GO into the hydrogel due to the increased ionic bonds. Remarkably, A1D0.5G2 sample displays a highest Young’s modulus of ~1056 kPa. The toughness of hydrogels reflected by energy dissipation is shown in Figure 3d. Without addition of GO, the toughness of hydrogels decreases from ~4.12 MJ m-3 for A1D0.5G0 to ~0.67 MJ m-3 for A1D2G0, due to the decreased hydrogen bonds of AM. After introduction of GO, the toughness improves compared to that of corresponding pristine hydrogels and further increases as GO content increases. At the same GO content, A1D2Gz samples display higher toughness than that of A1D1Gz, whereas lower than that of A1D0.5Gz, indicating that the hydrogen bonds are dominated in A1D0.5Gz series and ionic bonds are more obvious in A1D2Gz series due to the changes of AM and DAC ratio (Table S1). Noted that at 2 wt% GO content the considerable 14

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interaction between GO and polymer chain leads to slight decrease in toughness and self-healing efficiency as discussed below. A1D0.5G2 shows the highest Young’s modulus of ~1056 kPa, highest tensile strength of ~2.1 MPa at elongation of ~810% and toughness of ~9.3 MJ m-3 (Table S1). These mechanical properties are much better than those of polymer/graphene composite hydrogels29, 35 or elastomer38, and comparable to those of polyampholyte hydrogels and tough DN hydrogels24, 49. For hydrogels, the stiffness and toughness of polymer networks are often inversely related. According to the Lake−Thomas model, for example, as the cross-link density decreases, toughness increases, but stiffness decreases50. Most hydrogels are either stiff and brittle or tough and compliant with low elastic moduli on the order of 10 kPa. In the previous work, there are a few hydrogels with high stiffness at MPa level (about 1 ~ 2 MPa) and good toughness simultaneously24, 50. Different from the conventional hydrogels which usually possess high stiffness but low toughness, our hydrogels with reversible ionic bonds and hydrogen bonds allow for significant energy dissipation and display indeed enhanced mechanical strength and stiffness without compromising toughness. For example, A1D2G1 is strong enough to withstand high level deformation of bending (Figure 3e), knotting (Figure 3f). Short rods of A1D2G1 with diameter of 3 mm can withstand an external bending load of 1.2 kg (Figure 3g) and also can be highly stretched (Figure 3h, i). In addition to the good combination of high strength, high stiffness and high toughness, the P(AM-co-DAC)/GO composite hydrogels also exhibits outstanding fast recovery and fatigue resistance properties. The rate of stress relaxation is important factor for anti-fatigue materials. As shown in Figure 4a, small oscillatory shear strain at 1% was first applied to A1D2G1, then increased from 1% to 50% and maintained for 100 s, the storage energy (G’) is higher than loss energy (G’’), and they immediately (less than 10 s) recovered their original values once the strain goes back to 1%. Similarly, when we later applied the larger strains (100% and 200%) followed by small strain (1%), G’ also quickly restored the initial value, indicating the fast recovery of the hydrogel network (Figure 4a). 15

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Figure 4. (a) G’ and G’’ of the A1D2G1 under continuous strain sweep with alternate oscillation force at strain of 1%, 50%, 100% and 200%, respectively. (b) Cyclic tensile loading–unloading curves of A1D2G1 with different recovery time at a strain of 800%. (c) Twenty subsequent tensile loading-unloading cycles of A1D2G1 with 10 min recovery between each cycle. (d) Recovery degree of dissipated energy and Young’s modulus for A1D2G1 after 20 tensile loading-unloading cycles.

The recovery of deformation has been measured by cycling tensile tests. As shown in Figure 4b, A1D2G1 sample shows a large hysteresis loop and dissipated energy of ~0.97 MJ m-3 in the first loading-unloading cycle at strain of 800%. The second cycle without any resting time for recovery shows that the hysteresis loop becomes much smaller (~0.31 MJ m-3). When resting for 5 min of recovery, the dissipated energy recovers obviously. It is noticeable that the areas of hysteresis loops of A1D2G1 almost fully recover for the following successive loading-unloading cycles with 10 min of recovery (0.95 MJ m-3) (Figure 4b). To further explore tension cycle ability, we performed 20 subsequent tensile loading-unloading cycles on the same 16

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samples with 10 min of recovery between each cycle (Figure 4c). The cyclic tensile process was also shown in Movie S1. To be more quantitative, we define both stiffness and toughness recovery degree by calculating the ratios of Young’s modulus (representing a stiffness recovery) and dissipated energy loss (representing a toughness recovery) at different loading cycles to those values at the first one, respectively. It can be seen in Figure 4d that after 5 cycles, A1D2G1 hydrogel can recover its toughness by 97.5% and stiffness by 97.4%. Even after 20 cycles, toughness/stiffness recovery degrees can still maintain at 68.5%/90.5%, which demonstrate that A1D2G1 hydrogels possess good recovery and fatigue resistance properties.

Figure 5. (a) Images of compressed A1D2G1 hydrogels (up images) and A1D2G0 hydrogels (down images). (b) Compression curves of A1D2G1 at different compression strain. (c) 30 subsequent compressive loading-unloading cycles of A1D2G1 with 40 s recovery between each cycle. (d) Dissipated energy and recovery degree of A1D2G1 after 30 compression-relaxation cycles.

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The fast recovery and fatigue resistance properties are also reflected by compressive loading-unloading cycles. As shown in Figure 5a, A1D2G1 hydrogels can recover their original shape by manual compress-release cycle in several seconds. However, A1D2G0 can not complete recovery. Different strains were applied to A1D2G1 and it can completely recover its original state at large strain of 80% (Figure 5b). The recovery of the A1D2G1 hydrogel at strain of 80% were further rigorously tested by applying continuous compression-relaxation cycles to the same hydrogels for 30 cycles with 40 s waiting time between each cycle (Figure 5c). The cyclic compress-release process is shown in Movie S2. The stress–strain curves for all cycles are almost superimposable, indicating the plastic deformation is greatly removed. The relative dissipated energy and recovery degree in each cycle were summarized in Figure 5d. The amount of energy dissipated decreases and remains to be more than 66.7% while the recovery degree decreases slightly and remains to be more than 86% after 30 cycles. Such mechanical cycles do not cause any noticeable macroscopic changes to the hydrogel. These further confirm the reliable mechanical performance of the P(AM-co-DAC)/GO hydrogels in stressful working environments. Taken together, these results demonstrate that the simple one-pot polymerization could produce P(AM-co-DAC)/GO hydrogels with high stiffness, high tensile strength at large elongation and excellent fast recovery and fatigue resistance via facilely adjusting GO content and mass ratio of AM and DAC. Based on the reversible electrostatic interaction and hydrogen bonding, a remarkable self-healing ability of the hydrogels with the aid of water is achieved. As shown in Figure 6a, a pristine cylinder of A1D2G1 sample was cut into two halves and the cut surfaces were simply contacted each other without applying any pressure. After dropping a droplet of water on the rupture surface, the sample was allowed to stand for hours (Figure 6b). The healed sample can be stretched to a large strain by hand (Figure 6c). The cutting and healing process can be found in Movie S3. We also found that when two pieces of gels are put together without water assistance, they can adhere to each other at room temperature. However, the adhesion strength is weak and easily disrupts at the interface. The self-healing performances of the hydrogels with 18

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and without water assistance were shown in Figure S7. The fracture stress of A1D2G1 hydrogel in its original state is about 546.1 kPa. After self-healing with water assistance, A1D2G1 hydrogel reaches a stress of 503.4 kPa and the healing efficiency on the base of fracture stress is 92.3%. Without water assistance, the healed A1D2G1 hydrogel has a fracture stress of 248.9 kPa and the healing efficiency is about 45.6%. These results indicate that ionic bond and hydrogen bond can be reformed via water assistance.

Figure 6. Self-healing properties of hydrogels. A pristine cylinder of A1D2G1 sample was cut in half (a). The two halves were simply contacted and a drop of water was dropped on the cut surface (b). After standing for hours, the sample can be stretched to a large strain by hand (c). (d) Water fraction of A1D2G1 after dropping water with different waiting time. (e) Stress-strain curves of healed A1D2G1 after dropping water 19

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with different waiting time. (f) Stress-strain curves of A1D2Gz hydrogels before (solid lines) and after healing for 20 h at room temperature (dashed lines). (g) Healing efficiency of fracture stress of all composite hydrogels with different GO contents.

To explore the self-healing behavior systematically, first we studied the effect of water content and healing time of the hydrogels. After dropping water, the water content of the hydrogel increases from ~30 wt% to ~37 wt% due to swelling. At room temperature, the water in the hydrogel loses slowly with time. After 20h, the water content reaches to original state (about 29.8 wt%) (Figure 6d). The in-situ self-healing process of hydrogels at different waiting time was observed by optical microscopy (Figure S8). There is a broad crack at the beginning of contact. After addition of water, the surface of the hydrogel changes due to swelling. The addition of water promotes the movement of the polymer chains at the fracture surface. The dynamic ionic interactions and hydrogen bonding between (P(AM-co-DAC) and GO enable the polymer chains to migrate from one side to another side, leading to the healing of the hydrogels. With time the cracks at the cut surface become smaller and smaller. After 20h, it is almost no distinct fractures (Figure S8). We believe that not only the surface of the hydrogels is healed, but also the interior of the hydrogels complete the healing process. As shown in Figure 6e, the stress-strain curves of A1D2G1 hydrogels at different waiting time indicate that the hydrogels can recover their original state during 20h healing process. Next, the self-healing of all the hydrogels was conducted for 20 h. The addition of water will highly promote the healing of hydrogels, similar to the drying−reswelling process20. During the healing process, the preliminary contact of the freshly cut segments is critical for the diffusion of polymer chain across the interface. When a drop of water was dropped on the fracture interface, the hydrogels swelled at the interface and the diffusion of the polymer chains was promoted across the interface. The swelling process in the composite hydrogels has been suggested to reestablish the loose and weak contacts between polymer chains and oxygen groups on GO. The subsequent drying process drives the diffused polymer chains to closely 20

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interact with the oxygen group on the GO sheets and creates additional crosslinks in the final hydrogels, resulting in the reconstruction of a new, intact cross-linked structure between the two cut-off samples. The reestablished electrostatic interaction and hydrogen bonding should be responsible for the high mechanical strength of the hydrogels. Figure 6f reveals the stress-strain curves of original and healed A1D2Gz hydrogels. For A1D2G0, the fracture stress can be healed to 63.5% of original stress. After addition of 0.5 wt% GO, the fracture stress of A1D2G0.5 recovers to 92.8% of original value. This demonstrates that only relying hydrogen bonding of P(AM-co-DAC) polymer chains in the hydrogel to achieve self-healing is not enough, electrostatic interaction between GO and polymer chain after introduction of GO significantly improves the self-healing efficiency of hydrogel. When GO content reaches to 1 wt%, the self-healing efficiency of A1D2G1 reaches to 92.3% of fracture stress, 99.5% of the fracture strain and 93.6% of toughness (Figure S9). The tensile strength of healed A1D2G1 reaches to ~503.4 kPa at elongation of ~1592.6%, much higher than that of healed A1D2G0.5, further indicating the role of GO. However, as GO content further increases, the self-healing ability decreases (e.g. 61.7% and 47.8% of fracture stress for A1D2G1.5 and A1D2G2, respectively). This may be due to that the considerable interactions between GO and polymer chains impede the mobility of P(AM-co-DAC) chains to a greater extent, giving rise to a decline in flexibility of polymer chains39. Furthermore, the self-healing ability of hydrogels with different mass ratio of AM and DAC were also investigated (Figure S10, Table S1). As the content of DAC increases, the self-healing efficiency improves significantly. For example, when GO content is 1 wt%, the self-healing efficiency is 58.5%, 84% and 92.3% of fracture stress for A1D0.5G1, A1D1G1, and A1D2G1, respectively (Figure 6g). Higher DAC content leads higher self-healing efficiency due to more electrostatic interaction between DAC and GO. Our design can improve the mechanical strength without expense of self-healing ability by rationally tuning the composition of polymer hydrogels and GO content. We have compared our self-healed hydrogels with 21

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previously published data, our hydrogels display highest self-healing efficiency without sacrificing their mechanical properties (Table S2).

Figure 7. Photograph of an original dried A1D2G1 sample (a). Water was sprayed on the dried A1D2G1 (b). After standing for 40 min, the material can be manually stretched (c, d) (See Movie S4). (e) Stress-strain curves of original sample, recovered sample after absorbing water and self-healed sample by cutting the recovered sample.

More importantly, large amounts of water in hydrogels cause difficulties in transportation of hydrogels. Our hydrogels can be completely dried facilitating transportation and the mechanical properties can be recovered to their original values by simply spraying water on the dried samples. As shown in Figure 7a, A1D2G1 hydrogel was completely dried at 40 oC, thus it became very hard, without the original tensile, compressive and self-healing properties. After spraying water on the surface of the dried hydrogels, the sample recovers its original state with high stretchability after 40 min (Figure 7b-d). The recovery process of the dried A1D2G1 hydrogels was shown in Movie S4. The recovered sample displays similar fracture strain and fracture stress to that of the original one (Figure 7e). In addition, after cutting and healing of the recovered sample, the hydrogel still maintains a high self-healing efficiency of ~93% of fracture stress of original sample (Figure 7e).

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In summary, we have demonstrated that the simple one-pot polymerization could produce P(AM-co-DAC)/GO hydrogels with high stiffness (Young’s modulus: ~1.1 MPa), high tensile strength (2.1 MPa) at high elongation of 800-1700% and high fatigue resistance, as well as excellent self-healing efficiency (>92% of tensile strength, >99% of tensile strain and >93% of toughness). The mechanical properties of the hydrogels can be tailored by adjusting GO contents and mass ratio of AM and DAC. The reversible ionic bands and hydrogen bonds are responsible for the energy dissipation and network reformation. The combination of relatively high mechanical properties and high self-healing abilities, along with an easy process of synthesis, make these materials ideal candidates for a broad of applications.

Supporting Information Figure S1-S10, Table S1, S2 and movie S1-S4. This material is available free of charge via the Internet at http://pubs.acs.org/.

Conflict of interest The authors declare no conflict of interest

Acknowledgements We acknowledge the support by Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province, the National Natural Science Foundation of China (51702178) and the Natural Science Foundation of Shandong Province (ZR2017MB009).

Reference

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Graphene Oxide Nanocomposite Hydrogels with Excellent Self-Healing Performance.

Designing hydrogels with high mechanical properties without sacrificing their self-healing efficiencies remains great challenges. We have fabricated c...
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