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An Electrically and Mechanically Autonomic Self-healing Hybrid Hydrogel with Tough and Thermoplastic Properties Xiaoyan He,* Caiyun Zhang, Meng Wang, Yunlei Zhang, Liqin Liu, and Wu Yang* Key Lab of Bioelectrochemistry and Environmental Analysis of Gansu, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China S Supporting Information *

ABSTRACT: Conductive hydrogels are a class of composite materials that usually comprise hydrated polymers and conductive materials. Practical application requires the conductive hydrogels to have various properties such as high conductivity, toughness, self-healing, facile processing ability, and so on. Although challenging to have all the above-mentioned properties, a composite material composed of polymer hydrogel with embedded Au nanoparticles (i.e., P(NaSS)/P(VBIm-Cl)/ PVA@Au) was found to show the comprehensive properties above in this paper. For example, P(NaSS)/P(VBIm-Cl)/PVA@ Au exhibits mechanical and electrical self-healing properties at ambient conditions. In addition, P(NaSS)/P(VBIm-Cl)/PVA@ Au is tough and thermoplastic, potentially making it useful for a variety of applications. KEYWORDS: conductive hydrogel, self-healing, toughness, thermoplastic, dual network

1. INTRODUCTION Conductive polymer hydrogel is an important functional material due to its flexibility similar to polymer materials and high conductivity like metal materials. More importantly, the composition of the hydrogel implies high water content,1 swelling properties,2,3 biocompatibility4,5 in vitro and vivo, and the better property of diffusing small molecules when compared to other conductive materials. It has been widely used in neural prostheses,6 implantable biosensors,7,8 bioelectrodes,9,10 and electro-stimulated drug release devices11,12 et al. However, most hydrogels are mechanically weak, which largely restrict their application. The destruction of the conductive composite material during using process will result in performance failure. Therefore, a conductive hydrogel with self-healing and good toughness will help to address the above problems and the preparation of safer materials with longer cycle life. The commonly used approaches to preparing self-healable conductive hydrogels are based on using either supramolecular interactions, such as hydrogen-bonding,13−15 electrostatic interaction,16,17 host−guest recognition,18−20 metal−ligand coordination,21−24 hydrophobic association,25,26 and π−π stacking,27,28 or dynamic covalent bonds29−31 to construct the required network structure. However, repairing process requires the input of a stimulus or energy, such as light,32,33 heat,34,35 or pH change,36,37 which may limit their potential application, especially in the biomedical field. Furthermore, the potential as structural biomaterials or any application as load-bearing materials requires the conductive hydrogels to have a combination of © 2017 American Chemical Society

properties including thermoplastic, high toughness and selfhealing in addition to conductivity. Nokyoung Park et al.38 demonstrated a conductively thermoplastic hydrogel, which could be utilized to fabricate bendable, stretchable, and patternable electrodes directly on human skin, but the selfhealing procedure was completed under heating. Guihua Yu et al.39 developed a hybrid gel based on the self-assembled supramolecular gel and nanostructured polypyrrole. This hydrogel appeals mechanical and electrical self-healing property without any external stimuli, whereas the mechanical property was relatively weak and the molding was not investigated. Ali Reza Karimi et al.40 prepared a conductive gel with self-healing performance, while the time of self-healing was associated with the content of a conductive material. Thirty minutes are needed to complete the self-healing process when the conductivity of the gel reached a desirable value (2.94 × 10−1s.m−1). David ́ Diaz ́ et al.24 also achieved a supramolecular metallogel Diaz with self-healing, conductive, and moldable characteristics. Specifically, it could impart self-healing properties to other gel. However, the self-healing period is long and most work has been done on restoring only mechanical properties. Although some attempts have been made to this material which was able to restore conductivity, the performance was restored together, the repeated and rapid restoring without any stimulus still remains a challenge. Received: January 9, 2017 Accepted: March 9, 2017 Published: March 9, 2017 11134

DOI: 10.1021/acsami.7b00358 ACS Appl. Mater. Interfaces 2017, 9, 11134−11143

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ACS Applied Materials & Interfaces

Figure 1. Preparation procedure of the hydrogel of P(NaSS)/P(VBIm-Cl)/PVA@Au. temperature to 800 °C at the heating rate of 10 °C.min−1 under N2 atmosphere. Before the test, the sample was dried at 50 °C for 48h to obtain the dried lump. Tensile strength measurements were performed using an electrical universal material testing machine (EZ-Test, SHIMADZU) and the speed of test was 5 mm.min−1. The conductivity of the sample was measured by digital four point probe tester (SX1934) at room temperature. X-ray photoelectron spectroscopy (XPS) analysis was carried out on PHI-5702 multifunctional XPS. The content of Au was tested by Atomic Absorption Spectrophotometer (AAS) (AA240,Vairan).

Herein, we report a facile way of fabricating tough, selfhealable conductive hydrogel by polyelectrolyte gel compound with poly(vinyl alcohol) (PVA) and in situ reduced Au nanoparticles (NPS). This poly(ionic liquids)/PVA/Au NPS hybrid hydrogel combines the decent conductivity and is able to self-heal under ambient conditions without any external stimulus. We proved the flexible chains of polyelectrolyte are the key factors to form the weak and reversible metal−ligand coordination. In addition, the hydrogel is thermoplastic and tough. Mechanical properties and conductivity were repaired simultaneously, repeatedly and rapidly.

3. RESULTS AND DISCUSSION 3.1. The Preparation Procedure of the Hydrogel of P(NaSS)/P(VBIm-Cl)/PVA@Au. Figure 1 is the preparation procedure of the composite hydrogel. First, the semiinterconnected hydrogel was prepared by conventional radical polymerization. Polyionic liquids (P(VBIm-Cl)) and poly sodium p-styrenesulfonate hydrate(P(NaSS))were selected as the main chains to form the gel. BIS and KPS were used as cross-linking agent and initiator respectively in this chemical cross-linking process, in which the chain of P(VBIm-Cl) (Mn = 2.83 × 104 measured by GPC) were semi-interconnected in the net of the P(NaSS). This is very important for the self-healing ability without any stimulus, because more flexible chains of P(VBIm-Cl) are the key factors to form the weak and reversible metal−ligand coordination. Then the above-obtained matrix was injected into poly(vinyl alcohol) (PVA) solution, in which the physical cross-linking between PVA chains were formed after the freezing. As a consequence, the synthesis of the final hydrogel, which possess double-network as a result of physically cross-linked PVA and chemically cross-linked P(NaSS), was accomplished. It is worth noting that there are weak ionic bonds between P(NaSS) chain and P(VBIm-Cl) chain in the hydrogel. To increase the conductivity of the composite, Au NPS were selected as conductive filler. In detail, the prepared hydrogel was placed into HAuCl4 solution, the color of hydrogel changed from colorless and transparent to purple color gradually without any reductant. This showed the occurrence of the reduction reaction from AuIII to Au0 and this reduction procedure was also proven by XPS spectra (Figure 2). The reason for the reduction from AuIII to Au0 is due to a great deal of hydroxyl provided by PVA.41 The formation of double networked P(NaSS)/P(VBIm-Cl)/ PVA hydrogel and the interaction between PVA and the Au NPS in the composite hydrogel were investigated by FTIR

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium p-styrenesulfonate hydrate(NaSS) (Aladdin, 90%), methylimidazole (Aldrich, 99%), 4-vinylbenzyl chloride (Acros, 90%), poly(vinyl-alcohol) (PVA)(1799, Aladdin), potassium persulfate (KPS), N,N′-methylenebis(acrylamide)(BIS) (Aldrich, 98.0%), HAuCl4 (Alfa Aesar, 98%), and all other chemicals were analytic grade and used as received. 2.2. Hydrogel Preparation. The polymer of poly(vinyl-alcohol) (PVA)(0.3092g) was dissolved in 1 mL deionized water, sealed and stirred in oil bath at 95 °C to completely melt. The mixture of 0.1546g (i.e., 0.74 mmol) the monomer of sodium p-styrenesulfonate hydrate (NaSS) and 0.1759g the polymer of 1-methyl-3-(4-vinylbenzyl) imidazolium chloride(P(VBIm-Cl)) (synthetic steps was recorded in Supporting Information (SI)) were dissolved in 1 mL deionized water. The cross-linking agent N,N′-methylenebis(acrylamide)(BIS)(0.0093g,0.06 mmol) was added during the stirring to the mixture of NaSS and P(VBIm-Cl), and the obtained mixture was injected into the PVA solution. Twenty minutes later, the 0.2 mL solution of K2S2O8 with the concentration of 0.3 mol·L−1 was injected into the mixture quickly and the reaction proceeded in oil path at 60 °C for 2 h. After the polymerization, the result mixture was transferred to a freezer for 6 h and the P(NaSS)/P(VBIm-Cl)/PVA hydrogel was obtained. The prepared P(NaSS)/P(VBIm-Cl)/PVA hydrogel was placed into a flask containing HAuCl4 solution(50 mL 0.48 mmol L−1). After 36 h, the color of the gel changed from colorless to purple and the synthesis of the hybrid hydrogel of P(NaSS)/P(VBIm-Cl)/PVA@Au was accomplished. 2.3. Physical and Chemical Characterization. The morphology of the samples was observed by scanning electron microscope(SEM) (Quanta 200, Philips-FEI). Before the test, the sample was freeze-dried by vacuum freeze-dryer (FD-1−50) and sprayed with gold for 20 min. The X-ray Diffractometer (D/max-2400,Rigaku) was used to analyze the different crystal distribution strength of the sample and the sample was operated at 40 kV and 150 mA. Scans were made from 2 to 80°(2θ) at the step of 0.02. Thermogravimetric analysis was performed using simultaneous thermal analyzer (STA449C, Netzsch) from room 11135


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slightly shifted to 3424.87 cm−1. Because the binding of Au nanoparticles by PVA is likely achieved through Au−O bond, this shift is an evidence for that the coordination of metallic nanoparticles with PVA.42 Besides FTIR characterization, XPS spectrum results were measured to examine the different valence state of gold element in the hydrogel of P(NaSS)/P(VBIm-Cl)/PVA@Au and provided a further evidence to prove the coordination behavior between gold nanoparticles and the OH group of the PVA chains by monitoring the energy shift of the core electrons during the formation of the interface.43 C, N, O, and Au peaks were presented in XPS spectrum clearly (Figure 2). As shown in the graph, the peak of Au 4f could be split into two peaks located at the BEs of about 84.2 and 88.0 eV, which is attributed to the presence of Au0. This result clearly showed the reduction of AuIII by PVA. The broad peak of O 1s located at 532.0 eV was contributed to the OH groups of PVA. Generally, according to the standard spectrum gallery of XPS, the binding energy of Au0 was located at 84.0ev and 87.7ev and the binding energy of OH group of PVA was located at 532.4ev. The shift of binding energy together supported the formation of the coordination between Au nanoparticles and OH groups originating from the PVA. The hydrogels of P(NaSS)/P(VBIm-Cl)/PVA and P(NaSS)/P(VBIm-Cl)/PVA@Au were further characterized by X-ray Diffractometer(XRD) in Figure 3b. From the XRD patterns, a shape diffraction peak at d = 4.68 Å (2θ = 19.4°) was contributed to the typical crystalline PVA hydrogel. After loading of Au NPS, there were other diffraction peaks corresponding to the (111), (200), (220), (311) which assigned to Au NPS, these results demonstrate that the Au NPS successfully immobilized in the hydrogel. The morphology and microstructure of hybrid hydrogels of P(NaSS)/P(VBIm-Cl)/PVA and P(NaSS)/P(VBIm-Cl)/ PVA@Au were investigated by scanning electron microscope (SEM). Figure 4(a−c) shows the SEM images of freeze-dried P(NaSS)/P(VBIm-Cl)/PVA hydrogel at different magnification. It can be seen that the hydrogel show well-defined interconnected 3D porous networks. The hybrid hydrogel of P(NaSS)/P(VBIm-Cl)/PVA@Au also shows the porous structure and interleaved 3D networks clearly in Figure 4(d− f). The size of pores in the hydrogel ranges from several

Figure 2. XPS survey spectra of the P(NaSS)/P(VBIm-Cl)/PVA@Au hydrogel.

spectroscopy. Compared with the related monomers, the absorption of P(NaSS)/P(VBIm-Cl)/PVA(Figure3a(3)) is different. The peak at 1095.84 cm−1 was ascribed to the C− C−C stretching vibration responsible for the degree of crystallinity of PVA has strengthened obviously as a result of freezing. The presence of absorption band at 1046.27 cm−1 stems from the sulfonic group of NaSS and the peak at 1630.02 cm−1 has been clearly broaden because of the superposition of stretching vibration of CN in VBIm-Cl and deformation vibration of hydroxyl in PVA. The peaks at 989.26 and 909.62 cm−1 which responsible for the CC in NaSS were disappeared as a result of polymerization. In general, all of the above-mentioned IR bands demonstrate the success of double-network hydrogel synthesis. The characteristic absorption bands of the Au entrapped hydrogel(Figure 3a(4)) were quite similar to those of P(NaSS)/P(VBIm-Cl)/PVA. However, the O−H stretching band located at 3423.82 cm−1 was

Figure 3. (a) FTIR spectra of NaSS(1), PVA(2), P(NaSS)/P(VBIm-Cl)/PVA(3), and P(NaSS)/P(VBIm-Cl)/PVA@Au(4); (b) XRD patterns of the hydrogels of P(NaSS)/P(VBIm-Cl)/PVA (black line) and P(NaSS)/P(VBIm-Cl)/PVA@Au (red line). 11136


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Figure 4. SEM images of the hydrogels of P(NaSS)/P(VBIm-Cl)/PVA (a,b,c) and P(NaSS)/P(VBIm-Cl)/PVA@Au (d,e,f) at different magnifications.

Figure 5. Mechanical tests: (a) Stress−strain curves of the hydrogels of P(NaSS)/P(VBIm-Cl)/PVA (1) and P(NaSS)/P(VBIm-Cl)/PVA@Au(2); (b) recyclability of self-healing properties of P(NaSS)/P(VBIm-Cl)/PVA@Au hydrogel, Original hydrogel (0), 1 cycle time(1), 2 cycle times(2), 3 cycle times(3).

micrometers to larger than 10 μm as a result of doping Au NPS. The hierarchical porous structures provide both large open channels between the branches and nanoscale porosities within the structures, which facilitate the transportation of water molecules. 3.2. Mechanical Properties of the Hydrogels. Due to its intrinsic structural inhomogeneity or lack of effective energy dissipation mechanism, hydrogels usually do not show enough mechanical strength and this problem largely limits their applications in many fields. Just as we designed, the hydrogels with the half interpenetrating network structure were prepared in the first step formed by covalent cross-linking and the ionic interaction. The second precursor of PVA was doped and the physical network was completed after freezing, which formed the double-network hydrogel of P(NaSS)/P(VBIm-Cl)/PVA with covalent cross-linking and physical cross-linking. This unique structure dissipates the energy effectively and results in

that the obtained hydrogel has a good mechanical property with the maximum tensile stress of 6.14 MPa in Figure 5a(1). With the addition of gold nanoparticles, the coordination effect between gold nanoparticles and OH group in PVA gradually formed. As shown in Figure 5a(2), this coordination and the addition of the inorganic nanoparticles further improved the mechanical properties of the hydrogel with the maximum tensile stress to 7.26 MPa. The results above demonstrated that our hydrogels were mechanical tough. The ratio between the chemical cross-linking contributed to P(NaSS) and physical cross-linking assigned to PVA varied with the concentration of PVA had a great influence on the mechanical properties of the hydrogel as the result shown in Figure 6. The mechanical property of the hydrogel presents a gradual weakening trend with the decreasing of the content of PVA, because the stress of the rupture decreased from 7.74 to 4.14 MPa as the concentration of PVA decreased from 80% to 11137


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ability of the hydrogel of P(NaSS)/P(VBIm-Cl)/PVA was evaluated by cutting a test sample into two pieces using a sharp knife as shown in Figure 7(b), followed by rapidly contacting the freshly created two fracture surfaces in refrigerator for freezing. We thawed the sample at room temperature after 4 h and found that the cut mark on the surface was still visible, while the interface in the sample disappeared almost completely. The self-healed hydrogel could withstand certain mechanical forces such as pulling to a large extension without failure at the interface, as shown in Figure 7(c). The result may stem from the physical network of PVA that drew support from the extensive hydrogen bonding between the hydroxyl side groups. The self-healing property of the above hydrogel was achieved by freezing, which limited the widespread use of the material. The shortcoming was overcome by the adding Au NPS in this work, while the self-healing performance of the hydrogel of P(NaSS)/P(VBIm-Cl)/PVA@Au was also evaluated by the same way. There were some differences when brought freshly created fracture surfaces of the hydrogel into contact rapidly. The interface in the sample was disappeared completely without any stimulation and only 10 seconds was needed (Figure 7(f)), especially when it was processed under room temperature(SI Movie 1). This fascinating self-healing efficiency was attributed to the reversible metal−ligand coordination between Au nanoparticles and OH group. The special structure of the hydrogel also contributes to the room temperature self-healing ability, and the flexible chains of P(VBIm-Cl) were chosen to prepare the composite hydrogel. It is well-known that the good polymer chain mobility favors chain diffusion across an interface of cut or fractured surfaces and predominantly influences the efficiency of self-healing, which is proved by the comparative test. During the process, VBIm-Cl and NaSS were selected as the monomer, BIS and KPS were used as cross-linking agent and initiator respectively, P(VBIm-Cl) and P(NaSS) were completely cross-linked to form the hydrogel of P((VBIm-Cl)-(NaSS))/PVA@Au. The hydrogels prepared by this method did not show the self-

Figure 6. Effect of the content of PVA on the mechanical property of the hydrogel measured from the mechanical test at room temperature.

40%. This phenomenon can be reasonably explained due to the physical cross-linking density was weakened and the chemical was enhanced with the decreasing of PVA concentration. Consequently, physical cross-linking played a leading part in mechanical property of the hydrogel. It is noticeable that the hydrogel containing 50% PVA was better than the hydrogel of 60% in mechanical property. When the content of PVA reached 50%, the chemical and physical cross-linking density achieved a state of equilibrium. Meanwhile, two kinds of weak forces contributed to the electrostatic interaction between polyelectrolyte chains and the coordination between hydroxyl and Au NPS in hydrogel were enhanced gradually. This synergistic effect from the two interaction in mechanical property were maximized at this time. However, when the chemical crosslinking become big enough for the mechanical property of the gel, the mechanical property of the hydrogel decreased rapidly until not being formed as shown in the inset of Figure 6 (the concentration of PVA was 20%). 3.3. Mechanical and Conductive Properties during the Novel Self-Healing of the Hydrogels. Self-healing

Figure 7. Self-healing behavior: P(NaSS)/P(VBIm-Cl)/PVA hydrogel in freezing condition (a,b,c); P(NaSS)/P(VBIm-Cl)/PVA@Au hydrogel at room temperature(d,e,f). 11138


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Figure 8. (a) Stress−strain curves of the self-healed hydrogel at different concentrations of P(VIBm-Cl). (b) Effect of the content of P(VIBm-Cl) on the self-healing efficiency of the hydrogel measured from the mechanical test at room temperature.

Figure 9. Self-healing process of the hydrogel of P(NaSS)/P(VBIm-Cl)/PVA@Au.

property. As shown in Figure 5b, the original hydrogel possesses an excellent mechanical properties and its stress and strain at rupture reached 7.26 MPa and 99.64%, respectively(Figure 5b(0)). The value of strain also perfectly explained why our hydrogel can be drawn into a thin line(SI Figure S4). The interval of self-healing was set as 20 min after cutting the test samples of hydrogel into two pieces and bringing the two fracture surfaces into contact rapidly. After 1 cycle time the fracture stress reached 6.42 MPa, which was about 88% of the original uncut hydrogel as shown in Figure 5b(1). On the same scar, the fracture stress also reached 5.72 MPa and about 78% of the original gel after two-times selfhealing(Figure 5b(2)). After three cycle times the fracture stress even reached 4.62 MPa and about 63% of original hydrogel(Figure 5b(3)). The results above further illustrated the fast self-healing ability and recycled self-healing properties of the gel. Figure 9 was the illustration of the self-healing process of our hydrogel of P(NaSS)/P(VBIm-Cl)/PVA@Au. The conductivity of the hydrogel of P(NaSS)/P(VBIm-Cl)/ PVA@Au was measured by a four-point probe technique and the electrical conductivity reached at ≈2s·m−1, in which the content of Au was only 0.97%(measured by AAS). The value

healing ability under room temperature, because the restricted chains could not form the weak and reversible metal−ligand coordination. (see SI Movie 2) To further analyze the self-healing effect varied with the reaction component, stress−strain test of the self-healed hydrogels at different concentration of P(VIBm-Cl) were investigated. As shown in Figure 8(a) with the self-healing time was set as 20 min, increasing the amount of P(VIBm-Cl) results in an increase of the healing efficiency. As we know, the selfhealing property of the hydrogel was contributed to the weak and reversible binding. When the concentration of P(VIBm-Cl) increase, the random chains of the polymer and reversible binding of Au−OH increased together. When it reached 25% (the content of PVA was 50%), the malleability of the hydrogel was enhanced to a maximum value and consequently results in the wonderful self-healing property of the hydrogel with 25% P(VIBm-Cl). In recent years, a number of self-healable materials have been prepared, whereas the recyclability of self-healing properties of the obtained hydrogel was rarely considered. In this work, the recycled self-healing of obtained hydrogel of P(NaSS)/ P(VBIm-Cl)/PVA@Au was characterized by the mechanical 11139


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the hydrogel were reformed by mold-dialysis and injecting, respectively. Figure 12(a) is the original shape of the hydrogel. To start the process, melted P(NaSS)/P(VBIm-Cl)/PVA@Au hydrogel (110 °C) was placed into a heart-shaped mold and cooled to room temperature, then the heart-shaped hydrogel was obtained. Transformation of the heart-shaped hydrogel into a strip-shaped was carried out by melting the hydrogel to sol, injecting and cooled at room temperature. A series of melting-gelation-mold transfer steps can be carried out so that the same hydrogel can be reused and reshaped multiple times by using appropriate templates. This characteristic improves the potential of wide utilization of the P(NaSS)/P(VBIm-Cl)/ PVA@Au hydrogel. All of this flexibility in processing can be explained by the characteristic of reversible sol−gel transition shows in Figure 12(d,e). Coordination behavior between Au and OH group explained why the obtained hydrogel can undergo gel-to-sol transition as following. When the temperature elevated at the critical solution temperature of 110 °C, the coordination bond between Au and OH group was destroyed completely and the hydrogel transformed to sol, and then to gel when cooled at room temperature. What is more important is that after hot working, the characteristics of our hydrogel (such as electrical conductivity, and self-healing properties) were still existing the same as original hydrogel of P(NaSS)/P(VBIm-Cl)/PVA@Au, which were proved by the experiment. From the Figure 12(f−i), it showed that the LED was lighten on immediately when the strip-shaped hydrogel made by injecting was linked into circuit and was lighten off when the circuit was disconnected (Figure 12(f,g)). The result provided a powerful evidence proving that the P(NaSS)/P(VBIm-Cl)/PVA@Au hydrogel still has a very good electrical conductivity after hot working. The mechanical self-healing and self-healable conductivity of the hydrogel were also measured. As shown in figure, thus the LED was lighten off when strip-shaped hydrogel was cut into two fractures (Figure 12(h)) and light on immediately when two fractures were brought into connecting (Figure 12(i)). We have made complete comparison of previous works, by comparing with the other conductive hydrogels with selfhealing property, and emphasized some advantages in Table 1.

displayed that we can expect our composite hydrogel to be applied to electrodes in electronic devices. The electrical conductivity of the hydrogel was found to be highly dependent on the P(VIBm-Cl) concentration. As can be seen in Figure 10,

Figure 10. Effect of the content of P(VIBm-Cl) on the conductivity of the hydrogel measured at room temperature.

the conductivity of the hydrogel increased from 0.45s·m−1 to 2.92s·m−1 with concentration of P(VIBm-Cl) increasing from 10% to 30%, which contributed to the increasing loading ratio of the Au NPS increased with the P(VIBm-Cl) concentration. To investigate the electrical self-healing ability, the composite hydrogel was used in an electronic circuit of LED with conducting wires. The P(NaSS)/P(VBIm-Cl)/PVA@Au hydrogel was cut in half by a sharp knife (seeing in Figure 11(b)). The light-emitting diodes was light on again instantly and the current of the circuit was increased synchronously (Figure 11 (c) and SI movie 3) immediately after the two fractured surfaces was brought into connecting. This result demonstrates the restoration of electrical performance of the P(NaSS)/ P(VBIm-Cl)/PVA@Au hydrogel. 3.4. Thermoplasticity of the Hydrogel. As discussed in the Introduction, practical application requires the hydrogel to combine various properties including toughness, stiffness, and conductivity. In addition, a facile processing ability is also very important for industrial manufacturing. Due to thermoplastic property, the obtained hydrogel can reform easily into different shapes. Using the simple methods as shown in Figure 12(a−c),

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CONCLUSIONS In summary, we have presented a relatively simple method with chemical cross-linking and physical cross-linking to prepare a kind of unique dual network structure hydrogel, which shows

Figure 11. Conductive behavior and conductive function self-healing behavior of the P(NaSS)/P(VBIm-Cl)/PVA@Au hydrogel at room temperature. Original hydrogel (a), Broken hydrogel (b), Self-healed hydrogel (c). 11140


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Figure 12. Thermoplasticity of the P(NaSS)/P(VBIm-Cl)/PVA@Au hydrogel. Original hydrogel (a), heart-shaped hydrogel (b), strip-shaped hydrogel(c); reversible sol−gel transition of the hydrogel of P(NaSS)/P(VBIm-Cl)/PVA@Au(d,e); Conductive and self-healing properties of P(NaSS)/P(VBIm-Cl)/PVA@Au hydrogel after hot working(f−i).

Table 1. Literature Survey of the Other Conductive Hydrogels with Self-Healing Property gel

self-healing

1 2 3 4 5 6 7 8 9 10

condition sonication automatically automatically dropped of water with pressure heating 120 °C automatically automatically freezing automatically

conductivity s·m−1 time 10 min immediately 24 h 5 min 12 h 30 min 1 min 30 min 4h immediately

mechanical stress

1.4 × 10−2 10−1 9 15 0.67 × 10−2 35 12 2.94 × 10−1 0.2 2

various properties (such as electrical conductivity, good mechanical property, and self-healing function) at the same time. The self-healing ability was originated from hydrogen bonds between hydroxyl side groups and coordination effect between Au NPS and OH group. The self-healing and conductivity properties of our hydrogel can realize repeatedly restoring and the conductivity was almost not losing during the restoring process. Moreover, the conductive hydrogel prepared by us is mechanically tough with the maximum 7.26 MPa compressive strength. This paper shows that the hydrogel of P(NaSS)/P(VBIm-Cl)/PVA@Au can undergo gel-to-sol transition, which is convenient to process original gel into different shapes, as a result of reversibility of coordination behavior between Au and OH group.

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thermoplasticity moldable moldable moldable

>400 kPa ≈4.2 MPa 2.62 MPa 15 000 Pa

>700% 0.073% 90% 25% 50%

6.14 MPa 7.28 MPa

88% 99%

moldable

moldable

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reference strain 24 44 45 46 47 38 39 40 our work our work

The comparison of self-healing behavior of the hydrogels (movie 1.avi) The importance of special structure for self-healing behavior of the hydrogel (movie 2.avi) The self-healing of conductive function of the hydrogel (movie 3.avi) The effect of deformation on the conductive performance (movie 4.avi) (ZIP)

AUTHOR INFORMATION

Corresponding Authors

*(X.H.) [email protected]. *(W.Y.) [email protected]. ORCID

Xiaoyan He: 0000-0002-4396-7903 Author Contributions

ASSOCIATED CONTENT

All authors have given approval to the final version of the manuscript.

S Supporting Information *

Notes

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00358. Synthetic procedure and 1H NMR spectrum of the cationic liquid monomer, synthesis of poly(ionic liquids), thermal gravity analysis of the hydrogels, the TEM Analysis of the hydrogel, the superior performance of the hydrogel in strain, healing efficiency of the hydrogel of P(VIBm-Cl)-25% at various healing times (PDF)

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (NSFC) (No. 21164010), the Foundation for Distinguished Young Scholars of Gansu Province (No. 145RJDA326). 11141


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