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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 8245−8257

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Robust and Mechanically and Electrically Self-Healing Hydrogel for Efficient Electromagnetic Interference Shielding Weixing Yang, Bowen Shao, Tianyu Liu, Yiyin Zhang, Rui Huang, Feng Chen,* and Qiang Fu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China S Supporting Information *

ABSTRACT: Autonomously self-healing hydrogels have received considerable attentions due to their capacity for repairing themselves spontaneously after suffering damage, which can provide a better stability and a longer life span. In this work, a robust and mechanically and electrically self-healing hydrogel with an efficient electromagnetic interference (EMI) shielding performance was successfully fabricated via the incorporation of multiwalled carbon nanotubes (MWCNTs) into the hydrophobically associated polyacrylamide (PAM) hydrogels by using cellulose nanofiber (CNF) as the dispersant. It was been found that CNF could not only assist the homogeneous dispersion of MWCNTs but also effectively enhance the mechanical property of the resultant hydrogels. As a result, the optimal tensile strength (≈0.24 MPa), electrical conductivity (≈0.85 S m−1), and EMI shielding effectiveness (≈28.5 dB) were achieved for the PAM/CNF/MWCNT composite hydrogels with 1 wt % MWCNTs and 0.3 wt % CNF, which showed 458, 844, and 90% increase over (≈0.043 MPa, ≈0.09 S m−1, and ≈15 dB, respectively) the PAM hydrogel. More encouragingly, these composite hydrogels could rapidly restore their electrical conductivity and EMI shielding effectiveness after mechanical damage at room temperature without any external stimulus. With outstanding mechanical and selfhealing properties, the prepared composite hydrogels were similar to human skin, but beyond human skin owing to their additional satisfactory electrical and EMI shielding performances. They may offer promising and broad prospects in the field of simulate skin and protection of precision electronics. KEYWORDS: self-healing hydrogel, multiwalled carbon nanotubes, cellulose nanofiber, EMI shielding

1. INTRODUCTION The development of polymeric hydrogels with new functionalities is increasingly becoming aspirations in the view of their promising broad applications in various fields such as wastewater treatment, sensors, supercapacitors, soft robotics, and artificial skins.1−5 Particularly, self-healing hydrogels are receiving considerable attention because their self-healing capacities are essential to enhance the structural safety and lifetime of the materials.6−8 As for the preparation of selfhealing hydrogels, the dominating strategies are built on the usage of either dynamic covalent bonds9 or supramolecular interactions, such as hydrogen bonding, host−guest recognition, electrostatic interaction, π−π stacking, metal−ligand coordination, and hydrophobic association.10−19 Attractively, hydrophobically associated (HA) hydrogels, based on the hydrophobic association, show prominent superiorities owing to their autonomously self-healing capability without imputing any stimulus or energy, as well as simple preparation method in which complicated structural or molecular design could be well avoided.20 Generally, the HA hydrogels are successfully constructed via the formation of HA domains and micellar copolymerization with the existence of surfactants and © 2018 American Chemical Society

hydrophobic and hydrophilic monomers. The HA domains could not only act as physical cross-linkers but also promote the hydrophilic polymer chain cross-linking in the HA hydrogels.21−23 Additionally, the excellent and autonomous selfhealing capability of the HA hydrogels at room temperature is achieved by the hydrophobic association, which is attributed to the formation of these above-mentioned HA domains. These self-healing HA hydrogels show a great potential for practical applications in which long-term reliability and safety of materials are of great importance. However, expanding the practical applications of these self-healing HA hydrogels is enormously hindered by their poor mechanical properties and lack of functionality. Endowing HA hydrogels with electrical conductivity is a promising way for expanding their practical applications. The conductive self-healing HA hydrogels would be potentially applied in many advanced electronics such as electronic skin, electrical conductors, and batteries to improve the reliability of Received: December 8, 2017 Accepted: January 30, 2018 Published: January 30, 2018 8245

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ACS Applied Materials & Interfaces these devices greatly.24 Particularly, the conductive self-healing HA hydrogels could be used as electromagnetic interference (EMI) shielding materials to effectively suppress EMI pollution, which has emerged as one of the serious concerns for modern society in tandem with extensive usage of modern electrical devices.25 Although various high-performance EMI shielding materials have been developed to address the EMI problems, the development of EMI shielding materials that can restore the initial mechanical, electrical, and EMI shielding properties after mechanical damage still remains an unexplored strategy.26−29 The reliability and lifetime of EMI shielding materials would be enormously hindered by the structure or mechanical damage due to frequent deformation in the practical applications. Therefore, combined good EMI shielding property with excellent self-healing property, the conductive HA hydrogels would be indeed novel promising candidates for self-healing EMI shielding materials. However, merely making the HA hydrogels conductive is not enough for the preparation of high-performance self-healing EMI shielding materials. The inferior mechanical property that originated from the single cross-linked network seriously impeded their practical application. For example, the tensile strengths of the prepared HA polyacrylamide and HA poly(acrylic acid) were only 12 and 47 kPa, respectively.20,30 Thus, a novel strategy is highly desirable to significantly ameliorate both mechanical strength and electrical conductivity of the HA hydrogels for fabricating autonomously self-healing EMI shielding materials. It has been reported that the HA composite hydrogels incorporated with functional nanofillers could well inherit the excellent self-healing capability, and encouragingly, achieve the favorable mechanical strength and new functionality.23,31 For instance, Ran et al. reported that the addition of graphene oxide into the HA hydrogels not only endowed the obtained HA composite hydrogels with excellent mechanical properties and dye adsorption capacities but also commendably inherited autonomously self-healing ability.23 Thus, it is reasonable to expect that introducing conductive nanofillers into HA hydrogels would be a facile and promising method to construct autonomously self-healing EMI shielding materials with desirable mechanical and self-healing properties for practical applications. Carbon nanotube (CNT), as an extremely strong and conductive nanofiller, is an ideal alternative for improving the mechanical property and electrical conductivity of the HA hydrogels, which has been broadly applied for the preparation of high-performance conductive materials.32−35 To enhance the mechanical and electrical properties of the HA hydrogel by taking advantage of CNT, the key is to achieve a high-quality dispersion of the CNT in the HA hydrogel. Recently, it has been reported that cellulose nanofiber (CNF) can be used as a green dispersant for CNT in an aqueous solution and completely preserve the electronic properties of CNT.36−38 Moreover, CNF could be served as a green nanofiller to effectively enhance the mechanical properties of composites.39 Herein, an autonomously self-healing EMI shielding material is successfully prepared by incorporating multiwalled carbon nanotubes (MWCNTs) into the hydrophobically associated polyacrylamide (PAM) hydrogel, in which the CNF used as a green and effective dispersant for MWCNTs. Through this simple, green, and effective approach, the mechanical and electrical properties of the HA PAM hydrogel have been greatly improved and its autonomously self-healing capacity has been well inherited. More importantly, the HA PAM hydrogel was

endowed with novel functions, including electrical and EMI shielding performance, thus fabricating a mechanically and electrically self-healing EMI shielding material successfully. The resultant composite hydrogels reveal a prominent and autonomic self-healing ability at ambient conditions, which could enable the restoration of mechanical, electrical, and EMI shielding properties after mechanical damage. Taking into account the unity of the excellent mechanical property, like human skin, and outstanding electrical and EMI shielding performances, together with the superior self-healing capacity, it was reasonable to expect that the great application prospect in the field of simulate skin of intelligent robots and protection of precision modern electronics could be exploited immensely for the obtained composite hydrogels.

2. EXPERIMENTAL SECTION 2.1. Materials. Multiwalled carbon nanotubes (MWCNTs) and microfibrillated cellulose (MFC) were provided by Nanocyl S.A., Belgium and Daicel Chemical Industries, Ltd., Japan, respectively. 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO, 98 wt %), acquired from Sigma-Aldrich, was used as received. Sodium hypochlorite (NaClO) solution, acrylamide (AM), sodium bromide (NaBr), sodium dodecyl benzene sulfonate (SDBS), and potassium persulfate (KPS) were purchased from Kelong Chemical Reagent Factory (Chengdu, China). Stearyl methacrylate (SMA) used in this work was supplied by J&K Scientific Co., China. All of these above-mentioned reagents were used as received. 2.2. Hydrogel Preparation. In this work, polyacrylamide (PAM)/ CNF/MWCNT hydrophobically associated hydrogels were fabricated by means of simple and effective in situ polymerization. Also, the MFC was used to prepare the CNF used in our work according to the reported reference.40 In the Supporting Information, the specific preparation method of the CNF is shown in detail. The CNF was used as a reinforcing filler for the hydrogels and a dispersant for MWCNTs as well. The MWCNTs were used as conductive fillers in our work owing to their extraordinary mechanical and electrical properties. The mature production technique and low cost of MWCNTs are beneficial to the commercial applications of the fabricated hydrogels. As for the preparation of the PAM/CNF/MWCNT composite hydrogels, the first and foremost thing is to prepare a stable and homogeneous CNF/ MWCNT dispersion with different concentration ratios. Exactly, the mixture of the obtained CNF dispersion (0.63 wt %), a certain mass of MWCNT and water, was continuously magnetically stirred for 1 h first and then sonicated for 1 h. Next, the surfactant SDBS (3 wt %) and hydrophilic monomer AM (10 wt %) were incorporated into the stirring CNF/MWCNT dispersion and then the hydrophobic monomer SMA (1.42 wt %) was added into the system. A uniform and homogeneous solution was obtained through 12 h stirring. Also, the initiator KPS solution (0.5 wt % relative to the total mass of SMA and AM) was added into the mixture. An ampoule was used to hold the solution and the solution was bubbled by nitrogen for 15 min and then the ampoule was flame-sealed and placed at 50 °C for 4 h for polymerization. Finally, PAM/CNF/MWCNT hydrogel samples were obtained. In our work, the optimal CNF dosage was investigated, which was determined at 0.3 wt % (Figure S1). Also, the maximum MWCNT content was determined at 1 wt % as shown in Figure S2. When the MWCNT loading was higher than 1 wt %, it was difficult to incorporate the AM and SMA into the MWCNT/CNF dispersion and fabricate composite hydrogels successfully. The reason was that the high total mass of MWCNT and CNF would result in gel state of MWCNT/CNF dispersion after ultrasonic treatment. In this work, PAMF x T y was used to abbreviate the PAM/CNF/MWCNT composite hydrogels. The x and y represent the proportion of the amount of CNF and MWCNTs to the total mass of hydrogel, respectively. Moreover, the pure PAM hydrogel was fabricated through the same preparation process for comparison. 2.3. Characterization. Transmission electron microscope (TEM) (JEOL, Japan) was employed to observe the morphology of CNF and 8246

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Figure 1. Digital images of the CNF dispersion (a), MWCNT dispersion without CNF (b), and MWCNT/CNF (weight ratio = 10:3) dispersion (c). (d) TEM images of CNF dispersion. (e) TEM images of CNT dispersion. (f) TEM images of MWCNT/CNF (weight ratio = 10:3) dispersion. All of these dispersion were obtained by sonication for 1 h. MWCNTs and to affirm the assisted dispersing effect of CNF on MWCNTs. In the subsequent characterization, the TEM was also used to confirm the dispersion of MWCNTs in the PAM matrix and the formation of MWCNT conductive network. Scanning electron microscope (SEM, FEI) was applied to characterize the morphology of the hydrogels. The tensile tests were carried out by means of the Instron 5576 universal with 100 N load cell. The specific stretching speed was 100 mm min−1 and the diameter and length of the samples were uniformly 10.0 and 60.0 mm, respectively. The Instron 5576 universal was also used to measure the compressive properties of these hydrogels (40 mm length) with a 1 kN load cell under a 2 mm min−1 cross-head speed. When it comes to the electrical properties, the measurements were performed by a Keithley 6487 picoammeter at ambient temperature. Also, the values of EMI shielding effectiveness (EMI SE) for these composite hydrogels were measured by the APC-7 connected to an Agilent N5230. It was noteworthy that the range of testing frequency was 8.2−12.4 GHz (at the X-band). Also, the diameter and thickness of all of the tested hydrogels were 13 and 2 mm, respectively. Actually, the coefficients for transmission (T), absorption (A), and reflection (R) were calculated from the measured scattering parameters (S11 and S12). Then, the total EMI SE can be obtained. The total EMI SE is defined as SEtotal, which is the summation of the absorption of electromagnetic microware (SEA), the reflection from the materials surface (SER), and the multiple internal reflections of microware (SEM). The measuring process and calculations of SEtotal, SEA, SER, and SEM in detail are represented in the Supporting Information. The digital microscope (VHX-1000,

KEYENCE) was employed to observe the micromorphologies of the PAMF0.3T1 hydrogel before and after the self-healing process.

3. RESULTS AND DISCUSSION 3.1. Dispersion of CNTs by CNF in Water. Cellulose nanofiber (CNF), generated from the abundant and renewable cellulose, has been regarded as a kind of inexhaustible biopolymers, thanks to its biological origin and sustainable nature.41,42 CNF, prepared through TEMPO-oxidation of MFC in this work, represents a wonderful dispersion in water, and the CNF dispersion is transparent and stable as shown in Figure 1a. Explicably, the introduction of a great many carboxyl groups onto the surface of CNFs during the TEMPO-oxidation process gives rise to the electrostatic repulsion between the CNFs, thus facilitating the homogeneous and stable CNF dispersion.43 However, the MWCNTs, used as conductive nanofillers in our work, presented extremely poor dispersion in water, even precipitating to the bottom of the glass (Figure 1b). Excitingly, the NFC-assisted dispersed MWCNT dispersions, even when the MWCNT/CNF weight ratio was 10:3, displayed favorable homogeneity and stability, as shown in Figure 1c. The successful stabilization of the MWCNT/CNF dispersion is ascribed to the hydrophobic interactions between CNF and MWCNTs and electrostatic repulsion and fluctuations of the 8247

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Figure 2. Photographs of PAMF0.3T1 showing excellent mechanical property: (a) original shape, (b) convolving, (c) bending, (d) knotting, (e) stretching shapes after being knotted, and (f) recovering shape.

Figure 3. SEM images of PAM (a), PAMF0.3 (b), PAMF0.3T0.1 (c), PAMF0.3T0.5 (d), and PAMF0.3T1 (e).

counterions on the surface of the CNF.36,38 To convincingly prove that the uniform dispersion of MWCNTs is assisted by CNF in water, the TEM is further applied to investigate the morphology of pristine MWCNTs, CNF, and MWCNT/CNF from the obtained dispersion. As shown in Figure 1d, the dimension of the prepared CNF is around 5 nm. The average length and diameter of the MWCNTs are about 1.5−3 μm and 10−20 nm, respectively. Also, the pristine MWCNT dispersion shows a huge entanglement morphology (Figure 1e). From the

TEM results (Figure 1f), it is clearly revealed that MWCNTs are separated into individual nanotubes without an obvious agglomeration and CNF is adsorbed on the MWCNT surface, which demonstrates an efficient dispersion capacity of the CNF for MWCNTs. Notably, when compared with other dispersion routes such as chemical grafting, using surfactants or water-soluble polymers, the CNF would not destroy the electronic structure of MWCNTs and impede the overlap between MWCNTs to 8248

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Figure 4. (a) Stress−strain curves of the prepared hydrogels. (b) Comparison of tensile strength of the prepared hydrogels. (c) Photographs illustrating the extraordinary mechanical property of PAMF0.3T1, in which the diameter of PAMF0.3T1 is 10.0 mm.

extremely poor mechanical property. Also, there is a large deformation of pure PAM hydrogel after going through the same deformation process as undergone by PAMF0.3T1 hydrogel in Figure 2 and it cannot recover its original shape. All of the results indicate that the incorporation of CNF and MWCNTs endow the PAMF0.3T1 hydrogel with a superior mechanical property when compared to the pure PAM hydrogel. In summary, the fantastic integration of good ductility, flexibility, and shape self-sustaining ability would helpfully broaden the scope of application of these fabricated composite hydrogels in practice. To intensively explore the structure morphology of these obtained hydrogels, the freeze-drying method is used to remove the water and maintain the internal structures of all of these samples. The fracture surfaces are directly characterized by SEM, and the results are shown in Figure 3. Obviously, the fracture surfaces of all of the fabricated hydrogels represent porous structures. Hence, the quantitative statistics of the pore sizes of these fabricated hydrogels have also been carried out, and the results are represented in Figure S4. As for pure PAM, the careful observation indicates that the distribution of pores is heterogeneous and the pore size is relatively large, manifesting a loose structure within the PAM (Figures 3a and S4a). After incorporating 0.3 wt % CNF, a more homogeneous pore

transport electron, sequentially achieving higher electrical conductivity.38 Meanwhile, as a kind of nanofillers with excellent mechanical property, the CNF can significantly enhance the mechanical properties of the materials. Definitely, assisting the dispersion of MWCNTs by using CNF as a dispersant is an environment-friendly and cost-effective approach. 3.2. Morphology and Structure. On the account of a uniform and stable MWCNT/CNF dispersion, the PAMF0.3T composite hydrogels with various MWCNT content are subsequently fabricated through simple and feasible one-pot polymerization. Satisfyingly, the prepared composite hydrogels all possess outstanding ductility and flexibility. As shown in Figure 2, the PAMF0.3T1 composite hydrogels have a favorable capacity to withstand huge deformation such as convolving (Figure 2b), bending (Figure 2c), knotting (Figure 2d), and knotted stretching (Figure 2e). Moreover, after undergoing these different deformation processes, they could rapidly recover their original shape (Figure 2f), which manifested the excellent shape self-sustaining ability of the PAMF 0.3T1 hydrogel. For comparison, the photographs of pure PAM hydrogel for the presentation of mechanical property have been recorded as well. As shown in Figure S3, the pure PAM hydrogel exhibit 8249

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Figure 5. (a) Compressive stress−strain curves of the prepared hydrogels. (b) Comparison of compressive strength of the prepared hydrogels at 90% compressive strain. (c) Cyclic compressive stress−strain curves of PAMF0.3T1. (d) Photographs revealing the notable compression resistance of PAMF0.3T1.

hydrogels, their stress−strain curves are shown in Figure 4a. Obviously, the pure PAM hydrogel exhibits an extremely low tensile strength (≈0.043 MPa) and a good ductility (≈3250% elongation at break). With 0.3 wt % CNF into the PAM system, the tensile strength of the PAMF0.3 composite hydrogel exhibits a 229% increase compared to that of the pristine PAM hydrogel, but its elongation at break represents a relatively small reduction, which indicates that the incorporation of CNF could greatly enhance the tensile property of the PAM hydrogel without a great deterioration of ductility. Further investigation shows that the tensile strength of PAMF0.3T hydrogel is progressively increased with the increasing MWCNT content, as represented in Figure 4b. With a low content of MWCNTs (0.1 wt %), the tensile property is improved to 0.158 MPa, and the maximum could be up to 0.24 MPa for PAMF0.3T1, which is almost quintuple higher than that of the pure PAM hydrogel. Also, the PAMF0.3T1 hydrogel possesses excellent tear resistance (Figure S5). The reason may be that CNF and MWCNTs could form physical cross-linking junctions with polyacrylamide molecular chains, and it is exactly this strong interaction that could commendably heighten the tensile strength of the composite hydrogels. For elongation at break, it decreases with an increase in the MWCNT content. Particularly, the elongation at the break of

distribution is observed and the pore size exhibits a decreasing tendency (Figures 3b and S4b). Furthermore, the SEM observations of the PAMF0.3T hydrogels show that the incorporation of MWCNTs has a positive effect on the narrow pore distribution and the reduction of pore size. Apparently, as presented in Figure 3c−e, the higher MWCNT content gives rise to a smaller pore size and a narrower pore size distribution, consequently leading to a more compact and denser porous structure. Actually, it is attributed to the fact that CNF and MWCNTs act as physical cross-linking points in the PAM hydrogel. Owing to the homogeneous dispersion of CNF and MWCNTs in these composite hydrogels, the increasing content of CNF and MWCNTs would give rise to the more homogeneous and denser physical cross-linking points, thus greatly restraining the growth of these irregular pores and leading to a smaller and a more homogeneous pore size along with a narrower distribution of pore sizes.44−46 Hence, it is reasonable to believe that the gradual compact and homogeneous porous structure achieved by incorporation of increasing MWCNT concentration would be beneficial to improve the performances of these composite hydrogels.23 3.3. Mechanical Property. Favorable mechanical property is of great significance for hydrogels in practical applications. To quantitatively calculate the tensile properties of these 8250

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Figure 6. (a) Electrical conductivity of the prepared hydrogels. Unlighted LED connected with pure PAM under no voltage (b), lighted LED connected with pure PAM (c), and PAMF0.3T1 (d) under a voltage of 2 V.

resultant composite hydrogels (Figure S6). Above all, in accord with the results of the tensile strength test, these results manifest the effective enhancement effect of the CNF and MWCNTs on the PAM hydrogel, which is ascribed to the strong interaction between CNF or MWCNTs and PAM molecular chains along with the formation of a compact porous network structure (Figure 3e). Simultaneously, the PAMF0.3T1 hydrogel possesses an outstanding fatigue resistance as presented in Figure 5c. The digital photos in Figure 5d admirably represent the deformation process for PAMF0.3T1 during the compressive test. Apparently, the hydrogel did not fracture under large strain (90%) and no water overflowed from the hydrogel, indicating the excellent compression resistance and water-retaining capacity of the PAMF0.3T1 hydrogel. Moreover, after the removal of stress, the PAMF0.3T1 composite hydrogel can rapidly recover the original shape within 10 min. To sum up, the PAMF0.3T1 composite hydrogel possesses excellent mechanical properties. 3.4. Electrical Conductivity. The variation in the electrical conductivities of the prepared hydrogels is exhibited in Figure 6. Similar to the other CNT-based composites reported so far,27,47−49 the increasing MWCNT content would enhance the electrical conductivities of these prepared hydrogels gradually. As revealed in Figure 6a, the value of the electrical conductivity is relatively low (approximately 0.09 S m−1) for pure PAM hydrogel, which shows little increase after the introduction of 0.3 wt % CNF (merely 0.1 S m−1). The electrically conductive behaviors of these hydrogels without MWCNT could be well explained because there are a great amount of ions (such as K+, Na+, and SO42−) that could transport swiftly inside these hydrogels. Then, MWCNTs, as a kind of conductive fillers with good intrinsic electrical conductivity, are introduced into the PAM hydrogel to enhance the electrical property. Obviously, the electrical conductivities of the obtained composite hydrogels rise gradually in the wake of the increasing MWCNT content. The electrical conductivity of the prepared PAMF0.3T0.1 hydrogel is enhanced to 0.24 S m−1. Also, the highest value of the electrical conductivity could reach up to 0.85 S m−1 for PAMF0.3T1 hydrogel, which satisfyingly manifest

the PAMF0.3T1 hydrogel is still maintained at a relatively high level (≈1980%), indicating that the PAMF0.3T1 composite hydrogel with significantly enhanced tensile strength still possesses favorable ductility. Certainly, with the gradual increment in the CNF and MWCNT loadings, the continuous downtrend of elongation at break and ductility is ascribed to the fact that the increasing interaction between CNF or MWCNT and PAM molecular chains impede the mobility of molecular chains, consequently resulting in the decline in flexibility of the obtained composite hydrogels. Furthermore, the Figure 4c picturesquely exhibits the mechanical property of the PAMF0.3T1 composite hydrogel. Obviously, the PAMF0.3T1 could lift the 2 kg weights up and recover its original shape after the removal of the weights. All of these results manifested that the obtained PAMF0.3T1 possesses excellent mechanical property. Similarly, the compressive stress−strain behaviors of these resultant hydrogels have been investigated as well, and the results are presented in Figure 5a. At the same compressive strain, it is observable that the compression stress increases with increasing CNF and MWCNT contents for these composite hydrogels. Also, the rising tendency becomes sharper when the strain is above 70%. In this work, the maximum compressive strain for the measurement is fixed at 90% because, for the hydrogels in our tests, no fracture appears during the test. As shown in Figure 5b, the pure PAM hydrogel represents an extremely low compression strength (around 0.12 MPa). Also, then it can be obviously seen that the compression strength of the PAMF0.3 hydrogel is greatly improved to 0.59 MPa, which demonstrates that the addition of CNF can enhance the PAM significantly. Subsequently, it has been found that the compression strength further rises with the increasing MWCNT content and the optimal compressive property is observed for the PAMF0.3T1 composite hydrogel, in which the extremely high compressive strength (1.91 MPa) exhibits 1492% increase compared with that of pure PAM. The rheology measurements of the composite hydrogels also demonstrate that the incorporation of MWCNTs and CNF could effectively enhance the mechanical properties of the 8251

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Figure 7. (a) TEM images of MWCNT dispersions in PAMF0.3T1. (b) A partial enlarged image of (a).

Figure 8. (a) EMI SE of the prepared hydrogels. (b) The SER, SEA, and SEtotal for the prepared hydrogels at the frequency of 10 GHz. (c) Schematic of the microwaves shielding process for the composite hydrogel with various MWCNT contents.

the unlighted light-emitting diode (LED) connected to pure PAM under no voltage and the lighter LED connected to PAMF0.3T1 compared to that of PAM under the same voltage of 2 V, which demonstrates that the higher electrical conductivity is acquired for PAMF0.3T1 by the addition of MWCNTs. Furthermore, the TEM is employed to confirm the dispersion of MWCNTs and the formation of conductive MWCNT network in the PAMF0.3T1. An ultrathin section of the specimen for TEM observation is prepared by the freezedrying method to remove water and maintain the internal structures of the PAMF0.3T1 hydrogel followed by using a

a great improvement compared to that of the PAM and PAMF0.3 hydrogels. It is well-accepted that the increasing MWCNT content would give rise to more overlap between MWCNTs, consequently leading to more perfect conductive pathways. As a result, the higher electrical conductivity could be obtained. More importantly, it is noteworthy that the porous structure suffuses with a great deal of water inside the PAMF0.3T hydrogels and close packing of MWCNTs within the pore walls will bring about excluded volume effect, resulting in the more perfect conductive pathways with a relatively low MWCNT content.27 Apart from the quantitative analysis of the electrical conductivity, Figure 6b−d vividly record the photos of 8252

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Figure 9. Digital photos and micrographs of self-healing process for PAMF0.3T1. (a) The initial PAMF0.3T1. (b) The PAMF0.3T1 was cut into two pieces. (c) The two pieces of the PAMF0.3T1 could merge into an integrated hydrogel after a week’s autonomously self-healing process.

Figure 10. (a) Self-healing PAMF0.3T1 after mechanical damage, which could sustain a sizable stretching strain without fracture. (b) Stress−strain curves of the initial and healed PAMF0.3T1 hydrogel. (c) Comparison of EMI SE of initial PAMF0.3T1, PAMF0.3T1-h-7 (self-healing for 7 days), and PAMF0.3T1-h-7 after repeated bending 1000 times. (d) Comparison of tensile strength, electrical conductivity, and EMI SE between PAMF0.3T1 and PAMF0.3T1-h-7.

properties, the obtained PAMF0.3T1 hydrogel possesses great application potential as the EMI shielding materials. 3.5. EMI Shielding Performance. Based on the analysis of the electrical properties of these prepared hydrogels, the EMI shielding performances have also been characterized. The inset in Figure 8a records the outstanding flexibility and shape selfsustaining ability of PAMF0.3T1. The variation in EMI SE as a function of CNF and MWCNT contents for the resultant hydrogels are presented in Figure 8a. As a result of the relatively

microtome. Elaborative observation about Figure 7 informs that MWCNTs are homogeneously dispersed in the pore wall of the PAMF0.3T1 hydrogel. Moreover, the admirable dispersion of MWCNTs also facilitates the close interconnection between MWCNTs to form a strong and well-established conductive network throughout the pore wall of the PAMF0.3T1 hydrogel, consequently, resulting to the outstanding electrical property of PAMF0.3T1. In view of the excellent electrical and mechanical 8253

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that the PAMF0.3T1 hydrogel possesses an outstanding selfhealing performance. Certainly, the mechanical, electrical, and EMI shielding performances of the self-healed PAMF0.3T1 are of great importance as well. As represented in Figure 10a, the selfhealed PAMF0.3T1 could sustain a sizable stretching strain without fracture. To evaluate the self-healing ability, the tensile tests are carried out for the initial and self-healed PAMF0.3T1 hydrogel, respectively. Self-healing efficiency is defined as the ratio of elongation at the break of the self-healed sample to the initial sample. Obviously, the self-healing efficiency increases following the increasing self-healing time. More specifically, when the cut PAMF0.3T1 sample is automatically healed for 4 and 7 days, the self-healing efficiency reaches 63.4 and 77.2%, respectively (Figure 10b). Above all, the excellent self-healing performance of the PAMF 0.3T1 composite hydrogel is attributed to the synergistic effect between the recombination of hydrophobic and readsorption of the PAM polymer chains onto the surfaces of MWCNTs and CNF. PAMF0.3T1, as a kind of functional material, is expected to restore not merely its interior structure and mechanical property but also its electrical and EMI shielding performances. Hence, the comparison of the EMI shielding performances and the electrical conductivities between the sample before and after self-healing is characterized. As shown in Figure 10c, the EMI SE value of the self-healing PAMF0.3T1 exhibits wonderful stability. More interestingly, the EMI shielding performance of the self-healing PAMF0.3T1 composite hydrogel is almost kept unchanged even after being bended for 1000 cycles. Moreover, Figure 10d reveals that the electrical conductivity and EMI SE of the self-healing PAMF0.3T1remained invariability when compared to those of the initial PAMF0.3T1. To sum up, the PAMF0.3T1 hydrogel possesses excellent mechanical and self-healing properties, which are exactly similar to those of human skin. With regard to the mechanical and selfhealing properties, although there have been some published researches about the self-healing hydrogels with excellent mechanical properties along with high efficiency and the composite hydrogels in our work do not possess great superiority, the achievement of the excellent mechanical and self-healing properties of these published hydrogels generally require complicated molecular or structural design as well as external stimulus or energy.55−58 However, the HA PAM hydrogels possess the advantages of simple preparation method and excellent autonomous self-healing capacity at room temperature. In our work, we develop a facile route to effectively enhance and functionalize the HA PAM hydrogels. The mechanical properties of the resultant composite hydrogels in our work remain a leading position when compared to those of the reported HA hydrogels.18,20,23,59,60 Also, the resulting hydrogel well inherited the excellent self-healing capacity of the HA PAM hydrogels. More importantly, the incorporation of MWCNTs endows the HA PAM hydrogels with excellent electrical and EMI shielding performances. The PAMF0.3T1 hydrogel could spontaneously restore its mechanical, electrical, and EMI shielding performances after being subjected to the mechanical damage. These features can protect the electronics against harm from external forces and EMI, endowing the PAMF0.3T1 hydrogel great application prospect in the field of simulate skin of intelligent robot, portable devices, and E-skin, where outstanding flexibility, high strength, and excellent EMI shielding performance are required.

low electrical conductivity, the PAM hydrogel demonstrates a limited EMI shielding performance (≈15 dB). Then, to distinguish the curves of PAM and PAMF0.3, the testing results of these samples are delicately presented in Figure S7, and the tiny difference between their curves indicate that the incorporation of CNF has little influence on the EMI shielding performance. Encouragingly, with the introduction of MWCNTs into the PAM matrix, the largely enhanced EMI shielding performances are obtained. As for the PAMF0.3T composite hydrogel with only 0.1 wt % MWCNTs, the value of the EMI SE is improved from 15 to 18.5 dB and the gradual increment in EMI SE can be acquired by increasing the content of MWCNTs. Satisfyingly, when the MWCNT content is 1 wt %, an optimal EMI shielding performance is obtained (≈28.5 dB), which is double that of pristine PAM. For the purpose of a theoretical explanation of the EMI shielding performances, SER, SEA, and SEtotal are elaborately exhibited in Figure 8b. As for SER, there is a tiny increment with the rising CNF and MWCNT concentrations. The reason is that the amount of mobile charge carriers plays a decisive role in the value of SER and the ascending MWCNT content would provide more mobile charge carriers to interact with the incoming microwaves. Then, the SEA increases vastly in the wake of the increasing MWCNT content because the gradual increase in the MWCNT concentration would result in the higher complex permittivity and more conductive networks to serve as dissipating mobile charge carriers.50,51 To explore the mechanism behind the EMI shielding behaviors, the power coefficients of the prepared hydrogels as a function of the MWCNT loading at the X-band are shown in Figure S8. Definitely, in the microwave shielding process by these hydrogels, both absorption and reflection occupy vital positions. In Figure 8c, the EMI shielding processes are graphically depicted. Owing to the porous structure, the existence of the MWCNT conductive network in pore walls and the icons in the water inside the pores cause the incoming microwaves to be repeatedly absorbed and reflected. The incoming microwaves are trapped in the porous structure of the hydrogels. Therefore, the majority of the incoming microwaves are absorbed and dissipated, whereas an extremely tiny portion could pass through the hydrogels. Moreover, the increasing MWCNT content not only improves the electrical conductivity of these hydrogels but also decreases the pore sizes, thus leading to the higher pore density. Also, the improvement in pore density will increase the pore−matrix interfaces. Actually, the increasing pore−matrix interfaces and electrical conductivities cause more incoming microwaves to be reflected, multireflected, and absorbed, consequently trapping more microwaves in the porous structure and attenuating them effectively to achieve the optimization of the EMI shielding performances.52−54 3.6. Self-Healing Property. The novelty of this PAMF0.3T hydrogel is not only reflected in its excellent mechanical property, electrical and EMI shielding performances but also in its outstanding automatically self-healing capacity. As represented in Figure 9, the PAMF0.3T1 was cut into two pieces and then it could be spontaneously self-healed after the two sections contacting for a week. The micrographs in Figure 9c indicate that the two pieces of the PAMF0.3T1 hydrogel could come in contact and merge into an integrated hydrogel after the self-healing process at room temperature for a week. Furthermore, it is difficult to observe the crack in the magnified micromorphology, which manifests 8254

DOI: 10.1021/acsami.7b18700 ACS Appl. Mater. Interfaces 2018, 10, 8245−8257

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ACS Applied Materials & Interfaces

Coated Mesh for Oil/Water Separation. Adv. Mater. 2011, 23, 4270− 4273. (2) Liu, Y.-J.; Cao, W.-T.; Ma, M.-G.; Wan, P. Ultrasensitive Wearable Soft Strain Sensors of Conductive, Self-healing, and Elastic Hydrogels with Synergistic “Soft and Hard” Hybrid Networks. ACS Appl. Mater. Interfaces 2017, 9, 25559−25570. (3) Xu, Y.; Lin, Z.; Huang, X.; Liu, Y.; Huang, Y.; Duan, X. Flexible Solid-State Supercapacitors Based on Three-Dimensional Graphene Hydrogel Films. ACS Nano 2013, 7, 4042−4049. (4) Zheng, W. J.; An, N.; Yang, J. H.; Zhou, J.; Chen, Y. M. Tough Al-alginate/Poly(N-isopropylacrylamide) Hydrogel with Tunable LCST for Soft Robotics. ACS Appl. Mater. Interfaces 2015, 7, 1758− 1764. (5) Tee, B. C. K.; Wang, C.; Allen, R.; Bao, Z. An electrically and mechanically self-healing composite with pressure- and flexionsensitive properties for electronic skin applications. Nat. Nanotechnol. 2012, 7, 825. (6) Zhang, H.; Xia, H.; Zhao, Y. Poly(vinyl alcohol) Hydrogel Can Autonomously Self-Heal. ACS Macro Lett. 2012, 1, 1233−1236. (7) Wei, Z.; Yang, J. H.; Zhou, J.; Xu, F.; Zrinyi, M.; Dussault, P. H.; Osada, Y.; Chen, Y. M. Self-healing gels based on constitutional dynamic chemistry and their potential applications. Chem. Soc. Rev. 2014, 43, 8114−8131. (8) Pan, C.; Liu, L.; Chen, Q.; Zhang, Q.; Guo, G. Tough, Stretchable, Compressive Novel Polymer/Graphene Oxide Nanocomposite Hydrogels with Excellent Self-Healing Performance. ACS Appl. Mater. Interfaces 2017, 9, 38052−38061. (9) Deng, G.; Li, F.; Yu, H.; Liu, F.; Liu, C.; Sun, W.; Jiang, H.; Chen, Y. Dynamic Hydrogels with an Environmental Adaptive Self-Healing Ability and Dual Responsive Sol−Gel Transitions. ACS Macro Lett. 2012, 1, 275−279. (10) Gong, Z.; Zhang, G.; Zeng, X.; Li, J.; Li, G.; Huang, W.; Sun, R.; Wong, C. High-Strength, Tough, Fatigue Resistant, and Self-Healing Hydrogel Based on Dual Physically Cross-Linked Network. ACS Appl. Mater. Interfaces 2016, 8, 24030−24037. (11) Cui, J.; Campo, A. D. Multivalent H-bonds for self-healing hydrogels. Chem. Commun. 2012, 48, 9302−9304. (12) Kakuta, T.; Takashima, Y.; Nakahata, M.; Otsubo, M.; Yamaguchi, H.; Harada, A. Preorganized Hydrogel: Self-Healing Properties of Supramolecular Hydrogels Formed by Polymerization of Host−Guest-Monomers that Contain Cyclodextrins and Hydrophobic Guest Groups. Adv. Mater. 2013, 25, 2849−2853. (13) Nakahata, M.; Takashima, Y.; Yamaguchi, H.; Harada, A. Redoxresponsive self-healing materials formed from host-guest polymers. Nat. Commun. 2011, 2, No. 511. (14) Guo, Y.; Zhou, X.; Tang, Q.; Bao, H.; Wang, G.; Saha, P. A selfhealable and easily recyclable supramolecular hydrogel electrolyte for flexible supercapacitors. J. Mater. Chem. A 2016, 4, 8769−8776. (15) Sun, J.-Y.; Zhao, X.; Illeperuma, W. R. K.; Chaudhuri, O.; Oh, K. H.; Mooney, D. J.; Vlassak, J. J.; Suo, Z. Highly stretchable and tough hydrogels. Nature 2012, 489, 133. (16) Xu, Y.; Wu, Q.; Sun, Y.; Bai, H.; Shi, G. Three-Dimensional SelfAssembly of Graphene Oxide and DNA into Multifunctional Hydrogels. ACS Nano 2010, 4, 7358−7362. (17) Shafiq, Z.; Cui, J.; Pastor-Pérez, L.; San Miguel, V.; Gropeanu, R. A.; Serrano, C.; del Campo, A. Bioinspired Underwater Bonding and Debonding on Demand. Angew. Chem. 2012, 124, 4408−4411. (18) Akay, G.; Hassan-Raeisi, A.; Tuncaboylu, D. C.; Orakdogen, N.; Abdurrahmanoglu, S.; Oppermann, W.; Okay, O. Self-healing hydrogels formed in catanionic surfactant solutions. Soft Matter 2013, 9, 2254−2261. (19) Tuncaboylu, D. C.; Sahin, M.; Argun, A.; Oppermann, W.; Okay, O. Dynamics and Large Strain Behavior of Self-Healing Hydrogels with and without Surfactants. Macromolecules 2012, 45, 1991−2000. (20) Tuncaboylu, D. C.; Sari, M.; Oppermann, W.; Okay, O. Tough and Self-Healing Hydrogels Formed via Hydrophobic Interactions. Macromolecules 2011, 44, 4997−5005.

4. CONCLUSIONS In summary, a robust, flexible, and self-healable EMI shielding material based on MWCNTs and HA PAM hydrogel has been prepared via a simple method. The MWCNTs were homogeneously dispersed in the PAMF0.3T composite hydrogels by using CNF as the dispersant. The obtained PAMF0.3T composite hydrogels possessed significantly enhanced mechanical properties including outstanding tensile and compressive strengths by the incorporation of MWCNTs and CNF, which also represented excellent flexibility and shape self-sustaining ability. Besides, the well-defined conductive MWCNT network endowed the PAMF0.3T composite hydrogels with outstanding electrical and EMI shielding performances. Specifically, the optimum of electrical conductivity (≈0.85 S m−1) and EMI SE (≈28.5 dB) could be achieved for PAMF0.3T1 composite hydrogel. More importantly, the resultant PAMF0.3T1 hydrogel possessed extraordinary autonomic self-healing capacity, with the ability to restore not only the mechanical property but also the electrical and EMI shielding performances after suffering severe mechanical damage.

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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/acsami.7b18700. Preparation of CNF; EMI shielding measurement; investigation of the optimal CNF dosage to disperse MWCNTs; electrical conductivity of the PAMFT hydrogels and digital images of the MWCNT/CNF dispersion with 0.5 wt % CNF and 1 wt % MWCNTs (Figure S1); photographs of the MWCNT/CNF dispersion with 1.2 wt % MWCNTs after ultrasonic treatment (Figure S2); photographs of PAM hydrogel showing poor mechanical property (Figure S3); diameter distribution histograms of prepared hydrogels (Figure S4); excellent tear resistance of PAMF0.3T1 hydrogel (Figure S5); rheology properties of the prepared hydrogels (Figure S6); comparison of EMI SE of PAM and PAMF0.3 hydrogels (Figure S7); coefficients of transmission, reflection, and absorption for PAM and PAMF0.3Ty hydrogels (Figure S8) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel/Fax: +86-28-85460690 (F.C.). *E-mail: [email protected]. Tel/Fax: +86-28-8546 1795 (Q.F.). ORCID

Qiang Fu: 0000-0002-5191-3315 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (Grant Nos. 51573102 and 51721091).

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