Robust and Mechanically and Electrically Self-Healing Hydrogel for

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A Robust, 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18700 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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ACS Applied Materials & Interfaces

A Robust, Mechanically and Electrically Self-Healing Hydrogel for Efficient Electromagnetic Interference Shielding Weixing Yang,a Bowen Shao,a Tianyu Liu,a Yiyin Zhang,a Rui Huang,a Feng Chen*a and Qiang Fu*a a

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials

Engineering, Sichuan University, Chengdu 610065, China.

ABSTRACT: Autonomously self-healing hydrogels have received considerable attentions owe to their capacity for repairing themselves spontaneously after suffering damage, which can provide better stability and longer lifespan. In this work, a robust, mechanically and electrically self-healing hydrogel with efficient electromagnetic interference (EMI) shielding performance was successfully fabricated via the incorporation of multi-walled carbon nanotubes (MWCNTs) into the hydrophobically associated polyacrylamide (PAM) hydrogels by using cellulose nanofiber (CNF) as dispersant. It has 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 than those (≈ 0.043 MPa, ≈ 0.09 S m-1 and ≈15 dB, respectively) of 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 self-healing properties, the prepared composite hydrogels are 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, Multi-walled carbon nanotubes, Cellulose nanofiber, EMI shielding

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1. INTRODUCTION The development of polymeric hydrogels with new functionalities is becoming increasing aspirations in 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 achieve considerable attentions because their self-healing capacities are essential to enhance the structural safety and lifetime of materials.6-8 As for the preparation of self-healing 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. 19

10-

Attractively, hydrophobically associated (HA) hydrogels, based on hydrophobic

association, appear 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, HA hydrogels are successfully constructed via the formation of HA domains and micellar copolymerization with existence of surfactants, hydrophobic and hydrophilic monomer. The HA domains could not only act as physical crosslinkers but also promote hydrophilic polymer chain cross-linking in the HA hydrogels.21-23 Additionally, the excellent and autonomous self-healing capability of HA hydrogels at room temperature are achieved by the hydrophobic association which is attributed to the formation of these above-mentioned HA domains. These self-healing HA hydrogels show 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 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-


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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 HA hydrogels conductive is not enough for the preparation of high-performance self-healing EMI shielding materials. The inferior mechanical property 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 KPa and 47 KPa, respectively.20,30 Thus a novel strategy is highly desirable to significantly ameliorate both the mechanical strength and electrical conductivity of HA hydrogels for fabricating autonomously self-healing EMI shielding materials. It has been reported that the HA composite hydrogels incorporated with functional nano-fillers 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 GO 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 nano-fillers to 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 HA hydrogels, which has been broadly applied for the preparation of the high-performance conductive materials.32-35 To enhance the mechanical and electrical properties of HA hydrogel by taking advantage of CNT, the key is achieving high-quality dispersion of the CNT in the HA hydrogel. Recently, it has been reported that cellulose nanofiber (CNF) can be used as green dispersant for CNT in an aqueous solution and completely preserve electronic properties of CNT.36-38 Moreover, CNF



could be served as a green nano-filler to effectively enhance the mechanical properties of composites.39 Herein, an autonomously self-healing EMI shielding material was successfully prepared by incorporating multi-walled carbon nanotubes (MWCNTs) into hydrophobically associated polyacrylamide (PAM) hydrogel, in which CNF used as 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 revealed 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, 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. Multi-walled 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 these abovementioned 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. And the MFC was used to prepare the CNF used in


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our work according to the reported reference.40 In Supporting Information, the specific preparation method of CNF is shown in detail. CNF was used as reinforcing filler for hydrogels and dispersant for MWCNTs as well. The MWCNTs were used as the 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 the 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 magnetic stirred for 1h firstly and then one hour’s sonication was employed for the mixture. 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 though 12 hours’ stirring. And 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, then the ampoule was flame-sealed and placed at 50℃ for 4 h for the 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). And 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, PAMFxTy is used to abbreviate 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 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, USA) 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 mm and 60.0 mm, respectively. The Instron 5576 universal was also used to measure compressive properties of these hydrogels (40mm length) with a 1 kN load cell under a 2 mm min-1 crosshead speed. When it comes to the electrical properties, the measurements were performed by Keithley 6487 picoammeter at ambient temperature. And the values of EMI shielding effectiveness (EMI SE) for these composite hydrogels were measured by the APC-7 connected with Agilent N5230. It was noteworthy that the range of testing frequency was 8.2-12.4 GHz (at the X-band). And the diameter and thickness of all the tested hydrogels were 13 mm 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 were represented in Supporting Information. The digital microscope (VHX-1000, KEYENCE) was employed to observe the micromorphologies of the PAMF0.3T1 hydrogel before and after 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, represented wonderful dispersion in water and the CNF dispersion was transparent and stable as shown in Fig 1a. Explicably, the introduction of a great many carboxyl groups onto the surface of CNFs during the TEMPO-oxidation process gave rise to the electrostatic repulsion between the CNFs, and thus facilitating the homogeneous and stable CNF dispersion.43 However, the MWCNTs, used as


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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 these dispersion were obtained by sonication for 1h.

conductive nano-fillers in our work, presented extremely poor dispersion in water even precipitated to the bottom of the glass (Fig 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 Fig 1c. The successful stabilization of the MWCNT/CNF dispersion is ascribed to the hydrophobic interactions between CNF and MWCNTs, electrostatic repulsion and fluctuations of the counterions on the surface of the CNF.36,38 To convincingly prove that uniform dispersion of MWCNTs assisted by CNF in water, the TEM was 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 was around 5 nm. The average length and diameter of the MWCNTs were about 1.5-3 μm and 10-20 nm, respectively. And the pristine MWCNT dispersion showed huge entanglement morphology (Figure 1e). From the TEM results (Figure 1f), it clearly revealed that MWCNTs were separated into individual nanotube without obvious agglomeration and CNF was adsorbed on the MWCNT surface, which demonstrated



the efficient dispersion capacity of 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 transport electron, sequentially achieve the higher electrical conductivity.38 Meanwhile, as a kind of nano-fillers with excellent mechanical property, CNF could significantly enhance the mechanical properties of materials. Definitely, assisting the dispersion of MWCNTs by using CNF as dispersant is an environmentally friendly and costeffective approach. 3.2. Morphology and structure. On account of the uniform and stable MWCNT/CNF dispersion, the PAMF0.3T composite hydrogels with various MWCNT content were subsequently fabricated through simple and feasible one-pot polymerization. Satisfyingly, the prepared composite hydrogels all possessed outstanding ductility and flexibility. As shown in Figure 2, the PAMF0.3T1 composite hydrogels have favorable capacity to withstand the 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 to their original shape (Figure 2f), which manifested excellent shape self-sustaining ability of the PAMF0.3T1 hydrogel.

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.


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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 represented extremely poor mechanical property. And there was a large deformation for pure PAM hydrogel after the same deformation process as PAMF0.3T1 hydrogel underwent in Figure 2 and it could not recover to its original shape. All the results indicated that the incorporation of CNF and MWCNTs endowed the PAMF0.3T1 hydrogel with 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 was used to remove the water and maintain the internal structures of all these samples. The fracture surfaces were directly characterized by SEM and the results are shown in Figure 3. Obviously, the fracture surfaces of all the fabricated hydrogels represented porous structures. Hence, the quantitive 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 indicated that the distribution of pores was heterogeneous and the pore size was relatively large, manifesting a loose structure within the PAM (Figure 3a and S4a). After incorporating 0.3 wt% CNF, a more homogeneous pore distribution was observed and the pore size exhibited a decreasing tendency (Figure 3b and S4b). Furthermore, the SEM observations of the PAMF0.3T hydrogels showed that the incorporation of MWCNTs had a positive effect on the narrow pore distribution and the

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



reduction of pore size. Apparently, as presented in Figure 3c-e, the higher MWCNT content gave rise to the smaller pore size and the narrower pore size distribution, consequently leading to the more compact and denser porous structure. Actually, it was attributed to the fact that CNF and MWCNTs acted as physical crosslinking points in the PAM hydrogel. Owing to the homogenous dispersion of CNF and MWCNTs in these composite hydrogels, the increasing content of CNF and MWCNTs would give rise to the more homogenous and denser physical cross-linking points, thus greatly restrained the growth of those irregular pores and led to the smaller and more homogenous pore size along with narrower distribution of pore sizes.44-46 Hence, it is reasonable to believe that the gradually compact and homogeneous porous structure by incorporating with the increasing MWCNT concentration would be beneficial to improve 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 hydrogels, their stress-strain curves are shown in Figure 4a.

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 PAMF 0.3T1, in which the diameter of PAMF0.3T1 is 10.0 mm.


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Obviously, the pure PAM hydrogel exhibited extremely low tensile strength (≈ 0.043 MPa) and good ductility (≈ 3250 % elongation at break). With 0.3 wt% CNF into the PAM system, the tensile strength of the PAMF0.3 composite hydrogel exhibited a 229% increase compared to that of pristine PAM hydrogel, but its elongation at break represented a relatively small reduction, which indicated that the incorporation of CNF could greatly enhance the tensile property of PAM hydrogel without a great deterioration of ductility. Further investigation found that the tensile strength of PAMF0.3T hydrogel was progressively increased with the increasing MWCNT content as represented in Figure 4b. With a low content of MWCNTs (0.1 wt%), the tensile property was improved to 0.158 MPa, and the maximum could up to 0.24 MPa for PAMF0.3T1, which was almost quintuple higher than that of pure PAM hydrogel. And the PAMF0.3T1 hydrogel possessed 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 which could commendably heighten the tensile strength of the composite hydrogels. For elongation at break, it decreased with the augment of the MWCNT content. Particularly, the elongation at break of PAMF0.3T1 hydrogel still maintained at a relatively high level (≈ 1980 %), which indicated the PAMF0.3T1 composite hydrogel with significantly enhanced tensile strength still possessed favorable ductility. Certainly, with the gradual increment of CNF and MWCNT loadings, the continuous downtrend of elongation at break and ductility was ascribed to the fact that the increasing interaction between CNF or MWCNT and PAM molecular chains impeded the mobility of molecular chains, consequently resulting in the decline in flexibility of the obtained composite hydrogels. Furthermore, the Figure 4c picturesquely exhibited the mechanical property of PAMF0.3T1 composite hydrogel. Obviously, the PAMF0.3T1 could lift the 2Kg weights up and recover to its original shape after the removal of the weights. All these results manifested that the obtained PAMF0.3T1 possessed 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 was observable that the compression stress increased with the increasing CNF and MWCNT contents for these composite hydrogels. And the rising tendency became sharper when the strain was above 70%. In this work, the maximum compressive strain for measurement was fixed at 90% because, for the hydrogels in



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.

our tests, no fracture appeared during the test. As shown in Figure 5b, the pure PAM hydrogel represented extremely low compression strength (around 0.12 MPa). And then it could be obviously seen that the compression strength of PAMF0.3 hydrogel was greatly improved to 0.59 MPa, which demonstrated that the addition of CNF can enhance the PAM significantly. Subsequently, it has been found that the compression strength was further rising with the increasing MWCNT content and the optimal compressive property was observed for the PAMF0.3T1 composite hydrogel, in which the extremely high compressive strength (1.91 MPa) exhibited 1427% increase compared with that of pure PAM. The rheology measurements of the composite hydrogels also demonstrated that the incorporation of MWCNTs and CNF could effectively enhance the mechanical properties of the resultant composite hydrogels (Figure S6). Above all, in accord with the results of tensile strength test, these results manifested the effective enhancement effect of CNF and MWCNTs on the PAM hydrogel, which was ascribed to the strong interaction between CNF or MWCNTs and PAM molecular chains along with the formation of compact porous network


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structure (Figure 3e). Simultaneously, the PAMF0.3T1 hydrogel possessed outstanding fatigue resistance as presented in Figure 5c. The digital photos in Figure 5d admirably represented 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 could rapidly recover to the original shape within 10 min. To sum up, the PAMF0.3T1 composite hydrogel possessed excellent mechanical properties. 3.4. Electrical conductivity. The variation of 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 electrical conductivities of these prepared hydrogels gradually. As revealed in Figure 6a, the value of electrical conductivity is relatively low (approximately 0.09 S m-1) for pure PAM hydrogel, which showed 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 were a great amount of ions (such as K+, Na+ and SO42-) which could transport swiftly inside these hydrogels. And then, MWCNTs, as a kind of conductive fillers with good intrinsic electrical conductivity, were introduced into the PAM hydrogel to enhance the electrical

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 PAMF 0.3T1 (d) under a voltage of 2 V.



property. Obviously, the electrical conductivities of the obtained composite hydrogels rose gradually in the wake of the increasing MWCNT content. The electrical conductivity of the prepared PAMF0.3T0.1 hydrogel was enhanced to 0.24 S m-1. And the highest value of electrical conductivity could reach up to 0.85 S m-1 for PAMF0.3T1 hydrogel, which satisfyingly manifested a great improvement compared to that of PAM and PAMF0.3 hydrogels. It was 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 was noteworthy that the porous structure suffused with a great deal of water inside PAMF0.3T hydrogels and close packing of MWCNTs within pore walls would bring about excluded volume effect, resulting in the more perfect conductive pathways with relatively low MWCNT content.27 Apart from the quantitative analysis about the electrical conductivity, the Figure 6b-d vividly recorded the photos of the unlighted LED connected with pure PAM under no voltage and the lighter LED connected with PAMF0.3T1 compared to that of PAM under the same voltage of 2V, which demonstrated the higher electrical conductivity was acquired for PAMF0.3T1 by the addition of MWCNTs. Furthermore, the TEM was 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 was prepared by freeze-drying method to remove the water and maintain the internal structures of PAMF0.3T1 hydrogel followed by using a microtome. Elaborative observation about Figure 7 informed that MWCNTs were homogenously dispersed in the pore wall of the PAMF0.3T1 hydrogel.

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


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Moreover, the admirable dispersion of MWCNTs also facilitated the close interconnection between MWCNTs to form the 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 properties, the obtained PAMF0.3T1 hydrogel possessed great application potential as EMI shielding materials. 3.5. EMI shielding performance. Based on the analysis about the electrical properties of these prepared hydrogels, the EMI shielding performances have also been characterized. The inset in Figure 8a recorded the outstanding flexibility and shape self-sustaining ability of PAMF0.3T1. The variation of 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 low electrical conductivity, PAM hydrogel appeared a limited EMI shielding performance (≈15dB). And then, to distinguish the curves of PAM and PAMF0.3, the testing results of these samples were delicately presented in Figure S7 and the tiny difference between their curves indicated that the incorporation of CNF had little influence on the EMI shielding performance. Encouragingly, with the introduction of MWCNTs into the PAM matrix, the largely enhanced EMI shielding performances were obtained. As for PAMF0.3T composite hydrogel with only 0.1 wt% MWCNTs, the value of EMI SE was improved from 15 dB to 18.5 dB and the gradual

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



increment of EMI SE could be acquired by increasing the content of MWCNTs. Satisfyingly, when the MWCNT content was 1 wt%, the optimal EMI shielding performance was obtained (≈ 28.5 dB), which was double of that of pristine PAM. For the purpose of a theoretical explanation of the EMI shielding performances, the SER, SEA and SETotal were elaborately exhibited in Figure 8b. As for SER, there was a tiny increment with the rising CNF and MWCNT concentrations. The reason was that the amount of mobile charge carriers played a decisive role on the value of SER and the ascending MWCNT content would provide more mobile charge carriers to interact with the incoming microwaves. And then, the SEA increased vastly in the wake of the increasing MWCNT content because the gradual increase in MWCNT concentration would result in the higher complex permittivity and more conductive networks to serve as dissipating mobile charge carriers.50,51 In order to explore the mechanism behind the EMI shielding behaviors, the power coefficients of the prepared hydrogels as a function of the MWCNT loading at X-band are shown in Figure S8. Definitely, in the microwave shielding process by these hydrogels, both the absorption and reflection occupied vital positions. In Figure 8c, the EMI shielding processes are graphically depicted. Owing to the porous structure, the existence of MWCNT conductive network in pore walls and the icons in the water inside the pores made the incoming microwaves repeatedly absorbed and reflected. The incoming microwaves were trapped in the porous structure of the hydrogels. Therefore, the majority of the incoming microwaves were absorbed and dissipated while an extremely tiny portion could pass through the hydrogels. Moreover, the increasing MWCNT content not only improved the electrical conductivity of these hydrogels, but also decreased the pore sizes, thus led to the higher pore density. And the improvement of pore density would increase the pore-matrix interfaces. Actually, the increasing pore-matrix interfaces and electrical conductivities made more incoming microwaves reflected, multireflected and absorbed, consequently trapped more microwaves in the porous structure and attenuated them effectively to achieve the optimization of 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.


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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 sizeable stretching strain without fracture. (b) Stress-strain curves of 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 PAMF0.3T1h-7 after repeated bending 1000 times. (d) Comparison of tensile strength, electrical conductivity and EMI SE between PAMF0.3T1 and PAMF0.3T1-h-7.

The micrographs in Figure 9c indicated that the two pieces of the PAMF0.3T1 hydrogel could contact and merge into an integrated hydrogel after the self-healing process at room temperature for a week. Furthermore, it was difficult to observe the crack in the magnified micro-morphology, which manifested that the PAMF0.3T1



hydrogel possessed outstanding self-healing performance. Certainly, the mechanical, electrical and EMI shielding performances of self-healed PAMF0.3T1 are of great importance as well. As represented in Figure 10a, the selfhealed PAMF0.3T1 could sustain sizeable stretching strain without fracture. In order to evaluate the self-healing ability, the tensile tests were carried out for the initial and self-healed PAMF0.3T1 hydrogel, respectively. Self-healing efficiency was defined as the ratio of elongation at break of self-healed sample to the initial sample. Obviously, the self-healing efficiency increased following the increasing self-healing time. More specifically, when the cut PAMF0.3T1 sample automatically healed for 4 and 7 days, the self-healing efficiency reached to 63.4% and 77.2%, respectively (Figure 10b). Above all, the excellent self-healing performance of PAMF0.3T1 composite hydrogel was attributed to the synergistic effect between the recombination of hydrophobic and re-adsorption of PAM polymer chains onto the surfaces of MWCNTs and CNF. PAMF0.3T1, as a kind of functional materials, was expected to restore not merely its interior structure and mechanical property but also electrical and EMI shielding performances. Hence, the comparison of EMI shielding performances and the electrical conductivities between the sample before and after self-healing was characterized. As show in Figure 10c, the EMI SE value of self-healing PAMF0.3T1 exhibited wonderful stability. More interestingly, the EMI shielding performance of the self-healing PAMF0.3T1 composite hydrogel almost kept unchanged even after being bended for 1000 cycles. The results revealed that the values of electrical conductivity and EMI SE at X-band of the self-healing sample remained invariability when compared to those of initial sample (Figure 10d). To sum up, the PAMF0.3T1 hydrogel possessed excellent mechanical and selfhealing properties, which was exactly similar to those of human skin. With regard to the mechanical and self-healing 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 did not possess great superiority, the achievement of the excellent mechanical and self-healing properties of these published hydrogels generally required 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 developed a facile route to effectively enhance and functionalize the HA PAM hydrogels. The


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mechanical properties of the resultant composite hydrogels in our work remained a leading position when compared to those of the reported HA hydrogels.18,20,23,59,60 And the resulting hydrogel well inherited the excellent self-healing capacity of HA PAM hydrogels. More importantly, the incorporation of MWCNTs endowed 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 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.

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 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 welldefined 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, which have the ability to restore not only the mechanical property but also electrical and EMI shielding performances after suffering severe mechanical damage.

 ASSOCIATED CONTENT Supporting Information. Supporting information related to this article is available free of charge on the ACS Publications website.



Preparation of CNF; EMI shielding measurement; Investigation of the optimal CNF dosage to disperse MWCNTs; Figure S1: Electrical conductivity of the PAMFT hydrogels and digital images of the MWCNT/CNF dispersion with 0.5 wt% CNF and 1 wt% MWCNTs; Figure S2: Photographs of the MWCNT/CNF dispersion with 1.2 wt% MWCNTs after ultrasonic treatment; Figure S3: Photographs of PAM hydrogel showing poor mechanical property; Figure S4: Diameter distribution histograms of prepared hydrogels; Figure S5: Excellent tear resistance of PAMF0.3T1 hydrogel; Figure S6: Rheology properties of the prepared hydrogels; Figure S7: The comparison of EMI SE of PAM and PAMF0.3 hydrogels; Figure S8: Coefficients of transmission, reflection, and absorption for PAM and PAMF0.3Ty hydrogels (PDF)

 AUTHOR INFORMATION Corresponding author E-mail: [email protected] (Q. Fu), Tel. /Fax: +86- 28-8546 1795. E-mail: [email protected] (F. Chen), Tel. /Fax: +86-28-85460690.

 ACKNOWLEDGMENT We are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 51573102 and 51721091).

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Table Of Contents Graphic In this work, a robust, mechanically and electrically self-healing hydrogel with efficient EMI shielding performance was successfully fabricated via the incorporation of MWCNTs into the HA PAM hydrogels by using CNF as dispersant.


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Robust and Mechanically and Electrically Self-Healing Hydrogel for

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