Ultrasoft Self-Healing Nanoparticle-Hydrogel Composites with

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 6395−6403

pubs.acs.org/journal/ascecg

Ultrasoft Self-Healing Nanoparticle-Hydrogel Composites with Conductive and Magnetic Properties Kai Liu,*,† Xiaofeng Pan,† Lihui Chen,† Liulian Huang,† Yonghao Ni,†,‡ Jin Liu,† Shilin Cao,† and Hongping Wang§ †

College of Material Engineering, Fujian Agriculture and Forestry University, No. 15 Shangxiadian Road, Cangshan District, Fuzhou City, Fujian Province 350002, China ‡ Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick E3B5A3, Canada § Jinshan College, Fujian Agriculture and Forestry University, No. 15 Shangxiadian Road, Cangshan District, Fuzhou City, Fujian Province 350002, China ABSTRACT: Recently, integration of two or more important properties into a hydrogel has been a challenge in the preparation of the multifunctional hydrogel. Herein, in order to impart conductive and magnetic properties to the self-healing PVA hydrogel at the same time, the nanofibrillated cellulose (NFC) was used as the substrate. The polyaniline was coated on the NFC surface by in situ chemical polymerization, and the MnFe2O4 nanoparticles were synthesized and loaded on the NFC by the chemical co-precipitation method. The multifunctional PVA hydrogel was prepared by incorporating the NFC/PAni/MnFe2O4 nanocomposites with the PVA hydrogel. The magnetic and conductive property tests of the multifunctional PVA hydrogel showed that the maximum saturation magnetization and conductivity were 5.22 emu·g−1 and 8.15 × 10−3 S·cm−1, respectively. Moreover, the multifunctional PVA hydrogel exhibited excellent self-healing and ultrasoft properties, which could be self-healed completely after the pieces of the hydrogel were put together for several minutes at room temperature. Due to the self-healing ability, conductivity, and magnetism, the novel hydrogel was expected to be used in many practical applications, such as electrochemical display devices, rechargeable batteries, and electromagnetic interference shielding. More importantly, we proved a facile template approach to the preparation of a stable polymer and nanoparticle composites using NFC as substrates that imparted different properties to hydrogels. KEYWORDS: Self-healing, Hydrogel, Nanocomposite, Conductivity, Magnetism

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INTRODUCTION

functional polymer materials, and the most commonly used conductive polymers were polyaniline and polypyrrole due to the simple polymerization process. Recently, a few self-healing hydrogels with conductivity have been successfully prepared by some researchers. Hur et al. developed a new class of moldable, stretchable, and self-healable conductive hydrogels by utilizing an oxidizing agent as a mediator to induce homogeneous polymerization of the pyrrole monomer inside the threedimensional agarose hydrogel network.17 In addition to the conductivity, magnetism was another important property for the hydrogel because magnetic hydrogels were highly sensitive to the response in magnetic fields; thus, most magnetic hydrogels have great potential in biomedical applications.18 Most studies have focused on developing magnetic hydrogels by incorporating magnetic nanoparticles into hydrogels, such as Fe3O4 and MnFe2O4 nanoparticles.19 Thus, conductivity and

Self-healing hydrogels have received much attention in recent years due to their automatic self-repair capability after damage; thus, they have been widely used in many fields, such as drug delivery,1 supercapacitor,2 and biomedical materials.3,4 A variety of polymers, such as chitosan,5,6 gelatin,7 agarose,8,9 guar gum,10 poly(ethylene glycol),11 poly(acrylic acid),12,13 and poly(vinyl alcohol) (PVA),14,15 are available for the preparation of self-healing hydrogels. Among these polymers, PVA, a nontoxic, environmentally friendly, biodegradable, and watersoluble polymer, is very suitable for the preparation of hydrogel by a simple process. For example, Zhang et al. prepared a PVA hydrogel using the freezing/thawing method and found that the PVA hydrogel can self-repair at room temperature without the need for any stimulus or healing agent.16 Such a simple preparation process made PVA a promising polymer for the preparation of a self-healing hydrogel in industry. In order to broaden the application field of the self-healing hydrogel, some important properties have been imparted to the hydrogel. For example, conductivity was necessary for many © 2018 American Chemical Society

Received: January 14, 2018 Revised: March 26, 2018 Published: April 4, 2018 6395

DOI: 10.1021/acssuschemeng.8b00193 ACS Sustainable Chem. Eng. 2018, 6, 6395−6403

Research Article

ACS Sustainable Chemistry & Engineering

Table 1. Amounts of the NFC, Aniline, FeCl3·6H2O, MnSO4.H2O, PVA, and Borax Used in the Preparation of the Multifunctional PVA Hydrogels and Their Conductivities sample

NFC (wt%)

aniline (wt%)

FeCl3.6H2O + MnSO4·H2O (wt%)

PVA (wt%)

borax (wt%)

conductivity (S·cm−1)

PVA-0 PVA-1 PVA-2 PVA-3

0 3 4 5

0 6.87 9.16 11.45

0 30.5 40.6 50.75

5 5 5 5

0.2 0.2 0.2 0.2

4.3 × 10−3 ± 8.1 × 10−5 5.43 × 10−3 ± 2.3 × 10−4 8.15 × 10−3 ± 1.6 × 10−4

were dissolved in 2 mL of distilled water and then added into 40 g of a 0.5% (w/w) NFC suspension. The mixture was cooled to 4 °C and stirred at a speed of 200 r/min for 30 min. About 0.286 g of ammonium persulfate was dissolved in 2 mL of distilled water and added dropwise into the above NFC mixture. After being reacted for 30 min, the aniline was polymerized and coated on the NFC surface. The NFC/PAni composites were obtained by centrifugation and washing with distilled water three times. The MnFe2O4 NPs were synthesized by the chemical coprecipitation method.25 First, about 1.55 g of FeCl3·6H2O and 0.48 g of MnSO4·H2O (the molar ratio of Fe:Mn is 2:1) were dissolved in the 0.5% (w/w) NFC/PAni composites and heated to 80 °C for 3 h. Then an 8 mol/L NaOH solution was added to the above mixture to adjust the pH to about 10.5. After being reacted for 10 min at 80 °C, the mixture was cooled to room temperature. The NFC/PAni/ MnFe2O4 nanocomposites were obtained by centrifugation and washing with distilled water three times. Preparation of the Multifunctional PVA Hydrogel. About 0.5 g of PVA powder was dissolved in 9.5 mL of distilled water with continuous stirring at 95 °C until the PVA was completely dissolved. Then, the NFC/PAni/MnFe2O4 nanocomposites were added to the PVA solution. The mixture was kept at 95 °C and stirred at a speed of 200 r/min until the NFC/PAni/MnFe2O4 nanocomposites were well dispersed in the PVA solution. About 0.02 g of borax was dissolved in 2 mL of distilled water and added to the PVA solution. After the mixture was cooled to room temperature, the multifunctional PVA hydrogels were formed in several minutes. The amounts of the NFC, aniline, FeCl3·6H2O, MnSO4·H2O, PVA, and borax used in the preparation of the multifunctional PVA hydrogels are listed in Table 1. Characterization of the NFC/PAni/MnFe2O4 Nanocomposites. In order to observe the NFC/PAni/MnFe2O4 nanocomposites, the morphology of the NFC/PAni and NFC/PAni/MnFe2 O 4 nanocomposites was investigated using a scanning electron micrograph (Nova Nano SEM 230, FEI) at an accelerating voltage of 5 kV. Prior to observation, a drop of sample was deposited on a monocrystalline silicon piece and allowed to dry at room temperature. The surface of each sample was then coated with gold on an ion sputter coater. The FTIR spectra of the NFC and NFC/PAni composites were studied using Fourier transform infrared spectroscopy (Thermo Scientific Nicolet iS50) at a resolution of 4 cm−1 in the spectral region of 500−4000 cm−1. Each sample was mixed with KBr powder and pressed into pellets. The zeta potential of the NFC and NFC/PAni nanocomposites was measured using a Malvern zeta sizer Nano ZS90 (UK). Rheological Characterization of the Multifunctional PVA Hydrogel. All the rheological experiments were performed using a MARS III Haake rheometer (Thermo Scientific, Germany) with a parallel plate system (diameter: 35 mm). Frequency sweep tests were carried out from 0.01 to 10 Hz with the strain of 1% at 25 °C. For time sweep tests, the PVA-3 was first cut into 4 pieces using a knife and then self-healed to form one single hydrogel, the storage modulus G′ and loss modulus G′′ of the original and the healed PVA-3 were measured at a frequency of 1.0 Hz and strain of 1%, respectively. Characterization of the Multifunctional PVA Hydrogel. In order to observe the NFC/PAni/MnFe2O4 nanocomposites incorporated in the PVA hydrogel, the morphology of the cross-section of the multifunctional PVA hydrogel and the pure PVA hydrogel was investigated using a scanning electron micrograph (Nova Nano SEM 230, FEI) at an accelerating voltage of 5 kV. All of the samples were

magnetism can be seen as two different kinds of important and useful properties for the hydrogel, it would be greatly expected to develop multifunctional self-healing hydrogels for a wider variety of applications. In general, it was difficult to develop stable multifunctional self-healing hydrogels by adding the magnetic nanoparticles and functional polymer into hydrogels directly, because the magnetic nanoparticles and functional polymer tended to aggregate in the nanocomposites due to the electrostatic interactions or hydrogen bonds between them.20 In our previous studies, we have prepared triclosan or Fe 3 O 4 nanoparticles in the templates of nanofibrillated cellulose (NFC) or cellulose nanocrystals (CNC) and found that the function polymer (triclosan) or magnetic nanoparticles (Fe3O4) can be well dispersed and kept stable in solution, due to the network structure formed by nanocellulose and electrostatic repulsion caused by nanocellulose.21,22 Therefore, the nanocellulose with network structure and negative charge should be an effective template for the preparation of stable nanocomposites containing both a function polymer and magnetic nanoparticles. In order to impart conductive and magnetic properties to the PVA hydrogel at the same time, the NFC with biocompatibility, biodegradability, and high specific strength was used as the substrate and template. The polyaniline (PAni) was coated on the NFC surface by the facile in situ chemical polymerization, and the magnetic MnFe2O4 nanoparticles (NPs) were synthesized with the NFC as the template by the chemical co-precipitation method. The NFC functioned as the carrier and dispersant to prevent the MnFe2O4 NPs from aggregating. The stable NFC/PAni/MnFe2O4 nanocomposites were incorporated into the PVA hydrogel and imparted both conductive and magnetic properties to the self-healing PVA hydrogel. Although some composites with both conductive and magnetic properties had been developed in the previous research, such as nanoparticle NiZnFe2O4/PPy composites prepared by Saafan et al.,23 the self-healing hydrogel with both conductive and magnetic properties was rare. Therefore, the novel self-healing hydrogel with conductivity and magnetism prepared in this study can be used in many new applications, such as electrochemical display devices, rechargeable batteries, and electromagnetic interference shielding.23

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EXPERIMENTAL SECTION

Materials. NFC oxidized by TEMPO-mediated oxidation (1.5%, w/w) was from Tianjin Haojia Cellulose Co., Ltd. (China). Aniline, ammonium persulfate, phytic acid (70%, w/w in water), poly(vinyl alcohol) (PVA) (DS: 1700 ± 50, Mw: ∼75000 g/mol), borax (sodium tetraborate decahydrate), ferric chloride hexahydrate (FeCl3.6H2O), and manganese sulfate monohydrate (MnSO4.H2O) were purchased from Aladdin Reagent Co., Ltd. (China). All other chemicals were of analytical grade and used without further purification. Preparation of the NFC/PAni/MnFe2O4 Nanocomposites. The polyaniline was synthesized according to the reference with some modification.24 First, 0.658 mL of phytic acid and 0.458 mL of aniline 6396


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Figure 1. (a) Schematic illustration of the preparation process of the multifunctional PVA hydrogel via in situ chemical polymerization and chemical co-precipitation (photographs show the NFC, NFC/PAni composites, NFC/PAni/MnFe2O4 nanocomposites, and multifunctional PVA hydrogel). (b) Synthesis of the polyaniline on the NFC surface via in situ chemical polymerization. immersed in liquid nitrogen and cryo-fractured. The fractured surface of each sample was then coated with gold on an ion sputter coater. The magnetic property of the multifunctional PVA hydrogel was measured using a vibrating sample magnetometer (Quantum Design PPMS-9T, USA) at 300 K. The conductivity of the multifunctional PVA hydrogel with 30 (l) × 10 (w) × 1 mm (t) was tested using a four-point probe. For each sample, at least five replicates were tested, and the results were presented as the average of the tested samples. The conductivity σ in S·cm−1 was calculated by eq 1:17,26

σ=

1 Rt

persulfate. It should be noted that the TEMPO-oxidized NFC has a negative surface and can be well dispersed in solution because of the electrostatic repulsion.21 After the polyaniline is coated on the NFC surface, the surface charge of NFC should be changed. According to the measurement of the zeta potential of the NFC before and after polyaniline coating, it was found that the TEMPO-oxidized NFC before coating possessed a negative zeta potential of −51.2 mV. After polyaniline was coated on, the zeta potential increased to −14.1 mV. Although the polyaniline coating resulted in the increase of the zeta potential, the NFC/PAni composites also kept a negative surface in solution; thus, they can be well dispersed in solution due to the electrostatic repulsion (Figure 1a). A similar trend of the zeta potential could also be found when cellulose nanocrystals were coated with the polypyrrole. Wu et al. prepared conductive cellulose nanocrystals (C− CNCs) by the polymerization of polypyrrole (PPy) on individual CNC surfaces via a facile in situ chemical polymerization technique and found that the coating of PPy on the CNC surface led to the increase in the zeta potential of cellulose nanocrystals from −41 to −29 mV.28 In addition, the NFC can also function as the carrier for loading MnFe2O4 NPs, because NFC with a web-like network structure can prevent the nanoparticles from aggregating. In this study, the magnetic MnFe2O4 NPs were synthesized by the chemical co-precipitation method in the presence of NFC. It can be seen from Figure 1a that NFC prevented the MnFe2O4 NPs from aggregating, and the NFC/PAni/MnFe2O4 nanocomposites can be well dispersed in solution. Figure 2a,b showed the SEM images of NFC/PAni composites and NFC/ PAni/MnFe2O4 composites, respectively. It can be seen from Figure 2a that several nanofibrillated cellulose fibers were

(1)

where R is the resistance of the sample, and t is the thickness of the sample.

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RESULTS AND DISCUSSION Preparation of the Multifunctional PVA Hydrogel. The preparation process of the multifunctional hydrogel is illustrated in Figure 1a. First, the TEMPO-oxidized NFC was used as the substrate, and the polyaniline was synthesized via in situ chemical polymerization and coated on the NFC surface. The synthesis of the polyaniline on the NFC surface is illustrated in Figure 1b. According to the oxidation by TEMPOmediated oxidation, the carboxylic groups were introduced to the NFC surface;27 thus, the aniline monomers can be adsorbed on the NFC surface by the hydrogen bonding between the amino groups on the aniline and the carboxylic groups on NFC. Furthermore, more aniline monomers may be adsorbed on the NFC through the electrostatic adsorption between the aniline and NFC because of the protonation of the aniline monomer. The polyaniline was synthesized on the NFC surface via in situ chemical polymerization in the presence of ammonium 6397


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nanoparticles in the hydrogel obstructed the interconnections between PVA chains. FTIR Analysis of the NFC/PAni Composites. The FTIR spectra of the NFC and NFC/PAni nanocomposites are shown in Figure 3. For the spectrum of NFC, some typical bands of

Figure 3. FTIR spectra of (a) NFC and (b) NFC/PAni composites.

TEMPO-oxidized cellulose can be clearly observed. For instance, the characteristic peaks at 3333, 2900, and 1600 cm−1 correspond to the O−H stretching, C−O stretching, and CO stretching, respectively.28 After PAni was coated on the NFC surface, two characteristic peaks at 1568 and 1482 cm−1 corresponding to the stretching vibration of the quinoid ring and benzenoid ring, respectively, in the PAni can be found.24 It was noted that the characteristic peak (CO stretching) was shifted from 1600 to 1606 cm−1, and the peak at 3340 cm−1 (N−H stretching vibration) replaced the peak at 3333 cm−1 (O−H stretching) after the coating of PAni on the NFC surface. These changes of peaks indicated that the combination of PAni and NFC was closely related to the hydrogen bonding between the carboxylic groups of NFC and the amine groups of PAni. A similar phenomenon was also found when cellulose nanocrystals were coated with the polypyrrole.28 Rheological and Tensile Properties of the Multifunctional PVA Hydrogel. The multifunctional PVA hydrogel prepared in this study was very soft and like “plasticine”. As shown in Figure 4a, butterfly and elephant-like multifunctional PVA hydrogels were casted from the molds. The as-prepared composite hydrogel was so soft that it could not be clamped or tightened on the tensile strength tester. So the tensile properties of the hydrogel were only tested “by hand”. Figure 4b,c shows the toughness of the pure and multifunctional PVA hydrogels by the tensile experiments. It can be seen from Figure 4b that no more than 2 cm of the elongation can be reached for the pure PVA hydrogel sample with a length of about 1 cm after the sample was stretched by hand. By contrast, the elongation of the PVA-3 sample with a length of about 1 cm can reach 10 cm without fracture in the same process of stretch (Figure 4c), indicating the improvement of the tensile property of the PVA hydrogel after the incorporation of the NFC/PAni/MnFe2O4 nanocomposites. This may be due to the cross-linking between the PVA and NFC in the hydrogel. Similar results were also found in the previous study reported by Lu et al., who prepared

Figure 2. SEM images of (a) NFC/PAni composites and (b) NFC/ PAni/MnFe2O4 composites. SEM images of the cross sections of the (c) pure PVA hydrogel and (d) multifunctional hydrogel (PVA-3).

coated with a thin film (PAni). After MnFe2O4 NPs were loaded, a lot of nanoparticles could be found and dispersed on the NFC surface in the NFC/PAni/MnFe2O4 composites (Figure 2b). Finally, the NFC/PAni/MnFe2O4 nanocomposites were incorporated into the PVA hydrogel for imparting both conductive and magnetic properties to the PVA hydrogel. The SEM images of the cross sections of the pure PVA hydrogel and the multifunctional PVA hydrogel (PVA-3) are shown in Figure 2. It can be seen from Figure 2c that the crosssection of the pure PVA hydrogel presented a relatively smooth texture. With the incorporation of the NFC/PAni/MnFe2O4 nanocomposites, the NFC with a very clear network structure can be seen from the cross-section of the multifunctional PVA hydrogel (Figure 2d). It should be noted that the NFC with the network structure seen from the multifunctional hydrogel exhibited a very rough surface. This is due to the coating and loading of the polyaniline and MnFe2O4 nanoparticles on the NFC surface. Pure PVA hydrogels were, in general, not soluble in water. However, the as-prepared multifunctional PVA hydrogel was found to be soluble and dispersed in water in a short time. This may be due to the reason that the MnFe2O4 6398


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Figure 4. (a) Reformability of the multifunctional PVA hydrogel (PVA-3). (b) Tensile properties of the pure PVA hydrogel (PVA-0) and (c) the multifunctional PVA hydrogel (PVA-3). (d) Storage modulus G′ and loss modulus G′′ of PVA-0, PVA-1, PVA-2, and PVA-3 versus frequency.

Figure 5. (a) A piece of multifunctional PVA hydrogel (PVA-3) was held (left panel) and bent in response to the magnet (right panel). (b) The hysteresis loop of the multifunctional PVA hydrogel (PVA-3) at 300 K. 6399


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Figure 6. Multifunctional PVA hydrogel (PVA-3) used as the conductive bulk in a simple circuit through the cutting/healing operations.

Conductive Property of the Multifunctional PVA Hydrogel. The conductivities of the multifunctional PVA hydrogels incorporated with different amounts of the NFC/ PAni/MnFe2O4 nanocomposites have been tested using the four-probe method. As can be seen in Table 1, the conductivity of the PVA-1 hydrogel was only 4.3 × 10−3 S·cm−1 when the loading amount of PAni in the hydrogel (0.75 mmol/g hydrogel) was low. However, the conductivity of the hydrogel increased significantly with the increase of the loading amount of PAni. Finally, the conductivity of the PVA-3 hydrogel can achieve to be 8.15 × 10−3 S·cm−1 when the loading amount of PAni in the hydrogel is 1.25 mmol/g. The conductivity of the hydrogel has achieved or exceeded the conductivities of some hydrogels containing PAni found in the previous studies, such as the conductive hemicellulose hydrogels (1.12 × 10−6 S· cm−1)31 and the polyacrylate/polyaniline hydrogels (2.33 × 10−3 S·cm−1).32 The excellent conductive property of the multifunctional PVA hydrogel can be ascribed to the reason that the NFC coated with the conductive polyaniline possessed network structure and can be well dispersed in the PVA hydrogel, thus providing a continuous transporting path for electrons.33 Self-Healing Property of the Multifunctional PVA Hydrogel. In addition to the magnetic and conductive properties, the self-healing property is another significant function for the multifunctional PVA hydrogel. In order to demonstrate the self-healing property of the multifunctional PVA hydrogel, the PVA-3 was used as a conductive bulk in a simple circuit, in which a LED bulb as the indicator light and two dry batteries (1.5 V) as the power source were included.33,34 It can be seen from Figure 6 that the LED bulb can be lighted when the PVA-3 was used as a conductive bulk material to make the circuit close. After the first cut of the PVA3 to make the circuit open, the LED bulb was extinguished. The first self-healing test was carried out by putting the two pieces of the PVA-3 together at room temperature without any external treatment. After about 3 min, the damaged region in the PVA-3 can be self-healed and form one single hydrogel

the microfibrillated cellulose (MFC)-reinforced PVA-borax hydrogels and found the tensile property of the hydrogel was improved significantly after adding the MFC.29 In addition, the rheological properties of the pure PVA hydrogel and the multifunctional hydrogels containing different contents of the NFC/PAni/MnFe2O4 nanocomposites were investigated and are shown in Figure 4d. It can be seen that the storage moduli G′ was significantly greater than the loss modulus G′′ at high frequency for all of the samples, and the G′ and G′′ curves of all of the samples exhibited almost frequency independent trends, indicating all of the prepared hydrogels exhibited an elastic gel-like character. Compared with PVA-0, the G′ and G′′ of PVA-1 decreased significantly when a low amount of the NFC/PAni/MnFe2O4 nanocomposites was added, due to the reason that the MnFe2O4 nanoparticles may obstruct the cross-linking of the PVA and borax. However, the G′ and G′′ values increased with the increase of the nanocomposites content, because of the entanglement and cross-linking performance of NFC in the PVA hydrogels.29 It was also found that the G′ nearly approached the G′′ at low frequency for the multifunctional hydrogels, indicating there was a typical cross-linked network in the multifunctional hydrogels.29,30 Magnetic Property of the Multifunctional PVA Hydrogel. After MnFe2O4 NPs were loaded, the multifunctional PVA hydrogel possessed magnetism. Figure 5a shows that a piece of multifunctional PVA hydrogel (PVA-3) can be magnetically actuated by a small household magnet. In addition, the magnetic property of the multifunctional PVA hydrogel was measured at 300 K. As shown in Figure 5b, the hydrogel exhibited a typical hysteresis loop in the magnetic behavior, and the maximum saturation magnetization (Ms) of the PVA hydrogel was found to be 5.22 emu·g−1, which was lower than that of the pure MnFe2O4 NPs (Ms: 19.3 emu·g−1 at 300 K) .25 This was mainly due to the fact that the MnFe2O4 content of the multifunctional PVA hydrogel (PVA-3) was much lower than that of the pure MnFe2O4 NPs. 6400


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Figure 7. (a) Photographs of the self-healing process of PVA-3. The G′ and G′′ of (b) the original PVA-3 and (c) the self-healed PVA-3 in a time sweep test (frequency: 1.0 Hz; strain: 1%). (d) The self-healing mechanism of the multifunctional PVA hydrogel (PVA-3).

autonomously, making the LED bulb light up again. Then, the second and third cutting/self-healing tests were carried out in sequence, and the results showed that the PVA-3 can be selfhealed and recover to almost the initial state of the original hydrogel. Actually, no matter how many times the multifunctional PVA hydrogels were damaged, they could be self-healed completely after putting the pieces of the hydrogel together for several minutes at room temperature, demonstrating the excellent self-healing property of the multifunctional PVA hydrogel. To further investigate the self-healing property of the multifunctional PVA hydrogel, the rheological test was carried out to analyze the self-healing process of the hydrogel, and the results are shown in Figure 7b,c. It can be seen from Figure 7b that the storage modulus G′ was much greater than the loss modulus G′′ for the original PVA-3, indicating the PVA-3 possessed the elastic gel-like character. After the PVA-3 was cut into 4 pieces and then self-healed to form a single one (Figure 7a), the storage modulus G′ and loss modulus G′′ of the healed PVA-3 were found to recover to similar values to that of the original PVA-3 (Figure 7c), indicating the complete recovery of the inner network structure of the multifunctional PVA hydrogel.29,30

The self-healing mechanism of the multifunctional PVA hydrogel is illustrated in Figure 7d. First, the borax played a key role for the self-healing property of the PVA hydrogel, because the borax and PVA can form the didiol-borax complex by a cross-linking reaction and resulted in the gelation of the PVA solution.29 In addition, the hydrogen bonding between PVA− PVA and PVA−NFC also contributed to the formation of the PVA hydrogel. After the multifunctional PVA hydrogel was cut into several pieces, some hydrogen bonds and didiol-borax complexes were broken.35 When the pieces of the PVA hydrogel were put together, some of these broken hydrogen bonds and didiol-borax complexes could reform in a short time, as well as some new hydrogen bonds and didiol-borax complexes may form, resulting in the accomplishment of the self-healing process of the multifunctional PVA hydrogel. It should be noted that the PVA hydrogel exhibited excellent conductivity after it was self-healed, according to Figure 6. This may be due to the 3D continuous network nanostructure formed by the PAni in the hydrogel, according to the research by Pan et al.,24 and the unique nanostructure of PAni resulted in not only the superior conductivity performance but also the fast recovery of the interconnections between PAni chains during the healing process of the hydrogel. 6401


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CONCLUSIONS In summary, an ultrasoft and self-healing PVA hydrogel with conductive and magnetic properties was prepared using NFC as the substrate by a facile approach. Polyaniline was coated on the NFC surface, and the MnFe2O4 nanoparticles were precipitated on the NFC. The well-dispersed NFC/PAni/ MnFe 2 O 4 nanocomposites were used to impart both conductive and magnetic properties to the self-healing PVA hydrogel. The maximum saturation magnetization and conductivity of the multifunctional PVA hydrogel were found to be 5.22 emu·g−1 and 8.15 × 10−3 S·cm−1, respectively. Furthermore, the PVA hydrogel exhibited an excellent selfhealing property, which can be self-healed completely after the pieces of the hydrogel were put together for several minutes at room temperature. Thus, the novel multifunctional PVA hydrogel demonstrated the significant potential for many practical applications, such as electrochemical display devices, rechargeable batteries, and electromagnetic interference shielding. More importantly, nanofibrillated cellulose is sustainable and easy to prepare in large quantities from wood and other plants by simple methods.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kai Liu: 0000-0001-7833-2839 Liulian Huang: 0000-0003-3158-593X Yonghao Ni: 0000-0001-6107-6672 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the national key research and development plan (Grant 2017YFB0307900). REFERENCES

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