Ultrasoft Self-Healing Nanoparticle-Hydrogel Composites with

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Ultra-soft 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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00193 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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ACS Sustainable Chemistry & Engineering

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Ultra-soft self-healing nanoparticle-hydrogel composites with

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conductive and magnetic properties

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Kai Liu a,*, Xiaofeng Pan a, Lihui Chen a, Liulian Huang a, Yonghao Ni a,b, Jin Liu a,

4

Shilin Cao a, Hongping Wang c

5

a

Shangxiadian Road, Cangshan District, Fuzhou City, Fujian Province 350002, China

6 7

b

10

Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick E3B5A3, Canada

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College of Material Engineering, Fujian Agriculture and Forestry University, No. 15

c

Jinshan College, Fujian Agriculture and Forestry University, No. 15 Shangxiadian Road, Cangshan District, Fuzhou City, Fujian Province 350002, China

11 12

Corresponding author:

13

Kai Liu

14

College of Material Engineering, Fujian Agriculture and Forestry University, No. 15

15

Shangxiadian Road, Cangshan District, Fuzhou City, Fujian Province 350002, China.

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E-mail: [email protected]

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ACS Paragon Plus Environment

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Abstract: Recently, integration of two or more important properties into a hydrogel

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has been a challenge in the preparation of the multi-functional hydrogel. Herein, in

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order to impart conductive and magnetic properties to the self-healing PVA hydrogel

27

at the same time, the nanofibrillated cellulose (NFC) was used as substrates. The

28

polyaniline was coated on the NFC surface by in situ chemical polymerization and the

29

MnFe2O4 nanoparticles were synthesized and loaded on the NFC by the chemical

30

co-precipitation method. The multi-functional PVA hydrogel was prepared by

31

incorporating the NFC/PAni/MnFe2O4 nanocomposites with the PVA hydrogel. The

32

magnetic and conductive properties tests of the multi-functional PVA hydrogel

33

showed that the maximum saturation magnetization and conductivity were 5.22

34

emu.g-1 and 8.15×10-3 S.cm-1, respectively. Moreover, the multi-functional PVA

35

hydrogel exhibited excellent self-healing and ultra-soft properties, which can be

36

self-healed completely after the pieces of the hydrogel were put together for several

37

minutes at room temperature. Due to the self-healing ability, conductivity, and

38

magnetism, the novel hydrogel was expected to be used in many practical applications

39

such as electrochemical display devices, rechargeable batteries, and electromagnetic

40

interference shielding. More importantly, we proved a facile template approach to the

41

preparation of stable polymer and nanoparticle composites using NFC as substrates

42

that imparted different properties to hydrogels.

43

Keywords: self-healing, hydrogel, nanocomposite, conductivity, magnetism

44 45


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Introduction

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Self-healing hydrogels have received much attention in recent years due to their

48

automatic self-repair capability after damage, thus have been widely used in many

49

fields, such as drug delivery,1 supercapacitor,2 and biomedical materials.3, 4 A variety

50

of polymers, such as chitosan,5,

51

glycol) ,11 poly(acrylic acid) ,12, 13 and poly(vinyl alcohol) (PVA) 14, 15 are available for

52

the preparation of self-healing hydrogels. Among these polymers, PVA, a nontoxic,

53

environmentally friendly, biodegradable and water-soluble polymer, is very suitable

54

for the preparation of hydrogel by a simple process. For example, Zhang et al.

55

prepared poly(vinyl alcohol) (PVA) hydrogel using the freezing/thawing method and

56

found that the PVA hydrogel can self-repair at room temperature without the need for

57

any stimulus or healing agent.16 Such simple preparation process made PVA a

58

promising polymer for the preparation of self-healing hydrogel in industry.

6

gelatin,7 agarose,8,

9

guar gum,10 poly(ethylene

59

In order to broaden the application field of the self-healing hydrogel, some

60

important properties have been imparted to the hydrogel. For example, conductivity

61

was necessary for many functional polymer materials, and the most commonly used

62

conductive polymers were polyaniline and polypyrrole due to the simple

63

polymerization process. Recently, a few self-healing hydrogels with conductivity have

64

been successfully prepared by some researchers. Hur et al. developed a new class of

65

moldable, stretchable, and self-healable conductive hydrogels by utilizing an

66

oxidizing agent as a mediator to induce homogeneous polymerization of the pyrrole

67

monomer inside the three-dimensional agarose hydrogel network.17 In addition to the



68

conductivity, magnetism was another important property for the hydrogel because

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magnetic hydrogels were highly sensitive to response in magnetic fields, thus most

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magnetic hydrogels have great potential in biomedical applications.18 Most studies

71

have focused on developing magnetic hydrogels by incorporating magnetic

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nanoparticles into hydrogels, such as Fe3O4 and MnFe2O4 nanoparticles.19 Thus,

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conductivity and magnetism can be seen as two different kinds of important and

74

useful properties for the hydrogel, it would be greatly expected to develop

75

multi-functional self-healing hydrogels for a wider variety of applications.

76

In general, it was difficult to develop stable multi-functional self-healing

77

hydrogels by adding the magnetic nanoparticles and functional polymer into

78

hydrogels directly, because the magnetic nanoparticles and functional polymer tended

79

to aggregate in the nanocomposites due to the electrostatic interactions or hydrogen

80

bonds between them.20 In our previous studies, we have prepared triclosan or Fe3O4

81

nanoparticles in the templates of nanofibrillated cellulose (NFC) or cellulose

82

nanocrystals (CNC) and found that the function polymer (triclosan) or magnetic

83

nanoparticles (Fe3O4) can be well dispersed and kept stable in solution due to the

84

network structure formed by nanocellulose and electrostatic repulsion caused by

85

nanocellulose.21, 22 Therefore, the nanocellulose with network structure and negative

86

charge should be an effective template for the preparation of stable nanocomposites

87

containing both function polymer and magnetic nanoparticles.

88

In order to impart conductive and magnetic properties to the PVA hydrogel at the

89

same time, the nanofibrillated cellulose (NFC) with biocompatibility, biodegradability,


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90

and high specific strength was used as substrates and templates. The polyaniline (PAni)

91

was coated on the NFC surface by the facile in situ chemical polymerization and the

92

magnetic MnFe2O4 nanoparticles (NPs) were synthesized with the NFC as templates

93

by the chemical co-precipitation method. The NFC functioned as carriers and

94

dispersants

95

NFC/PAni/MnFe2O4 nanocomposites were incorporated into the PVA hydrogel, and

96

imparting both conductive and magnetic properties to the self-healing PVA hydrogel.

97

Although some composites with both conductive and magnetic properties have be

98

developed in the previous research, such as nanoparticle NiZnFe2O4/PPy composites

99

prepared by Saafan et al.,23 the self-healing hydrogel with both conductive and

100

magnetic properties was rare. Therefore, this novel self-healing hydrogel with

101

conductivity and magnetism prepared in this study can be used in many new

102

applications, such as electrochemical display devices, rechargeable batteries, and

103

electromagnetic interference shielding. 23

104

Experimental section

105

Materials

to

prevent

the

MnFe2O4 NPs

from

aggregating.

The

stable

106

Nanofibrillated cellulose (NFC) oxidized by TEMPO-mediated oxidation (1.5%,

107

w/w) was from Tianjin Haojia Cellulose Co., Ltd (China). Aniline, ammonium

108

persulfate, phytic acid (70%, w/w in water), poly(vinyl alcohol) (PVA) (DS:1700±50,

109

Mw:~75000 g/mol), borax (sodium tetraborate decahydrate), ferric chloride

110

hexahydrate (FeCl3.6H2O), and manganese sulfate monohydrate (MnSO4.H2O) were

111

purchased from Aladdin reagent Co., Ltd (China). All other chemicals were of



112

analytical grade and used without further purification.

113

Preparation of the NFC/PAni/ MnFe2O4 nanocomposites

114

The polyaniline was synthesized according to the reference with some

115

modification.24 Firstly, 0.658 mL phytic acid and 0.458 mL aniline were dissolved in 2

116

mL distilled water, and then added into 40 g of 0.5% (w/w) NFC suspension. The

117

mixture was cooled to 4 °C and stirred at a speed of 200 r/min for 30 min. About

118

0.286 g ammonium persulfate was dissolved in 2 mL distilled water and dropwise

119

added into the above NFC mixture. After reaction for 30 min, the aniline was

120

polymerized and coated on the NFC surface. The NFC/PAni composites were

121

obtained by centrifuging and washing with distilled water for three times.

122

The MnFe2O4 NPs were synthesized by the chemical co-precipitation method.25

123

Firstly, about 1.55 g FeCl3.6H2O and 0.48 g MnSO4.H2O (the molar ratio of Fe:Mn is

124

2:1) were dissolved in the 0.5% (w/w) NFC/PAni composites and heated to 80 °C for

125

3 h. Then 8 mol/L NaOH solution was added to the above mixture to adjust the pH to

126

about 10.5. After reacting for 10 min at 80 °C, the mixture was cooled to room

127

temperature. The NFC/PAni/MnFe2O4 nanocomposites were obtained by centrifuging

128

and washing with distilled water for three times.

129

Preparation of the multi-functional PVA hydrogel

130

About 0.5 g PVA powder was dissolved in 9.5 mL distilled water with continuous

131

stirring at 95 °C until the PVA was completely dissolved. Then the

132

NFC/PAni/MnFe2O4 nanocomposites were added to the PVA solution. The mixture

133

was kept at 95 °C and stirred at a speed of 200 r/min until the NFC/PAni/MnFe2O4


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134

nanocomposites were well dispersed in PVA solution. About 0.02 g borax was

135

dissolved in 2 mL distilled water and added to the PVA solution. After the mixture

136

was cooled to room temperature, the multi-functional PVA hydrogels were formed in

137

several minutes. The amounts of the NFC, aniline, FeCl3.6H2O, MnSO4.H2O, PVA,

138

and borax used in the preparation of the multi-functional PVA hydrogels were listed in

139

Table 1.

140

Characterization of the NFC/PAni/MnFe2O4 nanocomposites

141

In order to observe the NFC/PAni/MnFe2O4 nanocomposites, the morphology of

142

the NFC/PAni and NFC/PAni/MnFe2O4 nanocomposites was investigated using a

143

scanning electron micrograph (Nova Nano SEM 230, FEI) at an accelerating voltage

144

of 5 kV. Prior to observation, a drop of sample was deposited on a monocrystalline

145

silicon piece and allowed to dry at room temperature. The surface of each sample was

146

then coated with gold on an ion sputter coater.

147

The FTIR spectra of the NFC and NFC/PAni composites were studied using a

148

Fourier transform infrared spectroscopy (Thermo Scientific Nicolet iS50) at a

149

resolution of 4 cm-1 in the spectral region of 500-4000 cm-1. Each sample was mixed

150

with KBr powder and pressed to pellets.

151

The zeta potential of the NFC and NFC/PAni nanocomposites was measured using

152

a Malvern zeta sizer Nano ZS90 (UK).

153

Rheological characterization of the multi-functional PVA hydrogel

154

All the rheological experiments were performed using a MARS III Haake

155

rheometer (Thermo Scientific, Germany) with a parallel plate system (diameter:



156

35mm). Frequency sweep tests were carried out from 0.01 to 10 Hz with the strain of

157

1% at 25 °C. For time sweep tests, the PVA-3 was first cut into 4 pieces using a knife

158

and then self-healed to form one single hydrogel, the storage modulus G’ and loss

159

modulus G” of the original and the healed PVA-3 were measured at a frequency of 1.0

160

Hz and strain of 1%, respectively.

161

Characterization of the multi-functional PVA hydrogel

162

In order to observe the NFC/PAni/MnFe2O4 nanocomposites incorporated in the

163

PVA hydrogel, the morphology of the cross-section of the multi-functional PVA

164

hydrogel and the pure PVA hydrogel was investigated using a scanning electron

165

micrograph (Nova Nano SEM 230, FEI) at an accelerating voltage of 5 kV. All the

166

samples were immersed in liquid nitrogen and cryo-fractured. The fractured surface of

167

each sample was then coated with gold on an ion sputter coater.

168 169

The magnetic property of the multi-functional PVA hydrogel was measured using a vibrating sample magnetometer (Quantum Design PPMS-9T, USA) at 300 K.

170

The conductivity of the multi-functional PVA hydrogel with 30 (l) × 10 (w) × 1

171

mm (t) was tested using a four-point probe. For each sample, at least five replicates

172

were tested, and the results were presented as the average of the tested samples. The

173

conductivity σ in S.cm-1 was calculated by equation 1:17, 26

174

σ = ோ௧

175

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

ଵ

(1)

176 177


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178

Table 1. Amounts of the NFC, aniline, FeCl3.6H2O, MnSO4.H2O, PVA and borax used

179

in the preparation of the multi-functional PVA hydrogels and their conductivities. Sample

NFC

Aniline

FeCl3.6H2O+

PVA

Borax

Conductivity

(wt%)

(wt%)

MnSO4.H2O

(wt%)

(wt%)

(S.cm-1)

(wt%) PVA-0

0

0

0

5

0.2

-

PVA-1

3

6.87

30.5

5

0.2

4.3×10-3±8.1×10-5

PVA-2

4

9.16

40.6

5

0.2

5.43×10-3±2.3×10-4

PVA-3

5

11.45

50.75

5

0.2

8.15×10-3±1.6×10-4

180

Results and discussion

181

Preparation of the multi-functional PVA hydrogel

182

The preparation process of the multi-functional hydrogel was illustrated in Figure

183

1a. Firstly, the TEMPO-oxidized NFC was used as substrates and the polyaniline was

184

synthesized via in situ chemical polymerization and coated on the NFC surface. The

185

synthesis of the polyaniline on the NFC surface was illustrated in Figure 1b.

186

According to the oxidizing by TEMPO-mediated oxidation, the carboxylic groups

187

were introduced to the NFC surface,27 thus the aniline monomers can be adsorbed on

188

the NFC surface by the hydrogen bonding between the amino groups on aniline and

189

the carboxylic groups on NFC. Furthermore, more aniline monomers may be

190

adsorbed on the NFC through the electrostatic adsorption between aniline and NFC

191

because of the protonation of the aniline monomer. The polyaniline was synthesized

192

on the NFC surface via in situ chemical polymerization in the presence of ammonium

193

persulfate. It should be noted that the TEMPO-oxidized NFC has negative surface and



Page 10 of 27

194

can be well dispersed in solution because of the electrostatic repulsion.21 After coating

195

of the polyaniline on the NFC surface, the surface charge of NFC should be changed.

196

According to the measurement of zeta potential of the NFC before and after

197

polyaniline coating, it was found that the TEMPO-oxidized NFC before coating

198

possessed a negative zeta potential of -51.2 mV. After polyaniline coating, the zeta

199

potential increased to -14.1 mV. Although the polyaniline coating resulted in the

200

increase of the zeta potential, the NFC/PAni composites also kept negative surface in

201

solution, thus can be well dispersed in solution due to the electrostatic repulsion

202

(Figure 1a). Similar trend of zeta potential can also be found when cellulose

203

nanocrystals were coated with the polypyrrole. Wu et al. prepared conductive

204

cellulose nanocrystals (C-CNCs) by the polymerization of polypyrrole (PPy) on

205

individual CNC surfaces via a facile in situ chemical polymerization technique and

206

found that the coating of PPy on the CNC surface led to the increase in zeta potential

207

of cellulose nanocrystals from -41 to -29 mV.28

208

In addition, the NFC can also function as carriers for loading MnFe2O4 NPs

209

because NFC with a web-like network structure can prevent the nanoparticles from

210

aggregating. In this study, the magnetic MnFe2O4 NPs were synthesized by the

211

chemical co-precipitation method in the presence of NFC. It can be seen from Figure

212

1a

213

NFC/PAni/MnFe2O4 nanocomposites can be well dispersed in solution. Figure 2a,b

214

showed the SEM images of NFC/PAni composites and NFC/PAni/MnFe2O4

215

composites, respectively. It can be seen from Figure 2a that several nanofibrillated

that

NFC

prevented

the

MnFe2O4

NPs

from


aggregating

and

the

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216

cellulose fibers were coated with a thin film (PAni). After loading MnFe2O4 NPs, a lot

217

of nanoparticles can be found and dispersed on the NFC surface in the

218

NFC/PAni/MnFe2O4 composites (Figure 2b).

219

Finally, the NFC/PAni/MnFe2O4 nanocomposites were incorporated into the PVA

220

hydrogel for imparting both conductive and magnetic properties to the PVA hydrogel.

221

The SEM images of the cross-sections of the pure PVA hydrogel and the

222

multi-functional PVA hydrogel (PVA-3) were shown in Figure 2. It can be seen from

223

Figure 2c that the cross-section of the pure PVA hydrogel presented a relatively

224

smooth texture. With incorporation of the NFC/PAni/MnFe2O4 nanocomposites, the

225

NFC with a very clear network structure can be seen from the cross-section of the

226

multi-functional PVA hydrogel (Figure 2d). It should be noted that the NFC with

227

network structure seen from the multi-functional hydrogel exhibited a very rough

228

surface. This is due to the coating and loading of the polyaniline and MnFe2O4

229

nanoparticles on the NFC surface. Pure PVA hydrogels were in general not soluble in

230

water. However, the as-prepared multi-functional PVA hydrogel was found to be

231

soluble and dispersed in water in a short time, this may be due to the reason that the

232

MnFe2O4 nanoparticles in the hydrogel obstructed the interconnections between PVA

233

chains.

234 235



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236

237 238

Figure 1. (a) Schematic illustration of the preparation process of the multi-functional

239

PVA hydrogel via in situ chemical polymerization and chemical co-precipitation

240

(photographs

241

nanocomposites, and multi-functional PVA hydrogel); (b) Synthesis of the polyaniline

242

on the NFC surface via in situ chemical polymerization.

show

the

NFC,

NFC/PAni

composites,


NFC/PAni/MnFe2O4

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243

244 245

Figure 2. SEM images of (a) NFC/PAni composites and (b) NFC/PAni/MnFe2O4

246

composites. SEM images of the cross-sections of (c) pure PVA hydrogel and (d)

247

multi-functional hydrogel (PVA-3).



248

FTIR analysis of the NFC/PAni composites

249

The FTIR spectra of the NFC and NFC/PAni nanocomposites were shown in

250

Figure 3. For the spectrum of NFC, some typical bands of TEMPO-oxidized cellulose

251

can be clearly observed. For instance, the characteristic peaks at 3333, 2900, and 1600

252

cm−1 were corresponding to the O-H stretching, C-O stretching, and C=O stretching,

253

respectively.28 After coating of PAni on the NFC surface, two characteristic peaks at

254

1568 and 1482 cm−1 corresponding to the stretching vibration of the quinoid ring and

255

benzenoid ring, respectively in the PAni can be found.24 It was noted that the

256

characteristic peak (C=O stretching) was shifted from 1600 to 1606 cm-1, and the

257

peak at 3340 cm−1 (N-H stretching vibration) replaced the peak at 3333 cm−1 (O-H

258

stretching) after the coating of PAni on the NFC surface. These changes of peaks

259

indicated that the combination of PAni and NFC was closely related to the hydrogen

260

bonding between the carboxylic groups of NFC and the amine groups of PAni.

261

Similar phenomenon was also found when cellulose nanocrystals were coated with

262

the polypyrrole.28


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a b

(a)

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NFC NFC/PAni

2900 1600

3333

(b)

1606 1482 1568

3340

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumbers (cm ) 263 264

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

265

Rheological and tensile properties of the multi-functional PVA hydrogel

266

The multi-functional PVA hydrogel prepared in this study was very soft and like

267

“plasticine”. As shown in Figure 4a, a butterfly and elephant-like multi-functional

268

PVA hydrogels were casted from the molds. The as-prepared composite hydrogel was

269

so soft that it cannot be clamped or tightened on the tensile strength tester. So the

270

tensile properties of the hydrogel were only tested “by hand”. Figure 4b,c showed the

271

toughness of the pure and multi-functional PVA hydrogels by the tensile experiments.

272

It can be seen from Figure 4b that no more than 2 cm of the elongation can be reached

273

for the pure PVA hydrogel sample with a length of about 1 cm after the sample was

274

stretched by hand. By contrast, the elongation of the PVA-3 sample with a length of

275

about 1 cm can reach 10 cm without fracture in the same process of stretch (Figure

276

4c), indicating the improvement of the tensile property of the PVA hydrogel after the

277

incorporation of the NFC/PAni/MnFe2O4 nanocomposites. This may be due to the

278

cross-linking between the PVA and NFC in the hydrogel. Similar results were also



279

found in the previous study reported by Lu et al., who prepared the microfibrillated

280

cellulose (MFC) reinforced PVA-borax hydrogels and found the tensile property of

281

the hydrogel was improved significantly after adding the MFC.29

282

In addition, the rheological properties of the pure PVA hydrogel and the

283

multi-functional hydrogels containing different contents of the NFC/PAni/MnFe2O4

284

nanocomposites were investigated and shown in Figure 4d. It can be seen that the

285

storage moduli G’ was significantly greater than the loss modulus G” at high

286

frequency for all samples and the G’ and G” curves of all samples exhibited almost

287

frequency independent trends, indicating all the prepared hydrogels exhibited elastic

288

gel-like character. Compared with PVA-0, the G’ and G” of PVA-1 decreased

289

significantly when a low amount of the NFC/PAni/MnFe2O4 nanocomposites was

290

added, due to the reason that the MnFe2O4 nanoparticles may obstruct the

291

cross-linking of the PVA and borax. However, the G’ and G” values increased with the

292

increase of the nanocomposites content, because of the entanglement and

293

cross-linking performance of NFC in the PVA hydrogels.29 It was also found that the

294

G’ nearly approached to the G” at low frequency for the multi-functional hydrogels,

295

indicating there was a typical cross-linked network in the multi-functional

296

hydrogels.29, 30

297


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Page 17 of 27

298 PVA-0,G' PVA-0,G" PVA-1,G' PVA-1,G" PVA-2,G' PVA-2,G" PVA-3,G' PVA-3,G"

(d) 3000 2500

G', G" (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2000 1500 1000 500 0

0

2

4

6

8

10

Frequency (Hz)

299 300

Figure 4. (a) Re-formability of the multi-functional PVA hydrogel (PVA-3). (b)

301

Tensile properties of the pure PVA hydrogel (PVA-0) and (c) the multi-functional PVA

302

hydrogel (PVA-3). (d) Storage modulus G’ and loss modulus G” of PVA-0, PVA-1,

303

PVA-2, and PVA-3 versus frequency.

304

Magnetic property of the multi-functional PVA hydrogel

305

After loading of MnFe2O4 NPs, the multi-functional PVA hydrogel possessed

306

magnetism. Figure 5a showed that a piece of multi-functional PVA hydrogel (PVA-3)

307

can be magnetically actuated by a small household magnet. In addition, the magnetic

308

property of the multi-functional PVA hydrogel was measured at 300 K. As shown in



309

Figure 5b, the hydrogel exhibited a typical hysteresis loop in the magnetic behavior,

310

and the maximum saturation magnetization (Ms) of the PVA hydrogel was found to be

311

5.22 emu.g-1, which was lower than that of the pure MnFe2O4 NPs (Ms:19.3 emu.g-1

312

at 300 K) .25 This was mainly due to the fact that the MnFe2O4 content of the

313

multi-functional PVA hydrogel (PVA-3) was much lower than that of the pure

314

MnFe2O4 NPs.

315

(b) Magnetization (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6 4 2 0 -2 -4 -6 -80000

316

-40000

0

40000

80000

Magnetic Field (Oe)

317

Figure 5. (a) A piece of multi-functional PVA hydrogel (PVA-3) was held (left panel)

318

and bended in response to the magnet (right panel). (b) The hysteresis loop of the

319

multi-functional PVA hydrogel (PVA-3) at 300 K

320

Conductive property of the multi-functional PVA hydrogel

321

The conductivities of the multi-functional PVA hydrogels incorporated with

322

different amounts of the NFC/PAni/MnFe2O4 nanocomposites have been tested using


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323

the four-probe method. As can be seen in Table 1, the conductivity of the PVA-1

324

hydrogel was only 4.3×10-3 S.cm-1 when the loading amount of PAni in the hydrogel

325

(0.75 mmol/g hydrogel) was low. However, the conductivity of the hydrogel increased

326

significantly with the increase of the loading amount of PAni. Finally, the

327

conductivity of the PVA-3 hydrogel can achieve to 8.15×10-3 S.cm-1 when the loading

328

amount of PAni in the hydrogel was 1.25 mmol/g. The conductivity of the hydrogel

329

have achieved or exceeded the conductivities of some hydrogels containing PAni

330

found in the previous studies, such as the conductive hemicellulose hydrogels

331

(1.12×10-6 S.cm-1) 31 and the polyacrylate/polyaniline hydrogels (2.33×10-3 S.cm-1). 32

332

The excellent conductive property of the multi-functional PVA hydrogel can be

333

ascribed to the reason that the NFC coated with the conductive polyaniline possessed

334

network structure and can be well dispersed in the PVA hydrogel, thus providing

335

continuous transporting path for electrons.33

336

Self-healing property of the multi-functional PVA hydrogel

337

In addition to the magnetic and conductive properties, the self-healing property is

338

another significant function for the multi-functional PVA hydrogel. In order to

339

demonstrate the self-healing property of the multi-functional PVA hydrogel, the

340

PVA-3 was used as a conductive bulk in a simple circuit, in which a LED bulb as the

341

indicator light and two dry batteries (1.5 V) as the power source were included.33, 34 It

342

can be seen from Figure 6 that the LED bulb can be lighted when the PVA-3 was used

343

as a conductive bulk material to make the circuit close. After the first cut of the

344

PVA-3 to make the circuit open, the LED bulb was extinguished. The first self-healing

345

test was carried out by putting the two pieces of the PVA-3 together at room



346

temperature without any external treatment. After about 3 min, the damage region in

347

the PVA-3 can be self-healed and formed one single hydrogel autonomously, making

348

the LED bulb lighted up again. Then the second and third cutting/self-healing tests

349

were carried out in sequence, and the results showed that the PVA-3 can be

350

self-healed and recover to almost the initial state of the original hydrogel. Actually, no

351

matter how many times the multi-functional PVA hydrogels were damaged, they can

352

be self-healed completely after putting the pieces of the hydrogel together for several

353

minutes at room temperature, demonstrating the excellent self-healing property of the

354

multi-functional PVA hydrogel.

355

To further investigate the self-healing property of the multi-functional PVA

356

hydrogel, the rheological test was carried out to analyze the self-healing process of the

357

hydrogel and the results were shown in Figure 7b,c. It can be seen from Figure 7b that

358

the storage modulus G’ was much greater than the loss modulus G” for the original

359

PVA-3, indicating the PVA-3 possessed the elastic gel-like character. After the PVA-3

360

was cut into 4 pieces and then self-healed to form a single one (Figure 7a), the storage

361

modulus G’ and loss modulus G” of the healed PVA-3 were found to recover to

362

similar values to that of the original PVA-3 (Figure 7c), indicating the complete

363

recovery of the inner network structure of the multi-functional PVA hydrogel.29, 30

364

The self-healing mechanism of the multi-functional PVA hydrogel can be

365

illustrated in Figure 7d. Firstly, the borax played a key role for the self-healing

366

property of the PVA hydrogel, because the borax and PVA can form the didiol-borax

367

complex by a cross-linking reaction and resulted in the gelation of PVA solution.29 In


Page 20 of 27

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368

addition, the hydrogen bonding between PVA-PVA and PVA-NFC also contributed to

369

the formation of the PVA hydrogel. After the multi-functional PVA hydrogel was cut

370

into several pieces, some hydrogen bonds and didiol-borax complexes were broken.35

371

When the pieces of the PVA hydrogel were put together, some of these broken

372

hydrogen bonds and didiol-borax complexes can reform in a short time, as well as

373

some new hydrogen bonds and didiol-borax complexes may form, resulting in the

374

accomplishment of the self-healing process of the multi-functional PVA hydrogel. It

375

should be noted that the PVA hydrogel exhibited excellent conductivity after it was

376

self-healed according to Figure 6. This may be due to the 3D continuous network

377

nanostructure formed by the PAni in the hydrogel according to the research by Pan et

378

al.,

379

conductivity performance but also the fast recovery of the interconnections between

380

PAni chains during the healing process of the hydrogel.

24

and the unique nanostructure of PAni resulted in not only the superior



381 382

Figure 6. The multi-functional PVA hydrogel (PVA-3) using as the conductive bulk in

383

a simple circuit through the cutting/healing operations.

384


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385 386

Figure 7. (a) Photographs of the self-healing process of PVA-3. The G’ and G” of (b)

387

the original PVA-3 and (c) the self-healed PVA-3 in a time sweep test (frequency: 1.0

388

Hz; strain: 1%). (d) The self-healing mechanism of the multi-functional PVA hydrogel

389

(PVA-3).

390

Conclusions

391

In summary, an ultra-soft and self-healing PVA hydrogel with conductive and

392

magnetic properties was prepared using nanofibrillated cellulose (NFC) as substrates

393

by a facile approach. Polyaniline was coated on the NFC surface, and the MnFe2O4

394

nanoparticles were precipitated on the NFC. The well-dispersed NFC/PAni/MnFe2O4

395

nanocomposites were used to impart both conductive and magnetic properties to the



396

self-healing PVA hydrogel. The maximum saturation magnetization and conductivity

397

of the multi-functional PVA hydrogel were found to be 5.22 emu.g-1 and 8.15×10-3

398

S.cm-1, respectively. Furthermore, the PVA hydrogel exhibited excellent self-healing

399

property, which can be self-healed completely after the pieces of the hydrogel were

400

put together for several minutes at room temperature. Thus, the novel multi-functional

401

PVA hydrogel demonstrated the significant potential for many practical applications

402

such as electrochemical display devices, rechargeable batteries, and electromagnetic

403

interference shielding. More importantly, nanofibrillated cellulose is sustainable and

404

easy to prepare in large quantities from wood and other plants by simple methods.

405

Acknowledgements

406

The authors are grateful to the national key research and development plan (Grant No.

407

2017YFB0307900).

408

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512

For Table of Contents Use Only

513

TOC Graphic

514 515

This research presents a facile strategy for preparing ultra-soft self-healing

516

nanoparticle-hydrogel composites with conductive and magnetic properties.

517


Ultrasoft Self-Healing Nanoparticle-Hydrogel Composites with

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