Lignin-Containing Cellulose Nanofibril-Reinforced Polyvinyl Alcohol

Subscriber access provided by Kaohsiung Medical University

Lignin Containing Cellulose NanofibrilReinforced Polyvinyl Alcohol Hydrogels Huiyang Bian, Liqing Wei, Chunxiang Lin, Qianli Ma, Hongqi Dai, and J.Y. Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04172 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 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

ACS Sustainable Chemistry & Engineering

1 2 3 4 5 6 7 8 9 10 11 12

Lignin Containing Cellulose Nanofibril-Reinforced Polyvinyl Alcohol Hydrogels Huiyang Bian a,b, Liqing Wei b, Chunxiang Lin b,c, Qianli Ma b,d, Hongqi Dai a, J.Y. Zhu b,* a

b

Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China

Forest Products Laboratory, U.S. Forest Service, U.S. Department of Agriculture, Madison, WI 53726, USA c

College of Environment and Resources, Fuzhou University, Fuzhou 350108, China d

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China

13 14

ABSTRACT

15

Two lignin-containing cellulose nanofibril (LCNF) samples, produced from two

16

unbleached kraft pulps with very different lignin contents, were used to produce reinforced

17

polyvinyl alcohol (PVA) hydrogels. The effects of LCNF loading (0.25-2 wt%) and lignin

18

content on the rheological and mechanical properties of the reinforced hydrogels were

19

investigated. The 2 wt% LCNF-reinforced PVA hydrogels exhibited up to a 17-fold increase

20

in storage modulus and a 4-fold increase in specific Young’s modulus over that of pure PVA

21

hydrogel. Both the mechanical and rheological properties of LCNF-reinforced PVA hydrogels

22

can be tuned by varying LCNF loading and LCNF lignin content. During LCNF production,

23

lignin reduced cellulose depolymerization, resulting in LCNF with high aspect ratios that

24

promoted entanglement and physical bridging of the hydrogel network. Free lignin particles

25

generated during LCNF production, acted as multifunctional nano-spacers that increased

26

porosity of the hydrogels. Because LCNFs were produced from unbleached chemical pulps,

27

which have high yields and do not require bleaching, this study provides a more sustainable

28

approach to utilize lignocelluloses to produce biomass-based hydrogels than by methods

29

using commercial bleached pulps.

30

____________________________________________

31

KEYWORDS: Lignin containing cellulose nanofibrils (LCNFs), Hydrogel, Polyvinyl

32

alcohol (PVA), Rheological and mechanical properties

33 34

* Corresponding author email: [email protected]

35 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

36

INTRODUCTION

37

Hydrogels are soft and three-dimensional structural materials with very high water

38

content, and when made using lignocelluloses, they are non-toxic, biocompatible and

39

biodegradable with anti-biofouling properties.1 As a result, hydrogels have been widely

40

applied in many fields, for example, as scaffolds for tissue engineering, as vehicles for drug

41

delivery, and 3D matrices for cell cultures.2-4 Hydrophilic polyvinyl alcohol (PVA) can form

42

physically and/or chemically crosslinked hydrogels, owing to the hydroxyl groups in each of

43

the repeating molecular units.5-8 However, PVA hydrogels have poor mechanical properties,

44

which can impede their applications.9-10

45

Incorporating carbon nanotubes, clay, or metallic particles can enhance the mechanical

46

properties of hydrogels.11-13 Cellulose nanomaterials (CNM), such as cellulose nanocrystals

47

(CNCs) and nanofibrils (CNFs), are advantageous over inorganic nanomaterials owing to

48

their reactive surfaces, high aspect ratios, and excellent mechanical properties.14-16 The

49

reinforcing effects of CNM on mechanical strength, cross-linking, and rheological properties

50

of PVA hydrogels have been studied.17-19 These CNC or CNF reinforced PVA hydrogels were

51

crosslinked through hydrogen bonds after cyclic freezing-thawing with or without

52

crosslinking agents (Table 1).20 They exhibited higher strength and storage modulus than

53

those prepared by irradiation techniques9, 21; however, multiple cyclic freezing-thawing is

54

tedious and time-consuming.18 Surface-oxidized cellulose whiskers have also been used as a

55

reinforcing agent to crosslink PVA hydrogels at a high loading of 3.88%.16 Introducing a

56

reversible crosslinking agent, such as borax, was an efficient pathway to generate network

57

structure between polymer molecule chains.22-23 Lu et al. found that incorporating

58

microfibrillated cellulose (MFC) from bamboo pulp into the PVA-borax system resulted in

59

reinforced hydrogels with self-healing ability and pH-responsive abilities.22 The effects of

60

particle size, aspect ratio, crystal structure, and surface charge of CNM on the rheological 2


Page 2 of 25

Page 3 of 25 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


61

properties of the composite hydrogels formed with borax were investigated. A plausible

62

mechanism for complexation between CNM, PVA and borax was proposed.24

63

CNC or CNF used in reported studies on PVA hydrogels such as those mentioned above

64

were all produced from fully bleached chemical pulp fibers. We are not aware of publicly

65

available studies on the production of lignin containing cellulose nanofibril (LCNF)

66

reinforced PVA hydrogel. Producing lignin containing cellulose nanomaterials such as LCNF

67

is more environmentally sustainable by elimination of bleaching chemicals, which also

68

results in higher yields.25 Furthermore, LCNF can be directly produced from raw

69

lignocellulosic biomass using low cost and environmentally friendly fractionation without the

70

need of using commercial pulps.26-27 Moreover, lignin serves as a binder to hold the major

71

biopolymers in lignocelluloses together, therefore, LCNF can be a promising reinforcement

72

polymer for PVA matrix.

73

The objective of the present study is to demonstrate the utility of lignin containing

74

carboxylated LCNF as a polymer reinforcement for producing PVA hydrogels through an

75

efficient and facile one freeze-thawing cycle, rather than multiple cycles in the literature. The

76

small amount of free lignin separated from starting wood fibers through mechanical

77

fibrillation during LCNF production could play a positive role, acting as nano-spacers to

78

increase the porosity of 3D-hydrogel networks. The effects of aspect ratio and the loadings of

79

LCNF on the density, water content, viscoelastic and compression properties of hydrogels

80

were investigated. The development of physically crosslinked PVA structure reinforced by

81

carboxylated LCNF may extend this technology to polymer chemistry and open new

82

application potentials for valorization of lignocelluloses.

83 84 85

MATERIALS AND METHODS Materials. Lignin-free and lignin-containing cellulose nanofibrils were produced from 3



86

bleached dry lap eucalyptus kraft pulp (Aracruz Cellulose, Brazil) and unbleached virgin

87

mixed hardwood pulp fibers (complimentary provided by International Paper Company,

88

USA), respectively. The CNF and LCNF production process included maleic acid hydrolysis

89

at 60 wt% acid concentration and 120 °C for 120 min followed by 5 passes through a

90

microfluidizer (M-110EH, Microfluidics Corp., Westwood, MA), as described previously.25

91

These cellulose nanofibril samples were labelled as CNF (lignin-free), LCNF-lL (low lignin

92

content of 4.6%) and LCNF-hL (high lignin content of 17.2%). The basic properties of these

93

(L)CNF samples were presented in a previous study

94

Fig. S1. The CNF and LCNF suspensions were sealed in plastic containers and stored at 5 °C

95

in a refrigerator until used. Polyvinyl alcohol (PVA, Mw = 89000-98000 g/mol, 99%

96

hydrolyzed) was purchased from Sigma Aldrich (St. Louis, MO, USA) and used as received.

97

Deionized (DI) water was used in the preparation of all solutions.

25

and their morphologies are shown in

98

Preparation of Hybrid PVA-CNF and PVA-LCNF Hydrogels. Hybrid hydrogels of PVA

99

with varied contents of CNF or LCNF were prepared via a one-pot reaction system as

100

schematically shown in Fig. 1. CNF or LCNF aqueous suspension (0.25-2 wt%) were added

101

into a 25 mL conical flask with continuous stirring at room temperature for 30 min. PVA

102

powder was gently added into the stirred suspension. The final PVA loading was 4 wt% in the

103

(L)CNF suspension for all hydrogels. The mixtures were stirred for another 30 min to avoid

104

the formation of PVA lumps. After complete swelling of the PVA powder, the solution was

105

heated at 90 °C in an oil bath with magnetic stirring for 2 h. A hermetically sealed flask was

106

used for heating to prevent water evaporation. The PVA powder was gradually dissolved and

107

the solution became homogeneous with increasing time. Finally, the solution was cooled to

108

ambient temperature to form a stable PVA-(L)CNF aqueous solution. No additional

109

reinforcement agents were added throughout the entire process. The solution was then placed

110

in an ultrasonic water bath for 5 min to remove air bubbles. The resultant uniform solution 4


Page 4 of 25

Page 5 of 25 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


111

was poured into a cylindrical plastic beaker and stored in a freezer at -18 °C overnight to

112

freeze the sample. Only one freeze-thawing cycle was applied, which can substantially

113

increase productivity and therefore overall production cost.

114

Water Content of Hydrogels. Each hydrogel specimen was first taken out of the sealed

115

vials and then remove excess water using a tissue paper and weighed. The water content (Wc)

116

of the hydrogel was calculated using Eq. 1: =

− × 100% 1

117

Where Wi is the initial weight of hydrogel and Wd is the weight of hydrogel after oven drying

118

at 105 °C overnight.

119

Dynamic Viscoelastic Analyses. The rheological behavior of pure PVA and PVA-CNF

120

(or LCNF) hydrogels were analyzed using an Anton Paar MCR302 Rheometer (Anton Paar,

121

Ashland, VA, USA) with a parallel plate geometry of 25 mm in diameter. Samples were

122

prepared in the form of disks with a diameter of approximately 20 mm and a thickness of 1

123

mm. The dynamic strain sweep from 0.01 to 100% at angular frequency ω = 10 rad/s was

124

first performed at 25 °C. The storage modulus (G’) was recorded to define the linear

125

viscoelastic region (LVR) within which the storage modulus is independent of strain.

126

Oscillatory frequency sweeps were measured over the ω range of 0.1-100 rad/s at 25°C.

127

Compression Tests of Hydrogels. Uniaxial compression stress-strain measurements

128

were performed using an Instron 5544 mechanical tester (Instron Canton, MA, USA)

129

equipped with a 1000 N load cell. Due to the slightly uneven top surface of the hydrogels, a

130

preload of 0.1 N was used to stabilize the cylindrical hydrogel samples (approximately 34

131

mm in diameter based on the vial internal diameter and cut with a knife into 8 mm in height).

132

Each specimen was compressed vertically at a rate of 0.2 mm/s at 25 °C in air.28 Compression

133

stress, strain, as well as energy absorption values were calculated from measured forces and

134

sample displacements based on the initial dimensions of the hydrogels. 5



Page 6 of 25

135

FE-SEM. To observe the pore distribution and microstructure of pure PVA and

136

PVA-CNF (or LCNF) hydrogels, the cross section was exposed by fracturing the hydrogel in

137

liquid nitrogen. The morphologies were obtained by field-emission scanning electron

138

microscopy (FE-SEM, Nova NanoSEM 230, FEI, USA). The liquid nitrogen fractured

139

hydrogel was freeze-dried, then cut with a razor blade and coated with gold before imaging.

140

FTIR. FTIR spectra of the hydrogels were obtained on a commercial Fourier-transform

141

infrared

spectrometer

(Spectrum

Two,

PerkinElmer,

UK)

using

a

universal

142

attenuated-total-reflection (ATR) probe over the wavenumber range of 4000-400 cm-1. All

143

samples were freeze-dried prior to analyses.

144

X-ray Diffraction (XRD). X-ray diffraction patterns of the pure PVA and PVA-CNF (or

145

LCNF) hydrogels were obtained using an X-ray diffactometer (Bruker D8 Discover, Bruker

146

Co., Billerica, MA) equipped with Cu Kα radiation in the range of 10-38° in steps of 0.02°.

147

All freeze-dried samples were pressed at 180 MPa to make pellets and were dried again in a

148

vacuum oven at 40 °C for 24 h to remove moisture before measurements.

149 150

RESULTS AND DISCUSSIN

151

Water Content and Dynamic Viscoelasticity of Hydrogels. As listed in Table 2,

152

the water content of the hydrogels decreased with increasing solid content of CNF or LCNF;

153

however, they remained at a high level of roughly 94% due to the strong hydrogen bonds and

154

sufficient interaction between the hydroxyl groups of adjacent polymer strands. It was also

155

noted that the PVA-LCNF hydrogels were translucent or opaque due to the presence of lignin,

156

while the pure PVA and PVA-CNF hydrogels have good transparencies.

157

To understand the influence of the types of cellulose nanofibrils on the viscoelastic

158

properties of hydrogels, a shear strain (γ) of 0.1% was chosen in the dynamic strain sweep

159

tests to ensure that the dynamic oscillatory deformation of each sample was within the linear 6


Page 7 of 25 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


160

viscoelastic region (Fig. S2). Both shear storage modulus G’ and loss storage modulus G’’

161

values decreased with the increase in shear strain (γ) (Table 2.) As expected, owing to the

162

entanglement and cross-linking performance of CNF (or LCNF) in PVA hydrogels, CNF (or

163

LCNF)-reinforced hydrogels exhibited significantly higher G’ and G’’ values, especially

164

when the CNF or LCNF loading was increased to 2 wt% (Fig. S2). For instance, the G’max of

165

PVA hydrogel with 2 wt% LCNF-hL was 7725 Pa, or 15 times greater than 492 Pa with 0.25

166

wt% LCNF-hL loading. Compared with hydrogels reinforced by CNF and LCNF-lL under

167

the same CNF or LCNF contents, PVA-LCNF-hL hydrogel has a higher G’max. At a higher

168

lignin content, lignin molecules restrict the motion of PVA chains which results in a

169

significant increase in G’. The nanofibrils act as physical cross-linkers by embedding

170

themselves within the PVA chains, and above a specific CNF or LCNF loading, known as the

171

rheological percolation threshold, all PVA chains become connected. This could hinder the

172

mobility of individual polymer chains under shear force.

173

The changes in the G’ (elasticity) and G’’ (viscosity) of the hydrogels in the angular

174

frequency range of 0.1-100 rad/s were depicted in Fig. 2a1-a3. No crossover of G’ and G’’

175

occurred in the examined full frequency range, exhibiting typical solid-like characteristics.

176

Before the incorporation of CNF or LCNF, G’ and G’’ of pure PVA (4 wt%) hydrogels were

177

significantly lower throughout the entire frequency range. With the increase in the

178

concentration of CNF, the G’ and G’’ values of the CNF reinforced hydrogels measured at ω

179

= 10 rad/s were 17 and 24 times greater than those of the pure PVA hydrogel, respectively

180

(Table 2). Furthermore, PVA-LCNF-hL hydrogels showed the highest G’ and G’’ values

181

among all reinforced hydrogels with the same CNF or LCNF loading. Apparently, the

182

entanglements of the cellulose nanofibrils, the physical restriction of polymer mobility by

183

lignin, and the interactions between nanofibrils and lignin in the PVA matrix system are

184

responsible for the observed viscoelastic properties of hydrogels.24 7



185 186

Compression Stress-strain Behavior of Hydrogels. The hydrogel compressive

187

strength is an important property for many applications. All hydrogels exhibited an

188

exponential relationship between compression stress and strain without an obvious plateau

189

(Fig. 2b1-b3), which caused difficulties in determining the maximal stress. The compressive

190

stress and strain data can be well fitted by an exponential growth function y = A[exp(Bx)-1]

191

(Fig. S3, fitting parameters are listed in Table S1). The compressive stress and the energy

192

absorption at 20 % strain were used for comparing hydrogel mechanical properties. Within 20%

193

strain, stress and strain showed a linear relationship. It can been clearly seen that increasing

194

the (L)CNF loading resulted in higher compression stress of hydrogel. Similar improvement

195

in compression strength and energy absorption was also reported by Abitbol et al. using CNC

196

at 1.5 wt% loading.17 It was worth noting that the composites deformed greatly at low load of

197

below 20 kPa, however, further increase in load resulted in much less deformation. The

198

deformation at a low load range was mostly contributed by the compacting of cellulose fibrils

199

with PVA in the matrix and the lost of free water that are not completely entrapped in the

200

hydrogel matrix. Once this compacting caused deformation reaches its limit, further

201

deformation becomes difficult because the hydrogel is made of mostly water, an

202

incompressible liquid. It is well known that lignin in plant cell wall protects cellulose

203

degradation or depolymerization. This can be seen from the AFM images (Fig. S1) of the two

204

LCNF samples that have longer fibrils than the CNF sample. Furthermore, the LCNF-hL

205

sample with a higher lignin content than LCNF-lL has even longer fibrils. The AFM

206

topographic measurements of number-averaged heights were 13.4, 9.6 and 7.1 for CNF,

207

LCNF-lL, and LCNF-hL, respectively.25 The longer but thinner fibrils (based on AFM mean

208

heights) of LCNF, especially LCNF-hL, certainly promote fibril entanglement (Fig. S1).

209

Therefore, incorporating LCNF into PVA hydrogels can improve rheological and mechanical 8


Page 8 of 25

Page 9 of 25 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


210

properties.

211

The entangled cellulose nanofibrils act as physical crosslinker to maintain mechanical

212

integrity of the hydrogels.23 Visual observations during experiments indicated that CNF or

213

LCNF were well-dispersed in the aqueous PVA systems under continuous stirring without

214

forming visible aggregates; this dispersion facilitated the transfer of compressive stress from

215

PVA polymer chains to CNF or LCNF and inhibited the growth of microcracks.29 The

216

microstructures of the fractured cross sections of the PVA hydrogel and its composites

217

containing 2.0 wt% CNF or LCNF with different lignin contents were observed by

218

field-emission scanning electron microscopy (FE-SEM) as shown in Fig. 3. The fracture

219

surface of a pure PVA hydrogel was rather flat and smooth with regular layered structure in

220

the cross sectional direction (Fig. 3a, 3a’), while the addition of CNF or LCNF to the PVA

221

hydrogel resulted in a fine, micron porous network structure (Fig. 3b-3d). Layered structure

222

was still visible from the hydrogels produced with CNF (Fig. 3b’) or LCNF-lL (Fig. 3c’).

223

When CNF or LCNF were added into the PVA matrix, flocculation occurred during ice

224

crystal growth. It is worth noting that PVA with LCNF showed less layered structure and

225

appeared more porous; this is especially apparent by comparing the pure PVA hydrogel with

226

LCNF-hL reinforced PVA hydrogel (Fig. 3d and d’). It is plausible that free lignin particles

227

(Fig. S1), separated from LCNF during fibrillation, behaved as spacers to avoid

228

self-aggregation of cellulose nanofibrils as shown in Fig. S4. The improved porous structure

229

and the enhanced interactions between CNF and the PVA matrix lead to superior compressive

230

performance for LCNF reinforced PVA hydrogels. It should be pointed out that quantification

231

of free lignin in a LCNF sample is very difficult. The free lignin and the lignin in LCNFs are

232

chemically identical. The diameters of free lignin particles are in the same range of those

233

LCNFs. Moreover, the amount of free lignin is much lower than LCNFs, i.e., only a couple of

234

percent of LCNF based on the highest lignin content in LCNF of 17% and the amount of free 9



235

lignin of around 10% or lower of the LCNF lignin. Therefore, good separation free lignin

236

from LCNF for quantifying free lignin using chemical or physical methods is very challenge.

237

Future study is needed to examining the effect of free lignin particles on PVA-CNF hydrogels

238

by purposely spike various levels free lignin in the hydrogels.

239

The specific stiffness values determined from derivatives of the fitted exponential

240

stress-strain relation at different strains (Table S2) were used to further characterize hydrogel

241

mechanical properties.30 At a constant PVA loading of 4 wt%, increasing the CNF or LCNF

242

loading also increased hydrogel specific shear storage and loss modulus as well as the

243

specific compression stiffness as shown in Fig. 4. A plausible mechanism for this

244

phenomenon was presented in Fig. 1. CNF and PVA suspension could form physical

245

crosslinking upon mixing.31 The growth of ice crystals in the freezing-thawing process can

246

form a gel-like structure from cross-linked aggregates.32 The presence of PVA and CNF can

247

form three types of complexes, namely PVA-PVA, PVA-CNF and CNF-CNF, throughout the

248

whole process. The lignin nanoparticles separated from LCNF in the system acted as spacers

249

to support a porous network structure as revealed by FE-SEM (Fig. 3). The presence of small

250

pores in the hydrogel improved surface area and also could withstand a stronger compression

251

force.33 It is the combination of the improved reinforcement by LCNF and the porous

252

structure by the small nanoparticles of cellulose fibrils and lignin that resulted in an improved

253

compression strength of LCNF-PVA hydrogel (Fig. 2a-c). However, results from testing

254

single sample cannot discern the effects of lignin on hydrogel specific stiffness (Fig. 4c, Table

255

S2). Future studies is needed.

256

FTIR and XRD Analysis. As displayed in Fig. 5a, the absorption bands of PVA

257

occurring at 838 cm-1, 919 cm-1, 1088 cm-1 and 1424 cm-1 were attributed to the stretching of

258

C-O, bending of –CH2, rocking of –CH and stretching of –CH, respectively.34 The FTIR

259

spectra illustrated the presence of CNF or LCNF in PVA-CNF (or LCNF) hydrogels, 10


Page 10 of 25

Page 11 of 25 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


260

demonstrated by some characteristic peaks at 1034 cm-1 and 1059 cm-1. These peaks were

261

associated with the vibration stretch of C-OH bonds in secondary and primary alcohols of

262

cellulose, respectively.34-35 A weak band was observed around 1586 cm-1 in the

263

PVA-LCNF-lL and PVA-LCNF-hL samples that was not present in PVA and PVA-CNF

264

hydrogels. This band corresponds to the aromatic skeletal vibration of lignin.36 Furthermore,

265

the decreased peak intensities at 1088 and 838 cm-1 for the PVA-CNF (or LCNF) hydrogels

266

suggested changes in the crystallization of PVA due to the addition of CNF (or LCNF). This

267

result might demonstrate the expected interaction of hydroxyl groups of PVA matrix with the

268

hydrophilic surfaces of cellulose nanofibrils.

269

Diffractions patterns of PVA and PVA-CNF (or LCNF) hydrogels are shown in Fig. 5b

270

and Fig. S5. The pure PVA hydrogel showed diffraction peak at 2θ = 19.4°, being assigned to

271

the orthorhombic lattice structure of semi-crystalline PVA.24 With the incorporation of CNF

272

(or LCNF) into the PVA system, some new diffraction peaks formed, which are located at

273

around 16.6°, 22.6° and 34.5°, corresponding to the (110), (200) and (004) reflection planes

274

of typical cellulose I structure, respectively.37 Note that the diffraction peaks of PVA became

275

weaker with the increase in lignin and CNF or LCNF contents (Figs. 5b and S3). This

276

suggests the strong interactions between the hydroxyl radicals of PVA with CNF and LCNF,

277

as well as the complexation between them which destroyed the well-organized PVA structure.

278

Furthermore, the presence of lignin in PVA-LCNF hydrogels had no effect on the cellulose

279

crystal structure of composite hydrogels, but weakened the intensity of diffraction peaks.

280 281

CONCLUSIONS

282

LCNF-reinforced PVA hydrogels were successfully produced in an aqueous medium

283

without adding initiators or chemicals. By respectively incorporating three kinds of cellulose

284

nanofibrils (i.e., CNF, LCNF-lL and LCNF-hL) to PVA aqueous systems, the viscoelastic 11



285

properties and specific Young’s modulus of the composite hydrogels were remarkably

286

improved, especially using the LCNF with a high lignin content of 17%. The well-dispersed

287

LCNF in the polymer matrix acted as multifunctional cross-linking agents to enhance the

288

interactions between LCNF and PVA. Lignin, especially the lignin nanoparticles separated

289

from LCNF during LCNF production, served as nano-spacers, which effectively prevented

290

the aggregation of polymers and increased the PVA porosity. Both the mechanical and

291

rheological properties of LCNF reinforced PVA hydrogels can be tailored by varying the

292

LCNF loading and lignin content. Because LCNF can be more sustainably produced than

293

CNF with high yield and without bleaching, this study has practical significance in improving

294

the utilization of lignocellulosic materials for a variety of applications.

295 296

ASSOCIATED CONTENT

297

Supporting Information

298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319

Fig. S1 AFM topographic images of CNF (a), LCNF-lL (b), and LCNF-hL (c), respectively, All scale bar = 1 µm. Free lignin separated from LCNF are visible in LCNFs: (b) and (c) Fig. S2 Strain dependence of shear storage modulus (G’) and loss modulus (G’’) of CNF or LCNF reinforced PVA (4wt% loading) hydrogels: (a) PVA-CNF; (b) PVA-LCNF-lL; (c) PVA-LCNF-hL measured at angular frequency ω = 10 rad/s. Fig. S3 Typical fits of compression stress-strain data by an exponential growth function y = A[exp(B.x)-1)] (fitting parameters are listed in Table S1) Fig. S4 An AFM image of the suspension of hydrogel 4PVA-2LCNF-hL showing the presence of free lignin particles as spacer between cellulose nanofibrils as well as within the PVA matrix. Fig. S5 X-ray diffractograms of PVA (4wt% loading) hydrogels reinforced by CNF or LCNF at varied loadings. Table S1. Fitting results of hydrogel compression stress and strain data using an exponential function: y = A[exp(B.x)-1)] Table S2. Hydrogel stiffness at different strain rates calculated from the fitted stress function: 12


Page 12 of 25

Page 13 of 25 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

320


y’ = A.B.exp(B.x)

321 322 323 324

AUTHOR INFORMATION

325

Corresponding Author

326

*Email: [email protected]; Tel. +1-608 231 9520.

327

ORCID:

328

J.Y. Zhu: 0000-0002-5136-0845

329

Notes: The authors declare no competing financial interest.

330 331

ACKNOWLEDGMENTS

332

This work was partially supported by US Forest Service, the National Natural Science

333

Foundation of China (Project No. 31470599), and the Doctorate Fellowship Foundation of

334

Nanjing Forestry University. We also would like to acknowledge Dr. Goyal Gopal and his

335

colleagues of International Paper Company for complimentarily providing us the unbleached

336

mixed hardwood pulp samples, and Jane O’Dell, Carlos Baez and Sara Fishwild of our

337

Laboratory for assisting in rheology testing, XRD analyses, and mechanical testing,

338

respectively.

13



339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382

REFERENCES (1) Li, L.; Yan, B.; Yang, J.; Chen, L.; Zeng, H. Novel mussel-inspired injectable self-healing hydrogel with anti-biofouling property. Advanced materials 2015, 27 (7), 1294-1299. (2) Drury, J. L.; Mooney, D. J., Hydrogels for tissue engineering: scaffold design variables and applications. Biomaterials 2003, 24 (24), 4337-4351. (3) Hoare, T. R.; Kohane, D. S., Hydrogels in drug delivery: Progress and challenges. Polymer 2008, 49 (8), 1993-2007. (4) Tibbitt, M. W.; Anseth, K. S., Hydrogels as extracellular matrix mimics for 3D cell culture. Biotechnology & Bioengineering 2009, 103 (4), 655-663. (5) Mansur, H. S.; Sadahira, C. M.; Souza, A. N.; Mansur, A. A. P. FTIR spectroscopy characterization of poly (vinyl alcohol) hydrogel with different hydrolysis degree and chemically crosslinked with glutaraldehyde. Materials Science and Engineering: C 2008, 28 (4), 539-548. (6) Hassan, C. M.; Ward, J. H.; Peppas, N. A. Modeling of crystal dissolution of poly(vinyl alcohol) gels produced by freezing/thawing processes. Polymer 2000, 41 (18), 6729-6739. (7) Peppas, N. A.; Merrill, E. W. Crosslinked poly(vinyl alcohol) hydrogels as swollen elastic networks. Journal of Applied Polymer Science 1977, 21 (7), 1763-1770. (8) Aloui, H.; Khwaldia, K.; Hamdi, M.; Fortunati, E.; Kenny, J. M.; Buonocore, G. G.; Lavorgna, M. Synergistic Effect of Halloysite and Cellulose Nanocrystals on the Functional Properties of PVA Based Nanocomposites. ACS Sustainable Chemistry & Engineering 2016, 4 (3), 794-800. (9) Yang, X.; Liu, Q.; Chen, X.; Yu, F.; Zhu, Z. Investigation of PVA/ws-chitosan hydrogels prepared by combined γ-irradiation and freeze-thawing. Carbohydrate Polymers 2008, 73 (3), 401-408. (10) Lu, C.; Blackwell, C.; Ren, Q.; Ford, E. Effect of the Coagulation Bath on the Structure and Mechanical Properties of Gel-Spun Lignin/Poly(vinyl alcohol) Fibers. ACS Sustainable Chemistry & Engineering 2017, 5 (4), 2949-2959. (11) Shin, M. K.; Spinks, G. M.; Shin, S. R.; Kim, S. I.; Kim, S. J. Nanocomposite Hydrogel with High Toughness for Bioactuators. Advanced materials 2009, 21 (17), 1712-1715. (12) Peng, N.; Hu, D.; Zeng, J.; Li, Y.; Liang, L.; Chang, C. Superabsorbent Cellulose–Clay Nanocomposite Hydrogels for Highly Efficient Removal of Dye in Water. ACS Sustainable Chemistry & Engineering 2016, 4 (12), 7217-7224. (13) Wu, D.; Gao, Y.; Li, W.; Zheng, X.; Chen, Y.; Wang, Q. Selective Adsorption of La3+ Using a Tough Alginate-Clay-Poly(n-isopropylacrylamide) Hydrogel with Hierarchical Pores and Reversible Re-Deswelling/Swelling Cycles. ACS Sustainable Chemistry & Engineering 2016, 4 (12), 6732-6743. (14) Zhu, H. L.; Luo, W.; Ciesielski, P. N.; Fang, Z. J.; Zhu, J. Y.; Henriksson, G.; Himmel, M. E.; Hu, L. B. Wood-Derived Materials for Green Electronics, Biological Devices, and Energy Applications. Chem. Rev. 2016, 116 (16), 9305-9374. (15) Li, M.-C.; Wu, Q.; Song, K.; Lee, S.; Qing, Y.; Wu, Y. Cellulose Nanoparticles: Structure–Morphology–Rheology Relationships. ACS Sustainable Chemistry & Engineering 2015, 3 (5), 821-832. (16) Lu, Q.; Cai, Z.; Lin, F.; Tang, L.; Wang, S.; Huang, B. Extraction of Cellulose 14


Page 14 of 25

Page 15 of 25 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

383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426


Nanocrystals with a High Yield of 88% by Simultaneous Mechanochemical Activation and Phosphotungstic Acid Hydrolysis. ACS Sustainable Chemistry & Engineering 2016, 4 (4), 2165-2172. (17) Mihranyan, A. Viscoelastic properties of cross-linked polyvinyl alcohol and surface-oxidized cellulose whisker hydrogels. Cellulose 2013, 20 (3), 1369-1376. (18) Abitbol, T.; Johnstone, T.; Quinn, T. M.; Gray, D. G. Reinforcement with cellulose nanocrystals of poly(vinyl alcohol) hydrogels prepared by cyclic freezing and thawing. Soft Matter 2011, 7 (6), 2373. (19) Gonzalez, J. S.; Luduena, L. N.; Ponce, A.; Alvarez, V. A. Poly(vinyl alcohol)/cellulose nanowhiskers nanocomposite hydrogels for potential wound dressings. Materials science & engineering. C, Materials for biological applications 2014, 34, 54-61. (20) Chang, C.; Lue, A.; Zhang, L. Effects of Crosslinking Methods on Structure and Properties of Cellulose/PVA Hydrogels. Macromolecular Chemistry and Physics 2008, 209 (12), 1266-1273. (21) Chang, C.; Zhang, L. Cellulose-based hydrogels: Present status and application prospects. Carbohydrate Polymers 2011, 84 (1), 40-53. (22) Lu, B.; Lin, F.; Jiang, X.; Cheng, J.; Lu, Q.; Song, J.; Chen, C.; Huang, B. One-Pot Assembly of Microfibrillated Cellulose Reinforced PVA–Borax Hydrogels with Self-Healing and pH-Responsive Properties. ACS Sustainable Chemistry & Engineering 2017, 5 (1), 948-956. (23) Spoljaric, S.; Salminen, A.; Luong, N. D.; Seppälä, J. Stable, self-healing hydrogels from nanofibrillated cellulose, poly(vinyl alcohol) and borax via reversible crosslinking. European Polymer Journal 2014, 56, 105-117. (24) Han, J.; Lei, T.; Wu, Q. High-water-content mouldable polyvinyl alcohol-borax hydrogels reinforced by well-dispersed cellulose nanoparticles: Dynamic rheological properties and hydrogel formation mechanism. Carbohydrate Polymers 2014, 102, 306-316. (25) Bian, H.; Chen, L.; Dai, H.; Zhu, J. Y. Integrated production of lignin containing cellulose nanocrystals (LCNC) and nanofibrils (LCNF) using an easily recyclable di-carboxylic acid. Carbohydrate Polymers 2017, 167, 167-176. (26) Bian, H.; Chen, L.; Gleisner, R.; Dai, H.; Zhu, J. Y. Producing wood-based nanomaterials by rapid fractionation of wood at 80 °C using a recyclable acid hydrotrope. Green Chem. 2017, 19 (14), 3370-3379. (27) Chen, L.; Dou, J.; Ma, Q.; Li, N.; Wu, R.; Bian, H.; Yelle, D. J.; Vuorinen, T.; Fu, S.; Pan, X.; Zhu, J. Y. Rapid and near-complete dissolution of wood lignin at ≤80°C by a recyclable acid hydrotrope. Science Advances 2017, 3, e1701735. (28) Han, J.; Lei, T.; Wu, Q. Facile preparation of mouldable polyvinyl alcohol-borax hydrogels reinforced by well-dispersed cellulose nanoparticles: physical, viscoelastic and mechanical properties. Cellulose 2013, 20 (6), 2947-2958. (29) Zhou, C.; Wu, Q. A novel polyacrylamide nanocomposite hydrogel reinforced with natural chitosan nanofibers. Colloids and surfaces. B, Biointerfaces 2011, 84 (1), 155-162. (30) Gawryla, M. D.; van den Berg, O.; Weder, C.; Schiraldi, D. A. Clay aerogel/cellulose whisker nanocomposites: a nanoscale wattle and daub. Journal of Materials Chemistry 2009, 19 (15), 2118. (31) Wang, L.-Y.; Wang, M.-J. Removal of Heavy Metal Ions by Poly(vinyl alcohol) and 15



427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445

Carboxymethyl Cellulose Composite Hydrogels Prepared by a Freeze–Thaw Method. ACS Sustainable Chemistry & Engineering 2016, 4 (5), 2830-2837. (32) Yang, X.; Cranston, E. D. Chemically Cross-Linked Cellulose Nanocrystal Aerogels with Shape Recovery and Superabsorbent Properties. Chemistry of Materials 2014, 26 (20), 6016-6025. (33) Liu, D.; Ma, Z.; Wang, Z.; Tian, H.; Gu, M. Biodegradable poly(vinyl alcohol) foams supported by cellulose nanofibrils: processing, structure, and properties. Langmuir : the ACS journal of surfaces and colloids 2014, 30 (31), 9544-9550. (34) Liu, D.; Sun, X.; Tian, H.; Maiti, S.; Ma, Z. Effects of cellulose nanofibrils on the structure and properties on PVA nanocomposites. Cellulose 2013, 20 (6), 2981-2989. (35) Lam, N. T.; Chollakup, R.; Smitthipong, W.; Nimchua, T.; Sukyai, P. Utilizing cellulose from sugarcane bagasse mixed with poly(vinyl alcohol) for tissue engineering scaffold fabrication. Industrial Crops and Products 2017, 100, 183-197. (36) Schwanninger, M.; Rodrigues, J. C.; Pereira, H.; Hinterstoisser, B. Effects of short-time vibratory ball milling on the shape of FT-IR spectra of wood and cellulose. Vibrational Spectroscopy 2004, 36 (1), 23-40. (37) Jia, C.; Chen, L.; Shao, Z.; Agarwal, U. P.; Hu, L.; Zhu, J. Y. Using a fully recyclable dicarboxylic acid for producing dispersible and thermally stable cellulose nanomaterials from different cellulosic sources. Cellulose 2017, 24 (6), 2483-2498.

446

16


Page 16 of 25

Page 17 of 25 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


447

List of Figures

448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471

Fig. 1 A schematic illustration of the preparation and synthesis process of the PVA-CNF (or LCNF) hydrogels via freezing-thawing method. Fig. 2 Left panel: Angular frequency dependence of shear storage modulus (G’) and loss modulus (G’’) of PVA hydrogels reinforced by CNF or LCNF at various loadings: (a1) PVA-CNF, (a2) PVA-LCNF-lL, and (a3) PVA-LCNF-hL, measured at shear strain γ = 0.1%. Right panel: Compression stress-strain curves of the same hydrogels in the left panel: (b1) PVA-CNF, (b2) PVA-LCNF-lL, and (b3) PVA-LCNF-hL. Fig. 3 Field-emission scanning electron microscopy (FE-SEM) images of the fractured cross sections of pure PVA hydrogel (a, a’) and PVA hydrogel reinforced by 2 wt% CNF (b, b’), LCNF-lL (c, c’) and LCNF-hL (d, d’). Scale bar = 200 µm (top panel, a-d) and 20 µm (bottom panel, a’-d’). Fig. 4 Effects of CNF or LCNF loading on (a) shear storage modulus (G’), (b) loss modulus (G’’), and (c) specific Young’s modulus of reinforced PVA hydrogels measured at shear strain γ = 0.1% and angular frequency ω = 10 rad/s. PVA loading was 4 wt% for all samples. Fig. 5 (a) FTIR spectra and (b) X-ray diffractograms of pure PVA and PVA reinforced by 2 wt% CNF (or LCNF) hydrogels. PVA loading was 4 wt% for all samples.

472

List of Tables

473 474 475 476 477 478 479 480 481 482

Table 1 Comparisons of production conditions between this study and literature work for producing cellulose nanofibrils reinforced PVA hydrogels. Table 2 Physical, rheological and mechanical properties of CNF or LCNF reinforced PVA hydrogels at different CNF or LCNF loadings.

17


ACS Sustainable Chemistry & Engineering 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

Fig. 1 A schematic illustration of the preparation and synthesis process of the PVA-CNF (or LCNF) hydrogels via freezing-thawing method.

18


Page 18 of 25

Page 19 of 25

10000

140

PVA PVA-0.25CNF PVA-0.5CNF PVA-1CNF PVA-2CNF

120

1000

100 80

100

60

1 10000

G' PVA G'' PVA G' PVA-0.25CNF G'' PVA-0.25CNF G' PVA-0.5CNF G'' PVA-0.5CNF

G' PVA-1CNF G'' PVA-1CNF G' PVA-2CNF G'' PVA-2CNF

40

1000

100

10

1

G' PVA G'' PVA G' PVA-0.25LCNF-lL G'' PVA-0.25LCNF-lL G' PVA-0.5LCNF-lL G'' PVA-0.5LCNF-lL

20

(a1)

G' PVA-1LCNF-lL G'' PVA-1LCNF-lL G' PVA-2LCNF-lL G'' PVA-2LCNF-lL

(a2)

Compression stress (kPa)

10

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


(b1)

140 0

PVA PVA-0.25LCNF-lL PVA-0.5LCNF-lL PVA-1LCNF-lL PVA-2LCNF-lL

120 100 80 60 40 20

(b2)

0 140

10000

PVA PVA-0.25LCNF-hL PVA-0.5LCNF-hL PVA-1LCNF-hL PVA-2LCNF-hL

120

1000

100 80

100 60

10 1 0.1

G' PVA G'' PVA G' PVA-0.25LCNF-hL G'' PVA-0.25LCNF-hL G' PVA-0.5LCNF-hL G'' PVA-0.5LCNF-hL

1

G' PVA-1LCNF-hL G'' PVA-1LCNF-hL G' PVA-2LCNF-hL G'' PVA-2LCNF-hL

40

(a3) 10

100

20

(b3)

0 0

20

40

60

80

Strain (%)

Angular frequency (rad/s)

Fig. 2 Left panel: Angular frequency dependence of shear storage modulus (G’) and loss modulus (G’’) of PVA hydrogels reinforced by CNF or LCNF at various loadings: (a1) PVA-CNF, (a2) PVA-LCNF-lL, and (a3) PVA-LCNF-hL, measured at shear strain γ = 0.1%. Right panel: Compression stress-strain curves of the same hydrogels in the left panel: (b1) PVA-CNF, (b2) PVA-LCNF-lL, and (b3) PVA-LCNF-hL.

19



Page 20 of 25

(a)

(b)

(c)

(d)

(a’)

(b’)

(c’)

(d’)

Fig. 3 Field-emission scanning electron microscopy (FE-SEM) images of the fractured cross sections of pure PVA hydrogel (a, a’) and PVA hydrogel reinforced by 2 wt% CNF (b, b’), LCNF-lL (c, c’) and LCNF-hL (d, d’). Scale bar = 200 µm (top panel, a-d) and 20 µm (bottom panel, a’-d’).

20


Page 21 of 25

Storage modulus G' (Pa)

10000 PVA PVA-CNF PVA-LCNF-lL PVA-LCNF-hL

1000

(a)

100 PVA PVA-CNF PVA-LCNF-lL PVA-LCNF-hL

Loss modulus G'' (Pa)

1000

100

(b)

10

Specific stiffness (kPa.cm3/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


PVA PVA-CNF PVA-LCNF-lL PVA-LCNF-hL

18 16 14 12 10 8 6 4 2

(c)

0 0.0

0.5

1.0

1.5

2.0

CNF or LCNF loading (wt%) Fig. 4 Effects of CNF or LCNF loading on (a) shear storage modulus (G’), (b) loss modulus (G’’) measured at shear strain γ = 0.1% and angular frequency ω = 10 rad/s, and (c) specific compression stiffness of reinforced PVA hydrogels measured at 20% strain. PVA loading was 4 wt% for all samples.

21



1059

PVA PVA-2CNF PVA-2LCNF-lL PVA-2LCNF-hL

(a)

1088 1034 919 838

1424 1586

1800

1600

1400

1200

1000

800

600

-1

Wavenumber (cm ) 19.4o 22.6o

(b) 16.6o

Intensity (a.u.)

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

PVA PVA-2CNF PVA-2LCNF-lL PVA-2LCNF-hL 34.5o

10 12 14 16 18 20 22 24 26 28 30 32 34 36 38

2θ (o)

Fig. 5 (a) FTIR spectra and (b) X-ray diffractograms of pure PVA and PVA reinforced by 2 wt% CNF (or LCNF) hydrogels. PVA loading was 4 wt% for all samples.

22


Page 22 of 25

Page 23 of 25


1 2 3 4 5 6 Table 1 Comparisons of production conditions between this study and literature work for producing cellulose nanofibrils reinforced PVA 7 hydrogels. 8 9 10 Thawing PVA molecular weight and Nanocellulose Crosslinking Maximum G’ with Energy Process T (°C) 11 time (h) loading loading agent LVR, G’max (Pa) consumption 12 Freezing-thawing (-20 °C), Approximately DP = 1750±50; 13 Chang et 60 0.5 CNF; 4 wt% None High 14 12h, seven cycles 4 wt% 5000 al.20 15 Abitbol et Freezing-thawing, 90 6 25 kDa; 15 wt% CNC; 0-3 wt% None Not detected High 16 18 2-3 cycles al. 17 Freezing-thawing (-18 °C), 98000 g/mol; TEMPO oxidized Approximately 18Mihranyan17 60 0.5 None Medium overnight, one cycle 10 wt% MCC; 3.88 wt% 10000 19 20Han et al.24, 146000-186000 g/mol; CNC and CNF; Borax; Chemical crosslinking 90 2 1715 Low 21 28 2 wt% 1 wt% 0.4 wt% 22 One pot, 75000-80000 g/mol; Borax; 23 Lu et al.22 90 2 MFC; 0.5-5 wt% 11380 Medium chemical crosslinking 4 wt% 0.4 wt% 24 25 CNF, LCNF-lL and Freezing-thawing (-20 °C), 89000-98000 g/mol; 26 90 2 LCNF-hL; None 7724 Low This study 27 4h, one cycle 4 wt% 0.25-2 wt% 28 29 30 31 32 33 34 35 36 37 38 39 40 23 41 42 43 44 ACS Paragon Plus Environment 45 46 47


Page 24 of 25

Table 2 Physical, rheological and mechanical properties of CNF or LCNF reinforced PVA hydrogels at different CNF or LCNF loadings.

Material

PVA PVA-CNF PVA-0.25CNF PVA-0.5CNF PVA-1CNF PVA-2CNF PVA-LCNF-lL PVA-0.25LCNF-lL PVA-0.5LCNF-lL PVA-1LCNF-lL PVA-2LCNF-lL PVA-LCNF-hL PVA-0.25LCNF-hL PVA-0.5LCNF-hL PVA-1LCNF-hL PVA-2LCNF-hL

G’’ with LVR at ω = 10 rad/s (Pa)

94.7

Maximum G’ with LVR at ω = 10 rad/s, G’max (Pa) 237.2

Young’s modulus, E (kPa)

Specific E (N m/kg)

Compressive stress at ɛ = 20% (kPa)

26.0

1.787

1.770

0.44

Energy absorption at ɛ = 50% (kJ/m3) 0.46

95.3 95.2 94.8 93.6

456.6 645.7 840.3 4513.8

58.8 83.3 120.2 841.0

2.404 2.418 3.918 8.187

2.380 2.394 3.879 8.106

0.55 0.56 0.88 1.72

0.55 0.57 0.78 1.26

95.5 95.2 94.7 93.6

547.5 874.2 1468.5 4403.3

72.2 107.7 244.8 901.3

2.466 2.759 4.417 7.504

2.441 2.732 4.374 7.430

0.59 0.64 0.97 1.64

0.60 0.64 0.84 1.47

95.3 95.1 94.4 93.0

492.3 1287.3 2701.6 7724.8

66.4 203.4 500.0 1426.7

2.539 2.985 4.696 8.531

2.513 2.955 4.650 8.447

0.58 0.68 0.98 1.81

0.57 0.66 0.86 1.62

Water content (wt%)

24


Page 25 of 25 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


A schematic diagram shows the synthesis and physical structure of PVA-LCNF hydrogels


Lignin-Containing Cellulose Nanofibril-Reinforced Polyvinyl Alcohol

Recommend Documents