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High Yield Synthesis of Functionalized Cellulose Nanocrystals for Nano Bio-composites Qilin Lu, Linna Lu, Yonggui Li, Yuxin Yan, Zhaofeng Fang, Xin Chen, and Biao Huang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00048 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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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.

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ACS Applied Nano Materials

1 2 3

High Yield Synthesis of Functionalized Cellulose Nanocrystals for Nano Bio-composites

4 5

Qilin Lu, † Linna Lu, † Yonggui Li,† Yuxin Yan, ‡ Zhaofeng Fang, ‡ Xin Chen, ‡

6

Biao Huang,*,‡

7

†Fujian

8

Clothing and Design Faculty, Minjiang University, Fuzhou 350108, China

9

‡College

Key Laboratory of Novel Functional Textile Fibers and Materials,

of Material Engineering, Fujian Agriculture and Forestry University,

10

Fuzhou 350002, China

11

ABSTRACT: A facile versatile green one-step procedure to fabricating

12

functionalized

13

thermostability was put forward via molten oxalic acid hydrolysis by aid of

14

simultaneous microwave and sonication. The synchronized hydrolysis and

15

esterification of cellulose took place in the one-pot solvent-free reaction, and

16

thus we developed an environmentally benign and scalable concurrent acid

17

hydrolysis/Fischer esterification method to produce acid functionalized CNCs

18

using microwave and sonication in 85.5% yield. Compared to classical

19

methods for the functionalization of CNCs, the presented study avoided the

cellulose

nanocrystals

(CNCs)

1 ACS Paragon Plus Environment

with

high

yields

and

ACS Applied Nano Materials 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

20

extensive consumption of water and solvent, and achieved zero waste liquid

21

discharge. Moreover, the used solid acid could easily be recycled to make it

22

possible for the large-scale and cleaner production of CNCs. The resultant

23

CNCs with high thermal stability and excellent dispersion stability are

24

excellent for nano bio-composite applications.

25 26

KEYWORDS: cellulose nanocrystals, high efficiency, mechanochemistry,

27

molten oxalic acid, synergetic effects

28 29

INTRODUCTION

30

Cellulose nanocrystals (CNCs) have attracted great attention recently due to their

31

low density (1.5 g cm-3), high mechanical strength (Young’s modulus of 140 GPa),

32

good biodegradability, and unique optical properties.1 CNCs have been fostered for a

33

myriad of applications including biomaterials, drug delivery, tough nanocomposites,

34

and optical devices.2,3 The top-down manufacture of CNCs was usually carried out

35

through acid hydrolysis, the amorphous regions were removed, and individual

36

cellulose crystals were obtained and purified by centrifugation and dialysis.4 Existing

37

approaches to prepare CNCs including acid vapour,5 oxidative degradation,6

38

acetylation in ionic liquids,7 microwave-assisted hydrothermal treatment,8 or a

39

combination of electron beam irradiation and high-pressure homogenization.9

40

Unfortunately, the relatively low yields, excessive consumption of water or solvents, 2 ACS Paragon Plus Environment

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tedious separation process hindered the mass production of CNCs. Therefore,

42

high-yield, economic and environmentally sustainable fabrication of CNCs is the key

43

to achieve the mass production of CNCs-based bio-nanocomposites.

44

Some assisted methods such as microwave and sonication are usually used as

45

mechanochemical activation of cellulose fibers to acquire high efficiency through the

46

intensification of heat and mass transfer. On the one hand, microwave irradiation can

47

strengthen heat transfer and enhance the reaction activity of cellulose. On the other,

48

sonication can improve mass transfer efficiency among cellulosic fibrils. Therefore,

49

the combination of these two techniques in a single-step process would be a promising

50

approach to manufacture functionalized CNCs under mild conditions. Simultaneous

51

microwave and sonication assisted process was generally applied in nanoparticles

52

preparation, organic synthesis or extraction,10 but to the best of our knowledge, it has

53

not yet been used in the synchronous nanocrystallization and carboxylation of

54

cellulose under oxalic acid hydrolysis.

55

Herein, a simple, viable and green one-step procedure to produce highly

56

thermostable functionalized CNCs with high yields was put forward. The

57

nanocrystallization and functionalization of cellulose took place simultaneously in a

58

one-pot solvent-free reaction under microwave and sonication synergy. The

59

hydrolysis of cellulose was carried out by using molten oxalic acid, meanwhile, the

60

formed CNCs reacted with carboxyl groups through Fischer esterification to form

61

carboxylated CNCs. Oxalic acid has low water solubility at room temperature and 3 ACS Paragon Plus Environment


62

therefore can be easily recovered by crystallization. Consequently, it is expected to

63

achieve the high-yield, green and large-scale production of functionalized CNCs

64

under the synergetic effects induced by heat transfer augmentation of microwave and

65

mass transfer enhancement of sonication. The obtained CNCs can be used in nano

66

bio-composite to improve the mechanical performance, thermostability, and optical

67

properties due to their improved crystallinity, high thermal stability, and better

68

interfacial interaction.

69 70

EXPERIMENTAL SECTION

71

Materials. Dissolving bamboo pulp (α-cellulose content ≥95 wt%) as

72

cellulose raw material was supplied by Nanping Paper Co., Ltd. (Nanping,

73

Fujian, China), and oxalic acid dihydrate (C2H2O4·2H2O) was purchased from

74

Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). All used chemicals

75

were of analytical grade and the used water was deionized water.

76

Preparation of Functionalized CNCs. The functionalized CNCs were

77

prepared by molten oxalic acid hydrolysis under simultaneous microwave and

78

sonication assisted, and the solvent-free one-pot tandem reaction procedure

79

is illustrated in Scheme 1. The extraction process was carried out in a

80

computer-controlled microwave-ultrasound instrument for synthesis and

81

solvent extraction (XH-300A, Beijing Xianghu Science and Technology

82

Development Co., Ltd., China). 2 g dissolving bamboo pulp (DBP) and 50 g 4 ACS Paragon Plus Environment

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83

oxalic acid were added into a 250 mL three-necked flask and then loaded into

84

the microwave-ultrasound instrument equipped with reflux condensation and

85

magnetic stirring. The mixture was heated to 115 ℃ for a designated time

86

(15-75 min) at the microwave power of 500 W and ultrasonic power of 800 W

87

under continuous stirring and reflux condensation. The reaction was

88

terminated by adding 400 mL deionized water to precipitate CNCs from the

89

acid hydrolysate. The separated CNCs were purified by successive

90

centrifugations at 9000 rpm for 10 min with deionized water until attained

91

neutrality. The supernatant acid hydrolysate was collected to recycle oxalic

92

acid by crystallization and then the formed acid crystals were separated from

93

liquid by vacuum filtration for reuse.

94

As a control, the hydrolysis of cellulose with molten oxalic acid was carried

95

out in an oil bath without microwave and sonication. The reaction parameters

96

and subsequent processing after terminated reaction were the same as

97

mentioned above and the obtained products as control sample.

98 99 100 101 102 103



104 105 106

107 108

Scheme 1 Schematic of one-pot solvent-free preparation of functionalized

109

CNCs and illustration of reaction mechanism.

110


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111

Characterization. The morphology and size of CNCs were analyzed by

112

XL30 ESEM-FEG model (FEI Co., Ltd., USA) field emission scanning electron

113

microscopy (FESEM) and Hitachi-H7650 (Hitachi, Ltd., Japan) transmission

114

electron microscope (TEM). The dimension and aspect ratio of CNCs were

115

determined by NanoScope Ⅲa MultiMode (Veeco Instruments, Inc., USA)

116

atomic force microscope (AFM).

117

The particle size distribution and dispersion stability of CNCs suspensions

118

were explored through a Zetasizer ZEN3690 instrument (Malvern Instruments

119

Ltd., UK) which provides multi-angle particle size analysis by dynamic light

120

scattering (DLS) and low-angle zeta potential analysis by electrophoretic light

121

scattering (ELS). The measurements were performed at 25 ℃ with the CNCs

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suspensions of 1.5 mg·mL-1. The average particle size and zeta potential

123

value were obtained from the average of three runs of measurements.

124

The chemical structure of CNCs was analyzed by FT-IR and CP/MAS

13C

125

NMR spectroscopy. FT-IR spectra were obtained with a Nicolet 380 FT-IR

126

spectrometer (Thermo Electron Instruments Co., Ltd., USA) in the frequency

127

range of 4000-400 cm-1 with a resolution of 4 cm-1. Solid-state

128

(cross-polarization-magic angle spinning) measurement was performed on a

129

Bruker AVANCE Ⅲ 500 NMR spectrometer (Bruker Biospin AG, Fallanden,

130

Switzerland) at a resonant frequency of 125.73 MHz, a magic angle spinning


13C

NMR


Page 8 of 32

131

(MAS) rate of 5.0 kHz and a contact time of 2 ms. Over 1024 scans were

132

accumulated for each spectrum.

133

The carboxyl group content (CGC) and the degree of substitution (DS) of

134

functionalized CNCs were determined by conductometric titration.11 Briefly,

135

0.2 g CNCs was suspended in 40 mL of 0.01 M HCl solution and sonicated for

136

20 min to disperse CNCs uniformly, and then the suspension was titrated

137

against 0.01 M NaOH solution. The CGC was determined according to Eq.

138

(1):

139

CGC  (V 2  V 1)C W

(1)

140

where (V2 -V1) is the volume of NaOH (L) required to deprotonate the

141

carboxylic acids groups, C is the concentration of NaOH (M), W is the weight

142

of CNCs.

143 144

The DS of CNCs was calculated based on Eq. (2): DS 

162(V 2  V 1)C W  72(V 2  V 1)C

(2)

145

where 162 and 72 correspond to the molecular weight of an anhydroglucose

146

unit (AGU) and the difference between the molecular weight of an AGU and

147

that of the oxalates of a glucuronic acid moiety, respectively.

148

The crystalline structure of CNCs was investigated by X-ray diffraction (XRD)

149

analysis on an X’Pert Pro MPD X-ray diffractometer (Philips-FEI, Netherlands)

150

with Cu Kα radiation. The scattered radiation was detected in the range of


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151

2θ=6-90° at a scanning rate of 0.1°·s-1. The crystallinity (CrI) of the samples

152

was calculated using Eq. (3):

153

CrI 

I 002  Iam  100 I 002

(3)

154

where I002 is the intensity of the 002 lattice diffraction at 2θ about 22°,

155

representing both crystalline and amorphous regions of cellulose, and Iam is

156

the intensity of diffraction at 2θ about 18°, representing the amorphous region

157

of cellulose.12

158

The thermal stability of CNCs were studied by thermogravimetric analysis

159

(TGA) with a thermal gravimetric analyzer (NETZSCH STA 449 F3 Jupiter®,

160

Germany). 5 mg of each sample was put into an alumina crucible and heated

161

from 25 ℃ to 700 ℃ at a heating rate of 10 ℃·min-1 under nitrogen

162

atmosphere with a flow rate of 20 mL·min-1.

163 164

RESULTS AND DISCUSSION

165

Effects of Reaction Parameters on CNCs. The accurate information of the

166

yields, morphology and dimension of CNCs can be used to regulate product

167

quality, guide application direction and facilitate large-scale production. The

168

effects of reaction parameters on the yields and size distribution of CNCs are

169

shown in Table 1. The yield of CNCs in each fraction was calculated by the

170

weight of CNCs in each fraction divided by the initial weight of DBP. The

171

obtained particle size was average diameter calculated from the assumption 9 ACS Paragon Plus Environment


172

that CNCs were spherical by DLS. Obviously, the yield of CNCs up to 85.5%

173

was achieved higher than that of 13.8% for conventional oil bath heating.

174

Under the assistance of microwave and sonication, the yield of CNCs

175

increased sharply to 80% at a reaction time of 30 min. However, CNCs could

176

not be obtained at the reaction time of 30 min without microwave and

177

sonication, and even the reaction time prolonged to 360 min, the yield could

178

only reach 13.8%. These results confirmed that, the processing with

179

microwave and sonication significantly diminished the reaction period and was

180

high-efficient. The mechanochemical synergetic effects created by microwave

181

and sonication gave rise to rotation of the polar hydroxyl and disrupted the

182

strong hydrogen bonding within cellulose network,13 contributing to the

183

diffusion of oxalic acid into amorphous regions of cellulose. Molten oxalic acid

184

immersed rapidly into the disordered accessible regions of cellulose,

185

accelerating the hydrolysis and esterification of amorphous regions. Thus,

186

time-consuming process could be avoided and production efficiency could be

187

improved significantly.

188

At an acid dosage of 15 g·g-1 and ultrasonic power of 800 W, the yield of

189

CNCs increased from 73.3% to 80.5% and the size decreased from 200 nm to

190

100 nm with the reaction time increasing from 15 min to 60 min. This was

191

attributed to the character of microwave irradiation arising from thermal or

192

non-thermal effects, which was induced by heating rate, hot spots, 10 ACS Paragon Plus Environment

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193

acceleration of ions and collision with other molecules or rapid rotation of

194

dipoles.14 The mass transfer resistance gradually decreased with the

195

cleavage of strong hydrogen bonds caused by amorphous regions hydrolysis

196

under microwave and sonication synergy. Accordingly, the increase of mass

197

transfer rate could further enhance hydrolysis efficiency, indicating that the

198

synergetic effects between microwave and sonication played a crucial role in

199

the hydrolysis and esterification of cellulose. Furthermore, an increase in the

200

acid dosage from 25 g·g-1 to 30 g·g-1 led to a decrease in the yield from 85.5%

201

to 77.9%, the size from 50 nm to 30 nm, and the color of CNCs suspension

202

turned from white to gray. This could be explained by the fact that the

203

crystalline structure of CNCs was gradually destructed with acid dosage

204

further increasing, resulting in excessive hydrolysis and even carbonization.

205

The yield of CNCs increased from 76.2% to 85.5% with the ultrasonic power

206

increasing from 600 W to 1000 W, which might be attributed to the ultrasonic

207

cavitation

208

approximately 10-100 kJ·mol-1, which is within the hydrogen bond energy

209

scale,16 and thus can effectively break the inter- and intra-molecular hydrogen

210

bonds between cellulose chains, accelerating the hydrolysis of amorphous

211

area.17

effect.15

The

energy

provided

by

ultrasonic

cavitation

is

212

The optimum reaction parameters were chosen for obtaining relatively high

213

yield of CNCs, namely, a reaction time of 30 min, an acid dosage of 25 g·g-1, 11 ACS Paragon Plus Environment


Page 12 of 32

214

and an ultrasonic power of 800 W. The reusability of oxalic acid was verified

215

by using recovered oxalic acid to hydrolyze cellulose under the optimum

216

reaction parameters. The result showed that oxalic acid could be recovered

217

and reused for at least five times for CNCs fabrication, and after reusing five

218

times, CNCs yield still could reach 75%.

219 220

Table 1. Yield, Size Distribution and Zeta Potential Value of the Functionalized

221

CNCs under Different Reaction Parameters. Sample

Reaction

Acid-to-DBP

Time

(g·g-1)

(min)

CNCs

Control

Ultrasonic Power (W)

Yielda (%)

Size

Zeta

Distributionb

Potentialb

(nm)

(mV)

15

15

800

73.3  0.2

156-200

-35.6  0.4

30

15

800

79.7  0.1

100-132

-37.3  0.5

60

15

800

80.5  0.1

93-115

-38.7  0.3

75

15

800

75.6  0.0

52-79

-39.1  0.6

30

20

800

81.0  0.3

60-76

-41.3  0.7

30

25

800

85.5  0.0

37-53

-42.9  0.2

30

30

800

77.9  0.2

21-35

-42.5  0.5

30

25

600

76.2  0.1

45-58

-39.7  0.4

30

25

1000

81.6  0.3

30-46

-40.8  0.6

60

25

-

-

-

-

120

25

-

5.3  0.1

350-453

-30.7  0.7

240

25

-

8.2  0.2

232-345

-32.3  0.4

360

25

-

13.8  0.1

120-167

-33.6  0.5


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222

aYield

was calculated using the weight of CNCs divided by the initial weight of

223

DBP. bSize distribution and zeta potential were calculated using the DLS and

224

ELS particle size analyzer.

225 226

Morphology, Dimension and Suspensions’ Stability. In order to investigate

227

the morphology changes of cellulose fibrils in reaction process, SEM was

228

conducted. FESEM image shows that the raw material DBP fiber presents

229

curled and flat shape, and the surface is rough with lengths of several microns

230

(Figure 1a). Performing the hydrolysis reaction in an oil bath without

231

microwave and sonication, the obtained sample has widths of dozens of

232

nanometers and lengths of hundreds of nanometers, meanwhile, the surface

233

becomes relatively even and smooth (Figure 1b). Under the synergy of

234

microwave and sonication, the cellulose fibrils are cleaved into small fibers,

235

having their dimensions on the nanoscale (Figure 1c). As seen from TEM

236

(Figure 2a, Figure 2b) and AFM images (Figure 2c, Figure 2d), the

237

dimensions for CNCs are 285 nm and 17 nm in length and width respectively,

238

by contrast, the control sample has lengths of 610 nm and widths of 25 nm.

239

CNCs are rod-like with smooth surface, and these rod-shaped nanocrystals

240

intertwine to form a web-like network structure, which is the reason for their

241

enhancement function in composite materials.18 Additionally, the aspect ratio

242

of CNCs (16.8) is lower than that of control sample (24.4), owing to the 13 ACS Paragon Plus Environment


243

damage of the CNCs to some extent during the microwave and sonication

244

process. Nevertheless, the aspect ratio of the as-prepared CNCs is larger

245

than that of sulfuric acid method (10).19 This can be attributed to the retention

246

of most of the crystalline region which can withstand attack by mild oxalic acid

247

to a certain extent. Conversely, sulfuric acid can destroy the crystalline

248

component of CNCs, leading to a small size and low yield.

249

250 251 252

Figure 1 FESEM images of (a) DBP, (b) control sample and (c) CNCs.

253 254

The slight aggregation of CNCs can also be found from TEM and AFM

255

images, which derives from the strong hydrogen bonding between cellulose 14 ACS Paragon Plus Environment

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256

nanoparticles.20 However, the agglomeration is less than that of hydrochloric

257

acid

258

nanocrystals due to the existence of carboxyl groups. Generally, the lack of

259

surface charges for CNCs can quite easily result in agglomeration, which is

260

disadvantageous in nanocomposites production where the continuous matrix

261

is hydrophilic polymer. The dispersion stability of CNCs in water was further

262

investigated by zeta potential tests. It was generally considered that the zeta

263

potential value greater than -15 mV signified the onset of flocculation or

264

agglomeration whereas lower than -30 mV meant the sufficient mutual

265

repulsion rendered good stability to the colloidal suspension.21 For CNCs

266

suspensions, the smaller the value of zeta potential is, the better dispersion

267

stability that can be achieved. An average zeta potential value of -42.9 mV for

268

CNCs can be obtained (Table 1), which is comparable to that of sulfuric acid

269

method (-33.8 mV),22 and smaller than hydrochloric acid method (-6.7 mV).23

270

The result demonstrates that CNCs suspensions have fairly good dispersion

271

stability, and is consistent with the morphology observation.

method

because

of

the

large

electrostatic


repulsion

between


272 273 274

Figure 2 TEM images of (a) control sample, (b) CNCs; AFM images of (c)

275

control sample, (d) CNCs.

276 277

Chemical Structure. The FI-IR spectra of CNCs are shown in Figure 3a. The

278

presence of bands at 1645, 1430, 1162, 1059, 896 and 710 cm-1 suggests

279

that the obtained CNCs are mainly in the form of cellulose Iβ.24 The band at

280

1430 cm-1 is identified as the crystalline absorption band from the symmetric

281

bending vibration of -CH2.25 It is worth noting that the bands at 1059 cm-1

282

assigned to C-O stretching vibration of pyranose and 1113 cm-1 assigned to

283

glucose ring skeletal vibration become stronger for CNCs, demonstrating the

284

increase in crystalline cellulose content. Owing to the hydrolysis of amorphous


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285

regions, more hydroxyl groups are exposed, and thus the peak intensity of

286

3347 cm-1 for CNCs becomes higher than that of DBP. The existence of

287

carbonyl groups in CNCs is clearly evident from the C=O stretching of

288

carbonyl groups at 1732 cm-1.26 The signal indicates that oxalic acid reacted

289

with hydroxyl groups of cellulose and generated esterification successfully

290

during hydrolysis process. Nevertheless, the intensity of 1732 cm-1 is low,

291

implying that the degree of esterification is moderate, or probably the formed

292

ester is further hydrolyzed over time. Due to the overlap of absorption peaks

293

at 1732 cm-1, FT-IR cannot differentiate ester bonds (-COO) or carboxylic

294

acids (-COOH), so the structure of CNCs was further investigated by 13C NMR

295

spectroscopy.

296

Figure 3b shows that the

13C

NMR spectra of CNCs display typical signals

297

of celluloseⅠ. The peaks of C1 (104.5 ppm), C2, C3, C5 (70 ppm, 75 ppm), C4

298

(88.5 ppm) and C6 (64.5 ppm) correspond to the carbons of glucopyranose

299

rings in the crystalline parts, whereas the peaks of C4 (83.5 ppm) and C6 (62

300

ppm) are assigned to the carbons of glucopyranose rings in the amorphous

301

regions.27 In contrast with DBP, two characteristic peaks at 174 and 157 ppm

302

were detected, which corresponded to the carbons of carboxylic acid (-COOH)

303

and ester groups (-COO), respectively. This result implies that the

304

esterification occurred simultaneously during the hydrolysis of cellulose and



305

as a result, the covalent ester bonds were formed, meanwhile, the carboxylic

306

acid groups were also existed in CNCs.

307

308 309 310

Figure 3 (a) FTIR and (b) 13C NMR spectra of DBP and CNCs.

311 312

Degree of Esterification. The carboxyl group content (CGC) and the degree

313

of substitution (DS) were quantified by conductometric titration. As shown in

314

Table 2, the control sample has a low CGC of 0.42 mmol·g-1 and DS of 0.07. 18 ACS Paragon Plus Environment

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315

However, a high efficiency was achieved when microwave and sonication was

316

conducted, leading to a CGC of 1.23 mmol·g-1 and DS of 0.22. This benefitted

317

from the increase of mass transfer rate caused by microwave and sonication

318

synergy, which enhanced the reactivity of oxalic acid and accelerated

319

sequential hydrolysis and esterification reactions. The degrees of substitution

320

for the obtained CNCs are similar to other literature, which reported the

321

production of functional nanocrystals by combining acid hydrolysis and

322

Fischer esterification with various organic acids.28 In addition, as for the

323

modification of cellulose, various DS can be obtained through different

324

preparation methods. It has been reported that cellulose nanofibers were

325

prepared by mechanochemical esterification in an organic solvent and

326

obtained a DS of 0.6 after 24 h of ball milling.29 However, in the current study,

327

the DS for CNCs was lower than this value, indicating that the permeation of

328

oxalic acid towards the crystalline regions of cellulose was limited under the

329

experimental conditions, only parts of hydroxyl groups attended the

330

esterification.

331 332

Table 2. Size, CGC and DS for the Functionalized CNCs Obtained through

333

Various Reaction Time. Sample

Length (nm)a

Width (nm)a

Aspect Ratio


CGC (mmol·g-1)b

DSb


Page 20 of 32

SMU-15

300 ± 6

22 ± 5

13.6

0.97

0.17

SMU-30

285 ± 7

17 ± 3

16.8

1.08

0.19

SMU-45

276 ± 6

15 ± 4

18.4

1.23

0.22

SMU-60

269 ± 8

15 ± 5

17.9

1.15

0.20

SMU-75

262 ± 9

15 ± 3

17.5

1.06

0.19

Control

610 ± 9

25 ± 5

24.4

0.42

0.07

334

aThe

length and width were calculated from TEM and AFM images.

335

bThe

carboxyl group content (CGC) and the degree of substitution (DS) of

336

CNCs were determined by conductometric titration.

337 338

Crystal Structure. The crystal structure of CNCs was also explored by

13C

339

NMR spectroscopy. The crystallinity of CNCs can be calculated according to

340

evaluate the C4 (88.5 ppm) peak. Compared with DBP, the intensity of C4

341

(88.5 ppm) peak for CNCs has no significantly change (Figure 3b), suggesting

342

that the crystalline part of cellulose is not altered. XRD analysis was carried

343

out for further studying the crystal structure of CNCs, as shown in Figure 4.

344

DBP and CNCs all display the typical celluloseⅠ diffraction peaks at 2θ=15°,

345

16.5°, 22.7° and 34.8°, assigned to the (1-10), (110), (200) and (004)

346

crystallographic planes of cellulose Iβ lattice, respectively.30 It means that the 20 ACS Paragon Plus Environment

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347

hydrolysis and esterification reactions did not alter the crystal form of

348

cellulose. To gain further insights into the effects of microwave and sonication

349

on the crystal structure of CNCs, XRD spectra were measured at different

350

reaction time (Figure 4a). It is clear that the peak intensity of (200)

351

crystallographic plane increases with reaction time increasing, confirming the

352

gradual breakage of hydrogen bonded network and the fracture of glycosidic

353

bonds under the intensive impact of microwave irradiation and ultrasound.

354

The crystallinity of SMU-30 increases from 55.24% to 78.31% compared to

355

that of DBP due to the efficient removal of amorphous parts. Nevertheless,

356

with the esterification of CNCs, new groups were introduced which hindered

357

the intermolecular interaction of CNCs, and as a result the crystallinity

358

decreased.

359

As shown in Figure 4b, the peak shapes of DBP, control sample and CNCs

360

are similar, indicating that microwave and sonication have no remarkable

361

impact on the inner crystal structure of cellulose. It should be noted that CNCs

362

have a higher crystallinity of 78.31% than 67.80% for control sample, resulting

363

from a more intense hydrolysis of disordered regions induced by synergistic

364

microwave and sonication. Moreover, oxalic acid is less aggressive to

365

cellulose than inorganic acids, and thus, it may not be able to further attack

366

the ordered crystalline regions after the removal of disordered area, so a

367

higher crystallinity can be achieved. Higher crystallinity for CNCs is associated 21 ACS Paragon Plus Environment


368

with higher strength and thermal stability, which is expected to be beneficial

369

for manufacturing advanced bio-composites with high strength and heat

370

resistance.31

371

372 373 374

Figure 4 (a) XRD patterns of CNCs obtained through microwave and

375

sonication for 15 min, 30 min, 45 min, 60 min and 75 min, labeled as SMU-15,

376

SMU-30, SMU-45, SMU-60 and SMU-75, respectively; (b) XRD spectra of

377

DBP, control sample and CNCs.

378 379

Thermal Property. The TGA and DTG curves of CNCs are shown in Figure

380

5a and Figure 5b, respectively. CNCs have a drastic weight loss in the range

381

of 250-380 ℃ due to the thermal decomposition of glucose rings. Compared

382

with DBP, the onset decomposition temperature (Ti) of CNCs increases from

383

293 ℃ to 331 ℃ (Figure 5a), and the maximum decomposition temperature


Page 22 of 32

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384

(Tmax) increases from 338 ℃ to 356 ℃ (Figure 5b), offering direct evidence

385

that CNCs have enhanced thermo-stability. The thermal stability of CNCs is

386

associated with crystallinity, i.e. more ordered regions of cellulose need more

387

energy to thermal decomposition.32 The high crystallinity of the as-prepared

388

CNCs provides them with great thermal performance. Additionally, the

389

thermo-stability of the obtained CNCs is higher than that of sulfuric acid

390

method. For CNCs by sulfuric acid hydrolysis, the adherent sulfated groups

391

induce the dehydration reaction and accelerate low-temperature thermal

392

decomposition.33 However, the CNCs by presented method have no such

393

defect. The high thermal stability can broaden the application fields of CNCs,

394

especially in bio-composites that have high requirement for thermo-stability.

395

396 397 398

Figure 5 (a) TGA and (b) DTG curves of DBP, control sample and CNCs.

399

400

CONCLUSIONS 23 ACS Paragon Plus Environment


Page 24 of 32

401

Functionalized CNCs with high yields and excellent thermo-stability were

402

fabricated via a solvent-free acid hydrolysis of cellulose under synchronous

403

microwave and sonication assistance. In the one-pot procedure, the

404

synergetic effects of mechanochemistry as a way of process intensification

405

accelerated the simultaneous hydrolysis and carboxylation of cellulose.

406

Thermally stable CNCs with a high yield of 85.5% were achieved, which

407

present enhanced uniformity and good stability. The excellent performance

408

makes CNCs have potential application in nano bio-composite to enhance the

409

mechanical property and thermal stability. The convenient, green and

410

high-efficient approach benefits from the mechanochemical activation,

411

avoided intermediates’ separation, and no consumption of solvents. The new

412

pathway is amenable to the high-yield and large-scale production of

413

high-quality CNCs that will carry significant benefits in terms of economy and

414

sustainability.

415 416

AUTHOR INFORMATION

417

Corresponding Author

418

*

419

[email protected].

Tel.:

+86

591

88160598.

Fax:

+86

591

420 421

Notes 24 ACS Paragon Plus Environment

85715175.

E-mail:

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422


The authors declare no competing financial interest.

423 424

ACKNOWLEDGMENTS

425

This work was financially supported by the Special Scientific Research Fund

426

for Public Service Sectors of Forestry (Grant number 201504603), Talent

427

Introduction Program of Minjiang University (Grant number MJY18010) and

428

Open Project Program of Fujian Key Laboratory of Novel Functional Textile

429

Fibers and Materials (Minjiang University) (Grant number FKLTFM1803).

430 431

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Table of Contents

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High Yield Synthesis of Functionalized Cellulose Nanocrystals for Nano Bio-composites

551

Qilin Lu, † Linna Lu, † Yonggui Li,† Yuxin Yan, ‡ Zhaofeng Fang, ‡ Xin Chen, ‡

552

Biao Huang,*,‡

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High Yield Synthesis of Functionalized Cellulose ... - ACS Publications

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