Acid-Free Preparation of Cellulose Nanocrystals by TEMPO Oxidation

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Acid-free Preparation of Cellulose Nanocrystals by TEMPO Oxidation and Subsequent Cavitation Yaxin Zhou, Tsuguyuki Saito, Lennart Bergström, and Akira Isogai Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.7b01730 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017

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Biomacromolecules

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Acid-free Preparation of Cellulose Nanocrystals by

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TEMPO Oxidation and Subsequent Cavitation

3

Yaxin Zhou,† Tsuguyuki Saito,† Lennart Bergström,†,‡ and Akira Isogai*,†

4 5 6

†

7

University of Tokyo, Tokyo 113-8657, Japan

Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The

8 9 10

‡

Department of Materials and Environmental Chemistry, Stockholm University, Svante

Arrhenius väg 16C, S-106 91 Stockholm, Sweden

11 12

ABSTRACT: Softwood bleached kraft pulp (SBKP) and microcrystalline cellulose (MCC)

13

were oxidized using a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated system. The

14

TEMPO-oxidized SBKP prepared with 10 mmol/g NaClO (SBKP-10) had a higher mass

15

recovery ratio and higher carboxylate content than the other prepared celluloses including the

16

TEMPO-oxidized MCCs. The SBKP-10 was then exposed to cavitation-induced forces

17

through sonication in water for 10–120 min to prepare aqueous dispersions of needle-like

18

TEMPO-oxidized cellulose nanocrystals (TEMPO-CNCs) with homogenous width of 3.5–3.6

19

nm and average lengths of ~200 nm. The average chain lengths of the cellulose molecules that

20

make up the TEMPO-CNCs were less than half the average lengths of the TEMPO-CNCs.

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Compared with conventional CNCs prepared by acid hydrolysis, the TEMPO-CNCs prepared

22

by the acid-free and dialysis-free process exhibited higher mass recovery ratios, significantly

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Biomacromolecules 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

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higher amounts of surface anionic groups, and smaller and more homogeneous widths.

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KEYWORDS: Cellulose nanocrystal, TEMPO-mediated oxidation, Average length, Average

26

width, Cavitation

27 28

INTRODUCTION

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Nanocelluloses, prepared from abundant plant biomass resources, have received much

30

attention as new bio-based nanomaterials in this decade. The surface nano-structures,

31

morphologies, dimensions, and properties of nanocelluloses are affected by the cellulose

32

sources used as starting materials, any pretreatment before nanofibrillation, and the

33

nanofibrillation apparatuses and conditions. Nanocelluloses are roughly categorized into

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cellulose nanofibrils (CNFs) (or cellulose nanofibers) or cellulose nanocrystals (CNCs) (or

35

cellulose nanowhiskers, nanocellulose crystals, etc.) depending on their lengths or aspect ratios.

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Nanocellulose bulk materials such as hydrogels, aerogels, forms, and films, and

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nanocellulose-containing composites offer unique and excellent properties in some cases,

38

which originate from the characteristic nanosizes, nanostructures, and properties of the

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nanocelluloses.

40

characterization, and applications can be found in the literature.1–6

Several

reviews

of

nanocelluloses

focusing

on

their

preparation,

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CNFs with high aspect ratios >100 contribute to efficient improvement of mechanical,

42

thermal, and other properties when used in composites with polymer matrices or inorganic

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materials such as nanoclays and carbon nanotubes. In these cases, homogeneous and individual

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distributions of CNFs without agglomerations are required in the matrices. CNF hydrogels,

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aerogels, forms, and films also exhibit unique air-filtration, gas-barrier/gas-separation, and


2

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Biomacromolecules

insulation

properties1–6

46

thermal

when

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2,2,6,6-Tetramethylpiperidine-1-oxyl

48

phosphorylation9 of wood cellulose under adequate conditions, followed by mechanical

49

disintegration in water, enables the preparation of highly viscous and transparent CNF gels.

50

These gels consist of completely individualized CNF elements with homogeneous ~3-nm

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widths. However, it is generally difficult to increase the solid contents of these CNFs with high

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aspect ratios up to, for example, 5% in water or organic solvent dispersion while maintaining

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the individually nanodispersed states. Each long CNF element has some kinks,10 which act as

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defects in the bulk and composite materials.

radical

prepared

under

adequate

(TEMPO)-mediated

conditions.

oxidation3,7,8

and

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CNCs with low aspect ratios have therefore been preferable for certain preparation

56

conditions at the industrial level and for applications as functional bionanomaterials.1,4,5 CNCs

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are prepared at the laboratory scale from wood and cotton linters celluloses by acid hydrolysis

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with, for example, 65% H2SO4 at 70 °C for 30 min, followed by washing with water using

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centrifugation and dialysis against water.11–13 Because most of the disordered and some

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crystalline regions in native celluloses are removed during acid hydrolysis, the mass recovery

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ratios decrease to ~40% when prepared at the laboratory level. The sulfate ester contents of

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these CNCs are ~0.3 mmol/g.11–13 Homogeneous CNC/water dispersions are then prepared by

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sonication of CNC/water mixtures. These CNCs have mostly spindle-like morphologies with

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widths of 10–20 nm. The CNC/water dispersions can be spray-dried as powders, which are

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easily re-dispersible at the original CNC level by disintegration in water.

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TEMPO-oxidized CNFs with ~3-nm widths and high aspect ratios >100, which are prepared

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from, for example, softwood bleached kraft pulp (SBKP) by TEMPO-mediated oxidation in

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water,3,7,8 can be converted into CNCs by acid hydrolysis. In this case, the acid molecules

69

present after hydrolysis must be removed from the acid-containing CNC mixtures using, for ACS Paragon Plus Environment

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example, dialysis.14 The average length of a CNC prepared using this approach was reported to

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be 168 nm.14 The carboxylate contents of the original TEMPO-oxidized CNFs partly decreased,

72

and the original nanofibril widths of ~2.6 nm increased to ~3.8 nm during acid hydrolysis.

73

TEMPO-mediated oxidation has been applied to SBKP and cotton linters cellulose.15 However,

74

dialysis was required to isolate and purify the oxidized products, which were mostly

75

agglomerated because of no sonication treatment. Tunicate nanowhiskers were prepared by

76

acid hydrolysis, which were then TEMPO-oxidized to prepare TEMPO-oxidized tunicate

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nanowhiskers.16 In this case also, dialysis was required to isolate and purify the products.

78

Sonication-induced TEMPO-mediated oxidation of plant celluloses has been reported for

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efficient surface carboxylation of cellulose microfibrils.17‒21 However, in this oxidation system,

80

the completely individualized CNCs formed during the oxidation/ultrasonication cannot be

81

recovered as solids in high yields.18,21 Thus, the CNC fractions sufficiently nanodispersed in

82

the oxidation mixtures must be isolated and purified using, for example, dialysis to separate the

83

CNCs from TEMPO and other water-soluble reagents/compounds present in the mixtures.

84

The mass recovery ratios, carboxylate contents, viscosity-average degrees of polymerization

85

(DPv), and crystallinities and crystal widths of cellulose I for the TEMPO-oxidized SBKPs and

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MCCs were measured. The TEMPO-oxidized SBKPs and MCCs were then nano-dispersed in

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water by cavitation force treatment using ultrasonication for 10–120 min to prepare

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TEMPO-oxidized CNCs. Cavitation treatment has been previously applied by our group to

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TEMPO-oxidized wood and tunicate celluloses to measure the limiting lengths for calculation

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of the tensile strengths of single TEMPO-CNFs.22 However, comprehensive investigations to

91

prepare TEMPO-CNCs and characterize their properties have not yet been performed. In this

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study, the crystallinities and crystal widths of cellulose I, average widths and lengths, and their

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distributions, and DPv values for the TEMPO-CNCs were measured. The characteristics of the ACS Paragon Plus Environment

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Biomacromolecules

TEMPO-CNCs are also discussed and compared with those of conventional CNCs.

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MATERIALS AND METHODS

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Materials. Two celluloses were used as starting materials: SBKP and MCC. The never-dried

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SBKP with a ~90% α-cellulose content was provided by Nippon Paper Industries Co. Ltd.,

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Tokyo, Japan. The commercially available MCC had an α-cellulose content of 99%, produced

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from wood cellulose by dilute acid hydrolysis and successive grinding to an ~80-µm particle

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size (Funakoshi Co. Ltd., Tokyo, Japan). Other chemicals and solvents of laboratory grade

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were purchased from Wako Pure Chemicals (Tokyo, Japan) or Sigma-Aldrich (Saint Louis,

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MO, USA), and were used without prior purification.

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TEMPO-Mediated

Oxidation.

The

SBKP

and

MCC

were

oxidized

by the

105

TEMPO/NaBr/NaClO system in water at pH 10.7,8,23 The SBKP or MCC (3 g) was suspended in

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water (300 mL) containing TEMPO (0.048 g) and sodium bromide (0.3 g). Then, 1.7 M NaClO

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(10 or 20 mmol per gram of cellulose) was added to the cellulose slurry containing TEMPO and

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NaBr at room temperature under magnetic stirring. The slurry was gently agitated using a

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magnetic stir bar at pH 10 with the continuous addition of 0.5 M NaOH using a pH stat

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(AUT-501, DKK-TOA, Tokyo, Japan), and the oxidation continued until no NaOH

111

consumption was observed for 4.5 and 7 h for the NaClO additions of 10 and 20 mmol/g,

112

respectively. After oxidation, the TEMPO-oxidized cellulose was post-reduced with NaBH4 (0.3

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g) in the same container at room temperature for 3 h to reduce small amounts of C6-aldehydes

114

and C2/C3-ketones present in the oxidized cellulose to hydroxy groups,24 followed by thorough

115

washing with water by filtration and centrifugation for SBKP and MCC, respectively. The

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TEMPO-oxidized celluloses prepared from SBKP (MCC) with 10 and 20 mmol/g NaClO are


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abbreviated hereafter as SBKP-10 and SBKP-20, respectively (MCC-10 and MCC-20,

118

respectively).

119

Mechanical Disintegration. A 300-W ultrasonication system with a ߶ 26 titanium tip

120

(US-300E, Nihon Seiki Co. Ltd., Tokyo, Japan) was used for nanofibrillation of the

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TEMPO-oxidized celluloses. The ultrasonication was performed at a frequency of 19.5 kHz

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with an oscillation amplitude of 90% and an on/off interval of 1 min. The TEMPO-oxidized

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cellulose (0.1 g) was suspended in distilled water (100 mL) and sonicated for 10, 30, 60, or 120

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min while the temperature was maintained at 4 °C using a cooling system (CTP-1000, Eyela,

125

Co. Ltd., Tokyo, Japan) to avoid excessive heating caused by cavitation. The resulting

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dispersions were centrifuged at 12,700×g for 10 min to remove small amounts of unfibrillated

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cellulose fractions and metal particles formed from the sonication tip during disintegration. The

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TEMPO-oxidized nanocelluloses prepared from, for example, SBKP-10 by sonication in water

129

for 10, 30, 60, and 120 min are abbreviated as SBKP-10-10, SBKP-10-30, SBKP-10-60, and

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SBKP-10-120, respectively.

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Length and Width Measurements. The lengths and widths of isolated nanocellulose

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elements were measured using atomic force microscopy (AFM; NanoScopeV/MultiMode 8,

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Bruker, Co., Billerica, USA). The AFM images were obtained in the PeakForce QNM imaging

134

mode using a cantilever with a spring constant of 0.4 N/m and a SCANASYST-AIR probe

135

(Bruker) at a resonance frequency of 70 kHz. The nanocellulose widths were estimated from

136

the AFM height images and defined as the largest dimension measured along each

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nanocellulose, perpendicular to its long axis by NanoScope Analysis 1.70 software.22 The

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nanocellulose lengths were measured from the AFM images using ImageJ software. The width

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and length measurements were performed for 74–100 and 640–817 independent and different

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nanocelluloses, respectively, from their AFM images. ACS Paragon Plus Environment

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Biomacromolecules

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Other Analyses. The carboxylate contents of the oxidized celluloses were determined using

142

an electric conductivity titration method and calculated according to the procedure described in

143

a previous report.7,8,23,25 The TEMPO-oxidized SBKP and MCC were suspended in water at a

144

0.1% consistency, and the mixtures were sonicated for 10 min. The dispersions were subjected

145

to ζ-potential measurement after dilution with water to 0.05% consistency using a ζ-potential

146

analyzer (Delsa Nano HC, Beckman Coulter, Germany). The TEMPO-oxidized celluloses were

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soaked in dilute HCl solution to convert the sodium carboxylate groups into protonated

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carboxy groups,26 followed by washing with water and freeze-drying. Fourier‒transform

149

infrared (FTIR) spectra of the freeze-dried samples were recorded from 4000 to 400 cm–1 using

150

an FT/IR-600 Plus (JASCO, Co., Tokyo, Japan). The freeze-dried samples (approximately 0.1

151

g each) were pressed into pellets with densities of 1.4–1.5 g/cm3 at a pressure of 800 MPa for 1

152

min. X-ray diffraction (XRD) patterns of the pellets were collected using an X-ray

153

diffractometer (RINT 2000, Rigaku, Co., Tokyo, Japan) equipped with Ni-filtered Cu Kα

154

radiation (λ = 0.1548 nm) and operated at 40 kV and 40 mA. The crystallinities of cellulose I

155

were measured from the XRD patterns using the Segal method.27 The crystal widths of

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cellulose I were calculated from the full widths at half maximums of the (2 0 0) diffraction peaks

157

using Scherrer’s equation.28 The freeze-dried samples (0.04 g each) were dissolved in 0.5 M

158

copper ethylenediamine (20 mL) for 30 min under stirring, and the intrinsic viscosities [η] were

159

measured using a Cannon‒Fenske capillary viscometer in water bath at 25 °C. The obtained [η]

160

values were converted into viscosity-average molar masses (Mv) and viscosity-average degrees

161

of polymerization (DPv) using the following equations: [η] = 0.094×Mv0.67 and DPv = Mv/162,

162

respectively.29

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RESULTS AND DISCUSSION

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Preparation of TEMPO-Oxidized Cellulose from SBKP and MCC. SBKP and MCC

167

were oxidized using the TEMPO/NaBr/NaClO system in water at pH 10 with 10 and 20

168

mmol/g NaClO. The TEMPO-oxidized celluloses were then sonicated in water for 10–120 min

169

to prepare TEMPO-CNCs. The preparation and analytical scheme of the TEMPO-oxidized

170

celluloses and TEMPO-CNCs from SBKP and MCC and their corresponding abbreviations are

171

shown in Figure 1.

172 173 174 175

Softwood bleached kraft pulp (SBKP) Microcrystalline cellulose (MCC) TEMPO/NaBr/NaClO oxidation in water at pH 10 with NaClO of 10 and 20 mmol/g Reduction with NaBH4 at pH 10 SBKP-10, SBKP-20 MCC-10, MCC-20

176

Mass recovery ratio Carboxylate content DPv XRD XRD crystallinity XRD crystal size FT-IR ξ-Potential

Sonication in water for 10, 30, 60, 120 min

177 178 179 180 181

SBKP-10-10, SBKP-10-30, SBKP-10-60, SBKP-10-120 SBKP-20-10, SBKP-20-30, SBKP-20-60, SBKP-20-120 MCC-10-10, MCC-10-30, MCC-10-60, MCC-10-120 MCC-20-10, MCC-20-30, MCC-20-60, MCC-20-120 AFM image Average length, length distribution Average width, width distribution DPv XRD XRD crystallinity XRD crystal size

182

Figure 1. Preparation and analytical scheme of TEMPO-oxidized celluloses and

183

TEMPO-CNCs prepared from SBKP and MCC.

184 185

Table 1 shows that the mass recovery ratios and fundamental properties of the

186

TEMPO-oxidized celluloses are summarized in Table 1. The mass recovery ratios of SBKP-10

187

and SBKP-20 were 94% and 93%, respectively, indicating that the non-crystalline

188

hemicelluloses originally present in SBKP were preferentially removed during the oxidation ACS Paragon Plus Environment

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Biomacromolecules

water-washing

processes.30

189

and

The

SBKP

significantly

depolymerized

during

190

TEMPO-mediated oxidation and the DPv values of the TEMPO-oxidized SBKPs decreased

191

from 2900 to 800 and 650 at NaClO additions of 10 and 20 mmol/g, respectively. Thus,

192

TEMPO-mediated oxidation of SBKP is suitable for the preparation of TEMPO-CNCs with

193

low crystal lengths.

194 195

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Table 1. Mass Recovery Ratios and Properties of TEMPO-Oxidized Celluloses

a

Sample

Mass recovery (%)

Carboxylate content (mmol/g)

DPv

XRD crystallinity (%)

XRD crystal width (nm)

ξ-Potential (mV)a

Conductivity (µs/cm)a

SBKP

‒

0.01

2900

82

3.9

‒

‒

SBKP-10

94

1.74

800

81

3.6

‒54

121

SBKP-20

93

1.57

650

83

3.6

‒50

119

MCC

‒

0.06

270

90

4.7

‒

‒

MCC-10

70

1.24

250

88

4.3

‒60

70

MCC-20

69

1.12

240

88

4.1

‒55

57

Measured for 0.05% aqueous dispersions at pH 7.0–7.2 after sonication for 10 min.

197 198

When MCC was used as the starting material, the mass recovery ratios decreased to 70% and

199

69% when the amounts of NaClO added were 10 and 20 mmol/g, respectively. A part of the

200

cellulose molecules in MCC are presumably oxidized and depolymerized to become

201

water-soluble degradation products, resulting in lower mass recovery ratios than those for the

202

TEMPO-oxidized SBKPs.15

203

The crystal widths of cellulose I was 4.7 and 3.9 nm for MCC and SBKP, respectively. Thus,

204

the lower sodium carboxylate contents of the TEMPO-oxidized MCCs compared to the

205

TEMPO-oxidized SBKP may be related to the lower specific surface area of cellulose I crystal


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for MCC than SBKP.31

207

The crystallinities and crystal widths of the original SBKP and MCC were mostly unchanged

208

after the TEMPO-mediated oxidation. This lack of change was due to the oxidation occurring

209

position-selectively at the C6-OH groups exposed on the crystalline cellulose microfibril

210

surfaces.3 FTIR spectra of SBKP and MCC and their TEMPO-oxidized products are presented

211

in Figure S1 in Supporting Information. TEMPO-oxidized celluloses with protonated carboxy

212

groups had large absorption band at 1724 cm–1 due to C=O stretching vibration.23 The band at

213

~1623 cm–1 is due to bending vibration of water molecules adsorbing in the samples.

214

Preparation and Characterization of TEMPO-Oxidized Celluloses after Sonication in

215

Water. The TEMPO-oxidized SBKPs and MCCs were sonicated in water to prepare 0.1%

216

(w/v) nanocellulose/water dispersions. All the sonicated dispersions had ~99% light

217

transmittances at a wavelength of 600 nm before centrifugation. The DPv values of the original

218

SBKP and MCC, TEMPO-oxidized SBKP and MCC, and those after sonication for 10–120

219

min followed by freeze-drying are shown in Figure 2.

220

The DPv values of SBKP-10 and SBKP-20 decreased to 370, 250, 170, and 110–130 after

221

sonication for 10, 30, 60, and 120 min, respectively. The DPv values of MCC-10 and MCC-20

222

also decreased to 210–220, 170–190, 160–170, and 130 after sonication for 10, 30, 60, and 120

223

min, respectively. It is interesting to note that the DPv values of the TEMPO-oxidized SBKPs

224

and MCCs became quite similar after sonication in water for 60 and 120 min. The amount of

225

NaClO added had a negligible effect on the DPv values after sonication for both SBKP and

226

MCC. Hence, we have limited the detailed investigation on the effect of sonication to

227

SBKP-10 and MCC-10.

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230 2800

231

800

DPv

232 233

SBKP-10 SBKP-20 MCC-10 MCC-20

2400

600 400

234

200

235

0

236

Original cellulose

TEMPOoxidized cellulose

TEMPO-oxidized cellulose after sonication for 10 min

30 min

60 min

120 min

237

Figure 2. DPv values of the original SBKP and MCC, TEMPO-oxidized SBKPs and MCCs,

238

and those after sonication in water for 10–120 min.

239

Crystal width Crystallinity

240

242 243 244 245 246

SBKP MCC

8

100

80

6

60 4 40 2

Crystallinity (%)

241

Crystal width of (2 0 0) plane (nm)

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

Biomacromolecules

20

0

0 Original cellulose

TEMPOoxidized cellulose

TEMPO-oxidized cellulose after sonication for 10 min

30 min

60 min

120 min

247

Figure 3. Crystal widths of (2 0 0) plane and Segal crystallinities of the original SBKP and

248

MCC, TEMPO-oxidized SBKP and MCC, and those sonicated for 10–120 min.

249 250

The XRD patterns of the original SBKP and MCC, TEMPO-oxidized SBKP and MCC, and

251

those sonicated in water for 10–120 min are presented in Figure S2 in Supporting Information.

252

The crystal widths of the (2 0 0) plane and Segal crystallinities of cellulose I for these samples

253

are depicted in Figure 3. The crystal widths of cellulose I for the original SBKP and

254

TEMPO-oxidized SBKPs were 3.9 and 3.6 nm, respectively, and these values decreased to ACS Paragon Plus Environment

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2.2–2.3 nm after sonication in water for 10–120 min. The crystal widths of cellulose I for the

256

original MCC and TEMPO-oxidized MCCs were 4.7 and 4.1–4.3 nm, respectively, and these

257

values decreased to 3.5–3.6 nm after sonication in water for 10–120 min. Thus, the

258

TEMPO-oxidized SBKPs result in cellulose I crystal widths that are smaller than those

259

obtained using the TEMPO-oxidized MCCs.

260

Width and Length Distributions of TEMPO-CNCs. The AFM images of six

261

TEMPO-CNCs are presented in Figure 4. The SBKP-10-10 and MCC-10-10 contained some

262

entangled or agglomerated nanoelements without complete individualization; therefore, the

263

AFM images of these samples were excluded from Figure 4. The SBKP-10-30 contained not

264

only needle-like TEMPO-CNCs but also some long fibrils with or without kinks. Extending the

265

sonication time to 60 and 120 min resulted in the formation of needle-like nanocrystals with

266

homogeneous widths. The nanocelluloses prepared from MCC-10 had needle-like or

267

spindle-like nanocrystals with heterogeneous widths.

268

Figure 5 shows that the number-average widths of SBKP-10-30, SBKP-10-60, and

269

SBKP-10-120 were almost constant at 3.5–3.6 nm with standard deviations of 0.7–0.8 nm, and

270

their width-average widths were also 3.5–3.6 nm. The width polydispersities, i.e., the ratios of

271

the width-average width/number-average width, were quite small (1.04–1.06), with quite small

272

width distributions. The number-average widths of MCC-10-30, MCC-10-60, and MCC-10-120

273

were 4.6–5.5 with standard deviations of 1.5–2.4 nm. Their width-average widths were 5.1–6.4

274

nm, and the corresponding polydispersities were 1.10–1.18. Thus, the use of SBKP-10

275

produced TEMPO-CNCs with smaller and more homogeneous widths than those prepared

276

using MCC-10. The number-average widths of the TEMPO-CNCs shown in Figure 5 are

277

consistent with the crystal widths of cellulose I measured from the XRD patterns in Figure 3.

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279 280 281 282 283 284 285 286 287 288 289 290

Figure 4. AFM height images of TEMPO-CNCs prepared from TEMPO-oxidized SBKP and

291

MCC by sonication in water for 30, 60, and 120 min.

292 293 10 Number-average width with standard deviation

294

Width-average width

8

295 296 297

Width (nm)

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

Biomacromolecules

6

4

2

298 299

0

SBKP-10-30 SBKP-10-120 MCC-10-60 SBKP-10-60 MCC-10-120 MCC-10-30

300

Figure 5. Number-average width with standard deviation and width-average width of

301

TEMPO-CNCs, determined from AFM images in Figure 4.

302 303

The MCCs are produced from wood cellulose (dissolving pulp) by dilute acid hydrolysis, ACS Paragon Plus Environment

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and, therefore, they have high cellulose contents of ~99%.32 Most of the hemicelluloses in the

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original wood dissolving pulp are removed during acid hydrolysis. It is probable that the

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adjacent cellulose microfibrils are tightly adhered to each other through the formation of

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numerous hydrogen bonds during acid hydrolysis and successive spray drying. This abundant

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inter-fibrillar hydrogen bond formation probably caused the slight increase in the cellulose

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microfibril widths for the MCC, TEMPO-oxidized MCCs, and TEMPO-CNCs compare with

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SBKP

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In contrast, SBKP contains ~10% hemicelluloses, which probably behave like sponge layers

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present between crystalline cellulose microfibrils. In TEMPO-mediated oxidation, the oxidized

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TEMPO molecules with the N-oxoammonium structure (TEMPO+) have to form covalent

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bonds with the C6-OH groups present on crystalline cellulose microfibril surfaces to convert

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them into sodium C6-carboxylates through C6-aldehydes. Therefore, the presence of

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disordered hemicellulose molecules between crystalline cellulose microfibrils in SBKP

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efficiently enhances the oxidation, compared with that using MCC with almost no

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hemicelluloses.3

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The length distribution histograms of the TEMPO-CNCs prepared from SBKP and MCC are

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presented in Figures S3 and S4, respectively, in Supporting Information. Figure 6 shows that

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the length-average length of the TEMPO-CNCs prepared from SBKP decreased from 292 to

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185 nm upon increasing the sonication time from 30 to 120 min. Similarly, the length-average

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length of the TEMPO-CNCs prepared from MCC decreased from 231 to 190 nm upon

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increasing the sonication time from 30 to 120 min. The SBKP-10-120 and MCC-10-120 had

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similar average lengths and length distributions, which corroborates the previous result that the

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initial differences in polymerization between SBKP and MCC were eliminated by extensive

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sonication. ACS Paragon Plus Environment

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Acid-Free Preparation of Cellulose Nanocrystals by TEMPO Oxidation

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