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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
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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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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
34
cellulose nanofibrils (CNFs) (or cellulose nanofibers) or cellulose nanocrystals (CNCs) (or
35
cellulose nanowhiskers, nanocellulose crystals, etc.) depending on their lengths or aspect ratios.
36
Nanocellulose bulk materials such as hydrogels, aerogels, forms, and films, and
37
nanocellulose-containing composites offer unique and excellent properties in some cases,
38
which originate from the characteristic nanosizes, nanostructures, and properties of the
39
nanocelluloses.
40
characterization, and applications can be found in the literature.1–6
Several
reviews
of
nanocelluloses
focusing
on
their
preparation,
41
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
43
materials such as nanoclays and carbon nanotubes. In these cases, homogeneous and individual
44
distributions of CNFs without agglomerations are required in the matrices. CNF hydrogels,
45
aerogels, forms, and films also exhibit unique air-filtration, gas-barrier/gas-separation, and
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Biomacromolecules
insulation
properties1–6
46
thermal
when
47
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
51
widths. However, it is generally difficult to increase the solid contents of these CNFs with high
52
aspect ratios up to, for example, 5% in water or organic solvent dispersion while maintaining
53
the individually nanodispersed states. Each long CNF element has some kinks,10 which act as
54
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
57
are prepared at the laboratory scale from wood and cotton linters celluloses by acid hydrolysis
58
with, for example, 65% H2SO4 at 70 °C for 30 min, followed by washing with water using
59
centrifugation and dialysis against water.11–13 Because most of the disordered and some
60
crystalline regions in native celluloses are removed during acid hydrolysis, the mass recovery
61
ratios decrease to ~40% when prepared at the laboratory level. The sulfate ester contents of
62
these CNCs are ~0.3 mmol/g.11–13 Homogeneous CNC/water dispersions are then prepared by
63
sonication of CNC/water mixtures. These CNCs have mostly spindle-like morphologies with
64
widths of 10–20 nm. The CNC/water dispersions can be spray-dried as powders, which are
65
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
67
from, for example, softwood bleached kraft pulp (SBKP) by TEMPO-mediated oxidation in
68
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
71
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
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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
79
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.
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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
86
MCCs were measured. The TEMPO-oxidized SBKPs and MCCs were then nano-dispersed in
87
water by cavitation force treatment using ultrasonication for 10–120 min to prepare
88
TEMPO-oxidized CNCs. Cavitation treatment has been previously applied by our group to
89
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
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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
100
from wood cellulose by dilute acid hydrolysis and successive grinding to an ~80-µm particle
101
size (Funakoshi Co. Ltd., Tokyo, Japan). Other chemicals and solvents of laboratory grade
102
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
106
water (300 mL) containing TEMPO (0.048 g) and sodium bromide (0.3 g). Then, 1.7 M NaClO
107
(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
109
magnetic stir bar at pH 10 with the continuous addition of 0.5 M NaOH using a pH stat
110
(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
113
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).
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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
121
TEMPO-oxidized celluloses. The ultrasonication was performed at a frequency of 19.5 kHz
122
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
127
cellulose fractions and metal particles formed from the sonication tip during disintegration. The
128
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
137
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
139
and length measurements were performed for 74–100 and 640–817 independent and different
140
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
147
soaked in dilute HCl solution to convert the sodium carboxylate groups into protonated
148
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
156
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
166
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
196
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
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consistent with the crystal widths of cellulose I measured from the XRD patterns in Figure 3.
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Figure 4. AFM height images of TEMPO-CNCs prepared from TEMPO-oxidized SBKP and
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MCC by sonication in water for 30, 60, and 120 min.
292 293 10 Number-average width with standard deviation
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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
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Figure 5. Number-average width with standard deviation and width-average width of
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TEMPO-CNCs, determined from AFM images in Figure 4.
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The MCCs are produced from wood cellulose (dissolving pulp) by dilute acid hydrolysis, ACS Paragon Plus Environment
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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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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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