Accepted Manuscript Title: Bio-based nanocomposites obtained through covalent linkage between chitosan and cellulose nanocrystals Authors: Jo˜ao P. de Mesquita, Claudio L. Donnici, Ivo F. Teixeira, Fabiano V. Pereira PII: DOI: Reference:
S0144-8617(12)00446-8 doi:10.1016/j.carbpol.2012.05.025 CARP 6622
To appear in: Received date: Revised date: Accepted date:
31-1-2012 3-5-2012 5-5-2012
Please cite this article as: de Mesquita, J. P., Donnici, C. L., Teixeira, I. F., & Pereira, F. V., Bio-based nanocomposites obtained through covalent linkage between chitosan and cellulose nanocrystals, Carbohydrate Polymers (2010), doi:10.1016/j.carbpol.2012.05.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Bio-based nanocomposites obtained through covalent linkage
2
between chitosan and cellulose nanocrystals
3
ip t
4
João P. de Mesquita, Claudio L. Donnici, Ivo F. Teixeira and Fabiano V. Pereira*
cr
5 6
us
7
Departamento de Química - Universidade Federal de Minas Gerais. Av. Antônio
9
Carlos, 6627 - Pampulha - Belo Horizonte – MG. CEP 31270-901.
an
8
10
M
11 12 *
Corresponding author: Tel.: +55 31 3409 5753 ; Fax: +55 313409 5700
d
13
Departamento de Química
15
Universidade Federal de Minas Gerais
16
Av. Antônio Carlos, 6627 - Pampulha
17
CEP: 31270-901
18
E-mail: [email protected]
20 21
Ac ce p
19
te
14
Abbreviations 1, 2, 3
22 23 1. CNC: cellulose nanocrystal 2. CH: chitosan 3. MAC: methyl adipoyl chloride
Page 1 of 36
24 25
Abstract Bio-based nanocomposites were obtained through covalent linkage between cellulose nanocrystals (CNCs) and the natural polymer chitosan (CH). The CNCs
27
were first functionalized with methyl adipoyl chloride (MAC) and the reactive end
28
groups on the surface of the CNCs were reacted with the amino groups of the CH
29
biopolymer in an aqueous medium. The functionalized CNCs and the resulting
30
nanocomposites were characterized using FTIR, TEM, XRD, and elemental
31
analyses. Characterization of the functionalized CNCs showed that up to 8 % of the
32
hydroxyl groups in the nanocrystals were substituted by the MAC residue. The
33
covalent linkage between the CNCs and CH was confirmed by FTIR spectroscopy.
34
The nanocomposites demonstrated a significant improvement in the mechanical
35
performance and a considerable decrease in the hydrophilicity relative to the neat
36
chitosan. The approach used in this work can be extended to other natural polymers.
39 40 41 42 43 44
cr
us
an
M
d te
38
Keywords: chitosan; cellulose nanocrystals; bio-based nanocomposites.
Ac ce p
37
ip t
26
45 46 47 48
2
Page 2 of 36
49
1. Introduction
50
In the preparation of nanocomposites using different class of nanoparticles (such as nanoclays, carbon nanotubes or cellulose nanocrystals, CNCs), the most
52
important property is the formation of a percolation network within the matrix.
ip t
51
In the case of composite materials prepared with polysaccharide nanocrystal
54
such as CNCs or chitin nanocrystals (Muzzarelli, 2011), the achievement of an
55
appropriate percolation network is a particular issue because the nanoparticles
56
exhibit strong hydrogen bonding interactions between one another (Dufresne, 2010;
57
Wang et al., 2012). These interactions can create large undesirable agglomerates
58
within the polymer but, at the same time, the same interactions play an important role
59
to develop the desirable filler network inside the matrix. Thus, to obtain an
60
appropriate percolation of the CNCs in a nanocomposite, it is necessary to disperse
61
the nanocrystals within a polymer matrix and to maximize the interfacial adhesion
62
between the dispersed CNCs and the polymer, while still maintaining the filler/filler
63
hydrogen bonding interactions (Dufresne, 2010).
us
an
M
d
te
Several strategies have been developed during the last few years to achieve
Ac ce p
64
cr
53
65
the dispersion of the nanocrystals in different polymers, including the use of
66
surfactants or using the chemical surface modification of the nanowhiskers (de
67
Menezes, Siqueira, Curvelo & Dufresne, 2009; Petersson, Kvien & Oksman, 2007).
68
However, the use of surfactants to coat the high specific area of the nanocrystals
69
prohibits the use of this strategy (Samir, Alloin & Dufresne, 2005). Additionally,
70
previous results indicated that nanocomposites prepared from modified nanoparticles
71
typically exhibited a significant decrease in the mechanical performance because the
72
surface groups can insulate the nanoparticles, thus preventing the desirable filler/filler
73
interactions (Capadona et al., 2007).
3
Page 3 of 36
74
An alternative method to prepare nanocomposites with CNCs consists of a polymer chain surface modification of the nanoparticles (Dufresne, 2010; Habibi,
76
Goffin, Schiltz, Duquesne, Dubois & Dufresne, 2008; Lin, Chen, Huang, Dufresne &
77
Chang, 2009; Morandi, Heath & Thielemans, 2009). Generally speaking, two different
78
strategies have been used to obtain grafted chains on the surface of the
79
nanoparticles: the “grafting from” and the “grafting onto” methods (Dufresne, 2010).
80
In the first strategy, the polymer chains are formed using a in situ surface-initiated
81
polymerization from the immobilized initiators on the substrate (Habibi, Goffin,
82
Schiltz, Duquesne, Dubois & Dufresne, 2008). The second method (grafting onto)
83
consists of attaching a pre-synthesized polymer chain to the surface of the
84
nanoparticle (Azzam, Heux, Putaux & Jean, 2010; Dufresne, 2010). Using this
85
strategy, Labet et al (Labet, Thielemans & Dufresne, 2007) prepared starch
86
nanoparticles grafted with poly(tetrahydrofuran), poly(caprolactone), and
87
poly(ethylene glycol) monobutyl ether chains using toluene 2,4-diisocyanate as the
88
linking agent. More recently, Azzam et al.(Azzam, Heux, Putaux & Jean, 2010)
89
described the preparation of thermally responsive polymer-decorated cellulose-
90
nanocrystals.
cr
us
an
M
d
te
Ac ce p
91
ip t
75
Herein, we describe the preparation of a bio-based nanocomposite obtained
92
through covalent linkage between cellulose nanowhiskers and a natural biopolymer,
93
chitosan (CH). We have chosen CH because it is one of the strongest natural
94
polymers and also because it contains reactive amino groups. Moreover, this
95
biopolymer exhibits some important properties, such as biocompatibility,
96
biodegradability and anti-bacterial properties, which make it a suitable material for
97
biomedical applications, including drug delivery, tissue engineering, wound healing
98
and various antimicrobial strategies (Dash, Chiellini, Ottenbrite & Chiellini, 2011). In
4
Page 4 of 36
99
addition to biomedical applications, CH films have also been used for food packaging, mainly because of their non-toxicity and biodegradability (de Azeredo,
101
2009). However, the relatively weak mechanical properties of this biopolymer (in
102
comparison to petroleum-based polymers) and its inherent water sensitivity restrict
103
the wide use of chitosan films, particularly in environments where moisture is present
104
(Sebti, Chollet, Degraeve, Noel & Peyrol, 2007).
cr
105
ip t
100
In this work the CNCs surface was functionalized, creating reactive end groups on the nanocrystals which reacted with the amino groups of the CH
107
biopolymer, leading to a bionanocomposite with covalent linkage between the CNCs
108
and the chitosan. The functionalized CNCs and the nanocomposites were
109
characterized using different techniques, including FTIR, TEM, XRD, and elemental
110
analyses.
M
an
us
106
d
111 2. Experimental Section
113
2.1. Materials
114
Highly deacetylated chitosan polymer was supplied by the Phytomare® Food
Ac ce p
te
112
115
Supplements. The degree of chitosan deacetylation (92%) was determined by
116
potentiometric titration, Fourier transform infrared spectrometry (FTIR) and 1H NMR.
117
Eucalyptus wood pulp was kindly supplied by the Bahia Pulp Company (Brazil).
118
Sulfuric acid and acetone were purchased from Synth. Methyl adipoyl chloride
119
(MAC), triethylamine and toluene were purchased from Sigma-Aldrich.
120 121
2.2. Preparation of Cellulose Nanocrystals
122
A sulfuric acid hydrolysis reaction was performed with eucalyptus wood pulp
123
according to a procedure described in the literature and in our previous work, with
5
Page 5 of 36
minor modifications (de Mesquita, Donnici & Pereira, 2010; de Mesquita, Patricio,
125
Donnici, Petri, de Oliveira & Pereira, 2011; de Rodriguez, Thielemans & Dufresne,
126
2006). After a bleach treatment of the pulp, a controlled acid hydrolysis was
127
performed by adding 10.0 g of cellulose to 160.0 mL of 64 wt% sulfuric acid under
128
strong mechanical stirring. Hydrolysis was performed at 50 °C for approximately 40
129
minutes. After hydrolysis, the dispersion was washed with three cycles of
130
centrifugation and the last wash was performed using dialysis against deionized
131
water until the dispersion reached a pH of ~6. Afterward, the dispersions were
132
ultrasonicated with a Cole Parmer Sonifier cell disruptor, equipped with a microtip for
133
5 minutes.
an
us
cr
ip t
124
M
134
2.3 Preparation of the bionanocomposites
136
The preparation of the nanocomposites with covalent linkage between
138
chitosan and cellulose nanocrystals (CH-c-CNCs) consisted of two steps.
te
137
d
135
Firstly, the CNCs were functionalized using the methyl adipoyl chloride (MAC) reagent and then, the functionalized CNCs were reacted with amino groups of the
140
chitosan to produce bionanocomposites with different concentrations of nanocrystals.
141
Typically, the chemical surface modification of the CNCs was performed in toluene at
142
60° C for 4 hours using constant mechanical stirring. The suspension of CNCs in
143
toluene was obtained using a solvent exchange procedure, (Siqueira, Bras &
144
Dufresne, 2009) where the original aqueous CNCs suspension was solvent
145
exchanged with acetone followed by toluene, using three successive centrifugation
146
and redispersion steps for each solvent. Two different functionalized CNC were
147
prepared and are named according to the MAC/CNC ratio (wt%) used in each
148
reaction: MA-CNC-1.9 (MAC/CNC=1.9) and MA-CNC-0.8 (MAC/CNC=0.8). For the
Ac ce p
139
6
Page 6 of 36
MA-CNC-1.9, 200 L of MAC and 300 L of triethylamine were added in a
150
suspension of nanowhiskers (~0.5 % m/v) containing 120 mg of cellulose whiskers.
151
For the MA-CNC-0.8, 80 L of MAC and 130 L of triethylamine were also added in a
152
suspension of nanowhisker (~0.5 % m/v) containing 120 mg of CNCs. Triethylamine
153
was used to catalyze the reaction and also to complex the HCl formed during the
154
reaction (Thielemans, Belgacem & Dufresne, 2006). After the reaction, a suspension
155
of functionalized CNCs in water was obtained using another solvent exchange
156
procedure, from toluene to acetone, and finally to water, using the same procedure
157
described previously. Then, the aqueous suspension of functionalized CNCs was
158
freeze-dried.
cr
us
an
Secondly, for the preparation of chitosan covalently bonded to the
M
159
ip t
149
functionalized nanocrystals, a CH solution was prepared adding the biopolymer in
161
acetic acid (3 % wt/v), and then, an appropriate amount of MA-CNC-1.9 or MA-CNC-
162
0.8 (0, 1.0, 5.0, 10, 20, 30, 40, 50 or 60 % wt/wt) was added. The reaction was
163
performed in the aqueous medium at room temperature, and the suspensions were
164
stirred for 24 hours. The bionanocomposites were named CH-c-CNC-1.9 or CH-c-
165
CNC-0.8 when the nanocomposites were prepared with MA-CNC-1.9 or MA-CNC-
166
0.8, respectively. For comparative purposes, nanocomposites were also prepared
167
using a simple mixture of unmodified CNCs with CH (detailed results obtained from
168
these samples are not shown here). These samples were named CH/CNCs.
te
Ac ce p
169
d
160
170
2.4 Characterization
171
Transmission electron microscopy (TEM) was performed on samples using a
172
FEI Tecnai G2-Spirit with a 120-kV acceleration voltage. The CNCs suspensions
173
were deposited from an aqueous dispersion on a carbon-Formvar-coated copper
7
Page 7 of 36
174
(300-mesh) TEM grids. The samples were subsequently stained with a 2% uranyl
175
acetate solution to enhance the microscopic resolution. Fourier transform infrared spectroscopy (FTIR) of the CNCs and the
176
nanocomposites was recorded using a Nicolet 380 FTIR spectrometer (Nicolet, MN).
178
The samples were prepared by using a KBr-disk.
ip t
177
cr
Thermogravimetric analysis (TG/DTG) was performed using a DTG60
179
SHIMADZU. The samples were heated from room temperature to 600°C using a
181
heating ramp of 15°C min-1 under N2 flow (200 mL.min-1).
us
180
XRD measurements were performed on a SHIMADZU (XRD6000) X-ray
183
diffractometer with a Cu-K radiation source (λ = 0.154 nm) and in the 2 range of 10
184
to 35° using a step intervals of 0.02°.
M
an
182
Elemental analysis was performed using a CHN Perkin-Elmer analyzer. The
185
results obtained from this technique were used to determine the degree of
187
substitution (DS), which is given by the number of hydroxyl groups substituted by
188
MAC per unit of anhydroglucose. The methodology used here was recently reported
189
by Siqueira et al (Siqueira, Bras & Dufresne, 2010). Using this methodology, the DS
190
was calculated through the equation:
191
DS
te
Ac ce p
192
d
186
72.07 C 162.14 , 143.15 C 84.08
(1)
In this equation (Siqueira, Bras & Dufresne, 2010), C is the relative carbon
193
content in the sample, and the numbers 72.07, 162.14, 143.15 and 84.08 are the
194
relative carbon mass of the anhydroglucose unit (C6H10O5), the molecular weight of
195
the anhydroglucose, the molecular weight of the MAC residue linked to the
196
anhydroglucose unit and the carbon mass of the MAC group, respectively. The
197
experimental values were corrected by considering unmodified CNCs as pure
198
cellulose, which correlates with a relative carbon content of 44.44 %. Thus, the 8
Page 8 of 36
relative carbon content of the unmodified CNC was converted to this value (using the
200
conversion factor = 1.113), and the same correction factor was applied to the
201
functionalized CNCs. The obtained values were an average of two measurements.
202
Mechanical properties were obtained using a Lloyd testing instrument (Model
ip t
199
LS 500). Film strips of 6 cm x 1.5 cm (length x width) were cut from each
204
preconditioned sample and mounted between the grips of the machine. The initial
205
grip separation and crosshead speed were set to 50 mm and 12.5 mm/min,
206
respectively. At least ten replicates of each specimen were averaged together.
us
The water uptake of the CH and the bionanocomposites was determined using
an
207
cr
203
the following procedure. The samples were cut into a 10 x 10 mm square and dried
209
overnight at 90°C. Next, the samples were submerged in distilled water for 48 h at
210
room temperature until reach the equilibrium state. The degree of swelling of each
211
sample was determined by the equation W(%) = (W – W o)/W o x 100, where W is
212
the weight after 48 of submersion in distilled water (using a filter paper to remove the
213
surface adsorbed water) and Wo is the initial dry weight of the sample. To compare
214
the water absorption of the nanocomposites with the absorption of the neat chitosan,
215
the W(%) values were converted into a relative degree of swelling, using the equation
216
W r= W/W QT x 100, where W QT is the water absorption of the pure chitosan.
218 219 220
d
te
Ac ce p
217
M
208
3. Results and Discussion 3.1. CNCs Functionalization To prepare the CH-c-CNCs nanocomposites, the nanocrystals were first
221
chemically-modified using methyl adipoyl chloride (MAC) reagent. The selective
222
functionalization of the CNCs was achieved via the reaction between the acyl
223
chloride moieties of MAC and the hydroxyl groups of the nanocrystals, yielding
9
Page 9 of 36
224
functionalized CNCs with methyl ester end groups. MAC reagent has already been
225
used in other selective syntheses (Moon et al., 2007; Regourd, Ali & Thompson,
226
2007). The reaction route utilized to functionalize the CNCs is provided in Scheme 1.
228
ip t
227 Insert Scheme 1
cr
229
It is important to mention here how the excess of unreacted MAC was
231
discarded from the CNCs after the reaction. The functionalized CNCs, dispersed in
232
toluene, were precipitated using centrifugation. The solid material was re-dispersed
233
in toluene and then washed two more times with the same solvent. After that, the
234
material was again washed with several cycles of centrifugation: three times with
235
acetone and three times with water in order to completely discarded the excess of
236
unreacted MAC reagent. This procedure was important to prove the covalent
237
character between the MAC residue and the nanocrystals, ruling out possible
238
adsorption of carboxylated MAC.
an
M
d
te
Thus, the functionalization of the nanocrystals was confirmed first by the FTIR
Ac ce p
239
us
230
240
technique (Figure 1). The FTIR analysis of the freeze-dried nanocrystals suspensions
241
revealed the appearance of an absorption band at 1725 cm-1, characteristic of the
242
C=O ester stretch band. This band was found for both types of functionalized CNCs,
243
i.e., MA-CNC-1.9 and MA-CNC-0.8 (Figure 1). In addition, the spectrum of the
244
modified CNCs samples exhibited a narrower O-H stretching band (3360 cm-1), due
245
to a decrease in the amount of the hydrogen bonding between the nanoparticles.
246
Moreover, the signal at 1797 cm-1, assigned to the acyl chloride function of the MAC
247
reagent, was not observed in the spectra of the functionalized CNCs, indicating first a
248
complete reaction between these groups with the hydroxyl groups of the CNCs and
10
Page 10 of 36
249
also a complete discard of the unreacted MAC reagent from the nanocrystals
250
surface.
251 Insert Figure 1
ip t
252 253
255
Figure 2 shows a characteristic TEM image of the unmodified-CNC (a) and of
cr
254
the functionalized-CNC, MA-CNC-0.8 (b).
Insert Figure 2
an
257
us
256
258
Using several TEM images, the mean values for the length (L) and diameter
M
259
(D) of the unmodified rod-like nanocrystals were estimated to be 145±25 nm and
261
6±1.5 nm, respectively. Because the morphology of the nanocrystals plays a pivotal
262
role in nanocomposite properties (Dufresne, 2010; Eichhorn, 2011), it is very
263
important to ensure that the morphology of the CNCs was not significantly altered as
264
a result of the chemical modification. The TEM results show that the morphology of
265
the individual CNCs did not change after functionalization with the MAC reagent. The
266
nanocrystals maintained their elongated morphology with the same length and
267
aspect ratio as that of the unmodified CNCs. The TEM images obtained for the MA-
268
CNC-1.9 were similar (not shown).
te
Ac ce p
269
d
260
The analysis of the crystal structures of the modified nanowhiskers is another
270
method to evaluate their structural integrity (Gousse, Chanzy, Excoffier, Soubeyrand
271
& Fleury, 2002; Habibi, Lucia & Rojas, 2010). X-ray diffraction measurements were
272
performed to determine whether the functionalization altered the crystallinity of the
273
nanowhiskers. The unmodified CNCs and the functionalized CNCs (MA-CNC-0.8 and
11
Page 11 of 36
MA-CNC-1.9) exhibited the same X-ray diffraction pattern (Supplementary data,
275
Figure S1). Three main peaks at 15º, 16º and 22.5º were observed for the three
276
samples and are characteristic of the X-ray diffraction peaks for cellulose I. The
277
crystallinity indexes of the different CNCs were estimated from the intensity ratio
278
equation (I002-IAM)/I002 X 100 (Moran, Alvarez, Cyras & Vazquez, 2008), where I002 is
279
the intensity value of the peak at 2 close to 23° (representing the crystalline
280
material), and Iam is the intensity value close to 2=18° (representing amorphous
281
material in the CNCs). Although this method has often been used to calculate the
282
crystallinity indexes, it has also been shown to underestimate the amorphous content
283
(Park, Baker, Himmel, Parilla & Johnson, 2010); therefore we only used it here for
284
comparative purposes. The crystallinity indexes values obtained for the samples
285
CNC, MA-CNC-0.8 and MA-CNC-1.9 were 84, 82 and 84%, respectively. These
286
results, together with the TEM analysis, suggest that the functionalization did not
287
alter the crystallinity or morphology of the nanoparticles and that the chemical
288
modification took place only on the surface of the nanocrystals.
cr
us
an
M
d
te
Because the hydroxyl groups are being replaced by the MAC residue, the
Ac ce p
289
ip t
274
290
functionalized nanostructures are expected to have a higher hydrophobic character
291
than the unmodified nanocrystals. Figure 3 shows the dispersions of the unmodified-
292
CNC and MA-CNC-1.9 in water and in different organic solvents.
293
All of the dispersions were prepared using the same concentration (c= 0.01%
294
m/v) and the photographs were taken 15 minutes after the preparation (with
295
sonication) of the suspensions. The dispersability of the different samples show clear
296
differences exist between the redispersion of modified-CNC and unmodified CNC
297
that is related to a higher hydrophobic character of the MA-CNC-1.9 sample.
12
Page 12 of 36
The unmodified CNCs were easily dispersed in water, whereas the MA-CNC-
299
1.9 suspensions exhibited significant turbidity 15 minutes after the preparation of the
300
dispersion in the aqueous medium. Comparing the dispersion obtained in moderately
301
polar organic solvents, i.e., acetone ( = 20.7), THF ( = 7.58) and ethyl acetate (
302
= 6.0), it can be observed that a complete phase separation and precipitation took
303
place for the unmodified CNCs (Figure 3a). On the other hand, in Figure 3b, even
304
though turbid dispersions were obtained in the same solvents, they did not present
305
complete phase separation or precipitation as observed in figure 3a, showing an
306
increase in the hydrophobic character of the functionalized CNCs. Similar results
307
were obtained for the MA-CNC-0.8 functionalized cellulose nanocrystals.
an
us
cr
ip t
298
Our main goal here is to use the functionalized nanoparticles as a precursor in
309
the preparation of a CH-c-CNCs bionanocomposites. However, because the modified
310
nanoparticles could be relatively dispersed in moderately polar organic solvents, the
311
simple method proposed here for surface modification of the nanocrystals can also
312
find use for the preparation of other nanocomposites with different hydrophobic
313
polymers.
315 316 317
d
te
Ac ce p
314
M
308
Insert Figure 3
Elemental analysis of the functionalized CNCs was performed to confirm the
318
successful grafting of the MAC residue on the surface of the nanoparticles and
319
mainly to estimate the degree of functionalization. Table 1 shows the theoretical data
320
(in parentheses) and the experimental data obtained from the elemental analysis.
321
Considering a relative carbon content of 44.44% for the unmodified CNC (the
322
theoretical value of the unmodified CNC as that of pure cellulose), the relative carbon
13
Page 13 of 36
content obtained experimentally for the samples was converted into a theoretical
324
value. The conversion factor obtained (f = 44.45/39.93) was 1.113. This factor was
325
then used to convert the experimental values into corrected values of %C for
326
samples MA-CNC-0.8 and MA-CNC-1.9.
ip t
323
327 Insert Table 1
cr
328 329
The DS values, (calculated from equation 1), were determined from the
us
330
corrected values. After the functionalization of nanocrystals, the relative carbon
332
content in the samples increased according to the amount of the MAC reagent used
333
to modify the nanoparticles (Table 1). These values changed from 44.44 %
334
(unmodified CNCs) to 46.15% and 47.08%, for the MA-CNC-0.8 and MA-CNC-1.9
335
samples, respectively. The corresponding calculated DS values were 0.15 for the
336
former and 0.25 for the latter. The interpretation of these data (Siqueira, Bras &
337
Dufresne, 2010) is that the number of MAC residue grafted for each 100
338
anhydroglucose units was 15 for MA-CNC-0.8 and 25 for MA-CNC-1.9. By
339
considering the amount of hydroxyl groups in each anhydroglucose unit, we
340
calculated that ~5% and ~8% of the hydroxyl groups in the nanocrystals were
341
substituted by the MAC residue for MA-CNC-0.8 and MA-CNC-1.9, respectively. The
342
DS obtained in this work can be considered as relatively high because it is known
343
that a low amount of grafted chains is generally sufficient to significantly modify the
344
surface energy of cellulose (Buschlediller & Zeronian, 1992; Siqueira, Bras &
345
Dufresne, 2010). Also, since the functionalization of the nanocrystals took place on
346
the surface of the nanoparticles, the amount of surface hydroxyl groups substituted
Ac ce p
te
d
M
an
331
14
Page 14 of 36
347
by the functionalized reagent is indeed significantly greater than 5% or 8%, changing
348
the hydrophilic character of the nanoparticles, as shown in Figure 3.
349
The thermal degradation behavior of the unmodified and functionalized CNCs was investigated using TGA measurements (Figure S2). The functionalization and
351
DS of the nanocrystals decreased the thermal stability of the nanoparticles at
352
temperatures above 300°C. This behavior can be explained by the decrease in the
353
number of the hydrogen bonds in the modified nanoparticles (de Menezes, Siqueira,
354
Curvelo & Dufresne, 2009), as the OH groups became substituted after the
355
functionalization reaction.
an
us
cr
ip t
350
356
3.2 Preparation of CH-c-CNCs bionanocomposites
358
To demonstrate that the functionalized-CNCs can react appropriately with a
M
357
natural-based polymer in the expected manner, we combine the modified
360
nanocrystals with chitosan biopolymer. The CH-c-CNCs bionanocomposites were
361
prepared by the reaction of the functionalized nanocrystals (whose surface was
362
decorated with methyl ester end-groups) with the reactive and nucleophilic amino
363
groups of chitosan. The bionanocomposites films were prepared by solution casting
364
immediately after the reaction. The reaction pathway is illustrated in Scheme 2.
366 367
te
Ac ce p
365
d
359
Insert Scheme 2
After the preparation of the bionanocomposites, FTIR measurements were
368
performed to verify the chemical bond formation between the functionalized CNCs
369
and CH matrix. Although the selected reaction (scheme 2) is highly expected to
370
occur, the intrinsic difficulty of performing covalent linkages between the cellulose
371
nanoparticles and the CH polymer chains should be considered. The main limitation
15
Page 15 of 36
in achieving a high density of covalent linkages is due to the steric hindrance and the
373
potential of blocking of reactive sites by the grafted polymer chains (Morandi, Heath
374
& Thielemans, 2009). Thus, a significant number of ungrafted CH chains are
375
presumably found in the final nanocomposites.
376
ip t
372
Some studies described in the literature that have prepared nanocomposites through covalent linkages between the polymer and nanoparticles discussed the
378
same intrinsic complexity that leads to a limited number of covalent linkages between
379
the polymer and the nanoparticle. Typically, the final nanocomposites obtained are a
380
mixture of the polymer chain-modified nanoparticles in a polymer matrix that forms a
381
continuous interphase with improved interfacial adhesion (Chang, Ai, Chen, Dufresne
382
& Huang, 2009; Habibi, Goffin, Schiltz, Duquesne, Dubois & Dufresne, 2008; Yu, Ai,
383
Dufresne, Gao, Huang & Chang, 2008). We achieved a similar material as described
384
in these works because the composites were prepared immediately after the
385
reaction, without additional separation process or purification. Thus, the
386
bionanocomposites are composed of a mixture of chitosan chain-modified
387
nanoparticles in a chitosan matrix.
us
an
M
d
te
Ac ce p
388
cr
377
In order to probe the permanent linkage between the functionalized
389
nanocrystals and CH, films of the bionanocomposites (prepared according to the
390
procedure described in the experimental section) were sectioned and re-dispersed in
391
acetic acid solution (5% v/v). This dispersion remained under stirring for five days to
392
remove the free chitosan that was not grafted on the surface of the CNCs. The CH-c-
393
CNCs particles were then separated by centrifugation at 8,000 RPM. The resulting
394
precipitate was again dispersed in acetic acid solution for 24 hours and centrifuged.
395
This procedure was repeated 2 times to completely remove the ungrafted polymer
396
(free chitosan) and the final product was dried using the acetone solvent. Figure 4
16
Page 16 of 36
shows the FTIR spectra obtained for the CH-c-CNC-1.9, separated according to the
398
procedure described above. In order to compare, it is also showed in this figure the
399
spectrum of the pure functionalized MA-CNC-1.9 nanocrystal and the spectrum of the
400
neat chitosan. In the proposed reaction pathway (scheme 2), the formation of amide
401
groups is expected in the nanocomposite. However, the direct detection of the
402
formation of the amide group through FTIR is rather difficult because the absorption
403
band of this group overlaps with the peaks for the acetamide groups that are present
404
in chitosan. Thus, for the purpose of comparison, we also conducted FTIR
405
experiments with a composite prepared using the simple mixture of chitosan and the
406
unmodified cellulose nanoparticles, CH/CNCs (Figure S3). This sample was also
407
prepared using the same acetic acid dispersion procedure, followed by centrifugation
408
cycles to remove chitosan. We also provide the spectrum of the pure unmodified
409
CNCs and neat CH in this figure.
413
d Ac ce p
412
Insert Figure 4
te
410 411
M
an
us
cr
ip t
397
The MA-CNC-1.9 spectrum was compared with that of the nanocomposite CH-
414
c-CNC-1.9, (Figure 4). An important change observed after the grafting process was
415
the disappearance of the C=O ester stretching vibration (at 1725 cm-1). This result
416
indicates the reaction of the ester end groups on the nanocrystals surface with the
417
amino groups of chitosan. We also observed intensification of the band centered at
418
1650 cm-1, which corresponds to the C=O stretching of the amide I group.
419
Furthermore, the FTIR spectrum of the CH-c-CNC-1.9 nanocomposite exhibits
420
remarkable differences from the spectrum obtained for the CH/CNCs mixtures. In the
421
case of the composite CH/CNCs, no reaction took place and the entire contents of all
17
Page 17 of 36
of the chitosan was removed; thus, the spectrum of this composite (which is actually
423
expected to be only the CNCs after the procedure used to remove CH from the
424
unreacted composite) appears very similar to the one obtained from the pure and
425
unmodified CNCs. These results indicate that the modified-cellulose nanofillers are
426
covalently attached to the CH matrix.
It is important to mention that attempts to prepare films containing only CH-c-
cr
427
ip t
422
CNCs (after removing the ungrafted polymer) were not successful. Because there
429
are many amino groups present throughout the chitosan matrix, the CH-c-CNCs
430
obtained probably exist in a highly crosslinked system, preventing the dissolution of
431
this material, which is necessary for processing the films using the casting technique.
an
us
428
To investigate the effect of the CNCs in the CH-c-CNC as reinforcing phase,
433
we performed the classical tensile test. Figure 5a shows stress-strain curves of the
434
CH-c-CNC-0.8 films prepared with different concentrations of the nanocrystals. A
435
remarkable increase in the tensile strength and modulus as a function of the
436
concentration of the nanocrystals was observed. The strain-to-failure of the
437
nanocomposites was similar to that of the pristine CH matrix when the concentration
438
of CNCs was relatively low (up to approximately 10%). For higher concentrations of
439
CNCs, a decrease in the strain-to-failure (from 12 % to the pristine CH to 5 % for the
440
highest concentration used in this work) was observed. This behavior can be
441
explained by the strong interfacial adhesion between the matrix and the cellulose
442
nanofillers, which restricts the motion of the matrix.
Ac ce p
te
d
M
432
443 444
Insert Figure 5
445
18
Page 18 of 36
446
The modulus and the tensile strength (TS) were determined from the stressstrain curves, and the results are depicted in Figure 5b, as a function of the CNCs
448
concentration. A nearly linear increase in the TS and the modulus was observed with
449
increasing nanowhiskers concentration. For the nanocomposite with the highest
450
amount of nanocrystals (60%), an increase in the TS of approximately 150% relative
451
to the pristine polymer was observed (from 45 MPa to 108 MPa), while the modulus
452
increased up to 160% (from 1.2 GPa to 3.7 GPa).
cr
ip t
447
For comparative purposes, we also carried out tensile tests with the
454
nanocomposites CH-c-CNC-1.9 and with CH/CNC. Surprisingly, the mechanical
455
properties were similar (not shown) to those obtained for the CH-c-CNC-0.8 (Figure
456
5). These results are explained in the following manner. First, the CH is a very
457
hydrophilic and polar matrix, and the nanowhiskers are sufficiently well dispersed in
458
the polymer, even without covalent linkages between the nanofillers and the CH
459
chains. Second, the strategy used here presents an intrinsic constraint regarding the
460
limited number of covalent linkages between the nanoparticles and the polymer
461
chains. Thus, the number of permanent linkages obtained between both species
462
(CNCs and matrix) should not change considerably the spatial distribution of the
463
nanoparticles within the matrix, resulting in a nanocomposite with similar mechanical
464
properties compared to that observed for a mixture of unmodified CNCs and CH
465
matrix (CH/CNCs composite).
an
M
d
te
Ac ce p
466
us
453
However, a substantial difference in the water absorption behavior was
467
observed when comparing the CH-c-CNC with the CH/CNC nanocomposites. CH is
468
insoluble in pure water but can easily swell in this medium, which considerably
469
decreases the mechanical properties and prevents its application and thus restricts
470
its use in moist environments (Sebti, Chollet, Degraeve, Noel & Peyrol, 2007). It has
19
Page 19 of 36
also been shown that chemical crosslinking inside a hydrophilic polymer matrix
472
inhibits solubility or water absorption (Welsh & Price, 2003; Welsh, Schauer, Qadri &
473
Price, 2002). In addition, CNCs can effectively decrease the accessibility of water in
474
hydrophilic matrixes. Previous results demonstrated that for starch/CNCs
475
nanocomposites and also for chitosan/CNCs systems, the cellulose nanocrystals
476
decreased the hydrophilicity of these matrixes, leading to a decrease in swelling
477
(Cao, Chen, Chang, Stumborg & Huneault, 2008; de Rodriguez, Thielemans &
478
Dufresne, 2006; Li, Zhou & Zhang, 2009). In a more recent study (de Paula, Mano &
479
Pereira, 2011), we demonstrated that a small amount of CNCs can be used to control
480
the hydrolytic degradation of a polylactide polymer. This behavior was explained in
481
terms of the high degree of crystallinity found in the nanocrystals which creates a
482
tortuous path for the permeation of water and retards the hydrolytic degradation of
483
the bionanocomposites.
d
M
an
us
cr
ip t
471
We evaluate here not only the effect of the addition of CNCs in a hydrophilic
485
matrix but also (and simultaneously), the effect of the covalent linkage between the
486
matrix and the nanocrystals in the water uptake. Water uptake measurements were
487
carried out using CH-c-CNC-0.8 and also with CH/CNCs. Both results are provided in
488
Figure 6. The water uptake for both nanocomposites decreased remarkably with
489
increasing CNC concentration. However, this effect was considerably more
490
pronounced for the CH-c-CNCs, mainly using low concentrations of nanofillers. At
491
low concentrations of CNCs in the system CH-c-CNCs (approximately 10% wt), the
492
water swelling decreased to approximately 25 % of the absorption observed for the
493
neat chitosan, while for the mixed system CH/CNCs, the water swelling only
494
decreased to approximately 90% of the value observed for pure chitosan.
Ac ce p
te
484
495
20
Page 20 of 36
496
Insert Figure 6
497 498
As discussed by Rodriguez et al. (de Rodriguez, Thielemans & Dufresne, 2006), the three-dimensional network formed by hydrogen bonds between CNCs
500
significantly inhibit the swelling and solubility of the nanocomposites in water.
In both nanocomposites compared in Figure 6, a three-dimensional network
cr
501
ip t
499
formed by hydrogen bonds between CNCs is expected to occur. In addition, the
503
decrease in the hydrophilicity of the nanocomposite CH-c-CNCs compared to CH-
504
CNCs can be explained not only by the chemical functionalization of the
505
nanocrystals, which increased the hydrophobic character of the nanoparticles, but
506
also by the extra network formed by linkages between the CH matrix and the CNCs.
507
Despite the limited number of such linkages, we believe this network can also plays a
508
role and effectively inhibit more water diffusion between the chains compared to a
509
composite without the presence of those linkages.
an
M
d
te
510
us
502
The results for water uptake measurements using the MA-CNC-1.9 are similar to the nanocomposite obtained using MA-CNC-0.8 because, as explained in the
512
mechanical properties results, the number of linkages is limited (for both, MA-CNC-
513
1.9 and MA-CNC-0.8), by the steric hindrance and the potential of blocking of
514
reactive sites by the grafted polymer chains.
515 516
Ac ce p
511
517 518 519 520
21
Page 21 of 36
521
4. Conclusion
522
Bio-based nanocomposites were obtained through covalent linkage between chitosan and functionalized cellulose nanocrystals. Characterization of the
524
functionalization of the nanocrystals showed that the fraction of substituted hydroxyl
525
groups was ~5% and ~8%, for MA-CNC-0.8 and MA-CNC-1.9, respectively. FTIR
526
spectroscopy showed that the chitosan was covalently bonded onto the surface of
527
the functionalized CNCs. The chitosan-c-CNCs nanocomposites demonstrated a
528
significant improvement in mechanical performance (increase in tensile strength of
529
up to 150%) compared to the neat chitosan. The results for water uptake
530
measurements showed a remarkable decrease in hydrophylicity of chitosan, being
531
this effect more pronounced for the CH-c-CNC nanocomposite compared to the
532
system CH/CNCs. The approach used here can be extended to other natural
533
polymers, such as collagen, to produce different bio-based nanocomposites with new
534
properties and possible applications.
te
d
M
an
us
cr
ip t
523
535
537
Acknowledgment
Ac ce p
536
Authors thank CAPES (Nanobiotec - EDT Nº 04/2008) and Pró-reitoria de
538
Pesquisa - UFMG for the financial support. J.P. d. M. thanks the CNPq for funding a
539
scholarship. The Centro de Microscopia-UFMG is also acknowledged for obtaining
540
the TEM and SEM images.
541 542
References
543
Azzam, F., Heux, L., Putaux, J.-L., & Jean, B. (2010). Preparation By Grafting
544
Onto, Characterization, and Properties of Thermally Responsive Polymer-Decorated
545
Cellulose Nanocrystals. Biomacromolecules, 11, 3652-3659.
22
Page 22 of 36
Buschlediller, G., & Zeronian, S. H. (1992). Enhancing the Reactivity and
546 547
Strength of Cotton Fibers. Journal of Applied Polymer Science, 45, 967-979. Cao, X., Chen, Y., Chang, P. R., Stumborg, M., & Huneault, M. A. (2008).
549
Green composites reinforced with hemp nanocrystals in plasticized starch. Journal of
550
Applied Polymer Science, 109, 3804-3810.
ip t
548
Capadona, J. R., Van Den Berg, O., Capadona, L. A., Schroeter, M., Rowan,
552
S. J., Tyler, D. J., & Weder, C. (2007). A versatile approach for the processing of
553
polymer
554
Nanotechnology, 2, 765-769.
self-assembled
nanofibre
templates.
us
with
Nature
an
nanocomposites
cr
551
Chang, P. R., Ai, F., Chen, Y., Dufresne, A., & Huang, J. (2009). Effects of
556
Starch Nanocrystal-graft-Polycaprolactone on Mechanical Properties of Waterborne
557
Polyurethane-Based Nanocomposites. Journal of Applied Polymer Science, 111,
558
619-627.
d
M
555
Dash, M., Chiellini, F., Ottenbrite, R. M., & Chiellini, E. (2011). Chitosan-A
560
versatile semi-synthetic polymer in biomedical applications. Progress in Polymer
561
Science, 36, 981-1014.
563 564
Ac ce p
562
te
559
de Azeredo, H. M. C. (2009). Nanocomposites for food packaging
applications. Food Research International, 42, 1240-1253. de Menezes, A. J., Siqueira, G., Curvelo, A. A. S., & Dufresne, A. (2009).
565
Extrusion and characterization of functionalized cellulose whiskers reinforced
566
polyethylene nanocomposites. Polymer, 50, 4552-4563. de Mesquita, J., Donnici, C., & Pereira, F. (2010). Biobased Nanocomposites
567 568
from
Layer-by-Layer
Assembly
569
Biomacromolecules, 11, 473-480.
of
Cellulose
Nanowhiskers
with
Chitosan.
23
Page 23 of 36
de Mesquita, J., Patricio, P., Donnici, C., Petri, D., de Oliveira, L., & Pereira, F.
571
(2011). Hybrid layer-by-layer assembly based on animal and vegetable structural
572
materials: multilayered films of collagen and cellulose nanowhiskers. Soft Matter, 7,
573
4405-4413.
ip t
570
de Paula, E. L., Mano, V., & Pereira, F. V. (2011). Influence of cellulose
575
nanowhiskers on the hydrolytic degradation behavior of poly(D,L-lactide). Polymer
576
Degradation and Stability, 96, 1631-1638.
580 581 582
us
an
579
whiskers reinforced polyvinyl acetate nanocomposites. Cellulose, 13, 261-270. Dufresne, A. (2010). Processing of Polymer Nanocomposites Reinforced with Polysaccharide Nanocrystals Molecules, 15, 4111-4128.
M
578
de Rodriguez, N. L. G., Thielemans, W., & Dufresne, A. (2006). Sisal cellulose
Eichhorn, S. J. (2011). Cellulose nanowhiskers: promising materials for advanced applications. Soft Matter, 7, 303-315.
d
577
cr
574
Gousse, C., Chanzy, H., Excoffier, G., Soubeyrand, L., & Fleury, E. (2002).
584
Stable suspensions of partially silylated cellulose whiskers dispersed in organic
585
solvents. Polymer, 43, 2645-2651.
Ac ce p
586
te
583
Habibi, Y., Goffin, A.-L., Schiltz, N., Duquesne, E., Dubois, P., & Dufresne, A.
587
(2008). Bionanocomposites based on poly(epsilon-caprolactone)-grafted cellulose
588
nanocrystals by ring-opening polymerization. Journal of Materials Chemistry, 18,
589
5002-5010.
590 591 592 593
Habibi, Y., Lucia, L. A., & Rojas, O. J. (2010). Cellulose Nanocrystals:
Chemistry, Self-Assembly, and Applications. Chemical Reviews, 110, 3479–3500. Labet, M., Thielemans, W., & Dufresne, A. (2007). Polymer grafting onto starch nanocrystals. Biomacromolecules, 8, 2916-2927.
24
Page 24 of 36
594
Li, Q., Zhou, J., & Zhang, L. (2009). Structure and Properties of the
595
Nanocomposite Films of Chitosan Reinforced with Cellulose Whiskers. Journal of
596
Polymer Science Part B-Polymer Physics, 47, 1069-1077. Lin, N., Chen, G., Huang, J., Dufresne, A., & Chang, P. R. (2009). Effects of
598
Polymer-Grafted Natural Nanocrystals on the Structure and Mechanical Properties of
599
Poly(lactic acid): A Case of Cellulose Whisker-graft-Polycaprolactone. Journal of
600
Applied Polymer Science, 113, 3417-3425.
cr
ip t
597
Moon, B. S., Park, M.-T., Park, J. H., Kim, S. W., Lee, K. C., An, G. I., Yang,
602
S. D., Chi, D. Y., Cheon, G. J., & Lee, S.-J. (2007). Synthesis of novel
603
phytosphingosine derivatives
604
enhancing radiation therapy. Bioorganic & Medicinal Chemistry Letters, 17, 6643-
605
6646.
an
M
their preliminary biological evaluation for
Moran, J. I., Alvarez, V. A., Cyras, V. P., & Vazquez, A. (2008). Extraction of
d
607
and
cellulose and preparation of nanocellulose from sisal fibers. Cellulose, 15, 149-159.
te
606
us
601
Morandi, G., Heath, L., & Thielemans, W. (2009). Cellulose Nanocrystals
609
Grafted with Polystyrene Chains through Surface-Initiated Atom Transfer Radical
610
Polymerization (SI-ATRP). Langmuir, 25, 8280-8286.
611
Ac ce p
608
Muzzarelli, R. A. A. (2011). Chitin Nanostructures in Living Organisms. In N. S.
612
Gupta (Ed.), Chitin: formation and diagenesis (pp. 1-34). Dordrecht: Springer
613
Netherlands.
614
Park, S., Baker, J. O., Himmel, M. E., Parilla, P. A., & Johnson, D. K. (2010).
615
Cellulose crystallinity index: measurement techniques and their impact on
616
interpreting cellulase performance. Biotechnology for Biofuels, 3,1-10.
25
Page 25 of 36
617
Petersson, L., Kvien, I., & Oksman, K. (2007). Structure and thermal
618
properties
of
poly(lactic
acid)/cellulose
whiskers
619
Composites Science and Technology, 67, 2535-2544.
nanocomposite
materials.
Regourd, J., Ali, A. A.-S., & Thompson, A. (2007). Synthesis and anti-cancer
621
activity of C-ring-functionalized prodigiosin analogues. Journal of Medicinal
622
Chemistry, 50, 1528-1536.
cr
ip t
620
Samir, M. A. S. A., Alloin, F., & Dufresne, A. (2005). Review of Recent
624
Research into Cellulosic Whiskers, Their Properties and Their Application in
625
Nanocomposite Field. Biomacromolecules, 6, 612-626.
an
us
623
Sebti, I., Chollet, E., Degraeve, P., Noel, C., & Peyrol, E. (2007). Water
627
sensitivity, antimicrobial, and physicochemical analyses of edible films based on
628
HPMC and/or chitosan. Journal of Agricultural and Food Chemistry, 55, 693-699.
M
626
Siqueira, G., Bras, J., & Dufresne, A. (2009). Cellulose Whiskers versus
630
Microfibrils: Influence of the Nature of the Nanoparticle and its Surface
631
Functionalization on the Thermal and Mechanical Properties of Nanocomposites.
632
Biomacromolecules, 10, 425-432.
te
Ac ce p
633
d
629
Siqueira, G., Bras, J., & Dufresne, A. (2010). New Process of Chemical
634
Grafting of Cellulose Nanoparticles with a Long Chain Isocyanate. Langmuir, 26,
635
402-411.
636 637 638
Thielemans, W., Belgacem, M. N., & Dufresne, A. (2006). Starch nanocrystals
with large chain surface modifications. Langmuir, 22, 4804-4810. Wang, J., Wang, Z., Li, J., Wang, B., Liu, J., Chen, P., Miao, M., & Gu, Q.
639
(2012).
Chitin
nanocrystals
grafted
with
poly(3-hydroxybutyrate-co-3-
640
hydroxyvalerate) and their effects on thermal behavior of PHBV. Carbohydrate
641
Polymers, 87, 784-789.
26
Page 26 of 36
642
Welsh, E. R., & Price, R. R. (2003). Chitosan cross-linking with a water-
643
soluble, blocked diisocyanate. 2. Solvates and hydrogels. Biomacromolecules, 4,
644
1357-1361. Welsh, E. R., Schauer, C. L., Qadri, S. B., & Price, R. R. (2002). Chitosan cross-linking
with
a
water-soluble,
647
Biomacromolecules, 3, 1370-1374.
blocked
diisocyinate.
1.
Solid
state.
cr
646
ip t
645
Yu, J., Ai, F., Dufresne, A., Gao, S., Huang, J., & Chang, P. R. (2008).
649
Structure and mechanical properties of poly(lactic acid) filled with (starch
650
nanocrystal)-graft-poly(epsilon-caprolactone).
651
Engineering, 293, 763-770.
658 659 660 661 662
an
and
te Ac ce p
657
Materials
d
654
656
Macromolecular
M
652 653
655
us
648
663 664 665 666 667
27
Page 27 of 36
Figure Captions
668 669
671
Scheme 1. Depiction of the reaction used to functionalize the CNCs with a methyl ester end group.
ip t
670
672
674
Figure 1. FTIR spectra of the MAC reagent (I), MA-CNC-1.9 (II), MA-CNC-0.8
cr
673
(III) and the unmodified CNC (IV).
677
Figure 2. TEM images of the unmodified-CNC (a) and functionalized-CNC,
an
676
us
675
MA-CNC-0.8 (b).
M
678
Figure 3. Dispersibility of (a) unmodified-CNCs and (b) MA-CNC-1.9 in water
680
and different organic solvents. All photographs were taken 15 minutes after the
681
preparation of the suspensions (c = 0.01% m/v) and sonication.
te
d
679
682
684 685 686 687 688
Scheme 2. Covalent linkage of chitosan on the surface of functionalized CNCs
Ac ce p
683
containing terminal ester groups.
Figure 4. FTIR spectrum of the functionalized MA-CNC-1.9 nanocrystal (I),
CH-c-CNC-1.9 nanocomposite (II) and neat chitosan (III).
689
Figure 5. (a) Stress-strain curves of pristine CH and chitosan-c-CNC-1.9 with
690
different concentrations of CNCs and (b) Tensile strength () and elastic modulus (□)
691
as a function of the CNC-1.9 concentration (b). The dotted lines in (b) highlight the
692
observed trends.
28
Page 28 of 36
693 694 695
Figure 6. Water uptake results for CH/CNCs and for CH-c-CNC-0.8 nanocomposites, as a function of CNC concentration.
ip t
696 Table Title
697
cr
698
Table 1. Elemental analysis data for the unmodified CNCs and functionalized
700
CNCs with MAC reagent. The corrected elemental weight compositions are in
701
parentheses.
an
us
699
702
M
703 704
708 709 710 711 712 713
te
707
Ac ce p
706
d
705
714 715 716 717
29
Page 29 of 36
Figures
718
M
an
us
cr
ip t
719
721
Scheme 1
Ac ce p
te
722
d
720
723 724
Figure 1 30
Page 30 of 36
725
Figure 2
an
727
us
cr
ip t
726
728
730
Ac ce p
te
d
M
729
731 732
Figure 3
733 734 735
31
Page 31 of 36
ip t us
cr 737
an
736 Scheme 2
Ac ce p
te
d
M
738
739 740
Figure 4
741 742 743
32
Page 32 of 36
ip t us
cr 745 746 747
Ac ce p
te
d
M
an
744
Figure 5
748 749 750 751
33
Page 33 of 36
ip t cr us an
752 Figure 6
M
753
757 758 759 760 761 762
te
756
Ac ce p
755
d
754
763 764 765 766 767
34
Page 34 of 36
Tables
768
MA-CNC-0.8 MA-CNC-1.9
%H
%O
%N
39.93
5.43
54.14
0.50
(44.44)
(6.05)
(49.5)
(0)
41.46
5.92
52.28
0.06
(46.15)
(6.59)
(47.26)
(0)
42.28
5.49
52.14
(47.08)
(6.11)
(46.81)
770
--0.15
0.09 (0)
0.25
us
769
DS
ip t
NCCs
%C
cr
Samples
Table 1
Ac ce p
te
d
M
an
771
35
Page 35 of 36
771 772 773 774
Highlights Functionalized cellulose nanocrystals (CNCs) with ester end group were prepared.
775 The functionalized CNCs reacted with the amino groups of chitosan (CH).
ip t
776 777 778
Bionanocomposites based on chitosan covalently bonded to CNCs were obtained.
780
cr
779
Improvements in mechanical and water resistance properties were achieved.
782
us
781
The approach used in this work can be extended to other natural polymers.
an
783 784 785
M
786
Ac ce p
te
d
787
36
Page 36 of 36
S0144-8617(12)00446-8 doi:10.1016/j.carbpol.2012.05.025 CARP 6622
To appear in: Received date: Revised date: Accepted date:
31-1-2012 3-5-2012 5-5-2012
Please cite this article as: de Mesquita, J. P., Donnici, C. L., Teixeira, I. F., & Pereira, F. V., Bio-based nanocomposites obtained through covalent linkage between chitosan and cellulose nanocrystals, Carbohydrate Polymers (2010), doi:10.1016/j.carbpol.2012.05.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Bio-based nanocomposites obtained through covalent linkage
2
between chitosan and cellulose nanocrystals
3
ip t
4
João P. de Mesquita, Claudio L. Donnici, Ivo F. Teixeira and Fabiano V. Pereira*
cr
5 6
us
7
Departamento de Química - Universidade Federal de Minas Gerais. Av. Antônio
9
Carlos, 6627 - Pampulha - Belo Horizonte – MG. CEP 31270-901.
an
8
10
M
11 12 *
Corresponding author: Tel.: +55 31 3409 5753 ; Fax: +55 313409 5700
d
13
Departamento de Química
15
Universidade Federal de Minas Gerais
16
Av. Antônio Carlos, 6627 - Pampulha
17
CEP: 31270-901
18
E-mail: [email protected]
20 21
Ac ce p
19
te
14
Abbreviations 1, 2, 3
22 23 1. CNC: cellulose nanocrystal 2. CH: chitosan 3. MAC: methyl adipoyl chloride
Page 1 of 36
24 25
Abstract Bio-based nanocomposites were obtained through covalent linkage between cellulose nanocrystals (CNCs) and the natural polymer chitosan (CH). The CNCs
27
were first functionalized with methyl adipoyl chloride (MAC) and the reactive end
28
groups on the surface of the CNCs were reacted with the amino groups of the CH
29
biopolymer in an aqueous medium. The functionalized CNCs and the resulting
30
nanocomposites were characterized using FTIR, TEM, XRD, and elemental
31
analyses. Characterization of the functionalized CNCs showed that up to 8 % of the
32
hydroxyl groups in the nanocrystals were substituted by the MAC residue. The
33
covalent linkage between the CNCs and CH was confirmed by FTIR spectroscopy.
34
The nanocomposites demonstrated a significant improvement in the mechanical
35
performance and a considerable decrease in the hydrophilicity relative to the neat
36
chitosan. The approach used in this work can be extended to other natural polymers.
39 40 41 42 43 44
cr
us
an
M
d te
38
Keywords: chitosan; cellulose nanocrystals; bio-based nanocomposites.
Ac ce p
37
ip t
26
45 46 47 48
2
Page 2 of 36
49
1. Introduction
50
In the preparation of nanocomposites using different class of nanoparticles (such as nanoclays, carbon nanotubes or cellulose nanocrystals, CNCs), the most
52
important property is the formation of a percolation network within the matrix.
ip t
51
In the case of composite materials prepared with polysaccharide nanocrystal
54
such as CNCs or chitin nanocrystals (Muzzarelli, 2011), the achievement of an
55
appropriate percolation network is a particular issue because the nanoparticles
56
exhibit strong hydrogen bonding interactions between one another (Dufresne, 2010;
57
Wang et al., 2012). These interactions can create large undesirable agglomerates
58
within the polymer but, at the same time, the same interactions play an important role
59
to develop the desirable filler network inside the matrix. Thus, to obtain an
60
appropriate percolation of the CNCs in a nanocomposite, it is necessary to disperse
61
the nanocrystals within a polymer matrix and to maximize the interfacial adhesion
62
between the dispersed CNCs and the polymer, while still maintaining the filler/filler
63
hydrogen bonding interactions (Dufresne, 2010).
us
an
M
d
te
Several strategies have been developed during the last few years to achieve
Ac ce p
64
cr
53
65
the dispersion of the nanocrystals in different polymers, including the use of
66
surfactants or using the chemical surface modification of the nanowhiskers (de
67
Menezes, Siqueira, Curvelo & Dufresne, 2009; Petersson, Kvien & Oksman, 2007).
68
However, the use of surfactants to coat the high specific area of the nanocrystals
69
prohibits the use of this strategy (Samir, Alloin & Dufresne, 2005). Additionally,
70
previous results indicated that nanocomposites prepared from modified nanoparticles
71
typically exhibited a significant decrease in the mechanical performance because the
72
surface groups can insulate the nanoparticles, thus preventing the desirable filler/filler
73
interactions (Capadona et al., 2007).
3
Page 3 of 36
74
An alternative method to prepare nanocomposites with CNCs consists of a polymer chain surface modification of the nanoparticles (Dufresne, 2010; Habibi,
76
Goffin, Schiltz, Duquesne, Dubois & Dufresne, 2008; Lin, Chen, Huang, Dufresne &
77
Chang, 2009; Morandi, Heath & Thielemans, 2009). Generally speaking, two different
78
strategies have been used to obtain grafted chains on the surface of the
79
nanoparticles: the “grafting from” and the “grafting onto” methods (Dufresne, 2010).
80
In the first strategy, the polymer chains are formed using a in situ surface-initiated
81
polymerization from the immobilized initiators on the substrate (Habibi, Goffin,
82
Schiltz, Duquesne, Dubois & Dufresne, 2008). The second method (grafting onto)
83
consists of attaching a pre-synthesized polymer chain to the surface of the
84
nanoparticle (Azzam, Heux, Putaux & Jean, 2010; Dufresne, 2010). Using this
85
strategy, Labet et al (Labet, Thielemans & Dufresne, 2007) prepared starch
86
nanoparticles grafted with poly(tetrahydrofuran), poly(caprolactone), and
87
poly(ethylene glycol) monobutyl ether chains using toluene 2,4-diisocyanate as the
88
linking agent. More recently, Azzam et al.(Azzam, Heux, Putaux & Jean, 2010)
89
described the preparation of thermally responsive polymer-decorated cellulose-
90
nanocrystals.
cr
us
an
M
d
te
Ac ce p
91
ip t
75
Herein, we describe the preparation of a bio-based nanocomposite obtained
92
through covalent linkage between cellulose nanowhiskers and a natural biopolymer,
93
chitosan (CH). We have chosen CH because it is one of the strongest natural
94
polymers and also because it contains reactive amino groups. Moreover, this
95
biopolymer exhibits some important properties, such as biocompatibility,
96
biodegradability and anti-bacterial properties, which make it a suitable material for
97
biomedical applications, including drug delivery, tissue engineering, wound healing
98
and various antimicrobial strategies (Dash, Chiellini, Ottenbrite & Chiellini, 2011). In
4
Page 4 of 36
99
addition to biomedical applications, CH films have also been used for food packaging, mainly because of their non-toxicity and biodegradability (de Azeredo,
101
2009). However, the relatively weak mechanical properties of this biopolymer (in
102
comparison to petroleum-based polymers) and its inherent water sensitivity restrict
103
the wide use of chitosan films, particularly in environments where moisture is present
104
(Sebti, Chollet, Degraeve, Noel & Peyrol, 2007).
cr
105
ip t
100
In this work the CNCs surface was functionalized, creating reactive end groups on the nanocrystals which reacted with the amino groups of the CH
107
biopolymer, leading to a bionanocomposite with covalent linkage between the CNCs
108
and the chitosan. The functionalized CNCs and the nanocomposites were
109
characterized using different techniques, including FTIR, TEM, XRD, and elemental
110
analyses.
M
an
us
106
d
111 2. Experimental Section
113
2.1. Materials
114
Highly deacetylated chitosan polymer was supplied by the Phytomare® Food
Ac ce p
te
112
115
Supplements. The degree of chitosan deacetylation (92%) was determined by
116
potentiometric titration, Fourier transform infrared spectrometry (FTIR) and 1H NMR.
117
Eucalyptus wood pulp was kindly supplied by the Bahia Pulp Company (Brazil).
118
Sulfuric acid and acetone were purchased from Synth. Methyl adipoyl chloride
119
(MAC), triethylamine and toluene were purchased from Sigma-Aldrich.
120 121
2.2. Preparation of Cellulose Nanocrystals
122
A sulfuric acid hydrolysis reaction was performed with eucalyptus wood pulp
123
according to a procedure described in the literature and in our previous work, with
5
Page 5 of 36
minor modifications (de Mesquita, Donnici & Pereira, 2010; de Mesquita, Patricio,
125
Donnici, Petri, de Oliveira & Pereira, 2011; de Rodriguez, Thielemans & Dufresne,
126
2006). After a bleach treatment of the pulp, a controlled acid hydrolysis was
127
performed by adding 10.0 g of cellulose to 160.0 mL of 64 wt% sulfuric acid under
128
strong mechanical stirring. Hydrolysis was performed at 50 °C for approximately 40
129
minutes. After hydrolysis, the dispersion was washed with three cycles of
130
centrifugation and the last wash was performed using dialysis against deionized
131
water until the dispersion reached a pH of ~6. Afterward, the dispersions were
132
ultrasonicated with a Cole Parmer Sonifier cell disruptor, equipped with a microtip for
133
5 minutes.
an
us
cr
ip t
124
M
134
2.3 Preparation of the bionanocomposites
136
The preparation of the nanocomposites with covalent linkage between
138
chitosan and cellulose nanocrystals (CH-c-CNCs) consisted of two steps.
te
137
d
135
Firstly, the CNCs were functionalized using the methyl adipoyl chloride (MAC) reagent and then, the functionalized CNCs were reacted with amino groups of the
140
chitosan to produce bionanocomposites with different concentrations of nanocrystals.
141
Typically, the chemical surface modification of the CNCs was performed in toluene at
142
60° C for 4 hours using constant mechanical stirring. The suspension of CNCs in
143
toluene was obtained using a solvent exchange procedure, (Siqueira, Bras &
144
Dufresne, 2009) where the original aqueous CNCs suspension was solvent
145
exchanged with acetone followed by toluene, using three successive centrifugation
146
and redispersion steps for each solvent. Two different functionalized CNC were
147
prepared and are named according to the MAC/CNC ratio (wt%) used in each
148
reaction: MA-CNC-1.9 (MAC/CNC=1.9) and MA-CNC-0.8 (MAC/CNC=0.8). For the
Ac ce p
139
6
Page 6 of 36
MA-CNC-1.9, 200 L of MAC and 300 L of triethylamine were added in a
150
suspension of nanowhiskers (~0.5 % m/v) containing 120 mg of cellulose whiskers.
151
For the MA-CNC-0.8, 80 L of MAC and 130 L of triethylamine were also added in a
152
suspension of nanowhisker (~0.5 % m/v) containing 120 mg of CNCs. Triethylamine
153
was used to catalyze the reaction and also to complex the HCl formed during the
154
reaction (Thielemans, Belgacem & Dufresne, 2006). After the reaction, a suspension
155
of functionalized CNCs in water was obtained using another solvent exchange
156
procedure, from toluene to acetone, and finally to water, using the same procedure
157
described previously. Then, the aqueous suspension of functionalized CNCs was
158
freeze-dried.
cr
us
an
Secondly, for the preparation of chitosan covalently bonded to the
M
159
ip t
149
functionalized nanocrystals, a CH solution was prepared adding the biopolymer in
161
acetic acid (3 % wt/v), and then, an appropriate amount of MA-CNC-1.9 or MA-CNC-
162
0.8 (0, 1.0, 5.0, 10, 20, 30, 40, 50 or 60 % wt/wt) was added. The reaction was
163
performed in the aqueous medium at room temperature, and the suspensions were
164
stirred for 24 hours. The bionanocomposites were named CH-c-CNC-1.9 or CH-c-
165
CNC-0.8 when the nanocomposites were prepared with MA-CNC-1.9 or MA-CNC-
166
0.8, respectively. For comparative purposes, nanocomposites were also prepared
167
using a simple mixture of unmodified CNCs with CH (detailed results obtained from
168
these samples are not shown here). These samples were named CH/CNCs.
te
Ac ce p
169
d
160
170
2.4 Characterization
171
Transmission electron microscopy (TEM) was performed on samples using a
172
FEI Tecnai G2-Spirit with a 120-kV acceleration voltage. The CNCs suspensions
173
were deposited from an aqueous dispersion on a carbon-Formvar-coated copper
7
Page 7 of 36
174
(300-mesh) TEM grids. The samples were subsequently stained with a 2% uranyl
175
acetate solution to enhance the microscopic resolution. Fourier transform infrared spectroscopy (FTIR) of the CNCs and the
176
nanocomposites was recorded using a Nicolet 380 FTIR spectrometer (Nicolet, MN).
178
The samples were prepared by using a KBr-disk.
ip t
177
cr
Thermogravimetric analysis (TG/DTG) was performed using a DTG60
179
SHIMADZU. The samples were heated from room temperature to 600°C using a
181
heating ramp of 15°C min-1 under N2 flow (200 mL.min-1).
us
180
XRD measurements were performed on a SHIMADZU (XRD6000) X-ray
183
diffractometer with a Cu-K radiation source (λ = 0.154 nm) and in the 2 range of 10
184
to 35° using a step intervals of 0.02°.
M
an
182
Elemental analysis was performed using a CHN Perkin-Elmer analyzer. The
185
results obtained from this technique were used to determine the degree of
187
substitution (DS), which is given by the number of hydroxyl groups substituted by
188
MAC per unit of anhydroglucose. The methodology used here was recently reported
189
by Siqueira et al (Siqueira, Bras & Dufresne, 2010). Using this methodology, the DS
190
was calculated through the equation:
191
DS
te
Ac ce p
192
d
186
72.07 C 162.14 , 143.15 C 84.08
(1)
In this equation (Siqueira, Bras & Dufresne, 2010), C is the relative carbon
193
content in the sample, and the numbers 72.07, 162.14, 143.15 and 84.08 are the
194
relative carbon mass of the anhydroglucose unit (C6H10O5), the molecular weight of
195
the anhydroglucose, the molecular weight of the MAC residue linked to the
196
anhydroglucose unit and the carbon mass of the MAC group, respectively. The
197
experimental values were corrected by considering unmodified CNCs as pure
198
cellulose, which correlates with a relative carbon content of 44.44 %. Thus, the 8
Page 8 of 36
relative carbon content of the unmodified CNC was converted to this value (using the
200
conversion factor = 1.113), and the same correction factor was applied to the
201
functionalized CNCs. The obtained values were an average of two measurements.
202
Mechanical properties were obtained using a Lloyd testing instrument (Model
ip t
199
LS 500). Film strips of 6 cm x 1.5 cm (length x width) were cut from each
204
preconditioned sample and mounted between the grips of the machine. The initial
205
grip separation and crosshead speed were set to 50 mm and 12.5 mm/min,
206
respectively. At least ten replicates of each specimen were averaged together.
us
The water uptake of the CH and the bionanocomposites was determined using
an
207
cr
203
the following procedure. The samples were cut into a 10 x 10 mm square and dried
209
overnight at 90°C. Next, the samples were submerged in distilled water for 48 h at
210
room temperature until reach the equilibrium state. The degree of swelling of each
211
sample was determined by the equation W(%) = (W – W o)/W o x 100, where W is
212
the weight after 48 of submersion in distilled water (using a filter paper to remove the
213
surface adsorbed water) and Wo is the initial dry weight of the sample. To compare
214
the water absorption of the nanocomposites with the absorption of the neat chitosan,
215
the W(%) values were converted into a relative degree of swelling, using the equation
216
W r= W/W QT x 100, where W QT is the water absorption of the pure chitosan.
218 219 220
d
te
Ac ce p
217
M
208
3. Results and Discussion 3.1. CNCs Functionalization To prepare the CH-c-CNCs nanocomposites, the nanocrystals were first
221
chemically-modified using methyl adipoyl chloride (MAC) reagent. The selective
222
functionalization of the CNCs was achieved via the reaction between the acyl
223
chloride moieties of MAC and the hydroxyl groups of the nanocrystals, yielding
9
Page 9 of 36
224
functionalized CNCs with methyl ester end groups. MAC reagent has already been
225
used in other selective syntheses (Moon et al., 2007; Regourd, Ali & Thompson,
226
2007). The reaction route utilized to functionalize the CNCs is provided in Scheme 1.
228
ip t
227 Insert Scheme 1
cr
229
It is important to mention here how the excess of unreacted MAC was
231
discarded from the CNCs after the reaction. The functionalized CNCs, dispersed in
232
toluene, were precipitated using centrifugation. The solid material was re-dispersed
233
in toluene and then washed two more times with the same solvent. After that, the
234
material was again washed with several cycles of centrifugation: three times with
235
acetone and three times with water in order to completely discarded the excess of
236
unreacted MAC reagent. This procedure was important to prove the covalent
237
character between the MAC residue and the nanocrystals, ruling out possible
238
adsorption of carboxylated MAC.
an
M
d
te
Thus, the functionalization of the nanocrystals was confirmed first by the FTIR
Ac ce p
239
us
230
240
technique (Figure 1). The FTIR analysis of the freeze-dried nanocrystals suspensions
241
revealed the appearance of an absorption band at 1725 cm-1, characteristic of the
242
C=O ester stretch band. This band was found for both types of functionalized CNCs,
243
i.e., MA-CNC-1.9 and MA-CNC-0.8 (Figure 1). In addition, the spectrum of the
244
modified CNCs samples exhibited a narrower O-H stretching band (3360 cm-1), due
245
to a decrease in the amount of the hydrogen bonding between the nanoparticles.
246
Moreover, the signal at 1797 cm-1, assigned to the acyl chloride function of the MAC
247
reagent, was not observed in the spectra of the functionalized CNCs, indicating first a
248
complete reaction between these groups with the hydroxyl groups of the CNCs and
10
Page 10 of 36
249
also a complete discard of the unreacted MAC reagent from the nanocrystals
250
surface.
251 Insert Figure 1
ip t
252 253
255
Figure 2 shows a characteristic TEM image of the unmodified-CNC (a) and of
cr
254
the functionalized-CNC, MA-CNC-0.8 (b).
Insert Figure 2
an
257
us
256
258
Using several TEM images, the mean values for the length (L) and diameter
M
259
(D) of the unmodified rod-like nanocrystals were estimated to be 145±25 nm and
261
6±1.5 nm, respectively. Because the morphology of the nanocrystals plays a pivotal
262
role in nanocomposite properties (Dufresne, 2010; Eichhorn, 2011), it is very
263
important to ensure that the morphology of the CNCs was not significantly altered as
264
a result of the chemical modification. The TEM results show that the morphology of
265
the individual CNCs did not change after functionalization with the MAC reagent. The
266
nanocrystals maintained their elongated morphology with the same length and
267
aspect ratio as that of the unmodified CNCs. The TEM images obtained for the MA-
268
CNC-1.9 were similar (not shown).
te
Ac ce p
269
d
260
The analysis of the crystal structures of the modified nanowhiskers is another
270
method to evaluate their structural integrity (Gousse, Chanzy, Excoffier, Soubeyrand
271
& Fleury, 2002; Habibi, Lucia & Rojas, 2010). X-ray diffraction measurements were
272
performed to determine whether the functionalization altered the crystallinity of the
273
nanowhiskers. The unmodified CNCs and the functionalized CNCs (MA-CNC-0.8 and
11
Page 11 of 36
MA-CNC-1.9) exhibited the same X-ray diffraction pattern (Supplementary data,
275
Figure S1). Three main peaks at 15º, 16º and 22.5º were observed for the three
276
samples and are characteristic of the X-ray diffraction peaks for cellulose I. The
277
crystallinity indexes of the different CNCs were estimated from the intensity ratio
278
equation (I002-IAM)/I002 X 100 (Moran, Alvarez, Cyras & Vazquez, 2008), where I002 is
279
the intensity value of the peak at 2 close to 23° (representing the crystalline
280
material), and Iam is the intensity value close to 2=18° (representing amorphous
281
material in the CNCs). Although this method has often been used to calculate the
282
crystallinity indexes, it has also been shown to underestimate the amorphous content
283
(Park, Baker, Himmel, Parilla & Johnson, 2010); therefore we only used it here for
284
comparative purposes. The crystallinity indexes values obtained for the samples
285
CNC, MA-CNC-0.8 and MA-CNC-1.9 were 84, 82 and 84%, respectively. These
286
results, together with the TEM analysis, suggest that the functionalization did not
287
alter the crystallinity or morphology of the nanoparticles and that the chemical
288
modification took place only on the surface of the nanocrystals.
cr
us
an
M
d
te
Because the hydroxyl groups are being replaced by the MAC residue, the
Ac ce p
289
ip t
274
290
functionalized nanostructures are expected to have a higher hydrophobic character
291
than the unmodified nanocrystals. Figure 3 shows the dispersions of the unmodified-
292
CNC and MA-CNC-1.9 in water and in different organic solvents.
293
All of the dispersions were prepared using the same concentration (c= 0.01%
294
m/v) and the photographs were taken 15 minutes after the preparation (with
295
sonication) of the suspensions. The dispersability of the different samples show clear
296
differences exist between the redispersion of modified-CNC and unmodified CNC
297
that is related to a higher hydrophobic character of the MA-CNC-1.9 sample.
12
Page 12 of 36
The unmodified CNCs were easily dispersed in water, whereas the MA-CNC-
299
1.9 suspensions exhibited significant turbidity 15 minutes after the preparation of the
300
dispersion in the aqueous medium. Comparing the dispersion obtained in moderately
301
polar organic solvents, i.e., acetone ( = 20.7), THF ( = 7.58) and ethyl acetate (
302
= 6.0), it can be observed that a complete phase separation and precipitation took
303
place for the unmodified CNCs (Figure 3a). On the other hand, in Figure 3b, even
304
though turbid dispersions were obtained in the same solvents, they did not present
305
complete phase separation or precipitation as observed in figure 3a, showing an
306
increase in the hydrophobic character of the functionalized CNCs. Similar results
307
were obtained for the MA-CNC-0.8 functionalized cellulose nanocrystals.
an
us
cr
ip t
298
Our main goal here is to use the functionalized nanoparticles as a precursor in
309
the preparation of a CH-c-CNCs bionanocomposites. However, because the modified
310
nanoparticles could be relatively dispersed in moderately polar organic solvents, the
311
simple method proposed here for surface modification of the nanocrystals can also
312
find use for the preparation of other nanocomposites with different hydrophobic
313
polymers.
315 316 317
d
te
Ac ce p
314
M
308
Insert Figure 3
Elemental analysis of the functionalized CNCs was performed to confirm the
318
successful grafting of the MAC residue on the surface of the nanoparticles and
319
mainly to estimate the degree of functionalization. Table 1 shows the theoretical data
320
(in parentheses) and the experimental data obtained from the elemental analysis.
321
Considering a relative carbon content of 44.44% for the unmodified CNC (the
322
theoretical value of the unmodified CNC as that of pure cellulose), the relative carbon
13
Page 13 of 36
content obtained experimentally for the samples was converted into a theoretical
324
value. The conversion factor obtained (f = 44.45/39.93) was 1.113. This factor was
325
then used to convert the experimental values into corrected values of %C for
326
samples MA-CNC-0.8 and MA-CNC-1.9.
ip t
323
327 Insert Table 1
cr
328 329
The DS values, (calculated from equation 1), were determined from the
us
330
corrected values. After the functionalization of nanocrystals, the relative carbon
332
content in the samples increased according to the amount of the MAC reagent used
333
to modify the nanoparticles (Table 1). These values changed from 44.44 %
334
(unmodified CNCs) to 46.15% and 47.08%, for the MA-CNC-0.8 and MA-CNC-1.9
335
samples, respectively. The corresponding calculated DS values were 0.15 for the
336
former and 0.25 for the latter. The interpretation of these data (Siqueira, Bras &
337
Dufresne, 2010) is that the number of MAC residue grafted for each 100
338
anhydroglucose units was 15 for MA-CNC-0.8 and 25 for MA-CNC-1.9. By
339
considering the amount of hydroxyl groups in each anhydroglucose unit, we
340
calculated that ~5% and ~8% of the hydroxyl groups in the nanocrystals were
341
substituted by the MAC residue for MA-CNC-0.8 and MA-CNC-1.9, respectively. The
342
DS obtained in this work can be considered as relatively high because it is known
343
that a low amount of grafted chains is generally sufficient to significantly modify the
344
surface energy of cellulose (Buschlediller & Zeronian, 1992; Siqueira, Bras &
345
Dufresne, 2010). Also, since the functionalization of the nanocrystals took place on
346
the surface of the nanoparticles, the amount of surface hydroxyl groups substituted
Ac ce p
te
d
M
an
331
14
Page 14 of 36
347
by the functionalized reagent is indeed significantly greater than 5% or 8%, changing
348
the hydrophilic character of the nanoparticles, as shown in Figure 3.
349
The thermal degradation behavior of the unmodified and functionalized CNCs was investigated using TGA measurements (Figure S2). The functionalization and
351
DS of the nanocrystals decreased the thermal stability of the nanoparticles at
352
temperatures above 300°C. This behavior can be explained by the decrease in the
353
number of the hydrogen bonds in the modified nanoparticles (de Menezes, Siqueira,
354
Curvelo & Dufresne, 2009), as the OH groups became substituted after the
355
functionalization reaction.
an
us
cr
ip t
350
356
3.2 Preparation of CH-c-CNCs bionanocomposites
358
To demonstrate that the functionalized-CNCs can react appropriately with a
M
357
natural-based polymer in the expected manner, we combine the modified
360
nanocrystals with chitosan biopolymer. The CH-c-CNCs bionanocomposites were
361
prepared by the reaction of the functionalized nanocrystals (whose surface was
362
decorated with methyl ester end-groups) with the reactive and nucleophilic amino
363
groups of chitosan. The bionanocomposites films were prepared by solution casting
364
immediately after the reaction. The reaction pathway is illustrated in Scheme 2.
366 367
te
Ac ce p
365
d
359
Insert Scheme 2
After the preparation of the bionanocomposites, FTIR measurements were
368
performed to verify the chemical bond formation between the functionalized CNCs
369
and CH matrix. Although the selected reaction (scheme 2) is highly expected to
370
occur, the intrinsic difficulty of performing covalent linkages between the cellulose
371
nanoparticles and the CH polymer chains should be considered. The main limitation
15
Page 15 of 36
in achieving a high density of covalent linkages is due to the steric hindrance and the
373
potential of blocking of reactive sites by the grafted polymer chains (Morandi, Heath
374
& Thielemans, 2009). Thus, a significant number of ungrafted CH chains are
375
presumably found in the final nanocomposites.
376
ip t
372
Some studies described in the literature that have prepared nanocomposites through covalent linkages between the polymer and nanoparticles discussed the
378
same intrinsic complexity that leads to a limited number of covalent linkages between
379
the polymer and the nanoparticle. Typically, the final nanocomposites obtained are a
380
mixture of the polymer chain-modified nanoparticles in a polymer matrix that forms a
381
continuous interphase with improved interfacial adhesion (Chang, Ai, Chen, Dufresne
382
& Huang, 2009; Habibi, Goffin, Schiltz, Duquesne, Dubois & Dufresne, 2008; Yu, Ai,
383
Dufresne, Gao, Huang & Chang, 2008). We achieved a similar material as described
384
in these works because the composites were prepared immediately after the
385
reaction, without additional separation process or purification. Thus, the
386
bionanocomposites are composed of a mixture of chitosan chain-modified
387
nanoparticles in a chitosan matrix.
us
an
M
d
te
Ac ce p
388
cr
377
In order to probe the permanent linkage between the functionalized
389
nanocrystals and CH, films of the bionanocomposites (prepared according to the
390
procedure described in the experimental section) were sectioned and re-dispersed in
391
acetic acid solution (5% v/v). This dispersion remained under stirring for five days to
392
remove the free chitosan that was not grafted on the surface of the CNCs. The CH-c-
393
CNCs particles were then separated by centrifugation at 8,000 RPM. The resulting
394
precipitate was again dispersed in acetic acid solution for 24 hours and centrifuged.
395
This procedure was repeated 2 times to completely remove the ungrafted polymer
396
(free chitosan) and the final product was dried using the acetone solvent. Figure 4
16
Page 16 of 36
shows the FTIR spectra obtained for the CH-c-CNC-1.9, separated according to the
398
procedure described above. In order to compare, it is also showed in this figure the
399
spectrum of the pure functionalized MA-CNC-1.9 nanocrystal and the spectrum of the
400
neat chitosan. In the proposed reaction pathway (scheme 2), the formation of amide
401
groups is expected in the nanocomposite. However, the direct detection of the
402
formation of the amide group through FTIR is rather difficult because the absorption
403
band of this group overlaps with the peaks for the acetamide groups that are present
404
in chitosan. Thus, for the purpose of comparison, we also conducted FTIR
405
experiments with a composite prepared using the simple mixture of chitosan and the
406
unmodified cellulose nanoparticles, CH/CNCs (Figure S3). This sample was also
407
prepared using the same acetic acid dispersion procedure, followed by centrifugation
408
cycles to remove chitosan. We also provide the spectrum of the pure unmodified
409
CNCs and neat CH in this figure.
413
d Ac ce p
412
Insert Figure 4
te
410 411
M
an
us
cr
ip t
397
The MA-CNC-1.9 spectrum was compared with that of the nanocomposite CH-
414
c-CNC-1.9, (Figure 4). An important change observed after the grafting process was
415
the disappearance of the C=O ester stretching vibration (at 1725 cm-1). This result
416
indicates the reaction of the ester end groups on the nanocrystals surface with the
417
amino groups of chitosan. We also observed intensification of the band centered at
418
1650 cm-1, which corresponds to the C=O stretching of the amide I group.
419
Furthermore, the FTIR spectrum of the CH-c-CNC-1.9 nanocomposite exhibits
420
remarkable differences from the spectrum obtained for the CH/CNCs mixtures. In the
421
case of the composite CH/CNCs, no reaction took place and the entire contents of all
17
Page 17 of 36
of the chitosan was removed; thus, the spectrum of this composite (which is actually
423
expected to be only the CNCs after the procedure used to remove CH from the
424
unreacted composite) appears very similar to the one obtained from the pure and
425
unmodified CNCs. These results indicate that the modified-cellulose nanofillers are
426
covalently attached to the CH matrix.
It is important to mention that attempts to prepare films containing only CH-c-
cr
427
ip t
422
CNCs (after removing the ungrafted polymer) were not successful. Because there
429
are many amino groups present throughout the chitosan matrix, the CH-c-CNCs
430
obtained probably exist in a highly crosslinked system, preventing the dissolution of
431
this material, which is necessary for processing the films using the casting technique.
an
us
428
To investigate the effect of the CNCs in the CH-c-CNC as reinforcing phase,
433
we performed the classical tensile test. Figure 5a shows stress-strain curves of the
434
CH-c-CNC-0.8 films prepared with different concentrations of the nanocrystals. A
435
remarkable increase in the tensile strength and modulus as a function of the
436
concentration of the nanocrystals was observed. The strain-to-failure of the
437
nanocomposites was similar to that of the pristine CH matrix when the concentration
438
of CNCs was relatively low (up to approximately 10%). For higher concentrations of
439
CNCs, a decrease in the strain-to-failure (from 12 % to the pristine CH to 5 % for the
440
highest concentration used in this work) was observed. This behavior can be
441
explained by the strong interfacial adhesion between the matrix and the cellulose
442
nanofillers, which restricts the motion of the matrix.
Ac ce p
te
d
M
432
443 444
Insert Figure 5
445
18
Page 18 of 36
446
The modulus and the tensile strength (TS) were determined from the stressstrain curves, and the results are depicted in Figure 5b, as a function of the CNCs
448
concentration. A nearly linear increase in the TS and the modulus was observed with
449
increasing nanowhiskers concentration. For the nanocomposite with the highest
450
amount of nanocrystals (60%), an increase in the TS of approximately 150% relative
451
to the pristine polymer was observed (from 45 MPa to 108 MPa), while the modulus
452
increased up to 160% (from 1.2 GPa to 3.7 GPa).
cr
ip t
447
For comparative purposes, we also carried out tensile tests with the
454
nanocomposites CH-c-CNC-1.9 and with CH/CNC. Surprisingly, the mechanical
455
properties were similar (not shown) to those obtained for the CH-c-CNC-0.8 (Figure
456
5). These results are explained in the following manner. First, the CH is a very
457
hydrophilic and polar matrix, and the nanowhiskers are sufficiently well dispersed in
458
the polymer, even without covalent linkages between the nanofillers and the CH
459
chains. Second, the strategy used here presents an intrinsic constraint regarding the
460
limited number of covalent linkages between the nanoparticles and the polymer
461
chains. Thus, the number of permanent linkages obtained between both species
462
(CNCs and matrix) should not change considerably the spatial distribution of the
463
nanoparticles within the matrix, resulting in a nanocomposite with similar mechanical
464
properties compared to that observed for a mixture of unmodified CNCs and CH
465
matrix (CH/CNCs composite).
an
M
d
te
Ac ce p
466
us
453
However, a substantial difference in the water absorption behavior was
467
observed when comparing the CH-c-CNC with the CH/CNC nanocomposites. CH is
468
insoluble in pure water but can easily swell in this medium, which considerably
469
decreases the mechanical properties and prevents its application and thus restricts
470
its use in moist environments (Sebti, Chollet, Degraeve, Noel & Peyrol, 2007). It has
19
Page 19 of 36
also been shown that chemical crosslinking inside a hydrophilic polymer matrix
472
inhibits solubility or water absorption (Welsh & Price, 2003; Welsh, Schauer, Qadri &
473
Price, 2002). In addition, CNCs can effectively decrease the accessibility of water in
474
hydrophilic matrixes. Previous results demonstrated that for starch/CNCs
475
nanocomposites and also for chitosan/CNCs systems, the cellulose nanocrystals
476
decreased the hydrophilicity of these matrixes, leading to a decrease in swelling
477
(Cao, Chen, Chang, Stumborg & Huneault, 2008; de Rodriguez, Thielemans &
478
Dufresne, 2006; Li, Zhou & Zhang, 2009). In a more recent study (de Paula, Mano &
479
Pereira, 2011), we demonstrated that a small amount of CNCs can be used to control
480
the hydrolytic degradation of a polylactide polymer. This behavior was explained in
481
terms of the high degree of crystallinity found in the nanocrystals which creates a
482
tortuous path for the permeation of water and retards the hydrolytic degradation of
483
the bionanocomposites.
d
M
an
us
cr
ip t
471
We evaluate here not only the effect of the addition of CNCs in a hydrophilic
485
matrix but also (and simultaneously), the effect of the covalent linkage between the
486
matrix and the nanocrystals in the water uptake. Water uptake measurements were
487
carried out using CH-c-CNC-0.8 and also with CH/CNCs. Both results are provided in
488
Figure 6. The water uptake for both nanocomposites decreased remarkably with
489
increasing CNC concentration. However, this effect was considerably more
490
pronounced for the CH-c-CNCs, mainly using low concentrations of nanofillers. At
491
low concentrations of CNCs in the system CH-c-CNCs (approximately 10% wt), the
492
water swelling decreased to approximately 25 % of the absorption observed for the
493
neat chitosan, while for the mixed system CH/CNCs, the water swelling only
494
decreased to approximately 90% of the value observed for pure chitosan.
Ac ce p
te
484
495
20
Page 20 of 36
496
Insert Figure 6
497 498
As discussed by Rodriguez et al. (de Rodriguez, Thielemans & Dufresne, 2006), the three-dimensional network formed by hydrogen bonds between CNCs
500
significantly inhibit the swelling and solubility of the nanocomposites in water.
In both nanocomposites compared in Figure 6, a three-dimensional network
cr
501
ip t
499
formed by hydrogen bonds between CNCs is expected to occur. In addition, the
503
decrease in the hydrophilicity of the nanocomposite CH-c-CNCs compared to CH-
504
CNCs can be explained not only by the chemical functionalization of the
505
nanocrystals, which increased the hydrophobic character of the nanoparticles, but
506
also by the extra network formed by linkages between the CH matrix and the CNCs.
507
Despite the limited number of such linkages, we believe this network can also plays a
508
role and effectively inhibit more water diffusion between the chains compared to a
509
composite without the presence of those linkages.
an
M
d
te
510
us
502
The results for water uptake measurements using the MA-CNC-1.9 are similar to the nanocomposite obtained using MA-CNC-0.8 because, as explained in the
512
mechanical properties results, the number of linkages is limited (for both, MA-CNC-
513
1.9 and MA-CNC-0.8), by the steric hindrance and the potential of blocking of
514
reactive sites by the grafted polymer chains.
515 516
Ac ce p
511
517 518 519 520
21
Page 21 of 36
521
4. Conclusion
522
Bio-based nanocomposites were obtained through covalent linkage between chitosan and functionalized cellulose nanocrystals. Characterization of the
524
functionalization of the nanocrystals showed that the fraction of substituted hydroxyl
525
groups was ~5% and ~8%, for MA-CNC-0.8 and MA-CNC-1.9, respectively. FTIR
526
spectroscopy showed that the chitosan was covalently bonded onto the surface of
527
the functionalized CNCs. The chitosan-c-CNCs nanocomposites demonstrated a
528
significant improvement in mechanical performance (increase in tensile strength of
529
up to 150%) compared to the neat chitosan. The results for water uptake
530
measurements showed a remarkable decrease in hydrophylicity of chitosan, being
531
this effect more pronounced for the CH-c-CNC nanocomposite compared to the
532
system CH/CNCs. The approach used here can be extended to other natural
533
polymers, such as collagen, to produce different bio-based nanocomposites with new
534
properties and possible applications.
te
d
M
an
us
cr
ip t
523
535
537
Acknowledgment
Ac ce p
536
Authors thank CAPES (Nanobiotec - EDT Nº 04/2008) and Pró-reitoria de
538
Pesquisa - UFMG for the financial support. J.P. d. M. thanks the CNPq for funding a
539
scholarship. The Centro de Microscopia-UFMG is also acknowledged for obtaining
540
the TEM and SEM images.
541 542
References
543
Azzam, F., Heux, L., Putaux, J.-L., & Jean, B. (2010). Preparation By Grafting
544
Onto, Characterization, and Properties of Thermally Responsive Polymer-Decorated
545
Cellulose Nanocrystals. Biomacromolecules, 11, 3652-3659.
22
Page 22 of 36
Buschlediller, G., & Zeronian, S. H. (1992). Enhancing the Reactivity and
546 547
Strength of Cotton Fibers. Journal of Applied Polymer Science, 45, 967-979. Cao, X., Chen, Y., Chang, P. R., Stumborg, M., & Huneault, M. A. (2008).
549
Green composites reinforced with hemp nanocrystals in plasticized starch. Journal of
550
Applied Polymer Science, 109, 3804-3810.
ip t
548
Capadona, J. R., Van Den Berg, O., Capadona, L. A., Schroeter, M., Rowan,
552
S. J., Tyler, D. J., & Weder, C. (2007). A versatile approach for the processing of
553
polymer
554
Nanotechnology, 2, 765-769.
self-assembled
nanofibre
templates.
us
with
Nature
an
nanocomposites
cr
551
Chang, P. R., Ai, F., Chen, Y., Dufresne, A., & Huang, J. (2009). Effects of
556
Starch Nanocrystal-graft-Polycaprolactone on Mechanical Properties of Waterborne
557
Polyurethane-Based Nanocomposites. Journal of Applied Polymer Science, 111,
558
619-627.
d
M
555
Dash, M., Chiellini, F., Ottenbrite, R. M., & Chiellini, E. (2011). Chitosan-A
560
versatile semi-synthetic polymer in biomedical applications. Progress in Polymer
561
Science, 36, 981-1014.
563 564
Ac ce p
562
te
559
de Azeredo, H. M. C. (2009). Nanocomposites for food packaging
applications. Food Research International, 42, 1240-1253. de Menezes, A. J., Siqueira, G., Curvelo, A. A. S., & Dufresne, A. (2009).
565
Extrusion and characterization of functionalized cellulose whiskers reinforced
566
polyethylene nanocomposites. Polymer, 50, 4552-4563. de Mesquita, J., Donnici, C., & Pereira, F. (2010). Biobased Nanocomposites
567 568
from
Layer-by-Layer
Assembly
569
Biomacromolecules, 11, 473-480.
of
Cellulose
Nanowhiskers
with
Chitosan.
23
Page 23 of 36
de Mesquita, J., Patricio, P., Donnici, C., Petri, D., de Oliveira, L., & Pereira, F.
571
(2011). Hybrid layer-by-layer assembly based on animal and vegetable structural
572
materials: multilayered films of collagen and cellulose nanowhiskers. Soft Matter, 7,
573
4405-4413.
ip t
570
de Paula, E. L., Mano, V., & Pereira, F. V. (2011). Influence of cellulose
575
nanowhiskers on the hydrolytic degradation behavior of poly(D,L-lactide). Polymer
576
Degradation and Stability, 96, 1631-1638.
580 581 582
us
an
579
whiskers reinforced polyvinyl acetate nanocomposites. Cellulose, 13, 261-270. Dufresne, A. (2010). Processing of Polymer Nanocomposites Reinforced with Polysaccharide Nanocrystals Molecules, 15, 4111-4128.
M
578
de Rodriguez, N. L. G., Thielemans, W., & Dufresne, A. (2006). Sisal cellulose
Eichhorn, S. J. (2011). Cellulose nanowhiskers: promising materials for advanced applications. Soft Matter, 7, 303-315.
d
577
cr
574
Gousse, C., Chanzy, H., Excoffier, G., Soubeyrand, L., & Fleury, E. (2002).
584
Stable suspensions of partially silylated cellulose whiskers dispersed in organic
585
solvents. Polymer, 43, 2645-2651.
Ac ce p
586
te
583
Habibi, Y., Goffin, A.-L., Schiltz, N., Duquesne, E., Dubois, P., & Dufresne, A.
587
(2008). Bionanocomposites based on poly(epsilon-caprolactone)-grafted cellulose
588
nanocrystals by ring-opening polymerization. Journal of Materials Chemistry, 18,
589
5002-5010.
590 591 592 593
Habibi, Y., Lucia, L. A., & Rojas, O. J. (2010). Cellulose Nanocrystals:
Chemistry, Self-Assembly, and Applications. Chemical Reviews, 110, 3479–3500. Labet, M., Thielemans, W., & Dufresne, A. (2007). Polymer grafting onto starch nanocrystals. Biomacromolecules, 8, 2916-2927.
24
Page 24 of 36
594
Li, Q., Zhou, J., & Zhang, L. (2009). Structure and Properties of the
595
Nanocomposite Films of Chitosan Reinforced with Cellulose Whiskers. Journal of
596
Polymer Science Part B-Polymer Physics, 47, 1069-1077. Lin, N., Chen, G., Huang, J., Dufresne, A., & Chang, P. R. (2009). Effects of
598
Polymer-Grafted Natural Nanocrystals on the Structure and Mechanical Properties of
599
Poly(lactic acid): A Case of Cellulose Whisker-graft-Polycaprolactone. Journal of
600
Applied Polymer Science, 113, 3417-3425.
cr
ip t
597
Moon, B. S., Park, M.-T., Park, J. H., Kim, S. W., Lee, K. C., An, G. I., Yang,
602
S. D., Chi, D. Y., Cheon, G. J., & Lee, S.-J. (2007). Synthesis of novel
603
phytosphingosine derivatives
604
enhancing radiation therapy. Bioorganic & Medicinal Chemistry Letters, 17, 6643-
605
6646.
an
M
their preliminary biological evaluation for
Moran, J. I., Alvarez, V. A., Cyras, V. P., & Vazquez, A. (2008). Extraction of
d
607
and
cellulose and preparation of nanocellulose from sisal fibers. Cellulose, 15, 149-159.
te
606
us
601
Morandi, G., Heath, L., & Thielemans, W. (2009). Cellulose Nanocrystals
609
Grafted with Polystyrene Chains through Surface-Initiated Atom Transfer Radical
610
Polymerization (SI-ATRP). Langmuir, 25, 8280-8286.
611
Ac ce p
608
Muzzarelli, R. A. A. (2011). Chitin Nanostructures in Living Organisms. In N. S.
612
Gupta (Ed.), Chitin: formation and diagenesis (pp. 1-34). Dordrecht: Springer
613
Netherlands.
614
Park, S., Baker, J. O., Himmel, M. E., Parilla, P. A., & Johnson, D. K. (2010).
615
Cellulose crystallinity index: measurement techniques and their impact on
616
interpreting cellulase performance. Biotechnology for Biofuels, 3,1-10.
25
Page 25 of 36
617
Petersson, L., Kvien, I., & Oksman, K. (2007). Structure and thermal
618
properties
of
poly(lactic
acid)/cellulose
whiskers
619
Composites Science and Technology, 67, 2535-2544.
nanocomposite
materials.
Regourd, J., Ali, A. A.-S., & Thompson, A. (2007). Synthesis and anti-cancer
621
activity of C-ring-functionalized prodigiosin analogues. Journal of Medicinal
622
Chemistry, 50, 1528-1536.
cr
ip t
620
Samir, M. A. S. A., Alloin, F., & Dufresne, A. (2005). Review of Recent
624
Research into Cellulosic Whiskers, Their Properties and Their Application in
625
Nanocomposite Field. Biomacromolecules, 6, 612-626.
an
us
623
Sebti, I., Chollet, E., Degraeve, P., Noel, C., & Peyrol, E. (2007). Water
627
sensitivity, antimicrobial, and physicochemical analyses of edible films based on
628
HPMC and/or chitosan. Journal of Agricultural and Food Chemistry, 55, 693-699.
M
626
Siqueira, G., Bras, J., & Dufresne, A. (2009). Cellulose Whiskers versus
630
Microfibrils: Influence of the Nature of the Nanoparticle and its Surface
631
Functionalization on the Thermal and Mechanical Properties of Nanocomposites.
632
Biomacromolecules, 10, 425-432.
te
Ac ce p
633
d
629
Siqueira, G., Bras, J., & Dufresne, A. (2010). New Process of Chemical
634
Grafting of Cellulose Nanoparticles with a Long Chain Isocyanate. Langmuir, 26,
635
402-411.
636 637 638
Thielemans, W., Belgacem, M. N., & Dufresne, A. (2006). Starch nanocrystals
with large chain surface modifications. Langmuir, 22, 4804-4810. Wang, J., Wang, Z., Li, J., Wang, B., Liu, J., Chen, P., Miao, M., & Gu, Q.
639
(2012).
Chitin
nanocrystals
grafted
with
poly(3-hydroxybutyrate-co-3-
640
hydroxyvalerate) and their effects on thermal behavior of PHBV. Carbohydrate
641
Polymers, 87, 784-789.
26
Page 26 of 36
642
Welsh, E. R., & Price, R. R. (2003). Chitosan cross-linking with a water-
643
soluble, blocked diisocyanate. 2. Solvates and hydrogels. Biomacromolecules, 4,
644
1357-1361. Welsh, E. R., Schauer, C. L., Qadri, S. B., & Price, R. R. (2002). Chitosan cross-linking
with
a
water-soluble,
647
Biomacromolecules, 3, 1370-1374.
blocked
diisocyinate.
1.
Solid
state.
cr
646
ip t
645
Yu, J., Ai, F., Dufresne, A., Gao, S., Huang, J., & Chang, P. R. (2008).
649
Structure and mechanical properties of poly(lactic acid) filled with (starch
650
nanocrystal)-graft-poly(epsilon-caprolactone).
651
Engineering, 293, 763-770.
658 659 660 661 662
an
and
te Ac ce p
657
Materials
d
654
656
Macromolecular
M
652 653
655
us
648
663 664 665 666 667
27
Page 27 of 36
Figure Captions
668 669
671
Scheme 1. Depiction of the reaction used to functionalize the CNCs with a methyl ester end group.
ip t
670
672
674
Figure 1. FTIR spectra of the MAC reagent (I), MA-CNC-1.9 (II), MA-CNC-0.8
cr
673
(III) and the unmodified CNC (IV).
677
Figure 2. TEM images of the unmodified-CNC (a) and functionalized-CNC,
an
676
us
675
MA-CNC-0.8 (b).
M
678
Figure 3. Dispersibility of (a) unmodified-CNCs and (b) MA-CNC-1.9 in water
680
and different organic solvents. All photographs were taken 15 minutes after the
681
preparation of the suspensions (c = 0.01% m/v) and sonication.
te
d
679
682
684 685 686 687 688
Scheme 2. Covalent linkage of chitosan on the surface of functionalized CNCs
Ac ce p
683
containing terminal ester groups.
Figure 4. FTIR spectrum of the functionalized MA-CNC-1.9 nanocrystal (I),
CH-c-CNC-1.9 nanocomposite (II) and neat chitosan (III).
689
Figure 5. (a) Stress-strain curves of pristine CH and chitosan-c-CNC-1.9 with
690
different concentrations of CNCs and (b) Tensile strength () and elastic modulus (□)
691
as a function of the CNC-1.9 concentration (b). The dotted lines in (b) highlight the
692
observed trends.
28
Page 28 of 36
693 694 695
Figure 6. Water uptake results for CH/CNCs and for CH-c-CNC-0.8 nanocomposites, as a function of CNC concentration.
ip t
696 Table Title
697
cr
698
Table 1. Elemental analysis data for the unmodified CNCs and functionalized
700
CNCs with MAC reagent. The corrected elemental weight compositions are in
701
parentheses.
an
us
699
702
M
703 704
708 709 710 711 712 713
te
707
Ac ce p
706
d
705
714 715 716 717
29
Page 29 of 36
Figures
718
M
an
us
cr
ip t
719
721
Scheme 1
Ac ce p
te
722
d
720
723 724
Figure 1 30
Page 30 of 36
725
Figure 2
an
727
us
cr
ip t
726
728
730
Ac ce p
te
d
M
729
731 732
Figure 3
733 734 735
31
Page 31 of 36
ip t us
cr 737
an
736 Scheme 2
Ac ce p
te
d
M
738
739 740
Figure 4
741 742 743
32
Page 32 of 36
ip t us
cr 745 746 747
Ac ce p
te
d
M
an
744
Figure 5
748 749 750 751
33
Page 33 of 36
ip t cr us an
752 Figure 6
M
753
757 758 759 760 761 762
te
756
Ac ce p
755
d
754
763 764 765 766 767
34
Page 34 of 36
Tables
768
MA-CNC-0.8 MA-CNC-1.9
%H
%O
%N
39.93
5.43
54.14
0.50
(44.44)
(6.05)
(49.5)
(0)
41.46
5.92
52.28
0.06
(46.15)
(6.59)
(47.26)
(0)
42.28
5.49
52.14
(47.08)
(6.11)
(46.81)
770
--0.15
0.09 (0)
0.25
us
769
DS
ip t
NCCs
%C
cr
Samples
Table 1
Ac ce p
te
d
M
an
771
35
Page 35 of 36
771 772 773 774
Highlights Functionalized cellulose nanocrystals (CNCs) with ester end group were prepared.
775 The functionalized CNCs reacted with the amino groups of chitosan (CH).
ip t
776 777 778
Bionanocomposites based on chitosan covalently bonded to CNCs were obtained.
780
cr
779
Improvements in mechanical and water resistance properties were achieved.
782
us
781
The approach used in this work can be extended to other natural polymers.
an
783 784 785
M
786
Ac ce p
te
d
787
36
Page 36 of 36
