Accepted Manuscript Title: Creep behavior of starch-based nanocomposite films with cellulose nanofibrils Author: Meng Li Dong Li Li-jun Wang Benu Adhikari PII: DOI: Reference:
S0144-8617(14)01028-5 http://dx.doi.org/doi:10.1016/j.carbpol.2014.10.023 CARP 9376
To appear in: Received date: Revised date: Accepted date:
12-6-2014 4-10-2014 6-10-2014
Please cite this article as: Li, M., Li, D., Wang, L.-j., and Adhikari, B.,Creep behavior of starch-based nanocomposite films with cellulose nanofibrils, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.10.023 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.
*Highlights (for review)
Highlights ► Starch-cellulose nanofibrils composites were produced by solution casting method. ► Presence of CNFs increased the creep resistance of the nanocomposite films.
Ac
ce pt
ed
M
an
us
cr
ip t
► Presence of CNFs increased the storage and loss moduli of nanocomposite films.
Page 1 of 31
*Manuscript Click here to view linked References
1
Creep behavior of starch-based nanocomposite films with
2
cellulose nanofibrils
3
Meng Li a, Dong Li a, *, Li-jun Wang b, *, Benu Adhikari c a
College of Engineering, National Energy R & D Center for Non-food Biomass,
ip t
4
China Agricultural University, P. O. Box 50, 17 Qinghua Donglu, Beijing
6
100083, China b
College of Food Science and Nutritional Engineering, Beijing Key Laboratory of
us
7
cr
5
Functional Food from Plant Resources, China Agricultural University, Beijing,
9
China
12 13 14
School of Applied Sciences, RMIT University, City Campus, Melbourne, VIC 3001, Australia
M
11
c
* Corresponding authors. Tel. / fax: +86 10 62737351. E-mail addresses: [email protected] (Dong Li), [email protected] (Li-jun Wang).
ed
10
an
8
Abstract
Nanocomposite films were successfully prepared by incorporating cellulose
16
nanofibrils (CNFs) from sugar beet pulp into plasticized starch (PS) at CNFs
17
concentration of 5% to 20%. The storage (G’) and loss (G”) moduli, creep and
18
creep-recovery behavior of these films were studied. The creep behavior of these
19
films at long time frame was studied using time-temperature superposition (TTS). The
20
CNFs were uniformly distributed within these films up to 15% of CNFs. The PS-only
21
and the PS/CNFs nanocomposite films exhibited dominant elastic behavior. The
22
incorporation of CNFs increased both the G’ and G”. The CNFs improved the creep
23
resistance and reduced the creep recovery rate of the PS/CNFs nanocomposite films.
24
TTS method was successfully used to predict the creep behavior of these films at
Ac
ce pt
15
1
Page 2 of 31
longer time frame. Power law and Burgers model were capable (R2>0.98) of fitting
26
experimental G’ versus angular frequency and creep strain versus time data,
27
respectively.
28
Keywords: Starch; Cellulose nanofibrils; Composite films; Frequency sweep; Creep
29
behavior; Morphology; Time-temperature superposition (TTS)
30
Nomenclature
31
αT shift factor
32
Ea activation energy (kJ/mol)
33
R
34
Tr reference temperature (K)
35
T
test temperature (K)
36
ε
strain of suspension
37
ε’
creep rate (s-1)
38
σ0 constantly applied compressive stress (MPa)
39
EK modulus of the Kelvin spring (MPa)
40
EM modulus of the Maxwell spring (MPa)
41
τ
retardation time (s)
42
t
loading time (s)
43
ηM viscosity of the Maxwell dashpot (MPa·s)
44
ηK viscosity of the Kelvin dashpot (MPa·s)
45
J
46
G’ storage modulus (Pa)
47
G’’ loss modulus (Pa)
48
K’ power law model constant (Pa·sn)
49
n’ frequency exponent (dimensionless)
us
cr
ip t
25
Ac
ce pt
ed
M
an
universal gas constant (J·K-1·mol-1)
creep compliance (μm2/N)
2
Page 3 of 31
50
R2 correlation coefficient (dimensionless)
51
ω
52
1. Introduction
angular frequency (rad/s)
Growing ecological concerns and energy demands have made it imperative to
54
develop bio-based and environment-friendly packaging materials that have good
55
mechanical properties. Natural biodegradable polymers have more advantages over
56
the synthetic ones because they are comparatively inexpensive and derived from
57
renewable raw materials, agro-industrial by-products and agricultural waste (Lu,
58
Weng & Cao, 2005).
us
cr
ip t
53
Among the naturally occurring renewable biopolymers, starch is one of the most
60
abundant and low cost biodegradable materials (Mohanty, Misra & Hinrichsen, 2000).
61
Starch based biodegradable composites have drawn considerable attentions of
62
scientific community over the last two decades due to their potential applications in
63
many industries including food (Laohakunjit & Noomhorm, 2004), packaging (Avella,
64
De Vlieger, Errico, Fischer, Vacca & Volpe, 2005) and medicine (Krogars,
65
Heinamaki, Antikainen, Karjalainen & Yliruusi, 2003). However, compared with
66
conventional synthetic materials, starch-based biodegradable products exhibit many
67
disadvantages such as poor resistance to water, brittleness and poor elasticity. All of
68
these disadvantages can be attributed to the highly hydrophilic characteristics of
69
starch (Curvelo, de Carvalho & Agnelli, 2001). One of the effective methods to
70
improve the above mentioned properties is to incorporate various fillers. If these
71
fillers are also derived from renewable resource, the biodegradability of the
72
starch-based biocomposites will be preserved. Materials such as potato pulp
73
microfibrils (Dufresne & Vignon, 1998), bleached leafwood fibers (Averous &
74
Boquillon, 2004), tunicin whiskers (Mathew & Dufresne, 2002; Anglès & Dufresne,
Ac
ce pt
ed
M
an
59
3
Page 4 of 31
75
2001, 2000) and lignin (Lepifre et al., 2004) are used to improve the physicochemical
76
and mechanical properties of starch-based biocomposites. In recent years, natural cellulose nanofibrils (CNFs) are increasingly used as a
78
loading-bearing constituent in thermoset and thermoplastic polymeric matrices (Faruk,
79
Bledzki, Fink & Sain, 2012; Satyanarayana, Arizaga & Wypych, 2009). CNFs possess
80
unique characteristics, such as high aspect ratio, high tensile strength, high Young’s
81
modulus, and low coefficient of thermal expansion (Azizi Samir, Alloin & Dufresne,
82
2005; Nishino, Matsuda & Hirao, 2004). Attempts have been made in the past in
83
producing starch-based nanocomposites reinforced with CNFs (Anglès & Dufresne,
84
2001, 2000; Brinchi, Cotana, Fortunati & Kenny, 2013). Lu, Weng and Cao (2005)
85
reported that the cellulose nanocrystallites derived from cottonseed linter were able to
86
increase the Young’s modulus and tensile strength of starch films due to strong
87
interactions between starch and the cellulosic nanocrystallities. The CNFs obtained
88
from wheat straw were also found to improve the glass transition temperature (Tg) of
89
nanocomposite films compared to pure thermoplastic starch film (Alemdar & Sain,
90
2008).
ce pt
ed
M
an
us
cr
ip t
77
Most of the publications on the CNFs-reinforced nanocomposites report the
92
mechanical properties in terms of tensile strength and compressive strength. However,
93
viscoelastic properties are also very important characteristics of polymers which
94
indicate the time-dependent deformation of materials as a function of temperature,
95
stress and strain (Famá, Rojas, Goyanes & Gerschenson, 2005; Chen & Lai, 2008). In
96
order to determine the viscoelastic properties of composites, transient and dynamic
97
tests can be carried out by using dynamic thermomechanical analysis (DMA). Creep
98
and creep-recovery are the typical methods used to quantify the viscoelastic behavior
99
of composite materials (Del Nobile, Chillo, Mentana & Baiano, 2007). The
Ac
91
4
Page 5 of 31
100
creep-recovery behavior, quite common and useful in the life cycle of the composites,
101
is affected by the composition of polymeric substances. Creep is a time and temperature dependent behavior; hence, quantification of this
103
behavior is important for assessing the durability and reliability of composite
104
materials (Aifantis, 1987; Krempl & Khan, 2003). The Kelvin and Burgers models are
105
most commonly used to explain the creep behavior of materials under a fixed stress
106
(Del Nobile, Chillo, Mentana & Baiano, 2007). Kelvin model consists of a spring and
107
a dashpot in parallel. This model represents the start or early stage of creep behavior
108
and it does not incorporate the stress relaxation aspect of a viscoelastic body. The
109
Burgers model consists of a spring, a dashpot and a Kelvin component; hence, it is
110
able to describe the immediate elastic deformation (εSM), the delayed elastic strain
111
(εKV), and the Newtonian flow (ε∞) when the composite materials are subjected to a
112
static stress. Acha, Reboredo and Marcovich (2007) reported that the incorporation of
113
jute fibers in polypropylene (PP)-jute composites significantly improved their creep
114
resistance. A good agreement between experimental data and theoretical predictions
115
(Burgers model) were obtained in the creep region.
ce pt
ed
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an
us
cr
ip t
102
The frequency sweep test carried out using DMA provides melting property of
117
materials and this test is usually carried out to liquid samples. Many material experts
118
use DMA to study the temperature, strain rate or stress rate dependence of mechanical
119
properties; however, only few of them study the frequency sweep. For a viscoelastic
120
test conducted by DMA, the applied frequency plays a vital role to the mechanical
121
response of composite materials. The viscous or liquid-like behavior predominates
122
when the materials are subjected to a low frequency range. When the frequency is
123
high, the materials will exhibit elastic property and appear to be stiff. This variation in
124
viscoelastic property due to the increase in frequency is similar to the variation in the
Ac
116
5
Page 6 of 31
viscoelastic property due to decrease in temperature (Menard, 1999). Some studies on
126
the frequency sweep tests (from 0.01 to 200 Hz) of the poly (vinyl alcohol)
127
(PVA)-gelatin films containing 10-40% PVA were carried out by Mendieta-Taboada,
128
Sobral, Carvalho and Habitante (2008). These authors found that the value of storage
129
modulus increased with the increase in PVA content between 10% and 30%.
ip t
125
In this work, we aimed at producing nanocomposite films containing starch and
131
cellulose nanofibrils (CNFs) and investigating the viscoelastic property of these films.
132
The creep and creep-recovery were investigated using dynamic thermomechanical
133
analyzer (DMA). The frequency dependence of the storage and loss moduli was
134
determined by using frequency sweep tests of DMA. Power law model was used to
135
predict the frequency dependence of storage modulus. The Burgers model was used to
136
predict the creep recovery. The creep behavior of these films at longer time frame was
137
predicted using time-temperature superposition (TTS).
138
2. Materials and methods
139
2.1. Materials
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cr
130
Sugar beet pulp (SBP) was obtained from Xinjiang province of China. Corn
141
starch was obtained from Hebei Zhangjiakou Yujing Food Co. Ltd. (Hebei, China).
142
Xylitol (food grade) was purchased from Tianjin Jinguigu Science & Technology
143
Development Co. Ltd (Tianjin, China). Sodium chlorite (NaClO2) was purchased
144
from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Benzene,
145
absolute ethanol, sodium hydroxide (NaOH), glycerol and other chemicals were of
146
analytical grade and were supplied by Beijing Chemical Works (Beijing, China).
147
2.2. Preparation of cellulose nanofibrils
Ac
ce pt
140
148
Cellulose nanofibrils (CNFs) were produced by a method which combined
149
chemical treatment and high pressure homogenization. A detail of this method has 6
Page 7 of 31
been reported in our previous study (Li, Wang, Li, Cheng & Adhikari, 2014). Briefly,
151
the de-pectinated sugar beet pulp (DSBP) powder was treated with 4% (w/v) NaOH
152
solution at 80 oC for 2 h. This treatment was followed by bleaching (using sodium
153
chlorite and glacial acetic acid) which was carried out at 75-80 oC for 1h. The
154
bleaching treatment was repeated 3 times. The residue obtained after bleaching was
155
filtered and washed with distilled water until its pH was neutral. The bleached fibers
156
were subsequently passed through a high-pressure homogenizer (AH 100D, ATS
157
Engineering Inc, Italy) 10 times (passes) at 800 bar. The cellulose nanofibrils were
158
obtained after this high pressure homogenization step.
159
2.3. Preparation of plasticized starch (PS)/CNFs nanocomposite films
an
us
cr
ip t
150
The PS/CNFs nanocomposite films were prepared using solution casting method
161
as previously used by Fu, Wang, Li, Wei & Adhikari (2011) to prepare PS films. Corn
162
starch, plasticizers (glycerol: xylitol ratio =1:1), CNFs and deionized water were
163
mixed together to form starch-plasticizer-CNFs suspension with 5.0% (w/v) solid
164
concentration. The total concentration of the plasticizers was 30% (w/w) of the total
165
solid content. Each batch of the suspension was thoroughly mixed by stirring at
166
300 rpm and was heated at 100 oC for 1 h to allow complete gelatinization of starch
167
component. The evaporation of water during this gelatinization process was
168
minimized by covering the beakers with six layers of water resistant films. The
169
resulting paste was degassed under vacuum to remove the trapped air.
ed
ce pt
Ac
170
M
160
Films were prepared by syringing 13 mL of the paste into 9 cm diameter
171
polycarbonate petri dishes followed by drying at 45 oC for 6 h. A series of PS/CNFs
172
films were prepared by varying the concentration of CNFs from 5% to 20% at 5%
173
increment and designated as PS/CNFs-5, PS/CNFs-10, PS/CNFs-15 and PS/CNFs-20,
174
respectively. These films had a thickness of 10 µm to 50 µm. Plasticized starch only 7
Page 8 of 31
films (PS films) which did not contain CNFs were prepared in the same manner and
176
were used as control. All of these films were equilibrated at 20 oC and 43% RH for
177
one week using controlled humidity chambers before further tests. This equilibration
178
to a single RH value was essential to avoid the effect of relative humidity on the
179
viscoelastic behavior of starch-based films (Müller, Laurindo & Yamashita, 2009).
180
2.4. Frequency sweep tests
ip t
175
The frequency sweep tests were conducted in all of the films in tension mode by
182
using a dynamic mechanical analyzer (DMA, Q800, TA Instruments, New Castle,
183
USA). These films were cut into 30 mm × 7 mm rectangular strips. One end of the
184
film strip was clamped to the fixed grip and the other end was clamped to the movable
185
grip of the DMA furnace. Then, a small preload (0.02 N) was applied to keep the film
186
gently stretched (Pandini et al., 2012) and to establish desired film length between
187
both ends. After equilibrating the film at 35 oC for 2 min, the frequency sweep tests
188
were performed from 0.1 to 50 Hz. The samples were subjected to 0.05% strain which
189
is sufficiently small to ensure that the test remained within the linear viscoelastic
190
regime. Storage (G’) and loss (G’’) moduli were determined as a function of frequency.
191
All of these tests were carried out at least three times and the average values are
192
reported.
193
2.5. Creep and creep-recovery measurements
us
an
M
ed
ce pt
Ac
194
cr
181
A dynamic mechanical analyzer (DMA, Q800, TA Instruments, New Castle,
195
USA) was used to determine the creep and creep-recovery of PS/CNFs composite
196
films and the PS-only (control) films. These tests were carried out in tension mode
197
and at different stress levels. The specification of the films, the process of clamping or
198
loading to the furnace the applied preload were identical to those used in frequency
199
sweep tests (Section 2.4). The force track was set at 125%. After equilibrating at 35oC 8
Page 9 of 31
200
for 2 min, a stress of 8 MPa was supplied to the films for 5 min. The stress was
201
removed after 5 min and the films were recovered. The strain and creep compliance
202
data was recorded as a function of time.
203
2.6. Time-temperature superposition (TTS) Time-temperature superposition (TTS) was used to obtain creep deformation of
205
the PS/CNFs and PS-only films in longer time frame. The creep tests for both types of
206
films were carried out at six temperatures (30 oC, 35 oC, 40 oC, 45 oC, 50 oC, and 55
207
o
208
of 8 MPa for 5 min. All the individual creep curves obtained at different temperatures
209
were shifted along the logarithmic time axis to superimpose to a master curve. The
210
horizontal shift factor αT was calculated according to the Arrhenius equation given by
211
equation (1) (Yao, Wu, Chen & Zhang, 2013).
cr
log T
M
an
us
C) and the creep curves were obtained. These tests were carried out at a static stress
Ea 1 1 2.303R T Tr
(1)
ed
212
ip t
204
where, Eα (kJ/mol), R (JK-1 mol-1), Tr (K), and T(K) are the activation energy,
214
universal gas constant, reference temperature, and test temperature, respectively.
215
Regression analysis was performed on the experimental data to fit Eq. (1). SPSS 21.0
216
(SPSS Inc., Chicago, USA) was used to determine the activation energy (Eα) and the
217
associated correlation coefficient (R2). Fifty-five (55) oC was used as the reference
218
temperature for this purpose.
219
2.7. Statistical analysis
Ac
ce pt
213
220
All the experiments were carried out in triplicate. The experimental DMA data
221
was obtained directly from the Universal Analysis 2000 data analysis software (TA
222
Instruments, New Castle, USA). Each datum is expressed as mean standard
223
deviation. Duncan’s multiple comparison tests were used to determine the significance 9
Page 10 of 31
224
difference between two mean values using SPSS 21.0 software (SPSS Inc., Chicago,
225
USA) at 95% (P < 0.05) confidence level.
226
3. Results and discussion
227
3.1. The storage and loss moduli of PS/CNFs films The storage (G’) and loss (G”) moduli of starch films with and without CNFs as a
229
function of frequency are presented in Fig. 1. Both moduli in these films increased
230
with the increase in frequency; however, the elastic modulus (G’) increased much
231
prominently compared to the viscous modulus (G”). This is due to the fact that these
232
composite films had longer time to relax at low frequency; hence, they appeared less
233
elastic (less solid-like). At high frequency range, these films had less time to relax and
234
they exhibited more elastic (solid-like) characteristics. It can also be seen from this
235
figure that the values of storage modulus are much higher than the corresponding
236
values of the loss modulus over the entire frequency range. These results indicate that
237
both the starch-only and starch/CNFs composite films exhibit dominant elastic
238
behavior than the viscous behavior (Cespi, Bonacucina, Mencarelli, Casettari &
239
Palmieri, 2011). The incorporation of CNFs into the starch matrix significantly
240
(p
S0144-8617(14)01028-5 http://dx.doi.org/doi:10.1016/j.carbpol.2014.10.023 CARP 9376
To appear in: Received date: Revised date: Accepted date:
12-6-2014 4-10-2014 6-10-2014
Please cite this article as: Li, M., Li, D., Wang, L.-j., and Adhikari, B.,Creep behavior of starch-based nanocomposite films with cellulose nanofibrils, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.10.023 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.
*Highlights (for review)
Highlights ► Starch-cellulose nanofibrils composites were produced by solution casting method. ► Presence of CNFs increased the creep resistance of the nanocomposite films.
Ac
ce pt
ed
M
an
us
cr
ip t
► Presence of CNFs increased the storage and loss moduli of nanocomposite films.
Page 1 of 31
*Manuscript Click here to view linked References
1
Creep behavior of starch-based nanocomposite films with
2
cellulose nanofibrils
3
Meng Li a, Dong Li a, *, Li-jun Wang b, *, Benu Adhikari c a
College of Engineering, National Energy R & D Center for Non-food Biomass,
ip t
4
China Agricultural University, P. O. Box 50, 17 Qinghua Donglu, Beijing
6
100083, China b
College of Food Science and Nutritional Engineering, Beijing Key Laboratory of
us
7
cr
5
Functional Food from Plant Resources, China Agricultural University, Beijing,
9
China
12 13 14
School of Applied Sciences, RMIT University, City Campus, Melbourne, VIC 3001, Australia
M
11
c
* Corresponding authors. Tel. / fax: +86 10 62737351. E-mail addresses: [email protected] (Dong Li), [email protected] (Li-jun Wang).
ed
10
an
8
Abstract
Nanocomposite films were successfully prepared by incorporating cellulose
16
nanofibrils (CNFs) from sugar beet pulp into plasticized starch (PS) at CNFs
17
concentration of 5% to 20%. The storage (G’) and loss (G”) moduli, creep and
18
creep-recovery behavior of these films were studied. The creep behavior of these
19
films at long time frame was studied using time-temperature superposition (TTS). The
20
CNFs were uniformly distributed within these films up to 15% of CNFs. The PS-only
21
and the PS/CNFs nanocomposite films exhibited dominant elastic behavior. The
22
incorporation of CNFs increased both the G’ and G”. The CNFs improved the creep
23
resistance and reduced the creep recovery rate of the PS/CNFs nanocomposite films.
24
TTS method was successfully used to predict the creep behavior of these films at
Ac
ce pt
15
1
Page 2 of 31
longer time frame. Power law and Burgers model were capable (R2>0.98) of fitting
26
experimental G’ versus angular frequency and creep strain versus time data,
27
respectively.
28
Keywords: Starch; Cellulose nanofibrils; Composite films; Frequency sweep; Creep
29
behavior; Morphology; Time-temperature superposition (TTS)
30
Nomenclature
31
αT shift factor
32
Ea activation energy (kJ/mol)
33
R
34
Tr reference temperature (K)
35
T
test temperature (K)
36
ε
strain of suspension
37
ε’
creep rate (s-1)
38
σ0 constantly applied compressive stress (MPa)
39
EK modulus of the Kelvin spring (MPa)
40
EM modulus of the Maxwell spring (MPa)
41
τ
retardation time (s)
42
t
loading time (s)
43
ηM viscosity of the Maxwell dashpot (MPa·s)
44
ηK viscosity of the Kelvin dashpot (MPa·s)
45
J
46
G’ storage modulus (Pa)
47
G’’ loss modulus (Pa)
48
K’ power law model constant (Pa·sn)
49
n’ frequency exponent (dimensionless)
us
cr
ip t
25
Ac
ce pt
ed
M
an
universal gas constant (J·K-1·mol-1)
creep compliance (μm2/N)
2
Page 3 of 31
50
R2 correlation coefficient (dimensionless)
51
ω
52
1. Introduction
angular frequency (rad/s)
Growing ecological concerns and energy demands have made it imperative to
54
develop bio-based and environment-friendly packaging materials that have good
55
mechanical properties. Natural biodegradable polymers have more advantages over
56
the synthetic ones because they are comparatively inexpensive and derived from
57
renewable raw materials, agro-industrial by-products and agricultural waste (Lu,
58
Weng & Cao, 2005).
us
cr
ip t
53
Among the naturally occurring renewable biopolymers, starch is one of the most
60
abundant and low cost biodegradable materials (Mohanty, Misra & Hinrichsen, 2000).
61
Starch based biodegradable composites have drawn considerable attentions of
62
scientific community over the last two decades due to their potential applications in
63
many industries including food (Laohakunjit & Noomhorm, 2004), packaging (Avella,
64
De Vlieger, Errico, Fischer, Vacca & Volpe, 2005) and medicine (Krogars,
65
Heinamaki, Antikainen, Karjalainen & Yliruusi, 2003). However, compared with
66
conventional synthetic materials, starch-based biodegradable products exhibit many
67
disadvantages such as poor resistance to water, brittleness and poor elasticity. All of
68
these disadvantages can be attributed to the highly hydrophilic characteristics of
69
starch (Curvelo, de Carvalho & Agnelli, 2001). One of the effective methods to
70
improve the above mentioned properties is to incorporate various fillers. If these
71
fillers are also derived from renewable resource, the biodegradability of the
72
starch-based biocomposites will be preserved. Materials such as potato pulp
73
microfibrils (Dufresne & Vignon, 1998), bleached leafwood fibers (Averous &
74
Boquillon, 2004), tunicin whiskers (Mathew & Dufresne, 2002; Anglès & Dufresne,
Ac
ce pt
ed
M
an
59
3
Page 4 of 31
75
2001, 2000) and lignin (Lepifre et al., 2004) are used to improve the physicochemical
76
and mechanical properties of starch-based biocomposites. In recent years, natural cellulose nanofibrils (CNFs) are increasingly used as a
78
loading-bearing constituent in thermoset and thermoplastic polymeric matrices (Faruk,
79
Bledzki, Fink & Sain, 2012; Satyanarayana, Arizaga & Wypych, 2009). CNFs possess
80
unique characteristics, such as high aspect ratio, high tensile strength, high Young’s
81
modulus, and low coefficient of thermal expansion (Azizi Samir, Alloin & Dufresne,
82
2005; Nishino, Matsuda & Hirao, 2004). Attempts have been made in the past in
83
producing starch-based nanocomposites reinforced with CNFs (Anglès & Dufresne,
84
2001, 2000; Brinchi, Cotana, Fortunati & Kenny, 2013). Lu, Weng and Cao (2005)
85
reported that the cellulose nanocrystallites derived from cottonseed linter were able to
86
increase the Young’s modulus and tensile strength of starch films due to strong
87
interactions between starch and the cellulosic nanocrystallities. The CNFs obtained
88
from wheat straw were also found to improve the glass transition temperature (Tg) of
89
nanocomposite films compared to pure thermoplastic starch film (Alemdar & Sain,
90
2008).
ce pt
ed
M
an
us
cr
ip t
77
Most of the publications on the CNFs-reinforced nanocomposites report the
92
mechanical properties in terms of tensile strength and compressive strength. However,
93
viscoelastic properties are also very important characteristics of polymers which
94
indicate the time-dependent deformation of materials as a function of temperature,
95
stress and strain (Famá, Rojas, Goyanes & Gerschenson, 2005; Chen & Lai, 2008). In
96
order to determine the viscoelastic properties of composites, transient and dynamic
97
tests can be carried out by using dynamic thermomechanical analysis (DMA). Creep
98
and creep-recovery are the typical methods used to quantify the viscoelastic behavior
99
of composite materials (Del Nobile, Chillo, Mentana & Baiano, 2007). The
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creep-recovery behavior, quite common and useful in the life cycle of the composites,
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is affected by the composition of polymeric substances. Creep is a time and temperature dependent behavior; hence, quantification of this
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behavior is important for assessing the durability and reliability of composite
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materials (Aifantis, 1987; Krempl & Khan, 2003). The Kelvin and Burgers models are
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most commonly used to explain the creep behavior of materials under a fixed stress
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(Del Nobile, Chillo, Mentana & Baiano, 2007). Kelvin model consists of a spring and
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a dashpot in parallel. This model represents the start or early stage of creep behavior
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and it does not incorporate the stress relaxation aspect of a viscoelastic body. The
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Burgers model consists of a spring, a dashpot and a Kelvin component; hence, it is
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able to describe the immediate elastic deformation (εSM), the delayed elastic strain
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(εKV), and the Newtonian flow (ε∞) when the composite materials are subjected to a
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static stress. Acha, Reboredo and Marcovich (2007) reported that the incorporation of
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jute fibers in polypropylene (PP)-jute composites significantly improved their creep
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resistance. A good agreement between experimental data and theoretical predictions
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(Burgers model) were obtained in the creep region.
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The frequency sweep test carried out using DMA provides melting property of
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materials and this test is usually carried out to liquid samples. Many material experts
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use DMA to study the temperature, strain rate or stress rate dependence of mechanical
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properties; however, only few of them study the frequency sweep. For a viscoelastic
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test conducted by DMA, the applied frequency plays a vital role to the mechanical
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response of composite materials. The viscous or liquid-like behavior predominates
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when the materials are subjected to a low frequency range. When the frequency is
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high, the materials will exhibit elastic property and appear to be stiff. This variation in
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viscoelastic property due to the increase in frequency is similar to the variation in the
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viscoelastic property due to decrease in temperature (Menard, 1999). Some studies on
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the frequency sweep tests (from 0.01 to 200 Hz) of the poly (vinyl alcohol)
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(PVA)-gelatin films containing 10-40% PVA were carried out by Mendieta-Taboada,
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Sobral, Carvalho and Habitante (2008). These authors found that the value of storage
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modulus increased with the increase in PVA content between 10% and 30%.
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In this work, we aimed at producing nanocomposite films containing starch and
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cellulose nanofibrils (CNFs) and investigating the viscoelastic property of these films.
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The creep and creep-recovery were investigated using dynamic thermomechanical
133
analyzer (DMA). The frequency dependence of the storage and loss moduli was
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determined by using frequency sweep tests of DMA. Power law model was used to
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predict the frequency dependence of storage modulus. The Burgers model was used to
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predict the creep recovery. The creep behavior of these films at longer time frame was
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predicted using time-temperature superposition (TTS).
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2. Materials and methods
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2.1. Materials
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Sugar beet pulp (SBP) was obtained from Xinjiang province of China. Corn
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starch was obtained from Hebei Zhangjiakou Yujing Food Co. Ltd. (Hebei, China).
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Xylitol (food grade) was purchased from Tianjin Jinguigu Science & Technology
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Development Co. Ltd (Tianjin, China). Sodium chlorite (NaClO2) was purchased
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from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Benzene,
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absolute ethanol, sodium hydroxide (NaOH), glycerol and other chemicals were of
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analytical grade and were supplied by Beijing Chemical Works (Beijing, China).
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2.2. Preparation of cellulose nanofibrils
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Cellulose nanofibrils (CNFs) were produced by a method which combined
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chemical treatment and high pressure homogenization. A detail of this method has 6
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been reported in our previous study (Li, Wang, Li, Cheng & Adhikari, 2014). Briefly,
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the de-pectinated sugar beet pulp (DSBP) powder was treated with 4% (w/v) NaOH
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solution at 80 oC for 2 h. This treatment was followed by bleaching (using sodium
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chlorite and glacial acetic acid) which was carried out at 75-80 oC for 1h. The
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bleaching treatment was repeated 3 times. The residue obtained after bleaching was
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filtered and washed with distilled water until its pH was neutral. The bleached fibers
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were subsequently passed through a high-pressure homogenizer (AH 100D, ATS
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Engineering Inc, Italy) 10 times (passes) at 800 bar. The cellulose nanofibrils were
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obtained after this high pressure homogenization step.
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2.3. Preparation of plasticized starch (PS)/CNFs nanocomposite films
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The PS/CNFs nanocomposite films were prepared using solution casting method
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as previously used by Fu, Wang, Li, Wei & Adhikari (2011) to prepare PS films. Corn
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starch, plasticizers (glycerol: xylitol ratio =1:1), CNFs and deionized water were
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mixed together to form starch-plasticizer-CNFs suspension with 5.0% (w/v) solid
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concentration. The total concentration of the plasticizers was 30% (w/w) of the total
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solid content. Each batch of the suspension was thoroughly mixed by stirring at
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300 rpm and was heated at 100 oC for 1 h to allow complete gelatinization of starch
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component. The evaporation of water during this gelatinization process was
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minimized by covering the beakers with six layers of water resistant films. The
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resulting paste was degassed under vacuum to remove the trapped air.
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M
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Films were prepared by syringing 13 mL of the paste into 9 cm diameter
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polycarbonate petri dishes followed by drying at 45 oC for 6 h. A series of PS/CNFs
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films were prepared by varying the concentration of CNFs from 5% to 20% at 5%
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increment and designated as PS/CNFs-5, PS/CNFs-10, PS/CNFs-15 and PS/CNFs-20,
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respectively. These films had a thickness of 10 µm to 50 µm. Plasticized starch only 7
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films (PS films) which did not contain CNFs were prepared in the same manner and
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were used as control. All of these films were equilibrated at 20 oC and 43% RH for
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one week using controlled humidity chambers before further tests. This equilibration
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to a single RH value was essential to avoid the effect of relative humidity on the
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viscoelastic behavior of starch-based films (Müller, Laurindo & Yamashita, 2009).
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2.4. Frequency sweep tests
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The frequency sweep tests were conducted in all of the films in tension mode by
182
using a dynamic mechanical analyzer (DMA, Q800, TA Instruments, New Castle,
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USA). These films were cut into 30 mm × 7 mm rectangular strips. One end of the
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film strip was clamped to the fixed grip and the other end was clamped to the movable
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grip of the DMA furnace. Then, a small preload (0.02 N) was applied to keep the film
186
gently stretched (Pandini et al., 2012) and to establish desired film length between
187
both ends. After equilibrating the film at 35 oC for 2 min, the frequency sweep tests
188
were performed from 0.1 to 50 Hz. The samples were subjected to 0.05% strain which
189
is sufficiently small to ensure that the test remained within the linear viscoelastic
190
regime. Storage (G’) and loss (G’’) moduli were determined as a function of frequency.
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All of these tests were carried out at least three times and the average values are
192
reported.
193
2.5. Creep and creep-recovery measurements
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cr
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A dynamic mechanical analyzer (DMA, Q800, TA Instruments, New Castle,
195
USA) was used to determine the creep and creep-recovery of PS/CNFs composite
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films and the PS-only (control) films. These tests were carried out in tension mode
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and at different stress levels. The specification of the films, the process of clamping or
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loading to the furnace the applied preload were identical to those used in frequency
199
sweep tests (Section 2.4). The force track was set at 125%. After equilibrating at 35oC 8
Page 9 of 31
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for 2 min, a stress of 8 MPa was supplied to the films for 5 min. The stress was
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removed after 5 min and the films were recovered. The strain and creep compliance
202
data was recorded as a function of time.
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2.6. Time-temperature superposition (TTS) Time-temperature superposition (TTS) was used to obtain creep deformation of
205
the PS/CNFs and PS-only films in longer time frame. The creep tests for both types of
206
films were carried out at six temperatures (30 oC, 35 oC, 40 oC, 45 oC, 50 oC, and 55
207
o
208
of 8 MPa for 5 min. All the individual creep curves obtained at different temperatures
209
were shifted along the logarithmic time axis to superimpose to a master curve. The
210
horizontal shift factor αT was calculated according to the Arrhenius equation given by
211
equation (1) (Yao, Wu, Chen & Zhang, 2013).
cr
log T
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C) and the creep curves were obtained. These tests were carried out at a static stress
Ea 1 1 2.303R T Tr
(1)
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where, Eα (kJ/mol), R (JK-1 mol-1), Tr (K), and T(K) are the activation energy,
214
universal gas constant, reference temperature, and test temperature, respectively.
215
Regression analysis was performed on the experimental data to fit Eq. (1). SPSS 21.0
216
(SPSS Inc., Chicago, USA) was used to determine the activation energy (Eα) and the
217
associated correlation coefficient (R2). Fifty-five (55) oC was used as the reference
218
temperature for this purpose.
219
2.7. Statistical analysis
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All the experiments were carried out in triplicate. The experimental DMA data
221
was obtained directly from the Universal Analysis 2000 data analysis software (TA
222
Instruments, New Castle, USA). Each datum is expressed as mean standard
223
deviation. Duncan’s multiple comparison tests were used to determine the significance 9
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difference between two mean values using SPSS 21.0 software (SPSS Inc., Chicago,
225
USA) at 95% (P < 0.05) confidence level.
226
3. Results and discussion
227
3.1. The storage and loss moduli of PS/CNFs films The storage (G’) and loss (G”) moduli of starch films with and without CNFs as a
229
function of frequency are presented in Fig. 1. Both moduli in these films increased
230
with the increase in frequency; however, the elastic modulus (G’) increased much
231
prominently compared to the viscous modulus (G”). This is due to the fact that these
232
composite films had longer time to relax at low frequency; hence, they appeared less
233
elastic (less solid-like). At high frequency range, these films had less time to relax and
234
they exhibited more elastic (solid-like) characteristics. It can also be seen from this
235
figure that the values of storage modulus are much higher than the corresponding
236
values of the loss modulus over the entire frequency range. These results indicate that
237
both the starch-only and starch/CNFs composite films exhibit dominant elastic
238
behavior than the viscous behavior (Cespi, Bonacucina, Mencarelli, Casettari &
239
Palmieri, 2011). The incorporation of CNFs into the starch matrix significantly
240
(p
