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gelatin hydrogels with enhanced mechanical strength
Accepted Manuscript Title: Rapid Shape Memory TEMPO-Oxidized Cellulose Nanofibers/Polyacrylamide/Gelatin Hydrogels with Enhanced Mechanical Strength Authors: Nan Li, Wei Chen, Guangxue Chen, Junfei Tian PII: DOI: Reference:
S0144-8617(17)30424-1 http://dx.doi.org/doi:10.1016/j.carbpol.2017.04.035 CARP 12223
To appear in: Received date: Revised date: Accepted date:
24-2-2017 31-3-2017 15-4-2017
Please cite this article as: Li, Nan., Chen, Wei., Chen, Guangxue., & Tian, Junfei., Rapid Shape Memory TEMPO-Oxidized Cellulose Nanofibers/Polyacrylamide/Gelatin Hydrogels with Enhanced Mechanical Strength.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2017.04.035 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
Rapid Shape Memory TEMPO-Oxidized Cellulose
2
Nanofibers/ Polyacrylamide/ Gelatin Hydrogels with
3
Enhanced Mechanical Strength
4
Nan Li a, Wei Chen b, *, Guangxue Chen a, *, Junfei Tiana
5
a
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Guangzhou 510640, P. R. China; b College of Engineering, Qufu Normal University, RiZhao 276826, China
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* Corresponding authors at South China University of Technology, Guangzhou 510640, P. R. China and
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College of Engineering, Qufu Normal University, RiZhao 276826, China.
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E-mail addresses: [email protected] (N.Li); [email protected] (W.Chen); [email protected]
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology,
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(G.X.Chen); [email protected] (J.F.Tian)
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TEL: (86)-020-22236485 ； Fax :( 86)-020-22236485
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Abstract
13
TEMPO-oxidized cellulose nanofibers/ polyacrylamide/ gelatin shape memory hydrogels
14
were successfully fabricated through a facile in-situ free-radical polymerization method, and
15
double network was formed by chemically cross-linked polyacrylamide (PAM) network and
16
physically cross-linked gelatin network. TEMPO-oxidized cellulose nanofibers (TOCNs)
17
were introduced to improve the mechanical properties of the hydrogel. The structure, shape
18
memory behaviors and mechanical properties of the resulting composite gels with varied gel
19
compositions were investigated. The results obtained from those different studies revealed
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that TOCNs, gelatin, and PAM could mix with each other homogeneously. Due to the
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thermoreversible nature of the gelatin network, the composite hydrogels exhibited attractive
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thermo-induced shape memory properties. In addition, good mechanical properties 1
23
(strength >200 kPa, strain >650%) were achieved. Such composite hydrogels with good
24
shape memory behavior and enhanced mechanical strength would be an attractive candidate
25
for a wide variety of applications.
26
Keywords: TEMPO-oxidized cellulose nanofibers (TOCNs), gelatin, hydrogels, mechanical
27
properties, shape memory.
28
1. Introduction
29
Shape memory hydrogels are kinds of special stimuli-responsive, soft, and wet material
30
that have the ability to fix a temporary deformation and memorize the original shape when
31
exposed to a specific external stimulus such as heat (Hao & Weiss, 2013), light (Xiao, Gong,
32
Kang, Jiang, Zhang & Li, 2016; Zhang, Wang, Hong, Sun, Liu & Tong, 2014), ultrasound (Li,
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Yan, Xia & Zhao, 2015), solvent (Xiao et al., 2016), electric field (Zhao, Tan, Cui, Deng,
34
Huang & Guo, 2014). They have many potential applications in the fields of biomedical
35
(Kabir et al., 2014), tissue engineering (Huang et al., 2012), and smart devices (Harmon,
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Tang & Frank, 2003). Shape memory hydrogels are most commonly triggered by heat.
37
Typically, shape memory hydrogels consist the permanent netpoints (hard domains) and
38
crystalline or glassy domains acting as temporary cross-links (switching domains).While the
39
netpoints determine the permanent shape of the hydrogel, the physical cross-links formed by
40
solidification of the switching domains below the glass or transition temperatures fix the
41
temporary shape gained by deformation of the gel sample at higher temperatures (Behl,
42
Razzaq & Lendlein, 2010). Nevertheless, the low mechanical properties of the shape memory
43
hydrogels limit their applications. In recent years, the filling of shape memory polymer
44
composites with different nanofillers broadens the varieties of shape memory hydrogels and 2
45
significantly expanding their applications. Nanofillers primarily include organic agents, such
46
as azobenzene derivatives, inorganic particles, such as nano clay, and polymers, such as poly
47
(ethylene glycol) (Meng & Li, 2013). Thanks to its biodegradability, renewability and
48
non-toxicity, nanocellulose is thereby considered as a promising bio-based material derived
49
from native cellulose sources with low impact on the environment.
50
At present, numerous studies have been carried out to reinforce hydrogels by
51
incorporating rigid, rod-like cellulose nanocrystals (CNCs), one class of nanocellulose that is
52
often isolated by strong acid hydrolysis (Habibi, Lucia & Rojas, 2010; Wang, Zhu, Reiner,
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Verrill, Baxa & Mcneil, 2012). These studies indicate that CNCs exhibit good reinforcing
54
effect (Hebeish, Farag, Sharaf & Shaheen, 2014). As another type of nanocellulose materials,
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cellulose nanofibers (CNFs) are marked by a three-dimensional network structure formed by
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the entanglement of flexible nanofibers typically having a higher aspect ratio and flexibility
57
than CNCs (Xu, Liu, Jiang, Zhu, Haagenson & Wiesenborn, 2013). To this effect, CNFs are a
58
favorable reinforcing candidate for the hydrogel. CNFs can be produced by first pretreated
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with TEMPO-mediated oxidation (2, 2, 6, 6, -tetrame-thylpipelidine-1-oxyl radical), and then
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multipass high-pressure homogenization. As a kind of important “green” filler, TOCNs have
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great potential in reinforcing various kinds of polymers (Wei, Chen, Liu, Du, Yu & Zhou,
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2016). Gelatin, a partial derivative of collagen, possesses distinctive characteristics, such as
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low cost, biocompatibility, and biodegradability. Besides, it could easily dissolve in water and
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subsequently form a thermal reversible physical gel at low temperature, in which the
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macromolecular chains recover the triple-helix structure of collagen (Bohidar & Jena, 1993;
66
Gornall & Terentjev, 2008). What’s more, constructing double network structures (DN gels) 3
67
is an ideal strategy to obtain high mechanical properties of hydrogels (Gong, 2010).
68
In this study, a type of protein/polymer-TOCNs shape memory hydrogel was successfully
69
synthesized through the in situ polymerization of acrylamide (Am) in a mixed suspension of
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gelatin and TOCNs. This composite gels including two regions: the chemically crosslinked
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polyacrylamide (PAM) function as the “permanent” cross-links; and physically crosslinked
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gelatin as the “temporary” cross-links. TOCNs were introduced to improve the mechanical
73
properties of the hydrogel. The compatibility, morphology, shape memory properties and
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mechanical properties of the resulting shape memory hydrogels with varying gel
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compositions were investigated.
76
2. Experimental details
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2.1 Materials
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Bleached hardwood pulp was kindly supplied from the Guangzhou Chenhui Paper Co.,
79
Ltd. (China). (2, 2, 6, 6-Tetramethylpiperidin-1-yl) oxyl (TEMPO, 98 wt %), sodium bromide
80
(NaBr), sodium hypochlorite (NaClO), gelatin, acrylamide (AM), potassium peroxidisulfate
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(KPS, K2S2O8, 99%, J&K), N, N-methylene bisacrylamide (MBA) and N, N, N′,
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N′-tetramethyldiamine (TEMED) were purchased from Sinopharm Chemical Reagent Co.
83
Ltd (SCRC) and used without further purification. The concentrations of the solutions of KPS
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and MBA were fixed at 20mg/mL. They were freshly prepared just before use.
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2.2 Preparation of TOCNs
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TEMPO-mediated oxidation was conducted according to a procedure described in the
87
literature (Saito, Kimura, Nishiyama & Isogai, 2007). Specifically, 20g dried hardwood pulp
88
was first dispersed in water with a mechanical stirrer until the solid content of 1 wt% was 4
89
achieved. Then 0.32g TEMPO and 2.32 g NaBr were dissolved into the slurry and the
90
reaction system was adjusted to pH 10 by the dropwise addition of 10% NaOH solution.
91
Oxidation began immediately after adding sodium hypochlorite (284g, 6.3% chlorine solution)
92
at 30 °C. A pH meter was used to monitor the pH and 10% NaOH solution was used to
93
maintain the pH at 10 during the reaction. The reaction time for the chemical oxidation was
94
approximately 3 h. After the TEMPO-mediated oxidation, the pulp suspensions were filtered
95
and thoroughly washed with distilled water until pH values of the filtrate reached neutral (i.e.,
96
unchanged). The TEMPO-oxidized fiber suspension mechanical treated by successively
97
passing it through a high-pressure homogenizer. The obtained transparent TOCNs was stored
98
at 4 °C without any treatment before future utilization.
99
2.3 Synthesis of TOCN/PAM/Gelatin Composite Hydrogels
100
The hydrogel was prepared through the in situ radical polymerizations of the monomer
101
(Am) in an aqueous suspension of TOCNs and gelatin with a small amount of MBA as a
102
co-cross-linker. Firstly, the desired amount of gelatin (0−10 mg/mL) was added into a 50mL
103
dried three-necked flask and dissolved in water at 45°C for 1 h. Successively, AM (6.4g),
104
MBA (76 μL, 20mg/mL), and TOCN (0−5 wt% to AM) were added. After being stirred at
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45 °C for 2 h, the mixture became uniform. The oxygen was removed from the solution with
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nitrogen gas for 10 min. Thereafter, KPS solution (1.8mL, 20mg/mL) and catalyst TEMED
107
(30 μL) were added to the solution and stirred for 1min. The mixture solution was rapidly
108
injected into a laboratory-made mold which consisted of two glass sheets with a 2mm rubber
109
spacer. Finally, the mold was cooled down to 6°C to form a gelatin network, followed by
110
polymerization at room temperature for 12 h to obtain the nanocomposite hydrogels. In this 5
111
article, the hydrogels are denoted as TOCNx/PAM/Gelatiny, which represents x wt% TOCN to
112
AM, and y w/v% gelatin. The total volume of the solution mixture was fixed at 30 mL with
113
deionized water. Detailed feed compositions are listed in Table S1.
114
2.4 Characterization.
115
Atomic force microscopy (AFM) was employed to investigate the morphology of TOCN
116
by using a Nanoscope III instrument (Veeco Co., Ltd., USA). X-ray diffraction (XRD)
117
patterns were recorded using an X-ray diffractometer at a range of 2 = 5−60 (D8
118
ADVANCE, Bruker, Germany). Attenuated total reflectance-fourier transform infrared
119
(ATR-FTIR) spectra of hydrogels were obtained on a VERTEX 70 FT-IR spectrometer
120
(Bruker, Germany) in the range of 600−3600cm-1. For SEM observations, the samples were
121
freeze-dried in a lyophilizer for 24 h at −50 °C and then coated with gold (S-3700, Hitachi,
122
Japan).
123
2.5 Shape Memory Behavior
124
A hydrogel strip (with 30mm length × 4mm width × 2mm thickness) was cut from the
125
as-prepared hydrogels. The gel strip was deformed into a helix after being immersed in 90 °C
126
water for 20 s. Then, it was immersed in ice water for 30 s to fix the shape. Quantitative
127
information on the shape memory properties was determined according to the reported
128
method (Chen, Peng, Liu, Wang, Shi & Wang, 2016; Huang, Zhao, Wang, Sun & Tong,
129
2016). The shape fixity ratios (Rf) were defined by the following equation (1):
130
=
× 100%
(1)
131
Where θi was the actually curled angle and θt was the maintained angle after fixing in ice
132
water and keeping it in air for 5 min. 6
133
The shape memory behavior was further observed by stretching a hydrogel strip. A gel
134
strip (with 36mm length × 10mm width × 2mm thickness) was first immersed in 90 ° C water
135
for 20 s and then stretched to an elongated shape. The fixation of the elongated gel strip was
136
conducted by keeping in ice water for 30s.
137
2.6 Equilibrium Swelling Degree of Hydrogels
138
Swelling experiments were performed at room temperature by immersing the dried
139
hydrogels in excess distilled water to obtain swelling equilibrium. The swelling ratio (SR) of
140
the hydrogel was calculated according to the following equation (2):
141
Swelling ratio =
142
Where
143
equilibrium state, respectively.
144
2.7 Dynamic Mechanical Analysis
and
(2) represent the weights of the dried hydrogel and the hydrogel at swelling
145
Dynamic mechanical analysis was carried out using a stress-controlled rheometer
146
RheoStress 600 (Thermo Fisher Instrument, USA) with a parallel plate fixture of the diameter
147
of 40 mm. Temperature sweeps were performed from 25 to 75 °C and then cooled down to
148
25 °C without waiting at a rate of 2 °C/min with a fixed strain amplitude γ = 0.5% and
149
angular frequency ω = 6.28 rad/s. Storage modulus (G′) and loss modulus (G″) were
150
calculated using the dynamic mechanical analysis software. Silicone oil was laid on the edge
151
of the plates to prevent water evaporation.
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2.8 Measurements of Mechanical Properties
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Tensile tests were performed on the as-prepared gel samples (with 30mm length × 4mm
154
width × 2mm thickness) using an Instron Universal Testing Machine 5565 with a crosshead 7
155
speed of 100mm/min at room temperature.
156
3. Results and discussion
157
3.1 Characterization of TOCNs.
158
AFM was used to investigate the morphology and dimension of TOCNs (Figure 1a). As
159
shown in Figure 1a, most TOCNs have diameters mainly distributed in the range of 15~25nm
160
and show entangled morphology. Upon oxidization of the surface hydroxyl groups, some
161
changes on the spectrum of TOCNs can be observed (Figure 1b). The most important change
162
is the appearance of the sodium carboxylate groups (−COONa) stretching band at 1605 cm−1.
163
Based on this analysis, the success of the chemical oxidization of cellulose was clearly
164
verified by FTIR.
(a)
(b) 200 nm
600 nm 165 166
Figure.1. (a) AFM image of TOCNs, and (b) FTIR spectrum of TOCNs
167
3.2 Preparation and Structural Characterization of the Formation of TOCN/PAM/Gelatin
168
Nanocomposite Gels
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Scheme 1a shows the proposed mechanism for the reversible triple helices mechanism of
170
gelatin. It can be seen that during cooling, the gelatin chains underwent the coil-helix
171
transition followed by the aggregation of triple helices. As a result, the first, physically 8
172
cross-linked gelatin network was formed. Upon KPS initiation, the chemically cross-linked
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polyacrylamide was formed as the second network through the radical polymerization
174
(Scheme 1b). Thus, the hydrogels with the double networks of polyacrylamide and gelatin
175
were obtained.
176 177
Scheme.1. (a) Heat-induced reversible triple helices mechanism of gelatin, (b) Synthesis scheme of
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TOCN/PAAM/Gelatin gels.
179
ATR-FTIR spectra of the hydrogel samples are presented in Figure 2a. The PAM curve
180
had a broadband around 3339 cm−1 and a characteristic peak at 3189 cm−1 due to the
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stretching vibration of N–H. The significant characteristic peaks at 1655 and 1612 cm−1 were
182
attributed to the carbonyl stretching vibration (amide I) and N–H bending vibration (amide II)
183
of the amide group, respectively (Zhou & Wu, 2011). In the ATR-FTIR spectra of the
184
composite
185
TOCN5/PAM/Gelatin10, the absorptions associated with characteristic groups of PAM were
186
clearly identified. Compared with pure PAM, there was no perceptible shift in positions
hydrogels
of
TOCN0/PAM/Gelatin10,
9
TOCN5/PAM/Gelatin0
and
187
occurred in the composite hydrogels. This may be due to the lower TOCN contents and its
188
characteristic absorption peaks were overlapped by those associated with PAM. Additionally,
189
there was no appearance of new vibration, indicating that no new covalent bond formed
190
between gelatin, PAM, and TOCN.
(a)
(b)
TOCN5/PAM/Gelatin10
Intensity (a.u.)
T (%)
TOCN5/PAM PAM/Gelatin10 PAM
3339
3600
191
3189
TOCN PAM TOCN5/PAM TOCN5/PAM/Gelatin10
1655 1612
3000 2400 1800 1200 Wavenumbers (cm-1)
600
5 10 15 20 25 30 35 40 45 50 55 60 2
192
Figure.2. Physicochemical characterization as elucidated by (a) ATR-FTIR spectra; and (b) XRD
193
patterns of the hydrogels.
194
Figure 2b shows the XRD patterns of TOCNs, PAM, TOCN5/PAM, and
195
TOCN5/PAM/Gelatin10 hydrogels. The diffraction pattern of PAM does not show an obvious
196
sharp peak of the crystalline structure; instead, a blunt peak centered at 2θ= 26.1° is observed.
197
It is noteworthy that in comparison with TOCN there is no characteristic TOCN peak at 22.6°
198
detected in the XRD patterns of TOCN5/PAM and TOCN5/PAM/Gelatin10 hydrogels, which
199
suggested that TOCNs were uniformly dispersed in the polymer gels without ordered
200
aggregation.
201
Lyophilization is known to preserve the structure and volume of the hydrogels even after
202
all of the solvent has been removed. Figure 3 shows the SEM images of the lyophilized PAM
203
and TOCN5/PAM/Gelatin10 hydrogels. It can be seen that the network of the net PAM
204
hydrogel was clear with a uniform macropore structure and the pore sizes ranging from ~30 10
205
to ~200 μm. For the network of TOCN5/PAM/Gelatin10 hydrogel (Figure 3c and d), it was
206
observed that the pore structure of the gels became more complex and with a smaller pore
207
size, thus implying a denser structure of the hydrogel. This can be attributed to the formation
208
of polyacrylamide-gelatin double network and the strong hydrogen-bonding interactions
209
between TOCNs and polar functional groups of the PAM side chains.
(b)
(a)
200 µ m
50 µ m
(c)
(d)
200 µ m
50 µ m
210 211
Figure.3. SEM micrographs of (a, b) PAM hydrogel and (c, d) TOCN5/PAM/Gelatin10 hydrogel.
212
3.3 Shape Memory Behavior
213
The hydrogels exhibited good shape memory behaviors. Figure 4b shows the thermally
214
triggered shape memory behavior of a composite hydrogel strip. The gel strip (Figure 4a) was
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curled into a tight spiral after being immersed in 90 °C water for 20s. Afterwards, the
216
deformed gel was fixed in ice water for 30s to obtain a temporary deformed shape (Figure 4b).
217
When the deformed gel helix was immersed into 90 °C water, it can recover its original shape
218
rapidly (Movie S1). These results indicate that the hydrogels containing gelatin exhibit the 11
219
cooling-fixed shape memory capability.
(a)
(b)
(c)
220 221
Figure.4. Photos of (a) original and recovered shape of the gel strip;(b) fixed temporary shape of the
222
hydrogel. (c) Shape fixity ratios (Rf) of the gels prepared with different contents of TOCNs and gelatin
223
after fixing in ice water and keeping in the room temperature for 5 min. The numbers in the shape fixity
224
ratios picture indicate the content of the corresponding components in Table S1.
225
The shape fixity ratios (Rf) of the gels were obtained with the simple bending
226
experiments performed. The Rf of the gels was not significantly affected by TOCN contents,
227
while the contents of gelatin had an obvious effect on them (Figure 4c). The Rf of the
228
TOCN5/PAM/Gelatin10 gels were mostly around 65%, higher than those of the corresponding
229
gels that have lower contents of gelatin. It is noteworthy that the TOCN5/PAM/Gelatin0
230
sample without gelatin cannot be reshaped. All of the fixed shapes could rapid recover their
231
original shapes when immersing in 90 °C water.
12
Original Shape (a)
Deformed Shape (b)
Recovered Shape (c)
232 233
Figure.5. Photos showing the heat-shrinkable shape memory behavior of TOCN0/PAM/Gelatin10
234
hydrogel. (a−c): A gel strip of 36mm×10mm×2mm in the original (a), elongated (b), and recovered
235
shapes (c). The fixation of the elongated gel strip was conducted by keeping in ice water for 30s. This
236
shape memory behavior was performed with as-prepared wet TOCN0/PAM/Gelatin10 hydrogel.
237
The composite hydrogel also exhibited a heat-shrinkable shape memory behavior
238
(Figure 5). This behavior was observed by an elongated TOCN0/PAM/Gelatin10 hydrogel strip.
239
A gel strip was first heated to 90 ° C and then stretched to an elongated shape. The fixation of
240
the elongated gel strip was conducted by keeping in ice water for 30s. After being immersed
241
in 90 °C water for 5s, the elongated part shrank quickly to a remaining length of 39 mm
242
(Figure 5c), very close to the original length of 36mm (Figure 5a).
243
To examine the repeatability of the shape memory capability of the hydrogels, the cycle
244
of shape fixing and thermo-induced shape recovery was repeated 10 times (Figure S1). The
245
hydrogel can remain almost the same shape fixity ratio (Rf) and recover to its original shape
246
after 10 cycles showing a good reproducible capability of shape memory.
247
Temperature-dependent dynamic mechanical properties of the composite hydrogels were
248
measured. As shown in Figure 6, the storage modulus (G′) was always higher than the
249
corresponding loss modulus (G″) in the entire temperature cycle of 25 °C→75 °C→25 °C,
250
suggesting the elastic nature of the TOCN/PAM/Gelation hydrogels. It can be seen that the 13
251
TOCN5/PAM/Gelatin0 hydrogel keeps almost constant in G′ and G″ (Figure 6a) during a
252
cycle of heating and cooling process because of the absence of gelatin. However, G′ is not a
253
constant for the TOCN5/PAM/Gelatin10 hydrogel (Figure 6b). As temperature increases, G′ of
254
the TOCN5/PAM/Gelatin10 hydrogel decrease rapidly at 35 °C. This was induced by the
255
helix-coil transition of the gelatin chains at 35 °C. When temperature is higher than 45 °C, G′
256
of the TOCN5/PAM/Gelatin10 hydrogel keep almost a constant, no matter heating or cooling.
257
This constant G′ is attributed to the cross-linking density of the chemically cross-linked PAM
258
network, which is not changed with temperature. Moreover, G′ increases greatly at about
259
35 °C and could almost recover its initial value during the process of cooling, implying the
260
thermoreversible sol-gel transition of the gelatin network. These findings clearly indicate that
261
the thermoreversible sol-gel transition of gelatin network regulates the moduli of the
262
hydrogels, endowing the shape-memory capability of these composite hydrogels.
263 264
Figure.6. Dependence of storage modulus (G′) and loss modulus (G″) on temperature during a
265
temperature sweep at a heating and cooling rate of 2 °C/min for (a) TOCN5/PAM/Gelatin0, and (b)
266
TOCN5/PAM/Gelatin10 hydrogels. An angular frequency ω=6.28 rad/s and a strain amplitude γ=0.5%
267
were adopted.
268
3.3 Swelling and Mechanical Properties of TOCN/PAAM/Gelatin Gels. 14
269
The swelling behaviors were investigated to further reveal the structural features of the
270
TOCNx/PAM/Gelatiny nanocomposite hydrogels. The results are shown in Figure S2. It was
271
well-known that the swelling behavior of the hydrogel depended on the crosslinked density of
272
the gel network. The swelling ratio of the TOCNx/PAM/Gelatiny hydrogels decreased with the
273
increasing TOCN contents (Figure S2a). This was attributed to the fact that TOCNs could
274
interact with the PAM network by means of hydrogen bonding to form a closer network
275
structure, thus causing a decrease in the swelling capacity. Meanwhile, the swelling ratios of
276
TOCN5/PAM/Gelatiny hydrogels decreased from 21.79 to 11.4 g/g with the percent of gelatin
277
increasing from 3 to 10 w/v% (Figure S2b). This behavior confirmed that the
278
TOCN5/PAM/Gelatin10 double network (DN) hydrogel had a more robust network than that
279
of the TOCN5/PAM/Gelatin3, TOCN5/PAM/Gelatin5 and TOCN5/PAM/Gelatin7 DN
280
hydrogels.
281
As shown in Figure 7, TOCN/PAM/Gelatin nanocomposite hydrogels exhibited
282
excellent tensile properties. The gels can be easily stretched (Figure 7a), twisted stretched
283
(Figure 7b) and stretching with a knot (Figure 7c) to at least 4 times their original length. The
284
physically linked networks could be easily extended and contributed to the stretchability of
285
DN gels. Moreover, TOCN/PAM/Gelatin gels can be readily adapted to different complex
286
shapes (Figure 7d-f), demonstrating their free-shapeable property.
15
(d) (a) 1cm
(e) (b) 1cm
(f) (c) 1cm
287 288
Figure.7. The composite hydrogels are very tough and flexible, with the ability to withstand different
289
high-level deformations of (a) stretching (b) twisted stretching, as well as (c) stretching with a knot. The
290
gels are also free shapeable into different complex shapes (d-f, dyed with methyl orange). All these
291
behaviors were performed with the as-prepared wet TOCN5/PAM/Gelatin10 hydrogels.
292
The mechanical properties of TOCNx/PAM/Gelatiny gels at room temperature are
293
showed in Figure 8, including tensile stress-strain curves (Figure. 8a, c) and elastic moduli (E)
294
(Figure. 8b, d).
295
The positive effect of the TOCN amount on the mechanical properties of hydrogel is
296
shown in Figure 8a. As TOCN contents increased, the maximum tensile stress (b), and
297
elastic modulus (E) of the DN gels consistently increased. TOCN0/PAM/Gelatin10 had b, and
298
E of 91 kPa, and 20.8 kPa respectively. Compared with TOCN0/PAM/Gelatin10,
299
TOCN5/PAM/Gelatin10 exhibited much stronger mechanical properties (b of 237 kPa, and E
300
of 45.5 kPa), indicating that the TOCNs are able to effectively bear the tensile load applied on 16
301
the PAM matrix and thus enhance the mechanical properties of hydrogels.
302
The introduction of gelatin increases the elastic modulus and tensile strength of the
303
composite hydrogel, further revealing the enhanced compatibility of the gelatin and the
304
TOCN/PAM
305
TOCN5/PAM/Gelatin10 was superior to TOCN5/PAM/Gelatin0 single network (SN) gel and
306
demonstrated its high toughness in tensile tests. It is worth noting that the increment of
307
gelatin contents could not only enhances the hydrogel strength but also increases the
308
shape-fixing percentage Rf in Figure 4.
system.
From
Figure
8
it
can
be
seen
that
the
hydrogel
of
309
The elongation (εb) at break, however, of the samples do display a different tendency
310
compared with the above properties. It is highly dependent on gelatin contents. The
311
elongation (εb) significantly and monotonically decreased from 1590% to 680% as gelatin
312
content increased from 3 w/v% to 10 w/v% (Figure 8c). The results indicate that the
313
elongation (εb) at break decreases with the increasing crosslinking density as expected. It can
314
be attributed to the increase of crosslinking density having a negative effect on the toughness
315
of polymers. Therefore, we speculated that a combination of double network and nanofillers
316
facilitated such good elastic and mechanical properties.
17
(a)
TOCN0 TOCN1
Stress /kPa
TOCN3
150
TOCN5
100 50 TOCNx/PAM/Gelatin10
0
250
0
200
400 600 Strain / %
800
Gelatin3 Gelatin5
Stress /kPa
Gelatin7
150
Gelatin10
100 50
TOCN5/PAM/Gelatiny
0
317
400
800 1200 Strain / %
1600
2000
TOCNx/PAM/Gelatin10
30 20 10
50
(c)
200
0
(b) 40
0
1000
Elastic modulus (Kpa)
200
50
Elastic modulus (Kpa)
250
TOCN0
(d)
TOCN1
TOCN3
TOCN5
TOCN5/PAM/Gelatiny
40 30 20 10 0
Gelatin3 Gelatin5 Gelatin7 Gelatin10
318
Figure.8. (a) Tensile stress−strain curves of NC gels and (b) the corresponding elastic modulus. All
319
gels were tested at the as-prepared state without swelling equilibrium in water solution.
320
4. Conclusions
321
In this study, cellulose nanofibers were isolated through TEMPO-oxidation pre-treatment
322
followed by mechanical disintegration. And then a series kinds of shape memory
323
TOCN/PAM/Gelatin hydrogels with varying TOCNs and gelatin contents were prepared
324
through a facile in-situ free-radical polymerization method. After the polymerization, TOCNs,
325
gelatin, and PAM were distributed homogeneously in the hydrogels. Importantly, there was
326
no new covalent bond formed between gelatin, PAM, and TOCNs. As to the shape memory
327
properties of TOCN/PAM/Gelatin composite hydrogels, the reversible nature of the gelatin
328
network imparts the TOCN/PAM/Gelatin composite hydrogels with fast cooling-fixed (ice
329
water for 30s) and thermo-induced shape recovery (in 90 °C water for 5s) capability. Good 18
330
mechanical properties (strength >200 kPa, strain >650%) were achieved. As TOCNs content
331
increased, the maximum tensile stress (b), and elastic modulus (E) of the DN gels
332
consistently increased. The introduction of gelatin contributed to increases in the elastic
333
modulus and tensile strength, further revealing the enhanced compatibility of the gelatin and
334
the TOCN/PAM system. Such a composite hydrogel with good shape memory behavior and
335
enhanced mechanical strengths would be an attractive candidate for a wide variety of
336
applications.
337
Acknowledgements
338
The authors are extremely grateful to financial support from the Science and Technology
339
Project of Guangzhou City in China (No.2016070220045), State Key Laboratory of Pulp and
340
Paper Engineering (2016 c01), State Key Laboratory of Pulp and Paper Engineering
341
(2015TS01), Natural Science Foundation of China (No.201404042), Shandong Natural
342
Science Foundation (No. ZR2015PC016), Science and Technology Planning Project of
343
Higher Education of Shandong Province (No. J14LD51), Doctoral Starting up Foundation of
344
Qufu Normal University (No. BSQD2012058), and Science and Technology Planning Project
345
of Qufu Normal University (No. xkj201413).
346 347
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Gornall, J. L., & Terentjev, E. M. (2008). Helix-coil transition of gelatin: helical morphology and stability. Soft Matter, 4(3), 544-549. Habibi, Y., Lucia, L. A., & Rojas, O. J. (2010). Cellulose nanocrystals: chemistry, self-assembly, and applications. Chemical Reviews, 110(6), 3479. Hao, J., & Weiss, R. A. (2013). Mechanically Tough, Thermally Activated Shape Memory Hydrogels. ACS Macro Letters, 2(1), 86-89. Harmon, M. E., Tang, M., & Frank, C. W. (2003). A microfluidic actuator based on thermoresponsive hydrogels. Polymer, 44(16), 4547-4556. Hebeish, A., Farag, S., Sharaf, S., & Shaheen, T. I. (2014). Thermal responsive hydrogels based on semi interpenetrating network of poly(NIPAm) and cellulose nanowhiskers. Carbohydrate Polymers, 102(4), 159-166. Huang, J., Zhao, L., Wang, T., Sun, W., & Tong, Z. (2016). NIR Triggered Rapid Shape Memory PAM-GO-Gelatin Hydrogels with High Mechanical Strength. ACS Applied Materials & Interfaces, 8 (19), 12384–12392. Huang, W. M., Song, C. L., Fu, Y. Q., Wang, C. C., Zhao, Y., Purnawali, H., Lu, H. B., Tang, C., Ding, Z., & Zhang, J. L. (2012). Shaping tissue with shape memory materials. Advanced Drug Delivery Reviews, 65(4), 515-535. Kabir, M. H., Hazama, T., Watanabe, Y., Jin, G., Murase, K., Sunada, T., & Furukawa, H. (2014). Smart hydrogel with shape memory for biomedical applications. Journal of the Taiwan Institute of Chemical Engineers, Volume 45(Issue 6), 3134-3138. Li, G., Yan, Q., Xia, H., & Zhao, Y. (2015). Therapeutic-Ultrasound-Triggered Shape Memory of a Melamine-Enhanced Poly(vinyl alcohol) Physical Hydrogel. ACS Applied Materials & Interfaces, 7(22), 12067. Meng, H., & Li, G. (2013). A review of stimuli-responsive shape memory polymer composites. Polymer, 54(9), 2199-2221. Saito, T., Kimura, S., Nishiyama, Y., & Isogai, A. (2007). Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules, 8(8), 2485-2491. Wang, Q. Q., Zhu, J. Y., Reiner, R. S., Verrill, S. P., Baxa, U., & Mcneil, S. E. (2012). Approaching zero cellulose loss in cellulose nanocrystal (CNC) production: recovery and characterization of cellulosic solid residues (CSR) and CNC. Cellulose, 19(6), 2033-2047. Wei, J., Chen, Y., Liu, H., Du, C., Yu, H., & Zhou, Z. (2016). Thermo-responsive and compression properties of TEMPO-oxidized cellulose nanofiber-modified PNIPAm hydrogels. Carbohydrate Polymers, 147, 201-207. Xiao, H., Lu, W., Le, X., Ma, C., Li, Z., Zheng, J., Zhang, J., Huang, Y., & Chen, T. (2016). A multi-responsive hydrogel with a triple shape memory effect based on reversible switches. Chemical Communications, 52(90). Xiao, Y. Y., Gong, X. L., Kang, Y., Jiang, Z. C., Zhang, S., & Li, B. J. (2016). Light-, pH- and thermal-responsive hydrogels with the triple-shape memory effect. Chemical Communications, 52(70). Xu, X., Liu, F., Jiang, L., Zhu, J. Y., Haagenson, D., & Wiesenborn, D. P. (2013). Cellulose nanocrystals vs. cellulose nanofibrils: a comparative study on their microstructures and effects as polymer reinforcing agents. ACS Applied Materials & Interfaces, 5(8), 2999-3009. Zhang, E., Wang, T., Hong, W., Sun, W., Liu, X., & Tong, Z. (2014). Infrared-driving actuation based on bilayer graphene oxide-poly(N-isopropylacrylamide) nanocomposite hydrogels. Journal of Materials Chemistry A, 2(37), 15633-15639. 20
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21
S0144-8617(17)30424-1 http://dx.doi.org/doi:10.1016/j.carbpol.2017.04.035 CARP 12223
To appear in: Received date: Revised date: Accepted date:
24-2-2017 31-3-2017 15-4-2017
Please cite this article as: Li, Nan., Chen, Wei., Chen, Guangxue., & Tian, Junfei., Rapid Shape Memory TEMPO-Oxidized Cellulose Nanofibers/Polyacrylamide/Gelatin Hydrogels with Enhanced Mechanical Strength.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2017.04.035 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
Rapid Shape Memory TEMPO-Oxidized Cellulose
2
Nanofibers/ Polyacrylamide/ Gelatin Hydrogels with
3
Enhanced Mechanical Strength
4
Nan Li a, Wei Chen b, *, Guangxue Chen a, *, Junfei Tiana
5
a
6
Guangzhou 510640, P. R. China; b College of Engineering, Qufu Normal University, RiZhao 276826, China
7
* Corresponding authors at South China University of Technology, Guangzhou 510640, P. R. China and
8
College of Engineering, Qufu Normal University, RiZhao 276826, China.
9
E-mail addresses: [email protected] (N.Li); [email protected] (W.Chen); [email protected]
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology,
10
(G.X.Chen); [email protected] (J.F.Tian)
11
TEL: (86)-020-22236485 ； Fax :( 86)-020-22236485
12
Abstract
13
TEMPO-oxidized cellulose nanofibers/ polyacrylamide/ gelatin shape memory hydrogels
14
were successfully fabricated through a facile in-situ free-radical polymerization method, and
15
double network was formed by chemically cross-linked polyacrylamide (PAM) network and
16
physically cross-linked gelatin network. TEMPO-oxidized cellulose nanofibers (TOCNs)
17
were introduced to improve the mechanical properties of the hydrogel. The structure, shape
18
memory behaviors and mechanical properties of the resulting composite gels with varied gel
19
compositions were investigated. The results obtained from those different studies revealed
20
that TOCNs, gelatin, and PAM could mix with each other homogeneously. Due to the
21
thermoreversible nature of the gelatin network, the composite hydrogels exhibited attractive
22
thermo-induced shape memory properties. In addition, good mechanical properties 1
23
(strength >200 kPa, strain >650%) were achieved. Such composite hydrogels with good
24
shape memory behavior and enhanced mechanical strength would be an attractive candidate
25
for a wide variety of applications.
26
Keywords: TEMPO-oxidized cellulose nanofibers (TOCNs), gelatin, hydrogels, mechanical
27
properties, shape memory.
28
1. Introduction
29
Shape memory hydrogels are kinds of special stimuli-responsive, soft, and wet material
30
that have the ability to fix a temporary deformation and memorize the original shape when
31
exposed to a specific external stimulus such as heat (Hao & Weiss, 2013), light (Xiao, Gong,
32
Kang, Jiang, Zhang & Li, 2016; Zhang, Wang, Hong, Sun, Liu & Tong, 2014), ultrasound (Li,
33
Yan, Xia & Zhao, 2015), solvent (Xiao et al., 2016), electric field (Zhao, Tan, Cui, Deng,
34
Huang & Guo, 2014). They have many potential applications in the fields of biomedical
35
(Kabir et al., 2014), tissue engineering (Huang et al., 2012), and smart devices (Harmon,
36
Tang & Frank, 2003). Shape memory hydrogels are most commonly triggered by heat.
37
Typically, shape memory hydrogels consist the permanent netpoints (hard domains) and
38
crystalline or glassy domains acting as temporary cross-links (switching domains).While the
39
netpoints determine the permanent shape of the hydrogel, the physical cross-links formed by
40
solidification of the switching domains below the glass or transition temperatures fix the
41
temporary shape gained by deformation of the gel sample at higher temperatures (Behl,
42
Razzaq & Lendlein, 2010). Nevertheless, the low mechanical properties of the shape memory
43
hydrogels limit their applications. In recent years, the filling of shape memory polymer
44
composites with different nanofillers broadens the varieties of shape memory hydrogels and 2
45
significantly expanding their applications. Nanofillers primarily include organic agents, such
46
as azobenzene derivatives, inorganic particles, such as nano clay, and polymers, such as poly
47
(ethylene glycol) (Meng & Li, 2013). Thanks to its biodegradability, renewability and
48
non-toxicity, nanocellulose is thereby considered as a promising bio-based material derived
49
from native cellulose sources with low impact on the environment.
50
At present, numerous studies have been carried out to reinforce hydrogels by
51
incorporating rigid, rod-like cellulose nanocrystals (CNCs), one class of nanocellulose that is
52
often isolated by strong acid hydrolysis (Habibi, Lucia & Rojas, 2010; Wang, Zhu, Reiner,
53
Verrill, Baxa & Mcneil, 2012). These studies indicate that CNCs exhibit good reinforcing
54
effect (Hebeish, Farag, Sharaf & Shaheen, 2014). As another type of nanocellulose materials,
55
cellulose nanofibers (CNFs) are marked by a three-dimensional network structure formed by
56
the entanglement of flexible nanofibers typically having a higher aspect ratio and flexibility
57
than CNCs (Xu, Liu, Jiang, Zhu, Haagenson & Wiesenborn, 2013). To this effect, CNFs are a
58
favorable reinforcing candidate for the hydrogel. CNFs can be produced by first pretreated
59
with TEMPO-mediated oxidation (2, 2, 6, 6, -tetrame-thylpipelidine-1-oxyl radical), and then
60
multipass high-pressure homogenization. As a kind of important “green” filler, TOCNs have
61
great potential in reinforcing various kinds of polymers (Wei, Chen, Liu, Du, Yu & Zhou,
62
2016). Gelatin, a partial derivative of collagen, possesses distinctive characteristics, such as
63
low cost, biocompatibility, and biodegradability. Besides, it could easily dissolve in water and
64
subsequently form a thermal reversible physical gel at low temperature, in which the
65
macromolecular chains recover the triple-helix structure of collagen (Bohidar & Jena, 1993;
66
Gornall & Terentjev, 2008). What’s more, constructing double network structures (DN gels) 3
67
is an ideal strategy to obtain high mechanical properties of hydrogels (Gong, 2010).
68
In this study, a type of protein/polymer-TOCNs shape memory hydrogel was successfully
69
synthesized through the in situ polymerization of acrylamide (Am) in a mixed suspension of
70
gelatin and TOCNs. This composite gels including two regions: the chemically crosslinked
71
polyacrylamide (PAM) function as the “permanent” cross-links; and physically crosslinked
72
gelatin as the “temporary” cross-links. TOCNs were introduced to improve the mechanical
73
properties of the hydrogel. The compatibility, morphology, shape memory properties and
74
mechanical properties of the resulting shape memory hydrogels with varying gel
75
compositions were investigated.
76
2. Experimental details
77
2.1 Materials
78
Bleached hardwood pulp was kindly supplied from the Guangzhou Chenhui Paper Co.,
79
Ltd. (China). (2, 2, 6, 6-Tetramethylpiperidin-1-yl) oxyl (TEMPO, 98 wt %), sodium bromide
80
(NaBr), sodium hypochlorite (NaClO), gelatin, acrylamide (AM), potassium peroxidisulfate
81
(KPS, K2S2O8, 99%, J&K), N, N-methylene bisacrylamide (MBA) and N, N, N′,
82
N′-tetramethyldiamine (TEMED) were purchased from Sinopharm Chemical Reagent Co.
83
Ltd (SCRC) and used without further purification. The concentrations of the solutions of KPS
84
and MBA were fixed at 20mg/mL. They were freshly prepared just before use.
85
2.2 Preparation of TOCNs
86
TEMPO-mediated oxidation was conducted according to a procedure described in the
87
literature (Saito, Kimura, Nishiyama & Isogai, 2007). Specifically, 20g dried hardwood pulp
88
was first dispersed in water with a mechanical stirrer until the solid content of 1 wt% was 4
89
achieved. Then 0.32g TEMPO and 2.32 g NaBr were dissolved into the slurry and the
90
reaction system was adjusted to pH 10 by the dropwise addition of 10% NaOH solution.
91
Oxidation began immediately after adding sodium hypochlorite (284g, 6.3% chlorine solution)
92
at 30 °C. A pH meter was used to monitor the pH and 10% NaOH solution was used to
93
maintain the pH at 10 during the reaction. The reaction time for the chemical oxidation was
94
approximately 3 h. After the TEMPO-mediated oxidation, the pulp suspensions were filtered
95
and thoroughly washed with distilled water until pH values of the filtrate reached neutral (i.e.,
96
unchanged). The TEMPO-oxidized fiber suspension mechanical treated by successively
97
passing it through a high-pressure homogenizer. The obtained transparent TOCNs was stored
98
at 4 °C without any treatment before future utilization.
99
2.3 Synthesis of TOCN/PAM/Gelatin Composite Hydrogels
100
The hydrogel was prepared through the in situ radical polymerizations of the monomer
101
(Am) in an aqueous suspension of TOCNs and gelatin with a small amount of MBA as a
102
co-cross-linker. Firstly, the desired amount of gelatin (0−10 mg/mL) was added into a 50mL
103
dried three-necked flask and dissolved in water at 45°C for 1 h. Successively, AM (6.4g),
104
MBA (76 μL, 20mg/mL), and TOCN (0−5 wt% to AM) were added. After being stirred at
105
45 °C for 2 h, the mixture became uniform. The oxygen was removed from the solution with
106
nitrogen gas for 10 min. Thereafter, KPS solution (1.8mL, 20mg/mL) and catalyst TEMED
107
(30 μL) were added to the solution and stirred for 1min. The mixture solution was rapidly
108
injected into a laboratory-made mold which consisted of two glass sheets with a 2mm rubber
109
spacer. Finally, the mold was cooled down to 6°C to form a gelatin network, followed by
110
polymerization at room temperature for 12 h to obtain the nanocomposite hydrogels. In this 5
111
article, the hydrogels are denoted as TOCNx/PAM/Gelatiny, which represents x wt% TOCN to
112
AM, and y w/v% gelatin. The total volume of the solution mixture was fixed at 30 mL with
113
deionized water. Detailed feed compositions are listed in Table S1.
114
2.4 Characterization.
115
Atomic force microscopy (AFM) was employed to investigate the morphology of TOCN
116
by using a Nanoscope III instrument (Veeco Co., Ltd., USA). X-ray diffraction (XRD)
117
patterns were recorded using an X-ray diffractometer at a range of 2 = 5−60 (D8
118
ADVANCE, Bruker, Germany). Attenuated total reflectance-fourier transform infrared
119
(ATR-FTIR) spectra of hydrogels were obtained on a VERTEX 70 FT-IR spectrometer
120
(Bruker, Germany) in the range of 600−3600cm-1. For SEM observations, the samples were
121
freeze-dried in a lyophilizer for 24 h at −50 °C and then coated with gold (S-3700, Hitachi,
122
Japan).
123
2.5 Shape Memory Behavior
124
A hydrogel strip (with 30mm length × 4mm width × 2mm thickness) was cut from the
125
as-prepared hydrogels. The gel strip was deformed into a helix after being immersed in 90 °C
126
water for 20 s. Then, it was immersed in ice water for 30 s to fix the shape. Quantitative
127
information on the shape memory properties was determined according to the reported
128
method (Chen, Peng, Liu, Wang, Shi & Wang, 2016; Huang, Zhao, Wang, Sun & Tong,
129
2016). The shape fixity ratios (Rf) were defined by the following equation (1):
130
=
× 100%
(1)
131
Where θi was the actually curled angle and θt was the maintained angle after fixing in ice
132
water and keeping it in air for 5 min. 6
133
The shape memory behavior was further observed by stretching a hydrogel strip. A gel
134
strip (with 36mm length × 10mm width × 2mm thickness) was first immersed in 90 ° C water
135
for 20 s and then stretched to an elongated shape. The fixation of the elongated gel strip was
136
conducted by keeping in ice water for 30s.
137
2.6 Equilibrium Swelling Degree of Hydrogels
138
Swelling experiments were performed at room temperature by immersing the dried
139
hydrogels in excess distilled water to obtain swelling equilibrium. The swelling ratio (SR) of
140
the hydrogel was calculated according to the following equation (2):
141
Swelling ratio =
142
Where
143
equilibrium state, respectively.
144
2.7 Dynamic Mechanical Analysis
and
(2) represent the weights of the dried hydrogel and the hydrogel at swelling
145
Dynamic mechanical analysis was carried out using a stress-controlled rheometer
146
RheoStress 600 (Thermo Fisher Instrument, USA) with a parallel plate fixture of the diameter
147
of 40 mm. Temperature sweeps were performed from 25 to 75 °C and then cooled down to
148
25 °C without waiting at a rate of 2 °C/min with a fixed strain amplitude γ = 0.5% and
149
angular frequency ω = 6.28 rad/s. Storage modulus (G′) and loss modulus (G″) were
150
calculated using the dynamic mechanical analysis software. Silicone oil was laid on the edge
151
of the plates to prevent water evaporation.
152
2.8 Measurements of Mechanical Properties
153
Tensile tests were performed on the as-prepared gel samples (with 30mm length × 4mm
154
width × 2mm thickness) using an Instron Universal Testing Machine 5565 with a crosshead 7
155
speed of 100mm/min at room temperature.
156
3. Results and discussion
157
3.1 Characterization of TOCNs.
158
AFM was used to investigate the morphology and dimension of TOCNs (Figure 1a). As
159
shown in Figure 1a, most TOCNs have diameters mainly distributed in the range of 15~25nm
160
and show entangled morphology. Upon oxidization of the surface hydroxyl groups, some
161
changes on the spectrum of TOCNs can be observed (Figure 1b). The most important change
162
is the appearance of the sodium carboxylate groups (−COONa) stretching band at 1605 cm−1.
163
Based on this analysis, the success of the chemical oxidization of cellulose was clearly
164
verified by FTIR.
(a)
(b) 200 nm
600 nm 165 166
Figure.1. (a) AFM image of TOCNs, and (b) FTIR spectrum of TOCNs
167
3.2 Preparation and Structural Characterization of the Formation of TOCN/PAM/Gelatin
168
Nanocomposite Gels
169
Scheme 1a shows the proposed mechanism for the reversible triple helices mechanism of
170
gelatin. It can be seen that during cooling, the gelatin chains underwent the coil-helix
171
transition followed by the aggregation of triple helices. As a result, the first, physically 8
172
cross-linked gelatin network was formed. Upon KPS initiation, the chemically cross-linked
173
polyacrylamide was formed as the second network through the radical polymerization
174
(Scheme 1b). Thus, the hydrogels with the double networks of polyacrylamide and gelatin
175
were obtained.
176 177
Scheme.1. (a) Heat-induced reversible triple helices mechanism of gelatin, (b) Synthesis scheme of
178
TOCN/PAAM/Gelatin gels.
179
ATR-FTIR spectra of the hydrogel samples are presented in Figure 2a. The PAM curve
180
had a broadband around 3339 cm−1 and a characteristic peak at 3189 cm−1 due to the
181
stretching vibration of N–H. The significant characteristic peaks at 1655 and 1612 cm−1 were
182
attributed to the carbonyl stretching vibration (amide I) and N–H bending vibration (amide II)
183
of the amide group, respectively (Zhou & Wu, 2011). In the ATR-FTIR spectra of the
184
composite
185
TOCN5/PAM/Gelatin10, the absorptions associated with characteristic groups of PAM were
186
clearly identified. Compared with pure PAM, there was no perceptible shift in positions
hydrogels
of
TOCN0/PAM/Gelatin10,
9
TOCN5/PAM/Gelatin0
and
187
occurred in the composite hydrogels. This may be due to the lower TOCN contents and its
188
characteristic absorption peaks were overlapped by those associated with PAM. Additionally,
189
there was no appearance of new vibration, indicating that no new covalent bond formed
190
between gelatin, PAM, and TOCN.
(a)
(b)
TOCN5/PAM/Gelatin10
Intensity (a.u.)
T (%)
TOCN5/PAM PAM/Gelatin10 PAM
3339
3600
191
3189
TOCN PAM TOCN5/PAM TOCN5/PAM/Gelatin10
1655 1612
3000 2400 1800 1200 Wavenumbers (cm-1)
600
5 10 15 20 25 30 35 40 45 50 55 60 2
192
Figure.2. Physicochemical characterization as elucidated by (a) ATR-FTIR spectra; and (b) XRD
193
patterns of the hydrogels.
194
Figure 2b shows the XRD patterns of TOCNs, PAM, TOCN5/PAM, and
195
TOCN5/PAM/Gelatin10 hydrogels. The diffraction pattern of PAM does not show an obvious
196
sharp peak of the crystalline structure; instead, a blunt peak centered at 2θ= 26.1° is observed.
197
It is noteworthy that in comparison with TOCN there is no characteristic TOCN peak at 22.6°
198
detected in the XRD patterns of TOCN5/PAM and TOCN5/PAM/Gelatin10 hydrogels, which
199
suggested that TOCNs were uniformly dispersed in the polymer gels without ordered
200
aggregation.
201
Lyophilization is known to preserve the structure and volume of the hydrogels even after
202
all of the solvent has been removed. Figure 3 shows the SEM images of the lyophilized PAM
203
and TOCN5/PAM/Gelatin10 hydrogels. It can be seen that the network of the net PAM
204
hydrogel was clear with a uniform macropore structure and the pore sizes ranging from ~30 10
205
to ~200 μm. For the network of TOCN5/PAM/Gelatin10 hydrogel (Figure 3c and d), it was
206
observed that the pore structure of the gels became more complex and with a smaller pore
207
size, thus implying a denser structure of the hydrogel. This can be attributed to the formation
208
of polyacrylamide-gelatin double network and the strong hydrogen-bonding interactions
209
between TOCNs and polar functional groups of the PAM side chains.
(b)
(a)
200 µ m
50 µ m
(c)
(d)
200 µ m
50 µ m
210 211
Figure.3. SEM micrographs of (a, b) PAM hydrogel and (c, d) TOCN5/PAM/Gelatin10 hydrogel.
212
3.3 Shape Memory Behavior
213
The hydrogels exhibited good shape memory behaviors. Figure 4b shows the thermally
214
triggered shape memory behavior of a composite hydrogel strip. The gel strip (Figure 4a) was
215
curled into a tight spiral after being immersed in 90 °C water for 20s. Afterwards, the
216
deformed gel was fixed in ice water for 30s to obtain a temporary deformed shape (Figure 4b).
217
When the deformed gel helix was immersed into 90 °C water, it can recover its original shape
218
rapidly (Movie S1). These results indicate that the hydrogels containing gelatin exhibit the 11
219
cooling-fixed shape memory capability.
(a)
(b)
(c)
220 221
Figure.4. Photos of (a) original and recovered shape of the gel strip;(b) fixed temporary shape of the
222
hydrogel. (c) Shape fixity ratios (Rf) of the gels prepared with different contents of TOCNs and gelatin
223
after fixing in ice water and keeping in the room temperature for 5 min. The numbers in the shape fixity
224
ratios picture indicate the content of the corresponding components in Table S1.
225
The shape fixity ratios (Rf) of the gels were obtained with the simple bending
226
experiments performed. The Rf of the gels was not significantly affected by TOCN contents,
227
while the contents of gelatin had an obvious effect on them (Figure 4c). The Rf of the
228
TOCN5/PAM/Gelatin10 gels were mostly around 65%, higher than those of the corresponding
229
gels that have lower contents of gelatin. It is noteworthy that the TOCN5/PAM/Gelatin0
230
sample without gelatin cannot be reshaped. All of the fixed shapes could rapid recover their
231
original shapes when immersing in 90 °C water.
12
Original Shape (a)
Deformed Shape (b)
Recovered Shape (c)
232 233
Figure.5. Photos showing the heat-shrinkable shape memory behavior of TOCN0/PAM/Gelatin10
234
hydrogel. (a−c): A gel strip of 36mm×10mm×2mm in the original (a), elongated (b), and recovered
235
shapes (c). The fixation of the elongated gel strip was conducted by keeping in ice water for 30s. This
236
shape memory behavior was performed with as-prepared wet TOCN0/PAM/Gelatin10 hydrogel.
237
The composite hydrogel also exhibited a heat-shrinkable shape memory behavior
238
(Figure 5). This behavior was observed by an elongated TOCN0/PAM/Gelatin10 hydrogel strip.
239
A gel strip was first heated to 90 ° C and then stretched to an elongated shape. The fixation of
240
the elongated gel strip was conducted by keeping in ice water for 30s. After being immersed
241
in 90 °C water for 5s, the elongated part shrank quickly to a remaining length of 39 mm
242
(Figure 5c), very close to the original length of 36mm (Figure 5a).
243
To examine the repeatability of the shape memory capability of the hydrogels, the cycle
244
of shape fixing and thermo-induced shape recovery was repeated 10 times (Figure S1). The
245
hydrogel can remain almost the same shape fixity ratio (Rf) and recover to its original shape
246
after 10 cycles showing a good reproducible capability of shape memory.
247
Temperature-dependent dynamic mechanical properties of the composite hydrogels were
248
measured. As shown in Figure 6, the storage modulus (G′) was always higher than the
249
corresponding loss modulus (G″) in the entire temperature cycle of 25 °C→75 °C→25 °C,
250
suggesting the elastic nature of the TOCN/PAM/Gelation hydrogels. It can be seen that the 13
251
TOCN5/PAM/Gelatin0 hydrogel keeps almost constant in G′ and G″ (Figure 6a) during a
252
cycle of heating and cooling process because of the absence of gelatin. However, G′ is not a
253
constant for the TOCN5/PAM/Gelatin10 hydrogel (Figure 6b). As temperature increases, G′ of
254
the TOCN5/PAM/Gelatin10 hydrogel decrease rapidly at 35 °C. This was induced by the
255
helix-coil transition of the gelatin chains at 35 °C. When temperature is higher than 45 °C, G′
256
of the TOCN5/PAM/Gelatin10 hydrogel keep almost a constant, no matter heating or cooling.
257
This constant G′ is attributed to the cross-linking density of the chemically cross-linked PAM
258
network, which is not changed with temperature. Moreover, G′ increases greatly at about
259
35 °C and could almost recover its initial value during the process of cooling, implying the
260
thermoreversible sol-gel transition of the gelatin network. These findings clearly indicate that
261
the thermoreversible sol-gel transition of gelatin network regulates the moduli of the
262
hydrogels, endowing the shape-memory capability of these composite hydrogels.
263 264
Figure.6. Dependence of storage modulus (G′) and loss modulus (G″) on temperature during a
265
temperature sweep at a heating and cooling rate of 2 °C/min for (a) TOCN5/PAM/Gelatin0, and (b)
266
TOCN5/PAM/Gelatin10 hydrogels. An angular frequency ω=6.28 rad/s and a strain amplitude γ=0.5%
267
were adopted.
268
3.3 Swelling and Mechanical Properties of TOCN/PAAM/Gelatin Gels. 14
269
The swelling behaviors were investigated to further reveal the structural features of the
270
TOCNx/PAM/Gelatiny nanocomposite hydrogels. The results are shown in Figure S2. It was
271
well-known that the swelling behavior of the hydrogel depended on the crosslinked density of
272
the gel network. The swelling ratio of the TOCNx/PAM/Gelatiny hydrogels decreased with the
273
increasing TOCN contents (Figure S2a). This was attributed to the fact that TOCNs could
274
interact with the PAM network by means of hydrogen bonding to form a closer network
275
structure, thus causing a decrease in the swelling capacity. Meanwhile, the swelling ratios of
276
TOCN5/PAM/Gelatiny hydrogels decreased from 21.79 to 11.4 g/g with the percent of gelatin
277
increasing from 3 to 10 w/v% (Figure S2b). This behavior confirmed that the
278
TOCN5/PAM/Gelatin10 double network (DN) hydrogel had a more robust network than that
279
of the TOCN5/PAM/Gelatin3, TOCN5/PAM/Gelatin5 and TOCN5/PAM/Gelatin7 DN
280
hydrogels.
281
As shown in Figure 7, TOCN/PAM/Gelatin nanocomposite hydrogels exhibited
282
excellent tensile properties. The gels can be easily stretched (Figure 7a), twisted stretched
283
(Figure 7b) and stretching with a knot (Figure 7c) to at least 4 times their original length. The
284
physically linked networks could be easily extended and contributed to the stretchability of
285
DN gels. Moreover, TOCN/PAM/Gelatin gels can be readily adapted to different complex
286
shapes (Figure 7d-f), demonstrating their free-shapeable property.
15
(d) (a) 1cm
(e) (b) 1cm
(f) (c) 1cm
287 288
Figure.7. The composite hydrogels are very tough and flexible, with the ability to withstand different
289
high-level deformations of (a) stretching (b) twisted stretching, as well as (c) stretching with a knot. The
290
gels are also free shapeable into different complex shapes (d-f, dyed with methyl orange). All these
291
behaviors were performed with the as-prepared wet TOCN5/PAM/Gelatin10 hydrogels.
292
The mechanical properties of TOCNx/PAM/Gelatiny gels at room temperature are
293
showed in Figure 8, including tensile stress-strain curves (Figure. 8a, c) and elastic moduli (E)
294
(Figure. 8b, d).
295
The positive effect of the TOCN amount on the mechanical properties of hydrogel is
296
shown in Figure 8a. As TOCN contents increased, the maximum tensile stress (b), and
297
elastic modulus (E) of the DN gels consistently increased. TOCN0/PAM/Gelatin10 had b, and
298
E of 91 kPa, and 20.8 kPa respectively. Compared with TOCN0/PAM/Gelatin10,
299
TOCN5/PAM/Gelatin10 exhibited much stronger mechanical properties (b of 237 kPa, and E
300
of 45.5 kPa), indicating that the TOCNs are able to effectively bear the tensile load applied on 16
301
the PAM matrix and thus enhance the mechanical properties of hydrogels.
302
The introduction of gelatin increases the elastic modulus and tensile strength of the
303
composite hydrogel, further revealing the enhanced compatibility of the gelatin and the
304
TOCN/PAM
305
TOCN5/PAM/Gelatin10 was superior to TOCN5/PAM/Gelatin0 single network (SN) gel and
306
demonstrated its high toughness in tensile tests. It is worth noting that the increment of
307
gelatin contents could not only enhances the hydrogel strength but also increases the
308
shape-fixing percentage Rf in Figure 4.
system.
From
Figure
8
it
can
be
seen
that
the
hydrogel
of
309
The elongation (εb) at break, however, of the samples do display a different tendency
310
compared with the above properties. It is highly dependent on gelatin contents. The
311
elongation (εb) significantly and monotonically decreased from 1590% to 680% as gelatin
312
content increased from 3 w/v% to 10 w/v% (Figure 8c). The results indicate that the
313
elongation (εb) at break decreases with the increasing crosslinking density as expected. It can
314
be attributed to the increase of crosslinking density having a negative effect on the toughness
315
of polymers. Therefore, we speculated that a combination of double network and nanofillers
316
facilitated such good elastic and mechanical properties.
17
(a)
TOCN0 TOCN1
Stress /kPa
TOCN3
150
TOCN5
100 50 TOCNx/PAM/Gelatin10
0
250
0
200
400 600 Strain / %
800
Gelatin3 Gelatin5
Stress /kPa
Gelatin7
150
Gelatin10
100 50
TOCN5/PAM/Gelatiny
0
317
400
800 1200 Strain / %
1600
2000
TOCNx/PAM/Gelatin10
30 20 10
50
(c)
200
0
(b) 40
0
1000
Elastic modulus (Kpa)
200
50
Elastic modulus (Kpa)
250
TOCN0
(d)
TOCN1
TOCN3
TOCN5
TOCN5/PAM/Gelatiny
40 30 20 10 0
Gelatin3 Gelatin5 Gelatin7 Gelatin10
318
Figure.8. (a) Tensile stress−strain curves of NC gels and (b) the corresponding elastic modulus. All
319
gels were tested at the as-prepared state without swelling equilibrium in water solution.
320
4. Conclusions
321
In this study, cellulose nanofibers were isolated through TEMPO-oxidation pre-treatment
322
followed by mechanical disintegration. And then a series kinds of shape memory
323
TOCN/PAM/Gelatin hydrogels with varying TOCNs and gelatin contents were prepared
324
through a facile in-situ free-radical polymerization method. After the polymerization, TOCNs,
325
gelatin, and PAM were distributed homogeneously in the hydrogels. Importantly, there was
326
no new covalent bond formed between gelatin, PAM, and TOCNs. As to the shape memory
327
properties of TOCN/PAM/Gelatin composite hydrogels, the reversible nature of the gelatin
328
network imparts the TOCN/PAM/Gelatin composite hydrogels with fast cooling-fixed (ice
329
water for 30s) and thermo-induced shape recovery (in 90 °C water for 5s) capability. Good 18
330
mechanical properties (strength >200 kPa, strain >650%) were achieved. As TOCNs content
331
increased, the maximum tensile stress (b), and elastic modulus (E) of the DN gels
332
consistently increased. The introduction of gelatin contributed to increases in the elastic
333
modulus and tensile strength, further revealing the enhanced compatibility of the gelatin and
334
the TOCN/PAM system. Such a composite hydrogel with good shape memory behavior and
335
enhanced mechanical strengths would be an attractive candidate for a wide variety of
336
applications.
337
Acknowledgements
338
The authors are extremely grateful to financial support from the Science and Technology
339
Project of Guangzhou City in China (No.2016070220045), State Key Laboratory of Pulp and
340
Paper Engineering (2016 c01), State Key Laboratory of Pulp and Paper Engineering
341
(2015TS01), Natural Science Foundation of China (No.201404042), Shandong Natural
342
Science Foundation (No. ZR2015PC016), Science and Technology Planning Project of
343
Higher Education of Shandong Province (No. J14LD51), Doctoral Starting up Foundation of
344
Qufu Normal University (No. BSQD2012058), and Science and Technology Planning Project
345
of Qufu Normal University (No. xkj201413).
346 347
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