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ORI GIN AL PA PER
Synthesis of cellulose/silica gel polymer hybridsvia in-situ hydrolysis method
Takeru Iwamura1,2 • Kenzo Akiyama2 •
Taiki Hakozaki1 • Masahiro Shino1 • Kaoru Adachi3
Received: 13 November 2016 / Revised: 16 March 2017 / Accepted: 23 March 2017 /
Published online: 27 March 2017
� Springer-Verlag Berlin Heidelberg 2017
Abstract Homogeneous cellulose/silica gel polymer hybrids were prepared by
hydrolysis of acetyl cellulose (AcCL) in a sol–gel reaction mixture of alkoxysilane
such as tetramethoxysilane (TMOS). To a mixture of AcCL and TMOS in a mixed
solvent of THF and methanol (v/v, 7/3), an HCl aqueous solution was added to
initiate hydrolysis and condensation of the alkoxysilane. The resulting mixture was
constantly stirred for 5 h and heated at 60 �C for two weeks to allow evaporation of
the solvents. Consequently, corresponding transparent and homogeneous polymer
hybrids could be obtained in a range of mass ratios (AcCL/TMOS = 1/5–1/2). In
the FT-IR spectra, the absorption peaks corresponding to the acetyl group decreased
as the amount of 0.1 M aqueous HCl solution increased, which indicates hydrolysis
of acetyl groups of AcCL, whereas the intensity of the Si–O-Si stretching vibration
peak increased. The thermal properties of the obtained polymer hybrids were
evaluated by TG/DTA and DSC measurements.
Keywords Cellulose � Alkoxysilane � Tetramethoxysilane � Polymer hybrid �Sol–gel reaction
Electronic supplementary material The online version of this article (doi:10.1007/s00289-017-2000-
8) contains supplementary material, which is available to authorized users.
& Takeru Iwamura
1 Department of Chemistry and Energy Engineering, Graduate School of Engineering, Tokyo
City University, 1-28-1 Tamazutsumi, Setagaya-ku, Tokyo 158-8857, Japan
2 Department of Chemistry and Energy Engineering, Faculty of Engineering, Tokyo City
University, 1-28-1, Tamazutsumi, Setagaya-ku, Tokyo 158-8557, Japan
3 Department of Chemistry and Materials Technology, Kyoto Institute of Technology,
Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
123
Polym. Bull. (2017) 74:4997–5009
DOI 10.1007/s00289-017-2000-8
Introduction
Cellulose, which is one of the most important natural polymers, is considered to be
the most abundant carbon neutral organic polymer [1], which is widely distributed
in higher plants or in several marine animals. Wood, consisting of up to 50%
cellulose, is the most important raw material source for cellulose. Most of the low-
value biomass such as cellulose, hemicellulose and lignin is termed lignocellulosic
biomass. In particular, cellulose is considered an almost inexhaustible source of raw
material that can satisfy an increasing demand for environmentally friendly and
biocompatible products [2].
The generic chemical formula of cellulose is (C6H12O5)n. Cellulose is a natural
linear polymer, in which D-glucose units are joined together though b(1 ? 4)
glucosidic linkages in which extensive intramolecular and intermolecular hydrogen
bonding networks exist [3, 4]. Therefore, cellulose is insoluble in common solvents
such as acetone, methanol, and water. This insoluble property in common solvents
makes cellulose difficult to handle for chemical modifications. However, cellulose
acetate, which is a semi-synthetic polymer, was synthesized for the first time by
Scheutzenberger, who heated cotton with acetic anhydride [5]. The acetylation of
cellulose opened up a way for the industrial utilization such as optical, structural and
other materials, thus promoting the use of biomass.
Composite materials consisting of organic polymer and inorganic materials
attract attention because new materials can be created without using new chemicals.
Especially, organic–inorganic polymer hybrids are one example of excellent
materials with high performance properties, including transparency, thermal
stability, mechanical property, and others [6–9]. Cellulose-silica composites which
were prepared with sodium silicate or alkoxysilane have been reported [10–12].
However, most of the reports are about improving physical or mechanical
properties, and there is little reference to transparency. There is report on the
preparation of relatively transparent composites using supercritical CO2, but this
method is not a simple method [13]. In hybridization of inorganic silica with organic
polymers, the sol–gel reaction of alkoxysilanes is the most useful and simple
method for the synthesis of these polymer hybrids. When organic–inorganic
polymer hybrids are prepared, intermolecular interactions between an organic
polymer and silica gel are the key which enables molecular-level hybridization. The
creation of organic–inorganic polymer hybrids have involved various intermolec-
ular interactions such as hydrogen bonding interactions [14–19], ionic interactions
[20], p–p interactions [21, 22], CH/p interactions [23, 24], and others [25, 26]. The
in situ hydrolysis method, in which hydrolysis of an organic polymer in a sol–gel
reaction mixture is carried out, is an excellent method to obtain transparent and
homogeneous organic–inorganic polymer hybrids [27] especially for the combina-
tion of inorganic materials and the specific organic polymer which is insoluble in a
reaction medium. Since unmodified cellulose is known to be insoluble in common
solvents, this method might be suitable for the synthesis of cellulose/silica gel
polymer hybrids. In this article, we report detailed results of the synthesis of
cellulose/silica gel polymer hybrids based on an in situ hydrolysis method
4998 Polym. Bull. (2017) 74:4997–5009
123
(Scheme 1). The utilization of this in situ hydrolysis method can easily provide
novel transparent cellulose/silica gel polymer hybrids.
Experimental
Materials
Acetyl cellulose (AcCL) was purchased from Wako Pure Chemical Industries, Ltd.
Before use, AcCL was confirmed by 1H NMR spectra (supporting information Fig
S3). Tetramethoxysilane (TMOS) was purchased from Tokyo Chemical Industry
Co., Ltd. All other solvents and reagents were purchased from Wako Pure Chemical
Industries, Ltd.
Measurements
The morphology of the obtained organic–inorganic polymer hybrids was observed
using a Hitachi S-4100 scanning electron microscope (SEM). Thermal analyses
were performed on Seiko Instruments TG/DTA200 and DSC210. The glass
transition temperature (Tg) by differential scanning calorimetry (DSC) was assumed
as the inflection point on a trace at a heating rate of 10 �C/min. A 10% weight loss
temperature (Td10) was determined by thermogravimetric analysis (TGA) at a
heating rate of 10 �C/min under a nitrogen atmosphere. FT-IR spectra were
obtained on a JASCO FT/IR-4200 infrared spectrometer. 1H NMR spectra were
recorded on an Oxford Instruments Pulsar (1H NMR: 60 MHz) spectrometer and a
JEOL JNM-EPC 300 (1H NMR: 300 MHz) spectrometer. Transmittance analyses
were performed on a JASCO V-730 iRM UV–visible/NIR spectrophotometer.
Transmission spectra were measured using air as reference. X-ray diffraction (XRD)
analysis was performed by a Bruker AXS D8 ADVANCE.
Preparation of cellulose/silica gel polymer hybrids (typical procedure)
AcCL (0.40 g) was dissolved in 20 mL of a mixed solvent of THF and methanol (v/
v, 7/3), and 2.00 g of TMOS was added. The mixture was stirred until AcCL had
Scheme 1 Synthesis of transparent cellulose/silica gel polymer hybrids
Polym. Bull. (2017) 74:4997–5009 4999
123
dissolved, and then 0.1 M aqueous HCl solution (0.02 mL) was added. After stirring
at the prescribed temperature for 5 h, the mixture was placed in a polypropylene
vessel covered with a wiping paper and left in air at 60 �C for 1 week. The obtained
polymer hybrid was dried in vacuo at 60 �C for 2 days.
Results and discussion
Cellulose and silica gel polymer hybrids were prepared by utilizing a sol–gel
reaction of TMOS in the presence of AcCL in organic solvent. The sol–gel reaction
proceeded via hydrolysis and condensation of alkoxysilane. In that condition,
hydrolysis of AcCL can proceed simultaneously. The results are summarized in
Table 1. AcCL was added to a mixed solvent of THF and methanol (v/v, 7/3) with
TMOS and subsequently the prescribed volume of 0.1 M HCl aq. which is varied
from 0.02 to 1.60 mL. The weight ratios of AcCL as the organic polymer to TMOS
were 1/2 (runs 1–4) and 1/5 (runs 5–12). The mixture was then heated in a
polypropylene vessel covered with a wiping paper at 60 �C for 1 week. Typical
optical images of the obtained polymer hybrids are shown in Fig. 1. In the case of
run 1 in which 0.02 mL of 0.1 M HCl aq. as a catalyst was added, the polymer
hybrid became turbid. In contrast, the polymer hybrid became transparent when
0.10 mL of 0.1 M HCl aq. was employed (run 2). In the cases of runs 3 and 4, in
which 0.40 and 1.50 mL of the catalyst were added, respectively, the optical
appearances of both the samples were translucent or turbid, suggesting a phase
separation of the organic and inorganic domains. In runs 5–12, transparent polymer
hybrids were obtained when 0.02 mL of 0.1 M HCl aq. was employed (runs 5 and
9).
The dispersity of the resulting polymer hybrid was also examined by SEM. As
shown in Fig. 2a, c, d, the samples prepared from AcCL/TMOS (w/w, 1/2) at 60 �Cfor 5 h showed a phase separation of silica and the organic polymer. On the other
hand, in the case of the transparent polymer hybrid (run 2), silica domains could not
be observed at a micrometer order (Fig. 2b). This result supports the homogeneous
polymer hybrid nature of run 2.
Figure 3 and 4 show SEM images of the samples prepared from AcCL/TMOS
(w/w, 1/5) with different reaction temperature. As shown in Fig. 3a, a transparent
polymer hybrid (run 5) prepared with 0.02 mL of the catalyst at 60 �C for 5 h
showed no recognizable segregation at this scale. In contrast, phase separation
appeared in the obtained polymer composites prepared with large amount of the HCl
aq., as shown in Fig. 3b, c, d. Also in the case when the sol–gel reaction was carried
out at room temperature, homogeneous polymer hybrid was obtained with 0.02 mL
of the catalyst, as no recognizable segregation was observed in the SEM image
(Fig. 4a). With large amount of HCl aq., translucent composite (run 10) showed
slightly recognizable segregation at this scale in the SEM image (Fig. 4b). In
contrast, phase separation appeared in the turbid polymer composites, as shown in
Fig. 4c, d. These results suggests larger amount of HCl promote segregation of the
organic component. Note that a three-dimensional silica network, which might be
5000 Polym. Bull. (2017) 74:4997–5009
123
Table
1P
rep
arat
ion
of
cell
ulo
se/s
ilic
ag
elp
oly
mer
hy
bri
ds
Ru
nA
cCL
a/T
MO
S(w
/w)
Rea
ctio
n
tem
per
atu
re(�
C)
0.1
MH
Cl
aq.
(mL
)
Ap
pea
ran
ceC
eram
icy
ield
(%)b
Td10
Tg
(�C
)D
egre
eo
f
hy
dro
lysi
s
(%)
(�C
)
Ob
s.C
al.
Ob
s/C
al.
1�
60
0.0
2T
urb
id2
.14
4.1
4.8
28
8.4
19
0.4
0
21/2
60
0.1
0T
ransp
aren
t33.7
44.3
76.0
318.7
ND
c4
4
31
/26
00
.40
Tra
nsl
uce
nt
39
.64
4.2
89
.73
22
.3N
Dc
45
41
/26
01
.60
Tu
rbid
37
.54
4.2
84
.93
24
.3N
Dc
57
51/5
60
0.0
2T
ransp
aren
t16.7
66.4
25.2
306.6
198.0
0
61
/56
00
.10
Tra
nsl
uce
nt
40
.86
6.5
61
.43
23
.6N
Dc
51
71
/56
00
.40
Tra
nsl
uce
nt
55
.16
6.4
82
.93
42
.1N
Dc
75
81
/56
01
.60
Tu
rbid
65
.06
6.4
97
.93
23
.5N
Dc
83
91/5
rt0.0
2T
ransp
aren
t58.8
66.3
88.7
322.3
ND
c0
10
1/5
rt0
.10
Tra
nsl
uce
nt
58
.26
6.4
87
.73
14
.5N
Dc
10
11
1/5
rt0
.40
Tu
rbid
64
.36
6.4
96
.83
36
.8N
Dc
33
12
1/5
rt1
.60
Tu
rbid
62
.36
6.5
93
.73
38
.6N
Dc
28
Co
nd
itio
ns:
AcC
L0
.4g
or
1.0
g,
TM
OS
2.0
gaT
d10=
32
5.0
�C,T
g=
18
9.8
�Cb
Cer
amic
yie
ld=
(wei
ght
per
cent
of
cera
mic
inpoly
mer
hybri
ds
obse
rved
by
TG
A)/
(cal
cula
ted
val
ue
of
wei
ght
per
cent
of
cera
mic
)c
Not
det
ecte
d
Polym. Bull. (2017) 74:4997–5009 5001
123
formed at a relatively low temperature by a hydrolysis and condensation reaction of
TMOS, was recognized in these SEM images.
The transmittance spectra of the transparent polymer hybrid (Run 2) and AcCL
cast film on glass substrate at wavelength range of 200–1100 nm are shown in
Fig. 5. The average transmittance between 400 and 1100 nm of the transparent
polymer hybrid was 93%, which is slightly higher than AcCL (90%). These results
of transmittance analyses are consistent with the optical appearance and SEM
image.
The phase of the cellulose in the transparent polymer hybrid (Run 2) and
authentic cellulose were also examined by XRD analysis. Figure 6 shows XRD
pattern of the transparent polymer hybrid. Only broad amorphous halos derived
from amorphous silica matrix and amorphous cellulose were observed in the pattern.
On the other hand, in the case of authentic cellulose, crystalline peaks were
detected. This result supports the suggestion that cellulose would be dispersed in the
silica gel matrix at the molecular level.
The FT-IR spectra of the samples are shown in Fig. 7. In the FT-IR spectra, the
absorption peaks resulting from the acetyl group were observed at 1751 cm-1 (C=O
stretching vibration peak) and at around 1240 cm-1 (C–O–C stretching vibration
peak). These peaks decreased as the amount of 0.1 M HCl aq. solution increased. In
contrast, the intensity of the Si–O–Si stretching vibration peak at around 1050 cm-1
increased as the amount of 0.1 M HCl aq. solution increased. These results indicate
cellulose was generated by hydrolysis of AcCL, i.e., the obtained polymer hybrids
Fig. 1 Typical optical images of transparent polymer hybrid, translucent composite and turbid compositeprepared from AcCL/TMOS = 1/2, drying at 60 �C: a Run 1; b Run 2; c Run 3; d Run 4
5002 Polym. Bull. (2017) 74:4997–5009
123
were cellulose/silica gel polymer hybrids. The degree of hydrolysis of AcCL in the
obtained polymer hybrids was evaluated by comparing the relative absorption
intensity of carbonyl groups with that of the Si–O–Si stretching vibration peak of
each polymer hybrid. The results are shown in Table 1. In the case of AcCL/
TMOS = 1/2 at 60 �C, the degree of hydrolysis reached 57% at the maximum (run
4). The degree of hydrolysis increased as the amount of 0.1 M aqueous HCl solution
increased. A similar trend was observed in runs 5–12. In the case of AcCL/
TMOS = 1/5 at 60 �C, the degree of hydrolysis reached 83% at the maximum (run
8). In contrast to these results, in the case of AcCL/TMOS = 1/5 at room
temperature, the degree of hydrolysis was low (runs 9–12). From this knowledge, it
is expected that the hydrolysis of AcCL in the polymer hybrid proceeded effectively
to generate cellulose by heating at 60 �C.
On the other hand, the interaction of cellulose and silica gel was confirmed in
Fig. 8. The FT-IR spectrum of AcCL showed the absorption band at 3454 cm-1
based on the OH stretching frequency. In the FT-IR spectrum of cellulose/silica gel
polymer hybrid (Run 2), this absorption band was observed at 3451 cm-1. Namely,
the OH stretching absorption band was shifted to lower frequency after
Fig. 2 SEM images of a the turbid composite (Run 1), b the transparent polymer hybrid (Run 2), c thetranslucent composite (Run 3), and d the turbid composite (Run 4) prepared from AcCL/TMOS = 1/2drying at 60 �C
Polym. Bull. (2017) 74:4997–5009 5003
123
hybridization. Such shift has also been reported in previously published articles
[28–30]. This result might indicate an existence of hydrogen bonding interaction
between cellulose and silica gel in the transparent polymer hybrids.
Ceramic yields in the sample were measured by TGA analysis in air (Table 1).
These results indicate that the sol–gel reaction was almost complete except for runs
1, 5 and 6. In these samples, when a small amount of HCl aq. solution was added,
the sol–gel reaction of TMOS was not fast enough compared with evaporation of
TMOS. Therefore, the ceramic yields of those samples were smaller than calculated
value. Td10 of the samples were also measured by TGA. Whereas Td10 of the AcCL
was observed at 325.0 �C, Td10 of the homogeneous polymer hybrid shifted to a
lower temperature. This might be due to the acidic environment of surroundings of
organic polymers. The glass transition temperature (Tg) of AcCL was observed at
189.8 �C in the DSC thermograms. The Tg of the homogeneous polymer hybrids
(runs 1 and 5) was 190.4 and 198.0 �C, respectively, which were higher than that of
AcCL. It is notable that Tg of the polymers in the silica matrix were not clearly
detected (Table 1). These results indicate the mobility of the organic polymer was
prevented by the silica gel matrix that formed by the sol–gel reaction of TMOS.
Consequently, Tg was not clear. These results support the homogeneous integration
Fig. 3 SEM images of a the transparent polymer hybrid (Run 5), b the translucent composite (Run 6),c the translucent composite (Run 7), and d the turbid composite (Run 8) prepared from AcCL/TMOS = 1/5 drying at 60 �C
5004 Polym. Bull. (2017) 74:4997–5009
123
of an organic polymer and silica gel. A typical TG-DTG curve of transparent
polymer hybrid (Run 2) is shown in Fig. 9. The TG/DTG curves indicate that the
thermal decomposition process occurs in 134.5–667.3 �C. On the DTG plot, the rate
of mass loss is shown to accelerate to the maximum rate at the peak temperature of
365.1 �C. A colorless silica residue of about 33.7% of the total mass loss remained
in the Pt sample pan at the end of the experiment, indicating that a black carbon
residue was not included.
Conclusion
Cellulose/silica gel polymer hybrids were prepared by an in situ hydrolysis method.
Transparent and homogeneous polymer hybrids were obtained by hydrolysis of
AcCL in an acid-catalyzed sol–gel reaction of TMOS. Consequently, it was clarified
that the in situ hydrolysis of AcCL in the sol–gel reaction mixture with TMOS could
be an effective method for the synthesis of homogeneous cellulose and silica gel
polymer hybrids. In recent years, the practical use of biomass has become
important. From the point of view of environmental and material sciences, we
Fig. 4 SEM images of a the transparent polymer hybrid (Run 9), b the translucent composite (Run 10),c the turbid composite (Run 11), and d the turbid composite (Run 12) prepared from AcCL/TMOS = 1/5drying at room temperature
Polym. Bull. (2017) 74:4997–5009 5005
123
Wavelength (nm)
%T
Fig. 5 Normalized transmission spectra (thickness: 10 lm) of a the transparent polymer hybrid preparedfrom AcCL/TMOS = 1/2, drying at 60 �C; 0.1 M HCl aq.: 0.10 mL (Run 2), and b AcCL
Fig. 6 XRD patterns of a the transparent polymer hybrid prepared from AcCL/TMOS = 1/2, drying at60 �C; 0.1 M HCl aq.: 0.10 mL (Run 2), and b authentic cellulose
5006 Polym. Bull. (2017) 74:4997–5009
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Fig. 7 Typical FT-IR spectra of transparent polymer hybrid, translucent composite and turbid compositeprepared from AcCL/TMOS = 1/2, drying at 60 �C; 0.1 M HCl aq.: 0.02 mL (Run 1); 0.10 mL (Run 2);0.40 mL (Run 3); 1.60 mL (Run 4)
Fig. 8 FT-IR spectra of transparent polymer hybrid prepared from AcCL/TMOS = 1/2, drying at 60 �C;0.1 M HCl aq.: 0.10 mL (Run 2) and AcCL
Polym. Bull. (2017) 74:4997–5009 5007
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believe that cellulose/silica gel polymer hybrids are potentially applicable to the
synthesis of novel transparent materials such as automotive windshield.
Acknowledgements The authors thank Ms. Emi Shindou, Mr. Naoki Hamamura, and Dr. Akira Yoshida
of Nanotechnology Research Center, Tokyo City University for their help in SEM and XRD
measurements.
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Temp °C 800.0600.0400.0200.0
TG %
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