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

iwamurat@tcu.ac.jp

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

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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

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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

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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

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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

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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

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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

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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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40.0

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