Synthesis of cellulose/silica gel polymer hybrids via in-situ hydrolysis method

Transcription

1 Polym. Bull. (2017) 74: DOI /s ORIGINAL PAPER Synthesis of cellulose/silica gel polymer hybrids via in-situ hydrolysis method Takeru Iwamura 1,2 Kenzo Akiyama 2 Taiki Hakozaki 1 Masahiro Shino 1 Kaoru Adachi 3 Received: 13 November 2016 / Revised: 16 March 2017 / Accepted: 23 March 2017 / Published online: 27 March 2017 Ó 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: /s ) contains supplementary material, which is available to authorized users. & Takeru Iwamura iwamurat@tcu.ac.jp Department of Chemistry and Energy Engineering, Graduate School of Engineering, Tokyo City University, Tamazutsumi, Setagaya-ku, Tokyo , Japan Department of Chemistry and Energy Engineering, Faculty of Engineering, Tokyo City University, , Tamazutsumi, Setagaya-ku, Tokyo , Japan Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto , Japan

2 4998 Polym. Bull. (2017) 74: 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 lowvalue 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 (C 6 H 12 O 5 ) 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 Scheützenberger, 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 CO 2, 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 intermolecular 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 combination 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

3 Polym. Bull. (2017) 74: Scheme 1 Synthesis of transparent cellulose/silica gel polymer hybrids (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 1 H 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 (T g ) 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 (T d10 ) 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. 1 H NMR spectra were recorded on an Oxford Instruments Pulsar ( 1 H NMR: 60 MHz) spectrometer and a JEOL JNM-EPC 300 ( 1 H 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

4 5000 Polym. Bull. (2017) 74: 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 C for 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

5 Polym. Bull. (2017) 74: Table 1 Preparation of cellulose/silica gel polymer hybrids Run AcCL a /TMOS (w/w) Reaction temperature ( C) 0.1 M HCl aq. (ml) Appearance Ceramic yield (%) b Td10 Tg ( C) Degree of hydrolysis (%) ( C) Obs. Cal. Obs/Cal Turbid / Transparent ND c / Translucent ND c / Turbid ND c / Transparent / Translucent ND c / Translucent ND c / Turbid ND c /5 rt 0.02 Transparent ND c /5 rt 0.10 Translucent ND c /5 rt 0.40 Turbid ND c /5 rt 1.60 Turbid ND c 28 b Conditions: AcCL 0.4 g or 1.0 g, TMOS 2.0 g T d10 = C, T g = C Ceramic yield = (weight percent of ceramic in polymer hybrids observed by TGA)/(calculated value of weight percent of ceramic) Not detected a c

6 5002 Polym. Bull. (2017) 74: Fig. 1 Typical optical images of transparent polymer hybrid, translucent composite and turbid composite prepared from AcCL/TMOS = 1/2, drying at 60 C: a Run 1; b Run 2; c Run 3; d Run 4 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 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

7 Polym. Bull. (2017) 74: Fig. 2 SEM images of a the turbid composite (Run 1), b the transparent polymer hybrid (Run 2), c the translucent composite (Run 3), and d the turbid composite (Run 4) prepared from AcCL/TMOS = 1/2 drying at 60 C 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 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

8 5004 Polym. Bull. (2017) 74: 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 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. T d10 of the samples were also measured by TGA. Whereas T d10 of the AcCL was observed at C, T d10 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 (T g ) of AcCL was observed at C in the DSC thermograms. The T g of the homogeneous polymer hybrids (runs 1 and 5) was and C, respectively, which were higher than that of AcCL. It is notable that T g 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, T g was not clear. These results support the homogeneous integration

9 Polym. Bull. (2017) 74: 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 = 1/5 drying at room temperature 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 C. On the DTG plot, the rate of mass loss is shown to accelerate to the maximum rate at the peak temperature of 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

10 5006 Polym. Bull. (2017) 74: %T Wavelength (nm) Fig. 5 Normalized transmission spectra (thickness: 10 lm) of a the transparent polymer hybrid prepared from 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 at 60 C; 0.1 M HCl aq.: 0.10 ml (Run 2), and b authentic cellulose

11 Polym. Bull. (2017) 74: Fig. 7 Typical FT-IR spectra of transparent polymer hybrid, translucent composite and turbid composite prepared 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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