- Email: [email protected]
Inorganic–organic hybrids produced from tetraethoxysilane and 2-hydroxybenzyl alcohol as studied by solid-state 13c and 29Si nmr spectroscopy
Materials Research Bulletin, Vol. 34, No. 1, pp. 63–70, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/99/$–see front matter
PII S0025-5408(98)00208-6
INORGANIC–ORGANIC HYBRIDS PRODUCED FROM TETRAETHOXYSILANE AND 2-HYDROXYBENZYL ALCOHOL AS STUDIED BY SOLID-STATE 13C AND 29Si NMR SPECTROSCOPY
Isao Hasegawa1*, Toshio Takayama2, and Shuichi Naito2 Department of Chemistry, Faculty of Engineering, Gifu University, Yanagido 1-1, Gifu-City, Gifu 501-1193, Japan 2 Department of Applied Chemistry, Faculty of Engineering, Kanagawa University, Kanagawa-Ku, Yokohama-City, Kanagawa 221-8686, Japan
1
(Refereed) (Received April 28, 1998; Accepted May 4, 1998)
ABSTRACT Tetraethoxysilane and 2-hydroxybenzyl alcohol were allowed to undergo a sol-gel reaction under acidic conditions. These compounds underwent condensation independently to give hybrids consisting of silica and phenolic resin components, which are applicable as precursors for silicon carbide powders and fibers when they are prepared in the fibrous form. The spin-lattice relaxation time measurement for the hybrids using solid-state 29Si NMR spectroscopy suggests that there is appreciable interaction between the silica and phenolic resin components in the hybrids through hydrogen bonding. © 1999 Elsevier Science Ltd
KEYWORDS: A. carbides, A. inorganic compounds, A. organic compounds, B. sol-gel chemistry, C. nuclear magnetic resonance (NMR) INTRODUCTION Inorganic polymer and inorganic– organic hybrid gel precursors have been applied successfully to the synthesis of non-oxide ceramics, such as silicon carbide (SiC). Wei et al. [1] synthesized SiC powders from gels produced from methyltrimethoxysilane and phenolic resin or sucrose by a sol-gel reaction, which is the first example of inorganic– organic hybrids
*To whom correspondence should be addressed. 63
64
I. HASEGAWA et al.
Vol. 34, No. 1
as precursors for non-oxide ceramic materials. Following this work, a few studies [2– 4] have been conducted on SiC powder synthesis from hybrids prepared from alkoxysilane and phenolic resins. The main reason for employing phenolic resins for SiC synthesis is that the resins give a large amount of carbonaceous residue upon pyrolysis. The hybrid gels undergo carbothermal reduction, with C forming from the resins upon heating at an elevated temperature in Ar, to give SiC powders. An overall reaction scheme for carbothermal reduction of silica (SiO2) to yield SiC is expressed as SiO2 1 3C 3 SiC 1 2CO Previously, we reported [5] that novolac-type phenolic resin and tetraethoxysilane [Si(OC2H5)4, TEOS] underwent a sol-gel reaction in acidic ethanolic solutions to yield SiO2-phenolic resin hybrid fibers. The hybrid fibers could be converted into SiC fibers when they were heat-treated at 1500°C in Ar [5,6] or into silicon nitride fibers upon heating at the temperature in flowing N2 [7]. The addition of a titania or zirconia component into the hybrid fibers was also possible, which gave Si–Ti–C [8,9] or Si–Zr–C fibers [10], respectively, by carbothermal reduction. One of the features of the hybrid fiber route to non-oxide ceramic fibers is that the ceramic fibers can be produced without cure of the hybrid fibers, although the phenolic resin itself melts at ca. 110°C. This suggests that the phenolic resin may play another role in the SiO2-phenolic resin hybrid fibers. Thus, a question arises about the interaction between the two components in the hybrid fibers, which has been obscure up to now. Consequently, this study has been aimed at investigating that interaction. In this study, 2-hydroxybenzyl alcohol has been used instead of the phenolic resins, and inorganic– organic hybrids have been prepared from this compound and TEOS by a sol-gel reaction under the same reaction conditions as those for preparing the SiO2-phenolic resin hybrid fibers. The reasons for the use of 2-hydroxybenzyl alcohol are that it corresponds to the repeating unit of the phenolic resins and reaction behavior of the resins during the sol-gel reaction is expected to be elucidated as well by investigating the hybrids synthesized using this rather simple compound. The hybrids thus prepared have been studied with solid-state 13 C and 29Si NMR spectroscopy. EXPERIMENTAL Synthesis of Hybrids from TEOS and 2-Hydroxybenzyl Alcohol and SiC from the Hybrids. Starting solutions were prepared by dissolving 2-hydroxybenzyl alcohol in ethanol (10 cm3), followed by the addition of TEOS (15 cm3), distilled water, and 1 mol dm23 hydrochloric acid. The TEOS:H2O:HCl molar ratio of the solutions was fixed at 1:2:0.01, and the atomic ratio of C in 2-hydroxybenzyl alcohol to Si in TEOS (abbreviated to the C*:Si ratio) at 2.0 and 4.0. The solutions were stirred for 5 min at room temperature for intimate mixing and then maintained at 65°C to obtain gels. In addition, gels were synthesized using tetramethoxysilane [Si(OCH3)4, TMOS] as a SiO2 source and methanol as solvent. The Si:H2O:HCl and C*:Si ratios of these starting solutions were the same as above. For comparison with the gels, SiO2-free products and SiO2 gels were prepared using the same amounts of ethanol, distilled water, 1 mol dm23 hydrochloric acid, and 2-hydroxybenzyl alcohol or TEOS, respectively, as above. Heat treatment of the obtained gels was conducted at 1500°C in Ar for 4 h in order to
Vol. 34, No. 1
INORGANIC–ORGANIC HYBRIDS
65
investigate whether they acted as precursors for SiC. The flowing rate of Ar was 100 cm3 min21 and the heating rate was 10°C min–1. Analytical Procedures. XRD patterns of products after the heat-treatment at 1500°C were obtained with a Rigaku RINT 1100 X-ray diffractometer using Ni-filtered Cu Ka radiation. 29 Si and 13C NMR spectra were obtained with a JEOL EX270WB spectrometer. Samples were inserted into a cylindrical rotor made of zirconia. Line narrowing was achieved by high-power 1H decoupling and magic-angle-spinning (MAS). The spinning rate was set at ca. 6 kHz. The observed frequencies of 29Si and 13C nuclei were 53.54 and 67.80 MHz, respectively. For the measurements with spin-lock cross-polarization sequence (CP/MAS), the following conditions were used: contact time, 5 ms; repetition time, 5 s; spectral width, 10 kHz; data point, 8192. Spectra usually were accumulated for 1000 –2000 times to achieve a reasonable signal-to-noise ratio. 29Si and 13C chemical shift values were at first given with reference to the 29Si and 13C NMR signals due to polydimethylsilane, respectively, and were then converted to the values from tetramethylsilane. The measurement for spin-lattice relaxation time was made using the cross-polarization method of Torchia [11]. In the 13C NMR measurements, the MAS/DL sequence, which means the CP/MAS method with dipolar diphasing, was used to distinguish CH3 and non-hydrogenated carbons from the other hydrogenated carbons.
RESULTS AND DISCUSSION When TEOS and 2-hydroxybenzyl alcohol were allowed to react in the acidic ethanolic solutions, the solutions increased in viscosity with time of soaking at 65°C, resulting in gelation at ca. 4 h. At ca. 3.5 h of soaking, fibers (ca. 30 cm in length) could be drawn by immersing a glass rod into the viscous solutions and then withdrawing the rod. When the fibrous or bulk gels were heated at 1500°C in Ar for 4 h, they gave pale-gray fibers or powders, respectively. The XRD pattern of the powdered products after the heat treatment (Fig. 1) exhibited peaks due to b-SiC phases. This implies that the gels served as precursors for SiC fibers or powders. If 2-hydroxybenzyl alcohol (mp: 83– 85°C) itself had been included in the fibrous gels, the gels would have melted and, therefore, resulted in the coalescence of the fibers. However, the products preserved the fibrous form even after the heat treatment at 1500°C, as was observed for the SiO2-phenolic resin hybrid fibers we reported previously [5,6]. Figure 2 shows the solid-state 13C CP/MAS NMR spectra of (a) 2-hydroxybenzyl alcohol and (b) the gels prepared from TEOS and 2-hydroxybenzyl alcohol. Assignment of signals from 2-hydroxybenzyl alcohol is given in this figure. They clearly show different profiles, indicating that 2-hydroxybenzyl alcohol itself underwent a reaction during the sol-gel reaction to give the gels. When 2-hydroxybenzyl alcohol alone was allowed to react in the acidic ethanolic solution, a resin-like product formed. The 13C NMR spectrum of the product is shown in Figure 2(e). It should be noted that its profile is almost the same as that of the spectrum shown in Figure 2(b) and is similar to that of the novolac-type phenolic resin [Fig. 2(d)] that we used as a starting material for synthesizing the hybrid fibers [5–10]. The signals at ca. 18 and 61 ppm seen in Figure 2(b) decrease in intensity in the spectrum of the gels after heating at 100°C in air for drying. In addition, these two signals are not
66
I. HASEGAWA et al.
Vol. 34, No. 1
FIG. 1 The XRD pattern of powdered products obtained by heating hybrids produced from TEOS and 2-hydroxybenzyl alcohol at 1500°C in Ar for 4 h.
observed in the 13C NMR spectrum of gels prepared using TMOS as a SiO2 source and methanol as solvent [Fig. 2(c)]; instead, a new signal appears at ca. 52 ppm. These facts imply that the signals at ca. 18 and 61 ppm are due to ethanol, which was used as solvent for the gel synthesis and formed from TEOS upon hydrolysis. Similarly, the signal at 52 ppm in Figure 2(c) is ascribable to methanol involved in the gels produced in the TMOS-2hydroxybenzyl alcohol–methanol system. The 13C MAS/DL NMR spectrum of the gels formed from TEOS and 2-hydroxybenzyl alcohol gives rise to only four signals, at ca. 18, 61, 129, and 151 ppm. This implies that the signals at 129 and 151 ppm are due to C with no attached H in the benzene ring. Thus, signals observed in both Figures 2(b) and 2(e) would be ascribable as follows on the basis of the similarity to the spectrum of the novolac-type phenolic resin: 36 ppm, CH2; 116 ppm, CH in the benzene ring; 129 ppm, C with no attached H in the benzene ring; and 151 ppm, C in the benzene ring connecting to the OH group. A signal at ca. 63 ppm due to the CH2OH group in 2-hydroxybenzyl alcohol is not seen in the spectrum of the resin-like product formed solely from 2-hydroxybenzyl alcohol [Fig. 2(e)], but, instead, a signal appears at ca. 36 ppm. The product was not soluble in THF or methanol and, therefore, molecular weight distribution of the product could not be measured; whereas, the novolac-type phenolic resin could be dissolved in either THF or methanol. The insolubility, however, would indicate the formation of higher molecular weight compounds. From this fact and the resemblance of the spectrum shown in Figure 2(e) to that of the novolac-type phenolic resin, it appears that 2-hydroxybenzyl alcohol undergoes condensation at the CH2OH group to form the CH2 linkage, leading to the formation of a kind of phenolic resins. Comparing the ratio of intensity of a signal due to C–H in the benzene ring at 116 ppm to that of a signal due to C with no attached H in the benzene ring at 129 ppm, the ratio is lower in the spectrum of the 2-hydroxybenzyl alcohol-derived phenolic resin than in that of the novolac-type phenolic resin, indicating that the number of the C with no attached H in the benzene ring is higher in the 2-hydroxybenzyl alcohol-derived phenolic resin. This would
Vol. 34, No. 1
INORGANIC–ORGANIC HYBRIDS
67
FIG. 2 The solid-state 13C CP/MAS NMR spectra of (a) 2-hydroxybenzyl alcohol, (b) hybrids prepared from TEOS and 2-hydroxybenzyl alcohol at a C*:Si ratio of 4.0, (c) hybrids produced from TMOS and 2-hydroxybenzyl alcohol at a C*:Si ratio of 4.0 using methanol as solvent, (d) the novolac-type phenolic resin, and (e) a resin-like product formed from 2-hydroxybenzyl alcohol. Asterisks indicate spinning side bands.
68
I. HASEGAWA et al.
Vol. 34, No. 1
FIG. 3 The solid-state 29Si CP/MAS NMR spectra of (a) hybrids prepared from TEOS and 2-hydroxybenzyl alcohol at a C*:Si ratio of 2.0 (1055 scans) and (b) SiO2 gels produced from TEOS (12260 scans).
mean that the 2-hydroxybenzyl alcohol-derived phenolic resin has a three-dimensional network, in other words, a higher degree of cross-linking. This would explain the reason for the insolubility of the 2-hydroxybenzyl alcohol-derived phenolic resin in the organic solvents. This fact further suggests that the resin with the cross-linked structure would be incorporated as the organic component of the gels prepared from TEOS and 2-hydroxybenzyl alcohol since their 13C NMR spectra reveal the same profile. The 29Si NMR spectrum of the gels prepared from a solution of TEOS and 2-hydroxybenzyl alcohol with a C*:Si ratio of 2.0, shown in Figure 3(a), gives rise to three signals, at ca. 292, 2101, and 2108 ppm, which can be assigned to Q2 [Si(OSi)2(O2)2], Q3 [Si(OSi)3(O2)], and Q4 [Si(OSi)4] silicate units, respectively. The Q3 signal is dominant in the spectrum, indicating that the structure of the gels is mainly comprised of the silicate unit. The solid-state 29Si NMR spectrum of SiO2 gels prepared from TEOS alone under the same reaction conditions [Fig. 3(b)] indicates a profile similar to that of the spectrum of the gels formed from TEOS and 2-hydroxybenzyl alcohol. This suggests that there is no significant difference in silicate structures of these gels. Since the organic component in the gels formed from TEOS and 2-hydroxybenzyl alcohol has a structure similar to that of the 2-hydroxybenzyl alcohol-derived phenolic resin, it can be stated that 2-hydroxybenzyl alcohol and hydrolysis products of TEOS undergo condensation independently and simultaneously in the acidic ethanolic solution to yield hybrids consisting of the SiO2 and phenolic resin components.
Vol. 34, No. 1
INORGANIC–ORGANIC HYBRIDS
69
FIG. 4 The solid-state 29Si CP/MAS NMR spectra of the hybrids prepared from the solution with a C*:Si ratio of 2:1 for the spin-lattice relaxation time measurement as a function of t. t 5 (a) 2, (b) 4, (c) 10, (d) 20, (e) 40, and (f) 80 s.
The C–OH group in the phenolic resin component of the hybrids is seen at ca. 151 ppm. The chemical shift value is identical to those of the group seen in the 13C NMR spectra of the 2-hydroxybenzyl alcohol-derived phenolic resin and the novolac-type phenolic resin. This means that there is no C–O–Si linkage in the hybrids. The spin-lattice relaxation time (T1) was measured regarding the Q3 signal in the 29Si CP/MAS NMR spectra of the hybrids and the SiO2 gels. These samples were dried by heating at 100°C for 5 h prior to the measurement. Figure 4 shows the spectra of the hybrids produced from the solution with a C*:Si ratio of 2.0 as a function of waiting time (t) used for the determination of T1 by the Torchia method [11]. Common methods such as the inversion recovery and saturation recovery were not practically applicable to the determination, because of considerably long relaxation times of the samples for the methods. The T1 values of the hybrids synthesized from the solutions with C*:Si ratios of 2.0 and 4.0 and the SiO2 gels were 100, 117, and 60 s, respectively. These values are much lower than that for Nacrite, a component of Kaolin, with a rigid three-dimensional structure (29Si T1, ca. 5000 s) [12]. Hydrogen atoms present as the OH group in these hybrids and SiO2 gels would enhance sensitivity of 29Si–1H cross-polarization, thereby shortening their 29Si spin-lattice relaxation times [13]. As described above, these samples have been demonstrated to possess similar siloxane networks. Nevertheless, the relaxation time of the SiO2 gels is even shorter than those of the
70
I. HASEGAWA et al.
Vol. 34, No. 1
hybrids. Considering that the relaxation time of the SiO2 gels would result from hydrogen bonding between silanol groups therein, the slightly longer relaxation times of the hybrids indicate the presence of another type of hydrogen bonding. The fact that the organic component of the hybrids possesses the phenolic hydroxyl group suggests that there would be hydrogen bonding between the silanol and phenolic hydroxyl groups in the hybrids, which would cause the slightly longer relaxation times of the hybrids. On the basis of this fact, it is assumed that the interaction between the SiO2 and phenolic resin components in the hybrids is due to appreciable hydrogen bonding. Such an interaction would prevent the SiO2–phenolic resin hybrid fibers from melting and coalescing before the phenolic resin component is pyrolyzed to C, resulting in the formation of SiC fibers after the heat treatment at 1500°C in Ar without cure of the hybrid fibers. CONCLUSIONS SiO2–phenolic resin hybrids produced from TEOS and 2-hydroxybenzyl alcohol under acidic conditions by sol-gel processing were studied by solid-state 13C and 29Si NMR spectroscopy. 2-Hydroxybenzyl alcohol underwent condensation at the CH2OH group to form phenolic resin with a higher degree of cross-linking, which was incorporated as the organic component of the hybrids. Simultaneously with the condensation, TEOS also underwent condensation following hydrolysis, to form a silicate network as the inorganic component of the hybrids. The spin-lattice relaxation time measurement by 29Si NMR spectroscopy suggested that the interaction between these two components in the hybrids is due to hydrogen bonding between the silanol and phenolic hydroxyl groups. REFERENCES 1. G.C. Wei, C.R. Kennedy, and L.A. Harris, Bull. Am. Ceram. Soc. 63, 1054 (1984). 2. H. Tanaka and Y. Kurachi, Ceram. Int. 14, 109 (1988). 3. H. Tanaka, K.Z. Jin, and K. Hirota, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi 98, 607 (1990). 4. K. Ono and Y. Kurachi, J. Mater. Sci. 24, 388 (1991). 5. I. Hasegawa, T. Nakamura, S. Motojima, and M. Kajiwara, J. Mater. Chem. 5, 193 (1995). 6. I. Hasegawa, T. Nakamura, S. Motojima, and M. Kajiwara, J. Sol-Gel Sci. Technol. 8, 577 (1997). 7. I. Hasegawa, T. Nakamura, S. Motojima, and M. Kajiwara, submitted to Proceedings of the 2nd International Meeting of Pacific Rim Ceramic Societies. 8. I. Hasegawa, T. Nakamura, and M. Kajiwara, Mater. Res. Bull. 31, 869 (1996). 9. I. Hasegawa, Y. Fukuda, and M. Kajiwara, J. Eur. Ceram. Soc. 17, 1467 (1997). 10. I. Hasegawa, Y. Fukuda, and M. Kajiwara, Ceram. Int., in press. 11. D.A. Torchia, J. Magn. Reson. 30, 613 (1978). 12. P.F. Barron, R.L. Frost, and J.O. Skjemstad, J. Chem. Soc., Chem. Commun. 581 (1983). 13. S. Hayashi and E. Akiba, Chem. Phys. Lett. 226, 495 (1994).
PII S0025-5408(98)00208-6
INORGANIC–ORGANIC HYBRIDS PRODUCED FROM TETRAETHOXYSILANE AND 2-HYDROXYBENZYL ALCOHOL AS STUDIED BY SOLID-STATE 13C AND 29Si NMR SPECTROSCOPY
Isao Hasegawa1*, Toshio Takayama2, and Shuichi Naito2 Department of Chemistry, Faculty of Engineering, Gifu University, Yanagido 1-1, Gifu-City, Gifu 501-1193, Japan 2 Department of Applied Chemistry, Faculty of Engineering, Kanagawa University, Kanagawa-Ku, Yokohama-City, Kanagawa 221-8686, Japan
1
(Refereed) (Received April 28, 1998; Accepted May 4, 1998)
ABSTRACT Tetraethoxysilane and 2-hydroxybenzyl alcohol were allowed to undergo a sol-gel reaction under acidic conditions. These compounds underwent condensation independently to give hybrids consisting of silica and phenolic resin components, which are applicable as precursors for silicon carbide powders and fibers when they are prepared in the fibrous form. The spin-lattice relaxation time measurement for the hybrids using solid-state 29Si NMR spectroscopy suggests that there is appreciable interaction between the silica and phenolic resin components in the hybrids through hydrogen bonding. © 1999 Elsevier Science Ltd
KEYWORDS: A. carbides, A. inorganic compounds, A. organic compounds, B. sol-gel chemistry, C. nuclear magnetic resonance (NMR) INTRODUCTION Inorganic polymer and inorganic– organic hybrid gel precursors have been applied successfully to the synthesis of non-oxide ceramics, such as silicon carbide (SiC). Wei et al. [1] synthesized SiC powders from gels produced from methyltrimethoxysilane and phenolic resin or sucrose by a sol-gel reaction, which is the first example of inorganic– organic hybrids
*To whom correspondence should be addressed. 63
64
I. HASEGAWA et al.
Vol. 34, No. 1
as precursors for non-oxide ceramic materials. Following this work, a few studies [2– 4] have been conducted on SiC powder synthesis from hybrids prepared from alkoxysilane and phenolic resins. The main reason for employing phenolic resins for SiC synthesis is that the resins give a large amount of carbonaceous residue upon pyrolysis. The hybrid gels undergo carbothermal reduction, with C forming from the resins upon heating at an elevated temperature in Ar, to give SiC powders. An overall reaction scheme for carbothermal reduction of silica (SiO2) to yield SiC is expressed as SiO2 1 3C 3 SiC 1 2CO Previously, we reported [5] that novolac-type phenolic resin and tetraethoxysilane [Si(OC2H5)4, TEOS] underwent a sol-gel reaction in acidic ethanolic solutions to yield SiO2-phenolic resin hybrid fibers. The hybrid fibers could be converted into SiC fibers when they were heat-treated at 1500°C in Ar [5,6] or into silicon nitride fibers upon heating at the temperature in flowing N2 [7]. The addition of a titania or zirconia component into the hybrid fibers was also possible, which gave Si–Ti–C [8,9] or Si–Zr–C fibers [10], respectively, by carbothermal reduction. One of the features of the hybrid fiber route to non-oxide ceramic fibers is that the ceramic fibers can be produced without cure of the hybrid fibers, although the phenolic resin itself melts at ca. 110°C. This suggests that the phenolic resin may play another role in the SiO2-phenolic resin hybrid fibers. Thus, a question arises about the interaction between the two components in the hybrid fibers, which has been obscure up to now. Consequently, this study has been aimed at investigating that interaction. In this study, 2-hydroxybenzyl alcohol has been used instead of the phenolic resins, and inorganic– organic hybrids have been prepared from this compound and TEOS by a sol-gel reaction under the same reaction conditions as those for preparing the SiO2-phenolic resin hybrid fibers. The reasons for the use of 2-hydroxybenzyl alcohol are that it corresponds to the repeating unit of the phenolic resins and reaction behavior of the resins during the sol-gel reaction is expected to be elucidated as well by investigating the hybrids synthesized using this rather simple compound. The hybrids thus prepared have been studied with solid-state 13 C and 29Si NMR spectroscopy. EXPERIMENTAL Synthesis of Hybrids from TEOS and 2-Hydroxybenzyl Alcohol and SiC from the Hybrids. Starting solutions were prepared by dissolving 2-hydroxybenzyl alcohol in ethanol (10 cm3), followed by the addition of TEOS (15 cm3), distilled water, and 1 mol dm23 hydrochloric acid. The TEOS:H2O:HCl molar ratio of the solutions was fixed at 1:2:0.01, and the atomic ratio of C in 2-hydroxybenzyl alcohol to Si in TEOS (abbreviated to the C*:Si ratio) at 2.0 and 4.0. The solutions were stirred for 5 min at room temperature for intimate mixing and then maintained at 65°C to obtain gels. In addition, gels were synthesized using tetramethoxysilane [Si(OCH3)4, TMOS] as a SiO2 source and methanol as solvent. The Si:H2O:HCl and C*:Si ratios of these starting solutions were the same as above. For comparison with the gels, SiO2-free products and SiO2 gels were prepared using the same amounts of ethanol, distilled water, 1 mol dm23 hydrochloric acid, and 2-hydroxybenzyl alcohol or TEOS, respectively, as above. Heat treatment of the obtained gels was conducted at 1500°C in Ar for 4 h in order to
Vol. 34, No. 1
INORGANIC–ORGANIC HYBRIDS
65
investigate whether they acted as precursors for SiC. The flowing rate of Ar was 100 cm3 min21 and the heating rate was 10°C min–1. Analytical Procedures. XRD patterns of products after the heat-treatment at 1500°C were obtained with a Rigaku RINT 1100 X-ray diffractometer using Ni-filtered Cu Ka radiation. 29 Si and 13C NMR spectra were obtained with a JEOL EX270WB spectrometer. Samples were inserted into a cylindrical rotor made of zirconia. Line narrowing was achieved by high-power 1H decoupling and magic-angle-spinning (MAS). The spinning rate was set at ca. 6 kHz. The observed frequencies of 29Si and 13C nuclei were 53.54 and 67.80 MHz, respectively. For the measurements with spin-lock cross-polarization sequence (CP/MAS), the following conditions were used: contact time, 5 ms; repetition time, 5 s; spectral width, 10 kHz; data point, 8192. Spectra usually were accumulated for 1000 –2000 times to achieve a reasonable signal-to-noise ratio. 29Si and 13C chemical shift values were at first given with reference to the 29Si and 13C NMR signals due to polydimethylsilane, respectively, and were then converted to the values from tetramethylsilane. The measurement for spin-lattice relaxation time was made using the cross-polarization method of Torchia [11]. In the 13C NMR measurements, the MAS/DL sequence, which means the CP/MAS method with dipolar diphasing, was used to distinguish CH3 and non-hydrogenated carbons from the other hydrogenated carbons.
RESULTS AND DISCUSSION When TEOS and 2-hydroxybenzyl alcohol were allowed to react in the acidic ethanolic solutions, the solutions increased in viscosity with time of soaking at 65°C, resulting in gelation at ca. 4 h. At ca. 3.5 h of soaking, fibers (ca. 30 cm in length) could be drawn by immersing a glass rod into the viscous solutions and then withdrawing the rod. When the fibrous or bulk gels were heated at 1500°C in Ar for 4 h, they gave pale-gray fibers or powders, respectively. The XRD pattern of the powdered products after the heat treatment (Fig. 1) exhibited peaks due to b-SiC phases. This implies that the gels served as precursors for SiC fibers or powders. If 2-hydroxybenzyl alcohol (mp: 83– 85°C) itself had been included in the fibrous gels, the gels would have melted and, therefore, resulted in the coalescence of the fibers. However, the products preserved the fibrous form even after the heat treatment at 1500°C, as was observed for the SiO2-phenolic resin hybrid fibers we reported previously [5,6]. Figure 2 shows the solid-state 13C CP/MAS NMR spectra of (a) 2-hydroxybenzyl alcohol and (b) the gels prepared from TEOS and 2-hydroxybenzyl alcohol. Assignment of signals from 2-hydroxybenzyl alcohol is given in this figure. They clearly show different profiles, indicating that 2-hydroxybenzyl alcohol itself underwent a reaction during the sol-gel reaction to give the gels. When 2-hydroxybenzyl alcohol alone was allowed to react in the acidic ethanolic solution, a resin-like product formed. The 13C NMR spectrum of the product is shown in Figure 2(e). It should be noted that its profile is almost the same as that of the spectrum shown in Figure 2(b) and is similar to that of the novolac-type phenolic resin [Fig. 2(d)] that we used as a starting material for synthesizing the hybrid fibers [5–10]. The signals at ca. 18 and 61 ppm seen in Figure 2(b) decrease in intensity in the spectrum of the gels after heating at 100°C in air for drying. In addition, these two signals are not
66
I. HASEGAWA et al.
Vol. 34, No. 1
FIG. 1 The XRD pattern of powdered products obtained by heating hybrids produced from TEOS and 2-hydroxybenzyl alcohol at 1500°C in Ar for 4 h.
observed in the 13C NMR spectrum of gels prepared using TMOS as a SiO2 source and methanol as solvent [Fig. 2(c)]; instead, a new signal appears at ca. 52 ppm. These facts imply that the signals at ca. 18 and 61 ppm are due to ethanol, which was used as solvent for the gel synthesis and formed from TEOS upon hydrolysis. Similarly, the signal at 52 ppm in Figure 2(c) is ascribable to methanol involved in the gels produced in the TMOS-2hydroxybenzyl alcohol–methanol system. The 13C MAS/DL NMR spectrum of the gels formed from TEOS and 2-hydroxybenzyl alcohol gives rise to only four signals, at ca. 18, 61, 129, and 151 ppm. This implies that the signals at 129 and 151 ppm are due to C with no attached H in the benzene ring. Thus, signals observed in both Figures 2(b) and 2(e) would be ascribable as follows on the basis of the similarity to the spectrum of the novolac-type phenolic resin: 36 ppm, CH2; 116 ppm, CH in the benzene ring; 129 ppm, C with no attached H in the benzene ring; and 151 ppm, C in the benzene ring connecting to the OH group. A signal at ca. 63 ppm due to the CH2OH group in 2-hydroxybenzyl alcohol is not seen in the spectrum of the resin-like product formed solely from 2-hydroxybenzyl alcohol [Fig. 2(e)], but, instead, a signal appears at ca. 36 ppm. The product was not soluble in THF or methanol and, therefore, molecular weight distribution of the product could not be measured; whereas, the novolac-type phenolic resin could be dissolved in either THF or methanol. The insolubility, however, would indicate the formation of higher molecular weight compounds. From this fact and the resemblance of the spectrum shown in Figure 2(e) to that of the novolac-type phenolic resin, it appears that 2-hydroxybenzyl alcohol undergoes condensation at the CH2OH group to form the CH2 linkage, leading to the formation of a kind of phenolic resins. Comparing the ratio of intensity of a signal due to C–H in the benzene ring at 116 ppm to that of a signal due to C with no attached H in the benzene ring at 129 ppm, the ratio is lower in the spectrum of the 2-hydroxybenzyl alcohol-derived phenolic resin than in that of the novolac-type phenolic resin, indicating that the number of the C with no attached H in the benzene ring is higher in the 2-hydroxybenzyl alcohol-derived phenolic resin. This would
Vol. 34, No. 1
INORGANIC–ORGANIC HYBRIDS
67
FIG. 2 The solid-state 13C CP/MAS NMR spectra of (a) 2-hydroxybenzyl alcohol, (b) hybrids prepared from TEOS and 2-hydroxybenzyl alcohol at a C*:Si ratio of 4.0, (c) hybrids produced from TMOS and 2-hydroxybenzyl alcohol at a C*:Si ratio of 4.0 using methanol as solvent, (d) the novolac-type phenolic resin, and (e) a resin-like product formed from 2-hydroxybenzyl alcohol. Asterisks indicate spinning side bands.
68
I. HASEGAWA et al.
Vol. 34, No. 1
FIG. 3 The solid-state 29Si CP/MAS NMR spectra of (a) hybrids prepared from TEOS and 2-hydroxybenzyl alcohol at a C*:Si ratio of 2.0 (1055 scans) and (b) SiO2 gels produced from TEOS (12260 scans).
mean that the 2-hydroxybenzyl alcohol-derived phenolic resin has a three-dimensional network, in other words, a higher degree of cross-linking. This would explain the reason for the insolubility of the 2-hydroxybenzyl alcohol-derived phenolic resin in the organic solvents. This fact further suggests that the resin with the cross-linked structure would be incorporated as the organic component of the gels prepared from TEOS and 2-hydroxybenzyl alcohol since their 13C NMR spectra reveal the same profile. The 29Si NMR spectrum of the gels prepared from a solution of TEOS and 2-hydroxybenzyl alcohol with a C*:Si ratio of 2.0, shown in Figure 3(a), gives rise to three signals, at ca. 292, 2101, and 2108 ppm, which can be assigned to Q2 [Si(OSi)2(O2)2], Q3 [Si(OSi)3(O2)], and Q4 [Si(OSi)4] silicate units, respectively. The Q3 signal is dominant in the spectrum, indicating that the structure of the gels is mainly comprised of the silicate unit. The solid-state 29Si NMR spectrum of SiO2 gels prepared from TEOS alone under the same reaction conditions [Fig. 3(b)] indicates a profile similar to that of the spectrum of the gels formed from TEOS and 2-hydroxybenzyl alcohol. This suggests that there is no significant difference in silicate structures of these gels. Since the organic component in the gels formed from TEOS and 2-hydroxybenzyl alcohol has a structure similar to that of the 2-hydroxybenzyl alcohol-derived phenolic resin, it can be stated that 2-hydroxybenzyl alcohol and hydrolysis products of TEOS undergo condensation independently and simultaneously in the acidic ethanolic solution to yield hybrids consisting of the SiO2 and phenolic resin components.
Vol. 34, No. 1
INORGANIC–ORGANIC HYBRIDS
69
FIG. 4 The solid-state 29Si CP/MAS NMR spectra of the hybrids prepared from the solution with a C*:Si ratio of 2:1 for the spin-lattice relaxation time measurement as a function of t. t 5 (a) 2, (b) 4, (c) 10, (d) 20, (e) 40, and (f) 80 s.
The C–OH group in the phenolic resin component of the hybrids is seen at ca. 151 ppm. The chemical shift value is identical to those of the group seen in the 13C NMR spectra of the 2-hydroxybenzyl alcohol-derived phenolic resin and the novolac-type phenolic resin. This means that there is no C–O–Si linkage in the hybrids. The spin-lattice relaxation time (T1) was measured regarding the Q3 signal in the 29Si CP/MAS NMR spectra of the hybrids and the SiO2 gels. These samples were dried by heating at 100°C for 5 h prior to the measurement. Figure 4 shows the spectra of the hybrids produced from the solution with a C*:Si ratio of 2.0 as a function of waiting time (t) used for the determination of T1 by the Torchia method [11]. Common methods such as the inversion recovery and saturation recovery were not practically applicable to the determination, because of considerably long relaxation times of the samples for the methods. The T1 values of the hybrids synthesized from the solutions with C*:Si ratios of 2.0 and 4.0 and the SiO2 gels were 100, 117, and 60 s, respectively. These values are much lower than that for Nacrite, a component of Kaolin, with a rigid three-dimensional structure (29Si T1, ca. 5000 s) [12]. Hydrogen atoms present as the OH group in these hybrids and SiO2 gels would enhance sensitivity of 29Si–1H cross-polarization, thereby shortening their 29Si spin-lattice relaxation times [13]. As described above, these samples have been demonstrated to possess similar siloxane networks. Nevertheless, the relaxation time of the SiO2 gels is even shorter than those of the
70
I. HASEGAWA et al.
Vol. 34, No. 1
hybrids. Considering that the relaxation time of the SiO2 gels would result from hydrogen bonding between silanol groups therein, the slightly longer relaxation times of the hybrids indicate the presence of another type of hydrogen bonding. The fact that the organic component of the hybrids possesses the phenolic hydroxyl group suggests that there would be hydrogen bonding between the silanol and phenolic hydroxyl groups in the hybrids, which would cause the slightly longer relaxation times of the hybrids. On the basis of this fact, it is assumed that the interaction between the SiO2 and phenolic resin components in the hybrids is due to appreciable hydrogen bonding. Such an interaction would prevent the SiO2–phenolic resin hybrid fibers from melting and coalescing before the phenolic resin component is pyrolyzed to C, resulting in the formation of SiC fibers after the heat treatment at 1500°C in Ar without cure of the hybrid fibers. CONCLUSIONS SiO2–phenolic resin hybrids produced from TEOS and 2-hydroxybenzyl alcohol under acidic conditions by sol-gel processing were studied by solid-state 13C and 29Si NMR spectroscopy. 2-Hydroxybenzyl alcohol underwent condensation at the CH2OH group to form phenolic resin with a higher degree of cross-linking, which was incorporated as the organic component of the hybrids. Simultaneously with the condensation, TEOS also underwent condensation following hydrolysis, to form a silicate network as the inorganic component of the hybrids. The spin-lattice relaxation time measurement by 29Si NMR spectroscopy suggested that the interaction between these two components in the hybrids is due to hydrogen bonding between the silanol and phenolic hydroxyl groups. REFERENCES 1. G.C. Wei, C.R. Kennedy, and L.A. Harris, Bull. Am. Ceram. Soc. 63, 1054 (1984). 2. H. Tanaka and Y. Kurachi, Ceram. Int. 14, 109 (1988). 3. H. Tanaka, K.Z. Jin, and K. Hirota, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi 98, 607 (1990). 4. K. Ono and Y. Kurachi, J. Mater. Sci. 24, 388 (1991). 5. I. Hasegawa, T. Nakamura, S. Motojima, and M. Kajiwara, J. Mater. Chem. 5, 193 (1995). 6. I. Hasegawa, T. Nakamura, S. Motojima, and M. Kajiwara, J. Sol-Gel Sci. Technol. 8, 577 (1997). 7. I. Hasegawa, T. Nakamura, S. Motojima, and M. Kajiwara, submitted to Proceedings of the 2nd International Meeting of Pacific Rim Ceramic Societies. 8. I. Hasegawa, T. Nakamura, and M. Kajiwara, Mater. Res. Bull. 31, 869 (1996). 9. I. Hasegawa, Y. Fukuda, and M. Kajiwara, J. Eur. Ceram. Soc. 17, 1467 (1997). 10. I. Hasegawa, Y. Fukuda, and M. Kajiwara, Ceram. Int., in press. 11. D.A. Torchia, J. Magn. Reson. 30, 613 (1978). 12. P.F. Barron, R.L. Frost, and J.O. Skjemstad, J. Chem. Soc., Chem. Commun. 581 (1983). 13. S. Hayashi and E. Akiba, Chem. Phys. Lett. 226, 495 (1994).