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Pressure effect on the polymerization kinetics of sol-gel process
Volume 2, number SB
August 1984
MATERIALS LETTERS
PRESSURE EFFECT ON THE POLYMERIZATION
KINETICS OF SOL-GEL
PROCESS
I. ARTAKI, S. SINHA and J. JONAS Department
of Chemistry, School of Chemical Sciences,
University of Illinois, Urbana, IL 61801,
USA
Received 14 June 1984
The effect of pressure on the polymerization kinetics of Si(OCH& sol-gel process has been investigated at a molecular level using 29Si NMR spectroscopy. The condensation subsequent to hydrolysis of the initiating Si-alkoxide reagent, Si(OCHs)4 was monitored as a function of elapsed time at different pressures. It has been shown that high pressures considerably enhance the reaction rates without altering the condensation mechanism which operates during the initial stages of the polymerization process.
1. Introduction In recent years, sol-gel processes for preparing oxide glasses have aroused an increased interest from both a scientific and a technological point of view [ 11. The “gel route” allows the manufacture of highly homogeneous glasses at considerably lower temperatures than those required in the conventional melting procedure [2-41. The preparation of oxide glasses involves first the preparation of a wet monolithic gel by hydrolysis and polymerization of a silicon&oxide solution. It is followed by heat treatment to remove all residual organics and water from the porous gel, and finally by the densification of the dry gel to form a monolithic dense glass. The physical and chemical properties of the resulting oxide glass are highly dependent upon the degree of homogeneity achieved during the gelling stage [5,6]. Therefore, considerable attention has been devoted to the parameters [6,7] which affect the structural characteristics of the polymer gel networks in order to promote the most successful sol to gel transformation. These parameters are: (i) Si concentration in solution, (ii) H20/alkoxide ratio, (iii) pH of the medium and (iv) reaction temperature. It is generally believed [4,6,8] that the formation of highly homogeneous monolithic gels requires a high degree of cross-linking during the dehydration polymerization step. However, the optimum solution conditions [8] to achieve this state ne448
cessitate almost prohibitively long gelation times, often in the order of days, even months. The object of the present study is to investigate the role of pressure on the polymerization kinetics of the Si(OCH3)4 sol-gel transition. The possiblity of reducing gelation times by elevated pressures has been explored at a molecular level, using high-resolution 2gSi spectroscopy. This represents the first study to carry out 2gSi NMR measurements under high pressure.
2. Experimental To avoid precipitation, clear sample solutions were prepared according to the procedure outlined by Sakka and Kamiya [9]. A mixture of CH30H/HCl was added dropwise with stirring to a solution of Si(OCH3)4 (Aldrich Co.) diluted in CHjOH. The mole ratios of all reagents were adjusted to yield a final solution pH of 3.7, Si concentration of 1.6 M and H20/alkoxide ratio of 10. Si(OCH3)4 was preferred over the more commonly used Si(OC2H,)4 as the initiating silicon-alkoxide reagent. This is because, in the former, the hydrolysis is completed before the commencement of the condensation, in contrast to the latter, where both hydrolysis and condensation occur simultaneously. The interpretation of the NMR results is greatly facilitated if the polymerization reaction can be independently monitored without interference from the hydrolysis reaction. 0 167-557x/84/$ 03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 2. number SB
MATERIALS LETTERS
2gSi NMR measurements were carried out at -3O”C, over a pressure range of 1 bar to 5 kbar, using a pulsed spectrometer operating at 35.760 MHz. The pulsed NMR spectrometer and the high-pressure rf probe have been described in detail elsewhere [ 111. Pressure was generated with an Enerpac hand pump and intensifier and measured by a 7 kbar Bourbon-Heise gauge, using CS2 as a pressure transmitting fluid. The temperature was controlled by a regulated circulating Kryomat (purchased from Lauda Instruments Division) with a precision of t0 .l “C. Due to the low sensitivity and natural abundance of 29Si, 17000 accumulations at 0.7 s, intervals were required to produce spectra with adequate S/N ratios. Pulses of 15 ps (30”) duration and a spectral width of 1600 Hz were used. The resulting spectra were analyzed to monitor quantitatively the time evolution of all condensed species in solution during the initial stages of the polymerization reaction. Excessive line-broadening and weakening of the NMR signal intensity precluded measurements during the final stages of polymerization just prior to gelation.
August 1984
(a) 80 1 bar
I
01
0
I 40
I
I
I
I
120
80 Time (hrs)
(b)
80 t
4.5 k bar
1
3. Results and discussion Figs. la and lb depict the concentration changes of all observable species as a function of elapsed time at 1 bar and 4.5 kbar pressure, respectively. Monosilicic acid, Si(OH), , designated as “monomer” is initially the only species present in solution, indicating the complete hydrolysis of Si(OCH,), . The precise nature of all condensed species in solution has been the subject of controversy for many years. However, it is now believed [ 12,131 that, under the present pH conditions, the monomer proceeds to react with a monomeric anion [14], Si(OH)gO- to form a dimer Si(OH), -Si(OH), , which may in turn react with an additional monomeric anion to form a linear trimer , or with an anionic dimer to form a cyclic tetramer. The linear trimer and the cyclic tetramer further condense into higher polymeric species. However, because of the semi-condensed nature of solution at the time of formation of these higher species as well as the stringent requirements of the high-pressure experiment, the NMR signal produced by these species was too weak to be accurately monitored by our high-resolution spectrometer. A comparison of figs. la and lb reveals that the ef-
0
40
120
80 Time (hrs)
Fig. 1. Time evolution of all condensed species during the polymerization process at (a) 1 bar and (b) 4.5 kbar pressure. o Rep-
I resents the concentration
of the monomer,
-y-.
resents the concentration
of the dimer, -Si*-0-Sp-
q
I
I
I
Rep-
I
of the end groups of the linear trimer -Si*-0-Si-0-Si*-.
and
I
I a Represents the concentration I 1. linear trimer, -Sr-0-Si*-O-y-
of the middle group of the I
and of the cyclic tetramer,
_ai*_O_Si*_
-ii*-O-ii*feet of pressure is to accelerate the condensation process without altering either the path of the polymerization reaction or the structure of any of the condensed 449
Volume 2, number 5B
MATERIALS LETTERS
species. At both ambient and higher pressures, the disappearance of the monomer, Si(OH), is accompanied by the formation of the dimer and subsequently that of the linear trimer and cyclic tetramer, which in turn leads to the gradual disappearance of the dimer. The formation of the higher polymeric species, although not shown in figs. la and lb for the reasons stated above, is indicated by the difference between the total Si concentration (1.6 M) and the sum of the concentrations of the lower polymeric species (monomer, dimer, trimer and cyclotetramer). At 1 bar, their presence is detected ~50 h after hydrolysis, whereas at 4.5 kbar they are detected during the initial 5 h. Another indication that the reaction pathway is unaffected by elevated pressures is that the maximum concentration of dimer, trimer and cyclotetramer achieved before depletion sets in is roughly the same in both cases. However, the maxima occur at much earlier times at elevated pressures, reflecting the accelerated rates of the reaction. As expected, the polymerization rate enhancement is not an exponential function of pressure, and therefore not describable by an activation volume concept. In fact, the effect of pressure is barely observable at 2 kbar, whereas one order of magnitude rate enhancement occurs at 5 kbar. The details of the kinetics at higher pressures will be analyzed in a later paper. This study suggests that pressure may be successfully used as an additional experimental variable to drastically reduce gelation times without affecting the homogeneity of the resulting glass precursor gel. To confirm this hypothesis and in order to investigate whether structural modifications have been introduced in the polymer-gel networks under elevated pressures, measurements will be extended into the final stages
450
of the polymerization troscopy.
August 1984
process using laser Raman spec-
Acknowledgement This work was partially supported by the Air Force Office of Scientific Research under Grant USAFOSR 81-0010.
References [l] S.P. Mukherjee, J. Non-Cryst. Solids 42 (1980) 477. [2] M. Yamane, S. Aso, S. Okano and T. Sakaino, J. Mat. Sci. 14 (1979) 607. [ 31 M. Nogami and Y. Moriya, J. NonCryst. Solids 37 (1980) 191. [4] B.E. Yoldas, J. Mat. Sci. 14 (1979) 1843. [5] B.E. Yoldas, J. NonCryst. Solids 51 (1982) 105. [6] C.J. Brinker, K.D. Keefer, D.N. Schaefer and AS. Ashley, J. NonCryst. Solids 48 (1982) 47. [7] B.E. Yoldas, J. NonCryst. Solids 63 (1984) 145. [8] L.C. Klein and G.J. Garvey, J. NonCryst. Solids 38/39 (1980) 45. [9] S. Sakka and K. Kamiya, J. Nontryst. Solids 48 (1982) 31. [IO] M. Yamane, S. move and A. Yasumari, J. Non-Cryst. Solids 63 (1984) 13. [ 111 J. Jonas, Proceedings NATO ASI, High Pressure Chemistry, ed. H. Kelm, pp. 65-110. [ 121 D. Hoebbel, G. Garzo, G. Engelhardt and A. TiII, Z. Anorg. AIlg. Chem. 450 (1977) 5. [ 131 R. Harris, G.T.G. Knight and D.N. Smith, J. Chem. Sot. Chem. Commun. (1980) 726. [ 141 C. Okkerse, in: Physical and chemical aspects of adsorbents and catalysts, eds. B.G. Linsen, J.M.H. Fortuin, C. Okkerse and J.J. Steggerda (Academic Press, Now York, 1970) p. 213.
August 1984
MATERIALS LETTERS
PRESSURE EFFECT ON THE POLYMERIZATION
KINETICS OF SOL-GEL
PROCESS
I. ARTAKI, S. SINHA and J. JONAS Department
of Chemistry, School of Chemical Sciences,
University of Illinois, Urbana, IL 61801,
USA
Received 14 June 1984
The effect of pressure on the polymerization kinetics of Si(OCH& sol-gel process has been investigated at a molecular level using 29Si NMR spectroscopy. The condensation subsequent to hydrolysis of the initiating Si-alkoxide reagent, Si(OCHs)4 was monitored as a function of elapsed time at different pressures. It has been shown that high pressures considerably enhance the reaction rates without altering the condensation mechanism which operates during the initial stages of the polymerization process.
1. Introduction In recent years, sol-gel processes for preparing oxide glasses have aroused an increased interest from both a scientific and a technological point of view [ 11. The “gel route” allows the manufacture of highly homogeneous glasses at considerably lower temperatures than those required in the conventional melting procedure [2-41. The preparation of oxide glasses involves first the preparation of a wet monolithic gel by hydrolysis and polymerization of a silicon&oxide solution. It is followed by heat treatment to remove all residual organics and water from the porous gel, and finally by the densification of the dry gel to form a monolithic dense glass. The physical and chemical properties of the resulting oxide glass are highly dependent upon the degree of homogeneity achieved during the gelling stage [5,6]. Therefore, considerable attention has been devoted to the parameters [6,7] which affect the structural characteristics of the polymer gel networks in order to promote the most successful sol to gel transformation. These parameters are: (i) Si concentration in solution, (ii) H20/alkoxide ratio, (iii) pH of the medium and (iv) reaction temperature. It is generally believed [4,6,8] that the formation of highly homogeneous monolithic gels requires a high degree of cross-linking during the dehydration polymerization step. However, the optimum solution conditions [8] to achieve this state ne448
cessitate almost prohibitively long gelation times, often in the order of days, even months. The object of the present study is to investigate the role of pressure on the polymerization kinetics of the Si(OCH3)4 sol-gel transition. The possiblity of reducing gelation times by elevated pressures has been explored at a molecular level, using high-resolution 2gSi spectroscopy. This represents the first study to carry out 2gSi NMR measurements under high pressure.
2. Experimental To avoid precipitation, clear sample solutions were prepared according to the procedure outlined by Sakka and Kamiya [9]. A mixture of CH30H/HCl was added dropwise with stirring to a solution of Si(OCH3)4 (Aldrich Co.) diluted in CHjOH. The mole ratios of all reagents were adjusted to yield a final solution pH of 3.7, Si concentration of 1.6 M and H20/alkoxide ratio of 10. Si(OCH3)4 was preferred over the more commonly used Si(OC2H,)4 as the initiating silicon-alkoxide reagent. This is because, in the former, the hydrolysis is completed before the commencement of the condensation, in contrast to the latter, where both hydrolysis and condensation occur simultaneously. The interpretation of the NMR results is greatly facilitated if the polymerization reaction can be independently monitored without interference from the hydrolysis reaction. 0 167-557x/84/$ 03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 2. number SB
MATERIALS LETTERS
2gSi NMR measurements were carried out at -3O”C, over a pressure range of 1 bar to 5 kbar, using a pulsed spectrometer operating at 35.760 MHz. The pulsed NMR spectrometer and the high-pressure rf probe have been described in detail elsewhere [ 111. Pressure was generated with an Enerpac hand pump and intensifier and measured by a 7 kbar Bourbon-Heise gauge, using CS2 as a pressure transmitting fluid. The temperature was controlled by a regulated circulating Kryomat (purchased from Lauda Instruments Division) with a precision of t0 .l “C. Due to the low sensitivity and natural abundance of 29Si, 17000 accumulations at 0.7 s, intervals were required to produce spectra with adequate S/N ratios. Pulses of 15 ps (30”) duration and a spectral width of 1600 Hz were used. The resulting spectra were analyzed to monitor quantitatively the time evolution of all condensed species in solution during the initial stages of the polymerization reaction. Excessive line-broadening and weakening of the NMR signal intensity precluded measurements during the final stages of polymerization just prior to gelation.
August 1984
(a) 80 1 bar
I
01
0
I 40
I
I
I
I
120
80 Time (hrs)
(b)
80 t
4.5 k bar
1
3. Results and discussion Figs. la and lb depict the concentration changes of all observable species as a function of elapsed time at 1 bar and 4.5 kbar pressure, respectively. Monosilicic acid, Si(OH), , designated as “monomer” is initially the only species present in solution, indicating the complete hydrolysis of Si(OCH,), . The precise nature of all condensed species in solution has been the subject of controversy for many years. However, it is now believed [ 12,131 that, under the present pH conditions, the monomer proceeds to react with a monomeric anion [14], Si(OH)gO- to form a dimer Si(OH), -Si(OH), , which may in turn react with an additional monomeric anion to form a linear trimer , or with an anionic dimer to form a cyclic tetramer. The linear trimer and the cyclic tetramer further condense into higher polymeric species. However, because of the semi-condensed nature of solution at the time of formation of these higher species as well as the stringent requirements of the high-pressure experiment, the NMR signal produced by these species was too weak to be accurately monitored by our high-resolution spectrometer. A comparison of figs. la and lb reveals that the ef-
0
40
120
80 Time (hrs)
Fig. 1. Time evolution of all condensed species during the polymerization process at (a) 1 bar and (b) 4.5 kbar pressure. o Rep-
I resents the concentration
of the monomer,
-y-.
resents the concentration
of the dimer, -Si*-0-Sp-
q
I
I
I
Rep-
I
of the end groups of the linear trimer -Si*-0-Si-0-Si*-.
and
I
I a Represents the concentration I 1. linear trimer, -Sr-0-Si*-O-y-
of the middle group of the I
and of the cyclic tetramer,
_ai*_O_Si*_
-ii*-O-ii*feet of pressure is to accelerate the condensation process without altering either the path of the polymerization reaction or the structure of any of the condensed 449
Volume 2, number 5B
MATERIALS LETTERS
species. At both ambient and higher pressures, the disappearance of the monomer, Si(OH), is accompanied by the formation of the dimer and subsequently that of the linear trimer and cyclic tetramer, which in turn leads to the gradual disappearance of the dimer. The formation of the higher polymeric species, although not shown in figs. la and lb for the reasons stated above, is indicated by the difference between the total Si concentration (1.6 M) and the sum of the concentrations of the lower polymeric species (monomer, dimer, trimer and cyclotetramer). At 1 bar, their presence is detected ~50 h after hydrolysis, whereas at 4.5 kbar they are detected during the initial 5 h. Another indication that the reaction pathway is unaffected by elevated pressures is that the maximum concentration of dimer, trimer and cyclotetramer achieved before depletion sets in is roughly the same in both cases. However, the maxima occur at much earlier times at elevated pressures, reflecting the accelerated rates of the reaction. As expected, the polymerization rate enhancement is not an exponential function of pressure, and therefore not describable by an activation volume concept. In fact, the effect of pressure is barely observable at 2 kbar, whereas one order of magnitude rate enhancement occurs at 5 kbar. The details of the kinetics at higher pressures will be analyzed in a later paper. This study suggests that pressure may be successfully used as an additional experimental variable to drastically reduce gelation times without affecting the homogeneity of the resulting glass precursor gel. To confirm this hypothesis and in order to investigate whether structural modifications have been introduced in the polymer-gel networks under elevated pressures, measurements will be extended into the final stages
450
of the polymerization troscopy.
August 1984
process using laser Raman spec-
Acknowledgement This work was partially supported by the Air Force Office of Scientific Research under Grant USAFOSR 81-0010.
References [l] S.P. Mukherjee, J. Non-Cryst. Solids 42 (1980) 477. [2] M. Yamane, S. Aso, S. Okano and T. Sakaino, J. Mat. Sci. 14 (1979) 607. [ 31 M. Nogami and Y. Moriya, J. NonCryst. Solids 37 (1980) 191. [4] B.E. Yoldas, J. Mat. Sci. 14 (1979) 1843. [5] B.E. Yoldas, J. NonCryst. Solids 51 (1982) 105. [6] C.J. Brinker, K.D. Keefer, D.N. Schaefer and AS. Ashley, J. NonCryst. Solids 48 (1982) 47. [7] B.E. Yoldas, J. NonCryst. Solids 63 (1984) 145. [8] L.C. Klein and G.J. Garvey, J. NonCryst. Solids 38/39 (1980) 45. [9] S. Sakka and K. Kamiya, J. Nontryst. Solids 48 (1982) 31. [IO] M. Yamane, S. move and A. Yasumari, J. Non-Cryst. Solids 63 (1984) 13. [ 111 J. Jonas, Proceedings NATO ASI, High Pressure Chemistry, ed. H. Kelm, pp. 65-110. [ 121 D. Hoebbel, G. Garzo, G. Engelhardt and A. TiII, Z. Anorg. AIlg. Chem. 450 (1977) 5. [ 131 R. Harris, G.T.G. Knight and D.N. Smith, J. Chem. Sot. Chem. Commun. (1980) 726. [ 141 C. Okkerse, in: Physical and chemical aspects of adsorbents and catalysts, eds. B.G. Linsen, J.M.H. Fortuin, C. Okkerse and J.J. Steggerda (Academic Press, Now York, 1970) p. 213.