hydrolysis-condensation processes of the tetra-alkoxysilanes tpos, teos and tmos in some alcoholic...

Journal of Non-Crystalline Solids 134 (1991) 1-13 1 North-Holland

Hydrolysis-condensation processes of the tetra-alkoxysilanes TPOS, TEOS and TMOS in some alcoholic solvents

T.N.M. Bernards, M.J. van Bommel and A.H. Boonstra Philips Research Laboratories, PO Box 80000, 5600 JA Eindhoven, Netherlands

Received 31 January 1991 Revised manuscript received 18 April 1991

The effects of methanol, ethanol, 1-propanol and 2-propanol on the kinetics and the mechanisms of the hydrolysis-condensation reactions of the silanes TPOS, TEOS and TMOS were studied. Also, the influence of the amount of water on the hydrolysis-condensation processes of TMOS and TPOS was investigated. Accordingly, hydrolysis time versus condensation time curves were recorded and 29Si-NMR investigations were performed. The hydrolyzability of the silanes in an acidic environment was found to decrease in the sequence TMOS > TEOS > TPOS. The effect of the alcohols, in the sequence methanol > ethanol, 1-propanol > 2-propanol, upon the hydrolysis rate of each of the silanes is explained by differences in degree of dissociation of HCI in the different alcohols. Also, the exchange of alkoxy groups, observed when an alcoholic solvent was used with an alkoxy group different from the alkoxy group of the silane, may influence the hydrolysis rate of the silane. Differences in dimerization are ascribed to differences in base strength of the activated silanol complexes with regard to those of the protonated alcohols.

1. Introduction

The preparation of silica glass by a sol-gel process is receiving world-wide interest [1-4]. At the moment, thin films deposited on substrates by a sol-gel procedure are produced in large quanti- ties. Particular attention is being accorded to the preparation of reflection-reducing films [5-6]. These reflection-reducing films are used for televi- sion sets and computer monitors [7].

For these purposes, coating solutions are used which consist of hydrolyzed alkoxysilane compounds in different solvents. Often a combination of solvents is used to obtain a solution with suita- ble rheological properties, necessary to obtain films with the desired characteristics. The use of different solvents may have a considerable influence on the hydrolysis-condensation behaviour of the alkoxysilane compounds. To obtain a good repro- ducibility of the properties of the films, a fundamental knowledge of the chemistry of the hydrolysis-condensation processes is essential.

It has been found that exchange of alkoxy groups occurs when an alcohohc solvent is used with an alkoxy group different from the alkoxy groups of the silane [8-12]. Brinker et al. [10] and Pouxviel et al. [11] reported such a transesterifica- tion on a system of TEOS and n-propanol, whereas Hasegawa and Sakka [12] studied the exchange reactions between TEOS and isomers of butanol. In this study the influence of methanol, ethanol, 1-propanol and 2-propanol on the hydrolysis-condensation processes of tetra-l-propoxysilane (TPOS), tetra-ethoxysilane (TEOS) and tetra- methoxysilane (TMOS) are investigated.

To obtain an understanding of the mechanisms of these processes many techniques have been used [13-19]. We found that the recording of the hydrolysis time versus gelation time of TEOS, ethanol and water mixtures gives insight into both the hydrolysis and the condensation reactions in this two-step sol-gel process of TE(~S [20-23]. Therefore, we used this procedure t~.investigate the influence of the different alcohols on the hy-

0022-3093/91/$03.50 1991 - Elsevier Science Publishers B.V. All rights reserved

2 T.N.M. Bernards et al. / Hydrolysis-condensation processes of TPOS, TEOS and TMOS

drolysis-condensation behaviour of methoxy-, ethoxy- and 1-propoxysilane.

Additionally, we used 29Si-NMR spectroscopy for these investigations. It has already been found that 29Si-NMR studies give chemical information on the initial stages of the hydrolysis-condensation processes, provided that these measurements are carried out at a low temperature. At room temperature these hydrolysis-condensation processes proceed substantially during measurements [24,25].

2. Experimental

2.1. Procedure

The influence of the amount of water upon the hydrolysis-condensation processes of TPOS and TMOS was investigated using the well-known two-step sol-gel process. The relation between the hydrolysis time in the acid step and the gelation time in the basic step was studied for the mixtures of a tetra-alkoxysilane, its corresponding alcohol and water with a final molar ratio of l :6 :a , where a = 1, 2, 3 and 4, respectively. The amounts of water were added in the acid step to exclude secondary reactions as a consequence of the addition of a basic water fraction. Therefore, 61- of the amount of alcohol was selected in which the required amount of ammonia was dissolved. To prevent immiscibility during the process, especially in the case of TPOS mixtures, in the acid step a molar ratio of the silane compound and its corresponding alcohol of 1 : 5 was used. The acidic catalyst concentration was adapted to obtain equal hydrolysis rates for the TPOS and TMOS mixtures. Also, the net ammonia concentration was chosen so that similar gelation times were obtained, taking into account the amount of water added in the acid step [24].

The effect of the alcoholic solvent on the kinetics and the mechanisms of the hydrolysis-condensation reactions of the silica sol-gel process was also studied. We investigated mixtures of methanol, ethanol, 1-propanol or 2-propanol with each of the silanes TPOS, TEOS and TMOS.

To compare the effect of the solvents, a con-

stant silane concentration in the mixtures is de- sirable. For these experiments a molar ratio of silane and water of I : 2.5 was selected. The amount of water was divided into an acidic (HC1) and a basic (NHaOH) fraction in a ratio of 4:1. The composition area of stable solutions of silane, alcohol and water was the smallest for mixtures of TPOS, 2-propanol and water. For these mixtures, immiscibility could only be prevented when at least a molar ratio of TPOS and 2-propanol of 1:5 was used. To obtain a constant volume for the different alcohols, we selected a molar ratio of silane and alcohol of 1 : 9 using methanol, of 1 : 6.5 for ethanol and of 1 : 5 using 1-propanol or 2-propanol. We renounced an extension of our investigations with butanols because the composition area of stable solutions of silane, butanol and water were found to be very small and correspond with mixtures of very low silane concentrations.

To simplify a comparison of the results obtained for the different mixtures, the experimental circumstances were matched as far as possible. The acid step and the basic step were performed at 50 o C. The water fractions were diluted with an equal weight of the applied alcohol to avoid immiscibility during the addition. The remainder of the alcohol was mixed with the selected silane. The acidic water-alcohol mixture was added in about 0.5 min to the well-stirred silane-alcohol mixture. The hydrolysis experiment was performed for about 200 min, in which period several samples were drawn regularly. To each of these aliquots the specific amount of the basic fraction was added and the gelation time of each sample was then detected. Because the gelation time depends largely on the net NH4OH concentration, a constant concentration was selected for each silane so that for all hydrolysis time versus gelation time curves a gelation time in the range between 5 min and 2 h was measured.

The hydrolysis time, tH, of each sample was defined as the time interval between the addition of the acidic fraction and the moment its basic fraction was added. The gelation time, t r , was defined as the time interval between the moment the basic fraction was added and the moment no more fluidity of the sol was observed when tilting the test tube. This state was always preceded by a

T.N.M. Bernards et al. / Hydrolysis-condensation processes of TPOS, TEOS and TMOS 3

short period of greatly increasing viscosity and therefore inaccuracy in gelation time was less than +_15 s.

2.2. Techniques

The results of the hydrolysis-condensation processes might be influenced by exchange reactions which may occur when a mixture of silane and alcohol containing different alkoxy groups was heated to 50 o C. To verify this, a sample was drawn from each of such mixtures just before the hydrolysis started and its composition determined using a capillary gas chromatography technique.

With 29Si-NMR measurements at -75C we followed the hydrolysis-condensation reactions in the acid step of the two-step sol-gel processes, drawing samples from these mixtures at different hydrolysis times. It had already been found that the drawn samples have to be cooled very rapidly to about -100C to reduce the reaction rate drastically. Also, during the measurements the low temperature has to be maintained. A detailed de- scription of the measuring procedures has already been given elsewhere [24,25]. With the 29Si-NMR technique, differences in degree of dimerization in the acid step, especially at prolonged hydrolysis times, were investigated as a function of the alcoholic solvent used. With this technique exchange effects could also be detected easily.

3. Results

The effect of the amount of water in the hydrolysing mixture upon the relation between the hydrolysis time in the acid step and the gelation time in the basic step is shown in fig. 1 for TPOS, 1-propanol and water mixtures and in fig. 2 for TMOS, methanol and water mixtures, both having a final molar ratio of 1 : 6 : a with a = 1, 2, 3 and 4. The acidic catalyst concentrations of the TPOS mixtures and of the TMOS mixtures were 2 x 10 2 M HC1 and 5 x 10 -4 M HC1 in water, respectively. For the TPOS mixtures, the net ammonia concentration was 6 x 10 -1 M for a = 1, 6 x 10 -2 Mfora=2and6x10-3 Mfor a=3and4. For the TMOS mixtures, the net ammonia eoncentra-

tion was 10 3 M for a = 1 and 2 x 10 -4 M for a = 2, 3 and 4. In the figures showing the relation between the hydrolysis time and the gelation time, through the measuring points, lines are drawn to guide the eye. The results of the TPOS-l -propanol mixtures agree closely with those found for TEOS-ethanol mixtures [24]. The considerable increase in gelation time at prolonged hydrolysis times had been ascribed to a reduction of the amount of silanols caused by condensation in the acid step [20]. The absence of such an increase in gelation time as observed for TMOS-methanol mixtures suggests that in these mixtures practically no reduction of silanols by condensation occurs in the acidic environment. To verify this, 29Si-NMR measurements were performed on samples drawn as a function of the hydrolysis time from these hydrolyzing silane-alcohol mixtures. As an example, the results are given for a = 2 in figs. 3 and 4, because for this amount of water the largest differences in gelation behaviour were measured. As shown, substantial condensation effects were observed for TPOS-l-propanol mixtures, whereas for TMOS-methanol mixtures only small condensation effects were found. In table 1 the 29Si chemical shifts of TMOS, TPOS and their hydroxy monomers are given.

The differences in condensation behaviour can be ascribed to differences in acid catalyst concentration, to the alcoholic solvent used and to the silanols formed during the hydrolysis. To dis- tinguish between these possibilities, first the effect of the alcoholic solvent was investigated for each of the silanes maintaining a constant catalyst concentration. In fig. 5 the effect of the alcoholic solvent upon the relation between the hydrolysis time in the acid step and the gelation time in the basic step is given for TPOS, alcohol and water mixtures with a TPOS concentration of 1.5 mol/1 and a final molar ratio of TPOS and H20 of 1:2.5. As the acidic catalyst, an HC1 concentration of 10 -3 M was used. For gelation, a net NH4OH concentration of 1.5 10 -2 M was chosen. It is shown that using the same HC1 concentration, the minimum gelation time was at- tained after a hydrolysis time of 6 min with methanol as the solvent and 16 rain with ethanol and 1-propanol, whereas with 2-propanol the

4 TN.M. Bernards et aL / Hydrolysis-condensation processes of TPOS, TEOS and TMOS

t-- o~

E 160

t

12080 40

0 " ' ' ' ' "~ # ' ' ' " ' + = 0 20 40 60 80 ~00

t H (min) Fig. 1. The gelation time, tG, as a function of the hydrolysis time, tH, of TPOS, 1-propanol and water mixtures of a final molar ratio

of 1 :(5+1): a with a =1 (A), 2 (), 3 (11) and 4 (+).

minimum gelation time was found only after a hydrolysis time of about 45 min. It was striking that after prolonged hydrolysis the largest increase

in gelation time was observed using 2-propanol as the solvent.

With TEOS or TMOS as the silane compound,

100 E

f 80

60

4O

2O

0 I I k I I

0 20 40 60 80 100

- ' t H (rain)

Fig. 2. The gelation time, tG, as a function of the hydrolysis time, tH, of TMOS, mcthano] and water mixtures of a final molar ratio of 1 :(5+1): a with a =1 (&), 2 (), 3 (11) and 4 (+).

T.N.M. Bernards et al. / Hydrolysis-condensation processes of TPOS, TEOS and TMOS

TPOS: 1 -PrOH:H20 1:5:2

I . . . . I . . . . r . . . . ~ l 120 z

/

/

/

~xx

i i i i { , ~ i

-70 -80 -90 -100 PPM

Fig. 3. 29Si-NMR spectra measured at -75C of samples, drawn after different hydrolysis times from a hydrolyzing mixture of TPOS, 1-propanol and water with a molar ratio of 1 : 5 : 2.

similar results were obtained, as shown in figs. 6 and 7, respectively. However, to realize hydrolysis t ime-gelat ion time curves comparable with those of TPOS, the acidic catalyst concentration has to

be reduced to 3x10 4 M HC1 us ingTEOS and to 7 10 -5 M HC1 with TMOS, which can be ascribed to differences in affinity for hydrolysis of the different silanes. For gelation, a net ammonia

TMOS:MeOH:H20 1:5:2

................ 60

"30

" 15 /2 '

-70 -80 -90 -100

PPM Fig. 4. 29Si-NMR spectra measured at -75 C of samples, drawn after different hydrolysis times from a hydrolyzing mixture of

TMOS, methanol and water with a molar ratio of 1 : 5 : 2.

6 T.N.M. Bernards et aL / Hydrolysis-condensation processes of TPOS, TEOS and TMOS

Table 1 29Si-NMR chemical shifts with respect to TMS in ppm measured at -75C of the monomers Si(OCH3)4_n(OH)n and Si(OCH2CH2CH3) 4 ,(OH),

n Si(OCH3)4_n(OH), Si(OCH2CH2CH3) 4 ,(OH),

0 -78.1_+0.2 -81.5_+0.2 1 -76.0 -78.5 2 -74.4 -76.1 3 - 73.1 - 74.0 4 -72.1 -72.2

concentration was used of 10 -2 M for the TEOS mixtures and of 2 x 10 -3 M for the TMOS mixtures.

As shown in figs. 5-7, similar influences of the different alcoholic solvents were found for each of the silanes. The effect of the alcohols may be caused by differences in degree of dissociation of HC1 in the different alcohols. Another possibility may be the phenomenon of exchanges of alkoxy groups between silane and alcohol. Therefore, a check on exchange reactions was made for all the mixtures of silane and alcohol having different alkoxy groups. Before the hydrolysis process of

such mixtures was started, by adding the acidic fraction, a sample was drawn from the mixture. This sample was investigated using capillary gas chromatography. For none of the mixtures was an exchange of alkoxy groups observed. Some of these mixtures were additionally heated to 50 C and kept at that temperature for 2 h under exclusion of water vapour. Even after this treatment, no exchange of alkoxy groups was observed.

The silane-alcohol mixtures were also investigated with respect to exchange reactions in the presence of different concentrations of the acidic catalyst under the exclusion of water. It was found that at 50 C fast exchange reactions already occur at moderate catalyst concentrations. There- fore, the samples were investigated with 29Si-NMR at -75 o C. It is shown in fig. 8 that for the acidic catalyst concentration, as used in the hydrolysis experiments, the exchange reactions are strong for a TPOS-methanol mixture (A) but negligible for a TMOS-l -propanol mixture (B). However, when the catalyst concentration for the TMOS-l -propanol mixture is increased to 10 -3 M, exchange reactions are also observed (C). From the occur- rence of the different peaks as a function of the

.---- 160 C

E

120

8O

0 ' - - - - A ~ ' ' ' ' ' ' 0 40 80 120 160 200

- ' tH (min)

Fig. 5. The gelation time, to, as a function of the hydrolysis time, tH, of TPOS, alcohol and water mix tures o f a final molar ratio of 1 : x : 2.5 with x = 9 for methanol (A), 6.5 for ethanol ( + ), 5 for 1-propanol (o) and 2-propanol (B).


E

E v

T

160

12ol

80

4O

o _A $ , _ - , _ _~ , , 0 40 80 120 160 200

--' tH (min) Fig. 6. The gelation time, tG, as a function of the hydrolysis time, t H, of TEOS, alcohol and water mixtures of a final molar ratio of

1 : x : 2.5 with x = 9 for methanol (A), 6.5 for ethanol ( + ), 5 for 1-propanol () and 2-propanol ( I) .

exchange time, the peaks could be labelled simply, as shown in table 2. We observed a chemical shift of about 0.85 ppm for each -OCH 3 ~-OCH 2 CHzCH 3 exchange.

The differences in condensation behaviour using different alcohols, as shown in figs. 5-7, were investigated with 29Si-NMR on mixtures of TMOS, with each of the four selected alcohols using a

"-"._C 160 - - - -

E ,

t 120 .a

8O

40

0 40 80 120 160 200

--' t . (min)

Fig. 7. The gelation time, tG, as a function of the hydrolysis time, tH, of TMOS, alcohol and water mixtures of a final molar ratio of 1 : x : 2.5 with x = 9 for methanol (A), 6.5 for ethanol ( + ), 5 for 1-propanol () and 2-propanol (11).


A

-78 -82 -78 -82 -78 -82 PPM PPM PPM

Fig. 8. 29Si-NMR spectra measured at -75C of samples, drawn after an exchange time of 2 min from a mixture of TPOS and methanol, using 10 -3 M HC1 (A) and after an exchange time of 300 rnin from mixtures of TMOS and 1-propanol using 710 -5 M HC1 (B) and 10 -3 M HC1 (C),

respectively.

Table 2 29Si-NMR chemical shifts with respect to TMS in ppm measured at - 75 o C of the monomers Si(OCH3) 4-,.(OCH 2CH 2 CH3)m

m

0 -78.10.2 1 -78.9 2 -79.8 3 - 80.7 4 -81.5

constant catalyst concentrat ion of 7 10 -5 M HC1. We invest igated TMOS mixtures because at the catalyst concentrat ion we used for the hydrolysis, exchange reactions in these mixtures were almost negligible when water was excluded (fig. 8). As an example, in fig. 9 29Si-NMR spectra are given of samples drawn at different hydrolysis times from an acidic TMOS, 1-propanol and water mixture with the molar ratio of 1 : 5 : 2. On com- par ing these spectra with those of TMOS, methanol and water mixtures of the same molar ratio as shown in fig. 4, large differences in ol igomerizat ion were observed. Also, the number of peaks in fig. 9 belonging to hydroxy monomers was much larger than expected. The results of fig. 9 suggest that in the presence of water, in addit ion

TMOS: I -P rOH:H20 1:5:2

rl[ '/ I . . . . i ' ' '

i . . . . I . . . . (10

-70 -80 -90 -100 PPM

I . . . . ,1 320

' ' , i 60 ~+ /

Fig. 9. 29Si-NMR spectra measured at -75C of samples, drawn after different hydrolysis times from a hydrolyzing mixture of TMOS, 1-propanol and water with a molar ratio of 1 : 5 : 2.


to the hydrolysis, a strong exchange of alkoxy groups between TMOS and 1-propanol also oc- curred, followed by hydrolysis of the newly formed silanes. At prolonged hydrolysis times, the condensation rate is influenced by all these types of hydroxy monomers.

The effect of the acidic catalyst concentration upon the exchange behaviour of hydrolysing mixtures of TMOS, 1-propanol and water with a molar ratio of 1:5:2.5 was also investigated. In fig. 10 the influence of the catalyst concentration upon the hydrolysis time-gelation time curves is given. Because of the low hydrolysis rates at low acidic catalyst concentrations, longer hydrolysis times were required to find representative curves. Using a TEOS mixture it was already found that the hydrolysis rate is inversely proportional to the HC1 concentration [20], except at very low acid concentrations [22]. Therefore the gelation time is plotted versus the product of hydrolysis time and catalyst concentration (tr~ CHCl). For gelation, a net concentration of 4 10 -s M NH4OH was used. In fig. 11 the 29Si-NMR spectra of samples of these hydrolysing TMOS, 1-propanol and water mixtures with different acidic catalyst concentra-

Table 3 29Si-NMR chemical shifts with respect to TMS in ppm measured at -75C of monomers present after hydrolysis of a TMOS-1-PrOH or a TPOS-MeOH mixture

S i (OCH 3 )4 - 78.1 + 0.2 Si(OCH 3) s OH - 76.0 Si(OCH 3) 2 (OH) 2 - 74.4 Si(OCH3)(OH) 3 - 73.1 Si(OH)4 - 72.1

Si(OCH2CH2CHg)(OCH3) 3 - 78.9 Si(OCH2CH 2CH3)(OCH 3)2 OH - 76.8 Si(OCH 2CH2 CH3)(OCH3)(OH) 2 - 75.3 Si(OCH 2CH 2CH 3)(OH) 3 - 74.0

Si(OCH2CH 2CH3)2(OCH3) 2 - 79,8 Si(OCH 2CH 2CH 3) 2 (OCH 3)OH - 77,7 Si(OCH 2CH 2CH 3 ) 2 (OH) 2 - 76.1

Si(OC H 2CH 2CH 3) 30CH 3 - 80.7 Si(OCH 2CH 2CH 3 ) 3 OH - 78.5

Si(OCH2CH2CH~) 4 - 81.5

tions but with a constant value of t n HCI are given. From the results it is shown that the decrease in the catalyst concentration is accompa- nied by a decrease in the condensation rate and at

1oo ~- [

- [ E t 80

40 '~ "}" "1" 5

O' __ 1 I _ _ 2

0 20 40 60 80 100

--. t . xC .c , ' l 0 4 (min-mole.1-1)

Fig. 10. The relation between the gclation time, t G, and the product of hydrolysis time and catalyst conccntration, t H x cHcl, of a TMOS, 1-propanol and water mixture with a final molar ratio of 1:5:2.5 using an HCI concentration, clio, in the acid step of

6.410 -5 M (1), 3.210 -5 M (2), 2,9x10 -5 M(3), 2.610 -5 M (4) and 2.1x10 -5 M (5).


TMOS:I-PrOH:HzO 1:5:2

I ' ' I ' ' ' ' I ' ' ' ' I

I . . . . I ' ' ~ ' I ' ' ' ' I

C

B

A I ' ' ' ' I ' ' ~ ' 1 . . . . I

-70 -80 -90 -100 PPM

Fig. i l . ZgSi-NMR spectra measured at -75C of samples, drawn at t H x CHCl = 25 X 10 -4 min mo1-1 from hydrolyzing mixtures of TMOS, 1-propanol and water in a final molar ratio of 1:5:2 with an HC1 concentration of 6.4X10 -5 M (A),

2.9 10- 5 M (B) and 2.1 x 10- 5 M (C).

even lower catalyst concentrations followed by a significant decrease of the hydrolysis rate. These results are in agreement with those already found for TEOS-ethanol mixtures [22].

We attempted to identify the peaks present in figs. 9 and 11. We succeeded in labelling all the monomers on the basis of the following two as- sumptions. First, a chemical shift of about 0.85 ppm for each -OCH 3 --~-OCH2CH2CH 3 exchange (table 2) and second that the chemical shift caused by the replacement of an -OCH 3 group by

an -OH group or of an -OCH2CH2CH 3 group by an -OH group as shown in table 1 is independent of the other alkoxy groups which are attached to the same silicon atom. The results of the monomer labelling are given in table 3.

4. Discussion

On comparing the results of the hydrolysis- condensation process of TPOS-l-propanol and TMOS-methanol mixtures, as shown in figs. 1-4, we found that under identical conditions the hydrolysis rate in the acid step of TMOS is much larger than that of TPOS. However, the dimerization rate in the acid step of silanols originating from TPOS is larger than that of silanols prepared from TMOS. The results of the TPOS-l-propanol mixtures are closely correlated with the results of TEOS-ethanol mixtures described elsewhere [20,24].

A larger amount of water in the mixture increases both the hydrolysis rate and the condensation rate. When no excess of water was used in the mixture (a < 2), the increase in gelation time found for TEOS-ethanol and TPOS-l -propanol mixtures was not observed for TMOS- methanol mixtures. As shown in fig. 4, practically no oligomerization effects were found in the acidic TMOS mixtures. Therefore, at prolonged hydrolysis times a constant maximum silanol concentration is maintained, resulting in a constant low gelation time in the basic step. Because of the high silanol concentration, a much lower NH4OH concentration is needed for gelation of the TMOS mixtures than for gelation of the TPOS mixtures.

The effects of the alcoholic solvent upon the hydrolysis-condensation processes of TPOS, TEOS and TMOS are given in figs. 5-7 for the solvents methanol, ethanol, 1-propanol and 2-propanol. For each silane, the hydrolysis rate in the acid step is the highest in methanol, whereas with 2-propanol as the solvent the lowest reactivity was observed and about equal hydrolysis rates were found for the solvents ethanol and l-propanol. This difference can be ascribed to differences in proton activity of the catalyst in the solvents or to exchange reactions. However, regarding the equal


hydrolysis rates in ethanol and 1-propanol as well as the differences in hydrolysis rates found for TEOS and TPOS, the effect of the proton activity of the catalyst is dominant. With respect to ethanol and 1-propanol, the activity of the catalyst in methanol will be larger, whereas for 2-propanol it will be smaller.

For the three silanes, the dimerization rates in the acid step are also dependent on the alcohol used, but now in the opposite sequence: 2-propanol > 1-propanol > ethanol > methanol, where the dimerization rate is very small using methanol. However, a significant increase in gelation time at prolonged hydrolysis times is observed for the TPOS-methanol mixture in fig. 5. This increase has to be ascribed to the high degree of exchange of alkoxy groups (fig. 8) in this mixture, resulting in the formation of different types of silanes (table 2) in the solvent, being a mixture of methanol and 1-propanol.

It has already been found [12] that in the absence even of traces of water, exchange reactions in silane-alcohol mixtures occur when an acidic catalyst is present. The rate of exchange depends strongly on the catalyst concentration and is negligible without the catalyst. We found that when the exchange reactions at low HCI concentrations were followed with 29Si-NMR, peaks of differing chemical shifts arose as a function of time which could be simply labelled. As shown in table 3, a chemical shift of about 0.85 ppm at -75C was measured for the -OCH 3 -OCH2CH2CH 3 exchange, independent of the type of group (alkoxy or -OH) also attached to that silicon atom. As a result of the exchange reactions we finally observed a chemical shift of

- 81.5 ppm at -75 C, corresponding with the chemical shift observed for TPOS. This value dif- fered from the results recorded by Pouxviel and co-workers [11] for mixtures of TEOS and n-propanol. In their 29Si-NMR spectra, measured at room temperature, they also found a constant chemical shift when an ethoxy group was replaced by a propoxy group. However, they reported for TPOS a chemical shift of - 85.9 ppm with respect to TMS whereas we found a chemical shift of -82.0 ppm at 25C for both TEOS and TPOS (tetra-l-propoxysilane). The use of 2-propanol in-

stead of 1-propanol in their mixture may be the explanation. The chemical shifts they reported can be realized simply by exchange reactions of mixtures of TEOS and 2-propanol.

From the labelling presented in table 3, it can also be concluded that the change in chemical shift caused by hydrolysis is practically independent of the nature of the alkoxy groups still attached to the silicon atom, but is largely dependent on the nature of the replaced alkoxy group and on the number of -OH groups already attached to the silicon atom.

From the results given in figs. 10 and 11 it can be concluded that at low catalyst concentrations the dimerization effect decreases considerably. At even lower HC1 concentrations, the hydrolysis rate also becomes very small. Similar results had already been reported for TEOS [22].

The differences in dimerization rate in the acid step can be explained starting from the dimerization mechanism of silanols in an acidic environment given by

(RO)3-Si -OH + H+~ (RO)3-Si-OH~-, (1)

followed by

(RO)3-S i -OH ~" + HO-Si - (OR)3

-o (RO)3-S i -O-S i - (OR)3 + H3 O+, (2)

where R is given by H, any alkyl group, or by Si-(OR)3. The dimerization rate depends on the concentration of the activated silanol complex, which in turn depends on the concentration and the degree of dissociation of the catalyst in the solvent.

The results presented in figs. 10 and 11 can therefore be ascribed to the decrease of the activated silanol complex concentration as a result of the decrease of the catalyst concentration. However, the differences in dimerization found for the different solvents, as shown in figs. 5-7, cannot be explained by differences in proton activity of the catalyst. For instance, using methanol, for which the proton activity of the catalyst is the strongest and therefore the hydrolysis rate the highest, practically no dimerization was found.

In our opinion, the protons from the catalyst

12 T.N.M. Bernards et al. / Hydrolysis--,condensation processes of TPOS, TEOS and TMOS

can react with silanols and also with water or alcohol to form an activated complex according to

H++ H:O ~ H30 + (3)

and

R ' -OH + H+~ R'-OH. (4)

The nature of the alkyl group R' will influence the equilibrium position of eq. 4 [26]. The sequence of protonation of the complexes is given by

H30+> CH3OH f > (RO)3-Si-OH~-

> CH3CH2OH2 ~ > CH3CHzCHzOH ~

> (CH3)2CHOH; .

The concentration of each activated complex depends on the composition of the mixture. The base strength of the (RO)3-Si-OH~- complexes depends on the nature of the -OR groups and is smaller when more non-polar alkoxy groups are present and greater with an increasing number of -OH groups.

Starting from these hypotheses, the results can be explained unambiguously. When water is present in the mixture, the H3 O+ complex is dominant [26], resulting in high hydrolysis rates of the different silanes. In the absence of water, the distri- bution of the protons of the catalyst over the complexes of eqs. (1) and (4), depends on the alcoholic solvent and the nature of the silanol complex according to

(RO)3-Si-OH2 ~ + R ' -OH

(RO)3-Si -OH + R'-OH~-. (5)

When methanol is used as the solvent, the concentration of the activated silanol complex is small and the concentration of the methanol complex high. The low concentration of the silanol complex results in small dimerization effects. Using 2-propanol as the solvent, the activated silanol complex concentration is high compared with that of the activated 2-propanol complex, resulting in strong dimerization effects. With ethanol or 1-propanol as the solvent, intermediate effects were observed.

The relative amount of water in the mixtures has a strong influence on the composition of the activated silanol complexes and therefore on their

strengths. A larger amount of water results in a silanol complex with a larger number of -OH groups. This effect increases the base strength of the activated complex and results in an increase in the concentration of this complex, thus decreasing the influence of the alcoholic complexes upon the hydrolysis-condensation processes of the silanes.

It is shown that by using an alcoholic solvent with an alkoxy group different from those of the silane, the number of types of silanol monomers may increase from four to ten monomers (tables 1 and 3). Also, the number of types of dimers etc., may increase drastically. When a mixture of more alcohols is used as the solvent, an even more complex situation may occur. In practice, to realize coating solutions with the required rheological properties, mixtures of several alcohols are used. As a consequence of exchange reactions, followed by hydrolysis of the newly formed products, the composition of such coating solutions may vary as a function of time, temperature, etc. Without a fundamental knowledge of the possible reactions, the properties of such coating solutions will change in an unpredictable way.

5. Conclusions

(1) The hydrolysis rates of the silanes in an acidic environment decreases in the sequence: TMOS > TEOS > TPOS.

(2) For all the silanes used it holds that an increase in the acidic catalyst concentration as well as an increase of the relative amount of water in the mixture results in a higher hydrolysis rate in the acid step.

(3) The alcoholic solvents have a remarkable influence on the hydrolysis rate of the silanes in the acid step in the sequence methanol > ethanol, 1-propanol > 2-propanol, caused by differences in the proton activity of catalyst in the alcohols.

(4) An opposite sequence for the influence of the alcoholic solvents was observed for the dimerization rate of silanols in the acid step. The differences in dimerization rate are ascribed to differences in base strength of the activated silanol complexes with regard to those of the protonated alcohols.


The authors l ike to thank J .M.E. Baken for many

va luab le d iscuss ions and W.P .M. Rut ten for per- fo rming the 29Si -NMR spect roscopy exper iments .

References

[1] J. Zarzycki, in: Glass ... Current Issues, eds. A.F. Wright and J. Dupuy (Nijhoff, Dordrecht, 1985) p. 203.

[2] S.H. Wang and L.L. Hench, in: Ultrastructure Processing of Ceramics, Glasses and Composites, eds. L.L. Hench and D.R. Ulrich (Wiley, New York, 1986) ch. 21.

[3] L.L. Hench and M.J.R. Wilson, J. Non-Cryst. Solids 121 (1990) 234.

[4] C.J. Brinker and G.W. Scherer, Sol-Gel Science (Academic Press, New York, 1990) ch. 3.

[5] L.C. Klein, Sol-gel Technology for Thin Films, Fibers, Preforms, Electronics and Speciality Shapes (Noyes, New Jersey, 1988) p. 50.

[6] M.A. Aegerter, J. Non-Cryst. Solids 121 (1990) 294. [7] T. Kawamura, H. Kawamura, K. Kobara and A. Saitoh,

SID Int. Symp. (Anaheim, CA, 1988) 395. [8] D.C. Bradley, R.C. Mehrotra and D.P. Gauer, Metal

Alkoxides (Academic Press, London, 1978) p. 33. [9] J.D. Mackenzie, in: Ultrastructure Processing of Ceramics,

Glasses and Composites, eds. L.L. Hench and D.R. Ulrich (Wiley, New York, 1986) ch. 12.

[10] C.J. Brinker, K.D. Keefer, D.W. Schaefer, R.A. Assink, B.D. Kay and C.S. Ashley, J. Non-Cryst. Solids 63 (1984) 45.

[11] J.C. Pouxviel, J.P. Boilot, J.C. Beloeil and J.Y. Lallemand, J. Non-Cryst. Solids 89 (1987) 345.

[12] I. Hasegawa and S. Sakka, Bull. Chem. Soc. Jpn. 61 (1988) 4087.

[13] R.A. Assink and B.D. Kay, Mater. Res. Soc. Symp. Proc. 32 (1984) 301.

[14] I. Artaki, M. Bradley, T.W. Zerda and J. Jonas, J. Phys. Chem. 89 (1985) 4399.

[15] L.W. Kelts, N.J. Effinger and S.M. Melpolder, J. Non- Cryst. Solids 83 (1986) 353.

[16] R.A. Assink and B.D. Kay, J. Non-Cryst. Solids 99 (1988) 359.

[17] T.W. Zerda, I. Artaki and J. Jonas, J. Non-Cryst. Solids 81 (1986) 365.

[18] A.H. Boonstra, T.P.M. Meeuwsen, J.M.E. Baken and G.V.A. Aben, J. Non-Cryst. Solids 109 (1989) 153.

[19] L.C. Klein and G.J. Garvey, J. Non-Cryst. Solids 38&39 (1980) 45.

[20] A.H. Boonstra and T.N.M. Bernards, J. Non-Cryst. Solids 105 (1988) 207.

[21] A.H. Boonstra and J.M.E. Baken, J. Non-Cryst. Solids 109 (1989) 1.

[22] A.H. Boonstra and J.M.E. Baken, J. Non-Cryst. Solids 122 (1990) 171.

[23] M.J. van Bommel, T.N.M. Bernards and A.H. Boonstra, J. Non-Cryst. Solids 128 (1991) 231.

[24] A.H. Boonstra and T.N.M. Bernards, J. Non-Cryst. Solids 108 (1989) 249.

[25] A.H. Boonstra, T.N.M. Bernards and J.J.T. Smits, J. Non-Cryst. Solids 109 (1989) 141.

[26] B.E. Conway, J. O'M. Bockris and H. Linton, J. Chem. Phys. 24 (1956) 834.

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