J. Phys. Chem. 1995, 99, 4648-4654
4648
Strained Siloxane Rings on the Surface on Silica: Their Reaction with Organosiloxanes, Organosilanes, and Water? A. Grabbe,* T. A. Michalske, and W. L. Smith Sandia National Laboratories, Department 1114, M.S.0344, Albuquerque, New Mexico 87185 Received: September 1, 1994; In Final Form: December 28, 1994@
The siloxane network on the surface of highly dehydroxylated silica (Cab-0-Sil) contains strained siloxane rings. The =(Si0)2= dimer rings are edge-shared tetrahedra, some of which have a lessened reactivity due to an attached silanol. We use infrared spectrometry to show that the dimer rings react with organosiloxane molecules or with water at comparable rates, depending on the structure of the organosiloxane. A polar bond component is a necessary condition for a rapid reaction with dimer rings, since nonpolar organosilanes react orders of magnitude slower than does water or organosiloxanes. Our data show that the important features of the reacting bond that control the rapidity of the reaction are its polarity, steric accessibility, and bond strain. The reactions require approximately 6 orders of magnitude of gas exposure to go from 1% to 99% completion, which shows that the surface is highly heterogeneous. We have applied a simple model of the surface’s heterogeneity to our kinetic data, incorporating a linearly distributed activation energy in the ring population. Using this model, a fit to the data shows the spread in activation energy is approximately 40 kJ/mol in all cases. Our results show that the rapid organosiloxane reaction with dimer rings produces hydrolytically stable coupling points that cannot be readily synthesized with conventional silane chemistry.
1.0. Introduction Adhesive bonding to silica surfaces is important in a wide range of technological applications. Examples include reinforced polymer matrix composites, laminated safety-glass materials, and window sealants. Under normal conditions, the as-formed silica surface has sufficient time to react with atmospheric water to become silanol (ESiOH) terminated. The silanol groups dominate the chemistry of the hydroxylated silica surface and provide sites for attachment of silane coupling agents according to the following water-mediated scheme: RSi(OMe),
-+ -
+ H,O + HOSiE
MeOH 2[RSi(OMe),]OSiz 2MeOH [RSi(OMe),]OSi=
[RSi(OMe),]OSiE
+ H,O
+H20 + =SiO[R( OMe)] SiOSi[R(OMe)]OSi=
-
-
H20 MeOH
+ =SiOSiR(OMe)OH
(1)
These reactions create a cross-linked siloxane network containing unreacted silanol and methoxy species; the organic functional groups (R) can react with and control adhesion to organic coatings.’ In some manufacturing processes, organic coatings must be applied to silica surfaces that are not fully hydroxylated. For example, optical cable is made by drawing and rapidly quenching a silica fiber from a pool of molten glass. A protective polymer cladding, rapidly applied before handling or storing the fiber, preserves the fiber’s high strength.2 Subsequent adhesive failure of the polymer cladding can cause local mechanical stresses that greatly increase the optical losses in the fiber.2 An adhesive bond that is too strong can hinder the stripping of the cladding, which is required to form connections
’
Sandia document SAND94-1983J. Work supported by the U.S. Dept. of Energy contract DE-AC04-94AL8500 and AT&T Laboratories. @Abstractpublished in Advance ACS Abstracts, March 1, 1995.
0022-365419512099-4648$09.00/0
between fibers. Therefore, it is important to predict and control the adhesive bond made to the pristine silica ~ u r f a c e . Unfor~ tunately, the fiber’s exposure to the air during processing is not long enough for ambient water vapor to react with the surface to form a saturation coverage of silanols. For this reason, it is difficult to predict and control the quality of the adhesive bond, which is currently made by adding conventional silane coupling agents to the polymer coating. In this work, we investigate a new class of coupling agents that is better suited than conventional silane coupling agents to react with the pristine silica surface. Most of the siloxane bonds that exist on the pristine silica surface are chemically stable and do not make good candidates for reaction with coupling agents. However, some fraction of the siloxane bonds exist either as strained edge-shared tetrahedra (Si0 dimer rings) or much less reactive S i 0 trimer ring^.^-^ The ring strain in these structures greatly increases the reactivity of their siloxane bonds, especially in the dimer rings. These structures form from silanols on silica surfaces, when the surfaces are heated below the melting point of silica. Heat enables migration and condensation of neighboring s i l a n ~ l s , ~ -making ‘~ new ring structures. Disproportionation reactions between (SiO), rings also occur below the melting point,”-I2 so dimer and trimer rings can form in the absence of silanols. Dimer rings exist in two form^,^-'^ shown below: H
These rings’ labels are d4 (no associated silanol) and d3 (one associated silanol), respectively. The bond angles in the ring are nearly 90°, indicative of a highly strained structure; the d4 species is the more reactive of the t ~ 0 . IBecause ~ gas-phase reactions eliminate dimer rings on dehydroxylated silica sur0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 13, 1995 4649
Strained Siloxane Rings on the Surface on Silica faces, it has been inferred that dimer rings only exist on the surface of solid ~ i l i c a . ~ , ' ~ Although conventional coupling agents have been shown to react with dimer rings, their reaction product is readily hydrolyzable. The reaction mechanismI4 and subsequent hydrolysis scheme is
The formation of hydrolyzable groups is undesirable, since they provide a nucleation site for the condensation of water that causes debonding at the silica-polymer i n t e r f a ~ e . ~ . ' ~ - ' ~ A simple thought experiment beginning with the desired reaction products (siloxane termination at both points provided by ring opening) and then reversing the reaction direction suggests that disiloxanes (R3Si-O-SiR3) may be more appropriate coupling agents for the pristine surface:
(4)
Using infrared spectrometry (IR) and temperature-programmed desorption mass spectrometry (TPD), we examined the reaction of various organosiloxaneswith strained dimer rings on thermally treated silica surfaces. Our results show that the reaction mechanism postulated in scheme 4 can proceed at a rate comparable to that of ring opening by hydrolysis. We compare data from a variety of reactant molecules to demonstrate the effects of bond polarity, bond strain, steric effects, and the surface's heterogeneity on the surface reaction rate.
2.0. Experimental Section 2.1. Equipment and Materials. The experiments were conducted in a turbo-pumped vacuum chamber that had a Torr base pressure. The chamber had KBr windows to transmit infrared light from a Fourier transform infrared (FTIR) spectrometer through the sample to a narrow band HgCdTe detector; the entire beam path was kept within a drybox. Capacitance manometers were used to measure chamber pressures." The chamber also had a residual gas analyzer for thermal desorption experiments. We obtained hexamethyldisiloxane ([(CH3)3Si]20, hMdS), hexamethylcyclotrisiloxane ([OSi(CH3)2]3, hMctS), tetramethyldisiloxane ([H(CH&Si]20, tMdS), and ethylsilane (H3SiC2H5, EtS) from Huls America'* and used them without further purification. The level of impurities in the organosilanes was determined by infrared spectrometry to be too small to affect any of the results. Water was purified to 18.5 MSZ resistivity by ion exchange. The water and silanes were degassed by the freeze-pump-thaw method. Colloidal silica (Cab-0-Si1 M-5) was obtained from Cabot.I9 Cab-0-Si1 M5 has a structure composed of aggregates of spherical particles (-7 nm radius) and has a surface area of 200 k 25 m2/g. 2.2. Experimental Procedures. We made samples by depositing -1-3 mg of Cab-0-Si1 sol (in methanol) onto a
0.53
"
"
~
'
"
'
~
'
'
"
~
"
i
'
'
t
?
O.4St
Figure 1. Infrared absorption spectra of a highly dehydroxylated silica sample before and after exposure to water vapor. The inset shows the 882 and 902 cm-' dimer ring peaks and the shoulder at 930 cm-'. The water exposure completely removed the dimer rings and created hydrogen-bonded silanols on the surface. The infrared signature of the silanols is increased and broadened by this treatment.
central stripe of a tungsten mesh. This geometry kept the silica particles far enough away from the electrical leads to ensure that the temperature across the sample would be uniform. The area covered was 1 cm2; the mean sample thickness was -8 pm. The electrically heated samples' temperature was controlled by using feedback from a K-type thermocouple. The samples were cleaned and dehydroxylated by heating them in oxygen at 875 K and were heated in vacuum for 1 h (13251375 K) to make dimer rings. Figure 1 shows the infrared spectrum of highly dehydroxylated silica before and after the dimer rings were removed by treatment with water. The absorption bands at 900 and 882 cm-I were used to note the presence of dimer rings. The experiments were performed at constant temperature; the samples were briefly (50-200 s) exposed to a reactant, pumped and scanned with the IR spectrometer. To span orders of magnitude in exposure, the process was cycled with exponentially increasing gas pressures. TPD experiments were performed by heating the supporting mesh at -1 Ws and simultaneously tracking as many as six desorbing species with the residual gas analyzer. 2.3. Comparisons between Molecules. We surveyed the reactivity of several compounds searching for the best compromise between the reaction rate, spectral line broadening, and product stability as a function of temperature. At 480 K, the temperature is low enough to prevent pyrolysis, and the reaction rate is measurable with all the chemicals. Higher temperatures cause significant line broadening in the infrared spectra, which complicates the analyses. Therefore, unless otherwise stated, all the reactions were done at 480 K. We have investigated the reaction kinetics of the four silanes, hMdS, tMdS, EtS, and hMctS, and compared the results to the reaction kinetics for water. Examining the reaction of rings with water helps show how well the silane reactions can compete with hydrolysis and provides data to compare with the literature. hMdS provides a reference with which to compare other siloxanes because it is reactive only along its Si0 bond. hMctS is a slightly strained ring, which may accelerate the reaction. tMdS is a molecule with technological potential. The S i 0 bond is more accessible in tMdS than in hMdS, and its silane (SiH) group can serve as a surface coupling point:24 =Si-H
iH,C=CH-R
uv
%3i-CH2-CH2-R
(5)
4650 J. Phys. Chem., Vol. 99, No. 13, 1995
Grabbe et al.
We also used EtS to separately examine the reactivity of the SiH bond toward dimer rings, enabling a better understanding of the reaction between dimer rings and tMdS.
3.0. Data Analysis
+ O't
3.1. Diffusion. Because of the porous nature of the samples, it was necessary to determine if gaseous diffusion would limit the overall reaction rate or affect TPD experiments. To calculate the diffusion constant (D), we used an appropriate model for molecular
a " Y
t
"^I 0.4
ua ,,I X
X
X
o
0.2 0
M is the molecular weight of the gas, and we can estimate the mean pore radius ( r ) of the sample by assuming the colloidal particles pack in a face-centered cubic array. D is -5 x cm2/s for tMdS at 480 K. With this value of D,about 0.1 s is enough time to exchange 99.9% of the gas between the pellet interior and the sample chamber. Since the dosing times were never less than -30 s, we are confident that the reactions are not diffusion limited. However, the diffusion time is long enough to affect thermal desorption experiments. Therefore, the TPD data can only be used to determine if the reactants were chemisorbed or physisorbed. 3.2. Kinetics. The heterogeneity of the surface is manifested in the kinetics; therefore, an appropriate model is necessary to take their measure. A simple model can be derived by assuming that the surface can be broken up into small elements upon which simple first-order kinetics occur and that the activation energy for each surface element's reaction varies linearly over the population of elements.21 Now consider, a surface of individual elements, 8,. For a first-order mechanism, we write the following rate equation:
0
la4
A
0.01
0 ,x0 ,;,; ,
x
,,,x
,,,,,,I
,
, , , ,,,,!
,
,
,
loo00
1 100 Exposure (Toncswonds)
Figure 2. Relative extents of reaction, expressed as the fraction, 0, of dimer rings lost versus total exposure. Key: EtS (x), hMdS (O),tMdS (D), water (0),hMctS (A). The solid line shows a fit to the tMdS data using eq 8. All the data in this figure was taken at 480 K. I
"
t
"
400
1OOO
1200
1400
Kehrln [dT/dt= 0.91 KIsl
(7) The elements 8, are distributed by ds, p(t) is the gas pressure as a function of time, the activation energy is a linearly increasing function of the distribution parameter s, and the heterogeneity parameter is a . To simplify the equations, the rate constant k' can be rewritten as k = Ke-EJRT, and the exposure can be defined as E = j p(t) dt. Deriving the total surface coverage (e), we separate the variables in eq 7, and then integrate over all the elements 8, by ds. Thus, 8 becomes:
where a' = a/RT. There are efficient algorithms which can be used to evaluate the second integral.22 Since our data fits eq 8 within experimental error, the model is adequate for our purposes. Because the exposure multiplies the rate constant, we can determine the ratios of rate constants for different reactions by scaling the data to overlay each other. This calculation is valid as long as the coverage dependence of the activation energy is the same in both reactions, otherwise the calculation is model independent. 3.3. Coverage. When we followed the reactions to completion, we could normalize the coverage to span 0- 1. In the other cases, the data was normalized by comparing the amount of dimer seen in different experiments at the start, and by the relative (integrated) intensities of the bands used to make a quantitative measure.23 Unfortunately,there is no guarantee that
Figure 3. Temperature-programmeddesorption from a 0.2 g pressed pellet of silica that had been exposed to 10 Torrs of tMdS. Qualitatively, the desorption maximum at 1200 K implies a covalent reaction
with the surface. The absence of any species heavier than mass 59 ([HSi(CH3)2]'+)rules out the possibility of unreacted tMdS being present in the sample.
the oscillator strengths of a given functional group are the same in a surface species as they are in a gas-phase species. Therefore, the data normalized in this fashion are approximations. This procedure was checked by applying it to experiments that had run to completion, and in all cases the result agreed to within 30% of the correct coverage.
4.0. Results
4.1. General Features. With the exception of hMctS, the reaction kinetics appear to be similar in all cases. That is to say, the shape of the coverages as a function of dosage all appear to have the same shape. Relative to water, the rates are EtS 1/4400, hMdS 1/1200, and tMdS 1/3 (see Figure 2), at 480 K. The common feature in these reactions is the need for a wide range of exposure to drive the reactions from 1% to 99% completion, suggesting that the silica surface is highly heterogeneous. We are certain that the siloxane-dimer ring reaction is covalent; th4dS-treated silica's main thermal desorption peak for mass 59 ([CH&SiH]*+) occurs at 1200 K (see Figure 3). We did not detect higher mass fragments, which means that tMdS was not physisorbed or reformed by the heating. The actual coverage, 8, in each case, was determined by the various means discussed below.
J. Phys. Chem., Vol. 99, No. 13, 1995 4651
Strained Siloxane Rings on the Surface on Silica
r
0.07
Dimer loss, SlOH galn
L
n,
0.06
4
0.05
f
amplitude
0.6
0.04
a
I
0.4
0.2
0 0
0.2
0.4
0.6
0.8
1
e Figure 4. Growth of free silanol due to the water-ring reaction. The free silanol is estimated by the 3749 cm-I amplitude and plotted against the loss of dimer rings (8). The difference from the line of slope 1 is the growth of hydrogen-bonded species. The hydrogen bonded silanol absorption band is centered at -3550 cm-I. It is not detectable until the reaction is -25% complete. The 930 cm-' band decays from the start of the reaction until -75% completion but does not correlate with the behavior of the silanols or the 882 and 902 cm-] peaks.
4.2. Water. The depletion of dimer rings by water was followed by integrating their IR bands at 882 and 902 cm-I. The increase in the silanol band matches the dimer loss, which is consistent with earlier observation^.'^ By least squares, eq 8 fits the data using two parameters (at 90% confidence a = 44 & 2 kJ/mol and k = 370 & 80 Torr-%'), within the noise level of the data. The exposure caused the total amount of silanol on the surface to increase by a factor of -3.7, permitting an estimate of the d4 to d3 ratio. The net increment in silanol concentration was 2.7, and since there are two silanols added per ring site, the net loss of dimer rings was 1.35. If we assume that all the original surface silanols were attached to d3 sites, then the starting concentration of d3 sites was 1, and that of d4 sites was 0.35. Therefore the fraction of d4 sites must have been no less than 0.3Y1.35 -25% of the population. Dimer rings with a d3 structure react with water to make geminal silanols that efficiently hydrogen bond.I3 Therefore, the IR band for hydrogen-bonded silanols roughly corresponds to the d3 sites, and the band for free silanols corresponds to the d4 sites. We can use this information to qualitatively estimate the relative reactivity of each kind of site. Although free silanols have a narrow infrared absorption peak, the hydrogen-bonded silanols have a characteristically broad absorption that is redshifted and slightly o ~ e r l a p p i n g .By ~ ~integrating the absorbance of the entire silanol band and subtracting the intensity of the 3749 cm-' absorption, we can estimate the growth of both free and hydrogen-bonded silanols. The hydrogen-bonded silanols grow in relatively slowly during the reaction (see Figure 4), suggesting that the d3 structure is somewhat less reactive than the d4 structure. The reaction causes an infrared absorption (Figure 5 ) at 930 cm-I to decay rapidly. The 930 cm-' absorption disappears when the overall reaction is -75% complete. We cannot correlate the time dependence of this peak either with the dimer rings or with the silanols. This peak might be due to a population of dimer rings that are bent by their connection to the lattice, changing their symmetry from D2h to CzV;bands that are normally Raman active thereby become IR active. This interpretation should be taken cautiously because the weak 930 cm-' band is superimposed on a steep shoulder, whose slight shifts in position are an unavoidable source of experimental
-0.014 ' 3850
'
'
'
f
1
~
3750
3650
3550
3450
3350
cm.1
Figure 5. Infrared absorption difference spectra of highly dehydroxylated silica exposed to 0.001 23 and 485.9 Torrs of water. We scale up the data at lesser (early) exposure by 16.53, comparing band shapes at high (late) and low exposures. A significant fraction of the reaction product causes an absorption loss at 934 cm-l (inset) at low exposures. This implies that the ring site's local environment is heterogeneous. The low-dosage spectrum shows a higher ratio of free OH (3749 cm-' peak) to hydrogen-bonded OH (3460-3620 cm-' region) than does the high-dosage spectrum.
error. There was no excess reaction with other surface species (e.g., trimer rings). 4.3. Ethylsilane (EtS). EtS reacts with rings only along its SiH bond. We were unable to complete the reaction because it was too slow. Therefore, the approximate methods described in the Experimental Section were used to normalize the data. The relative reaction rate is -1/2200th of the tMdS rate (see Figure 2), which suggests that a polar component in the reactive bond accelerates the reaction with dimer rings. A reaction with the silanols was not observed until we used doses in excess of 60 000 Torrs. At 60 000 Torrs the loss of silanols is barely detectable. This means that the SiH group in tMdS is unlikely to contribute to its net reactivity to silanols or dimer rings. 4.4. Hexamethyldisiloxane (hMdS). hMdS reacts with dimer rings only along its Si-0 bond, and the overall reaction rate is comparable to that of EtS. Although the reaction was not completed, the estimated rate is -1/6OOth of that of tMdS, suggesting that the S i 0 bond in hMdS is sterically hindered. Using eq 8 and neglecting mass effects in the pre-exponential factors, we estimate the difSerence in activation energy from the tMdS reaction to be 26 kJ/mol (Le., RT ln(600)). A portion of the 3749 cm-' silanol peak becomes red-shifted during the course of the reaction (Figure 6). This suggests that the reaction perturbs but does not eliminate the silanols attached to the d3 sites. By dividing the apparent silanol loss by the hydrocarbon gain, we can see (Figure 7) that this ratio increases logarithmically with exposure. This is consistent with our interpretation of the water reaction, that the less reactive population is associated with surface silanols (d3 sites). 4.5. Hexamethylcyclotrisiloxanes (hMctS). hMctS is a planar ring whose strain energyz6 is 61 kJ/mol. Due to an IR peak at 814 cm-I, hMctS has a weak IR absorbance tail in the 840-980 cm-l region. We used the hMctS gas-phase spectrum to subtract this tail from the data, permitting us to observe the loss of dimer rings. We did this by nulling the hydrocarbon band in the data. During the reaction, the apparent dimer ring signal eventually decays to a constant value, while the hydrocarbon bands continue to grow in. If the subtraction was wrong,
4652 J. Phys. Chem., Vol. 99, No. 13, 1995 O.Oi
Grabbe et al.
T
0.004
.0.012-l"' 3850
;
"
'
"
"
3750
3800
!
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:
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jegg
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"
' 1 , ' ;
"
,''',!
'''.'','!
100
1W
" ' ' , ' , I
10 1000 Exposun (Torr-mooonds)
"""!
'
Figure 8. Extents of reaction, 8, for tMdS at 480 K measured by silane (0)and hydrocarbon (0),for tMdS at 350 K measured by silane (W) and hydrocarbon (U), for hMctS at 480 K measured by dimer loss (e) and hydrocarbon gain (0),for hMctS measured by dimer loss (A)and hydrocarbon gain (A) on a sample pre-exposed to 280 Torrs of water. tMdS does not polymerize at 350 K. The polymerization of hMctS at 480 K is detectable at -60 Torrs of exposure. The reaction between hMctS and rehydroxylated silica is slow, and there is no detectable polymerization. We attribute the initial growth and subsequent decay of free silanol (inset) to condensation of chain ends (see main text). To determine the reactivity of the silanols toward hMctS, a sample was pretreated with water and then reacted with hMctS. A very slow reaction with hMctS occurred. Initially, the 3749 cm-' amplitude grows in, but the rest of the silanol band shows a loss. Later, the 3749 cm-' amplitude starts to decay until there is a net loss of silanol altogether (see inset, Figure 8). The reaction of a silanol with hMctS transfers the silanol to the end of the poly(dimethylsi1oxane) chain. This silanol may later react with another hMctS molecule, or with another silanol causing a net loss of silanols. The reaction scheme is:
1 "
1
0.01
Exposun (Ton Seconds)
Figure 6. Difference spectra in the OH region of silica samples, reacted with hMdS and tMdS at 480 K. The reference spectra in both cases is that of the dehydroxylated surface. The result of the hMdS reaction is to deplete the 3749 cm-' peak and grow in a red-shifted 3745-3650 cm-I band. This is the result of a reaction with d3 sites; the associated silanol is perturbed but not reacted. Exposure to tMdS depletes the sample of OH.
.0.2i ' 0.001
101
'
1on
Figure 7. Ratios of the extent of apparent OH loss (measured at 3749 cm-') to growth in total hydrocarbon: reactions of dehydroxylated surfaces with tMdS (W) and hMdS (e)at 480 K. The ratio of OH loss to SiH gain (0)for tMdS corrects for polymerization. The ratio increases logarithmically over the entire exposure range with hMdS but only during the latter stages of the tMdS (0) reaction. The polymerization of tMdS starts at about 2 Torrs, causing a decrease in this ratio (W) for exposures exceeding 200 Torrs. Inset: Ratio of OH loss to SiH gain, on a sample pretreated with 3500 Torrs of hMdS then reacted with tMdS. Although the early stage is noisy due to the division of small numbers, the gain of SiH is proportional to the loss of OH.
Si
Ill
then the apparent ring signal would have increased proportional to the hydrocarbon growth. Therefore, the subtractive method was correct. hMctS reacts about as fast as tMdS, although the shape of the coverage as a function of exposure is different (see Figure 2 ) . The differently shaped coverage suggests that hMctS responds differently to the heterogeneity of the surface; a comparison of the rates to obtain an estimate of the relative activation energy for the overall reaction is not possible. The hydrocarbon signal tracks the dimer signal very well before diverging at about 90 Torrs of exposure. Therefore, hMctS must be reacting with another kind of site, possibly trimer rings. Simpler explanations for this are a polymerization, or silanol reaction. Under proper conditions hMctS is known to polymerize; the commercial use of hMctS is as a monomer in the production of poly(dimethyl~iloxane).~~
Si
Ill
(9)
The mechanism in eq 9 is sufficient to explain these results; therefore, evidence for a reaction between hMctS and trimer rings is lacking. 4.6. Tetramethyldisiloxane (tMdS). In this reaction a growing 909 cm-' peak masks the dimer ring bands (see Figure 7). Although we could qualitatively observe the decay of the dimer rings, we could not quantitatively subtract the interfering peak, therefore we used the SiH peak (2160 cm-') to infer the coverage. A representative curve in Figure 8 shows the growth of SiH and hydrocarbon bands. The relative rate at 350 K (-0.14) implies a base activation energy of -20 kJ/mol, roughly onefifth the strain energy per bond of the dimer ring. A fit of the data at 480 K (Figure 2 ) to eq 8 yields a heterogeneity parameter close to the value obtained with water (with 90% confidence a
J. Phys. Chem., Vol. 99, No. 13, 1995 4653
Strained Siloxane Rings on the Surface on Silica 0.087
reacts with silanols along its SiH bond, not its S i 0 bond. However, this reaction is much faster than its analogue with EtS.
0.05
5.0. General Discussion
4000
2800
"1
400
le00
Figure 9. Difference spectrum of a dehydroxylated silica pellet before and after 10 Torrs of exposure to tMdS (350 K). The 909 cm-' peak is due to the reaction product, masking the loss at 885 cm-'. Because of the differing temperatures and environments of the Si(CH&H group in gas phase tMdS and in surface species, a spectral subtraction to determine the loss in dimer is not practical in this case, except to note its qualitative depletion.
= 41 & 2 kJ/mol and k = 117 f 19 Torr-'*s-' at 480 K; at 350 K a = 40 f 1 kJ/mol and k = 18 f 2 Torr-'*s-'). Although the SiH growth eventually slows and stops, at 480 K the hydrocarbon growth continues. At the same time, there is a steady loss of surface silanol that appears to track hydrocarbon growth. One interpretation is that a polymerization reaction occurs between suvace SiH and Si-0 bonds of gasphase tMdS according to the scheme yH3
-0-Si-H \
(313
y3
F3
7
(313
a
(3%
3
F3
+ H-Si-0-Si-H 4-0-Si-0-Si-H I \ I \ 3
(313
F 3 + H-Si-O-H
I
(313
(10)
The residual silanols could react in two ways:
5.1. Conditions for Reactivity. The strain energy of the dimer rings drives the chemistry described here, and other factors govem the kinetics. Since the SiH bond is approximately as polar as the S i c bond and the S i 0 vibration occurs at the same frequency in tMdS as in hMdS, we may conclude that the S i 0 bond in these molecules is nearly identical. The S i 0 bond in tMdS is more accessible than in hMdS, which is the simplest way to account for the -600 times faster reaction. We deduce from the EtS data that the reactivity of the SiH bond in tMdS is too small to complicate this argument. Nonpolar EtS reacts -2200 times slower than does tMdS; therefore a polar bond to initiate the reaction seems necessary. The ring strain in hMctS also accelerates the reaction, although we cannot model the kinetics. 5.2. Nature of the Reacting Sites. Our investigations provide strong evidence that the reactive sites are d4 and d3 rings and that residual silanols play a minimal role. We know that the d4 sites must account for at least 25% of the dimer rings. We also know a significant fraction must consist of d3, otherwise the reaction with hMdS would not create perturbed surface silanols, as seen in Figure 6. The bulk of our data is consistent with the idea that the d4 sites are more reactive than the d3 sites. The depletion of the 930 cm-' peak does not correlate with the behavior of any other spectral feature, except that it decays with increasing water exposure. We tentatively assign it to bent dimer rings of C2" symmetry. Therefore the overall picture of the dimer rings is that there are two populations: the more reactive d4, and the less reactive d3, both of which have fractions perturbed by the local lattice into CzVsymmetry. We can infer no reaction with trimer rings because they have no detectable IR active vibrations,6 and other mechanisms can explain all of the detected reactions. To the author's knowledge, there has been no prior attempt to measure the heterogeneity of dimer rings on silica. By fitting eq 8 either to water or tMdS data, we derived a spread in activation energy of -43 kJ/mol for reactions with dimers. The temperature dependence for the tMdS reaction suggests a -20 kJ/mol base activation energy; therefore, in the kinetic model, the sth surface element in the tMdS reaction has Ea(s) x 20 43s kJ/mol. The intensity of the SiH peak in a saturated sample and the estimated surface area were used to calculate the ring density, 0.2-0.5 per mi2. We cannot say that the concentration of dimer rings (d4 and d3) would be the same on a true melt surface as on dehydroxylated Cab-0-Sil; the higher temperatures involved and the lower degree of hydroxylation prevents us from making such predictions. This issue is therefore an object of further investigation. 5.3. Implications for Silica Properties. Since dimer rings are unstable in the it is hard to conceive of a better analogue of a dimer ring reacting with a network bond than a surface reaction with organosiloxanes. We made the dimer rings by heating the samples to the strain point for silica (-1373 K, depending on the level of impurities2*),where molecular flow occurs. Presumably, experiments on dimer rings caught in their high-temperature form ought to reflect the chemistry of rings in the melt. One property that dimer rings might affect is the intrinsic viscosity of silanol-free silica. Because silica's mechanism for
+
These reactions would increase the detected ratio of CH3 to SiH, but we did not observe them at 350 K. For exposures less than 0.1 Torrs there is no detectable depletion of silanol (Figure 7), which suggests that d4 sites are more reactive than d3 sites. By 0.05 T o r s of exposure, the reaction of tMdS with the dimer rings is -25% complete, and the silanols just begin to become depleted. This corresponds to the fraction of d4 sites previously estimated with water. TMdS is far more reactive to dimer rings than to surface silanols, which is consistent with the known properties of poly(dimethylsiloxane) and the SiH group, as demonstrated by the EtS and hMdS experiments. 4.7. Sequential Exposure of hMdS and tMdS. A sample was first exposed to a 3500 Torrs dose of hMdS, and afterwards a tMdS reaction was tracked. The relatively slow tMdS reaction correlated with the loss of surface silanol (see inset, Figure 7). The pretreatment removes most of the d sites, leaving intact all the original silanols on the surface. Since hMdS does not measurably directly react with silanols, we conclude that tMdS
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flow must involve bond breaking and bond making, it is not inconceivable that edge-shared tetrahedra may play a role in determining the intrinsic viscosity of silica glass, perhaps as intermediate structures. If the heterogeneity of dimer rings found on the surface is reflective of the population found in the melt, the heterogeneity should affect the temperature dependence of the intrinsic viscosity accordingly.
6.0. Conclusion Heat-treated (1350 K) Cab-0-Si1 silica surfaces contain dimer rings (edge-shared tetrahedra) at a coverage of approximately 0.2-0.4 nm-2. No more than 75% of the dimer rings exist in the d3 form. Qualitatively, d4 rings are more reactive than d3 rings, implying that attached silanols stabilize dimer rings. Some of the dimer rings may exist in a bent (G) form. A linear heterogeneous kinetics model shows that the activation energy for dimer ring reactions spans -43 kJ/mol. The reaction rate accelerates if the reactant contains a polar or strained bond; steric hindrance decelerates the reaction. The tetramethyldisiloxane (tMdS) reaction with dimer rings is complete by 100-200 Torrs at 480 K, almost as fast as water. Therefore at elevated temperatures and pressures this molecule would be an excellent candidate as coupling agent for highly dehydroxylated silica surfaces.
7.0. Appendix: Infrared Spectral Features of Organosiloxanes A characteristic 2160 cm-' peak appears in the spectra of SiH compounds.24 The peak's position and intensity are very sensitive to the environment surrounding the SiH group. Therefore, the gas-phase absorbance of the SiH functional group is not the same as it is on a surface compound. The S i 0 bond in siloxanes exhibits a 1074 cm-I absorption. In hMctS, the peak is red-shifted to 1034 cm-I, indicative of a weaker S i 0 bond. The hydrocarbon absorptions occur in their usual places: CH3 at 2964 and 2907 cm-I, CH2 at 2899 and 2879 cm-l. The Si-C bonds have absorptions at 1270 cm-'. Overlapping bands that cannot be unambiguously assigned occur from 700 to 1000 These bands are due to various modes of SiH and S i c groups: in hMdS 850, 832, and 762 cm-l; in tMdS 913, 887, 850, 837, and 773 cm-I; in hMctS 817 cm-I; in EtS 941 cm-' and 929 cm-I. In the hh4ctS spectrum, there is a weak absorbance from 840 to 940 cm-' due to tails of the peaks at 817 and 1034 cm-I.
References and Notes (1) Pluedemann, E. P. Silane Coupling Agents, 2nd ed.; Plenum: New York, 1991.
(2) Murata, H. Handbook of Optical Fibers and Cables; Marcel Dekker: New York, 1988. (3) Blyler, L. L., Jr.; DiMarcello, F. V. Encyclopedia of of Physical Science and Technology; Academic Press, 1987; Vol. 9. (4) Galleener, F. L. Solid State Commun. 1982, 44, 1037. (5) Revez, A. G.; Walrafen, G. E. J . Non-Cryst. Solids 1983,54, 4716. (6) Galleener, F. L.; Barrio, R. A,; Martinez, E.; Elliott, R. J. Phys. Rev. Lett. 1984, 53, (25). (7) Morrow, B. A,; Cody, I. A. J . Phys. Chem. 1976, 80, 1998. (8) Morrow, B. A,; Cody, I. A.; Lee, L. S. M. J . Phys. Chem. 1976, 80, 2761. (9) Michalske, T. A,; Bunker, B. C. J . Appl. Phys. 1984, 56, 2686. (10) Bunker, B. C.; Haaland, D. M.; Ward, K. J.; Michalske, T. A.; Smith, W. L.; Binkley, J. S.; Balfe, C. A. Surf. Sci. 1989, 210, 406-428. (11) Chiang, C.-M.; Zegarski, B. R.; DuBois, L. H. J . Phys. Chem. 1993, 97, 6948-6950. (12) Feuston, B. P.; Garofalini, S. H. J . Chem. Phys. 1989, 91, 564. (13) Bunker, B. C.; Haaland, D. M.; Michalske, T. A,; Smith, W. L. Surf. Sci. 1989,222, 95- 118. (14) Dubois, L. H.; Zegarski, B. R. J . Am. Chem. SOC. 1993,115, 11901191. (15) Michalske, T. A,; Bunker, B. C. J . Appl. Phys. 1987, 255, 122. (16) Michalske, T. A,; Smith, W. L.; Bunker, B. C. J . Am. Ceram. SOC. 1991, 74, 1993-6. (17) MKS Instruments Inc., Burlington, MA. We used 0.1 and 1.0 Torr to 1.0 Torr. The sensors, which provided a useful working range of 0.1 Torr sensor is sensitive to water adsorption. Readings could be high by as much as 8%, in the range from 1 to 10 mTorr, after the sensor had been exposed to water and immediately pumped out. This was not due to a virtual leak because an ionization gauge read in the lo-' Torr range at the same time. Therefore, we calibrated the sensor for the full range of water exposure and subtracted the offset from our data. This correction slightly affects the water data for doses less than 0.01 Torrs, but does not significantly affect the derived parameters from the curve fits. (18) Huls America Inc., Piscataway, NJ. (19) Cabot Corp., Tuscola, IL. (20) Szekely, J.; Evans, J. W.; Sohn, H. Y. Gas-Solid Reactions; Academic hess: New York, 1976. (21) Hayward, D. 0.; Trapnell, B. M. W. Chemisorption; Butterworths: London, 1964. Brunauer, s.;Love, K. s.; Keenan, R. G. J . Am. Chem. SOC.1942, 64, 751-758. (22) Press, W. H.; Teukolsky, S. A,; Vetterling, W. T.; Flannery, B. P. Numerical Recipies in C ; Cambridge University Press: Now York, 1988. Equation 8 can be efficiently evaluated in terms of the exponential integral (El),by using power series and continued fraction representations for arguments in different domains. (23) We took reference spectra of each reactant at 1.000 Torr. (24) Noll, W. Chemistry and Technology of Silicones; Academic Press: New York, 1968. (25) Hoffmann, P.; Knozinger, E. SurJ Sci. 1987, 188, 181-198. (26) Taken to be equivalent to the heat of polymerization. Silicon Compounds Regisfer and Review, 5th ed.; Huls America Inc.: Piscataway, NJ. (27) Garofalini, S. H. J . Non-Cryst. Solids 1990, 120, 1. (28) General Electric Co. Quartz Products Form 7700; 1990, NOS. (29) Smith, A. L.; Anderson, D. R. Appl. Spectrosc. 1984, 38 No.6, 822-825. JP942349C
