Langmuir 1986,2, 127-131
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Articles Quantitative Intermolecular Reaction of Hydrolyzed Trialkoxysilanes at Submonolayer, Monolayer, and Multilayer Surface Coverages James D. Miller and Hatsuo Ishida* Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106 Received May 31, 1985. I n Final Form: September 3, 1985 The intermolecular reaction of hydrolyzed [y-(methacry1oxy)propyljtrimethoxysilane(7-MPS) deposited on a low surface area particulate substrate has been quantitatively monitored by diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy. The reaction is the condensation of the hydrolyzed species to siloxane linkages at 25 "C and 1 atm. Sufficient sensitivity has been obtained to observe the reaction at submonolayer coverage. When this sensitivity is combined with spectral manipulation techniques which allow contributions of the substrate and the organofunctionality to be removed, an infrared spectrum of only the siloxane structure is produced. The condensation reaction is quantitatively followed as a function of surface coverage to produce isotherms which reflect the intermolecular reaction on the particulate surface. The reaction isotherm demonstrates that the intermolecular reaction is controlled by the degree of surface coverage and that there is little interlayer reaction between the monolayer species and subsequent layers.
Introduction Recently, numerous applications have appeared for systems involving intermolecular reactions in the thin adsorptive films. The cross-linking of Langmuirl monolayers for lithographic, device, and barrier applications is a good example. In these systems the chemical reactions are highly dependent on the physical structure of the film including the molecular orientation, density, and packing. Both steric and electrical factors influence these properties and a number of literature illustrations are a ~ a i l a b l e . ~ - l ~ As an extreme example, there are monomer systems that will not react in bulk yet, when formed as well-defined, highly oriented monolayers are capable of substantial polymerization.12 In our system of interest, functionally substituted silicon atoms are deposited on the surfaces of various materials as primers, surface modifiers, or coupling agents. A typical use of organofunctional trialkoxysilanes is to serve as practical adhesion promoters in reinforced polymer composites. In these systems the hybrid organic-inorganic silane molecules may chemically react with the inorganic substrate surface, with themselves, and with organic matrix (1) Langmuir, I. J. Am. Chem. SOC.1917, 39, 1848. (2) Adam Proc. R. SOC.London, Ser. A 1926, 112, 362. (3) Hughes; Rideal Proc. R . SOC.London, Ser. A 1933, 140, 253. (4) Mittelmann; Palmer Trans. Faraday SOC.1942, 38, 506. (5) Marsden; Rideal J. Chem. Soc. 1938, 1163. (6) Naaini; Mattei Gazz. Chim. Ital. 1941, 71,302. (7) Alexander, A. E.; Schulman Proc. Roy. SOC.London, Ser. A 1933, 143, 61. (8) Gee Proc. R . SOC.London, Ser. A 1935,153, 129. (9) Gee; Rideal Proc. Roy. SOC.London, Ser. A 1935, 153, 116. (10) Bresler; Judin; Talmud Acta Physicochim. URSS 1941, 14, 72. (11) Lando, J. B. In 'Contemporary Topics in Polymer Science"; Pearce, E. M., Schaefgen J. R., Eds.; Plenum Press: New York, 1977; Vol. 2. (12) Ringsdorf, H.; Schupp, H. J. Macromol. Sci., Chem. 1981, AI5, 1015. (13) Schupp, H.; Hupfer, B.; Van Wagenen, R. A.; Andrade, J. D.; Ringsdorf, H. Colloid Polymer Sci. 1982, 260, 262.
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polymers. Of these reactions, the intermolecular reaction between silanols is the least understood. This paper quantitatively describes the intermolecular anhydrocondensation reaction of hydrolyzed [y-(methacryloxy)propyl] trimethoxysilane on the surface of lead oxide at submonolayer, monolayer, and multilayer coverages. The degree and type of reaction as well as the resulting thin film characteristics are examined as a function of surface coverage. The use of lead oxide serves as a model surface but the results obtained are believed to be valid on many surface systems.
Experimental Section Materials. [y-(Methacryloxy)propyl]trimethoxysilanewas
purchased from Petrarch Systems, Inc., and used as received. Particulate lead(I1)oxide was purchased from Aldrich Chemical Co. with a reported purity of 99.9+ %. Surface Area. Surface area of the lead oxide substrate was measured to be 0.50 & 0.03 m2/g by N2 BET adsorption. Surface Modification. Particulate lead oxide samples were modified with different concentrationsof an aqueous alcohol silane solution. 1-Butanolwas the carrier alcohol and the molar ratio of twice distilled and deionized water to yMPS was 1 O : l . The solution was atomized and deposited on the particulates at a constant solution loading of 0.18 mL/g of substrate. Infrared Spectrophotometry. A Fourier transform infrared spectrophotometer (FTS-2OE) with a hemispherical diffuse reflectance attachment (DRA-100) and a liquid nitrogen cooled mercury cadmium telluride (MCT) detector was used at a resolution of 2 wavenumbers with coaddition of 100 scans. The spectrophotometer was continually purged with nitrogen to reduce atmospheric water vapor. All diffuse reflectance infrared spectra are plotted according to the Kubelka-Munk function14J5as follows: F(R,) = (1 - R,)=/2R,
F(R,) represents a ratio between absorption and scattering (14) Kubelka, P.; Munk, F. 2. Tech. Phys. 1931, 12, 593. (15) Kubelka, P.; Munk, F. J. Opt. SOC.Am. 1948, 38, 448.
0 1986 American Chemical Society
128 Langmuir, Vol. 2, No. 2, 1986
Miller and Ishida
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Figure 3. Diffuse reflectance infrared spectra from 1300 to 900 cm-' of y-MPS-modified lead oxide with the substrate contribution removed. The spectra represent the following increasing concentrations of the surface modification: 0.4,0.8, 1.2, 2.0,3.0,5.0, and 10.0 mg/g of substrate. The corresponding values of AR, are 0.019, 0.047, 0.082, 0.101, 0.168, 0.263, and 0.568, respectively. Table I. Assignment of the 1300-900-cm-' Absorption Bands of the Organofunctional Component of Hydrolyzed [y-(Methacryloxy)propyl]trimethoxysilane(Silylpropyl Methacrylate) frequency, cm-I intensity assignment ref
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sufficient sensitivity to examine these films a t submonolayer coverages on low surface area substrates.16 Figure 2 illustrates the iinfrared spectra obtained from the diffuse reflectance experiment. Spectrum A is the y-MPS-modified lead oxide substrate a t a concentration of 2.0 mg/g of substrate (approximately five average layers thick) in the mid-infrared region from 3400 to 900 cm-'. Spectrum B is the pristine lead oxide surface with basic lead carbonate (2PbC0,:Pb(OH)2) impurities, and spectrum C is the digital subtraction of B from A and represents only the adsorbate y-MPS molecules. Figure 2 illustrates the outstanding sensitivity that can be achieved when working with surface coatings down to submonolayer levels. In this work we are concerned only with the intermolecular reaction between hydrolyzed trialkoxysilane molecules. This interaction can be monitored by observing the siloxane (Si-0-Si) stretching vibrational modes in the frequency range 1200-1000 cm-'. The intermolecular reaction was monitored as a function of surface coverage by controlling adsorbate concentration. The [y-(methacryloxy)propyl]trimethoxysilane loading was varied from 0.01 to 20.0 mg/g of substrate. The upper limit of this coverage corresponds to an average thickness of approximately 50 1ayers.l6 Typical diffuse reflectance spectra in this concentration range are illustrated in Figure 3. Any spectral contributions due to the substrate material have been eliminated by digital subtraction using the methods described in our previous work.16 The bands observed are due to a convolution of siloxane stretching modes and absorption by the organofunctionality. The siloxane contribution is concentrated between approximately 1170 and 1000 cm-I. The remaining bands are typical of the (16)Miller, J. D.; Ishida, H.Surf. Sci. 1984, 148, 601
Langmuir, Vol. 2, No. 2, 1986 129
Reaction of Hydrolyzed Trialkoxysilanes
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ganofunctionality contributions removed. The spectra represent the following increasing concentrationsof the surface modification: 0.4, 0.8, 1.2, 2.0, 3.0, 5.0, and 10.0 mg/g of substrate. The corresponding values of m-are 0.008,0.011,0.017,0.033,0.087,0.169, and 0.418, respectively.
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silylpropyl methacrylate group. A discussion of these bands is seen in Table I (ref 17-23). Analysis of the band assignments was also aided by comparison of the infrared spectra of [ y-(methacryloxy)propyl] trichlorosilane, octyl methacrylate, octyldecyl methacrylate, methyl methacrylate, and 1,4-butanediol dimethacrylate. The infrared bands of the organofunctionality are linear in intensity with concentration of the adsorbate16 and their contribution to the spectrum can be fitted and removed by a combination of Levenberg-Marquardt nonlinear least-squares analysisz4and digital s u b t r a c t i ~ n . ~ Ap~-~~ plication of this analysis to the spectra of the adsorbate molecules in Figure 3 is illustrated in Figure 4. The result is the infrared difference spectra of only the intermolecular (17) Walton, W. L.; Hughes, R. B. Anal. Chem. 1956,28, 1388. (18) Walton, W. L.; Hughes, R. B. J. Am. Chem. SOC. 1957,79,3985. (19) Smith, A. L. Spectrochim. Acta 1960, 16, 87. (20) Young, C. W.; Servais, P. C.; Currie, C. C.; Hunter, M. J. J. Am. Chem. SOC.1948, 70, 3758. (21) Kaye, S.; Tannenbaum, S. J. Org. Chem. 1953, 18, 750. (22) Harvey, M. C.; Nebergall, H. W.; Peak, J. S. J. Am. Chem. SOC. 1954, 76, 4555. (23) Dirlikov, S.; Koenig, J. L. Appl. Spectrosc. 1979, 33, 551. (24) Marquardt, D. M. J. SOC.Ind. Appl. Math. 1963, 11, 431. (25) Koenig, J. L. In "Advances in Polymer Science"; Gordon, M., Ed.; Springer-Verlag: Berlin, 1983; Vol. 54. (26) Hirschfeld, T. In 'Fourier Transform Infrared Spectroscopy Applications to Chemical Systems"; Ferraro, J. R., Basile, L. J., Eds.; Academic Press: New York, 1978.
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fication in the range 0-2.0 mg/g of substrate. adsorbate reaction. Two major bands are seen a t 1130 and 1040 cm-l and are assigned to the antisymmetric stretching modes of the Si-0-Si bonds. Close inspection reveals that the position and relative contribution of the bands change a t low concentrations. Two absorption maxima in the range 1200-1000 cm-' are expected for the antisymmetric stretching vibrations of the siloxane linkages in linear and branched polysiloxanes, cyclosiloxanes with more than four siloxane linkages, and noncagelike silsesquioxanes. The higher frequency band is determined by the local configuration while the lower frequency band is caused by a chain vibration. Chain vibrations are infrared-active only in those modes where the repeating distance corresponds to a repeating distance in the chain c o n f i g u r a t i ~ n . ~ ~ Integration over the frequency range in Figure 4 yields a quantitative measure of the amount of intermolecular anhydrocondensation reaction. This integrated intensity is plotted as a function of the adsorbate concentration or surface coverage in Figure 5. The amount of intermolecular siloxane linkages present on the substrate surface appears to be near zero a t concentrations less than 1.0 mg/g of substrate. At higher concentrations, a linear (27) Brown, J. F. Polym. P r e p . (Am. Chem. SOC.,Diu. Polym. Chem.) 1961, 1, 112.
130 Langmuir, Vol. 2, No. 2, 1986
Miller a n d Ishida crease in reaction rate starting near 1.2 mg/g of substrate.
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illustrated. dependence of integrated intensity on adsorbate concentration is observed. The data in Figure 5 can be normalized by ratioing the integrated intensity of the siloxane bands against the integrated intensity of the carbonyl stretching absorption (1750-1700 cm-l).16 By ratioing against an internal standard band of the adsorbate molecule we obtain a value for the relative degree of intermolecular condensation per monomer unit. This integrated intensity ratio is plotted as a function of adsorbate concentration in Figures 6 and '7. Over the concentration range illustrated in Figure 6, there is a general rise in the extent of intermolecular reaction followed by a decrease in slope and an approach to an asymptotic value. A discontinuity near 1.0 mg/g of substrate is also noted. The data from 0 to 2.0 mg/g of substrate is replotted in Figure 7. The integrated intensity ratio (normalized extent of intermolecular reaction) has a value near zero at concentrations less than 0.1 mg/g of substrate. This is followed by an increase in the ratio up to a concentration of 0.5 mg/g of substrate. A plateau is observed and is followed by an increase in the value of the integrated intensity ratio. The differential rate of change in the integrated intensity ratio is plotted as a function of adsorbate concentration in Figure 8. The two plots represent the same adsorbate concentration ranges illustrated in Figures 6 and 7. The value d l / d C is a measure of the differential rate of intermolecular condensation per monomer unit. It is seen that this rate is a strong function of the surface coverage (adsorbate concentration). The plots in Figure 8 reflect the dynamics and kinetics of the intermolecular reaction occurring on the substrate surface. It appears in Figure 8A that the reaction scheme is somewhat symmetrical about a maximum near 2.0 mg/g of substrate except for the discontinuity at low adsorbate concentration. Figure 8B shows that the reaction recycles before a general in-
Discussion The analysis of the polymerization of multifunctional monomers on two-dimensional surfaces has specific experimental difficulties including (1)the necessity for extremely high sensitivity measurements and ( 2 ) influences of the substrate. The thin films produced on the substrate surface are often only one molecule thick, a coverage that corresponds to a typical value of only 2 X 1014repeat units per square centimeter. The difficulties related to surface studies are nowhere more true than for monolayer films of organofunctional trialkoxysilanes adsorbed on particulate or fibrous inorganic surfaces. Most often these substrates are siliceous in nature. A problem arises because the intermolecular condensation of hydrolyzed trialkoxysilanes produces a chemical functionality (the siloxane bond) similar to the bulk substrate and to possible bonds formed between the substrate and adsorbate. As a result, delineation of these individual reactions on low surface area siliceous surfaces by infrared spectroscopy at monolayer coverages has not been unambiguously demonstrated. By use of lead oxide as the substrate material neither the bulk substrate nor the formation of possible substrate-adsorbate linkages (Pb-0-Si) which have a lower frequency (960 cm-l) infrared a b s o r p t i ~ nwill ~ ~ interfere *~~ with the reaction of interest. The term monolayer is often confused and used to represent different physical phenomena. We use the term monolayer throughout the text to describe a limiting adsorption in which all the adsorbate molecules have a nearest-neighbor interaction with the substrate surface. This interaction is identified by a hydrogen bonding between adsorbate carbonyls and the substrate surface functionality.16 There are two factors to be considered when analyzing the infrared spectra of these siloxane vibrations. First, there is the influence of the substrate on the amount, rate, and type of siloxane condensation that occurs. Second, there is the influence of the substrate in modifying the frequency of the u,(Si-0-Si) bands due to either weak or strong coupling with the vibrations of the substrate lattice. In general, the first of these effects should be the much stronger of the two. However, at surface coverages of fractions of a monolayer it is reasonable to assume that the chemical and physical forces holding the molecule to the substrate surface also influence the frequency of the siloxane vibration. In either case it is unlikely that any such perturbations affect the molar absorptivity of the v,,(Si-0-Si) modes to any great extent, and, therefore, quantitative analysis of the amount of siloxane bonding is possible. Extremely small amounts (less than 50% fractional surface coverage) of adsorbate material on the substrate surface (not illustrated in Figures 3 and 4) indicate a spectral contribution near 1040 cm-l which is slightly stronger than the contribution of the band above 1100 cm-l. This band is extremely weak and can probably be assigned to an in-plane bending mode of unreacted silanol functionality. Unreacted silanol functionality has only a very weak spectral contribution near 1035 cm-' and is not expected to be a problem in the quantitative analysis of the siloxane region. 30331
(28) Schmidbauer, H.; Hussek, H. J. Organomet. Chem. 1964,1,257. (29) Miller, J. D.; Ishida, H. In "Silylated Surfaces"; Leyden, D. E., Collins, W.; Eds.; Gordon and Breach: New York, 1985. (30) Ishida, H.; Koenig, J. L. Appl. Spectrosc. 1978, 32, 469. (31) Miller, J. D.; Ishida, H. Anal. Chem. 1985, 57, 283.
Reaction of Hydrolyzed Trialkoxysilanes The condensed siloxane polymers resulting from the intermolecular reaction of hydrolyzed trialkoxysilanes are referred to as polysilsesquioxanes and have the general form RSiO,,, as seen in Figure 1. Close inspection of Figures 3 and 4 show that with increasing adsorbate concentration the peak apex of the siloxane band above 1100 cm-' gradually increases from 1115 to 1130 cm-'. There appears to be distinct spectral contributions near 1135, 1125, 1115, and 1110 cm-' with a relative increase in the higher frequency bands with increasing surface coverage. The shift may reflect an increase in the degree of structuring of the resultant polysilsesquioxane with distance from the substrate surface. The infrared spectra in Figure 4 indicate that the molecular structure of the condensed polysilsesquioxane at the higher loadings is that of a well-ordered ladder polymer as reflected by the resolved bands near 1130 and 1040 cm-1.32333 The results in Figures 5-8 demonstrate the relationship between the number of siloxane bonds formed and the adsorbate concentration. There are significant nonlinearities in the plots which can be related to specific physical phenomena occurring at the substrate surface. If there was no influence of the substrate, then in Figure 5 the plot would be strictly linear throughout the entire range of adsorbate concentration. The fact that the plot is relatively linear at higher concentrations reveals that the effect of the substrate decreases with distance from the surface. In Figure 6 the amount of siloxane bonds formed is normalized per monomer unit. The asymptotic value reached is within 10% of the integrated intensity ratio evaluated for a highly condensed bulk silsesquioxane, indicating that the substrate has a diminishing effect on the degree of intermolecular condensation reaction a t high adsorbate concentrations (greater than approximately 25 molecular layers in thickness). This does not indicate, however, that the substrate surface does not have other longer range effects which influence adsorbate properties a t high concentration^.^^ The discontinuity appearing in Figures 6 and 7 near 0.5 mg/g of substrate has physical significance and is related to the formation of discrete monolayers of the adsorbate molecules. The integrated intensity ratio does not increase gradually throughout the adsorbate concentration range but flattens to a slope of zero between 0.5 and 0.7 mg/g of substrate. The onset of this event corresponds to deposition of adsorbate material beyond one complete monolayer. This can be confirmed from other experimental results which have shown that complete monomolecular coverage of yMPS on lead oxide of the same surface area occurs at 0.38 mg/g of substrate.16 The (32) Brown, J. F.; Vogt, L. H.; Prescott, P.I. J.Am. Chem. SOC.1964, 86, 1120. (33) Miller, J. D.; Hoh, K.; Ishida, H.Polym. Compos. 1984, 5, 18.
Langmuir, Vol. 2, No. 2, 1986 131 normalized amount of siloxane linkages per monomer unit a t this monolayer coverage is 0.2 measured as the integrated intensity ratio. This value represents only 5% of the amount of siloxane linkages found in the completely condensed polysilsesquioxane. The 9570difference is due to the presence of interfacial bonds and residual silanols. The observation that few siloxane linkages are formed in the molecules deposited shortly after initial monomolecular formation indicates that there must be little interaction between adsorbate molecules in the first and subsequent layers. This can be attributed to the highly oriented nature of the adsorbate molecules in the first layerl6 which would make little if any hydrolyzed alkoxy moiety available for reaction. By the same analysis, the observation that a similar sharp drop in the slope of the integrated intensity ratio is not seen at any subsequent adsorbate concentration indicates that molecules beyond the first monolayer are not as highly oriented or restricted from interlayer reaction. In the concentration ranges 0-0.1 mg/g of substrate and 0.5-0.7 mg/g of substrate there is essentially no change in integrated intensity ratio (normalized amount of siloxane bonds) with increases in adsorbate concentration. The lack of intermolecular reaction in these concentration ranges is attributed to a surface concentration effect. Low fractional surface coverages (less than a complete monolayer) having well-distributed molecules will result in reactive functional groups that are spatially hindered from reaction. In Figure 8 the first derivative or slope of the integrated intensity ratio depicts the differential rate a t which siloxane bonds are forming as a function of adsorbate concentration. Figure 8A shows that a maximum in the differential rate of siloxane bond formation occurs a t a concentration of 2.0 mg/g of substrate or about five equivalent molecular layers. Figure 8B shows that there is a general increase in the rate of siloxane bond formation during buildup of the first molecular layer (up to 0.4 mg/g of substrate). This is immediately followed by a dramatic drop in the rate representing initial buildup of a second molecular layer. The second molecular layer has a general increase in the rate of siloxane bond formation which is practically superimposable on the behavior in the first molecular layer. Beyond buildup of a second molecular layer (greater than 1.0 mg/g of substrate) there is a continuous increase in the differential rate of siloxane bond formation to a maximum followed by a continuous decrease approaching a differential rate of zero. The continuous behavior past the second layer reflects the more random and mobile nature of these adsorbate molecules.
Acknowledgment. We gratefully acknowledge the partial financial support of Owens-Corning Fiberglas Corporation. Registry No. y M P S , 2530-85-0;lead oxide, 1317-36-8.
