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Notes Attenuated Total Reflection Fourier Transform Infrared Spectroscopy Study of Poly(methyl methacrylate) Adsorption on a Silica Thin Film: Polymer/Surface Interactions Jean-Marc Berquier* and Herve´ Arribart Laboratoire CNRS Saint-Gobain, “Surface du Verre et Interfaces” 39, Quai Lucien Lefranc, F-93304 Aubervilliers, France Received April 17, 1997. In Final Form: December 22, 1997
Introduction Infrared spectroscopy has been extensively used for the study of the adsorption of organic species onto high surface area silica.1-4 For example the formation of the hydrogen bonds between polymers containing carbonyl groups5-11 and the surface sites of dispersed silica has been studied by this technique. On the contrary, in the attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy studies about polymer adsorption,12-22 the characterization of the polymer/surface interactions only results from the investigation of the polymer bands, and the silanol bands are not studied. We present here the results of a study of the interactions of poly(methyl methacrylate) (PMMA) with a chemically grown silica thin film. We show that bands due to the SiOH adsorption sites can be observed in situ and that the number of silanols involved in the interaction with the polymer and their orientation can be estimated. * To whom correspondence should be addressed. (1) Hair, M. L. IR spectroscopy in surface chemistry; M. Dekker: New York, 1967. (2) Iler, R. K. The chemistry of silica; Wiley-Interscience: New York, 1979. (3) Rochester, C. H. Adv. Colloid Interface Sci. 1980, 12, 43. (4) Kno¨zinger, H. In The Hydrogen Bond; Schuster, P., Zundel, G., Sandorfy, C., Eds; North-Holland Publishers: Amsterdam, 1976; Vol. 3. (5) Kobayashi, K.; Araki, K.; Imamura, Y. Bull. Chem. Soc. Jpn. 1989, 62, 3421. (6) Thies, C. J. Polym. Sci., C 1971, 34, 201. (7) Kawaguchi, M.; Yamagiwa, S.; Takahashi, A.; Kato, T. J. Chem. Soc., Faraday Trans. 1990, 86, 1383. (8) Fontana, B. J.; Thomas, J. R. J. Phys. Chem. 1961, 65, 480. (9) Korn, M.; Killmann, E. J. Colloid Interface Sci. 1980, 76, 19. (10) Cohen Stuart, M. A.; Fleer, G. J.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1982, 90, 321. (11) Killmann, E.; Bergmann, M. Colloid Polym. Sci. 1985, 263, 372. (12) Kuzmenka, D. J.; Granick, S. Colloids Surf. 1988, 31, 105. (13) McGlinn, T. C.; Kuzmenka, D. J.; Granick, S. Phys. Rev. Lett. 1988, 60, 805. (14) Johnson, H. E.; Granick, S. Macromolecules 1990, 23, 3367. (15) Frantz, P.; Leonhardt, D. C.; Granick, S. Macromolecules 1991, 24, 1868. (16) van der Beek, G. P. Thesis, Landbouwuniversiteit, Wageningen, 1991. van der Beek, G. P.; Cohen Stuart, M. A.; Fleer, G. J. Macromolecules 1991, 24, 3553. (17) Van Alsten, J. G. Macromolecules 1992, 25, 3007. (18) Schneider, H. M.; Granick, S. Macromolecules 1992, 25, 5054. (19) Couzis, A.; Gulari, E. Macromolecules 1994, 27, 3580. (20) Zazzera, L. A.; Tirrell, M.; Evans, J. F. Mater. Res. Soc. Symp. Proc. 1993, 304, 125. Zazzera, L. A.; Tirrell, M.; Evans, J. F. J. Vac. Sci. Technol. 1993, 11, 2239. (21) Frantz, P.; Granick, S. Macromolecules 1995, 28, 6915. (22) Semenovitch, G.; Lipatov, Y.; Todosijchuk, T.; Chornaya, V. J. Colloid Interface Sci. 1996, 184, 131.
Experimental Section The internal reflection element (IRE) is a 50 × 10 × 0.5 mm3, 45°-trapezoidal single crystal from a silicon wafer, covered by a silica layer (∼2 nm thick) prepared as follows: HF-ultrapure water (1:10 by volume) until total dewetting of the solution, ultrapure water rinse, H2O2-NH4OH-ultrapure water (1:1:5 by volume) at 80 °C for 15 min, ultrapure water rinse, ultrapure water for 20 min in a sonicated bath, final drying in a N2 gas stream. Just before the adsorption experiment, the crystal is cleaned with the following procedure: detergent solution at 40 °C for 30 min in a sonicated bath, deionized water rinse, ethanol rinse, drying in a N2 gas stream, irradiation by UV light for 1 h in a O2 gas stream. After the crystal preparation, the thermostated (30 ( 1 °C) ATR flow cell (Harrick) is immediately assembled and aligned in the purged sample compartment of the spectrometer. The solvent (CCl4, Uvasol Merck), already in presence of molecular sieves (0.4 nm pore size), is pumped through the cell to dry the surface. According to Scott and Traiman,23 such a treatment can dry the surface as a mild heating. After 1 h background spectra are recorded. Then the PMMA solution (Mw ) 3000, Mw/Mn < 1.1, Polymer Laboratories Ltd.) or an acetone (Uvasol Merck) solution is rapidly pumped into the cell and spectra are collected. After the polymer adsorption is completed, solvent and several acetone solutions of different concentrations are pumped successively into the cell. These last spectra are used to calculate the adsorbed amount of polymer. Other experimental details can be found elsewhere.24-26
Results and Discussion We show in Figure 1 two wavenumber regions of a representative ATR spectrum recorded as the cell is filled with the PMMA solution. The negative (positive) bands correspond to vibrators that disappear (appear) when the polymer adsorption proceeds. In the ν(SiO-H) stretching vibration domain (Figure 1a) a negative band at 3675 cm-1 and a broad positive band around 3440 cm-1 appear. The band in Figure 1b is from the stretching vibration ν(CO) of the polymer carbonyls that are either in solution or adsorbed on the surface. It is well-known for dispersed silica27,28 that, during the adsorption of esters or polyesters onto the silica surface, the ester groups interact with the surface silanols through hydrogen bonds, the formation of which induces a frequency shift of the bands of the O-H and CdO groups. The isolated silica silanols show a band at 3685-3690 cm-1 when they are in contact with CCl428-33 and a band between 3410 and 3440 cm-1 5,8,28,31,34 when they are (23) Scott, R. P. W.; Traiman, S. J. Chromatogr. 1980, 196, 193. (24) Azzopardi, M.-J. Thesis, University Paris VI, 1994. (25) Azzopardi, M.-J.; Arribart, H. J. Adhes. 1994, 46, 103. (26) Berquier, J.-M. Thesis, University Paris VI, 1995. (27) Korn, M.; Killmann, E.; Eisenlauer J. J. Colloid Interface Sci. 1980, 76, 7. (28) Mills, A. K.; Hockey, J. A. J. Chem. Soc., Faraday Trans. 1975, 71, 2398. (29) Tripp, C. P.; Hair, M. L. Langmuir 1996, 12, 3952. (30) Griffiths, D. M.; Marshall, K.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1974, 70, 400. (31) Cross, S. N. W.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1979, 75, 2865. (32) Low, M. J. D.; Hasegawa, M. J. Colloid Interface Sci. 1968, 26, 95. (33) Bascom, W. D. J. Phys. Chem. 1972, 76, 3188. (34) Yamagiwa, S.; Kawaguchi, M.; Kato, T.; Takahashi, A. Macromolecules 1989, 22, 2199.
S0743-7463(97)00396-X CCC: $15.00 © 1998 American Chemical Society Published on Web 06/03/1998
Notes
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An estimation of the silanol orientation is done through the measurement of the ratio Ap/As of the intensities in the two polarizations of the ν(OH) band. The ratios measured in various experiments with PMMAs of different molecular weights range between 2.75 and 3.2. These values are significantly different from 2 (random orientation), and we can infer that the isolated silanol groups involved in the adsorption process were on average oriented relative to the interface plane. A ratio value of 3 means that if the OH vibrators of the silanol groups could be considered as being disposed on a cone, the cone angle would be equal to 45°. From the intensity of the free silanols band, the number of isolated silanol groups involved in the adsorption process can be estimated. The method, which is generally used for the calculation of the adsorbed amount, can also be applied for this estimation. In this method one uses the relation that can be written between the band intensity, the concentration of molecules in solution, and the amount of molecules adsorbed at the surface25,35
A i ) Ki a
Figure 1. In situ ATR-FTIR absorbance spectrum of 3000 Mw PMMA adsorbed onto the silica surface: (a) in the region of silanol absorption, bands due to the free and perturbed surface silanols and to the polymer CH (between 2800 and 3100 cm-1) groups appear in this range; (b) in the region of carbonyl absorption, solution (0.4 g/L) as well as adsorbed chains contribute to the ν(CO) band.
hydrogen-bonded to ester-bearing species. The shift to lower frequency (lower wavenumber) of the stretching vibration of the carbonyl groups due to hydrogen bonding ranges between 15 and 35 cm-1 8,14,20,21,23,28,31 and generally leads to the appearance of a shoulder on the ν(CO) band. Therefore the spectrum in Figure 1a indicates that hydrogen bonds between the polymer and the surface silanols are formed in our experiment. The ν(CO) band in Figure 1b is markedly asymmetric, which confirms the presence of the hydrogen bonds. 1. The SiO-H Bands of the Substrate. According to the literature, we attribute the band at 3675 cm-1 to the surface isolated silanols in contact with the solvent. This frequency is slightly lower than the one observed for dispersed silicas but is in agreement with the value reported by Azzopardi and Arribart25 for a similar substrate. The fact that this band is negative indicates that some of the surface isolated silanols that were in contact with the solvent when the background spectrum was recorded are not anymore in this configuration after the polymer adsorption. Therefore, we are led to the conclusion that these silanols are involved in the adsorption process. Accordingly, the band at 3438 cm-1 in the spectrum in Figure 1a can be assigned to these surface silanols, which are now hydrogen bonded with the ester groups of the polymer.
[cd2 + Q]
(1)
where Ai)s,p is the band absorbance acquired for the s (or p) polarization, a is the absorptivity, c is the concentration of vibrators in solution, d is the penetration depth (at 1717 cm-1, d ) 0.475 µm), Q is the density of vibrators in excess at the surface, and Ki)s,p is an experimental parameter. Once the value of K has been experimentally determined through the measurement of the spectra of solutions of known concentrations, in our case with acetone solutions, Q can be calculated. As K can be considered as being the same for the different bands in the 4000-1500 cm-1 range, the density of silanols at the surface can be estimated in a similar way from the intensity of the 3675 cm-1 band. We use for the absorptivity of this band the value given in the literature27 for surface silanols on dispersed silica in CCl4: a ) 17.9 mol-1 m2. The accuracy of our result depends on the reasonable assumption that this value obtained on dispersed silica can be used for flat silica. The knowledge of the Ap/As ratio allows the estimation of the silanol density from the band intensity in the nonpolarized spectrum of Figure 1. A silanol density of 1.5 ( 0.4 silanol/nm2 is obtained; that is, it is estimated that from 1 to 2 isolated silanols/nm2 interact with the adsorbed polymer. These values are in reasonable agreement with the density of isolated silanols measured on dispersed silicas.2,27 2. The CdO Band of the Polymer. The ν(CO) band of Figure 1b is made of several contributions. The main component at ca. 1733 cm-1 due to free carbonyl groups comes (i) from the carbonyls that are parts of the chains adsorbed on the surface but are not in interaction with the surface sites and (ii) from the carbonyl groups of polymer chains in solution. For the decompositions shown in Figure 2, the band obtained in a transmission experiment has been used to simulate this first component. The shoulder on the low-wavenumber side of the band is attributed to the carbonyl groups that are hydrogenbonded with the surface silanols. No correct decomposition of this shoulder can be achieved with a single Gaussian component (Figure 2a). Two Gaussians at 1716 and 1693 cm-1 have to be added in order to obtain the satisfactory decomposition shown in Figure 2b. The presence of two (35) Harrick, N. J. Internal Reflection Spectroscopy; Intersciences: New York, 1967. Mirabella, F. M., Jr.; Harrick, N. J. Internal Reflection Spectroscopy: review and supplement; Marcel Dekker: New York, 1985.
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Notes
Figure 3. In situ ATR-FTIR spectrum (1800-1500 cm-1 range) acquired with an acetone solution (3 × 10-3 M) in contact with the silica surface. The best simulated spectrum is shown: the bands at 1738 and 1717 cm-1 are from the acetone molecules in solution; the two shifted bands at 1705 and 1690 cm-1 are ascribed to acetone molecules adsorbed on the two different adsorption sites of Figure 4.
Figure 2. Two possible decompositions of the ν(CO) band presented in Figure 1: x, experimental data. The difference between the experimental and the simulated curves is also shown. (a) One Gaussian is used to simulate the hydrogenbonded carbonyls component: note the discrepancy around 1680 cm-1 between the experimental points and the simulated curve. (b) Two Gaussians are used.
different bands for the hydrogen-bonded carbonyls can be interpreted by comparison with the case of the adsorption of acetone molecules. In an auxiliary experiment, an acetone solution is passed in the ATR cell instead of the PMMA solution. A spectrum (Figure 3) is acquired as the silica layer is in contact with an acetone solution (ca. 3 × 10-3 M). The band at 1717 cm-1 is due to the free carbonyl stretching vibration of the acetone molecules in solution. In addition to this band, a broad band at lower wavenumber is present that can be assigned to the acetone molecules in interaction with the surface silanols through hydrogen bonds. The resulting ν(CO) band can only be decomposed by the addition of two Gaussians at 1705 and 1690 cm-1 to the band due to the solution molecules, as shown in Figure 3. The fact that two bands are present in the case of the acetone adsorption reinforces the validity of the decomposition of the PMMA carbonyl band that we propose above. The presence of two bands for the hydrogen-bonded carbonyls has already been reported in the literature for the adsorption of ester or cetone-containing molecules on dispersed silica.30,31,36 Rochester and co-workers reported two bands at 1705 and 1690 cm-1 for the adsorption of acetone30 and at 1712 and 1688 cm-1 for the adsorption of ethyl acetate31 on silica. These values are in good agreement with ours. According to these papers, we ascribed the two bands in our spectra to two different (36) Rochester, C. H.; Trebilco, D. A. J. Chem. Soc., Faraday Trans 1979, 75, 2211.
Figure 4. Scheme of the two adsorption processes for acetone molecules on the silica surface.
adsorption sites for the carbonyls. The first site, corresponding to the bands at 1716 cm-1 in the case of PMMA and at 1705 cm-1 in the case of acetone, is constituted by one silanol that is hydrogen-bonded with one carbonyl: the 1:1 site. The second site, corresponding to the bands at 1693 cm-1 in the case of PMMA and at 1690 cm-1 in the case of acetone, is made of two neighboring silanols that form two hydrogen bonds with the sets of oxygen lone-pair electrons of one carbonyl group: the 2:1 site (see Figure 4). The relative proportion of 1:1 and 2:1 sites depends on the distribution of the silanols as well as upon the concentration of carbonyl groups near the surface.30,31,36,37 For low acetone concentrations, as in Figure 3, the main adsorption site is the 2:1 site. As the acetone concentration increases, it is expected that the silanols which participate in the 2:1 sites tend to interact with more acetone molecules and give additional 1:1 sites. The 1:1 sites can then become predominant for high acetone concentrations. Such a behavior cannot be observed with our ATR experiment as the presence of a highly concentrated acetone solution results in an intense band that obscures other bands in the ATR spectrum and prevents any possibility of band decomposition. But in the case of PMMA adsorption, the concentration in carbonyls near the surface is very high even if the polymer concentration in solution is low. Such a high carbonyl concentration near the surface can explain why the 1:1 site is prevailing in the case of the polymer adsorption. The conformation of a polymer adsorbed at the solid/ liquid interface is usually described in terms of trains, (37) Gue´neau, L.; Berquier, J.-M. To be submitted for publication.
Notes
loops, and tails. The knowledge of the proportion of segments in the trains gives some idea of the polymer conformation. The larger the fractions of the attached segments per chain, the flatter the configuration is. As the contributions of the adsorbed and free carbonyl groups in the band absorbance can now be evaluated, the fraction of hydrogen-bonded segments or “bound fraction” can be determined. It is equal to 0.47 in this experiment. This means that our low molecular weight polymer is in a very flat conformation with no or only a few loops dangling in the solution. Values between 0.25 and 0.28 have been reported in similar conditions but for 90 000 Mw polymers.14,20,26 Such an increase of the bound fraction for (38) Fleer, G. J.; Cohen Stuart M. A.; Scheutjens J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at interfaces; Chapman and Hall: London, 1993.
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lower molecular weight is in agreement with theoretical predictions.38 Conclusions A very sensitive FT-IR ATR experiment has been used to study the adsorbed polymer layer at the surface of a thin silica film. Two types of PMMA/surface interactions have been identified. Bands due to the free and perturbated silanols at the silica surface are observed. The number of isolated silanols involved in the adsorption process has been estimated between 1 and 2 silanol/nm2, in good agreement with the literature data on dispersed silicas. This work is the first step to the characterization of the adsorption sites at the surface of silica and other oxide thin films by ATR-FTIRS. LA9703961
