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Adsorption of PMMA on Oxidized Al and Si Substrates: An Investigation by High-Resolution X-ray Photoelectron Spectroscopy J. F. Watts,*,† S. R. Leadley,†,‡ J. E. Castle,† and C. J. Blomfield§ School of Mechanical and Materials Engineering, University of Surrey, Guildford, Surrey GU2 5XH, U.K., and Kratos Analytical, Wharfside, Trafford Wharf Road, Manchester M17 1GP, U.K. Received November 4, 1998. In Final Form: September 29, 1999 The adsorption of medium molecular weight poly(methyl methacrylate) (PMMA) from an apolar solvent onto oxidized aluminum and silicon substrates has been studied by monochromated X-ray photoelectron spectroscopy (XPS). By recording the capacity of the solid surface for the polymer, by XPS, as a function of solution concentration, it has been possible to construct adsorption isotherms for the two systems under study. At low solution concentration, these isotherms are shown to conform to Langmuir adsorption and provide similar values of monolayer coverage, Γm, but significantly different values of the constant term, b, in the Langmuir equation were determined. At higher solution concentration, the adsorption isotherm was different due to an increase in Γm and a dramatic decrease in the constant b. Such observations are thought to result from a change in conformation of the adsorbed molecule at higher solution concentrations. Such effects are not associated with changes of conformation in solution but reflect the adsorption of a more compact molecule, which invariably forms fewer bonds with the substrate. The nature of the adsorption sites present in the oxidized metal substrates and their interactions with PMMA and conformation of the adsorbed molecules were interpreted from the high-resolution XPS spectra. These results were used to interpret the adsorption isotherms in terms of the acido-basic properties of the polymer and substrates, entropy of polymer solutions, and conformation of adsorbed molecules.
Introduction One of the most important properties of polymers is their ability to become readily adsorbed on to solid surfaces.1 In the adhesion of organic coatings or adhesives (usually applied as liquid solutions2) to inorganic substrates, various chemical and/or configurational changes may occur in the interphase region.3 One manner in which the performance of such systems can be improved is by a more complete understanding of such interactions. It may then become possible to engineer the exact characteristics of the interphase to provide superior properties. The work reported in this paper is part of an ongoing program to understand the interactions that occur at an organic/inorganic interface and develop methodologies for their study. Many polymers do not have specific anchoring groups that chemisorb irreversibly to a surface. Instead, each segment can bind to the surfaces via much weaker physical forces. Depending on polymer and substrate type, the nature of the solvent, and the solution concentration, various configurations ranging from “bristles” through “loops and trains” to compact “coils” are possible. Such adsorbed layers are highly dynamic, with individual segments continually attaching and detaching from the surfaces, and where whole molecules slowly exchange with those in the bulk solution.4 Fowkes observed that the * To whom correspondence should be addressed. † University of Surrey. ‡ Present address: Dow Corning Limited, Barry, South Glamorgan CF63 2YL, U.K. § Kratos Analytical. (1) Roe, R. Adhesion and Adsorption of Polymers; Lee, L. H. Ed.; Plenum Press: New York and London, 1980; Vol. 12B, p 629W. (2) Gutowski In Fundamentals of Adhesion; Lee, L.-H., Ed.; Plenum Press: New York, 1991. (3) Fowkes, F. M. J. Adhes. Sci. Technol. 1987, 1, 7.
adsorption of poly(methyl methacrylate) (PMMA) on silica and chlorinated poly(vinyl chloride) (CPVC) on calcium carbonate were found to be reversible.5 The adsorption of solutes at the solid-liquid interface can often be described by the Langmuir equation. This model assumes that the solute is reversibly adsorbed as a monolayer, with the absence of lateral interactions between adsorbed species. It is quite possible that these assumptions are not relevant in the case of polymers, as the adsorption of polymers at the solid-liquid interface differs from that of small molecules due to the macromolecular size and the flexibility of the polymers. This has resulted in several theoretical models having been developed concerning polymer adsorption, which have been reviewed elsewhere.4 Despite these sophisticated treatments having been developed, most polymer adsorption data fits the simple Langmuir equation. The traditional way of following adsorption processes is the construction of adsorption isotherms. This is relatively easy in the gas phase, but more difficult in the liquid phase since solute and solvent compete for the same sites. Thus solution analysis yields only relative adsorption isotherms. The use of X-ray photoelectron spectroscopy (XPS) analysis of the surface to construct adsorption isotherms was first reported for the adsorption of silane adhesion promoters onto hydroxylated iron surfaces.6 More recently, this method has been extended to determine the acidity of carbon fiber surfaces, through their exposure to silver and magnesium ions.7 In recent years both XPS8-10 (4) Israelachvili, J. Intermolecular & Surface Forces; Academic Press Ltd.: London, 1992. (5) Fowkes, F. M. In Physicochemical Aspects of Polymer Surfaces; Mittal, K. L., Ed.; Plenum Press: New York, 1983; Vol 2, pp 583-603. (6) Bailey, R.; Castle, J. E. J. Mater. Sci. 1977, 12, 4647. (7) Baillie, C. A.; Watts, J. F.; Castle, J. E. J. Mater. Chem. 1992, 2, 939.
10.1021/la981558b CCC: $19.00 © 2000 American Chemical Society Published on Web 01/04/2000
Adsorption of PMMA on Oxidized Substrates
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and computational chemistry11,12 have been used to investigate PMMA/metal oxide interfaces. It has been shown that the C 1s spectra of ultrathin PMMA films on oxidized metal substrates enable specific interactions at the interface to be identified spectroscopically.10,13 This study will show how XPS has been used to construct adsorption isotherms for a basic polymer (PMMA) on oxidized silicon and aluminum substrates. The PMMA films adsorbed from solution are thin enough to allow observation of the oxidized metal substrate in the XPS survey spectra. Therefore, through the use of the highresolution C 1s spectrum, information on the acid-base interactions at the interface between PMMA and the substrate can be deduced. This paper describes preliminary work, which shows that by making simple assumptions concerning polymer behavior and adsorption conditions, it is possible to develop a model to relate XPS data to the conformation of the adsorbed molecules. Experimental Section Sample Preparation. The substrates used were singlecrystal silicon wafer and polycrystalline aluminum of 99.0% purity. The substrates were placed in a VG Scientific ESCALAB MkII spectrometer and etched, using argon ion bombardment, to remove any oxide and surface contamination. On removal of the substrates from the spectrometer, natural oxide growth occurred, typically of the order of 2 nm in thickness. Polymer solutions of PMMA (medium molecular weight [MW ) 120 000, atactic, polydispersity ) 2.13], Aldrich Chemical Co.) in carbon tetrachloride were prepared in the range 0.01-0.1% w/w. This apolar solvent was used in order that polymer-solvent interactions could be eliminated, and although a theta solvent for PMMA, the theta temperature is 27 °C,14 some five degrees above the ambient laboratory temperature. The substrates were immersed in the polymer solutions for 2 min. On removal from the polymer solution, the substrates were washed in carbon tetrachloride. This washing procedure was used to remove any physisorbed PMMA from the surface. The samples were then placed in a vacuum desiccator prior to analysis. Spectroscopy. XPS spectra were acquired using a Kratos Axis 165 electron spectrometer. The instrument is equipped with a 165 mm mean radius hemispherical sector electron energy analyzer, integral automatic charge neutralizer, and magnetic lens. A monochromatic Al KR X-ray source was used for analysis, at a nominal power of 450 W. Although the PMMA films were extremely thin, minor charging effects were observed in the C 1s spectra, as shown in Figure 1. To gain the optimum spectral resolution all of the samples under investigation were analyzed with the automatic charge neutralization device switched on. Curve fitting of the C 1s spectra was performed using the Kratos Vision peak synthesis software. After peak fitting the C 1s spectra, all spectra were charge referenced so that the unfunctionalized aliphatic C 1s component occurs at 285.0 eV binding energy. Care was taken to ensure that sample degradation did not occur during analysis. In contrast to XPS with an achromatic source, where the Bremsstrahlung component of the radiation (with energies of up to 15kV) can be responsible for damage, XPS using a monochromatic source is known to be less damaging. Indeed published data15 indicate a reduction of only 10% in the C/O ratio of PMMA after 500 min exposure to monochromatic X-rays, (8) Beamson, G.; Bunn, A.; Briggs, D. Surf. Interface Anal. 1991, 17, 105. (9) Possart, W.; Schlett, V. J. Adhes. 1995, 48, 25. (10) Leadley, S. R.; Watts, J. F. J. Adhes. 1997, 60, 175. (11) Chakraborty, A. K.; Adriani, P. M. Macromolecules 1992, 25, 2470. (12) Shaffer, J. S.; Chakraborty, A. K. Macromolecules 1993, 26, 1120. (13) Leadley, S. R.; Watts, J. F. J. Electron Spectrosc. Relat. Phenom. 1997, 85, 107. (14) Fox, T. G. Polymer 1962, 3, 111. (15) Beamson, G.; Briggs, D. High-resolution XPS of organic polymers-The Scienta ESCA300 Database; J. Wiley and Sons: Chichester, 1992.
Figure 1. The C 1s spectrum of PMMA on silicon with and without charge neutralization. Table 1. Surface Composition of Oxidized Silicon Substrates following Adsorption Experiments, as a Function of Solution Concentration PMMA solution concn % w/w 0.01 0.02 0.04 0.05 0.07 0.10
mol of
elemental concn/atomic %
MMA/dm3
1.2 × 10-2 2.1 × 10-2 3.0 × 10-2 5.4 × 10-2 7.3 × 10-2 9.8 × 10-2
C
O
Si
29.1 29.8 30.2 29.1 35.2 39.2
26.7 26.5 28.5 30.2 27.8 29.5
44.2 43.6 41.3 40.7 37.0 31.4
Table 2. Surface Composition of Oxidized Aluminum Substrates following Adsorption Experiments, as a Function of Solution Concentration PMMA solution concn
elemental concn/atomic %
% w/w
mol of MMA/dm3
C
O
Al
0.01 0.02 0.04 0.05 0.07 0.10
1.2 × 10-2 2.1 × 10-2 3.0 × 10-2 5.4 × 10-2 7.3 × 10-2 9.8 × 10-2
20.6 24.5 24.4 29.0 28.2 34.5
51.9 50.8 51.5 47.8 48.7 44.9
27.6 24.7 24.0 23.2 23.1 20.6
more than an order of magnitude longer than the acquisition times employed for the spectra presented in this paper.
Results Adsorption Isotherms. All the survey spectra showed peaks for carbon and oxygen, as well as silicon or aluminum from the substrate. The elemental concentrations of carbon, oxygen, silicon, and aluminum are shown in Tables 1 and 2. The absence of chlorine in the sample spectra indicates that there was no measurable uptake and/or retention of the solvent. The modified Langmuir equation, suitable for use in XPS experiments, can be written as follows7
1 C C ) + Γ Γmb Γm
(1)
where C is the initial solute concentration, Γ is the
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Figure 3. Adsorption isotherms of PMMA on oxidized silicon and aluminum substrates.
calculated using the percentage carbon detected by XPS. Nevertheless, these values for Γm can be used to compare the relative aeric density of adsorption sites on each of the substrates under investigation. Thus, the values for Γm on oxidized silicon and aluminum were both calculated as 31 at low solution concentrations. This indicates that the coverage of PMMA on both oxidized silicon and aluminum is comparable. Having calculated Γm, it is possible to determine values of b using the intercept of the Langmuir plots, i.e.
intercept ) 1/Γmb
Figure 2. Langmuir adsorption isotherms of PMMA adsorbed on to (a) oxidized silicon and (b) aluminum substrates.
concentration of the polymer at the solid/liquid interface (retained on removal from solution and during the subsequent washing procedure), Γm is the number of adsorption sites for the solute, and b is a constant. The adsorption of each PMMA polymer chain is dependent on interactions of each methyl methacrylate (MMA) unit with adsorption sites on the oxidized metal substrate. In our approach, therefore, the molar concentration of MMA units in each polymer solution is used for the initial solute concentration, CPMMA. This parameter has the advantage of circumventing the problem of the polydispersity of the polymer and is a reflection of the number of bonds available for interaction with the substrate. A value of CPMMA ) 1 mol dm-3 is equivalent to 6 × 1023 bonds per liter of solution available for interaction, at a solution concentration of 100 g of PMMA per liter. The specific bonding between polymer and substrate, reported in previous work,10 will be sufficient to displace adventitious organic material from the inorganic surface. Therefore, the elemental concentration of carbon (% C) is used as a measure of the concentration of polymer at the solid/liquid interface, Γ. The values for CPMMA/Γ versus CPMMA were plotted for each substrate as shown in Figure 2, which show that for each substrate two linear correlations were drawn. The gradients of the Langmuir plots shown in Figure 2 can be used to calculate Γm, the number of adsorption sites for MMA units on each substrate. It is important to remember that these values do not represent the number of adsorption sites per unit area, as the polymer uptake was
(2)
Thus, the values for b on oxidized silicon and aluminum were calculated as 8.10 × 103 and 1.04 × 103, respectively. Once values for Γm and the constant b have been determined from the Langmuir plots in Figure 2, adsorption isotherms can be calculated for the uptake of PMMA on oxidized silicon and aluminum substrates in terms of the degree of coverage, θ (θ ) Γ/Γm). To determine whether monolayer coverage of PMMA is achieved over this range of solution concentrations, the Langmuir isotherms are plotted against coverage, Γ/Γm, as shown in Figure 3. In the construction of these isotherms very dilute solutions were not included as it was considered that data from such concentrations would not be reliable. Recent, as yet unpublished, work from this laboratory on epoxy systems has indicated that this is not the case, and with care, the level of confidence in this very important region of the isotherm can be substantially increased. Unfortunately such data are not available for the current investigation. It should be noted that for both substrates there are points at higher solution concentrations in Figure 3 that do not appear to conform to the Langmuir model of adsorption. The importance of these data will be considered further in the discussion section of this paper. Peak Fitting of the C 1s Spectra. By careful peak fitting of high-resolution C 1s spectra, it is now well established that the nature of specific interfacial interactions can be determined. The spectra generated in this work were fitted with four peaks, following a previously reported strategy.10 The four peaks represent the following PMMA groups: aliphatic hydrocarbon (C1-C/C1-H) (assumed to have a binding energy of 285.0 eV15), an ester oxygen induced β-shifted carbon16 (C2-CdO), the methyl (16) Pjipers, A. P.; Donners, W. A. B. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 453.
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peak fitting data in Table 3 show that the binding energy shift of the carboxyl carbon (C4dO) varied between 4.3 and 4.1 eV. Stoichiometry arguments would dictate that the areas of the methyl ester (C3-O) and the carboxyl carbon (C4dO) peaks should be in a 1:1 ratio. However, this is not always the case, particularly for very thin films, and the peak areas in Table 3 show that the carboxyl carbon peak (C4dO) is indeed smaller than expected. It is also interesting to note that the relative size of the carboxyl carbon peak (C4dO) increases as the PMMA solution concentration increased. Figure 5 shows representative C 1s spectra of the oxidized aluminum substrate exposed to PMMA solution. Figure 5a shows that the four-peak fit strategy enabled the experimental data to be fitted for the oxidized aluminum samples exposed to the most concentrated PMMA solution. However, the C 1s spectra of oxidized aluminum exposed to the less concentrated PMMA solutions required an additional peak of very low intensity to be included, Figure 5b, consistent with work of the current authors13 and others.17 This fifth peak had a binding energy shift of 3.1 eV. The peak fitting data in Table 4 shows that the binding energy shift of the carboxyl carbon (C4dO) varied between 4.1 and 4.0 eV. The peak areas in Table 4 show that, unlike oxidized silicon, the carboxyl carbon (C4dO) peak is approximately the same size as the methyl ester (C3-O) peak at higher concentrations, but shows a reduced intensity at lower concentrations (although not as marked as for the oxidized silicon substrate). Discussion
Figure 4. High-resolution XPS spectra of oxidized silicon exposed to (a) a high concentration of PMMA and (b) a low concentration of PMMA.
ester (C3-O), and the carboxyl carbon (C4dO). The β-shifted carbon and the carboxyl carbon components were linked in a 1:1 ratio, deviations from this theoretical value were sometimes inevitable, as reported by other authors.15 For clarity, the C 1s spectra acquired from the oxidized silicon and aluminum substrates will be discussed separately. Figure 4 shows representative C 1s spectra of the oxidized silicon substrate exposed to PMMA solution. Figure 4a shows that the four-peak fit strategy enabled the experimental data for oxidized silicon exposed to the more concentrated PMMA solutions to be fitted. However, the C 1s spectra of oxidized silicon exposed to 0.01 and 0.02% w/w PMMA solutions required a very small, additional, peak to be fitted,10,13 as shown in Figure 4b. This fifth peak had a binding energy shift of 3.2 eV. The
Adsorption Isotherms at Low Solution Concentration. The results have shown that the uptake of PMMA onto oxidized silicon and aluminum substrates at low solution concentrations can be described by Langmuir isotherms, as shown in Figure 3. The Langmuir isotherm that was plotted for PMMA adsorption on oxidized silicon substrates reaches a plateau at a surface coverage, Γ/Γm, of approximately 1, which indicates monolayer coverage. The isotherm plotted for PMMA adsorption on oxidized aluminum substrates has a plateau at a surface coverage of approximately 0.9, indicating that monolayer adsorption is not fully complete. Both of the Langmuir isotherms plotted in Figure 3 use a value of 31 for Γm, which indicates that oxidized silicon and aluminum substrates have an equivalent number of adsorption sites for PMMA. This is in agreement with Barr’s XPS study of the passivation of metals, where he found that both silicon and aluminum passivate in the same way.18 He observed that the passivated surfaces of silicon and aluminum consist mainly of SiO2 and Al2O3, respectively, incorporating some of the respective hydroxides.18 The nature of the adsorption sites present in the oxidized metal substrates and their interactions with PMMA can be interpreted from the high-resolution XPS spectra. It has been shown that the C 1s spectra of the ultrathin PMMA films are different from the bulk polymer.8,9 This was initially shown by an increase in the binding energy shift of the carboxyl (C4dO) carbon peak from 4.0 eV in bulk PMMA15 to ∼4.2 eV in ultrathin PMMA films on oxidized silicon and aluminum. The source of such a binding energy shift is thought to be a partial reduction of electron density in the valence states.9 A withdrawing force that the native metal oxide exerts on the carboxyl (17) Alexander, M. R.; Pyan, S.; Duc, T. M. Surf. Interface Anal. 1998, 26, 961. (18) Barr, T. L. J. Phys. Chem. 1978, 82, 1801.
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Table 3. C 1s Peak Fitting Data for PMMA Adsorbed on Oxidized Silicon C1-C/C1-H CPMMA (% w/w) 0.01 0.02 0.04 0.05 0.07 0.10
BE shift
C2-CdO
C3-O
C3a
fwhm
area
BE shift
fwhm
area
BE shift
fwhm
area
1.28 1.25 1.25 1.25 1.31 1.38
62.4 55.5 52.1 51.3 49.1 44.8
0.8 0.8 0.8 0.8 0.8 0.7
0.87 0.95 1.02 1.02 0.99 1.20
6.8 8.6 10.3 11.4 11.6 18.1
1.7 1.7 1.7 1.8 1.8 1.8
1.56 1.56 1.61 1.57 1.57 1.53
22.8 26.4 26.2 25.2 26.1 24.4
O-C4dO
BE shift
fwhm
area
3.3 3.2 3.0
1.28 1.25 1.25
1.5 0.7 0.6
BE shift
fwhm
area
4.3 4.3 4.2 4.2 4.2 4.2
0.83 0.87 0.93 0.93 0.90 0.84
6.5 8.8 10.8 12.2 13.2 12.7
shift of the carboxyl (C4dO) carbon peak with decreasing concentration of PMMA solution on both oxidized metal substrates. This indicates that acidic groups are present on both oxidized silicon and aluminum substrates. Previous studies have proposed that the surface hydroxides of alumina are capable of hydrolyzing the ester bond on the side chain of PMMA.19 This hydrolysis produces a carboxylate side chain, which bonds ionically with the surface of alumina as shown below.
Figure 5. High-resolution XPS spectra of oxidized aluminum exposed to (a) a high concentration of PMMA and (b) a low concentration of PMMA.
side group of PMMA possibly causes this. Leadley and Watts have attributed this to an acid-base interaction, such as hydrogen bonding between the carboxyl side group of PMMA and acidic hydroxide groups on the surface of the oxidized metal substrates.10 The results in Tables 3 and 4 also show that there is an increase in binding energy
Leadley and Watts observed that the C 1s spectra of ultrathin PMMA films on amphoteric substrates require a five-peak fitting protocol. The fifth peak was attributed to the formation of carboxylate side chains that interact with the oxidized metal substrate by forming strong ionic bonds. Peak-fitting of the C 1s spectra of the oxidized substrates exposed to the least concentrated solutions of PMMA required a five-peak fitting strategy. Fitting the C3a peak indicates that basic hydroxide groups were present in both oxidized metal substrates. The results have shown that it was necessary to fit an additional, low intensity, fifth (C3a) peak to the C 1s spectra of the thinnest PMMA films on the oxidized silicon substrate. However, in a previous study of PMMA on oxidized silicon using high-resolution XPS,10 a four-peak fitting strategy was sufficient. In a high-resolution XPS study of ultrathin poly(acrylic acid) (PAA) films, a fifth peak was also fitted to the C 1s spectrum of PAA on oxidized silicon.13 It was proposed that PMMA requires the cleavage of the methyl ester before a carboxylate anion can be formed, and therefore PMMA would not interact with very weak basic sites. In light of this investigation, it is possible that the resolution of the spectrometer used in the previous study was not sufficient to necessitate the additional peak. It is known that PMMA may degrade under XPS analysis via main-chain scission.20 It might be suggested that changes seen in the high-resolution C 1s spectra of ultrathin layers of PMMA on oxidized metal substrates are the result of X-ray damage. However, previous studies have shown that sample degradation in PMMA induced by Al KR radiation goes unnoticed in the C 1s spectrum.20 Therefore, it is our conclusion that changes seen in the high-resolution C 1s spectra of PMMA on the oxidized silicon and aluminum substrates are a result of acid(19) Mallik, R. R.; Pritchard, R. G.; Morley, C. C.; Comyn, J. Polymer 1985, 26, 551. (20) Buchwalter, L. P.; Czornyj, G. J. Vac. Sci. Technol., A 1990, 8, 781.
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Table 4. C 1s Peak Fitting Data for PMMA Adsorbed on Oxidized Aluminum C1-C/C1-H CPMMA (% w/w)
BE shift
0.01 0.02 0.04 0.05 0.07 0.10
C2-CdO
C3-O
O-C4dO
fwhm
area
BE shift
fwhm
area
BE shift
fwhm
area
BE shift
fwhm
area
BE shift
fwhm
area
1.36 1.28 1.20 1.20
64.5 57.1 50.9 49.9
0.7 0.7 0.7 0.7
1.30 1.14 1.07 1.07
13.1 12.5 14.6 15.2
1.9 1.8 1.8 1.8
1.36 1.50 1.43 1.44
9.2 16.1 17.9 17.7
2.9 3.0 3.1 3.1
1.36 1.28 1.20 1.02
1.1 1.1 1.1 0.7
4.2 4.1 4.1 4.1
1.18 1.09 1.02 1.02
12.1 13.3 15.6 16.6
1.12
46.4
0.7
0.98
17.4
1.8
1.40
19.1
4.1
1.01
17.1
base interactions at the interface, i.e., hydrogen bonding with acidic hydroxides and carboxylate formation by basic hydroxides. This accounts for the same Γm values being used for the Langmuir isotherms plotted in Figure 3, irrespective of the oxidized metal substrate. Although the Langmuir isotherms plotted in Figure 3 use the same value for Γm, the “knee” of each isotherm is different. The “knee” is much sharper in the isotherm representing PMMA uptake on oxidized silicon than in the isotherm for PMMA on oxidized aluminum. This indicates that the interactions between PMMA and adsorption sites on the oxidized silicon substrates are much stronger than interactions with adsorption sites present in the oxidized aluminum substrates. The shape of this “knee” region is related to the value for the constant b in the Langmuir equation used to plot the isotherm. The constant b in eq 1 is equated to the ratio of rate constants for desorption (k1) and adsorption (k2), and it is possible to construct equations, using the usual gas-phase adsorption approach,21 to represent these rate constants. The Langmuir constant, b, is then simply
b)
C3a
k2 [vj σn(θ - 1)M!] ) k1 θZmνMΓ exp(-Q/RT)
(3)
where σ is the sticking coefficient per bond of macromolecule on substrate, n is the degree of polymerization, (θ - 1) is the proportion of surface sites available for adsorption, vj is the mean velocity component of a molecule perpendicular to the surface, Zm is the number of substrate sites per unit area, ν is the vibration frequency of adsorbed atom within macromolecules, Γ is the surface concentration of macromolecule, M is the maximum number of bonds formed by adsorbed molecule with substrate, and Q is the interaction energy per mole of bonds of adsorbed molecule with the substrate. A factor 1/M! gives the probability that all the possible M bonds will be broken. The parameter M is a maximum value and in practice the actual number of bonds formed between adsorbate and substrate will depend on conformation, both in solution and in the adsorbed state. Following usual practice21 it is convenient to write b as
b ) bφ exp(Q/RT)
(4)
In contrast to the gas-phase situation for small molecules the constant bφ contains not only a frequency factor but also a stereochemistry factor. In the case of oxidized aluminum and silicon substrates, the parameters that influence the extent of adsorption (sticking coefficients and available sites) will, to a first approximation, be very similar as a result of similar ionic size and oxide characteristics. Thus the values of b will be an indication of differences in stereochemistry and in the interaction energy, Q, of the bonds between substrate (21) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley and Sons Inc.: New York, 1990; pp 595-598.
Figure 6. Plot of O-C4dO binding energy shift versus PMMA solution concentration.
and adsorbate. Although silicon and aluminum passivate in a similar manner, their acid-base character is different, i.e., silicon is acidic and aluminum is amphoteric. As PMMA is considered to be a basic polymer, it would be expected to have a greater affinity for the substrate with the most acidic adsorption sites, i.e., silicon. The values for b used in the Langmuir isotherms plotted in Figure 3 were 8.10 × 103and 1.04 × 103 for silicon and aluminum, respectively. Assuming that PMMA was adsorbed in the same conformation on both oxidized metal substrates, bφ in eq 3 would be a constant. Therefore, it would appear that the interaction energy per mole of bonds, Q, for PMMA on oxidized aluminum is 12% of Q for PMMA on oxidized silicon. As PMMA is a basic polymer, these values for the constant b are in agreement with the acid-base character of the oxidized metal substrates. This is confirmed by plotting the binding energy shift of the (C4dO) peaks versus the concentration of the PMMA solution, shown in Figure 6. This shows that the change in binding energy shift was more pronounced in the C 1s spectra of PMMA on oxidized silicon, as indicated by the steeper gradient of the upper curve. If, as proposed by Leadley and Watts, this change in binding energy shift is associated with hydrogen bonding, the effect would be more pronounced in the native oxide with greater acidic character, i.e., silicon. Nevertheless, the increased binding energy shifts of the C4dO peaks fitted to the C 1s spectra of PMMA adsorbed on oxidized aluminum indicate that acidic sites were present on the oxidized aluminum surface. However, it is assumed that these sites are less acidic than the adsorption sites on oxidized silicon, indicated by the less steep gradient of the lower curve. This raises the question as to whether the difference in acidity of the two substrates would result in an 88% difference in interaction energy, Q. It is also possible to comment on the orientation of the adsorbed polymer chains using the high-resolution C 1s spectra. Stoichiometry arguments would dictate that the areas of the methyl ester (C3-O) and the carboxyl carbon (C4dO) peaks should be in a 1:1 ratio; however this is not
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Figure 7. Plot of C3-O/O-C4dO peak area ratio versus PMMA solution concentration.
always the case, particularly for very thin films. The peak areas for PMMA on oxidized silicon in Table 3 show that the carboxyl carbon peak (C4dO) is indeed smaller than expected. The reason for this observation is the interaction of the carboxyl group with the substrate leading to a reorientation of the pendant groups yielding a surface rich in methylene and methyl groups. This phenomenon has been investigated by angular resolved XPS,22 and clear differences are seen when PMMA interacts with substrates that are rich in acidic groups and those devoid of such groups. It has been shown by a polarized attenuated total reflection (ATR) experiment that at low PMMA solution concentrations, the carbonyl functionalities orientate so that they are normal to the plane of a silicon surface.23 It is thought that this is due to polymer chains spreading over the surface plane to maximize the number of MMA segments interacting with adsorption sites. In the case of a medium molecular weight PMMA of MW ) 120 000 this will give rise to 1200 interactions (or bonds) per molecule. It would be expected that this orientation would result in a decrease in the relative intensity of the C4dO peaks fitted to the C 1s spectra of PMMA adsorbed from solutions of low concentration, as shown in Table 3. Figure 7 shows the ratio of the carboxyl (C4dO) peak to the methyl ester (C3-O) peak versus solution concentration, and it shows that the carboxyl component increases in size as the solution concentration increases. This indicates that the carbonyl functionalities are orientating away from the interface as the solution concentration increased. In the ATR experiment it was also observed that as more polymer chains are adsorbed on the surface, the initial orientation of the carbonyl functionalities is lost.23 The peak areas for PMMA on oxidized aluminum in Table 4 show that, unlike oxidized silicon, the carboxyl carbon (C4dO) peak is approximately the same size as the methyl ester (C3-O) peak at higher concentrations. Figure 7 also shows that the ratio of the carboxyl (C4dO) and methyl ester (C3-O) peaks for PMMA on oxidized aluminum remains constant over the range of solution concentrations used. This ratio is approximately 1, indicating that there was no preferred orientation of the carbonyl functionalites toward the surfaces of the oxidized aluminum substrates. This is in agreement with solidstate NMR data that show that PMMA has a flat orientation on alumina, which is due to ionic bonds formed between the surface and carboxylate anions in the polymer. This observation is entirely consistent with the acidbase properties of adsorbate and substrate. (22) Chehimi, M. M.; Watts, J. F. J. Electron Spectrosc. Relat. Phenom. 1993, 63, 393. (23) van Alsten, J. Macromolecules 1995, 25, 3007.
Figure 8. Adsorption isotherms of PMMA on oxidized silicon and aluminum substrates.
It is clear that at low concentrations of PMMA solution, the adsorbed PMMA molecules have different conformations on oxidized silicon and aluminum substrates. Polymer adsorption from solution, when considered on the basis of the individual macromolecules, involves a decrease in entropy, for the adsorbed polymer has fewer possible configurations in the adsorbed state than in solution. It is worthy of note, at this point, that the entropy “penalty” is much smaller for the adsorption of a polymer molecule than for a single smaller molecule. Although in terms of the overall entropy change of the system, the displacement of a molecule with M bonds with the surface being displaced by M small molecules, each with a single interaction is equivalent. The possible distributions of the M bonds of the macromolecule, or the M small molecules quite simply being M!, both M and M! contribute to bφ, as well as differences in conformation of the adsorbed molecule. Thus, bφ in eq 3 would be expected to be different for PMMA adsorbed on oxidized silicon and aluminum. Therefore, the differences in the values of the constant b used to plot the Langmuir isotherms in Figure 3 are due to difference in the stereochemistry, entropy, and interaction energy of the adsorbed PMMA molecules. Adsorption Isotherms at High Solution Concentration. The Langmuir isotherms plotted in Figure 3 do not represent the uptake of PMMA from the solutions of highest concentration. The general form of the uptake from these solutions can be represented by the Langmuir isotherms plotted in Figure 8, which provides a useful comparison with the adsorption characteristics at lower solution concentrations. These isotherms were calculated using Γm values of 68 and 66 and b constants of 87.8 and 67.4 for oxidized silicon and aluminum, respectively. Although the data are not as secure as the values from the lower concentrations, they are included here for comparative purposes. The points represented by these isotherms show a surface coverage of greater than 1, which apparently indicates that multilayer adsorption occurs on exposure to these more concentrated solutions of PMMA. Such an occurrence would clearly render the application of the Langmuir model totally inappropriate, as one of the key assumptions in this model is that adsorption does not proceed beyond one monolayer. Other assumptions implicit in the use of Langmuir’s model are that all adsorption sites are equivalent and that the heat of adsorption is independent of the fractional coverage, θ. These assump-
Adsorption of PMMA on Oxidized Substrates
tions, it must be recalled, are relevant to the adsorption of small molecules adsorbed onto a solid substrate from the gas phase. Such molecules, although they may show different orientations on adsorption, will not change dramatically in shape, as they are much too small to exhibit reptation, unlike macromolecules. One explanation for the increased surface coverage would be an increase in the packing density of the adsorbed molecules at higher PMMA solution concentrations. When mixed with a suitable solvent, the polymer disperses itself in the solvent and behaves as a liquid. If the solvent is highly compatible with the polymer, the solvent-polymer interactions expand the polymer coil from its unperturbed dimensions by an amount which is in proportion to the extent of these interactions;24 i.e., the excluded volume increases. Thus, in most dilute solutions it can be visualized that individual polymer coils are well separated from each other and occupy the maximum volume allowed by the quality of the solvent-polymer interactions. As the concentration of the solution increases, the probability of collisions between polymer coils will also increase. This will have the net effect of reducing the number of polymer-solvent interactions, which will reduce the expansion of the polymer coils toward the unperturbed dimension; i.e., the excluded volume will be reduced. Thus, the changes in the gradient of the curves of Figure 2 could be due to a reduction in the excluded volume of the individual PMMA polymer chains. This would allow an increase in the packing density at the substrate surface. A critical parameter in order to assess the worth of this argument is the question of whether there is a transition from a dilute to a more concentrated regime within the concentrations of polymer solutions used in this investigation. Calculations, using the approach of Kurata and Tsunashima,25 have confirmed that all solutions employed in this study can be considered dilute. Thus, the changes in conformation of the adsorbed molecules result from substrate-adsorbate interaction, rather than changes in the excluded volume of the macromolecule in solution. The ratio of the carboxyl (C4dO) peak to the methyl ester (C3-O) peak versus solution concentration plotted in Figure 8 shows a change in PMMA conformation at higher solution concentration. Such changes occur at a critical solution concentration, which is lower for the oxidized silicon substrate. Also the oxidized silicon substrate would be expected to have the stronger interactions with the adsorbed molecule, bringing about such a change in conformation at the lower concentration. This is consistent with the substrate-adsorbate interaction being influential on the conformation of the adsorbed polymer molecule. The apparent concentration of bonding sites, Γm, increases above this point, an observation consistent with the more compact morphology of the polymer molecules in the more concentrated solutions. The Γm values used for the isotherms plotted in Figure 8 were 68 and 66, which, once again, indicates that there were an equivalent number of adsorption sites for both substrates. The data plotted in Figure 7 indicate that the conformation of adsorbed PMMA was similar at higher solution concentrations on both oxidized metal substrates. Thus, the value of bφ in eq 3 can be considered a constant at higher PMMA solution concentrations. Therefore, the (24) Cowie, J. M. G. Polymers: Chemsitry and Physics of Modern Materials; Blackie, Academic and Professional Press: Glasgow, U.K., 1991. (25) Kurata, M.; Tsunashima, Y. In Polymer Handbook, 3rd ed.; Brandrup, J., Immergut, E. H., Eds.; John Wiley and Sons: New York, 1989.
Langmuir, Vol. 16, No. 5, 2000 2299
values for the constant b should be directly related to the interaction energy, Q, between the adsorbed polymer and the oxidized metal substrate. The values for the constant b used for the isotherms in Figure 8, as for lower solution concentrations, were greater for oxidized silicon substrates than for oxidized aluminum, which is in accord with the acid-base character of the two substrates. This indicates that the acid-base character of the substrates results in a 24% difference in the interaction energy per bond. One of the key assumptions in the use of Langmuir’s model is that the heat of adsorption is independent of the fractional coverage. Yet the difference in the values of the constant b at lower solution concentration was far greater than that at higher solution concentration. For PMMA adsorbed on oxidized silicon, the value for constant b at high solution concentration was 1% of the value for the constant b at low solution concentration. For PMMA adsorbed on oxidized aluminum, the value for constant b at high solution concentration was 6% of the value for the constant b at low solution concentration. Therefore it can be concluded that at lower solution concentrations bφ has a greater influence on uptake than acid-base interactions. A comparison of the values of the Langmuir constant b for the high and low concentration regimes gives some insight into the surface conformation of the adsorbed molecule. If we write b1 for the low concentration range and b2 for the high concentration range, we see that
b1 (M1 - 1)! ) b2 (M2 - 1)!
(5)
since all other terms in b (eq 4) remain constant (M1 and M2 are the maximum number of bonds formed between the adsorbed molecule and the substrate in the low- and high-concentration regimes respectively, i.e., the molecule in its extended and compact form). Assuming that M2 < M1 then
(M1 - 1)! (M2 - 1)!
g (M1 - 1)
(6)
The ratio of (M1 - 1)!/(M2 - 1)!, derived from the ratio of the b parameters, is equal to 100 for the silicon substrate and to 14 for the aluminum substrate. Thus it follows that (M1 - 1) cannot exceed this ratio for each respective substrate. We thus conclude that the conformations of the adsorbed macromolecules permit the formation of a maximum of 100 bonds in the case of adsorption on silicon, ca. 10% of the bonds available per molecules, and only 14, or approximately 1%, of the available bonds in the case of the aluminum substrate. The value for bφ is a function of entropy and adsorbate stereochemistry; however the data plotted in Figure 7 appears to indicate that the conformation of PMMA adsorbed on oxidized aluminum changed very little as a function of polymer solution concentration, as specific interactions can be identified spectroscopically. Yet it has been proposed that the values for bφ change dramatically as solution concentration changes. This indicates that there is a reduction in the entropy of the adsorption process as the solution concentration changes. This is a new result and was not observed in previous work that considered the adsorption of PMMA26,27 on low-energy surfaces of high specific area in the regime of lower concentrations. (26) Abel, M.-L.; Chehimi, M. M.; Brown, A. M.; Leadley, S. R.; Watts, J. F. J. Mater. Chem. 1995, 5, 845. (27) Abel, M.-L.; Vickers, P. E.; Turner, M. E.; Watts, J. F. Composites 1998, 29A, 1291.
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Conclusions High-resolution XPS has been used to construct adsorption isotherms of PMMA in an apolar solvent on oxidized silicon and aluminum substrates. The data are shown to conform to Langmuir adsorption, the substrates having equivalent capacities for the polymer molecules, although the silicon has a higher strength of interaction than the aluminum substrate. This latter observation is in complete agreement with the acido-basic properties of the materials involved: PMMA is a basic polymer, oxidized silicon is acidic while oxidized aluminum is amphoteric. Using the high-resolution C 1s spectra, it has been possible to comment on the polymer-substrate interactions and conformation of the adsorbed PMMA molecules. To the authors’ knowledge this is the first time that XPS data of adsorbed polymer layers have been used to infer changes in the conformation of the adsorbed species, and experiments are planned, using atomic force microscopy in the phase sensitive detection mode, to see if such observations can be confirmed directly. The Langmuir isotherms and detailed analysis of the high-resolution XPS spectra show that there is a change in the adsorption process as the solution concentration increases. This is observed as an increase in the number of adsorption sites for PMMA, Γm, and a dramatic decrease in the constant b. The constant b is a function of entropy, stereochemistry of the adsorbed molecule, and the interaction energy between the polymer and active sites on the substrate. Therefore it has been possible to draw conclu-
Watts et al.
sions as to the adsorption of dilute solutions of PMMA. At low solution concentrations, the interaction between the polymer and the substrate is dominated by the effects of entropy. As the solution concentration increases, there is a change in entropy. At a critical concentration the adsorbate conformation becomes more compact allowing an increase in adsorption sites. This allows an increase in the number of adsorbed molecules on the substrate surface. Thus at higher solution concentrations the interaction energy is dominated by the acid-base character of the oxidized metal substrate. Such an observation becomes important when dilute solutions are used to model the behavior of fully formulated commercial systems such as organic coatings and adhesive, as the density of bonding sites and the magnitude, although not the type, of chemical interaction may vary with solution concentration. The answer to this dilemma is to separate the diagnosis of bond type from the estimation of aeric density of bonding sites on the substrate, in the manner suggested by Leadley and Watts.13 Acknowledgment. The authors thank the EPSRC for financial support (Grants GR/J25949 and GR/L40090), and Dr. M.-L. Abel for helpful discussions regarding the conformation of molecules in polymer solutions and undertaking the Kurata and Tsunashima25 calculations. LA981558B
