Langmuir 1992,8, 1307-1317
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Segment Level Chemistry and Chain Conformation in the Reactive Adsorption of Poly(methy1 methacrylate) on Aluminum Oxide Surfaces K. Konstadinidis, B. Thakkar, A. Chakraborty,? L. W. Potts,T R. Tannenbaum,§ and M. Tirrell* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
J . F. Evans* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Received February 27,1990. I n Final Form: August 6, 1990 We have used X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy (IRRAS),and solid-state nuclear magnetic resonance spectroscopy (NMR) to study the interface between poly(methy1methacrylate) (PMMA) and amorphous aluminum oxide. For the first two techniques, the aluminum oxide was a native oxide grown in ambient on vapor-deposited aluminum metal films, while for the third, high surface area porous aluminum oxide was used. In all cases the polymer was adsorbed from solution. Our results have shown that when PMMA adsorbs on the aluminum oxide surface, the surface hydroxyl groups hydrolyze the ester bond in the side chain of the polymer. As a result of the reaction, a side chain carboxylate ion is formed which bonds ionically with the surface, while methanol is released as a byproduct. In the IR spectra the formation of carboxylate ions is indicated by the appearance of a new peak at 1670 cm-l, while in the NMR spectra the loss of the methoxy carbons is indicated by a significant decrease in the methoxy carbon peak. While XPS spectra are less specific, they consistently show a new peak in the O(2p) region of the adsorbed polymer spectrum. The strength and the specificity of this interaction drive conformational changes of the polymer molecules upon adsorption, which show up as shifts in peak positions in the NMR spectra. These changes suggest a relatively flat configuration of the extensively hydrolyzed molecules on the oxide surface. Introduction The segment-level chemical interactions between polymer molecules and metal or metal oxide surfaces, as well as the conformation of polymer molecules on such surfaces, are important factors in understanding the fundamentals of polymer-metal/metal oxide adhesion and, therefore, in finding ways to control it and further improve it. Interfaces between metals and polymeric materials appear in many industrial applications such as photoresists in integrated circuit fabrication, protective coatings in automobile parts, metal-insulator and semiconductor-insulator junctions, metal-filled polymer composites, metal-polymer laminates, and polymer-lined metal containers for protective food packaging.lI2 A feature that makes polymer adsorption different from small molecule adsorption is the chain nature of the molecules. A polymer molecule can occupy more than one site on the surface, and yet all its segments need not be attached.’ While the bonding mechanisms that lead to adsorption (i.e. chemical bonds, hydrogen bonds, dispersive forces, etc.) are the same for small molecules and macromolecules, at least two new questions have to be answered in polymer adsorption. These are: What is the fraction of directly adsorbed segments? What is the
* Authors to whom correspondence should be addressed. Present address: Department of Chemical Engineering, University of California, Berkeley, CA 94720. t Department of Chemistry, Gustavus Adolphus College, St. Peter, MN 56082. 8 Present address: Department of Chemistry, Israel Institute of Technology, Technion City, Haifa 32000,Israel. (1)Colletti, R. F.;Gold, H. S.; Dybowski, C. Appl. Spectrosc. 1987, 41(7), 1185. (2)Leidheiser, J. R.;Deck, P. D. Science (Washington, D.C.) 1988, 241,1176. t
configuration of the polymer molecules on the oxide surface? The fraction of segments that is attached to the surface and the configuration of the polymer chain on the surface are directly related. The larger the fraction of the attached segments per chain, the more flat the configuration will be. Distinction should be made between physisorption, where weak dispersive forces are involved, and chemisorption, where polymer molecules adsorb irreversibly by forming strong chemical bonds. Physical adsorption of polymers has been studied and modeled extensively via equilibrium statistical mechanics. Irreversible chemisorption is the concern of the present work. Like many non-noble metals, aluminum forms a passive, amorphous “oxide” upon exposure to atmosphere. The oxide layer, which is approximately 20A thick? is thought to consist of an amorphous oxide whose surface is decorated with a high density of hydroxyl functi~nalities.~-l~ These OH groups are thought to form from the dissociative adsorption of water onto anion vacancies at the surface (Lewis acid AP+ sites’l-l3). It has been observed experimentally that the OH groups are responsible for the chemi(3)Mathieu, H.J.; Datta, M.; Landolt, D. J. Vac. Sci. Technol. 1985, A3(2), 331. (4)Peri, J. B. J. Phys. Chem. 1965,69,220. (5)Hair, M. L.Infrared Spectroscopy in Surface Chemistry; Dekker: New York, 1967. (6)Knozinger, H. Hydrogen Bonds in Systemsof Adsorbed Molecules. In The Hydrogen Bond; Schuster, P., Zundel, G., Sandorfy, Eds.; NorthHolland: Amsterdam, 1976; Vol. 3, Chapter 27,pp 1263-1364. (7)Peri, J. B. J. Catal. 1976,41,227. (8)Knotzinger, H.; Ratnasamy, P. Catal. Rev. Sci. Eng. 1978,17,31. (9)Streyrer, R. Ph.D. Dissertation, Universitat Munchen, Munich, FRG, 1978. (10)Baumgarten, E.;Denecke, E. J. Catal. 1986,100, 377. (11)Paul, J.; Hoffman, F. M. J. Phys. Chem. 1986,21,5321. (12)Almy, D. B.; Foyt, D. C.; White, J. M. J. Electron Spectrosc. Relat. Phenom. 1977,11, 129.
0743-7463/92/2408-1307$03.00/00 1992 American Chemical Society
1308 Langmuir, Vol. 8, No. 5, 1992
sorption of small molecules (carboxylic acids) on amorphous aluminum oxide surfaces.'"16 It has long been realized by those working in chromatography and catalysis that the chemical activity of amphoteric aluminas (derived from various crystalline phases) can be modified by heat treatment, an effect which changes the type and distribution of these OH groups." Most would agree that the surface of the native oxide under study here is the most poorly understood aluminum oxide phase in terms of its structural detail. However, it is a surface that is of paramount technological importance. If one accepts the premise that this oxide is a highly defective modification having local structural similarities to many of the "transition" alumina phases, then it seems appropriate to consider the native oxide as a more highly hydroxylated material than the crystalline phases which have been exposed to atmospheric humidity. A reasonable model for which there is ample IR spectroscopic evidencela is one that depicts 7-10 pmol of OH groups per square meter of surface area ((4.2-6.0) X lOI4 OH/cm2) on previously dehydrated y- and a-Al203. These results are in good agreement with titrimetric results obtained by using pyridine to titrate the y phase after dehydration at 470 Ka9 For comparison, the surface coverage of OH for a fully developedmonolayer on these polycrystalline phases has been reported to be 23 pmol/m2 (1.4 X 1015OH/cm2).18 Finally, by way of further introduction to the present work, the adsorptive and/or reactive nature of the surface of aluminas might be expected to be strongly influenced by the types of interactions between OH groups in close proximity to the surface site under consideration. It is interesting to note recent IR results18which suggest that at surface coverages of less than 2 pmo1/m2,surface hydroxyl groups are unassociated. Above this coverage there is clear evidence for the onset of the development of a two-dimensional network, which is complete at 5 pmol/ m2. It is very likely that this hydrogen bonded network might contain sites that both are better hydrogen bond acceptors and are also more acidic than the isolated OH groups at low coverage. Another important feature of the native aluminum oxide surface is its high sorption activity, which is responsible for the adsorption of low molecular weight organics from the laboratory environment, as reported by Allara and Nuz20.~~ Using IR spectroscopy, they investigated the adsorption of n-alkanoic acids from solution onto oxidized aluminum surfaces. Their most important observation, as far as our work is concerned, is that the kinetics of the formation of the close-packed assemblies on the oxide surface are greatly influenced by surface defects as well as surface contaminants. They found that the latter must be displaced from the aluminum oxide surface before an oriented monolayer of the acid can be formed. The chemistry of the interaction between PMMA and aluminum oxide has been studied previously. Mallik et aLZoused inelastic electron tunneling spectroscopy (IETS) (13)Chen, J. G.; Crowell, J. E.; Yates, J. T., Jr. J. Chem. Phys. 1986, 84,5906. (14)Lewis, B. F.; Moseman, M.; Weinberg, W. H. Surf. Sci. 1974,41, 142. (15)Bowser, W.; Weinberg, W. H. Surf. Sci. 1977,64,377. (16)Brown, N. M. D.; Floyd, R. B.; Walmsley, D. G. J. Chem. Soc., Faraday Trans. 2 1979,75, 17. (17)Unger, K.K.;Trudinger, U. Oxide Stationary Phases. In High Performance Liquid Chromatography; Brown, P. R., Hartwick, P. A,, Eds.; John Wiley and Sons: New York, 1989; Chapter 3, pp 165-169. (18)Baumgarten, E.;Wagner, R.; Lentes-Wagner, C. Fresenius' Z . Anal. Chem. 1989,334,246. (19)Allara, D.L.; Nuzzo, R. G. Langmuir 1985,1,45. (20) Mallik, R.R.; Pritchard, R. G.; Horley, C. C. Polymer 1985,26, 551.
Konstadinidis et al.
to investigate the interface between PMMA and aluminum oxide films. In their experiments the oxide was formed in ambient conditions on an evaporated aluminum film and the polymer was spin cast from solution on the oxide surface, after which a tunneling junction was fabricated by vapor deposition of a lead thin film under vacuum. The IETS spectrum, when compared to the IR spectrum of the bulk polymer, showed a decrease in the intensity of the carbonyl peak (1677cm-') and the appearance of two new peaks (1460and 1621 cm-') corresponding to the carboxylate ion symmetric and asymmetric stretching vibrations, respectively. The authors suggested that the polymer undergoes ester cleavage on the oxide surface leading to the formation of carboxylate ions which bond ionically to the surface. Logvinenko and Gorokhovskii21 used IR spectroscopy to investigate the thermal degradation of PMMA on a-alumina. The polymer was adsorbed from solution on alumina tablets. They suggested that ester bond fracture occurs during adsorption, which leads to the formation of a donol-acceptor bond between the carboxylate ion and the metal cation. The formation of these bonds activates the thermal degradation of the polymer by weakening the carbon-carbon backbone bond. In similar studies and Colleti et al.' used IRRAS and IETS, respectively, to study the adsorption of poly(acrylic acid) (PAA)on aluminum oxide. Both studies suggested that PAA adsorption occurs by acid dissociation resulting in the formation of carboxylate ions bonded to the oxide surface. In all of the above cases the surface hydroxyl groups play an important role in the interface reaction. Despite these studies, further systematic work is needed as certain issues are unresolved. The aluminum oxide surface has to be thoroughly characterized before any conclusions on its interactions with PMMA are to be drawn. In view of Allara and Nuzzo's observations,'g the effect of the cleanliness of the surface both on the adsorption kinetics and on the adsorption mechanism has to be established. And, of course, issues concerning the polymeric nature of the adsorbates must be considered. Although the local chemistry that leads to polymer adsorption might be the same as for small molecules bearing the same functional groups, polymer molecules, due to their large size and multiple functionality, can adopt configurations on surfaces that are different compared to those in bulk. NMR is a suitable technique for this problem and is used in this work to attack it. Another issue that is polymeric in nature is the effect of the method used to bring the polymer into contact with the oxide surface. Adsorption from solution and spin coating are two very popular deposition methods, but they differ greatly in the time available for reaching equilibrium. While spin coating allows a few seconds before solvent evaporation and "freezing" of the polymer into its glassy, nonequilibrium state, adsorption from solution allows ample time for equilibration. Considering that we are dealing with surfaces exposed to the laboratory environment (i.e. contaminated), and the fact that polymer molecules equilibrate over much longer time scales than small molecules, the state and properties of the polymer/ oxide interface are generally affected by the polymer deposition method. In this work we report experimental results utilizing a variety of spectroscopic techniques that address the issues of bonding and near-surface chain conformations at (21)Logvinenko, P.N.;Gorokhovskii, G. A. Vysokomol. eoyed. 1980, A22 (No. 4),812. (22)Allara, D.L.Polym. Sci. Technol. 1980,12B,751.
PMMA on Aluminum Oxide Surfaces PMMA-aluminum oxide interfaces. Some of our results confirm (in general terms) earlier work with regard to the interfacial chemistry. However, we provide new information regarding the details of near-surface chain conformations induced by the strong and specific chemical interactions at the interface.
Experimental Section Materials. Narrow molecular weight distribution PMMA samples with the following properties were used: (1)Du Pont PMMA, MW = 7500, polydispersity < 1.04; (2) Pressure Chemical PMMA, MW = 61 800, polydispersity < 1.08, lot no. PM6-2; (3) Pressure ChemicalPMMA, MW = 330 000, polydispersity < 1.08, lot no. PM5-9; (4) Pressure Chemical PMMA, MW = 2 200 000, polydispersity < 1.08, lot no. PM8-11; (5) PMMA, MW = 47000, polydispersity 1.9 (determined with GPC). Two different kinds of surfaces were used. For the planar surfaces used in IRRAS and XPS experiments aluminum metal wires of 99.999% purity were used for the metal evaporation onto glass slides or silicon wafers. For the NMR experiments the aluminum oxide (ACS registry no. 1344-28-1)was purchased from Aldrich Chemical Co. (catalog no. 19,997-4) and used as received. The oxide was amorphous (from X-ray diffraction data), with surface area 155 m2/g, particle size 105 pm, and average pore diameter 58 A, as specified by the manufacturer. Spectroscopic grade chlorobenzene (Aldrich) was used as a solvent in the IR and XPS experiments and spectroscopic grade chloroform (Fisher Scientific) in the NMR experiments. Sample Preparation. XPS and IR Experiments. The aluminum oxide films were prepared by vapor deposition of aluminum onto glass or silicon substrates (1mm X 25 mm X 75 mm) to a thickness of about 1000 A. Prior to deposition the substrates were washed with nitric acid, rinsed with ethanol, and dried with nitrogen. The deposition pressure was about lo4 Torr, and the thickness of the deposited aluminum film was monitored with a quartz crystal thickness monitor. After deposition, the samples were exposed to laboratory ambient and a native aluminum oxide film was formed on the surface. To characterize the aluminum oxide surface in the absence of carbonaceous contamination adsorbed from the laboratory ambient, some samples were cleaned by exposure to a very low power water plasma discharge in an appendage reactor, which has been previously described.23 Such an approach allows for transfer to the analytical chamber of a surface analysis system without risk of recontamination of the plasma-cleaned sample. The conditions employed for rf (13.56 MHz) plasma cleaning are 1.5 W net power dissipated in a 1-Lreactorvolume through which a flowing stream water vapor at 300 mTorr is pumped. Cleaning times of 3 min or less are found sufficient to remove all traces of carbon-containing contamination as determined by XPS at low photoemission angle (loo). Under these conditions in the reactor configuration employed, the active glow extends to the surface such that the primary chemical etchant, atomic oxygen, and low energy positive ions (
