Infrared Study of the Kinetics and Mechanism of Adsorption of Acrylic

R. Tannenbaum*, S. King, J. Lecy, M. Tirrell, and L. Potts* ... on Polyurethane/Siloxane Cross-Linked Films for Hydrophobic Surfaces by the Sol–Gel ...
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Langmuir 2004, 20, 4507-4514

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Infrared Study of the Kinetics and Mechanism of Adsorption of Acrylic Polymers on Alumina Surfaces R. Tannenbaum,*,† S. King,† J. Lecy,‡ M. Tirrell,§ and L. Potts*,| School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, Department of Chemical Engineering, University of California at Santa Barbara, Santa Barbara, California 93106, and Department of Chemistry, Gustavus Adolphus College, St. Peter, Minnesota 56082 Received November 13, 2003. In Final Form: January 15, 2004 In this paper, we studied the kinetics of the adsorption of poly(methyl methacrylate), PMMA, onto native aluminum oxide surfaces by X-ray photoelectron spectroscopy and reflection-absorption infrared spectroscopy, with the intent of tracking the various changes observed in the infrared spectrum of the adsorbed polymer layer as a function of adsorption time. Specifically, we utilized the relative changes in the absorption bands of the carbonyl, carboxylic acid, and carboxylate groups to determine the sequence of events that culminate in the formation of bonds between carboxylate groups on hydrolyzed PMMA and specific sites on the aluminum oxide surface. We have shown that the adsorption process involves the hydrolysis of a fraction of the methoxy groups of the PMMA to generate COOH groups. Unlike previous assumptions, the formation of COOH groups on the PMMA chains does not constitute a sufficient condition for the actual chemisorption of the polymer chains onto the metal oxide surface. To promote bonding, the acid groups must undergo dissociation to form the carboxylate groups, followed subsequently by actual bond formation, that is, active anchoring, on the surface. This process is mediated by the aluminum oxide sites on the surface in the presence of water. Hence, the adsorption process occurs via a two-step mechanism, in which the first step, that is, the hydrolysis step, is a necessary but insufficient condition and the second step, that is, the anchoring step, is largely dependent on the type of interfacial chemistry possible for a particular polymer-metal oxide surface, the polymer conformation, the molecular weight, and the flexibility of the adsorbing molecules.

1. Introduction The interface between insulating polymers and metals or metal oxides is important in a wide variety of areas such as microelectronic devices, flexible interconnections, photovoltaics, microlithography, corrosion protection, polymer composites and in the adhesives, sealants, and coatings industries. The details of the physical interactions and solid-state chemical reactions between polymer surface atoms and the metal/metal oxide surface and nearsurface atoms will be essential to an eventual thorough understanding of the macroscopic properties of a variety of chemically bonded multilamellar dielectric and composite systems. When polymers are brought into contact with solids, the chemical reactions should be similar if not identical to those of their small molecule counterparts.1-18 The nature of the interactions between polymers and solids * Corresponding authors. E-mail: rina.tannenbaum@ mse.gatech.edu (R.T.); [email protected] (L.P.). † Georgia Institute of Technology. ‡ University of Minnesota. § University of California at Santa Barbara. | Gustavus Adolphus College. (1) Crispin, X.; Lazzaroni, R.; Geskin, V.; Baute, N.; Dubois, P.; Jerome, R.; Bredas, J. L. J. Am. Chem. Soc. 1999, 121 (1), 176-187. (2) Karpovich, D. S.; Blanchard, G. J. Langmuir 1997, 13 (15), 40314037. (3) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50 (6-8), 141. (4) Bizzotto, D. Electrochem. Soc. Interface 1996, 5 (1), 47-48. (5) King, E. M.; Clark, S. J.; Verdozzi, C. F.; Ackland, G. J. J. Phys. Chem. B 2001, 105 (3), 641-645. (6) Sasaki, M.; Yoshida, S. Appl. Surf. Sci. 1997, 121-122, 73-79. (7) Ehara, T.; Hirose, H.; Kobayashi, H.; Kotani, M. Synth. Met. 2000, 109 (1), 43-46. (8) Johansson, E.; Nyborg, L. Surf. Interface Anal. 2000, 30 (1), 333336.

depends on the unique characteristics of chain molecules. In a macromolecule with many identical segments, any interaction such as bonding, which occurs with one segment, gets multiplied by the large number of segments into a large macromolecule-to-surface effect. As a result, a polymer could bind very strongly to a solid, even though each segment-level interaction might be relatively weak.19,20 This means that molecular weight is an important variable in polymer adsorption. It is also reasonable to generalize that if the polymer molecule equilibrates with the surface, strong segmentlevel binding produces a more compact configuration, flattened into the surface plane of the solid, while weaker binding allows portions of the chain to extend normal to the solid in loops and tails. On the other hand, strong, (9) Losic, D.; Shapter, J. G.; Gooding, J. J. Langmuir 2002, 18 (14), 5422-5428. (10) Zou, S.; Williams, C. T.; Chen, E. K.-Y.; Weaver, M. J. J. Phys. Chem. B 1998, 102 (45), 9039-9049. (11) Barlow, S. M.; Haq, S.; Raval, R. Langmuir 2001, 17 (11), 32923300. (12) Zhao, H.; Kim, J.; Koel, B. E. Surf. Sci. 2003, 538 (3), 147-159. (13) Cai, L.; Xiao, X.; Loy, M. M. T. Surf. Sci. 2001, 492 (1-2), L688L692. (14) Zubkov, T.; Morgan, G. A., Jr.; Yates, J. T., Jr.; Kuhlert, O.; Lisowski, M.; Schillinger, R.; Fick, D.; Jansch, H. J. Surf. Sci. 2003, 526 (1-2), 57-71. (15) Rzeznicka, I. I.; Matsushima, T. J. Phys. Chem. B 2003, 107 (33), 8479-8483. (16) Ormerod, R. M.; Lambert, R. M.; Bennett, D. W.; Tysoe, W. T. Surf. Sci. 1995, 330 (1), 1-10. (17) Carrez, S.; Dragnea, B.; Zheng, W. Q.; Dubost, H.; Bourguignon, B. Surf. Sci. 1999, 440 (1-2), 151-162. (18) Lee, I.; Wool, R. P. Macromolecules 2000, 33 (7), 2680-2687. (19) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1991. (20) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953.

10.1021/la036137v CCC: $27.50 © 2004 American Chemical Society Published on Web 04/23/2004

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nonequilibrium binding may also lock chain segments into loops and entangle unreacted polymer chains. Loops and long tails of chains extending away from the surface produce a sort of molecular scale texture and mechanical interlocking that can be effective in transmitting stress between even molecularly smooth surfaces and their surroundings.21-27 The bonding strength, through its effect on the polymer configuration, exerts an influence on the degree of this interphase entanglement that can be accomplished by adherent macromolecules.19-31 While there are many internal configurational degrees of freedom in a macromolecule because of the large number of segments, chemically, a macromolecule is a connected set of reactive groups and has fewer degrees of freedom and more steric restrictions than the same number of disconnected equivalent reactive groups.19,20,28-31 Much of the published work regarding metal-polymer surface interactions and adhesion is concerned with the surface interactions of polyimide with a variety of metals, where the polymer acts as the substrate and the metal is deposited in a high vacuum environment.32-37 In these cases, the surface atoms of the polymer encounter individual zerovalent metal atoms and, hence, the interactions may be attributed uniquely to the metal-polymer chemistry at the evolving interface. When metal coverage on the polymer surface is more than a monolayer, most of the bond-forming interactions have already occurred and, hence, the interactions that develop in these systems are between polymer surfaces and discrete metal atoms, rather than metal surfaces. On the other hand, interfaces created in an unprotected environment involve the presence of a mixture of the metal oxide, metal hydroxide, and mixed oxyhydroxide species on the metal surface and, hence, the ensuing interfacial interactions are a result of the contact between the polymer and a variety of metal compounds.38 Our continued interest in the details of the interfacial interactions that take place between a native metal oxide surface and a polymer stems from some of our previous work regarding the mechanism of such processes.37-39 We (21) Konstadinidis, K.; Prager, S.; Tirrell, M. J. Chem. Phys. 1992, 97 (10), 7777-1180. (22) Brownstein, S. K.; Guiver, M. D. Macromolecules 1992, 25 (20), 5181-5185. (23) Zhang, H. F.; Leung, W. T.; Tsui, O. K. C. Proceedings of the 3rd Joint Meeting of Chinese Physicists Worldwide; World Scientific Publishing Co. Pte., Ltd.: Singapore, 2002; pp 174-175. (24) Joshi, Y. M.; Lele, A. K. J. Rheol. 2002, 46 (2), 427-453. (25) Simmons, E. R.; Chakraborty, A. K. J. Chem. Phys. 1998, 109 (19), 8667-8676. (26) Peters, M. H.; Ying, R. J. Chem. Phys. 1992, 98 (8), 6492-6503. (27) Tadd, E. H.; Zeno, A.; Zubris, M.; Dan, N.; Tannenbaum, R. Macromolecules 2003, 36 (17), 6497-6502. (28) McCafferty, E. J. Electrochem. Soc. 2003, 150 (7), B342-B347. (29) Landry, C. J. T.; Teegarden, D. M.; Coltrain, B. K. Polym. Mater. Sci. Eng. 1994, 71, 83-84. (30) Whitesides, G. M.; Ferguson, G. S.; Allara, D.; Scherson, D.; Speaker, L.; Ulman, A. Crit. Rev. Surf. Chem. 1993, 3 (1), 49-65. (31) Allara, D. L.; Fowkes, F. M.; Noolandi, J.; Rubloff, G. W.; Tirrell, M. V. Mater. Sci. Eng. 1986, 83 (2), 213-226. (32) Brown, H. R. Mater. Forum 2000, 24, 49-58 and pertinent references therein. (33) Jackman, R. J.; Brittain, S. T.; Adams, A.; Wu, H.; Prentiss, M. G.; Whitesides, S.; Whitesides, G. M. Langmuir 1999, 15 (3), 826-836. (34) Grunze, M.; Hahner, G.; Woll, Ch., Surf. Interface Anal. 1993, 20 (5), 393-401. (35) Ho, P. S. Appl. Surf. Sci. 1985, 41-42, 559-566 and pertinent references therein. (36) Zazzera, L.; Tirrell, M.; Evans, J. F. J. Vac. Sci. Technol., A 1993, 11, 2239-2243. (37) Tannenbaum, R.; Hakanson, C.; Zeno, A. D.; Tirrell, M. Langmuir 2002, 18 (14), 5592-5599. (38) Kostandinidis, F.; Thakkar, B.; Chakraborty, A. K.; Potts, L.; Tannenbaum, R.; Tirrell, M.; Evans, J. Langmuir 1992, 8, 1307-1317. (39) King, S.; Hyunh, K. Tannenbaum, R. J. Phys. Chem. B 2003, 107 (44), 12097-12104.

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have studied in some detail the behavior of poly(methyl methacrylate) (PMMA) when brought in contact, either by adsorption or by spin-coating, with several metal oxide surfaces, such as aluminum oxide,38 chromium oxide,37 copper oxide, and cobalt oxide.39 In all these cases, deposition of PMMA on the native oxides by spin-coating from solution resulted in no spectroscopically detectable chemical reaction between the polymer and the solid. On the other hand, adsorbing PMMA from solutions onto the same surfaces and allowing the solutions and surfaces to remain in contact for several days resulted in substantial hydrolysis of the polymer ester bonds and the formation of MAA segments. Comparison of the infrared spectrum of bulk PMMA with the infrared reflection-absorption spectroscopy (IRRAS) spectrum of a thin film of PMMA (30-80 Å) adsorbed from a solution revealed an important new feature, an absorption peak at ∼1670 cm-1, which comes from the asymmetric stretch of a carboxylate ion. As control experiments, exposing the native oxide to solvent alone or exposing thick PMMA films on metal oxide surfaces to surrounding solvent produced no such peak. Hence, this peak was considered evidence for the hydrolysis of the PMMA ester groups in the thin polymer layer near the surface. The lack of interaction in the case of spin-coated samples was attributed to the rapid evaporation of the solvent during spin-coating, resulting in the rapid vitrification of the PMMA, quenching most of the PMMA segments in positions where reaction with the surface was impossible. Estimates of the extent of reaction of the PMMA ester groups through the use of both Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) are in the range of 50-60%, consistent with a flattened conformation of the adsorbed polymer, in the indirect sense that, if all 60% of the reacted segments were in contact with the surface simultaneously, this would imply a flattened conformation. From the standpoint of adhesion science, we believe that there are several significant consequences of this preliminary work. Moreover, these results may be of interest also for areas diverse from adhesion, such as ceramics processing and corrosion protection. In the case of ceramic processing, it has been found that acrylic polymers used as binders, rheology modifiers, and dispersants undergo chemistry during the desired pyrolysis reactions for burning out the polymer from the ceramic, which appears to be consistent with the de-esterification mechanism we see in our studies.40 In the case of corrosion protection, use of acrylic-containing coatings in harsh environments may benefit from more detailed knowledge of this interfacial chemistry as well. The formation of a carboxylate-metal oxide ionic bond seems to be a general chemical phenomenon when acrylic polymers are brought into contact with oxidized metals. However, our observations so far have been largely qualitative because they pertain mainly to the identification of the various species that are formed at the interface between the metal oxide and the polymer, and on the basis of this information, we have been able to reach some conclusions regarding their mechanism of formation. One of the most important aspects of this adhesion process that remains unanswered is to what extent it is accurate to view the metal oxide-polycarboxylate surface bond as irreversible and as an inevitable consequence of the hydrolysis reaction and how strongly the polymer segments that contact the solid become locked into place. (40) Sun, Y. N.; Sacks, M. D.; Williams, J. W. Ceram. Trans. 1988, 1, 538-548 (Ceram. Powder Sci. 2, Part A, 1988).

Acrylic Polymers on Alumina Surfaces

This has ramifications in determining the conformation of the solid-bound polymer and its ability to transmit stress between the solid and the contacting polymer phase. In this paper, we study the dynamics of PMMA adsorption onto native aluminum oxide surfaces with the intent of tracking the various changes observed in the infrared spectrum of the adsorbed polymer layer as a function of adsorption time. Specifically, we will utilize the relative changes in the absorption bands of the carbonyl, carboxylic acid, and carboxylate groups to determine the sequence of events that culminate in the formation of bonds between carboxylate groups on hydrolyzed PMMA and specific sites on the aluminum oxide surface. 2. Experimental Section 2.1. Preparation of the Al-PMMA, Al-Poly(acrylic acid) PAA, and Au-PMMA Samples. A 1000-Å layer of aluminum was deposited onto Si wafers by electron beam evaporation, using an instrument equipped with a multisample holder mounted on a rotating carriage to ensure even deposition. The thickness of the metal film was measured with a quartz crystal monitor. Optimal depositions were performed under 2 × 10-6 Torr without heating the wafers. Prior to deposition, the silicon substrates were washed with nitric acid, rinsed with ethanol, and dried with nitrogen. Gold surfaces were obtained by the sequential evaporation of a 1000-Å adhesion layer of chromium onto Si substrates, followed by a 1000-Å layer of gold. Immediately upon removal from the evaporator, the freshly prepared aluminum and gold-coated substrates were immersed in a 1.0 wt % chlorobenzene solution of PMMA (Mead Technologies, Inc., M hn ) 60 000, 120 000, 250 000, and 330 000 g/mol, PDI < 1.08). In some cases, the PMMA solution contained 0.2 vol % added deionized water. The time lapse from the opening of the sample holder of the evaporator and the complete immersion of the samples in the PMMA solution was on the average less than 10 min. This allowed the formation of an amorphous native oxide layer on the aluminum surface.38 The PMMA solutions with the immersed metal oxide substrates were then placed in a desiccator, which was subsequently evacuated and filled with N2. The metal oxide substrates were left in the PMMA solution under an inert atmosphere and at room temperature for different periods of time, depending on the type of experiment performed. After the allotted time, the substrates were removed from the PMMA solution and dried under a vacuum at 130 °C for 24 h to remove the residual solvent and ensure the equilibrium adsorption of PMMA onto the metal surface. Subsequently, for most experiments, residual PMMA, not directly adsorbed onto the metal surface, was removed by immersing the samples in pure chlorobenzene for 4 h, followed by the drying process in the vacuum oven at 130 °C for 24 h. The thicknesses of the polymer films resulting from this process were determined by ellipsometry. This soaking time gives a chance for entrained but only lightly adsorbed chains to wash out and for more tightly adsorbed chains to adsorb even more, if they are able to reorient easily enough. Prior to the adsorption process, the optical parameters of the bare metal oxide layer and the refractive index of a spin-coated (nonadsorbing) PMMA film were determined. The PMMA film thickness at various different points on the substrate and for two different angles of incidence was determined to be ∼87 ( 8 Å. The film was thin enough to see the photoemission signal and avoid distortions of the vibrational band shapes that depend on the thickness and refractive index of the sample. In the experiment in which the PMMA adsorption was monitored as a function of time, no additional washing of the films was necessary and the polymer film thickness was measured directly on the dried samples. The thicknesses of the polymer films resulting from this process were determined by ellipsometry as well. Spin-coated films of PMMA were obtained by spin-coating the same polymer solutions onto the metal oxide substrates for different time intervals, to obtain different thicknesses. The optimal spin-coating conditions to allow the formation of an ∼87-

Langmuir, Vol. 20, No. 11, 2004 4509 Å-thick PMMA film (∼10 s at 3600 rpm) were determined by trial and error through a series of spin-coating experiments followed by the determination of the film thickness using ellipsometry. The bare metal oxide surfaces were characterized by transmission electron microscopy (TEM) and by XPS. TEM samples were obtained by gluing TEM copper sample grids (Ted Pella, 400 mesh, carbon-coated) onto the glass substrates prior to electron beam deposition by the use of a double-sided tape. After the metal deposition, the copper grids were very carefully removed from the tape to avoid tearing or bending. TEM analysis and electron diffractions were performed on both TEM (Hitachi HF2000 field emission gun and JEOL 2010) and high-resolution TEM (JEOL 4000EX). The operating voltage was 200 keV for all three microscopes. PAA films (Scientific Polymer Products, Inc., Ontario, NY, M hn ) 250 000 g/mol) were prepared by the adsorption of the polymer onto metal-coated Si substrates from dilute ethanol solutions (100 µg PAA/mL solution) similarly to the PMMA samples. 2.2. Surface-Sensitive IRRAS. The preliminary infrared experiments were conducted on an IBM-IR-44 (Bruker) FTIR spectrometer equipped with a Harrick variable-angle specular reflection (IRRAS) attachment.41 The cell chamber was purged with dry nitrogen for at least 1/2 h before collecting interferograms. The resolution was 2 cm-1, and 3000 scans were collected for the spin-coated and the adsorbed polymer samples. The majority of infrared experiments were conducted on a Nicolet Nexus 870, equipped with a polarization-modulation (PM)-IRRAS attachment. The polarization was achieved by using a 50-kHz ZnSe photoelastic modulator.42 The polarization-modulated IR light impinges on the sample surface near the grazing angle, ∼80°, and the reflected light from the sample surface is then collected by a BaF2 lens and focused onto a mercury cadmium telluride (cooled with liquid N2) detector. Also in these experiments, the resolution was 2 cm-1 and 3000 scans were collected for the spin-coated and the adsorbed polymer samples. In experiments in which the PMMA solution contained 0.2 vol % added deionized water, the spectrum of isolated water molecules (0.2 vol % deionized water dissolved in chlorobenzene with a typical OH stretching band at ∼3650 cm-1) was subtracted from the sample spectrum to eliminate all possibility of the wrong interpretation of the OH absorption bands. All spectra underwent ratio operations against a freshly measured background spectrum, and the subsequent subtraction of spectra was conducted without additional spectral manipulation or factoring. 2.3. XPS. The XPS experiments were carried out using a Perkin-Elmer model 5400 spectrometer equipped with an ultrahigh vacuum chamber using a double pass cylindrical mirror analyzer with nonmonochromatized Mg KR (1253.6 eV) radiation and an analyzer pass energy of 35.75 eV. The source was operated at 100 W to minimize the damage to the PMMA films in the samples that were examined. Under these conditions, C(1s) spectra of both spin-coated and adsorbed PMMA indicate that samples could be safely exposed to the source for about 15 min. The operating pressure of the vacuum chamber was 2 × 10-10 Torr. The measurements were made at the normal (90°) angle of incidence. The data analysis was carried out on a PerkinElmer 7000 computer. The details of the protocol used for curve fitting are given in several of our previous reports.37,38

3. Results and Discussion The FTIR spectra of PMMA films of equivalent thickness (∼87 Å) that were deposited by various techniques onto different substrates are shown in Figure 1a. All three spectra reveal common features: (a) The asymmetric and symmetric stretching modes of the CH3 (either on C or on O) and the CH2 groups in the 2850-3050 cm-1 frequency region; (b) the CdO stretching vibration in the 17401750 cm-1 frequency region; (c) and the CsO and CsC coupled stretching modes in the 1150-1300 cm-1 frequency region. However, the spectrum of a thin PMMA (41) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (42) Wang, B. Spectroscopy 1997, 12 (1), 30.

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Tannenbaum et al. Table 1. Summary of the Infrared Absorption Bands of Three Different Thin PMMA Layers Spin-Cast on Gold, Spin-Cast on Aluminum Oxide, or Adsorbed from Solution onto an Aluminum Oxide Surface adsorbed on Au ( 2 (cm-1) N/A 2982 2938 2910 2819 1748 N/A N/A N/A

spin-cast on Al2O3 ( 2 (cm-1) N/A 2996 2953 2919 2839 1751 N/A N/A N/A

adsorbed on Al2O3 ( 2 (cm-1)

peak assignment

3268 2996 2955 2912 2839 1745 1708 1685 1588

OH in COOH CsH in OsCH3 CsH in CsCH3 CsH in CsCH2sC CsH in >CHsC CdO CdO in COOH COO-(asymmetric) COO-(symmetric)

group, according to the following reaction:

Figure 1. (a) FTIR spectra of PMMA films of equivalent thickness (∼87 Å) that were deposited by various techniques onto different substrates. Spectrum A is a thin PMMA layer adsorbed from solution on a gold surface, spectrum B is a thin PMMA layer spin-coated on an aluminum oxide surface, and spectrum C is a thin PMMA layer that was adsorbed from solution on an aluminum oxide surface. (b) The direct comparison of the spectra of PMMA and PAA. Spectrum A is a very thin PMMA layer adsorbed on an aluminum oxide surface, and spectrum B is a very thin PAA layer, also adsorbed on an aluminum oxide surface.

layer that was adsorbed from solution on an aluminum oxide surface (spectrum C) exhibits two additional absorption bands, one in the 3200-3300 cm-1 region, corresponding to OsH stretching modes in organic acids, and two other bands in the 1600-1700 cm-1 region, corresponding to the stretching modes of hydrogen-bonded COOH and COO- groups. These bands were absent from the other two spectra, which were obtained from a thin PMMA layer spin-coated on an aluminum oxide surface (spectrum B) and a thin PMMA layer adsorbed from solution on a gold surface (spectrum A).43-45 A detailed summary of the various experimental infrared absorption bands of PMMA is shown in Table 1. For a better understanding of the differences between the spectra of the various samples, a direct comparison of the spectra of a very thin PMMA layer adsorbed on an aluminum oxide surface (spectrum A) and a very thin PAA (a carboxylic acid) layer, also adsorbed on an aluminum oxide surface (spectrum B), is shown in Figure 1b. As was previously suggested and as evidenced by the changes seen in the spectrum of the adsorbed PMMA (an ester) on amorphous aluminum oxide and the similarities with the PAA spectrum, the ester group in PMMA undergoes hydrolysis, facilitated by the presence of OH groups on the surface of the aluminum oxide surface, to produce either the COOH acid group or its conjugate COO- base (43) Allara, D. L.; Pryde, C. A. Org. Coat. Plast. Chem. 1978, 38, 638-45. (44) Allara, D. L. Polym. Sci. Technol. 1980, 12B, 751-756. (45) Allara, D. L.; Baca, A.; Pryde, C. A. Macromolecules 1978, 11 (6), 1215-1220.

This latter group can directly interact with the positively charged aluminum atoms to generate a Lewis acid-base chemisorbed polymer layer on the aluminum oxide surface. This type of reaction was observed to various extents also on other metal oxide surfaces, provided the metal oxide present was capable of hydration, according to the following reaction:46,47

M2O3 + H2O h 2MO(OH); M ) Al, Cr, Co, Cu

(2)

The concentration of the oxyhydroxide surface groups at equilibrium is a direct function of the amount of water available at the aluminum oxide surface. The involvement of the surface OH groups in the surfacemediated hydrolysis of PMMA is evident in the attenuation of the O(1s) core electrons peak of the metal oxyhydroxide OH group in the XPS experiments, at 531.7 eV, as shown in Figure 2a. The relative abundance of the OH species on the Al2O3/AlO(OH) surface, obtained by the curve fitting of the high-resolution contour spectrum,37,38 is 41%, while its concentration at the interface with the adsorbed PMMA is reduced to 18%. The presence of surface OH groups on the aluminum oxide surface is further demonstrated by the OsH stretching vibration at 3552 cm-1, as shown in the PM-IRRAS spectrum of the aluminum oxide surface, from which the PM-IRRAS spectrum of a gold surface was subtracted in Figure 2b. The positive adsorption peaks represent species that are present on the aluminum oxide surface but not on the gold surface. The reaction of these groups is supported by the disappearance of this OsH stretching vibration on the surface once a thin layer of PMMA has been adsorbed and the persistence of this weak peak in spin-coated samples. The fact that PMMA that was spin-coated onto an aluminum oxide surface does not appear to have undergone this surface-promoted hydrolysis is due to the rapid removal of the solvent and the resulting fast transformation of the polymer to its glassy state. Once in the glassy state, the polymer chains are restricted in their mobility and the reaction with the surface becomes kinetically hindered. The absence of surface-mediated hydrolysis in the PMMA sample adsorbed on gold supports the suggestion that there are two necessary conditions for the (46) Aboulayt, A.; Mauge, F.; Hoggan, P. E.; Lavalley, J. C. Catal. Lett. 1996, 39 (3, 4), 213-218. (47) Guillet, B.; Souchier, B. Pedologie 1979, 2, 16-37.

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Figure 3. Progress of the adsorption of PMMA from a solution containing 0.2 vol % of added deionized water on an aluminum oxide substrate, as a function of exposure time.

Figure 2. (a) High-resolution XPS spectra of the O(1s) core electrons of surface OH groups of the bare aluminum oxide surface and of the same surface covered with a thin layer of adsorbed PMMA. The gray curves are the experimental O(1s) contour spectra of the hydrated surface before and after PMMA adsorption, and the black curves are the specific curve-fitted O(1s) oxyhydroxide OH band before and after PMMA adsorption. (b) PM-IRRAS difference spectrum of an aluminum oxide surface and a gold surface. The positive adsorption peaks represent species that are present on the aluminum oxide surface but not on the gold surface.

occurrence of the reaction: (a) the availability of a reactive metal oxide surface capable of generating OH surface groups in the presence of H2O and (b) the ability of the polymer segments to reach the reactive metal oxide surface on time scales commensurate with the kinetics of the hydrolytic reaction. On gold substrates, no metal hydroxyl groups are generated even in the presence of added H2O and, hence, the PMMA layer is expected to adsorb weakly without undergoing any visible reaction. In spin-coated samples that were deposited on either gold or metal oxide substrates, no such reaction is expected as a result of the very short time scale for polymer chain motion before solvent removal. According to these requirements, the addition of water to a reactive aluminum oxide surface in the presence of the polymer would promote the hydrolysis of the ester group in PMMA and lead to an increase of the concentration of the carboxylate species in the polymer and, thus, will give rise to an extensive interaction between the polymer and the metal oxide surface. Figure 3 shows the progress of the adsorption of PMMA from a solution containing 0.2 vol % added deionized water on an aluminum oxide substrate as a function of exposure time. Clearly, the presence of the added water has promoted the surface-mediated hydrolysis reaction of PMMA, as observed by the increase in absorbance of the OH stretching bands in the 3200-3300 cm-1 spectral region, corresponding to the formation of PMAA. The formation of the acid as a result of hydrolysis is also evidenced by the appearance of the hydrogen-bonded stretching band of the COOH group at 1708 cm-1 and the 1685 and 1588 cm-1 bands of bound COO-, corresponding to the asymmetric and symmetric stretches, respectively. In the absence of reliable calibration data regarding the molar absorptivities of these bands, we have used the absorbance of the free CdO bond in the ester group of the

Figure 4. (a) Plot of the COOH/CdO ratio, represented by the relative intensities of the 3268 and 1741 cm-1 bands, and the COO-/CdO ratio, represented by the relative intensities of the 1685 and 1741 cm-1 bands, as a function of adsorption time. (b) The concentrations of the ester groups in PMMA, COOH groups in the hydrolyzed polymer, and carboxylate groups as a function of adsorption time.

adsorbed PMMA as an internal standard. If all the species resulting from the hydrolysis of PMMA are generated in similar concentrations, as would be expected according to Eq 1, then the ratio of the new bands formed to the ester CdO band of PMMA would increase linearly with the amount of adsorbed PMMA. This implies that the more PMMA is hydrolyzed and adsorbed on the aluminum oxide surface, the higher the ratios of the new infrared bands to the unaltered carbonyl ester band should be, that is, that the adsorption of PMMA is a linear function of the polymer concentration on the surface. However, Figure 4a reveals that the COOH/CdO ratio, as measured by the relative intensities of the 3268 and 1741 cm-1 bands (the CdO stretching vibration of the ester group in the waterrich experiments has shifted to a slightly lower frequency), is almost constant throughout the adsorption process. On the other hand, the COO-/CdO ratio, as measured by the relative intensities of the 1685 and 1741 cm-1 bands, exhibits a nonlinear behavior during the adsorption process. Examination of Figure 4b shows that, in the initial

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stages of the adsorption, hydrolyzed PMMA is deposited on the aluminum oxide surface, but the concentration of the carboxylate group is negligible. After an induction period of about 24 h, the appearance of the carboxylate peak at 1685 cm-1 begins to increase in parallel with the continuing increase in the concentration of the COOH groups and the CdO groups. After about 48 h, the concentration of the carboxylate groups on the surface remains constant, which translates into a decrease in the COO-/CdO ratio, while the concentration of the other main groups, COOH and ester CdO, continues to grow as adsorption proceeds. These observations lead to the following conclusions: (a) The hydrolysis of the PMMA molecules is a result of a reaction with the surface aluminum oxyhydroxide groups generated by the reaction of aluminum oxide and surface water. The fraction of methoxy groups in the PMMA that undergo hydrolysis must be constant (for a particular chain length), as it is probably dictated by the ability of the polymer chain to expose methoxy groups to the hydrated metal oxide surface. Hence, the hydrolyzed PMMA present near the aluminum oxide surface after the reaction has taken place results in a constant proportion of CdO and acid OH groups and (b) the formation of the carboxylate group occurs as a result of the molecular proximity of a COOH group in the hydrolyzed PMMA to a particular site on the aluminum oxide surface, resulting in an anchoring point, that is, a bond formation. The number of anchoring points for a particular polymer chain is dependent on, and limited by, the chain conformation, length, and strength of interaction with the surface. Hence, the COO-/CdO ratio may initially increase with the increase in the adsorption layer, but once a monolayer of polymer has been adsorbed, no additional carboxylate groups will be formed and, hence, the relative concentration of these groups in the adsorbed layer will decrease. The development of anchoring points at the metal oxide-polymer interface, that is, the formation of bonds between the carboxylate groups on the polymer and the surface oxide sites, generates polymer loops and trains, depending on the bonding strength between the polymer and the surface, the conformation of the chains confined near the surface, and the surface roughness.27,48-51 The measured effective thickness of the polymer layer, Leff, shown in Figure 5a, corresponds to the distance that the largest loop or chain fragment extends above the surface of the metal oxide. If we assume that the chain fragment forming the loop is less affected by the steric confinement on the surface than the fragments at or near the anchoring points, then at first approximation, this fragment may be viewed to have a random coil conformation. Hence, from the value of Leff, we can calculate the minimum average number of unadsorbed repeating units, nloop, that are required to form the loop between two anchoring points and that are a part of the chain that extends out and forms the thickness of the polymer layer:51

[

( )]

1 + cos θ Leff nloop ) 2neff - 1 ) 2 2(1 - cos θ) σl

2

Figure 5. (a) Measured effective thickness of the adsorbed polymer layer, Leff. (b) Number of anchoring points of PMMA per chain on the aluminum oxide surface, as a function of adsorption time.

coil segment of length Leff, θ is 109.5°, σ is the steric hindrance factor, which is 2.1 for PMMA at room temperature,52 and l is the CsC bond length of 1.54 Å. The length of the loop segment of the chain is, therefore, calculated as

x

Lloop ) σl

2nloop(1 - cos θ) 1 + cos θ

(4)

If the random coil average length of the whole polymer chain is Lchain, then the maximum number of anchoring points per polymer chain can be calculated as

Nanchor )

Lchain Lloop

(5)

Figure 5b shows the results for PMMA having an average molecular weight of 250 000 g/mol. It is interesting to note that there is a progressive weakening of the anchoring ability of the polymer chains with increasing adsorption. After ∼48 h, the number of anchoring points per chain with the surface decreases dramatically, indicating that additional chains that are deposited on the surface are too far from the surface to undergo the surface-mediated H2O

-1

(3)

where neff is the number of repeating units in a random (48) Evers, O. A.; Scheutjens, J. M. H. M.; Fleer, G. J. J. Chem. Soc., Faraday Trans. 1990, 86 (9), 1333-1340. (49) Whitmore, M. D.; Noolandi, J. Macromolecules 1990, 23 (13), 3321-3339. (50) Yang, Y.; Yan, X.; Cheng, R. J. Macromol. Sci., Phys. 1999, B38 (3), 237-249. (51) Chibowski, S.; Wisniewska, M.; Marczewski, A. W.; Pikus, S. J. Colloid Interface Sci. 2003, 267 (1), 1-8.

COOH {\} COO- + H3O+ transformation that can lead to bond formation. This agrees well with the observations shown in Figure 3, where the intensity of the absorbance band corresponding to the COO- group reaches a maximum after ∼48 h. Additional support for the two-step adsorption mechanism of PMMA onto aluminum oxide surfaces is obtained by the variation of the COO-/CdO absorbance ratio for (52) Pehlert, G. J.; Painter, P. C.; Coleman, M. M. Macromolecules 1998, 31 (23), 8423-8424.

Acrylic Polymers on Alumina Surfaces

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Figure 6. Variation of the COO-/CdO concentration ratio as evidenced by the absorbance ratio of the corresponding infrared bands (A1685/A1741) for different molecular weights of PMMA.

different molecular weights of the polymer, shown in Figure 6. In the presence of 0.2 vol % water in the polymer solution, the COO-/CdO absorbance ratio scales with the square of the molecular weight, that is, AbsCOO-/AbsCdO ∝M h n2, while the COOH/CdO absorbance ratio remains constant and is independent of M h n.53 This indicates, as concluded earlier from the results in Figure 4, that the formation of the COO- group occurs only at the anchoring point on the surface and is limited by the ability of the polymer to accommodate the conformation and permanent entropy changes resulting from the bond formation at the anchoring site. Because longer chains are more flexible than shorter chains, the stresses generated by conformational changes due to bond formation and anchoring will be better dissipated than those in shorter chains, allowing a more compact adsorbed layer and a more efficient adsorption. In the absence of added water, the extent of hydrolysis mediated by the aluminum oxide surface is more limited, resulting in a weaker hydroxide signature peak generated by the COOH group. Nevertheless, the amount of water vapor present in the ambient atmosphere is sufficient to promote the formation of AlO(OH) at the surface of the oxide layer, as evidenced by the weak absorbance at 3564 cm-1 observed in the spectrum of a thin layer of PMMA that was spin-cast onto an aluminum oxide substrate, as shown in Figure 7 (spectrum A). Despite its glassy state, a limited hydrolysis of the thin polymer layer has nevertheless taken place, as evidenced by the small and broad absorption band at 3236 cm-1, corresponding to the stretching vibration of the OH group in COOH. The shift in the position of this band to a lower frequency denotes a higher degree of hydrogen bonding, most likely with the free OH group on the aluminum oxide surface. This reaction has probably taken place immediately upon the contact of the polymer solution with the hydrated aluminum oxide surface but was further inhibited as a result of the rapid removal of the solvent. Under these circumstances, only few anchoring points are expected in the layer and, indeed, the appearance of the 1685 cm-1 absorption band of COO- is almost negligible. When the sample is heated to 60 °C and annealed at this temperature for several days, the spectrum undergoes a dramatic change and ultimately resembles the spectrum of a thin PMMA layer that was adsorbed on the alumina surface rather than spin-cast (spectrum B). While this temperature is quite far from the glass transition temperature for bulk PMMA (Tg ≈ 105 °C), the effective glass transition for a very thin polymer film is considerably lower and, hence, the polymer chains in the layer are able to exhibit viscous flow. Under such conditions, additional hydrolysis (53) Wu, C.; Gao, J. Macromolecules 1999, 32 (5), 1704-1705.

Figure 7. Influence of temperature on the interfacial interaction capabilities of a thin PMMA film that was originally spincast onto an aluminum oxide surface. Spectrum A is before the annealing process, and spectrum B is after annealing the sample at 60 °C for several days. The first inset is the difference spectrum between the two measurements, and the second inset is the difference spectrum in the 1650-1750 cm-1 spectral region.

becomes possible, but more importantly, as a result of an increase in the kinetic energy of the chains, a more extensive conformational rearrangement can take place at the surface, resulting in the surface-mediated formation of COO- groups, followed by bond formation and anchoring. Hence, the spectrum of the heated sample exhibits an increase in the OH absorption band with a shift to 3271 cm-1, the disappearance of the OH band originating from the aluminum oxide substrate, and a considerable increase of the 1685 cm-1 absorption band of COO-. The difference between the two spectra (before and after heating) is shown in the inset in Figure 7. The difference spectrum was obtained by subtracting spectrum A from spectrum B by simple spectral subtraction without factoring. The negative peaks denote features that were more intense in Spectrum A and the positive peaks denote features that were more intense in spectrum B. From the difference spectrum, it is possible to see that in the heated sample there was a decrease in the intensity of the bands at 2996 and 1745 cm-1, corresponding to the CH3 vibrations in the O-CH3 group and the CdO group in the ester, respectively. This supports the earlier conclusion regarding the hydrolysis of PMMA and the formation of PAA. The second inset in Figure 7 shows the carbonyl region of the difference spectrum, in which two new absorption bands are observed at 1708 and 1685 cm-1, corresponding to the hydrogen-bonded CdO group in COOH and the COO- group, respectively. 4. Summary In this paper, we have shown that the adsorption of PMMA on aluminum oxide surfaces that have been exposed to the ambient environment proceeds via a complex adsorption process. The first step in this process involves the hydrolysis of a fraction of the methoxy groups of the PMMA to generate COOH groups, by the reaction with hydroxide species on the aluminum oxide surface (aluminum oxyhydroxide), generated as a result of the presence of water molecules on the oxide surface. However, we have shown that, unlike previous assumptions, the formation of COOH groups on the PMMA chains does not constitute a sufficient condition for the actual chemisorption of the polymer chains onto the metal oxide surface.

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For bonding to occur, the acid groups must undergo dissociation to form the carboxylate groups, followed subsequently by actual bond formation, that is, active anchoring, on the surface. This process is mediated by the aluminum oxide sites on the surface in the presence of water. Hence, the adsorption process occurs via a twostep mechanism, in which the first step, that is, the hydrolysis step, is a necessary but insufficient condition and the second step, that is, the anchoring step, is largely dependent on the type of interfacial chemistry possible for a particular polymer-metal oxide surface, the polymer

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conformation, the molecular weight, and the flexibility of the adsorbing molecules. Acknowledgment. We thank the Surface Science Center at the University of Minnesota and its director, Dr. Raul Carreta, for assistance with the XPS measurements. We also thank Mr. Attir Khalid for his diligent typing of the manuscript. This work was supported by and ACS-Petroleum Research Fund Type AC award, PRF #40014-AC5M, to R.T., and by an IBM Materials Research Grant to M.T. LA036137V