504
Langmuir 2000, 16, 504-516
Epoxy-Terminated Self-Assembled Monolayers: Molecular Glues for Polymer Layers Igor Luzinov,† Daungrut Julthongpiput,† Andrea Liebmann-Vinson,‡ Tricia Cregger,§ Mark D. Foster,§ and Vladimir V. Tsukruk*,† Department of Materials Science & Engineering, Iowa State University, Ames, Iowa 50011, Becton Dickinson Research Center, Research Triangle Park, North Carolina 27709-2016, and Maurice Morton Institute of Polymer Science, The University of Akron, Akron, Ohio 44325-3909 Received April 26, 1999. In Final Form: July 10, 1999 Dense, homogeneous, and complete self-assembled monolayers (SAMs) with epoxy surface groups were fabricated from epoxysilanes to serve as a template for chemical anchoring of polymer layers. A combination of scanning probe microscopy, ellipsometry, XPS, X-ray reflectivity, and contact angle measurements was used to study their morphology and surface properties. Self-assembly of epoxysilane molecules resulted in the formation of homogeneous SAMs 0.85 nm thick with the surface roughness 0.22 nm. Epoxysilane SAMs were truly monomolecular films with a virtually normal molecular orientation of densely packed molecules, which were firmly attached to the substrate. The formation of stable polymer layers from carboxy-terminated polystyrenes on reactive SAM surfaces was demonstrated.
Introduction A variety of modern applications require highly ordered ultrathin coatings with molecularly controllable surface properties. Some recent examples are molecular lubrication for microelectromechanical systems (MEMSs) and biofunctionalized and biocompatible surfaces.1,2 Mechanical stability is always a critical issue for polymer molecular coatings and affects their long-term durability under alternating shear stresses. A concept of molecular lubrication through nanocomposite multimolecular layers has recently been explored.3,4 As a first example, amine- and sulfate-terminated self-assembled monolayers (SAMs) capped with various rigid polymers and monomers were studied.4 Composite molecular films firmly tethered to solid surfaces possessed superior tribological properties due to the combination of the low shear strength of a supporting compliant sublayer and the high hardness of the rigid polymer overlay.3 These first examples demonstrated that the formation of bilayer SAMs with the low shear strength/high surface hardness combination may result in significant reduction of dissipation of energy. Higher wear stability along with the preservation of the microroughness of grainy polysilicon surfaces of MEMS * Towhomcorrespondenceshouldbeaddressed.E-mail: vladimir@ iastate.edu. † Iowa State University. ‡ Becton Dickinson Research Center. § The University of Akron. (1) Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. Bhushan, B., Ed. Tribology Issues and Opportunities in MEMS; Kluwer Academic Publishers: Dordrecht, 1998. Srinivasan, U.; Houston, M. R.; Howe, R.; Maboudian, R. J. Microelectromech. Syst. 1998, 7, 252. Muller, R. S. In Micro/Nanotribology and Its Applications; Bhushan, B., Ed.; Kluwer Press: 1997; p 579. Komvopoulos, K. Wear, 1996, 200, 305. (2) Elam, J. H.; Nygren, H. J. Biomed. Mater. Res. 1984, 18, 953. Tsukruk, V. V. Prog. Polym. Sci. 1997, 22, 247. (3) Bliznyuk, V. N.; Everson, M. P.; Tsukruk, V. V. J. Tribol. 1998, 120, 489. (4) Tsukruk, V. V.; Nguyen, T.; Lemieux, M.; Hazel, J.; Weber, W. H.; Shevchenko, V. V.; Klimenko, N.; Sheludko, E. In Tribology Issues and Opportunities in MEMS; Bhushan, B., Ed.; Kluwer Academic Publishers: Dordrecht, 1998; p 608.
devices made these molecular coatings very promising as a new generation of molecular lubricants. The availability of various surface reactive groups to modify polysilicon surfaces is critical for testing new polymeric materials appropriate for the design of nanocomposite molecular layers. In the present paper, we focus on fabrication of epoxy-terminated self-assembled monolayers from epoxysilanes and their ability to bind polymer coatings. Epoxysilanes are classical compounds that have been used to enhance the stability and integrity of polymer/ inorganic interfaces.5,6 They are applied widely to a variety of glass-fiber reinforced polymer composite materials.6 These compounds also have found application in the biomedical sciences to provide a strong binding of biological polymers to glass surfaces for biocompatibilization of inorganic surfaces.7,8 However, their ability to form stable, smooth, and homogeneous monolayers has not yet been proven. Usually, for ordinary composite materials, relatively thick (several hundred nanometers) films of these compounds are applied to fabricate effective bonding conditions at interfaces. In several recent studies, attempts to build epoxysilane monolayer films by either dip-coating or vapor deposition have lead to the formation of films composed of at least aggregated molecular layers with unknown surface morphology and microstructure. For example, in ref 7 the thickness of a dip-coated epoxysilane film was determined to be 1.7 nm, which is much greater than the extended molecular length. In addition, no (5) Pluddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1991. Tsubokawa, N.; Kogure, A.; Maruyama, K.; Sone, Y.; Shimomura, M. Polym. J. 1990, 22, 827. (6) Lan, L.; Gnappi, G.; Montenero, A. J. Mater. Sci. 1993, 28, 2119. Xue, G.; Koening, J. L.; Wheeler, D. D.; Ishida, H. J. Appl. Polym. Sci. 1983, 28, 2633. Yamaguchi, M.; Nakamura, Y.; Iida, T. Polym. Polym. Compos. 1998, 6, 85. Hong, S. G.; Lin, J. J. J. Polym. Sci., Polym. Phys. Ed. 1997, 35, 2063. (7) Elender, G.; Kuhher, M.; Sackmann, E. Biosens. Bioelectron. 1996, 11, 565. (8) Salmon, L.; Thominette, F.; Pays, M. F., Verdu, J. Compos. Sci. Technol. 1997, 57, 1119. Petrunin, M. A.; Nazarov, A. P. Mater. Res. Soc. Symp. Proc. 1994, 351, 305. (9) Bierbaum, K.; Kinzler, M.; Woll, Ch.; Grunze, M.; Hahner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512.
10.1021/la990500+ CCC: $19.00 © 2000 American Chemical Society Published on Web 10/29/1999
Epoxy-Terminated SAMs Chart 1. Structure and Molecular Model of the Epoxysilane Compound Studied Here
detailed morphological studies on these layers were performed to prove their integrity and homogeneity. The formation of disordered and chemically heterogeneous molecular layers is a common problem for functional silanes due to the complicated hydrolysis/interaction/ adsorption/reactivity competition of the head-end reactive groups with the hydroxyl-terminated silicon oxide surfaces. The questions of completeness, smoothness, and ordering of chemisorbed layers from silanes with terminal functional groups should be addressed to prove their ability to serve as coupling agents for molecular layers. In this study, we report on the fabrication of truly monolayer epoxysilane films appropriate for chemical binding of composite molecular layers on silicon substrates.10 We especially focus on the surface morphology and microstructure of these SAMs as a function of fabrication conditions as a means to optimize the selfassembly process. Results of testing the epoxysilane SAMs’ surface morphology, microstructure, shear properties, and ability to tether functional terminated polymers are reported and discussed. An example of dense polymer layers firmly attached to the epoxysilane SAM is demonstrated. Experimental Section An epoxysilane compound, (3-glycidoxypropyl)trimethoxysilane (see Chart 1), was purchased from Gelest Inc.11 ACS grade toluene and ethanol were obtained from Aldrich and were used as received. The epoxysilane solutions in toluene were prepared in oven-dried glassware in a nitrogen-purged glovebox. Highly polished single-crystal silicon wafers of {100} orientation (SAS) were cut in pieces of approximately 1.5 × 2 cm before modification. The substrates were first cleaned in an ultrasonic bath for 30 min, placed in a hot piranha solution (3:1 concentrated sulfuric acid/ 30% hydrogen peroxide) for 1 h, and then rinsed several times with high-purity water (18 MΩ‚cm, Nanopure). After the rinsing, the substrates were dried under a stream of dry nitrogen, immediately taken into the nitrogen-filled glovebox, and immersed in epoxysilane solutions of different concentrations (from 0.1 to 1 vol %) for different periods of deposition time (from 1 min to 24 h). After the deposition was complete, the modified substrates were removed from solution and rinsed several times with toluene and ethanol. To remove unbound deposited materi(10) Tsukruk, V. V.; Luzinov, I.; Julthongpiput, D. Langmuir 1999, 15, 3029.
Langmuir, Vol. 16, No. 2, 2000 505 als, the substrates were additionally placed in ethanol in the ultrasonic bath for 20 min. The SAMs formed were dried overnight at ambient conditions before measurements. All sample preparations were performed inside a Cleanroom 100 facility (Laminaire Corporation). After preparation, samples were stored in a dry atmosphere in the cleanroom. The thickness, morphology, and contact angle of the fabricated films were observed to not change over several months after preparation. To check the thermal stability of the epoxysilane layers, selected samples were annealed for 24 h in a vacuum oven at 120 °C. The thickness and morphology were found virtually unchanged after the annealing. The contact angle increased by 8-10% after the thermal treatment. Monodispersed carboxy-terminated polystyrene (Mw ) 10 500, Mw/Mn ) 1.08, functionality ) 0.98) was obtained from Polymer Source, Inc. The polymer was spin-coated from a 1 wt % toluene solution onto the wafers covered with the epoxysilane SAM. The thickness of the polystyrene film measured by ellipsometry was about 40 ( 3 nm. The coated wafers were annealed for 18 h in a vacuum oven at 150 °C to enable the end groups to graft to the substrate. The polymer that had not grafted was removed by multiple washings with toluene. The thickness of the layer was measured by ellipsometry. After eight washes, the thickness of the layer did not decrease with additional washing in an ultrasonic bath. Film surfaces were examined by static contact angle (sessile droplet) measurements using a custom-designed optical microscopic system. Droplets (1.5-2 µL) of Nanopure water were placed randomly over the surface. Contact angles were determined within 1 min after droplet deposition. All reported values were an average of at least six measurements. The shape of the drop was observed with a microscope equipped with a CCD camera, and the contact angle was measured at a monitor screen. Ellipsometry was performed using a COMPEL automatic ellipsometer (InOmTech, Inc.) with an angle of incidence of 70°. The silicon oxide thickness was measured for each silicon wafer after the piranha solution treatment and before film deposition. The thickness of the silicon oxide layer was determined to be within 0.8-1.2 nm for different wafers. The indexes of refraction of the epoxysilane monolayer and the silicon oxide were considered to be constant and equal to the “bulk” values 1.429,11 and 1.46,12 respectively. All reported thickness values were averaged over six measurements from different parts of the substrate. Scanning probe microscopy (SPM) was used to obtain topographical, friction, and phase mode images in air. Studies were performed on a Dimension 3000 (Digital Instruments, Inc.) microscope according to the known procedure.13,14 Silicon nitride and silicon tips with spring constants from 0.2 N/m for contact mode to 50 N/m for tapping mode were used. Imaging was done at scan rates in the range 1-2 Hz, at normal loads ranging from several tens of newtons for contact mode to several newtons for tapping mode. For thickness evaluation from SPM data, we used a “scratch” test. Scratches were produced with a sharp steel needle at different loads or by multiple scannings with a stiff tip with a high normal load (several micronewtons). This approach is used frequently for SPM measurement of organic and polymer layers and produces reasonable results.15 Adhesive forces were measured from force-distance curves as pull-off forces. Loading data (friction forces versus normal load) were collected by using a friction loop and a standard procedure described elsewhere.16 We analyzed these data using a linear regression.17 Vertical and torsional spring constants were determined for V-shaped can(11) Catalog; Gelest, Inc.: Tullytown, PA 19007, 1998; p 173. (12) Handbook of Chemistry and Physics, Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1996. (13) Tsukruk, V. V. Rubber Chem. Technol. 1997, 70 (3), 430. (14) Ratner, B., Tsukruk, V. V., Eds. Scanning Probe Microscopy in Polymers; ACS Symposium Series; American Chemical Society: Washington, DC, 1998; Vol. 694. (15) Sheller, N. B.; Petrach, S.; Foster, M. D.; Tsukruk, V. V. Langmuir 1998, 14, 4535. (16) Tsukruk, V. V.; Bliznyuk, V. N.; Hazel, J.; Visser, D.; Everson, M. P. Langmuir 1996, 12, 4840. (17) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids; Claredon Press: Oxford, 1950. Rabinowicz, E. Friction and Wear of Materials; Wiley & Sons: New York, 1965.
506
Langmuir, Vol. 16, No. 2, 2000
tilevers from fundamental resonance frequencies using calibration plots developed earlier.18 Selected samples of the SAMs were studied in air with X-ray reflectivity (XR)19 using a rotating anode source (Rigaku, 12 kW, RU200) with Cu KR, radiation (λ ) 0.154 nm). The two-axis goniometer was outfitted with a pyrolytic graphite monochromator (δλ/λ ) 0.022), slit collimation (dθ/θ ) 0.002, where θ is the incident angle with respect to the surface), and a NaI scintillation detector. The structureless background was measured by recording the intensity with the detector position offset from the specular condition by a fixed angular amount. The background was then subtracted from the raw reflected intensities and the curve normalized to yield reflectivities as a function of scattering vector, q ) 4π sin θ/λ. Information about the SAM structure is deduced by nonlinear regression of the data using the simplest possible structural model,20 calculating the reflectivities from the model with a well-established matrix formalism. For the comparatively small samples (ca. 2 cm × 2 cm) measured here, at low q, discrepancies between the experimental and model reflectivities are due to the fact that the experimental data reflect geometric effects. The beam footprint is longer than the sample at these values of q. Electron spectroscopy for chemical analysis (ESCA) was performed on a Surface Science SSX-200 spectrometer using standard operating conditions with electron-gun charge neutralization. Survey scans probing binding energies from 0 to 1000 eV were used to determine the chemical elements present at the surface. Binding-energy-corrected high-resolution scans of the carbon region, using the binding energy of C-H ) 284.6 eV as the reference energy, were used to determine the nature and amount of C-O-containing groups at the sample surface. A fitting strategy for these scans was determined using literature values of binding energy shifts21 typically obtained for carbon associated with epoxy cycles, about 2.0 eV, and C-OH, about 1.55 eV. A molecular model of epoxysilane molecules was built with the CERIUS2 package on a Silicon Graphics workstation and minimized using the Dreiding force field,22 and the geometrical dimensions were measured for a fully extended conformation.
Results and Discussion Epoxysilane Films at Different Deposition Conditions. Different epoxysilane films were prepared by adsorption from toluene solutions with 0.1% to 1% of the epoxysilane compound. Figure 1 presents the apparent ellipsometric thickness and water contact angle for the films prepared for different concentrations and times of deposition. Generally, the thickness and contact angle are higher for longer deposition time (16 h). For a 10-min deposition, the contact angle increases gradually with epoxysilane solution concentration from 34° to 42°. After 16 h of deposition, the contact angle is 51° ( 1 for the films prepared from 0.25% to 1% solutions and equals 47° ( 1 for the 0.1% epoxysilane concentration. The contact angle is the highest for films deposited from the 1% epoxysilane solution at the longest deposition time. Therefore, contact angle measurements alone might indicate that the film formation is more complete if the solution with higher epoxysilane concentration is used. However, the thickness of the epoxysilane layer determined from ellipsometry, assuming a bulk refractive index, is the highest for deposition from the 0.1% solution and reaches 1.1 ( 0.15 nm as compared to 0.75 ( 0.1 nm for higher concentrations. Taking into account that this thickness is very close to the extended length of the epoxysilane molecules (0.95 nm, see the molecular model (18) Hazel, J. L.; Tsukruk, V. V. J. Tribol. 1998, 120, 814. Hazel, J. L.; Tsukruk, V. V. Thin Solid Films 1999, 339, 249. (19) Foster, M. D. Crit. Rev. Anal. Chem. 1993, 24, 179. (20) Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Wasserman, S. R.; Whitesides, G. M.; Axe, J. D. Phys. Rev. B 1990, 41, 1111. (21) Handbook of X-ray and Ultraviolet Photoelectron Spectroscopy; Briggs, D., Ed.; Heyden & Son Ltd.: 1977. (22) CERIUS2, Molecular Simulations, v. 3.8, 1998.
Luzinov et al.
Figure 1. Ellipsometric thickness (a) and water contact angle (b) after 10 min and 16 h of deposition for epoxysilane films versus concentration of epoxysilane solution.
in Chart 1b), one could conclude that the solution concentration below 0.25% is optimal for fabrication of a complete SAM. However, the lower contact angle for these films could indicate a more disordered state or lower surface coverage, and detailed analysis of microscopic data is required to draw final conclusions. Figures 2 and 3 show topographical SPM images of epoxysilane films formed at various concentrations of epoxysilane solution and times of their deposition. Comparison of the images revealed that, indeed, the films obtained from 0.1% and 1% solutions have very different surface morphologies. Epoxysilane films obtained from the 1% solution are complete, very smooth, and homogeneous with only a few aggregates observed on a micrometer size area. Unexpectedly, a decrease of the solution concentration resulted in a gradual increase of the concentration of globular aggregates. The surface concentration of these aggregates increases significantly for longer deposition times, as can be seen from comparison of Figures 2 and 3. The average lateral size, height, and concentration of aggregates are determined from the topography images obtained at several different locations. Figure 4a demonstrates the number of aggregates per square micrometer calculated for various films fabricated from epoxysilane solutions of different concentration at two different deposition times. As can be concluded from these data, the number of surface aggregates decreases sharply from 140 to 200 µm-2 to 6-8 µm-2 for higher epoxysilane concentrations. For the 1% solution concentration, a very smooth film surface is observed over regions of tens of micrometers across with rarely observed aggregates. The highest number of molecular aggregates is observed at low solution concentration. The aggregates have a relatively constant height of 2.0 ( 0.7 nm, which is close to the observed thickness in ref 7. This height corresponds roughly to the total length of two to three coupled
Epoxy-Terminated SAMs
Langmuir, Vol. 16, No. 2, 2000 507
Figure 2. SPM images of epoxysilane films after 10 min of deposition for different concentrations of epoxysilane solution: (a) 0.1 vol %; (b) 0.25 vol %; (c) 0.5 vol %; (d) 1 vol %. The vertical scale is 20 nm. The bright parts correspond to higher features.
molecules, and hence, formation of bi- and trilayers can be suggested. The apparent diameter of the aggregates varies within 19-30 nm with a tendency to smaller size at higher solution concentration. The actual lateral size of aggregates can be estimated taking into account a typical tip radius (20-40 nm). Their lateral sizes are in the range 10-20 nm, and thus, aggregates include 800-3000 molecules assuming a cross-sectional area of 0.25 nm2 per molecule. Thus, we can conclude that the formation of epoxysilane films follows very different routes at high (1%) and low (0.1-0.5%) concentrations of epoxysilane solutions. At high concentration, we observe the formation of smooth films with rarely occurring surfaces aggregates. However, at lower concentrations, the smooth film is replaced with the collection of tiny molecular aggregates composed of hundreds to thousands of molecules packed in bi- and
trilayers. These aggregates are loosely packed at high deposition times and cover about 70-80% of the surface area, making these films heterogeneous on a molecular scale. The formation of such multilayer aggregates explains the increase of the “apparent” ellipsometric thickness of the deposited films at very low solution concentration. The actual thickness of aggregated films close to the SPM value can be obtained from the ellipsometric results if the actual surface coverage is taken into account by reducing the average refraction index. Incomplete surface coverage with epoxysilane aggregates for the lower concentration solutions also explains the decrease of the contact angle observed for these films. To understand the reasons for different microstructures of epoxysilane films deposited from solutions with different concentrations, one should consider the hydrolysis reaction in the system.23 As is known from extensive studies of
508
Langmuir, Vol. 16, No. 2, 2000
Luzinov et al.
Figure 3. SPM topographical images of epoxysilane films after 16 h of deposition for different concentrations of epoxysilane solution: (a) 0.1 vol %; (b) 0.25 vol %; (c) 0.5 vol %; (d) 1 vol %. The vertical scale is 20 nm. The bright parts correspond to higher features.
self-assembly of alkylsilanes, a competition between hydrolization and polymerization of the methoxysilane groups in the bulk and on silicon surfaces determines the formation of surface films.24 Too low a water content in the bulk and at the interface leads to incomplete monolayers. On the other hand, high water concentrations result in fast polymerization in the bulk solution. The excess polymerization leads to aggregate formation and their deposition on the substrate. Only the residual unreacted silanol groups of the aggregates allow them to interact with the surface. (23) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532. Britcher, L. G.; Kehoe, D. C.; Matinsons, J. G.; Smart, R. St. C.; Swincer, A. G. Langmuir 1993, 9, 1609. (24) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607.
Several recent studies showed that bulk hydrolysis processes play a critical role in the formation of complete SAMs and an optimal amount of water is required to form a complete homogeneous monolayer.1,23,24 In our studies, the water content in the solutions is determined by the equilibrium water concentration in the solvent (0.03 wt %). The water layer at the surface at normal ambient conditions right after drying with dry nitrogen is minute and assumed to be 2-5 molecules thick.1 The very strong variation of the film morphology for various epoxysilane concentrations in solution shows that, in fact, bulk water content is crucial for the fabrication process. The molar ratio of water to epoxysilane molecules in the solvent phase decreases very fast with increasing epoxysilane concentration. This ratio decreases from three water molecules per epoxysilane molecule for the 0.1% concentration to
Epoxy-Terminated SAMs
Langmuir, Vol. 16, No. 2, 2000 509
min from close to zero (
