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Surface-Initiated Thermal Radical Polymerization on Gold Wenxi Huang, Skanth G., Gregory L. Baker,* and Merlin L. Bruening* Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824 Received September 15, 2000. In Final Form: December 28, 2000 We report surface-initiated thermal radical polymerization from cross-linked monolayers of azo-initiators on Au. Initial attempts to graft polymer layers from initiators attached to alkanethiol monolayers yielded polymer films with thicknesses less than 5 nm. Efficient grafting from such surfaces is not possible because initiator-containing monolayers are somewhat unstable under thermal radical polymerization conditions, and desorbed thiols may serve as efficient chain-transfer reagents that inhibit radical polymerization. In addition, reactive radicals can attack the Au-S bonds that link the initiator monolayer to the surface. To overcome these problems, we employed mercaptopropyltrimethoxysilanes to form an adhesion layer for initiator attachment. Cross-linked poly(siloxane) layers apparently stabilize the initiator layer, allowing well-defined surface radical polymerization to occur from Au substrates. We characterized the grafted polymer layers with ellipsometry, reflectance Fourier transform infrared spectroscopy, and atomic force microscopy.
Introduction Tethered polymer chains on solid surfaces are of theoretical interest and have potential applications in chemical separations, sensing, stabilization of colloidal suspensions, control of wetting and adhesion, corrosion resistance, and microelectronics.1,2 Synthesis of polymer layers covalently bound to a solid surface can be achieved by either the “grafting to” or the “grafting from” methods.3 The “grafting to” method involves the reaction of endfunctionalized polymers with appropriate surface sites.4,5 Initially grafted polymer layers, however, hinder the further attachment of polymer chains, thus limiting film thickness. In the “grafting from” approach, a reactive unit on the surface initiates the polymerization, and consequently the polymer chains grow from the substrate. This approach generally leads to higher grafting densities because monomers more easily diffuse to reactive sites than do macromolecules. Several groups reported surfaceinitiated syntheses of polymer films using techniques such as cationic,6-8 anionic,9 free-radical,3 living radical,10-13 ring-opening,14 and ring-opening metathesis polymeri* To whom correspondence should be addressed. E-mail:
[email protected],
[email protected]. Phone: (517) 355-9715 ext 237. Fax: (517) 353-1793. (1) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1991, 100, 31. (2) Sanchez, I. C. Physics of Polymer Surfaces and Interfaces; Butterworth: London, 1992. (3) (a) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592. (b) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 601. (4) Jordan, R.; Graf, K.; Riegler, H.; Unger, K. K. J. Chem. Soc., Chem. Comm. 1996, 9, 1025. (5) Zhao, W.; Krausch, G.; Rafailovich, M. H.; Sokolov, J. Macromolecules 1994, 27, 2933. (6) Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 243. (7) Zhao, B.; Brittain, W. J. J. Am. Chem. Soc. 1999, 121, 3557. (8) Ingall, M. D. K.; Honeyman, C. H.; Mercure, J. V.; Bianconi, P. A.; Kunz, R. R. J. Am. Chem. Soc. 1999, 121, 3607. (9) Jordan, R.; Ulman, A.; Kang, J. F.; Rafailovich, M. H.; Sokolov, J. J. Am. Chem. Soc. 1999, 121, 1016. (10) Husseman, M.; Malmstrom, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 1424. (11) Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 1998, 31, 5034. (12) 2) (a) Huang, X.; Wirth, M. J. Anal. Chem. 1997, 69, 4577. (b) Huang, X.; Wirth, M. J. Macromolecules 1999, 32, 1694. (13) Sedjo, R.; Mirous, B.; Brittain, W. J. Macromolecules 2000, 33, 1492.
zation.15-17 Of these techniques, radical polymerization is especially useful because it allows the use of a variety of monomers, including those with polar and unprotected functional groups. Covalent attachment of initiators to the substrate of interest is crucial for surface-initiated polymerization. The most common systems for anchoring reactive sites onto substrates are organic thiols on Au and organosilicon compounds on oxides because these molecules form welldefined surfaces. Such systems allow control of the reactivity, chemical composition, and density of initiators on a surface,18 and hence one can tailor properties of grafted films such as thickness and chain density. Previous research on surface-initiated thermal radical polymerization focused on the use of initiators tethered to siloxanebased monolayers to generate polymer layers on oxide surfaces.3,10-13 These monolayers allow grafting polymerization to be conducted at elevated temperatures. Surface-initiated polymerization on Au is highly desirable because Au surfaces are homogeneous and clean and are compatible with a broad variety of surface analytical techniques. In addition, contact-printing schemes allow patterning of layers on Au surfaces. For example, Hawker et al.14,19 reported the use of combined microcontact printing (µCP) and surface-initiated polymerization to generate patterned polymer films on surfaces for lithographic applications. The grafted polymer layers possess better barrier properties toward wet chemical etchants than the patterned monolayers from which they are grown. Surface-initiated radical polymerization from thiol selfassembled monolayers (SAMs) on Au is challenging because of the lability of the Au-S bond that links the (14) Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abott, N. L. Angew. Chem., Int. Ed. 1999, 38, 647. (15) Weck, M.; Jackiw, J. J.; Rossi, R. R.; Weiss, P. S.; Grubbs, R. H. J. Am. Chem. Soc. 1999, 121, 4088. (16) Jeon, N. L.; Choi, I. S.; Whitesides, G. M.; Kim, N. Y.; Laibinis, P. E.; Harada, Y.; Finnie, K. R.; Girolami, G. S.; Nuzzo, R. G. Appl. Phys. Lett. 1999, 75, 4201. (17) Seery, T. A. P.; Dhar, P.; Huber, D. L.; Vatansever, F. Polym. Prepr. 1999, 40 (2), 148. (18) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembled Monolayers; Academic Press: Boston, 1991. (19) Shah, R, R.; Merreceyes, D.; Husseman, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597.
10.1021/la001325w CCC: $20.00 © 2001 American Chemical Society Published on Web 02/01/2001
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initiator layers to the surface and because most radical polymerizations require the use of elevated temperatures. SAMs prepared from alkanethiols on Au surfaces are relatively fragile. The enthalpy of binding is in the range of 30-50 kcal/mol,20 whereas the free energy of binding is only 5 kcal/mol.21 These SAMs thermally desorb from Au surfaces when placed in contact with hot organic solvents.22 Thus, only a few reports describe surfaceinitiated thermal polymerization on Au surfaces. Ulman et al. reported cationic ring-opening polymerization of 2-ethyloxazoline from self-assembled initiator-containing monolayers on Au. Thin poly(N-propionylethylenimine) layers (9 nm) were obtained after a 7-day reaction in refluxing chloroform.6 Hawker et al. reported surfaceinitiated atom-transfer radical polymerization of various vinyl monomers on Au substrates.19 The initiator monolayers were assembled at elevated temperature so as to prevent their thermal desorption during polymerization. Free initiator was also added to the reaction solution to achieve a controlled grafting polymerization. This paper describes free-radical polymerization of styrene initiated from azo-initiators attached to a planar Au substrate. We also show that free-radical polymerization from Au is hindered by the stability of alkanethiol monolayers. Free radicals in solution accelerate the desorption of thiols from Au, and desorbed alkanethiols appear to serve as efficient chain-transfer reagents that hinder polymerization. To overcome monolayer instability, we utilize a simple cross-linking procedure to enhance the stability of SAMs and make thermal radical polymerization from Au surfaces facile. Experimental Section Materials. 3-Mercaptopropyltrimethoxysilane (MPS) was used as received from United Chemical Technologies. 11Mercaptoundecanoic acid (MUA), 11-mercaptundecanol (MUD), cystamine, 3-aminopropyltrimethoxysilane, and 1,3-dicyclohexylcarbodiimide (DCC) were purchased from Aldrich. Silane compounds were used under a nitrogen atmosphere. Toluene was distilled under a nitrogen atmosphere in the presence of sodium/ potassium alloy using benzophenone as indicator. Styrene was purified by extracting with 1 M NaOH three times and then Milli-Q water (18 MΩ cm) several times, drying over Na2SO4, passing through a basic alumina column, and distilling under reduced pressure just before polymerizations. All other solvents and chemicals were used as received from Aldrich. Au-coated wafers (electron-beam evaporation of 200 nm of Au on 20 nm of Ti on Si(100) wafers or sputter coating of 200 nm of Au on 20 nm of Cr on Si(100) wafers) and silicon wafers sputter-coated with 20 nm of aluminum were cleaned in a UV/O3 chamber for 15 min just before use. For the study of SAMs under UV exposure, we placed samples in vials under a long wavelength ultraviolet lamp (BLAK-RAY, model B, 100 W). The distance between the lamp and the Au slide was 16 cm. An 8 cm thick water filter was employed to eliminate infrared light from the beam. Characterization Methods. NMR spectra were collected in CDCl3 on a Varian Gemini-300 spectrometer. The chemical shifts were calibrated using residual CHCl3 and are reported relative to tetramethylsilane. Ellipsometric measurements were obtained with a rotating analyzer ellipsometer (model M-44, J. A. Woollam) using WVASE32 software. The angle of incidence was 75° for all experiments. For the calculation of layer thickness, a film refractive index of 1.50 was used. Reflectance Fourier transform infrared (reflectance FTIR) spectroscopy was performed using a Nicolet Magna-560 FTIR spectrometer containing a PIKE grazing angle (80°) attachment. The spectra were typically collected with 256 scans using a MCT detector. Atomic force microscopy (AFM) images were obtained in the tapping mode with a Nanoscope (20) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (21) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315. (22) Bain, C. D.; Troughton, E. B.; Tao, Y.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
Huang et al. Chart 1
IIIa instrument (Digital Instruments). A cantilever having a nominal spring constant of 20-100 N/m was used along with etched silicon tips. The tips have a nominal radius of curvature of 20-60 nm. Formation of MUA and MUD Monolayers on Au Substrates. Clean Au slides were immersed in a 2 mM ethanolic solution of MUA or MUD for 12 h. Slides were then rinsed with ethanol followed by Milli-Q water and dried with nitrogen. Formation of Initiator Monolayers on Various Substrates. Initiator monolayer A (Chart 1) was attached to Au substrates by coupling the asymmetric azo monocarboxylic acid CH3C(CH3)(CN)NdNC(CH3)(CN)CH2CH2COOH to a MUD monolayer. The coupling reaction occurs by immersing the monolayer in a solution containing 0.1 g of the azo compound and 0.14 g of DCC in 10 mL of N,N-dimethylformamide (DMF) with 20 µL of pyridine as a catalyst. After deposition, the wafers were sonicated in DMF and dried under nitrogen. The ellipsometric thickness of initiator monolayer A was 1.7 ( 0.2 nm. An ester carbonyl peak at 1737 cm-1 in the reflectance FTIR spectrum confirmed that coupling occurred. Initiator monolayer B (Chart 1) was attached to Au substrates by the same procedure as above except that MUD was replaced by cystamine. The ellipsometric thickness of initiator monolayer B was 1.4 ( 0.2 nm. Reflectance FTIR spectra showed the presence of amide peaks at 1652 and 1548 cm-1. Initiator monolayer C (Chart 1) was attached to aluminumcoated silicon wafers by immersing the wafers in a 0.2 wt % solution of 1 (Scheme 1) in toluene at 60 °C for 5 min under a nitrogen atmosphere, using 20 µL of pyridine as a catalyst. Following the deposition, the substrates were rinsed with toluene and dried with nitrogen. The ellipsometric thickness of initiator monolayer C was 1.8 ( 0.2 nm. Reflectance FTIR spectra showed the presence of amide peaks at 1650 and 1556 cm-1. An initiator layer similar to monolayer C was attached to modified Au substrates (Scheme 1) as follows: Immersion of a Au substrate into a 2 mM MPS solution in methanol for 12 h at room temperature resulted in a MPS monolayer. After deposition, the substrate was rinsed with 2 mL of methanol three times and dried with nitrogen. The attached silane monolayer was then hydrolyzed at room temperature in 0.1 M HCl for 15 h to afford a hydroxylated surface. The modified Au substrate was then treated with a 0.2 wt % solution of 1 in toluene at 60 °C for 5 min under nitrogen, using 20 µL of pyridine as a catalyst. Following the deposition, the substrates were rinsed with toluene and dried with nitrogen. Synthesis of the Trimethoxysilane-Substituted Azo Initiator (1). A 1.11 g (5 mmol) sample of the monocarboxylic acid precursor CH3C(CH3)(CN)NdNC(CH3)(CN)CH2CH2COOH, synthesized according to the method of Ru¨he,3 was coupled with 0.9 g (5 mmol) of 3-aminopropyltrimethoxysilane using 1.24 g (6 mmol) of DCC with pyridine as a catalyst. The reaction was
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Scheme 1
Figure 1. Integrated absorbance of the carbonyl peak in reflectance FTIR spectra of MUA monolayers versus time of exposure to toluene at 60 °C (0), 6 mM AIBN in toluene at 60 °C (9), toluene at room temperate under UV light (4), and 6 mM AIBN in toluene at room temperature under UV light (2). The absorbance was normalized by dividing by the integrated carbonyl absorbance before exposure.
carried out in 20 mL of anhydrous CH2Cl2 at room temperature for 12 h. After completion of the reaction, urea byproducts were removed by filtration of the CH2Cl2 solution of the product over anhydrous sodium sulfate, and the solvent was evaporated to afford a light-orange solid. The product was used without further purification because of its sensitivity to heat and water. 1H NMR analysis of the product revealed about 20 mol % of DCC along with the expected compound. (The Supporting Information contains NMR spectra of the compound and DCC.) Surface-Initiated Polymerizations. Polymerizations were performed in toluene/styrene mixtures (1/1, v/v) at 60 °C for different time periods. All solutions were carefully degassed through at least three freeze-pump-thaw cycles to remove traces of oxygen. After polymerization, the samples were rinsed with toluene and Soxhlet extracted with toluene for at least 12 h.
Results and Discussion Stability of SAMs under Polymerization Conditions. Because of concerns that the instability of alkanethiol SAMs prohibits thermal grafting of polymers from Au surfaces, we began studying the stability of SAMs under typical thermal polymerization conditions. We evaluated the thermal stability of MUA monolayers in toluene at 60 °C by measuring the integrated absorbance due to the acid carbonyl groups as a function of immersion time. The reflectance FTIR spectrum of a monolayer of MUA shows the expected methylene and carbonyl stretching bands at 2925, 2851, and 1724 cm-1, confirming the formation of the monolayer. We chose to monitor stability using the carbonyl peak as it is less sensitive to adventitious contamination than hydrocarbon peaks. As seen in Figure 1 (open squares), the carbonyl peak decreases rapidly during the first 2 h of immersion in hot toluene and then remains constant for up to 12 h. This result is in general agreement with studies by Hawker et al. and Schlenoff et al.,19,23 who demonstrated that a significant
fraction of thiols in a SAM do not desorb at elevated temperatures. From IR absorbances, we estimate that about 60% of the SAM remains on the surface after 12 h of immersion in toluene at 60 °C.24 Although SAMs of alkanethiols on Au partially desorb when immersed in hot toluene, the remaining fraction of the SAMs should still initiate surface polymerization if derivatized with an appropriate radical initiator. Therefore, we prepared two different types of monolayers that contain azo-initiators (Chart 1, A and B) on Au surfaces. Ellipsometry25 and reflectance FTIR spectra confirmed the presence of the azo-initiators on the surface. Initial attempts at graft polymerization of styrene by immersion of monolayers A and B in a styrene/toluene solution (1/1, v/v) at 60 °C for 12 h were unsuccessful. In all cases, the polystyrene layers were no more than 5 nm thick. When we attempted analogous experiments under identical polymerization conditions using an initiator bound to an aluminum-coated wafer (Chart 1, C), we obtained a 40 nm thick polystyrene film. One possible explanation for the lack of polymerization from Au is that desorbed thiols inhibit radical polymerization. To test this possibility, we repeated the polymerization of styrene from an azo-initiator bound to an aluminum-coated silicon wafer in the presence of Au/Si substrates coated with MUA and MUD SAMs. As shown in Table 1, the presence of a monolayer of MUA or MUD decreased film thickness on Al by 10-18 nm (20-40% decrease). Adding a bare Au slide to the polymerization solution had no significant effect on film thickness. To further confirm that desorbed thiols affect polymerization from surfaces, we added a trace amount (0.25 or 0.025 µM) of free thiols (expts 3 and 4, Table 1) to the polymerization solution. In these cases, polystyrene film thickness decreased by 16 and 8 nm, respectively. The above results confirm that desorbed thiols can inhibit surface radical polymerization. This is not com(23) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (24) We note that absorbances in reflectance FTIR spectra depend on the orientation of the transition dipole with respect to the surface normal (see: Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52.). Because partial desorption of the SAM may result in orientation changes of the remaining carbonyl groups attached to the surface, estimates of surface coverage from IR spectra are only approximate. (25) Thickness data for initiator layers suggest that coverage by initiator A on Au may be less than that of initiator C on aluminum, whereas initiator B on Au has a similar coverage to that of initiator C on aluminum. All of the initiator layers on gold should still be dense enough to initiate polymerization.
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Table 1. Thickness of Surface-Grafted Polystyrene Layers on Al When Polymerized in the Presence and Absence of Alkanethiolsa
expt
additives in the reaction vessel
1 2 3
nothing bare Au slide mercaptoundecanol in solution (0.25 µM in toluene) mercaptoundecanol in solution (0.025 µM in toluene) mercaptoundecanol monolayer on an Au slide mercaptoundecanoic acid monolayer on an Au slide
4 5 6
thickness of polystyrene on alumina (nm) 41 ( 3 40 ( 0.2 25 ( 4 33 ( 2 23 ( 3 31 ( 3
a Polymerization was performed for 12 h at 60 °C, and films were cleaned by Soxhlet extraction.
pletely unexpected because organic thiols are very effective chain-transfer reagents and are widely used to regulate molecular weights in radical polymerizations.26 Although alkanethiolates could desorb from Au as either disulfides or monothiols,23 both species have high chain-transfer constants and are known to inhibit radical polymerization in solution.26 The consequences of chain-transfer reactions on a surface and in solution are quite different. When polymerization occurs in solution, the active center is transferred to monomer, solvent, or a chain-transfer reagent and a new polymer chain is initiated. In polymerization from a substrate, such transfer reactions result in a termination of polymer growth from the surface and hence a decrease in film thickness. We tried to predict the effect of thiols on polymerization from a surface using chain-transfer rate constants measured in solution.26 Such calculations suggest that the concentrations of thiols used in our experiments should have little effect on polymerization. Under the conditions shown in Table 1, the ratio of monomer to thiol concentration is 107-108. Such a large ratio should result in negligible chain transfer to the thiol based on chaintransfer rate constants measured in solution.26 The experimental data, however, definitively show that thiols do have an effect on polymerization as the addition of even 0.025 µM of MUD leads to a reduction in film thickness by 8 nm. (If all of the thiol in a SAM desorbed from the Au surface, we estimate that the concentration of thiol in solution would be 0.27 µM.) Thiol-induced chain-transfer reactions at the surface should result in increased solution polymerization and perhaps more physisorption on the surface. Such physisorption might decrease polymer growth from the surface by blocking reactive sites. In an attempt to address this possibility, we investigated whether the amount of material physisorbed onto the polymer film correlates with the presence of the thiols. To do this, we measured the thickness of polymer films before and after removing physisorbed polymers through Soxhlet extraction. The ellipsometric thicknesses of a polymer film prepared in the absence of thiol (12 h polymerization from aluminum) decreased from 44 ( 3 to 40 ( 3 nm after a Soxhlet extraction. When polymerization occurred with thiol in solution (MUD, 0.025 µm), film thicknesses decreased from 34 ( 3 to 30 ( 3 nm after removal of physisorbed material. The amount of physisorbed material was not significantly different in the two cases. At this point, we do not understand why such small amounts of thiol have an effect on polymerization from a surface. In addition to the thermal instability of thiols on Au, a second possible explanation for our inability to initiate
Figure 2. Fate of surface-bound radicals on Au: (top) thermal desorption of surface-bound radicals and desorption induced by reaction of a radical with an Au-S bond, (bottom left) reaction with a chain-transfer agent, and (bottom right) initiation of polymerization.
substantial polymerization from thiol monolayers is that radicals accelerate the desorption of thiols through attack at the Au-S bond. To investigate this, we studied the stability of SAMs in the presence of radicals generated by thermal and photoinduced decomposition of azobisisobutyronitrile (AIBN) in solution. For the thermal reaction, we exposed a monolayer of MUA to a toluene solution (60 °C) containing AIBN (6 mM). After 6 h of immersion in this solution, reflectance FTIR spectra (carbonyl peak) showed
