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Polymer Layers through Self-Assembled Monolayers of Initiators O. Prucker† and J. Ru¨he* Macromolecular Chemistry II, University of Bayreuth, D-95440 Bayreuth, Germany Received September 12, 1997. In Final Form: June 12, 1998 A novel concept for the generation of molecularly thin polymer layers attached to a solid surface is described. In contrast to commonly used techniques, where functional groups of the polymers are reacted with appropriate sites on the surfaces of a substrate the polymer layers are formed in situ by using self-assembled monolayers of an initiator. As an example, the formation of polystyrene monolayers terminally attached to silicon oxide surfaces through radical chain polymerization which has been started by a self-assembled monolayer of an azo initiator is described. The thickness of the attached polymer films can be adjusted over a wide range up to values of several hundred nanometers by variation of polymerization parameters such as temperature and azo conversion. When suitable conditions are chosen, monolayers with thicknesses inaccessible by other techniques of preparation can be obtained.
Introduction Polymer films with thicknesses in molecular dimensions attached to solid substrates are expected to be very useful for a wide range of different applications ranging from protective coatings1 to improvement of the biocompatibilty of materials2,3 and the fabrication of electronic devices.4 Consequently, numerous deposition strategies and protocols have been studied that use either chemical or physical interactions between the surfaces and the coating materials. Examples for such processes are the Langmuir-Blodgett technique, preparation of self-assembled monolayers of low molecular weight organic compounds, and polymer adsorption from solution.5,6 Any system for the preparation of thin polymer films must fulfill several criteria. First, it should be possible to adjust the film thickness over a range as large as possible. Second, to allow fine-tuning of the film properties, a large variety of functional groups should be easily introduced. Another very important factor is to have a sufficient long-term stability of the films in different environments. This is especially important if materials with very different surface properties are combined, such as hydrophobic polymers on hydrophilic oxide surfaces. Otherwise dewetting will occur and the film properties will severely deteriorate. Clearly the best way to solve the adhesion problem is to establish a chemical bond between the polymer molecules and the surface of the substrate. Most approaches * To whom correspondence should be adressed. Current address: Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany. Phone: ++49-6131-379162. Fax: ++49-6131-379360. E-mail:
[email protected]. † Current address: Department of Chemical Engineering, Stanford University, Stanford, CA 94305-5025. (1) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: New York, 1992. (2) Ratner, B. J. Biomed. Mater. Res. 1993, 27, 837. (3) Dumitriu, S. Polymeric Biomaterials; Marcel Dekker: New York, 1994. (4) Bawden, M. J.; Turner, S. R. In Electronic and Photonic Applications of Polymers; Advance in Chemistry Series 218; American Chemical Society: Washington, DC, 1988. (5) Cosgrove, T.; Vincent, B.; Scheutjens, J. M. H. M.; Fleer, G. J.; Cohen-Stuart, M. A. Polymers at Interfaces; Chapman & Hall: London, 1993. (6) Ulman, A. Ultrathin Organic Films; Academic Press: San Diego, CA, 1991.
in that direction use (end) functionalized polymers, which can be reacted with appropriate surface sites.7-10 However, films generated by these “grafting-to” methods are usually limited to thicknesses of 1-5 nm. As soon as the surface becomes significantly covered with attached chains, the polymer concentration at the interface becomes larger than the concentration of the macromolecules in the solution. Additional chains, which try to reach the surface now, have to diffuse against this increasing concentration gradient rendering the immobilization at the surface energetically more and more unfavorable. Thus the rate of the attachment reaction levels off quickly, and no further polymer is linked to the substrate. Films generated by this technique are therefore intrinsically limited concerning the film thickness due to this kinetic hindrance. Additionally, even if this kinetic limitation is somehow circumvented, the attachment of chains to a strongly covered surface becomes also for thermodynamic reasons unfavorable. To understand this second barrier, it has to be considered that with increasing graft densities the surface-attached chains become more and more stretched. A chain, which is now becoming attached to the surface, strongly loses entropy as it has to change from a coil conformation in solution to a stretched (“brushlike”) conformation at the surface thus reducing the number of possible chain conformations significantly. This entropy penalty is only compensated by the establishment of one chemical bond, namely, the one connecting the polymer to the surface. Some limitations of “grafting-to” processes can be overcome if the polymer is directly generated at the surface of the substrate. In the work described here we prepared self-assembled monolayers (SAMs) of a radical chain initiator and generated the polymer in situ on the surface of the substrate (see Figure 1a). There is no significant diffusion barrier for these “grafting from” techniques as only low molecular weight compoundssthe monomerss have to reach the growing chain end during film formation. (7) Laible, R.; Hamann, K. Adv. Colloid Interface Sci. 1980, 13, 65. (8) Tsubokawa, N.; Hosoya, M.; Yanadori, K.; Sone, Y. J. Macromol. Sci. - Chem. 1990, A27, 445. (9) Bridger, K.; Vincent, B. Eur. Polym. J. 1980, 16, 1017. (10) Ben Ouada, H.; Hommel, H.; Legrand, A. P.; Balard, H.; Papirer, E. J. Colloid Interface Sci. 1988, 122, 441. (11) Fery, N.; Laible, R.; Hamann, K. Angew. Makromol. Chem. 1973, 46, 81.
10.1021/la971035o CCC: $15.00 © 1998 American Chemical Society Published on Web 10/30/1998
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and important kinetic parameters such as the radical chain efficiency are therefore inaccessible. Both problems are circumvented if a monofunctional and monodentate initiator monolayer is deposited in one reaction step as described here. To evaluate the chemical structure, the molecular weights, and molecular weight distributions, it is desirable to detach the covalently bonded macromolecules from the surfaces after polymerization and generation of the polymer monolayer and characterize them by using common techniques of polymer analysis. This can be achieved by introducing a third, cleavable group to the structure of the initiator, which can now act as a “breakseal” for the covalent bond between the polymer chain and the surface. Experimental Section
In the few systems known up to now where radical chain initiators are attached to surfaces (almost exclusively to high surface area silica gels) stepwise procedures are employed,11-15 sometimes requiring up to eight surface modification reactions.11 Due to incomplete conversion in the different reaction steps and side reactions, the graft densities of the initiators remain low and cannot be easily reproduced. Consequently, the amounts of immobilized polymer also remain low and not very reproducible. Therefore the kinetics and mechanisms of the polymerization reactions are not well understood. Additionally, several systems12-15 use symmetric, bidentate azo compounds of the general structure X-R-NdN-R-X, with X being the group that can bind to an appropriate surface site. It cannot easily be determined to which extent both ends or only one of them are attached to the surface. This leads to an unknown structure of the initiator monolayer,
Materials. Styrene was purified using an ALOX B column, distilled in vacuo from copper(I) chloride and stored under nitrogen at -30 °C. Toluene was distilled under a nitrogen atmosphere from sodium/potassium using benzophenone indicator. Triethylamine was refluxed over CaH2 for 12 h and then distilled. Both were stored under nitrogen. Technical grade toluene was used for Soxhlet extractions. For surface plasmon spectroscopy and optical waveguide spectroscopy (SPS and OWS)16 special substrates had to be employed. Onto a microscopy slide (BK7 glass) a ca. 50 nm thick silver film was thermally evaporated. This film was then topped by an approximately 10-30 nm SiOx layer by argon ion sputtering from a quartz target. Those substrates were cleaned prior to modification by rinsing with several solvents. Characterization. X-ray photoelectron spectra were measured using an instrument from Leybold Heraeus equipped with an electron analyzer EA 11/100 and a X-ray source RQ 20/38 (Mg KR). The survey spectra were recorded by accumulating 4 scans (step width, 200 meV; step time, 50 ms). For detail spectra 150 scans were taken (step width, 50 meV; step time, 200 ms). The signals were identified according to ref 17. The FTIR spectra were recorded using a Digilab Division BioRad 3240-SPC FTS-40 spectrometer at a resolution of 4 cm-1. All spectra are were recorded in transmission. As substrates, 1 mm thick silicon wafers were used. Typically 100 to 1000 scans were accumulated. SPS and OWS were performed using a setup in an ATR (attenuated total reflection) configuration,18 where s- or ppolarized laser light (HeNe, 632.8 nm) is coupled into a silver film deposited on the substrate slides using a glass prism (BK7; n ) 1.5151), as schematically depicted in Figure 4A. The reflected light intensity as function of the angle of incidence was detected by a photodiode. The atomic force microscopy (AFM) measurements were carried out on a Topometrix 2000 AFM in the constant force mode. Synthesis of the Initiator. The initiator 2′,4-azo-(2′cyanopropyl)(4-cyanopentanoxy-(3′′-chlorodimethylsilyl)propylate) (1) is obtained by esterification of the corresponding azo carboxylic acid with allyl alcohol followed by hydrosilation with dimethylchlorosilane. Details of the synthesis have been reported elsewhere.19,20 Immobilization of the Initiator. The initiator was immobilized at room temperature under inert conditions (atmosphere of dry nitrogen) using anhydrous toluene as solvent and dry triethylamine as catalyst. The samples were kept in this solution overnight and then cleaned by extensive rinsing with methanol and chloroform. Polymerizations. Polymerizations were performed in toluene/ styrene mixtures (1/2 v/v) at 50 or 60 °C. All solutions were
(12) Boven, G.; Oosterling, M. L. C. M.; Challa, G.; Schouten, A. J. Polymer 1990, 31, 2377. (13) Tsubokawa, N.; Hayashi, S. J. Macromol. Sci., Chem. 1995, A32, 525. (14) Tsubokawa, N.; Ishida, H.; Hashimoto, K. Polym. Bull. 1993, 31, 457. (15) Carlier, E.; Guyot, A.; Revillon, A. React. Polym. 1992, 16, 115.
(16) Knoll., W. MRS Bull. 1991, 16, 29. (17) Beamson, G.; Briggs, D. High-Resolution Electron Spectroscopy of Organic Polymers - The Scienta ESCA300 Database; J. Wiley & Sons: New York, 1992. (18) Kretschmann, E. Opt. Commun. 1972, 6, 185. (19) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 1, 592. (20) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 1, 602.
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Figure 1. (A) Concept for the synthesis of covalently attached polymer monolayers via radical graft polymerization (“grafting from” technique) using immobilized initiators. (B) Synthesis of monolayers of polystyrene terminally attached to SiOx surfaces by using a self-assembled monolayer of AIBN like azo compound with a chlorosilane headgroup.
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carefully degassed through at least three freeze-thaw cycles to remove all oxygen traces. After polymerization, every sample was extracted using a Soxhlet apparatus and toluene for at least 10 h (more than 100 extraction cycles). This procedure was found to be mandatory to remove all of the nonbonded polymer material that is formed in solution. The nonbonded polymer can originate from the nonimmobilized part of the initiator, from thermally initiated polymerization of styrene in solution, or from transfer reactions of the growing chains to solvent or monomer molecules. Details of experiments to determine the kinetic and mechanism of the polymerization reaction on highly dispersed silica gel are published elsewhere.19,20
Results and Discussion To demonstrate the concept depicted in Figure 1a, we first synthesized (Figure 1b) an azo compound with a chlorosilane headgroup.19,20 This group can then be reacted with the silanol moieties on the surface of SiO2 substrates to form the initiator monolayer. In the work described here we used a monochlorosilane as the anchor group as this molecule cannot oligomerize or cross-link and therefore forms chemically well-defined monolayers.21 The azo moiety of this molecule is designed in such a way that it is comparable to the structure of azobis(2isobutyronitrile) (AIBN), which is probably the most extensively studied initiator for radical chain polymerization reactions.22 This self-assembled initiator monolayer was used for the radical chain polymerization of styrene using toluene as solvent. At first we performed X-ray photoelectron spectroscopy (XPS) and FTIR measurements to characterize the initiator and polymer layers qualitatively and to verify that all modification reactions follow the scheme described above. Figure 2 summarizes the results of the XPS measurements. In Figure 2A examples of the survey spectra of an unmodified SiOx substrate (I), the initiator monolayer (II), and the polymer film (III) on that substrate are depicted. Parts B, C, and D of Figure 2 show detail scans of several areas of interest, such as the N(1s) signal at 400 eV (Figure 2B), which appears in the spectrum of the substrate with the self-assembled monolayer of the azo initiator. The XP spectrum after polymerization (Figure 2A (I) and Figure 2C) is dominated by the C(1s) signal at 286 eV due to the presence of the polystyrene layer. The thickness of the polymer layer of this sample was ca. 40 nm and therefore higher than the escape depth of the photoelectrons (ca. 10 nm). The fact that even upon accumulating many scans no signals caused by the underlying substrate (e.g., Si(2p) signal at approximately 100 eV) could be detected (Figure 2D), shows that the polymer layer covers the surface homogeneously. As will be shown in more detail further below, the described system can be used to grow polystyrene layers with thicknesses of several hundred nanometers on SiO2 surfaces. Films with such thicknesses can be characterized by routine transmission FTIR spectroscopy. There(21) Most studies in the area of self-assembled monolayers of silanes on silicon oxide surfaces are carried out with trichloro- or trialkoxysilanes. These silanes, however, not only react with the surface silanol groups but also can react with each other resulting in the formation of surface-attached networks. The degree of cross-linking in these systems is strongly depended on the amount of water (moisture) present during immobilization. As this parameter is not very well controllable, often highly disordered “multilayers” on the surface are obtained and the graft density varies considerably from run to run. See for example: (a) Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1983, 87, 5516. (b) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (22) Moad, G.; Solomon, D. H. In Comprehensive Polymer Science; Allen, G., Bevington, J. C., Eds.; Pergamon Press: Oxford, 1989; Vol. 3, p 97. Using highly dispersed silica gel as substrate, we found that the decomposition behavior of the immobilized initiator is comparable to that of AIBN. See reference 19 and 20.
Figure 2. (A) XP survey spectra of a PSP substrate (glass/ silver (50 nm)/SiOx (30 nm)) prior to modification (I), after immobilization of the initiator (II), and after deposition of a 40 nm thick polystyrene layer (III). (B) Detail spectrum of the N(1s) area of the initiator modified sample. (C) Detail spectrum of the C(2s) area of the polystyrene monolayer. (D) Detail spectrum of the Si(2p) area of the polystyrene monolayer.
Figure 3. Transmission FTIR spectrum of a silicon wafer to which a ∼100 nm thick polystyrene layer was immobilized on both sides: 256 scans, resolution 4 cm-1.
fore, we deposited approximately 100 nm thick layers on both sides of a 1 mm thick silicon wafer. The transmission infrared spectrum of this sample is shown in Figure 3. This spectrum shows vibrational bands typical for polystyrene such as the CH stretching vibrations around 3000 cm-1 and absorptions due to the CdC double bond stretching vibrations at 1600, 1500, and 1450 cm-1. The area below ca. 1400 cm-1 contains the broad signal of the solid-state vibrations of the silicon oxide layers. For quantitative studies of the thicknesses of polymer layers deposited on silicon oxide surfaces, we used surface plasmon (plasmon surface polaritons, PSP) and optical waveguide spectroscopy (OWS). PSP and OWS measurements were performed by monitoring the reflected in-
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Figure 4. (A) Schematic description of the setup for surface plasmon and wave guide spectroscopy. (B) Reflectivity scans of a sample prior (O) and after (b) formation of a self-assembled monolayer of the azo initiator. Full lines represent results of Fresnel calculations giving a thickness of the initiator layer of 1.3 nm. (C) Reflectivity scans of several samples after deposition of different thick polystyrene monolayers by varying polymerization time. Full lines represent results of Fresnel calculations. (D) Thickness of polystyrene layers as a function of polymerization times. Values are determined by PSP spectroscopy. (E) Reflectivity scans of a polystyrene monolayer obtained by polymerization at 50 °C for 96 h in toluene/styrene (1/1 v/v) after 40 h of extraction with toluene. For experimental details see ref 19.
tensity as a function of the angle of incidence θ. As an example, Figure 4B depicts the reflectivity curves of a sample before and after the deposition of the selfassembled monolayer of the initiator silane. Both scans
show a narrow dip in the reflectivity R, however the resonance angle of the sample after immobilization of the initiator is clearly shifted to a higher value. These measurements agree well with curves calculated using
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Figure 5. (A) AFM image of a 200 nm thick polystyrene layer prepared by the “grafting from” technique. (B) Height profile along a line shown in (A).
Fresnel equations and a simple box model consisting of a 50.0 nm thick silver layer, topped by the SiOx layer (10.5 nm) on which in the latter case a 1.3 ( 0.5 nm thick initiator layer (assuming n ) 1.5) is deposited. As mentioned above, any system for the deposition of polymer layers should be designed to allow for the adjustment of the layer thickness over a large range. In the system described here the film thickness can be adjusted by changing the graft density and molecular weight of the surface-attached polymer chains. This can be done, for example, by polymerizing for different periods of time at a chosen temperature. In experiments with silica gel substrates19,20 we found that the polymerization reaction is easily controlled at about 60 °C. Thus, we used several azo-modified substrates and polymerized at that temperature for different periods of time. Figure 4C shows some typical examples of the reflectivity curves obtained from the different samples. It can be seen clearly that the resonance minimum shifts to higher angles with increasing polymerization time. From these shifts the
film thicknesses of the deposited layers were calculated on the basis of the Fresnel equations. Again a simple box model was used, and the refractive index of the polystyrene layer was assumed to be the same as that in the bulk (n ) 1.591).23 The thicknesses of the obtained films (in the dried, collapsed state) are shown in Figure 4D as a function of the polymerization time. It can be seen that the film thickness increases if the polymerization time, and therefore the number of activated azo initiator molecules, is raised. Due to the fact that within the observed period less than half of the initiator has decomposed19,20 (halflife time at 60 °C ca. 21 h), no saturation limit of the layer thickness is visible. As each data point shown in this plot was evaluated on a different sample, these results show that the modification reactions can be performed in a wellreproducible manner. It should be specifically noted that all samples have been extracted for 15-48 h with boiling (23) Polymer Handbook; Brandrup, J., Immergut, E. H., Ed.; J. Wiley & Sons: New York, 1989; p V/83.
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solvent (toluene) in a continuos extraction setup (Soxhlet extractor), so that no nonattached polymer remained in the film. However, the layer thickness is a function not only of the number of immobilized chains but also of their length (i.e., the molecular weight of the attached chains). Thus, it should be possible to control the layer thickness also if the polymerization conditions are altered in such a way that higher molecular weights are obtained. A convenient way to achieve this is to decrease the polymerization temperature, which lowers the concentration of growing chains in the monolayer and therefore decreases the probability of deactivation of the polymer radicals by recombination or disproportionation. Figure 4e shows the reflectivity scans using s- and p-polarized light of a sample that was prepared by polymerization at 50 °C for 96 h. In Figure 5A the first waveguide mode for p-polarized laser light is shown; Figure 5B depicts the waveguide mode excited on the same sample with s-polarized light. From these curves a layer thickness of 360 nm can be calculated. The occurrence of waveguided modes (one s- and one p-mode) allows a simultaneous calculation of the film thickness and the refractive index without further assumptions. Again, the calculated and measured reflectivity curves agree when the bulk refractive index of n ) 1.591 is used. Additionally, the good agreement between the measured and calculated reflectivity curves indicates a good homogeneity and low roughness of the film. To get a more quantitative and detailed impression of the film morphology, AFM measurements were performed. Figure 5A shows an AFM image taken from a 5 × 5 µm2 large area of a polystyrene film attached to a silicon wafer. The film thickness is approximately 200 nm. It should be noted that the scale in z-direction is only 5 nm and that all data points are within this range. When a profile along an arbitrary chosen line is taken (Figure 5B), it can be seen that the strongest height differences (peak to valley value)
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are only about 2 nm. The root-mean-square roughness of this section of the film was measured to be smaller than 1 nm. Both values show that the film is very homogeneous and smooth on this length scale. Conclusions Although we restricted the scope of the work reported here to polystyrene monolayers on silicon oxide surfaces as a model system, the concept described here, which uses self-assembled initiator monolayers and polymerization in situ, is capable of the formation of molecularly thin films from any polymer that can be synthesized using a chain polymerization reaction. This method opens the door to the preparation of surfaces with precisely tailored properties. After deposition of the initiator, simply a suitable monomer (and/or comonomer) has to be chosen and polymerized. In addition to the chemical tuning of the attached layer the thickness of the polymer film can be adjusted over a wide range up to values not accessible by other techniques of monolayer deposition. Thus the formation of surface coatings in a very controlled way can be achieved. The tethering of the polymer molecules to the substrate surface allows the preparation of films that are thermally stable and resistant to exposure even to good solvents for the polymer. In subsequent publications we will describe details on the reaction mechanism and report on the immobilization of functional polymer molecules, copolymers, and polyelectrolytes. Acknowledgment. Financial support by the German Research Council (DFG) is gratefully acknowledged. J.R. is indebted to the “Fond der Chemischen Industrie” for a “Liebig” fellowship and to the German Research Council for a second fellowship (“Habiliationsstipendium”). A. Rothers is thanked for performing the AFM measurements. LA971035O
