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Multifunctional Silane Polymers for Persistent Surface Derivatization and Their Antimicrobial Properties Varun Sambhy, Blake R. Peterson, and Ayusman Sen* Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed March 18, 2008 We demonstrate a versatile methodology combining both covalent surface anchoring and polymer cross-linking that is capable of forming long-lasting coatings on reactive and nonreactive surfaces. Polymers containing reactive methoxysilane groups form strong Si-O-Si links to oxide surfaces, thereby anchoring the polymer chains at multiple points. The interchain cross-linking of the methoxysilane groups provides additional durability to the coating and makes the coatings highly resistant to solvents. By tailoring the chemical structure of the polymer, we were able to control the surface energy (wetting) of a variety of surfaces over a wide range of water contact angles of 30-140°. In addition, we synthesized covalently linked layer-by-layer polymeric assemblies from these novel methoxysilane polymers. Finally, antibacterial agents, such as silver bromide nanoparticles and triiodide ions, were introduced into these functional polymers to generate long-lasting and renewable antiseptic coatings on glass, metals, and textiles.
1. Introduction Polymer coatings on surfaces play a critical role in biomaterials, in controlling surface interfacial properties, in sensors, and in the synthesis of novel surface-responsive materials.1 For example, coating glass, ceramics, and metals with organic polymers to render them antimicrobial or antiadhesive can result in longlasting self-sterilizing surfaces. A wide variety of membrane disrupting cationic amphiphilic polymers can be used for this purpose.2–5 However, a variety of mechanisms lead to the eventual degradation and loss of the organic coating material from the surface. These processes include poor interfacial adhesion due to incompatibility between the hydrophobic polymer and hydrophilic substrate, water seepage into the polymer-surface interface, cleavage of covalent bonds anchoring the organic moiety to the surface, and so forth. Hence, the design of coating materials with strong adhesive properties is of vital importance in developing long-lasting and durable functional (e.g., antimicrobial) surfaces. One method to increase durability of the polymer coating is to covalently anchor the polymer to the surface of interest. Traditional methods used to covalently anchor polymers to oxide surfaces include (a) surface-initiated polymerization,6–8 (b) * To whom correspondence should be addressed. E-mail:
[email protected]. (1) (a) Verkholantsev, V. V. Eur. Coat. J. 2003, 18, 21–22. (b) Schmatloch, S.; Bach, H.; van Benthem, R. A. T. M.; Schubert, U. S. Macromol. Rapid Commun. 2004, 25, 95–107. (c) Potyrailo, R. A. Angew. Chem., Int. Ed. 2006, 45, 702–723. (d) Alexander, C.; Shakesheff, K. M. AdV. Mater. 2006, 18, 3321–3328. (e) Amirgoulova, E. V.; Groll, J.; Heyes, C. D.; Ameringer, T.; Roecker, C.; Moeller, M.; Nienhaus, G. U. ChemPhysChem. 2004, 5, 552–555. (2) (a) Ilker, M. F.; Nuesslein, K.; Tew, G. N.; Coughlin, E. B. J. Am. Chem. Soc. 2004, 126, 15870–15875. (b) Ishitsuka, Y.; Arnt, L.; Majewski, J.; Frey, S.; Ratajczek, M.; Kjaer, K.; Tew, G. N.; Lee, K. Y. C. J. Am. Chem. Soc 2006, 128, 13123–13129. (c) Sellenet, P. H.; Allison, B.; Applegate, B. M.; Youngblood, J. P. Biomacromolecules 2007, 8, 19–23. (d) Tang, H.; Doerksen, R. J.; Jones, T. V.; Klein, M. L.; Tew, G. N. Chem. Biol. 2006, 13, 427–435. (3) (a) Tiller, J. C.; Liao, C. J.; Lewis, K.; Klibanov, A. M. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 5981–5985. (b) Haldar, J.; An, D.; de Cienfuegos, L. A.; Chen, J.; Klibanov, A. M. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 17667–17671. (4) (a) Youngblood, J. P.; Andruzzi, L.; Ober, C. K.; Hexemer, A.; Kramer, E. J.; Callow, J. A.; Finlay, J. A.; Callow, M. E. Biofouling 2003, 19, 91–98. (b) Krishnan, S.; Ward, R. J.; Hexemer, A.; Sohn, K. E.; Lee, K. L.; Angert, E. R.; Fischer, D. A.; Kramer, E. J.; Ober, C. K. Langmuir 2006, 22, 11255–11266. (5) Ainslie, K. M.; Bachelder, E. M.; Borkar, S.; Zahr, A. S.; Sen, A.; Badding, J. V.; Pishko, M. V. Langmuir 2007, 23, 747–754. (6) Dronavajjala, K. D.; Rajagopalan, R.; Uppili, S.; Sen, A.; Allara, D. L.; Foley, H. C. J. Am. Chem. Soc. 2006, 128, 13040–13041.
covalent linking of polymer to chemically modified glass surfaces,9–11 and (c) covalent linking of polymers having endfunctionalized groups to surfaces.12 All of the above methods suffer from drawbacks. In both surface-initiated polymerization and covalent binding of polymer to surface, the surface has to go through complex treatment to generate appropriate surface functionalities which can react further. Moreover, surface-initiated polymerization often has to be performed under highly controlled conditions (absence of water, air). All these factors limit the type and size of substrate being coated. Grafting end-functionalized polymers is also very inefficient and slow, as the chain ends have to diffuse to the surface and react. Further, polymer films resulting from these methods usually have just a single point of attachment to the surface, and hence, the polymer chains can easily break away to expose the surface. Recently, Ryu et al. have described a novel cross-linking methodology based on benzocyclobutene-functionalized random copolymers to form durable ultrathin coatings on generalized surfaces.13 This cross-linking approach is especially versatile, as it does not require specific chemical interactions with the surface and instead utilizes interchain polymer cross-linking to form insoluble nanometer thick coats. Since the conditions required to promote benzocyclobutene cross-linking are somewhat harsh (250 °C), it would be desirable to have cross-linkable groups capable of reacting under milder conditions such that a wider diversity of functionalities (e.g., bioactive and/or temperaturesensitive groups) can be built into the polymer coating system. In addition, it would be advantageous to design systems combining both covalent surface-anchoring and cross-linking abilities to widen the range of polymer coatings on generalized surfaces. It (7) Ma, H.; Wells, M.; Beebe, T. P.; Chilkoti, A. AdV. Funct. Mater. 2006, 16, 640–648. (8) Cui, T.; Zhang, J.; Wang, J.; Cui, F.; Chen, W.; Xu, F.; Wang, Z.; Zhang, K.; Yang, B. AdV. Funct. Mater. 2005, 15, 481–486. (9) Park, D.; Wang, J.; Klibanov, A. M. Biotechnol. Prog. 2006, 22, 584–589. (10) Lin, J.; Qiu, S.; Lewis, K.; Klibanov, A. M. Biotechnol. Prog. 2002, 18, 1082–1086. (11) Tiller, J. C.; Lee, S. B.; Lewis, K.; Klibanov, A. M. Biotechnol. Bioeng. 2002, 79, 465–471. (12) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. J. Science 1997, 275, 1458. (13) (a) Ryu, D. Y.; Shin, K.; Drockenmuller, E.; Hawker, C. J.; Russell, T. P. Science 2005, 308, 236–239. (b) Ryu, D. Y.; Wang, J. Y.; Lavery, K. A.; Drockenmuller, E.; Satija, S. K.; Hawker, C.; Russell, T. P. Macromolecules 2007, 40, 4296–4300.
10.1021/la800858z CCC: $40.75 2008 American Chemical Society Published on Web 06/12/2008
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is well-known that methoxysilane groups condense irreversibly with free -OH or other -Si(OR)3 groups under mild conditions to form strong Si-O-Si linkages.14 This reaction is facile and is catalyzed by traces of water or added base or acid. The chemistry has been widely used in synthesizing bonded stationary phases for chromatography, modifying surface interfacial properties, and forming self-assembled monolayers (SAMs) on oxide surfaces.15 Hence, one can reasonably expect polymers having these reactive methoxysilane groups to covalently attach to oxide surfaces (glass, ceramics, cellulosics, metals) having free -OH groups via Si-O-Si linkages. In addition, interchain methoxysilane coupling would allow these polymers to form coatings on nonfunctional surfaces (noble metals, hydrocarbon polymers) due to the cross-linking effect. Here, we describe novel copolymers containing reactive methoxysilane groups, which form excellent and durable surface coatings irrespective of surface chemistry under mild conditions. Not only do the polymers form multiple-point covalently linked durable coats on reactive oxide surface, they also cross-link to form persistent solvent-resistant coats on nonreactive surfaces. We also show that, by tailoring the chemical structure of the polymer, we can control the surface energy (wetting) of a wide range of both reactive and nonreactive surfaces. In addition, antibacterial functionalities were introduced into these functional polymers to generate long-lasting and renewable antiseptic coatings. This strategy can be used as a general technique to coat and modify both reactive and nonreactive surfaces with durable coatings of a desired functional polymer.
2. Experimental Section 2.1. Materials and Instrumentation. Poly(4-vinylpyridine) (MW ) 60 000), pentafluorostyrene, styrene, 4-vinylpyridine, and silver nitrate (99+%) were purchased from Aldrich. Iodomethane, 1-bromohexane (99+%), potassium iodide, iodine, methanol, nitromethane, and dimethyl sulfoxide (DMSO) (all ACS grade) were purchased from Acros Organics. Chloroform and acetonitrile were purchased from EMD chemicals. 3-Bromopropyltrimethoxy silane was purchased from Gelest. Commercial glass cleaning detergent (Labtone) was purchased from VWR. All solvents were dried by distilling from CaH2 prior to use. Bacterial growth media and agar were purchased from Difco. 1H NMR and 13C NMR spectra were recorded on a Brucker DPX-300 or CDPX-360 instrument. X-ray photoelectron (XP) spectra were recorded on a Kratos Analytical Axis Ultra instrument. Fourier transform infrared (FTIR) spectra were recorded on a Bruker IFS 66/s spectroscope. A Laurell WS5000 spin coater was used for making polymer coatings on surfaces. 2.2. Synthesis of Polymers 1a-e. All reactions were performed in inert atmosphere in a glovebox. Poly(4-vinylpyridine) (MW ) 60 000 from Aldrich) was dried for 3 days under vacuum at 60 °C. One gram of polymer was then dissolved in 15 mL of dry nitromethane (distilled from CaH2) and 0.35 g of 3-bromopropyltrimethoxy silane was added to it. The contents were heated at 65 °C for 5 h. After this, required amounts of the 1-bromohexane or iodomethane were added. The contents were then heated at 65 °C for 18 h. These polymers can also be prepared by heating poly(4-vinylpyridine) polymer with the 1-bromopropylytrimethoxy silane and the 1-bromoalkane in a one-step reaction. The final polymers were precipitated in dry ethyl ether and were dried under vacuum for 2 days. Polymers were characterized by 1H NMR and 13C NMR spectroscopy. The polymers were stored under an atmosphere of dry nitrogen to prevent (14) (a) Schottner, G. Chem. Mater. 2001, 13, 3422–3435. (b) Ruckenstein, E.; Li, Z. F. AdV. Colloid Interface Sci. 2005, 113, 43–63. (c) Howarter, J. A.; Youngblood, J. P. Langmuir 2006, 22, 11142–11147. (d) Howarter, J. A.; Youngblood, J. P. Macromolecules 2007, 40, 1128–1132. (15) (a) Faria, A. M.; Jardim, I.; Collins, K. E.; Collins, C. H. J. Sep. Sci. 2006, 29, 782–789. (b) Curran, D. P. Synlett 2001, 9, 1488–1496. (c) Franco, P.; Senso, A.; Oliveros, L.; Minguillon, C. J. Chromatogr., A 2001, 906, 155–170. (d) Snyder, L. R.; Dolan, J. W.; Carr, P. W. Anal. Chem. 2007, 79, 3254–3262. (e) LaCourse, W. R.; Dasenbrock, C. O. Anal. Chem. 1998, 70, 37R52R.
Sambhy et al. interchain cross-linking. The degree of N-alkylation and polymer compositions was determined by 1H NMR peak integration. 2.3. Synthesis of Polymer 2b. 4-Vinylpyridine and pentafluorostyrene were distilled from CaH2. Vinylpyridine (3.51 g) and pentafluorostyrene (3.31 g) were added to a round-bottomed flask. AIBN (azobis(isobutyronitrile), 0.22 g; 50:1 monomer/initiator molar ratio) was dissolved in 10 mL of anhydrous CHCl3 and was added to the flask. The flask was immediately put under N2 and was cooled in a liquid N2 bath. The contents of the flask were then deoxygenated by three freeze-pump-thaw cycles. The reaction mixture was then stirred under N2 at 65 °C in a preheated oil bath for 6 h. The contents were then added dropwise to a stirring solution of hexane/ethyl ether, which caused the polymer to precipitate. The precipitated polymer was washed with hexane and was separated by suction filtration. The polymer was finally dried at room temperature (RT) under vacuum for 24 h. A total of 0.13 g of the pure dry polymer from step 1 above was dissolved in 10 mL of DMSO/acetonitrile/ CHCl3 (3/3/1, v/v), 3-bromopropyltrimethoxy silane (0.4 g) was added, and then the contents were heated at 65 °C for 24 h. The quarternized polymer was precipitated by dropwise addition to hexane/ether. The precipitated polymer was collected by centrifugation, dried under vacuum for 24 h, and then stored under inert atmosphere. Polymer 2a was prepared similarly by radical polymerization of vinylpyridine and styrene. The NMR spectra of the polymers with peak assignments are given in the Supporting Information. 2.4. Synthesis of AgBr/1b Composite. Polymer 1b was dissolved in methanol/DMSO. A required amount of AgNO3 (1:2 molar ratio with respect to polymer bromide ions) was dissolved in water/ methanol. AgNO3 solution was then added dropwise to the stirring polymer solution over a time period of 15 min. The solvent composition of the final polymer solutions was 99% methanol/1% water. This solution was coated on glass slides and was baked to give composite films containing AgBr. The slides had a very slight yellowish color. 2.5. Coating Characterization. Glass and silicon surfaces were cleaned with Piranha solution (30% H2O2/70% H2SO4) for 5 min. Metal surfaces and textile fabrics were cleaned with detergent solution (VWR Labtone). Polymer solutions were made in methanol for polymers 1a-1e, chloroform for polymer 2a, and 1:1 acetonitrile/ chloroform for polymer 2b. Water (1% v/v) was added to the polymer solutions to promote methoxysilane coupling. Various substrates were then coated by using a spin coating technique (3000 rpm for 45 s) for solid surfaces or dip coating for textile fabrics. The substrates were baked in an oven at 70 °C for 1 h. The substrates were rinsed rigorously with appropriate solvents for the respective polymers (methanol for polymers 1a-1e or CH3CN/CHCl3 for polymers 2a and 2b) for 24 h followed by washing with water/detergent (VWR Labtone detergent) at 60 °C for 24 h and finally water at 60 °C for 24 h. The XPS analysis was performed with a Kratos instrument using a monochromatic Al X-ray source (1486.7 eV) operated at 280 W and at a pressure of 10-9 Torr. The analysis was performed at a takeoff angle of 90° with respect to the sample surface plane, resulting in an approximate sampling depth of 50 Å. Quantification was performed using relative sensitivity factors (RSFs) given by the instrumentation facility. XPS data processing was carried out on CASAXPS software. The C-C carbon 1s peaks were shifted to 285.0 eV to correct for any charging effects. For FTIR measurements, gold and silicon surfaces were coated with respective polymers and washed as described above. Attenuated total reflection (ATR)-FTIR or reflectance FTIR were performed on polymer-coated silicon or gold samples, respectively. Single wave ellipsometry measurements were recorded at 632.8 nm (He-Ne laser) using a null ellipsometer (Gaertner Scientific Corporation, Skokie, IL) set at a 70° angle of incidence. Silicon wafers were used as substrates for polymer films. The experimental polarization angles were used to determine film thickness using well-established modeling methods. The two angles (incident and reflecting) give the phase shift between the parallel and perpendicular components (∆), and the change in the ratio of the amplitudes of
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Figure 1. Schematics of surface-binding methodology. (A) Incorporating hydrolytically unstable methoxysilane-substituted methacrylate units in a polymer failed to yield detergent resistant coatings. (B) Chemistry for incorporating hydrolytically stable methoxysilane functionalities (N-alkylmethoxysilane pyridinium) in a polymer. (C) Covalent multilayer polymer anchoring on surface.
the two components (tan Ψ). For simplicity, the thicknesses were based on a single, homogeneous, and isotropic film model, and the ∆ and Ψ values were related to the refractive index (RI) of the polymer films to find the thickness (assumed RI of 1.5). At least five ellipsometric measurements were made at different spots for each sample, and the thickness reported is the average of the measurements. The water contact angle measurements were performed by using the static sessile drop method with a contact angle goniometer having a reliability of (3°. Polymer coatings were formed on various flat surfaces as described previously. The coated surfaces were then rigorously washed to remove nonbound polymer, and images of water droplets on these surfaces were recorded using a chargecoupled device (CCD) camera attached to the contact angle goniometer. Contact angles were obtained by analyzing the images of the water droplets using Image J image processing software. A minimum of five measurements in five different spots were recorded for each substrate, and the values reported are the average of these measurements, with a reliability of (3°. 2.6. Antibacterial Testing. Escherichia coli DH5-R (Clontech) were grown at 37 °C and maintained on Luria-Bertani (LB) plates (LB broth, Lennox modification, with 1.5% agar). Bacillus cereus was grown at 30 °C. The relation between absorbance at 590 nm (OD590) and colony forming units (cfu) per milliliter determined using the plate count method was used for standardization of bacterial solution concentrations.29 A modified Kirby Bauer disk diffusion method was used to study the antibacterial activity of the coated surfaces toward surface-borne bacteria. Freshly grown bacteria were diluted by LB broth to yield stock solutions having an approximate concentration of 5 × 105 cfu/mL for E. coli. A total of 100 µL of this stock solution was plated on LB-agar growth plates. The coated surfaces were placed coating side down on top of the inoculated agar plates and incubated overnight at 37 °C. Colonies were visualized the next day, and digital images of the plates were captured. Image J 1.34n, a free software available via the NIH, was used to measure the zone of inhibition in digital pictures of the plates. For airborne testing, a saturated suspension of E. coli was centrifuged at 4000 rpm for 5 min. The cells were resuspended in nanopure water to yield a concentration of 106 cfu/mL. A sterilized chromatography sprayer (General Glassblowing, Richmond, CA) was used to spray a fine mist of the above cell suspension onto the partially coated slides. These slides were then placed into empty
polystyrene Petri dishes (VWR, 100 mm × 15 mm) and air-dried for five minutes at 37 °C. Autoclaved LB broth (with 1% agar) that had been allowed to cool to approximately 40 °C was poured onto the bacteria-sprayed slides. After the agar had solidified, the slides were incubated at 37 °C for 18 h and then colonies were quantified.
3. Results and Discussion 3.1. Polymer Synthesis and Characterization. The general strategy entails introducing reactive methoxysilane groups in a polymer of interest. Initial attempts to form polymers with pendant methoxysilane groups by free radical copolymerization of appropriate methoxysilane-substituted methacrylate failed to yield durable covalently linked polymer coatings on glass surfaces. For example, copolymers of styrene or methyl methacrylate with ∼10 mol % 3-(trimethoxysilyl)propyl methacrylate were synthesized by free radical polymerization. These copolymers containing reactive alkoxysilyl methacrylate functionalities were spin coated on Piranha cleaned glass and silicon surfaces. However, it was observed that the polymer coatings were completely removed upon rigorous washing with detergents (pH 12, 24 h). This was indicated by the low water contact angles (∼11°) of coated glass surfaces after detergent washing and the ellipsometry data indicating complete loss of the ∼30 nm thick coating on silicon after the washing step. Hydrolytic cleavage of the ester bond in the alkaline detergent solutions led to the eventual loss of the surface-anchored polymer film (Figure 1A). Upon further investigation, we found that N-alkyl pyridinium bonds are especially stable toward hydrolytic cleavage. The pyridine group readily reacts with n-haloalkane via nucleophilic displacement under mild conditions to form N-alkylpyridinium halides. This methodology was used to introduce reactive methoxysilane groups into several different polymers (Figure 1B and Figure 2). Polymers containing pendant pyridine units were reacted with 1-bromopropyltrimethoxy silane to introduce the reactive methoxysilane functionality. These polymers can condense with free -OH groups on oxide surfaces (glass, ceramics), metals, and cellulosics (wood, cotton, paper) to covalently anchor the polymer chains to the surface through
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Figure 2. Synthesis of library of surface-binding polymers with reactive methoxysilane groups. (A) Derivatives of poly(4-vinylpyridine). (B) Surfacebinding polymers synthesized by radical polymerization.
Si-O-Si and X-O-Si linkages (X ) surface) (Figure 1C). Moreover, methoxysilane groups on neighboring polymer chains can further react with each other to form uniform cross-linked polymer films which remain covalently anchored to the surface. This cross-linking process also enables the formation of polymer coats on unreactive surfaces (noble metals, polymers), in which the nanometer thick, insoluble cross-linked polymer coat remains tightly bound to the surface. The formation of robust coatings on unreactive surfaces may be attributed to the cross-linking insolublization of the polymer film and favorable noncovalent interactions between the coating and the substrate. A similar cross-linking methodology for coating unreactive surfaces (e.g., gold, plastic) has been previously described by Ryu et al.13 A library of polymers containing N-propyltrimethoxysilane pyridinium groups were synthesized as shown in Figure 2. Polymers 1a-1e were synthesized by sequential N-alkylation of the pyridine groups of commercially available poly(4-vinylpyridine) with 3-bromopropyltrimethoxysilane, followed by reaction with calculated amounts of iodomethane or 1-bromohexane. This yielded five polymers containing ∼10-15% of the reactive methoxysilane group and varying amounts of free and N-alkylated pyridine units. Polymers 1b-1e were used to generate potent long-lasting antimicrobial surfaces, as discussed later. Polymers 2a and 2b were prepared by free radical copolymerization of 4-vinylpyridine with styrene or pentafluorostyrene. The monomer feed ratios were tailored such that the resulting copolymers had ∼10-15% of vinylpyridine groups. The precursor copolymers were then N-alkylated with 3-bromopropyltrimethoxy silane to introduce the reactive methoxysilane functionalities. Perfluorinated polymer 2b was used to generate low energy surfaceanchored coatings (high water contact angle) on various surfaces. The polymers were soluble at room temperature in organic solvents such as DMSO, nitromethane, methanol (1a-1e); chloroform or toluene (2a); and 1:1 acetonitrile/chloroform (2b). Dilute solutions of the polymers remained stable with no observable precipitation for months. However, upon exposure to moist air, the solid polymers slowly cross-linked over weeks to yield insoluble gels. Hence, solid polymer samples were stored under dry atmosphere in a constantly purged desiccator. Copolymers were characterized by 1H NMR, 13C NMR, and 19F NMR spectroscopy. 1H NMR spectra were used to calculate
polymer compositions, for example, degree of N-alkylation and monomer content. (see the Supporting Information). Molecular weight analysis of polymers 2a and 2b by size exclusion chromatography indicated a monomodal distribution with Mn ) 30-40 kDa. 3.2. Analysis of Polymer Coatings. As determined by 1H NMR spectroscopy, the polymers contain around 5-15 mol% of the reactive methoxysilane groups on the pyridinium side chains. Hence, coatings of the polymers are expected to covalently link to surfaces having free hydroxyl (-OH) groups. Methoxysilane groups on neighboring polymer chains should also react with each other under mild conditions to form a uniform crosslinked polymer network which remains covalently anchored to the surface. In addition, this cross-linking effect would enable these polymers to form durable solvent-resistant coatings on nonreactive surfaces. We tested the ability of these polymer libraries to form durable coatings on a wide variety of surfaces: glass, silicon, metals such as stainless steel, copper, gold, and hydrocarbon polymers such as Parylene-C. Glass and silicon substrates were cleaned with Piranha solution (3:1 H2SO4/H2O2). Metal and polymer substrates were cleaned with detergent. Polymer solutions (1% w/v) were prepared in appropriate solvents (methanol or chloroform/acetonitrile), and the substrates were coated using the spin coating technique. To promote methoxysilane coupling to form Si-O-Si linkages, 1% water (v/v) was added to each solution just prior to substrate coating. The substrates were then placed in an oven set at 70 °C for 1 h. The baking step at mild temperature promoted the condensation of the -Si(OMe)3 groups to yield Si-O-Si covalent linkages both between the polymer and the surface and in-between the polymer chains. The surfaces were then washed exhaustively for 3 days: 24 h with appropriate solvent (methanol or chloroform), 24 h with water/detergent at 60 °C (pH 12), and finally 24 h with water at 60 °C. The substrates were dried in nitrogen stream and kept in Teflon boxes for further testing and characterization. Substrate surface analyses were carried out using contact angle measurements (CA), ellipsometry, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS).
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Table 1. Water Contact Angle and Ellipsometry Thickness of Polymer Coatings on Chemically Distinct Substrates sessile water droplet contact angle on substrate (°)b coating polymera
glass
steel
gold
brass
aluminum
Parylene-C
ellipsometry thickness on siliconc (nm)
baresubstrate 1a 1b 1c 1d 1e 2a 2b
10.5 55.0 60.8 74.2 50.5 33.0 81.3 101.5
27.3 54.3 59.5 73.3 49.0 34.3 83.0 102.4
62.0 53.2 58.5 72.7 51.5 -d 80.5 100.0
22.5 55.7 61.0 75.0 52.0 33.7 82.7 103.3
24.2 52.5 60.5 74.5 52.3 32.3 80.5 102.0
86.5 55.6 61.7 75.2 52.0 -d 79.3 104.5
34.9 (3.1) 29.3 (2.7) 27.5 (4.0) 20.7 (3.6) 31.2 (2.9) 33.3 (3.5) 23.1 (3.9)
a 1% (w/v) polymer solution spin coated on substrates (3000 rpm, 45 s). b Contact angles reported are the average of at least five measurements with a reliability of (3°. c Standard deviations reported in paranthesis. d Polymer 1e did not coat the substrate.
Ellipsometry was used to determine the thickness of polymer coatings on silicon substrates. A thickness of ∼20-30 nm was observed for single coats of different polymers spin coated from 1% w/v solutions (Table 1). Water contact angle measurements on substrates after rigorous solvent washing clearly indicated the presence of surface-anchored polymer coatings. The contact angles for different polymers on substrates are given in Table 1. As is clear, the contact angle for water droplets on each coated substrate was independent of the nature of the substrate and was dependent only on the chemical identity of the coated polymer. The small differences in contact angles for the same polymer on different surfaces can be attributed to the variations in surface roughness for different substrates. Control polymer coatings without the surface-binding methoxysilane groups on silicon surfaces were completely removed after the wash steps as indicated by zero thickness in ellipsometry and a very low water contact angle (
