Attachment of Polymer Films to Solid Surfaces via Thermal Activation

Aug 12, 2009 - G. K. Raghuraman, Kerstin Schuh, Oswald Prucker, and Jürgen Rühe*. Department of Microsystems Engineering (IMTEK), University of ...
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Attachment of Polymer Films to Solid Surfaces via Thermal Activation of Self-assembled Monolayers Containing Sulphonyl Azide Group G. K. Raghuraman, Kerstin Schuh, Oswald Prucker, and J€urgen R€uhe* Department of Microsystems Engineering (IMTEK), University of Freiburg, Georges-K€ ohler-Allee 103, 79110 Freiburg, Germany Received June 16, 2009. Revised Manuscript Received July 22, 2009 We report on a simple and effective way to attach thin polymer films to solid surfaces. The system is based on a thermosensitive sulphonyl azide derivative that is immobilized to SiO2 surfaces via chlorosilane anchoring group and subsequently covered with a polymer film. Upon heating the sulfonyl azide decomposes, leading to a C-H insertion reaction from the adjacent polymer chain resulting in a covalent attachment of the polymer to the surface. Any nonbound polymer can be removed by extraction. The method allows to attach a wide spectrum of polymers to solid surfaces. The film thickness of the monolayers can be tuned by adjusting the molecular weight of the polymer used and to some extent, the thermolysis conditions. The film thickness increases linearly with the radius of gyration of the polymers used for attachment. We have successfully attached thin layers of poly (styrene), poly (dimethylacryl amide) and poly (heptadecafluorodecylacrylate).

Introduction Modification of surfaces through the attachment of polymer films is an important means of controlling the properties of surfaces of materials and introducing functional groups. One pathway to apply such coatings is to simply deposit them from solution like a spraying process, dip-coating, spin-coating, or attaching them through adsorption processes.1 Although such methods are very simple to carry out, the resulting polymer films are intrinsically endangered against dissolution, displacement, dewetting or delamination.2 Especially, displacement is a rather critical issue as polymer coatings are not only used in the ultra clean conditions and so the polymer molecules have to compete against all sorts of contaminants present in the contacting environment.3-7 One way to enhance stability is to chemically attach the polymer molecules to the surfaces to be modified through the establishment of chemical bonds. A wide spectrum of different attachment strategies have been developed including the generation of surface-attached polymer chains through induction of the growth of polymer chains at surfaces “in situ”, through surfaceattached monolayers of initiators (a surface-initiated polymerization, “grafting from”), or with the help of surface-attached monomers (“grafting through”).8-14 As the immobilization of *correspondence should be addressed to: [email protected]. (1) Advincula, R. C.; Brittain, W. J.; Caster, K. C.; R€uhe, J., Eds. Polymer Brushes; Wiley-VCH: Weinheim, Germany, 2004. (2) Horn, J.; Hoene, R.; Hamann, K. Makromol. Chem. Suppl. 1975, 35, 329. (3) Hashimoto, K.; Fujisawa, T.; Kobayashi, M.; Yosomiya, R. J. Appl. Polym. Sci. 1982, 27, 4529. (4) Tsubokawa, N.; Kuroda, A.; Sone, Y. J. Polym. Sci. 1989, A27, 1701. (5) Tsubokawa, N.; Hosaya, M.; Yanadori, K.; Sone, Y. J. Macromol. Sci. Chem. 1990, A27, 445. (6) Kato, K.; Uchida, E.; Kang, E. T.; Uyama, Y.; Idaka, Y. Prog. Polym. Sci. 2003, 28, 209. (7) Prucker, O.; Ruhe, J. Macromolecules 1998, 31, 592–601. (8) Prucker, O.; Ruhe, J. Macromolecules 1998, 31, 602–613. (9) Bridger, K.; Vincent, B. Eur. Polym. J. 1980, 16(10), 1017. (10) Bridger, K.; Fairhurst, D.; Vincent, B. J. Colloid Interface Sci. 1979, 68(1), 190. (11) Prucker, O.; Naumann, C. A.; Ruhe, J.; Knoll, W.; Frank, W. C. J. Am. Chem. Soc. 1999, 121, 8766. (12) Pahnke, J.; R€uhe, J. Macromol. Rapid Commun. 2004, 25, 1396. (13) Raghuraman, G. K.; Dhamodharan, R.; Prucker, O.; Ruhe, J Macromolecules 2008, 41, 873. (14) Yan, M.; Ren, J. Chem. Mater. 2004, 16, 1627.

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preformed polymers is concerned, most approaches focus on the reaction of reactive moieties in the polymer chain with corresponding reactive sites at the surface of the substrate, mostly through condensation reactions. However, such a process seems not in all cases desirable, as reactive groups need to be present in the polymer. These reactive anchor groups might interfere with other functional groups simultaneously present in the polymer. In addition, such attachment reactions are not easily reproduced from a quantitative point of view as the yield of such surface reaction will vary, a feature that is more or less characteristic for all self-assembly processes as the steric situation at a surface depends strongly on the surface topography. In consequence, such a procedure leaves behind reactive sites both in the polymer and at the surface, which might be problematic for further use of the obtained layers. An interesting alternative to this is the polymer attachment to surfaces as depicted in Figure 1. Here, a self-assembled monolayer (SAM) of molecules containing a reactive moiety is first generated on the substrate surface. SAM formation is then followed by the deposition of a (functionalized) polymer layer. Upon photo- or thermal activation, the polymer is bound to the surface because of C-H bond insertion of the reactive group. An example for such monolayers is a benzophenone group containing silane forming a SAM at SiO2 surfaces. After deposition of polymers and photoactivation, the polymer is bound to the surface through triplet formation, hydrogen abstraction and radical-radical coupling process. For such processes, almost all polymers can be used and no specific composition is required.12-15 However, the polymer film is not accessible to light in all situations. In such a case, a thermally induced activation would be preferable. An interesting process that allows thermal activation was published by Mingdi Yan and co-workers, in which they have attached ultrathin polymer layers to surfaces through an insertion reaction using a pentafluorphenylazide group containing monolayer.14,15 Upon thermal activation, the azide group decomposes under release of nitrogen to yield nitrene groups, which are highly reactive and insert into almost any C-H bonds, provided that they (15) Liu, L.; Yan, M. Angew. Chem., Int. Ed. 2006, 45, 6207–6210.

Published on Web 08/12/2009

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Synthesis of 4-Hydroxybenzene Sulphonyl Chloride (3).

Figure 1. C-H bond insertion of functional group from a surfaceattached monolayer into an aliphatic group of the polymer leads to monolayer formation.

are in close vicinity. The pentafluorophenyl moiety was chosen because unsubstituted phenylazides have rather high decomposition temperature of typically more than 250 °C, making them impractical for a wide spectrum of applications. A drawback of pentafluorophenyl compounds, however, might be that they are rather nonpolar so that the resulting monolayers are not easily wetted by more hydrophilic polymers. In this paper, we propose the use of self-assembled monolayers of sulphonyl azide groups for the thermal immobilization of polymers at surfaces. We describe the synthesis of a chlorosilane containing such groups, the formation of SAMs of these molecules at silicon substrate surfaces, and the thermal binding of a variety of different polymers to the thus obtained modified substrates.

Experimental Section Materials. Allyl bromide, sodium-4-hydroxybenzene sulfonic acid, thionyl chloride, dimethyl formamide (DMF), sodium azide, dimethylchlorosilane, triethylamine, methanol, toluene, platinum activated on charcoal (10% Pt/C), and poly(sodium-4-styrene sulfonate) (PSS) were purchased (Sigma-Aldrich and Fluka, Germany). The solvents were dried according to standard drying procedures. Triethylamine (NEt3) was distilled over CaH2 prior to use and kept and handled unter nitrogen. All other chemicals and solvents (HPLC grade) were used as received. Polystyrene (PS) and poly(methacrylic acid) (PMAA) of known molecular weight used for the photochemical attachment experiments was used as purchased from Polymer Standard Service (Mainz, Germany). Poly(dimethyl acrylamide) (PDMAA) and poly(heptadecafluorodecyl acrylate) (PHFDA) were synthesized via free radical polymerization according to a literature process.16 Methods. The FT-IR spectra were recorded using a Biorad spectrometer at a resolution of 4 cm-1. NMR was performed on a Bruker (250 MHz) FT-NMR spectrometer using deuterated solvents. Film thicknesses were measured with a DRE-XO2 C ellipsometer operating with a 632.8 nm He/Ne laser at a 70° incident angle. Measurements were obtained on three different spots on each wafer with 3 measurements per spot. XPS analyses were performed on a Physical Electronics 5600 spectrometer equipped with a concentric hemispherical analyzer and using an Al KR X-ray source (15 KeV, filament current 20 mA). The samples were investigated under ultrahigh vacuum conditions (1  10-9 to 1  10-8 mbar). Advancing contact angles were measured using a OCA 20 system from Dataphysics GmbH, Germany with Millipore-filtered DI water as a test liquid, which was added to a sessile drop at a rate of 0.1 μL/s with a syringe pump. The contact angle was continuously monitored and found to plateau as the contact line began to move. Printing of the microarray were performed from aqueous solution (1 mg/mL polymer, 400 pL/spot) on a piezo-despensing noncontact printer from Scienion (SciFlexarrayer S3). Imaging ellipsometry was carried out at an angle of incidence = 60° and λ = 532 nm (solid-state laser) on an autonulling ellipsometer from Nanofilm (Nanofilm EP3). (16) Schuh, K.; Prucker, O.; Ruhe, J Macromolecules 2008, 41(23), 9284–9289.

770 DOI: 10.1021/la9018019

The synthesis of 4-hydroxybenzene sulphonyl chloride 3 was performed using a modified literature process.17 Thionyl chloride (44.3 mL, 604 mmol, 6.91 equiv) with a catalytic amount of DMF (1 mL) were quickly added to solid sodium 4-hydroxybenzenesulfonate 2 (20.3 g, 87.2 mmol, 1.00 equiv) and the reaction mixture was stirred at 60 °C for 4 h. After being cooled to room temperature, the mobile, nearly homogeneous mixture was poured over 160 g of ice with vigorous stirring. The resulting aqueous layer was extracted with dichloromethane (DCM, 1  60 mL, 2  40 mL) and the combined organic solutions were washed with 40 mL of ice water. After drying with Na2SO4, the solvent was removed in vacuo and the remaining oil (10.4 g, 53.8 mmol, 62%) was used in the second step without further purification. The analytics were performed with a product purified by column chromatography (hexane/ethyl acetate 3:1, Rf = 0.22) leading to a pale red solid. 1H NMR (250 MHz, CDCl3): δ (ppm) 7.93 (dt, J= 9 Hz, J= 2.5 Hz, 2H, C-Har ortho to SO2Cl), 7.02 (dt, J= 9 Hz, J= 2.5 Hz, 2H, C-Har ortho to OH), 5.84 (bs, 1H, OH). 13C NMR (63 MHz, CDCl3): δ (ppm) 161.8 (Car-OH), 135.9 (Car-SO2N3), 129.8 (2 x HCar-CarSO2N3), 116.4 (2 x HCar-CarOH). FTIR (neat): 3458, 3102, 3074, 1914, 1664, 1584, 1497, 1443, 1371, 1290, 1228, 1186, 1162, 1082, 836, 672, 573, 530 cm-1. HRMS (EI) calcd for C6H5SO3Cl, 192.6; found, 192.0. Anal. Calcd for C6H5SO3Cl: C, 37.41; H, 2.62; S, 16.65. Found: C, 37.42; H, 2.62; S, 16.65.

Synthesis of 4-Hydroxybenzene Sulphonyl Azide (4). 4-hydroxybenzenesulphonyl chloride 3 (3.76 g, 19.5 mmol, 1.00 equiv) was dissolved in acetone (50 mL), and an aliquot volume of water (50 mL) was added. The turbid reaction mixture was cooled to 0 °C and NaN3 (1.41 g 21.7 mmol, 1.11 equiv) was added in small portions. After the reaction mixture was allowed to stir for 1 h at 0 °C, the acetone was removed in vacuum (30 °C, 150 mbar) and the aqueous layer was extracted with DCM (3  30 mL). The combined organic layers were dried over Na2SO4 and the solvent was evaporated. The resulting oil (3.06 g, 15.4 mmol, 79%) was dried in a vacuum and used in the following step without further purification. Rf = 0.23 (hexane/ethyl acetate 3:1). 1 H NMR (250 MHz, CDCl3): δ (ppm) 7.85 (dt, J = 9 Hz, J = 2.5 Hz, 2H, C-Har ortho to SO2N3), 7.02 (dt, J = 9 Hz, J = 2.5 Hz, 2H, C-Har ortho to OH), 6.36 (bs, 1H, OH). 13C NMR (63 MHz, CDCl3): δ (ppm) 161.5 (Car-OH), 130.2 (2  HCarCarSO2N3), 129.5 (Car-SO2N3), 116.5 (2  HCar-CarOH). FTIR (neat): 3437, 3250, 2936, 2128, 1920, 1657, 1584, 1500, 1440, 1366, 1290, 1161, 1087, 839, 750, 675, 596 cm-1. HRMS (EI) calcd for C6H5SO3N3, 199.2; found, 199.0. Anal. Calcd for C6H5SO3N3: C, 36.48; H, 2.53; N, 21.10; S, 16.10. Found: C, 36.35; H, 2.61; N, 21.09; S, 15.89. Synthesis of 4-Allyloxybenzene Sulphonyl Azide (5). 4hydroxybenzenesulphonyl azide 4 (1.72 g, 8.63 mmol, 1.00 equiv) was dissolved in 17 mL of DMF and sodium hydroxide (354 mg, 8.85 mmol, 1.03 equiv) and allyl bromide (14.0 g, 116 mmol, 13.4 equiv) were added. After the solution was stirred at room temperature for 16 h, 50 mL of water was added and the product was extracted with diethyl ether (3  20 mL). The combined organic layers were dried over Na2SO4 and the solvent was evaporated at 30 °C. Purification with column chromatography (hexane/ethyl acetate 10:1) yielded 5 as a pale yellow oil (0.89 g, 32%). Rf = 0.24. 1H NMR (250 MHz, CDCl3): δ (ppm) 7.88 (dt, J = 9 Hz, J = 2.5 Hz, 2H, C-Har ortho to SO2N3), 7.06 (dt, J = 9 Hz, J = 2.5 Hz, 2H C-Har ortho to OCH2-), 6.12-5.97 (m, 1H, =C-H-), 5.48-5.33 (m, 2H, =CH2), 4.65 (dt, J = 5 Hz, J = 1.5 Hz 2H, OCH2). 13C NMR (CDCl3): δ (ppm) 163.6 (CarOCH2-), 131.7 (-CHdCH2) 129.8 (2  HCar-CarSO2N3, 1 x CarSO2N3), 118.7.(-CH=CH2), 115.5 (2  HCar-CarOCH2-), 69.3 (-OCH2). FTIR (neat): 3095, 3087, 2931, 2876, 2348, 2128, 1651, 1593, 1495, 1368, 1174, 1089, 990, 928, 833, 744, 657, 591, 555 cm-1. HRMS (EI) calcd for C9H9SO3N3, 239.3; found, 239.0. (17) Campbell, R. W.; Hill, H. W. J. Org. Chem. 1973, 38, 1047.

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Figure 2. Thermal attachment of polymers to surfaces using a sulphonyl azide monolayer.

Figure 3. Synthesis of chloro silane sulphonyl azide 1. Anal. Calcd for C9H9SO3N3: C, 45.18; H, 3.79; N, 17.56; S, 13.41. Found: C, 45.15; H, 3.84; N, 17.1; S, 13.33.

the resulting polystyrene layers were determined again by ellipsometry.

Synthesis of 4-(3-Chlorodimethylsilyl)propyloxybenzenesulphonyl Azide (1). 4-allyloxybenzenesulphonyl azide 5 was

Results and Discussion

placed in a Schlenk tube and 20 mL of freshly distilled dimethylchloro silane was added along with a catalytic amount of 10% Pt/C under a nitrogen atmosphere. The resulting mixture was refluxed at 40 °C. The reaction progress was followed by NMR and after completion of the reaction; the catalyst was filtered off and the excess dimethyl silane was removed by vacuum distillation yielding the desired product 1 in quantitative yields. 1H NMR (250 MHz, CDCl3) δ (ppm) 7.87 (d, J = 8.5 Hz, 2H, C-Har), 7.03 (d, J = 8.5 Hz, 2H, C-Har), 4.01 (t, 2H, -OCH2), 1.92-1.78 (m, 2H, -CH2-), 0.71-0.61 (t, 2H, -SiCH2), 0.11 (s, 6H, -SiCH3). 13C NMR (CDCl3) δ (ppm): 129.7 (2  HCar-CarSO2N3), 115.0 (2  HCar-CarOCH2-), 70.9 (-OCH2), 22.8 (-OCH2CH2-), 14.0 (CH2Si), 0.15 (2 x CH3). FTIR (neat): 2954, 2879, 2127, 1596, 1495, 1371, 1261, 1163, 1089, 835, 744, 600 cm-1.

Immobilization of Chlorosilane Sulphonyl Azide Moieties on Silicon Wafers. The substrates were cleaned by rinsing it in water, acetone and then with toluene. Precleaned substrates were taken in a Schlenk tube and dried in vacuum followed by filling with a stream of dry nitrogen. Dry toluene was added until the substrates were completely covered. 5.0 mL of sulphonyl azide chlorosilane 1 in toluene (0.1 mol/L) was added along with few drops of triethylamine (as an acid scavenger) under inert atmosphere and kept overnight. After the reaction, the samples were rinsed with toluene, methanol and acetone to remove excess of sulphonyl azide chlorosilane and the byproduct.

Preparation of Polymer Layers for Thermal Attachment. Thick overcoats (>100 nm) of PS were prepared by spin-casting a solution of PS in toluene (concentrations of 10 mg mL-1) at a typical spin speed of 1500 rpm for 1 min. The samples were then heated in an oven which preheated to the desired temperature. After the thermal treatment, the samples were extracted in a Soxhlet apparatus with toluene for 6 h to remove the nonbonded polymer. Similarly, PDMAA in ethanol (concentrations of 10 mg mL-1) and PHFDA in freon (concentrations of 10 mg mL-1) were spin-casted and then subsequently attached thermally. After the thermal treatment, the samples were then extracted in a Soxhlet apparatus with the respective solvents for 6 h to remove the nonbonded polymer. The thicknesses of Langmuir 2010, 26(2), 769–774

Sulphonyl azide derivatives are well-known for their high tendency to decompose through activation by heat or light into sulfonyl nitrenes with a concurrent loss of nitrogen. The resulting sulfonyl nitrenes in turn are highly reactive compounds, which insert into almost any C-H bond.16,18-22 We have utilized the robust nature of these sulphonyl azide derivatives for the thermal attachment of polymer monolayers. As shown in Figure 2, a SEM of a silane with a sulphonyl azide headgroup on a silicon surface was prepared, followed by the deposition of polymers. Upon heating, the polymers, which are in contact with the monolayer, become covalently attached to the surface. Chlorosilane sulphonyl azide 1 can be synthesized in four simple steps (Figure 3). First, sodium 4-hydroxylbenzenesulphonate 2 is converted into sulphonyl chloride 3 with thionyl chloride, followed by the chloride-azide exchange with sodium azide. Without further purification, 4-hydroxybenzene sulphonyl azide 4 was transferred into 4-allyloxybenzene sulphonyl azide 5 in a standard Williamson ether reaction with allyl bromide. Hydrosilylation of allyl ether 5 with dimethyl chlorosilane using platinum on charcoal (Pt-C, 10% Pt) as catalyst,23 was finally performed to get the desired sulphonyl azide chlorosilane 1. After removing the excessive dimethyl chlorosilane and filtration of the catalyst, silane 1 was directly dissolved in dry toluene. The use of hexacholoroplatinic acid (H2PtCl6) as catalyst in the hydrosilylation reaction results in a rapid reduction of the (18) Breslow, D. S.; Sloan, M. F.; Newburg, N. R.; Renfrow, W. B. J. Am. Chem. Soc. 1969, 91(9), 2273–2279. (19) Baker, D. A.; East, G. C.; Mukhopadhyay, S. K. J. Appl. Polym. Sci. 2001, 79, 1092–1100. (20) Jorgensen, J. K.; Ommundsen, E.; Stori, A.; Redford, K. Polymer 2005, 46, 12073–12080. (21) Ragab, A.; Al-Azhar, E.-S. Phosphorus, Sulfur Silicon Relat. Elem. 2004, 179, 237–266. (22) Marciniec, B. Comprehensive Handbook of Hydrosilylation; Pergamon Press: Oxford, U.K., 1992. (23) Boksanyi, L.; Liardon, O.; Kovats, E. s. Adv. Colloid Interface Sci. 1976, 6, 95.

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Figure 4. Influence of temperature and time for the attachment of PS (MW = 3 000 000). Thick overcoats of PS (≈ 100 nm) thickness were prepared (10 mg/mL, toluene) and then subsequently subjected to thermal attachment. (9) T = 120 °C; (0) T = 140 °C; (b) T = 180 °C. The respective lines are guides to the eye with respect to the trend of the plots.

sulphonyl azide group, which was not the case with platinum on charcoal. However, a longer reaction time (66 h at 40 °C) is needed to obtain quantitative addition. As known from other allyl ethers, this gave almost exclusively the Markovnikow product of the sulphonyl azide silane. Immobilization of silane 1 on SiO2 surfaces was achieved by immersing an appropriate substrate into a dilute solution of 1 in toluene, together with catalytic amount of dry NEt3.24,25 The obtained silane monolayers had a typical film thickness of ca. 1 nm. Onto the thus prepared substrates polymer layers were deposited by spin-coating while the initial film thicknesses were chosen to be >100 nm to avoid any influence on the final thickness of the covalently bound polymer monolayer. After the samples were heated and the polymers attached via C-H bond insertion of sulphonyl azides, all nonbonded polymer was removed by Soxhlet extraction with a good solvent for the polymer. The sulphonyl azide groups were activated by heating the samples in a conventional oven. Upon heating, sulphonyl azide 5 decomposed into a sulfonyl nitrene, which inserts into the C-H bonds of the polymers located in close vicinity of these moieties. The thus obtained polymer monolayers were smooth and homogeneous and had a very low surface roughness. An AFM image of a thus obtained featureless layer is shown in the Supporting Information (Figure SI1). In reference experiments, where no sulphonyl azide monolayer was attached, all nonbound polymer was removed during extraction. In a first set of experiments PS layers (MW = 3 000 000 g/mol) were attached to silicon surfaces at different temperatures. The results of the ellipsometric measurements of the film ticknesses after extraction as a function of time are plotted in Figure 4. Attached polymer layers were obtained from 120-180 °C. As expected, a strong temperature dependence of the growth of the film thickness with time is observed. For a temperature of 180 °C, the film thickness increases linearly in the beginning and then levels off to reach a constant value of around 10 nm after 2 h. The film thickness in the plateau regime depends mainly on the molecular weight of the polymer chains together with their particular conformation at the interface. For 120 and 140 °C the final film thickness is not reached in the plotted time frame, but would presumably be obtained with longer heating time. (24) Kallury, K. M. R.; Macdonald, P. M.; Thompson, M. Langmuir 1994, 10, 492. (25) Matsuda, T.; Sugawara, T. J. Biomed. Mater. Res. 1995, 29, 749.

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Figure 5. Arrhenius-plot for the azide decomposition in the sulphonyl azide monolayer determined from the initial increase of film thickness at different temperatures.

Figure 6. Influence of molecular weight (a) and Rg (b) of PS on bound layer thickness. (9) PS monolayers on sulphonyl azide modified substrates, (O) = reference experiments on blank substrates. Thermal attachment (T = 160 °C) were performed for 120 min. The lines are guide to the eyes with respect to the trend of the plot.

The initial slope from curves, where some selected ones are shown in Figure 4, is related to the rate constant k of azide decomposition at the respective temperature. From the values of k at different temperatures the activation energy Ea can be calculated according to the Arrhenius equation ln k ¼ const -Ea =RT

ð1Þ

The corresponding plot (Figure 5) gives us an activation energy of 68 kJ/mol required for azide decomposition, which is surprisingly small compared to the Ea values of nonsurface-attached sulphonyl azide compounds determined with DSC, which is about 147 kJ/mol.20 An alternative way to control the film thickness of the polymer monolayers is to vary the size of the contacting polymer. Thus, the influence of the molecular weight on the bound film thickness was studied with PS layers by varying the molecular weight from Mw = 30 000-3 000 000 g/mol with a heating time of 2 h at 160 °C. This long reaction time is chosen to ensure the formation of the bound film thickness. The results are shown in Figure 6, Langmuir 2010, 26(2), 769–774

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Figure 7. Influence of Rg/molecular weight of a polymer coil on the film thickness of the surface-attached monolayer: If the coil is not in contact with the surface, no attachment takes place.

Figure 9. Image and cross-sections from a microarray of PSS (upper spots, Mw = 1 000 000 g/mol) and PMAA (lower spots, Mw =700 000 g/mol) monolayers thermally attached to a silicon wafer with sulphonyl azide chemistry. The picture is taken by imaging ellipsometry and contrast enhanced for a better visualization. Polymers are printed from aqueous solution (1 mg/mL) with a contact-less printer. After thermal attachment (160 °C for 2 h), the sample was extracted for 16 h in water.

Figure 8. X-ray photoelectron survey spectra of thermally attached polymer monolayers on a silica surface together with the corresponding images from drops of water on these surfaces. (a) PDMAA, (b) PS, (c) PHFDA.

where a clear increase in film thickness with molecular weight (Figure 6a) is observed. This behavior gets more evident by plotting the thickness data against the radius of gyration (Rg) of the bulk polymer, where a linear increase of the film thickness is observed (Figure 6b). The radius of gyration can be calculated from the square root of the molecular weight (eq 2).11 Rg ¼ 0:03Mw 1=2

ð2Þ

As expected, no significant increase in the layer thickness is observed when the experiments are performed without a sulphonyl azide monolayer. Figure 7 schematically depicts this situation. The film thickness of the bound monolayer scales with the square root of the molecular weight of the polymer (and accordingly linearly with the size of the molecule, represented by the radius of gyration Rg) as seen in Figure 6. Only coils that are less than the radius of gyration of the polymers away from the surface can become attached. Chains that are statistically more than Rg away from the surface are not attacked in the insertion reaction and are subsequently washed away in the extraction step. As the molecular weight of a coil in a solvent-free film is proportional to the square root of Rg, a larger polymer coil brings about a higher mass of the attached polymer chains and accordingly higher film thickness. As thermal attachment takes place because of C-H bond insertion, this technique can be used for the covalent binding of a wide spectrum of different polymers, as long as they have accessible C-H bonds. Monolayers from PDMAA, PS, and PHFDA serve in the following as examples for hydrophilic and mildly and strongly hydrophobic monolayer coatings, respectively. The presence of the polymer layers onto the surfaces is confirmed by X-ray photoelectron spectroscopy (XPS) (Figure 8). The XP spectra of a PDMAA monolayer show the Langmuir 2010, 26(2), 769–774

expected signals due to the presence of oxygen [533 eV, O(1s)], nitrogen [402 eV, N(1s)], and carbon [287 eV, C(1s)] atoms. The PS monolayer, exhibits a strong signal of the corresponding carbon [287 eV, C(1s)] atom with its characteristic signature, and the PHFDA monolayer, shows strong, typical signals due to the presence of fluorine atoms (692 eV, F (1s), 603 eV (KLL Auger peak)). The spectra of polymer monolayers clearly show strongly attenuate spectral features of the substrate and the azide monolayer. Furthermore, the corresponding images of water drops on these surfaces are pictured in the same figure together with the associated water contact angles, which show typical values for the particular monolayer coated surfaces. The versatility of the process for the generation of chemical pattern in surfaces is also demonstrated in Figure 9, which shows an image of a section of an array from two different polymers obtained by imaging ellipsometry. For generation of the samples, simply polymer solutions from PMAA and PSS in water are printed onto a sulphonyl azide monolayer covering the whole substrate. After thermal activation and extraction, spots composed of monolayers of the corresponding polymers are obtained. Although this is a rather simple example the images illustrates, that it is easy to design microstructures of different polymer layers with different chemical compositions on the same substrate. In such simple printing processes, the exact shape of the obtained spot depends of course on the details of the wetting characteristics. Whereas the PSS spots were nicely round and flat, the PMAA spots had a “donut”-shape.

Conclusions We have described a simple and versatile way to attach polymer monolayers to a silicon surface. For this, a sulphonyl azides group containing silane is used to form a self-assembled monolayer on the surface. Subsequently this monolayer is coated with a relatively thick polymer layer and heated. During the thermal treatment, the sulphonyl azide groups decompose and the resulting nitrenes react with the contacting polymer molecules via C-H insertion. As sulphonyl azide monolayers are easily wetted by a wide spectrum of polymers and thermally activated under relatively DOI: 10.1021/la9018019

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mild conditions, this seems to be a rather general way to attach polymer molecules to surfaces, especially when light-triggered attachment is not possible. Acknowledgment. Andreas B€onisch is thanked for the printing of the microarray, Nicolas Schorr for the imaging ellipsometry measurements, and Dr. Alexey Kopyshev for performing the AFM measurements. Financial support by the

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Deutsche Forschungsgemeinschaft (DFG) through the PhD program “Micro Energy Harvesting” is gratefully acknowledged. Supporting Information Available: AFM measurement of a surface-attached PS-monolayer (Figure SI1) as well as H1 and 13C NMR and FT-IR spectra of compounds 1 and 3-5 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org/.

Langmuir 2010, 26(2), 769–774