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Grafting of Alkanethiol-Terminated Poly(ethylene glycol) on Gold S. Tokumitsu, A. Liebich, S. Herrwerth, W. Eck, M. Himmelhaus,* and M. Grunze Lehrstuhl fu¨ r Angewandte Physikalische Chemie der Universita¨ t Heidelberg, INF 253, 69120 Heidelberg, Germany Received May 2, 2002. In Final Form: August 14, 2002 The grafting of alkanethiol-terminated poly(ethylene glycol) [HS(CH2)11(OCH2CH2)n-OCH3; n ) 3456, MW ≈ 2224 Da] onto polycrystalline gold from dilute solutions was investigated by ellipsometry, X-ray photoelectron spectroscopy, infrared reflection-absorption spectroscopy, and in situ second harmonic generation. After immersion of a gold-coated Si wafer into a 50 µM dimethylformamide solution, the thickness of the grafted layer increases in a first rapid step up to ∼20 Å. After about 10 min, the thickness rises notably again and reaches saturation after ∼2 h at ∼120 Å. The kinetics of film formation clearly deviate from Langmuir kinetics, which is normally observed for the self-assembly of nonfunctionalized alkanethiols. Our observation can be explained by a conformational transition of the grafted poly(ethylene glycol) chains from amorphous coils to a brush morphology, predominantly consisting of helices with an orientation perpendicular to the surface. The second harmonic generation experiments show that the coverage at saturation of adsorption corresponds to ∼90% that of self-assembled monolayers of alkanethiols, indicating a densely packed film.
1. Introduction Grafting of a polymer brush onto a solid substrate has been the subject of many experimental and theoretical studies for more than two decades,1-5 ranging from industry-related problems, such as stabilization of colloidal dispersions,6 surface coatings for corrosion protection, nonfouling surfaces,7 and biomedical applications8 to the proof of fundamental laws in polymer science.3,9-12 Tethering of the molecular termini to the surface can be achieved in a number of different ways.3,5 Weak moleculesurface interactions, i.e., physisorption, can be mediated for example by selective adsorption of an insoluble anchor group from solution13-18 or electrostatic attraction between a charged or polar headgroup and the substrate.19-21 * Corresponding author: e-mail
[email protected]. (1) de Gennes, P. G. Adv. Colloid Interface Sci. 1987, 27, 189-209. (2) Milner, S. T. Science 1991, 251, 905-914. (3) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 33-71. (4) Uyama, Y.; Kato, K.; Ikada, Y. Adv. Polym. Sci. 1998, 137, 3-39. (5) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677-710. (6) Napper, D. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (7) Leckband, D.; Sheth, S.; Halperin, A. J. Biomater. Sci., Polym. Ed. 1999, 10, 1125-1147. (8) Harris, J. M. Poly(ethylene glycol) Chemistry; Plenum Press: New York, 1992. (9) Alexander, S. J. Phys. (Paris) 1977, 38, 983-987. (10) de Gennes, P. G. Macromolecules 1980, 13, 1069-1075. (11) Ou-Yang, H. D.; Gao, Z. J. Phys. II 1991, 1, 1375-1385. (12) Perahia, D.; Wiesler, D. G.; Satija, S. K.; Fetters, L. J.; Sinha, S. K.; Milner, S. T. Phys. Rev. Lett. 1994, 72, 100. (13) Hadziioannou, G.; Patel, S.; Tirrell, M. J. Am. Chem. Soc. 1986, 108, 2869-2876. (14) Tassin, J. F.; Siemens, R. L.; Tang, W. T.; Hadziioannou, G.; Swalen, J. D.; Smith, B. A. J. Phys. Chem. 1989, 93, 2106-2111. (15) Munch, M. R.; Gast, A. P. J. Chem. Soc., Faraday Trans. 1990, 86, 1341-1348. (16) Motschmann, H.; Stamm, M.; Toprakcioglu, Ch. Macromolecules 1991, 24, 3681-3688. (17) Huguenard, C.; Varoqui, R.; Pefferkorn, E. Macromolecules 1991, 24, 2226-2230. (18) Cosgrove, T.; Heath, T. G.; Phipps, J. S.; Richardson, R. M. Macromolecules 1991, 24, 94-98. (19) Davidson, N. S.; Fetters, L. J.; Funk, W. G.; Graessley, W. W.; Hadjichristidis, N. Macromolecules 1988, 21, 112-121.
Strong interaction is accomplished by formation of a chemical bond between the anchor group and the surface, i.e., chemisorption.22-24 In the latter case, the strong surface-polymer bond12 leads to excellent adhesion properties of the brush layer and thus to higher mechanical stability of the film as compared to grafting by physisorption, which may be utilized for example for adhesion promotion between a polymeric material and a solid substrate.25 The morphology of end-grafted polymers depends crucially on coverage, i.e., the average grafting density.2,9-11,26,27 At low coverage, intermolecular interactions are negligible and amorphous moieties form where the polymer chains adopt a coillike structure similar to that found in solution. With increasing grafting density, the molecules experience a lateral confinement that finally causes a transition into a stretched phase, the so-called brush regime. That way, those physical and chemical properties of the layer which depend on morphology, such as solvation behavior, water and ion uptake, stiffness, dichroism and refractive index, and chemical reactivity, become functions of coverage. The methods utilized for chemical grafting so far can roughly be divided into three different schemes: (1) direct grafting, where the desired polymer is directly attached to the surface via a suitable functional group;22-24,28-31 (2) (20) Taunton, H. J.; Toprakcioglu, C.; Fetters, L. J.; Klein, J. Nature 1988, 332, 712-714. (21) Kawaguchi, M.; Kawarabayashi, M.; Nagata, N.; Kato, T.; Yoshioka, A.; Takahashi, A. Macromolecules 1988, 21, 1059-1062. (22) Auroy, P.; Auvray, L.; Le´ger, L. Phys. Rev. Lett. 1991, 66, 719722. (23) Auroy, P.; Mir, Y.; Auvray, L. Phys. Rev. Lett. 1992, 69, 93-95. (24) Karim, A.; Satija, S. K.; Douglas, J. F.; Ankner, J. F.; Fetters, L. J. Phys. Rev. Lett. 1994, 73, 3407-3410. (25) Lee, L. H. Adhesion and Adsorption of Polymers; Plenum Press: New York, 1980. (26) Netz, R. R.; Schick, M. Europhys. Lett. 1997, 38, 37-42. (27) Szleifer, I. Europhys. Lett. 1998, 44, 721-727. (28) Koutsos, V.; v. d. Vegte, E. W.; Pelletier, E.; Stamouli, A.; Hadziioannou, G. Macromolecules 1997, 30, 4719-4726. (29) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458-1460.
10.1021/la0258953 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/19/2002
Grafting of Alkanethiol-Terminated PEG on Gold
indirect grafting, where the surface is first chemically modified, e.g., by chemical activation32 or functionalization with organic layers;33-38 and (3) surface polymerization,5 i.e., a precursor or monomer is chemisorbed on the surface as an initiator and the desired brushes are polymerized. Direct grafting is not always possible due to a lack of suitable surface chemistry for the desired polymer/ substrate system. Mostly, the polymers are end-functionalized by silane23,24,30,31 or thiol7,28 groups to mediate coupling, e.g., to glass, silicon, or metals. Indirect grafting widens the range of applicable surface chemistry and thus the number of polymers that can be grafted. However, due to the additional number of steps in the reaction path, it is more difficult to obtain a uniform, densely packed layer. Surface polymerization apparently overcomes this problem; however, it is more difficult to control. Up to now, it seems that all the different methods face problems in precisely controlling the morphology of the obtained brush layer, in particular at high grafting density and for high molecular weights. Recently, Zhu and co-workers32 have proposed an indirect grafting method for poly(ethylene glycol) (PEG) onto the Si(111) surface to improve the grafting process as compared to techniques for grafting on silicon or glass utilized before.30,31,33,34 A H-terminated Si surface is chlorinated via reaction with Cl2 and then reacted with the terminal OH-groups of the PEG molecules. Due to the single molecule-surface bond in this grafting process, multilayer formation, which is crucial in the case of methods based on silane coupling, is avoided. For molecular weights up to 300 Da, densely packed brushes were obtained. Above that value, coverage was found to be less dense and an amorphous layer was formed. In the present study we demonstrate that direct grafting of PEG via thiol termination provides the opportunity to precisely control the layer morphology as a function of coverage. Due to the strong coupling between sulfur and gold, brushes form despite the relatively high average molecular weight of the polymer of ∼2000 Da [HS(CH2)11(OCH2CH2)n-OCH3; n ) 34-56; PEG2000-SH]. In addition, at high grafting density, crystallinelike layers are observed. Of course, this method of grafting polymers is restricted to gold as a substrate and thiolterminated polymer molecules. However, as a more general conclusion, our experiments show that a strong anchor group-substrate interaction can drive the formation of densely packed polymer brushes via self-assembly from solution. In the following, we analyze the various structures adopted throughout film formation in more detail and give experimental evidence for (1) the transition from amorphous coils to brushes with increasing coverage, (2) formation of a densely packed quasi-crystalline monolayer (30) Yang, Z.; Galloway, J. A.; Yu, H. Langmuir 1999, 15, 84058411. (31) Papra, A.; Gadegaard, N.; Larsen, N. B. Langmuir 2001, 17, 1457-1460. (32) Zhu, X.-Y.; Jun, Y.; Staarup, D. R.; Major, R. C.; Danielson, S.; Boiadjiev, V.; Gladfelter, W. L.; Bunker, B. C.; Guo, A. Langmuir 2001, 17, 7798-7803. (33) Emoto, K.; Harris, J. M.; van Alstine, J. M. Anal. Chem. 1996, 68, 3751-3757. (34) Sofia, S. J.; Premnath, V.; Merrill, E. W. Macromolecules 1998, 31, 5059-5070. (35) Ebata, K.; Furukawa, K.; Matsumoto, N. J. Am. Chem. Soc. 1998, 120, 7367-7368. (36) Bergbreiter, D. E.; Franchina, J. G.; Kabza, K. Macromolecules 1999, 32, 4993-4998. (37) Luzinov, I.; Julthongpiput, D.; Malz, H.; Pionteck, J.; Tsukruk, V. V. Macromolecules 2000, 33, 1043-1048. (38) Shybanova, O.; Voronov, S.; Bednarska, O.; Medvedevskikh, Y.; Stamm, M.; Tokarev, V. Macromol. Symp. 2001, 164, 211-218.
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Figure 1. Route of synthesis for PEG2000-SH. For details see text.
at high coverage, and finally (3) chemisorption of the adsorbed molecules over the entire range of the adsorption process. 2. Experimental Section 2.1. Chemicals. Methanol, ethanol, acetone, dimethylformamide (DMF), tetrahydrofurane (THF), chloroform, and fuming hydrochloric acid (all analytical grade) were purchased from Sigma (Deisenhofen, Germany). Sodium hydride suspension (55% in mineral oil), 11-bromoundec-1-ene (97%), ethylenediaminetetraacetic acid (EDTA; 99%), thioacetic acid (97%), azoisobutyronitrile (AIBN; 98%), and poly(ethylene glycol) monomethyl ether (MW 2000) were obtained from Fluka (Deisenhofen, Germany). THF and poly(ethylene glycol) monomethyl ether were dried over molecular sieves for at least 2 days before use. All other chemicals were used as received. 2.2. Substrate Preparation. Thin films of polycrystalline gold were prepared by thermal evaporation of 3.5 nm of titanium as adhesion promoter and subsequent deposition of 70 nm of gold of 99.99% purity onto polished single-crystal silicon wafers (Silicon Sense). Evaporation was performed at a pressure of 2 × 10-7 Torr and a deposition rate of 0.5 nm/s. 2.3. Synthesis of PEG2000-SH. Figure 1 summarizes the synthetic route to the PEG2000-SH studied in this work. (1Mercaptoundec-11-yl) poly(ethylene glycol) monomethyl ether (CH3-(OCH2CH2)45-O-(CH2)11-SH) (1) was prepared according to a general procedure developed by Prime and Whitesides:39 (Undec-1-en-11-yl) Poly(ethylene Glycol) Monomethyl Ether [CH3-(OCH2CH2)45-O-(CH2)9-CHdCH2] (2). A sodium hydride suspension (436 mg, 10 mmol; 55% in mineral oil) was added carefully to a solution of 10 g (5 mmol) of poly(ethylene glycol) monomethyl ether (MW 2000) in 100 mL of THF under dry nitrogen. After the mixture was stirred for 24 h at room temperature, 2.25 g (10 mmol) of 11-bromoundec-1-ene was added slowly. After the addition, the reaction mixture was stirred for another 48 h. Nonconsumed sodium hydride was then destroyed by addition of 2-propanol. The solvent was reduced by rotary evaporation and the product was purified by column chromatography on silica gel (eluent methanol/chloroform 3:1) to yield 6.25 g (2.9 mmol, 58%) of 2 as a white solid. 1H NMR (300 MHz, CDCl3): δ ) 1.21-1.38 (m, 12H, 6 CH2), 1.47-1.61 (m, 2H, 1 CH2), 1.93-2.02 (m 2H, 1 CH2), 3,35 (s, 3H, 1 O-CH3), 3.413.46 (t, 2H, 1 CH2-O), 3.5-3.68 (m, 180H, 45 OCH2CH2-), 4.884.98 (m, 2H, dCH2), 5.71-5.84 (m, 1H, -CH)) (1-[(Methylcarbonyl)thio]undec-11-yl) Poly(ethylene Glycol) Monomethyl Ether [CH3-(OCH2CH2)45-O-(CH2)11-SCO-CH3] (3). The olefin 2 (6.25 g, 2.9 mmol) was dissolved in 500 mL of methanol, together with 682 mg (8.7 mmol) of thioacetic acid and 20 mg of AIBN. The solution was irradiated for 8 h with a mercury lamp (150 W) through Duran glassware. The solvent was reduced and the excess of thioacetic acid was removed by column chromatography on silica gel (eluent methanol/ethyl acetate 1:2, methanol/chloroform 2:1). The resulting solid was dissolved in THF, and 1 g of Norit A was added for discoloration. (39) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12-20.
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After being stirred for 30 min, the suspension was filtered and the solvent was reduced again. The product was purified by column chromatography on silica gel (methanol/chloroform 2:1) to yield 5 g (2,2 mmol) of 3 as a gray solid. 1H NMR (300 MHz, CDCl3): δ ) 1.21-1.38 (m, 14H, 7 CH2), 1.47-1.61 (m, 4H, 2 CH2), 2.38 (s 3H, 1 CH3), 2.80-2.85 (t, 2H, 1 S-CH2), 3.35 (s, 3H, 1 -OCH3), 3.41-3.46 (t, 2H, 1 CH2-O), 3.5-3.68 (m, 180H, 45 OCH2CH2-). (1-Mercaptoundec-11-yl) Poly(ethylene Glycol) Monomethyl Ether [CH3-(OCH2CH2)45-O-(CH2)11-SH] (1). Compound 3 (5 g, 2.2 mmol) and 100 mg of EDTA were dissolved in 200 mL of methanol under nitrogen. Fuming hydrochloric acid (10 mL, 37%) was added and the reaction mixture was refluxed for 30 h. The solvent was reduced by rotary evaporation and the crude product was purified by column chromatography on silica gel (eluent methanol/ethyl acetate 1:2; methanol/chloroform 2:1) to give 4.5 g (2.0 mmol) of 1 as a white solid. The purity of 1 was checked by 1H NMR and MALDI mass spectrometry. 1H NMR (300 MHz, CDCl3): δ ) 1.21-1.38 (m, 15H, 7 CH2 + 1 S-H), 1.47-1.61 (m, 4H, 2 CH2), 2.46-2.54 (m, 2H, 1 S-CH2), 3.35 (s, 3H, 1 -OCH3), 3.41-3.46 (t, 2H, 1 CH2-O), 3.5-3.68 (m, 180H, 45 OCH2CH2-). Molecular weights determined from MALDI: Mn (number average) ) 2199, Mw (weight average) ) 2224; Mw/Mn ) 1.01 2.4. Surface Grafting of PEG2000-SH. For ex situ studies, the substrates were cut into pieces of 15 × 20 mm2 and subsequently immersed into 30 mL of a 50 µM PEG2000-SH/ DMF solution for different periods of time. After removal from the solution, the samples were rinsed with DMF and sonicated in DMF (5 min twice) in order to remove physisorbed molecules. Then the samples were rinsed with ethanol to remove residual DMF, dried in a nitrogen stream, and stored under pure nitrogen. In case of the in situ SHG measurements, the samples were mounted into a liquid cell, which was filled with pure DMF. After 5-10 min, when the SHG signal was stable, the pure solvent was replaced by the 50 µM PEG2000-SH/DMF solution. 2.5. Measurements. The film thickness of the grafted polymer layer was measured with a spectral ellipsometer (J. A. Woollam, M-44). The optical properties of each gold substrate were determined immediately after removal of the wafer from the evaporation chamber in order to minimize the effect of contamination. The refractive index of the PEG2000-SH film was assumed as that of a Cauchy layer with the two lowest Cauchy parameters An ) 1.45 and Bn ) 0.01, corresponding to a refractive index of 1.4760 at λ ) 620.0 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed with a Leybold Max 200 spectrometer with an Al KR X-ray source operated at 200 W. Photoelectrons were collected by a spherical sector-type analyzer at a pass energy of 96 eV for the survey spectra and 48 eV for narrow-scanned spectra. The electron binding energies are referenced to the Au4f 7/ 2 peak at 84.0 eV. For calculation of the film thickness from quantification of the XPS data, we assumed a electron mean free path of λe ) 24.2 Å for C1s and λe ) 35.6 Å for the Au4f peak. For further details of the evaluation we refer to the literature.40 Infrared spectra were taken with a dry-air purged Bio-Rad spectrometer, model FTS 175c, and a Bruker IFS-66v vacuum spectrometer operated at 2 mbar. Both instruments were equipped with a liquid nitrogen-cooled MCT detector, a polarizer constructed of an aluminum wire grid on a KRS-5 substrate, and an accessory for grazing angle reflectance spectroscopy. Spectra shown here were obtained by collecting 1024 scans at a resolution of 4 cm-1 and subsequent normalization of thus obtained data against a spectrum of a n-C20D41S-SAM (self-assembled monolayer) formed on gold. The negative absorption bands in the spectra at 2050-2200 cm-1 are due to absorption of the C-D bands of the perdeuterated reference sample. The perdeuterated reference is basically free of contamination and does not absorb in the spectral regions of interest. The experimental setup for the in situ second-harmonic generation experiments was similar to the one described earlier.41 (40) Hansen, H. S.; Tougaard, S.; Biebuyck, H. J. Electron Spectrosc. Relat. Phenom. 1992, 58, 141-58. (41) Dannenberger, O.; Buck, M.; Grunze, M. J. Phys. Chem. B 1999, 103, 2202.
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Figure 2. Evolution of film thickness of PEG2000-SH films on gold after different immersion times into a 50 µM DMF solution as measured by ex situ ellipsometry (9). For samples from 5 min to 15 h of immersion time, the figure also shows the relative intensities of the COC stretching vibrations at 1118 (O) and 1152 cm-1 (0), respectively, as obtained by IRRAS, for comparison of film thickness and structure. A Nd:YAG laser with a fundamental wavelength of λ ) 1064 nm, a pulse duration of 40 ps, and a repetition rate of 10 Hz was used. The beam irradiated onto the substrate had a diameter of 2.5 mm, and the energy of the laser pulse incident on the sample was about 2.5 mJ. The intensity of the frequency-doubled signal at λ ) 532 nm generated at the gold surface in reflection was detected by a photomultiplier that was equipped with a monochromator and a highpass filter (type KG5, Schott, Germany) to reduce noise. The adsorption experiments were performed in a home-built liquid cell consisting of a Teflon body and a quartz glass window with a viton seal. The laser radiation was incident at an angle of 45° with respect to the surface of the cover glass. Prior to each experiment, the Teflon cell and the glass window were cleaned by immersion into a hot 3:1 (v/v) mixture of H2SO4 and 30% H2O2 aqueous solution (caution!), while the viton seal was immersed into pure 30% H2O2 solution. In any case, this cleaning procedure was followed by an extensive rinse with pure water and absolute ethanol and drying in a stream of nitrogen. After the substrate had been mounted in the cell, it was sealed with the window and filled with pure DMF to rinse residual contamination and to align the optics. To avoid major variations in the concentration of the solution, the cell was first emptied and then filled with the PEG2000-SH/ DMF solution. Twenty laser pulses were averaged to give one data point in the spectrum.
3. Results 3.1. Ellipsometry. The evolution of film thickness with increasing immersion time of the substrates into a 50 µM PEG2000-SH/DMF solution was measured ex situ by spectral ellipsometry. Figure 2 shows the thickness change of the adsorbed organic layer on a polycrystalline gold substrate as a function of immersion time. In the same figure, the relative intensities of the vibrational bands in the COC stretching region as measured by IR spectroscopy are drawn, which will be discussed later. At the initial stage of film formation, the film thickness is ∼20 Å and shows no change over the first 10 min. Due to the relatively high concentration of the solution, the kinetics of the adsorption process leading to this initial film cannot be resolved in an ex situ study. After ∼10 min of immersion, the thickness increases and finally reaches saturation after ∼2 h. The final film thickness is ∼120 Å, assuming Cauchy parameters An ) 1.45 and Bn ) 0.01 (nfilm ≈ 1.48). This adsorption process clearly differs from Langmuir kinetics observed for self-assembled monolayers of alkanethiols. There are two apparent explanations for this behavior: (1) Due to the molecular weight distribution in the PEG2000-SH, the shorter chain molecules may adsorb first to form a densely packed film because of their higher
Grafting of Alkanethiol-Terminated PEG on Gold
Figure 3. XPS analysis of PEG2000-SH films on gold as a function of immersion time in a 50 µM DMF solution: (a) survey spectra for samples after 10 min and 3 h of immersion; (b) film thickness as calculated from the C1s/Au4f ratio (9) in comparison with the ellipsometry data of Figure 2 (.); (c) C1s/O1s ratio as determined from the XPS data (9) and calculated under the assumption that in the beginning of the adsorption process a densely packed film of short-chain molecules forms that subsequently is replaced by long-chain moieties (O). The calculation assumes that the molecular axis of the methylene chains are tilted by 30° with respect to the surface normal and that the EG units have a helical conformation with an orientation perpendicular to the surface plane.
mobility in solution, and are subsequently replaced by long-chain molecules in a slowly progressing exchange reaction resulting in energy minimization. (2) The thickness change corresponds to a change in molecular conformation, i.e., the randomly adsorbed molecules transform from a random to a more oriented structure as coverage increases. To distinguish between these two possibilities, we estimate the expected thickness as a function of coverage for these two scenarios. The thickness in the initial stage of ∼20 Å corresponds to a film with an average number of 3-6 EG units, if we assume that the molecules are in a brushlike phase with the C11 spacer group in all-trans conformation and tilted by 30° as found for oligo(ethylene glycol)-terminated undecanethiolates on gold.42 Since mass spectrometry data taken for the PEG2000-SH do not show any low molecular weight components (below 25 EG units), we can rule out scenario 1. Further evidence for this conclusion is given by the XPS data in the following section. 3. 2. X-ray Photoelectron Spectroscopy. Figure 3a shows the change in the X-ray photoelectron spectra taken for PEG2000-SH films after 10 min and 3 h of immersion. As film thickness develops, the C1s and O1s emissions increase and the Au4f peak of the substrate is screened due to scattering of the photoelectrons within the organic adsorbate. Figure 3b displays the change of the film thickness as a function of immersion time calculated from the change of the Au4f and C1s intensities (9). The adsorption kinetics obtained from the XPS data are in good agreement with those from ellipsometry (.). The slight mismatch between the two methods in the values obtained for saturation coverage might be caused by the (42) Harder, P.; Grunze, M.; Dahint, R.; Whitsides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426.
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Figure 4. Infrared reflection absorption spectra (IRRAS) of the COC stretching and CH2 deformation region of PEG2000SH films on gold after different immersion times into a 50 µM DMF solution. For comparison, the spectra are vertically shifted and normalized to the film thickness as given by Figure 2.
uncertainty in the refractive index used for ellipsometry and/or the errors associated with the electron mean free path used in the analysis of the XPS data. Figure 3c displays the stoichiometric ratio of carbon to oxygen within the film as obtained from the C1s and O1s emission (9). This set of data provides further evidence for the validity of the second adsorption model discussed above. The PEG2000-SH molecule has 11 methylene units as a spacer between the SH group and the EG chain, whereas the length of the EG chain is variable (45 ( 11 EG units, where the deviation indicates the distribution’s e-2 boundaries) according to its polymeric character. If the average EG chain length in the adsorbed film would change from shorter to longer with increasing coverage, the C/O ratio is expected to decrease. The theoretical change of the C/O ratio according to this assumption is also displayed in Figure 3c (O); the calculation assumes that initially the surface is covered by a densely packed monolayer of short-chain molecules that subsequently is replaced by long-chain moieties. For all stages, the molecular axis of the methylene chains is assumed to be tilted by 30° with respect to the surface normal, and the EG units are assumed to be in a helical conformation with an orientation perpendicular to the surface plane.42 The experimental data clearly indicate that the C/O ratio does not change over the entire process of monolayer formation. Its average value amounts to 2.1 ( 0.1, matching the value of 2.2 for the mean chain length of the molecule (n ) 45) rather nicely. Thus, in agreement with the second model, we conclude that randomly adsorbed molecules transform from a random to a more oriented structure with increasing coverage, while the average chain length of the molecules is constant during the entire adsorption process. 3.3. Infrared Spectroscopy. To get further evidence for the second model, infrared reflection adsorption spectroscopy (IRRAS) was applied in order to investigate the conformation of the PEG2000-SH molecule as a function of coverage. Figure 4 displays the IRRA spectra
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Table 1. Assignment of the IR Bands of the COC Stretching and CH2 Deformation Regionsa mode assignment EG CH2 scissor
EG CH2 wag
EG CH2 twist C-O, C-C stretch
EG CH2 rocking
polarizationb
PEG, crystalline
PEG, amorphous
⊥ | | ⊥ ⊥ ⊥ |
1470 m 1463 m 1457 w 1453 w 1415 w 1364 m 1345 s
∼1460e m
⊥ | ⊥ ⊥ ⊥ | ⊥ |
1283 m 1244 m 1236 w 1149 s 1119 s 1102 vs 1062 m 963 s
PEG2000-SH/Au 15 hc
PEG2000-SH/Au 5 minc ∼1460e br
1463 m 1456 m
1352 m 1325 w 1296 m 1249 m
1345 s
1140 sh
1152 sh
1107 s 1038 m 945 m
1118 vs
a
1350 m
1243 m
1294 w 1252 w 1215 w 1148 s 1124 s ... 1085 shf 1039 b 949 w
965 s
EG6OH/Au (OEG-SAM)d ∼1464 m 1461 m
1348 m...s 1325 w 1296 m 1250 m 1145 sh 1130 s ... 1114 vsf 964 s
b
IR bands: w weak, m medium, s strong, vs very strong, br broad, sh shoulder. Transition dipole moment with respect to the helical axis in crystalline PEG. c PEG2000-SH adsorbed onto polycrystalline gold from a 50 µM DMF solution after 15 h or 5 min. d Self-assembled monolayer of oligo(ethylene glycol)-terminated alkanethiols. e Vibrational range 1470-1463 cm-1 with respect to crystalline PEG, to which the resonance is attributed. f Vibrational range 1119-1102 cm-1 with respect to crystalline PEG, to which the resonance is attributed.
in the COC stretching and the CH2 deformation region for PEG2000-SH films on gold adsorbed from a 50 µM DMF solution for different immersion times. To facilitate comparison of resonance intensities for samples with differing coverage, the spectra are normalized to the film thickness as determined by ellipsometry. As immersion time increases, several bands of the CH2 deformation and COC stretching modes develop. However, the bandwidth of the resonances decreases significantly, thus giving evidence for formation of a uniform structure. In Table 1 the assignments and positions for the various bands are given for spectra obtained after 5 min and 15 h of immersion and compared with those of crystalline, i.e., helical, PEG,43 amorphous PEG,42,44 and an oligo(ethylene glycol) self-assembled monolayer (OEG-SAM).42 Interestingly, almost all bands observed in the spectrum for 15 h of immersion match those of the crystalline PEG with an orientation of the transition dipole moment parallel to the helical axis of the molecular backbone, while the resonances perpendicular to this axis are absent. Due to the selection rule for IR spectroscopy at metal surfaces, which states that transition dipole moments oriented parallel to the surface are not detectable, the lack of modes with perpendicular orientation in the spectrum of the 15 h sample can be explained by assuming an orientation of the helices parallel to the surface normal. Thus, although we must assume that due to the polymeric character of the molecule, i.e., the variation in chain length, there must be some disorder at the very top of the film, at saturation coverage the body of PEG units is highly ordered. This conclusion is also supported by the narrow bandwidth of the various resonances at that stage (≈10 cm-1), which indicates a film of uniform structure, and the presence of the resonance at 1345 cm-1, which is known to be characteristic of the helical conformation of the EG units.42 Only the band at 1118 cm-1 needs some further discussion. In agreement with the assignments for the OEG-SAMs, we interpret this resonance as oriented parallel to the helical axis, although its position is very close to that at 1119 cm-1 of crystalline PEG, which is assigned to a perpendicular transition dipole moment. However, a closer look at the spectrum given by Miyazawa et al.43 reveals (43) Miyazawa, T.; Fukushima, K.; Ideguchi, Y. J. Chem. Phys. 1962, 37, 2764. (44) Harder, P. Ph.D. Thesis, Ruprecht-Karls-Universita¨t Heidelberg, Heidelberg, Germany, 1998.
that the spectrum in polarization parallel to the helical axis also exhibits significant intensity at 1119 cm-1. Thus, the assignment of this resonance is ambiguous at first instance. To our understanding, the various bands showing up in the range from 1107 to 1119 cm-1 are caused by different conformers of the EG units, an interpretation that is supported by the work of J. R. Fried,45 who put the various bands of poly(ethylene oxide) in relation to the conformations of the monomer. In the case of the COC stretching region, he states that all conformers of the monomer do contribute. In our case just two resonances with narrow bandwidth can be observed, indicating a uniform structure. We evaluated the intensities of these bands as a function of immersion time according to Figure 4 numerically, using Gaussian profiles. The obtained relative intensities are plotted together with the film thickness measured by ellipsometry in Figure 2. Obviously, in the initial adsorption stage both resonances show comparable intensity. However, with increasing film thickness, the band at 1118 cm-1 increases, while that at 1152 cm-1 decreases to a negligible intensity. Given that all other major bands in Figure 4 clearly can be related to a parallel orientation with respect to the helical axis, we therefore assign the band at 1118 cm-1 to parallel and that at 1152 cm-1 to a perpendicular orientation of the molecular backbone with respect to the surface. From Table 1 it becomes clear that the spectral features after 5 min of immersion are similar to those for amorphous PEG, and also, they are in line with those obtained for the amorphous phase of the OEG SAMs on gold and silver.42 Further, the bandwidths of the modes are increased significantly with respect to the final stage. All this suggests that the PEG chains have an amorphous structure at the beginning of film formation. The same effects can be observed in the region of the CH stretching modes. Figure 5 shows the spectra of the CH stretching region for the same set of samples as displayed in Figure 4. Again, the spectra are normalized to their respective film thickness as determined by ellipsometry. The assignments of the bands are summarized in Table 2. In general, the assignment in the CH stretching region is more ambiguous than that for the fingerprint region, as (1) the volume spectra of Miyazawa et al.43 show a significant overlap between bands with (45) Fried, J. R. Polymer Science and Technology; Prentice Hall: Englewood Cliffs, NJ, 1995.
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Table 2. Assignment of the IR Bands of the CH Stretching Regiona mode assignment EG CH2 asym stretch alkyl CH2 asym stretch EG CH2 sym stretch alkyl CH2 sym stretch EG combination vibration
polarizationb
PEG, crystalline
PEG, amorphous
PEG2000-SH/Au 15 hc
⊥
2950 m
2930 sh
| ⊥ |
2890 s 2885 s 2865 sh
2865 br
2956 w 2918 sh 2892 vs
⊥ ⊥ | ⊥
2825 sh 2805 sh 2740 2695
PEG2000-SH/Au 5 minc ∼2952 sh ∼2921 sh 2903 s ... 2870 se
2861 s
2740 sh
2740 m
∼2858 sh ∼2823 sh
EG6OH/Au (OEG-SAM)d ∼2930 sh 2921 s 2870 br ... 2894 s 2870 br; 2896 s ∼2856 sh 2740 w
a IR bands: w weak, m medium, s strong, vs very strong, br broad, sh shoulder. b Transition dipole moment with respect to the helical axis in crystalline PEG. c PEG2000-SH adsorbed onto polycrystalline gold from a 50 µM DMF solution after 15 h or 5 min. d Self-assembled monolayer of oligo(ethylene glycol)-terminated alkanethiols. e Vibrational range 2890-2865 cm-1 with respect to crystalline PEG, to which the resonance is attributed.
Figure 5. Infrared reflection absorption spectra (IRRAS) of the CH stretching region of PEG2000-SH films on gold after different immersion times into a 50 µM DMF solution. For comparison, the spectra are vertically shifted and normalized to the film thickness as given by Figure 2.
parallel and perpendicular orientation to the molecular axis of PEG, respectively, and (2) in our case the CH modes of the C11 spacer group are located in the same frequency region and thus add to this ambiguity. Despite that, the dominant features in the spectrum of the sample immersed for 15 h at 2740, 2861, and 2892 cm-1 are characteristic for the helical conformation of the PEG chains. These bands have transition dipole moments parallel to the PEG molecular axis, indicating that the PEG chains are oriented predominantly perpendicular to the surface. This holds in particular for the mode at 2740 cm-1, where the volume spectra of crystalline PEG exhibit a resonance solely for parallel orientation to the molecular axis.43 Thus, in combination with the selection rules for IR spectroscopy at metal surfaces, the evolution of the band at 2740 cm-1 in Figure 5 can be used as a direct measure for a transition from a disordered to an ordered state with an orientation of the molecular backbone perpendicular to the surface. In contrast, for 5 min of immersion, the spectrum shows two broad bands at 2870 and 2903 cm-1. These relatively broad spectral features for the EG CH2 stretching modes
have been also observed for amorphous OEG SAMs on gold and silver as well as amorphous PEG, indicating that the PEG chains are strongly disordered at this stage. Summarizing, the data obtained by IR spectroscopy support our assumption of a structural transition during the process of film formation, in which PEG2000-SH molecules adsorb as amorphous moieties and then undergo a transition into a brush-type structure as the coverage increases. In the final stage of film formation, the brush layer shows a uniform helical structure with a preferential orientation perpendicular to the surface. 3.4. In Situ Second Harmonic Generation. While above experiments have shown that the change in thickness during film formation corresponds to a transition from an amorphous to an oriented morphology of the adsorbate, it is not clear if the molecules are chemisorbed or physisorbed at the very beginning of the adsorption process. The latter hypothesis seems to be quite reasonable, as polymers dissolved in good solvents are known to adapt a coil structure for entropic reasons. If such a coil adsorbs on a surface with its alkanethiol termination buried within the coil, chemisorption might be sterically hindered. To clarify this point, i.e., to distinguish between physisorption and chemisorption in the intitial stage of film formation, we applied optical second harmonic generation (SHG), a second-order nonlinear optical technique that has proven to be a useful tool for monitoring the chemisorption process of alkanethiols on gold and silver. In the case of adsorption of aliphatic moieties onto a gold surface, the SH signal solely originates from the gold46 with the contribution of the free electrons being the dominating one.47 While physisorption of aliphatic moieties does not affect the free electron system significantly, chemisorption changes the electronic surface state density. This leads to a change of the SH signal with increasing thiolate coverage θ. For thiol adsorption onto gold, the SH intensity ISHG can be expressed as follows:41,46
ISHG(θ) ∝ |χsub - χint(θ)|2I02
(1)
with χint(θ) ) χintθ)1θ(t). Here, χsub describes the second-order susceptibility of the substrate, χint represents the susceptibility arising from the interaction between substrate and adsorbate, i.e., the chemisorption process, and ISHG and I0 are the intensities of the second harmonic and the fundamental light waves, respectively. For better comparison with the experimental (46) Buck, M.; Eisert, F.; Fischer, J.; Grunze, M.; Traeger, F. Appl. Phys. A 1991, A53, 552-556. (47) Liebsch, A. Phys. Rev. Lett. 1988, 61, 1233-1236.
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Figure 6. In situ study of the adsorption kinetics of PEG2000SH on gold from a 50 µM DMF solution by second harmonic generation (SHG). For comparison, the adsorption kinetics of C16SH and PEG2000-OH are shown. Note that the signal for PEG2000-OH shows no major change, well matching the fact that SHG only can track chemisorption of aliphatic molecules. From the drop in intensity, the relative coverage for PEG2000SH as compared to that of C16SH is calculated for the plateau region after ∼10 min of immersion as well as for final coverage.
data, eq 1 usually is normalized to the intensity of the second harmonic at zero coverage, i.e., ISHG(θ ) 0). That way, eq 1 transforms into
θ ) [1 - xISHG(θ)/ISHG(θ ) 0)]/R
(2)
where R ) χint (θ ) 1)/χsub.41 Figure 6 shows the evolution of the SH intensity of the gold surface exposed to (1) PEG2000-SH 50 µM DMF solution, (2) PEG2000-OH (without the alkanethiol spacer) 50 µM DMF solution, and (3) C16SH 5 µM DMF solution as a reference for the first 120 min of the adsorption process. In the latter case, the molarity of the solution was scaled down by a factor of 10 due to the high mobility of the short C16SH molecules in solution and thus to resolve the fast adsorption kinetics experimentally. In this case, the SH intensity shows an exponential decrease with increasing time due to chemisorption, i.e., it corresponds basically to a Langmuir adsorption isotherm, in good agreement with previous results for the adsorption of alkanethiols dissolved in ethanol and hexane.41 In contrast, the SH intensity in the case of PEG2000-OH does not decrease at all because this molecule does not chemically react with the gold surface. The reason for the slight increase in signal intensity with time is not clear yet, but it might be caused by changes of the Fresnel coefficients at the gold/DMF interface due to formation of a physisorption layer and thus can be attributed to a linear optical effect. The evolution of the SHG signal in the case of PEG2000SH appears to be different from both that of PEG2000OH and that of C16SH. The SH intensity rapidly decreases at the beginning of immersion, indicating that PEG2000SH molecules are chemisorbed on the gold surface in the very first stage of adsorption. This first adsorption saturates after ∼10 min and is followed by a slower decrease in SH intensity, which cannot be described by a single-exponential function, i.e., a Langmuir adsorption isotherm. We suppose that this deviation is caused by a blocking behavior of the amorphous moieties on the surface, shielding neighboring adsorption sites after the surface has been filled with polymer coils. Thus, in this
stage adsorption depends not only on the flux of molecules onto the surface but also on the coil-to-brush transition of the adsorbate. Assuming that C16SH forms a densely packed monolayer and that the change in signal intensity is solely caused by sulfur-gold bond formation, i.e., independent of the nature of the aliphatic chain, we can use eq 2 to estimate the relative coverage of PEG2000-SH. The only unknown parameter, R, can be calculated from the signal drop of the C16SH reference measurement for t f ∞ . With this normalization, the coverage for PEG2000-SH after the first adsorption step is ∼28% and for final coverage ∼88% of the densely packed thiolate monolayer (cf. Figure 6). The latter value is in good agreement with the packing density given by Harder et al.42 for oligo(ethylene glycol)terminated alkanethiolates (89%) and gives evidence for a densely packed layer at final coverage. From the relative coverage after the first adsorption step we can deduce two important facts. First, assuming a surface area per molecule of ∼22 Å2 for a densely packed thiolate layer, we obtain an average area of ∼78 Å2/coil for the saturation region of the first adsorption step. As adsorption intermediately slows down in this stage, we conclude that the molecules cover the whole surface occupying an average area of 78 Å2/molecule. Thus, the structure of the molecules must be amorphous. Further, an area of 78 Å2 corresponds to an average next-neighbor distance of ∼9 Å. However, the Flory radius, i.e., the radius of a dissolved polymer coil in a good solvent, for PEG2000-SH is ∼27 Å. Therefore, although the morphology is amorphous, the molecules experience a lateral confinement and their structure should be different from that in solution. 4. Discussion The analysis of the grafting process of alkanethiolterminated PEG on polycrystalline gold surfaces revealed a structural transition from an amorphous to a crystalline state, which is driven by free energy minimization of the solute/surface system. It seems that the gain in free energy due to bond formation between sulfur and gold and due to the intermolecular interactions between neighboring chains is high enough to compensate for the loss in entropy caused by lateral confinement of the molecules and thus to drive the transition from a disordered to an ordered state within the end-grafted polymer layer. From the viewpoint of SAM formation, the influence of the large tailgroup on the kinetics of film formation is striking, as it leads to a clear deviation from Langmuir kinetics, which is observed for the adsorption of alkanethiols onto gold. As shown in Figure 2, the most significant deviation from Langmuir kinetics is the almost constant film thickness over an extended period of time at the initial stage of the adsorption process. The results of in situ SHG (Figure 6) suggest that the molecules are chemisorbed and are not in a physisorbed precursor state. In addition, from the IRRAS data it becomes clear that the EG chains are disordered at this stage (Figure 7a). The existence of the plateau of nearly constant coverage is explained by the inhibition of further adsorption of the EG coils. After ∼10 min of immersion, the film thickness starts to increase. XPS indicates that the average EG chain length does not change during this process. Further, SHG gives evidence that the number of chemisorbed molecules increases. We therefore conclude that the increase of the film thickness corresponds to an increase in coverage and not to the substitution of shorter molecules by longer ones. IRRAS suggests that the EG chains start stretching. In the final stage of adsorption they are in an ordered helical
Grafting of Alkanethiol-Terminated PEG on Gold
Figure 7. Model of film formation for PEG2000-SH on polycrystalline gold: (a) in the initial stage, the molecules chemisorb as amorphous coils; (b) with increasing coverage, a transition from the coil to a brushlike state occurs; (c) in the final stage, the PEG chains are preferably in helical conformation with an orientation of the helical axis close to the surface normal.
conformation with an orientation of the helices perpendicular to the surface (Figure 7c), i.e., the structure of the EG chains changes from an amorphous to an ordered helical phase during film formation as schematically shown in Figure 7b. Only at the very top of the layer does some disorder remain, due to the chain length distribution of the polymer. The film thickness saturates at ∼120 Å and no multilayer formation is observed. By comparing the SHG results of the PEG2000-SH with those of C16SH, the coverage of the fully developed PEG2000-SH film on gold is ∼88% that of hexadecanethiolate (C16SH) films on Au, indicating that the molecules are closely packed despite their long PEG chains. From IRRAS it is clear that the conformation of the EG chains at saturation coverage is helical, and thus is the same as that of OEG-terminated thiol monolayers formed on gold. Assuming a well-ordered and oriented helical conformation for the PEG units and for the alkyl spacer a planar zigzag conformation tilted by 30° with respect to the surface normal, the film thickness in the final stage can be calculated. In the z-direction, the intramolecular distance between the methylene units in the alkyl spacer (dCH2) as well as the sulfur-gold bond distance (dS-Au) is ∼1.1 Å. The length of one EG unit in helical conformation (dEG) amounts to 2.78 Å. With 88% of the adsorption sites on the gold surface occupied by the molecules, the average film thickness is calculated to 0.88 × (dS-Au + 11dCH2 + 45dEG) ) 121.7 Å, in reasonable agreement with the results of our ellipsometry measurements (∼ 117 Å). The adsorption model suggests that the rate-determining step for adsorption into the ordered high-coverage phase is governed by the structural changes of the PEG chains from coillike to rodlike. At the initial stage of adsorption, coils are favored for entropic reasons. With increasing coverage and thus increasing intermolecular interaction, the rodlike structure is energetically favored due to the increase of van der Waals forces among the EG chains and the alkyl spacer. In addition, by shifting the structure of the EG chains from coillike to rodlike, adsorption sites that were blocked by physisorbed molecules become accessible and thus allow a further gain in free energy by additional chemisorption of molecules. That way, the package density is further increased, leading to a rise in surface pressure that forces other molecules into a brushlike conformation, so that finally a high-coverage state is obtained. Our findings agree well with the experimental and theoretical27,48-50 studies on the adsorption kinetics of polymer brushes onto solid surfaces found in the literature. (48) Johner, A.; Joanny, J. F. Macromolecules 1990, 23, 5299-5311. (49) Ligoure, C.; Leibler, L. J. Phys. France 1990, 51, 1313-1328. (50) Zajac, R.; Chakrabarti, A. Phys. Rev. E 1994, 49, 3069-3078.
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Experimental investigations of the liquid/solid interface were performed basically on block copolymers by means of surface plasmon resonance,14 internal reflection interferometry,15 ellipsometry,16,51,52 dynamic light scattering,11 radiolabeling,17 scanning angle reflectometry,53 and surface force apparatus.51,54 Although in physisorbed systems the molecule-surface interaction is rather weak, also in these cases a two-stage adsorption process with a pancaketo-brush transition was observed.11,14,16,51-53 At low grafting density, the molecules adsorb homogeneously, i.e., both types of blocks interact with the surface. At high grafting density, the soluble blocks stretch out into the solvent, while the insoluble anchors form a dense layer at the liquid/ solid interface. However, in contrast to the molecule presented in this article, a high-density state with crystallinelike characteristics was not reported. This is most probably due to the relatively large size of the physisorbed anchors, which prevents close packing and causes steric hindrance. Theory suggests that the pancaketo-brush transition is caused by repulsive interaction between grafting and grafted blocks of the block copolymers.27 To what extent repulsive forces between the C11SH anchors and the PEG chains play a crucial role for the observed conformational transitions of PEG2000-SH needs to be examined in more detail in future studies. Another interesting comparison can be made with recent experimental work on direct30,31 and indirect32-34 grafting of PEG on silicon or glass. While most of these studies relied on silane coupling reactions, Zhu et al.32 utilized chlorine-terminated silicone surfaces for grafting of PEGOH with an aim to improve grafting density and homogeneity of the monolayer. However, in agreement with the work of Yang et al.,30 who utilized direct grafting of silane-terminated PEG on glass, they found molecular weight moieties up to 300 Da to develop into brushlike structures, while molecules with molecular weights above 1000 Da form amorphous films. Yang et al. observed brush formation for PEG silanes with a molecular weight of 750 Da and amorphous films for 5000 Da. Thus, it seems that for grafting on glass or silicon the onset of steric hindrance for brush formation lies somewhere around 1000 Da, independent of the surface chemistry used. Further, none of the groups report about formation of crystallinelike monolayers, not even for low molecular weights. These differences from our findings might be caused by the complexity of the grafting methods they apply, which involve more preparation steps than necessary for thiolate formation on gold. Also, in the case of silane coupling, mobility of grafted moieties on silicon or glass might be lower than that of thiolates on gold, because each lateral movement requires simultaneous cleavage of up to three covalent bonds per molecule. Thus, an equilibrated and homogeneous adsorbate layer on the surface is more difficult to achieve. Some open questions remain for future studies. For example, from our experiments it is not clear yet which parameters are relevant for the incubation period before brush formation commences. In this ex situ study, we observed a certain sample-to-sample deviation for the onset of the transition to the brush state that has to be analyzed in more detail. Therefore, currently we are investigating the influence of sample preparation and (51) Schille´n, K.; Claesson, P. M.; Malmsten, M.; Linse, P.; Booth, C. J. Phys. Chem. B 1997, 101, 4238-4252. (52) Awan, M. A.; Dimonie, V. L.; Filippov, L. K.; El-Aasser, M. S. Langmuir 1997, 13, 130-139. (53) Leermakers, F. A. M.; Gast, A. P. Macromolecules 1991, 24, 718-730. (54) Pelletier, E.; Stamouli, A.; Belder, G. F.; Hadziioannou, G. Langmuir 1997, 13, 1884-1886.
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condition of the solvent, such as concentration, water and gas content, and temperature, on the growth rate. For comparison with theoretical predictions, in particular, the question of the role of repulsive forces between anchor groups and PEG chains seems to be interesting. Here, experiments where the C11SH group is substituted by a different spacer group, e.g., with different chain length or a rigid aromatic group, might give interesting insight into the mechanism of film formation in the case of thiolterminated linear polymers. Conclusions We have shown that alkanethiol-terminated poly(ethylene glycol) grafted on polycrystalline thin gold films from a 50 µM DMF solution forms monolayers with a coverage-dependent morphology. In the initial stage of the adsorption process, molecules chemisorb as coillike amorphous moieties that inhibit adsorption at nextneighbor sites by steric effects. After ∼10 min, the grafting density of the amorphous moieties reaches a level that induces a conformational change of the coils into brushes. This structural transition is completed after ∼2 h of immersion. The obtained high-coverage state is characterized by a helical structure of the PEG backbone with an orientation of its symmetry axis close to the surface
Tokumitsu et al.
normal. Only the very top of this quasi-crystalline PEG layer is expected to be amorphous due to the chain length distribution of the polymer. SAM formation by a polymeric molecule consisting of a relatively small C11 spacer and a tail group of ∼45 EG units, i.e., ∼134 segments, reveals several interesting aspects, such as deviation from Langmuir kinetics due to the blocking of adsorption sites by individual molecules in the beginning of film formation. The entropically unfavorable conformational transition from coil to brush is driven by (1) the gain in Helmholtz free energy due to chemisorption of the sulfur headgroups and (2) the relatively high surface mobility of the adsorbed molecules that induces a homogeneous surface pressure within the layer and thus statistical equilibrium between the various conformers. Acknowledgment. We gratefully acknowledge fruitful discussions with H. J. Kreuzer and thank G. Albert for the substrate preparation. This work was funded by the Deutsche Forschungsgemeinschaft, the Office of Naval Research, the Bayer AG, and the Fonds der Chemischen Industrie. S.T. thanks the Alexander von Humboldt Foundation for financial support. LA0258953
