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Swelling Behavior of Self-Assembled Monolayers of Alkanethiol-Terminated Poly(ethylene glycol): A Neutron Reflectometry Study J. Fick,† R. Steitz,‡ V. Leiner,§ S. Tokumitsu,† M. Himmelhaus,*,† and M. Grunze† Lehrstuhl fu¨ r Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany, Stranski-Laboratorium, Technische Universita¨ t Berlin, Straβe des 17. Juni 112, 10623 Berlin, Germany, BENSC am Hahn-Meitner Institut, Glienicker-Str. 100, 14109 Berlin, Germany, Institut fu¨ r Experimentalphysik IV, Ruhr-Universita¨ t Bochum, 44780 Bochum, Germany, and Institut Laue-Langevin, F-38042 Grenoble, France Received June 30, 2003 The swelling behavior of alkanethiol-terminated poly(ethylene glycol) with an average molecular weight of 2180 Da (i.e., ∼45 ethylene glycol, EG, units) in contact with water was investigated by neutron reflectometry as a function of the morphology of the PEG-SH layer. Amorphous films at a low grafting density show significant swelling with an increase of the film thickness from ∼25 Å in the dry state to ∼70 Å in contact with D2O, which corresponds to a total water uptake of ∼38 mass %. In contrast, quasicrystalline monolayers exhibit only a small amount of water penetrating into the film (∼8 mass %) with a corresponding change of the layer thickness from ∼110 to ∼125 Å. The water uptake per EG unit corresponds to the literature value of 1.5 for the amorphous layer and to only 0.25 in the case of the quasi-crystalline film.
Introduction The tethering of polymers to solid surfaces via chemical bonding provides an interesting way to modify the physical and chemical properties of technical surfaces and, therefore, is of significant interest in several applications, such as colloidal stabilization,1 corrosion inhibition, nonfouling surface technology,2 and biomedical science.3 Typically, the polymers are grafted to the surface via suitable linker groups designed to match the particular polymer/surface system. In some cases, the polymer is directly grafted from solution by covalent coupling.4-10 Alternatively, either the surface or the polymer is activated by intermediate preparative steps for polymer grafting, for example, by functionalization.11-16 Recently, surface po* Corresponding author: e-mail,
[email protected]. † Universita ¨ t Heidelberg. ‡ Technische Universita ¨ t Berlin and BENSC. § Ruhr-Universita ¨ t Bochum and Institut Laue-Langevin. (1) Napper, D. Polymeric stabilization of colloidal Dispersions; Academic Press: London, 1983. (2) Leckband, D.; Sheth, S.; Halperin, A. J. Biomater. Sci., Polym. Ed. 1999, 10, 1125. (3) Harris, J. M. Poly(ethylene glycol) Chemistry; Plenum Press: New York, 1992. (4) Auroy, P.; Auvray, L.; Le´ger, L. Phys. Rev. Lett. 1991, 66, 719. (5) Auroy, P.; Mir, Y.; Auvray, L. Phys. Rev. Lett. 1992, 69, 93. (6) Karim, A.; Satija, S. K.; Douglas, J. F.; Ankner, J. F.; Fetters, L. J. Phys. Rev. Lett. 1994, 73, 3407. (7) Koutsos, V.; Vegte, W. v. d.; Pelletier, E.; Stamouli, A.; Hadziioannu, G. Macromolecules 1997, 30, 4719. (8) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458. (9) Yang, Z.; Galloway, J. A.; Yu, H. Langmuir 1999, 15, 8405. (10) Papra, A.; Gadegaard, N.; Larsen, N. B. Langmuir 2001, 17, 1457. (11) Emoto, K.; Harris, J. M.; Alstine, J. M. V. Anal. Chem. 1996, 68, 3751. (12) Sofia, S. J.; Premnath, V.; Merrill, E. W. Macromolecules 1998, 31, 5059. (13) Ebata, K.; Furukawa, K.; Matsumoto, N. J. Am. Chem. Soc. 1998, 120, 7367.
lymerization was achieved by “living polymerization” using appropriate molecules.17 In general, it is difficult to obtain a high grafting density on solid substrates via grafting from solution because polymer molecules in good solvents adopt a coil-like structure that temporarily is maintained after adsorption, thus hindering close packing of the grafted layer.17 Therefore, formation of densely packed grafted films depends on the ability of the grafted molecules to transform from a coil-like to a brush morphology. This ability not only depends on the polymer itself but also is more likely a characteristics of the overall solvent/polymer/substrate system. So far, only few systems have been reported that exhibit a satisfactory transition behavior, that is, yield a high grafting density.17,18 This is all the more crucial because the morphology of the grafted layer determines many of its physical properties, such as solvation behavior, water and ion uptake, stiffness, dichroism, and refractive index, which is critical for specific applications. In a recent study, we have shown that poly(ethylene glycol) (PEG) can be effectively grafted to gold surfaces, if functionalized with an alkanethiol group.19 We use PEG with an average molecular weight of 2000 Da (i.e., ∼ 45 ethylene glycol, EG, units) and a C11-SH linker group attached to one chain terminus (PEG-SH; 2180 Da). This thiol termination allows for chemisorption of the molecules onto gold-coated surfaces and yields grafted polymer layers of different structures, just depending on the immersion time of the substrates into 50 µM N,N-dimethylformamide (14) Bergbreiter, E. D.; Franchina, J. G.; Kabza, K. Macromolecules 1999, 32, 4993. (15) Luzinov, I.; Julthongpiput, D.; Malz, H.; Pionteck, J.; Tsukruk, V. V. Macromolecules 2000, 33, 1043. (16) Shybanova, O.; Voronov, S.; Bednarska, O.; Medvedevskikh, Y.; Stamm, M.; Tokarev, V. Macromol. Symp. 2001, 164, 211. (17) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677. (18) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 33. (19) Tokumitsu, S.; Liebich, A.; Herrwerth, S.; Eck, W.; Himmelhaus, M.; Grunze, M. Langmuir 2002, 18, 8862.
10.1021/la049526d CCC: $27.50 © 2004 American Chemical Society Published on Web 04/03/2004
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Table 1. Results of the Evaluation of Neutron Reflectivity Data for Two Amorphous and Two Quasi-Crystalline PEG-SH Films on Gold-Coated Quartz under Different Ambient Conditionsa polymeric state sample 1 Ti thickness (Å) Au thickness (Å) PEG thickness in contact with N2 measured by NR (Å) PEG thickness measured against air by spectral ellipsometry (Å) PEG thickness in contact with water measured by NR (Å) roughness σ of the PEG/water interface (Å) sld of PEG in contact with D2O (10-6 Å-2) sld of PEG in contact with IMQ (10-6 Å-2) volume fraction of sorbed water (vol %) mass fraction of sorbed water (mass %) water molecules per EG unit
crystalline state
sample 2
sample 3
sample 4
21.0 ( 0.1b 718.2 ( 7.9b 32.2 ( 7.9
17.1 ( 0.2c 674.2 ( 7.2c 44 ( 19.3
22.5 ( 0.9b 632.0 ( 8.5b 62.2 ( 13.0
18.3 ( 0.9c 715.5 ( 1.8c 141 ( 41.0
22.2 ( 1.1
24.2 ( 1.2
100.2 ( 5.0
111.2 ( 5.6
71.0 ( 7.7
54.0 ( 6.7; 89.2 ( 11.0; mean, 71.6d 24.9 ( 4.2
124.3 ( 8.0
125.1 ( 4.0 18.7 ( 2.3
2.90 ( 0.3
2.17 ( 0.5
1.57 ( 0.4
1.20 ( 0.3
2.17 ( 0.3
2.33 ( 0.2
0.95 ( 0.3
0.94 ( 0.2
68.7 ( 3.7
66.2 ( 4.6
19.4 ( 6.6
11.1 ( 5.3
39.8 ( 4.6
35.7 ( 5.6
10.1 ( 5.8
6.7 ( 3.6
1.5 ( 0.2
0.25 ( 0.1
a
For the determination of the film thickness in the dry state, the ellipsometric results also are shown. b Average and standard deviation of four data sets. c Average and standard deviation of three data sets. d Here, two “best fits” with similar χ2 were found.
measurements and calculate the change in the film thickness and the fraction of sorbed water for the amorphous as well as the quasi-crystalline films in water contact.
(DMF) solution. After a short immersion of up to 10 min, amorphous films with coil-like molecules have formed, followed by a transition into a brush state at intermediate times. In the final stage of film formation, corresponding to immersion times of above 2 h, densely packed crystalline-like monolayers have self-assembled. The ease of preparation allows for the study of the physical properties of the grafted film as a function of its structure. In continuation of this work, we present here an analysis of the swelling behavior and water uptake of PEG-SH as a function of the structure of the grafted film. Our motivation is twofold. First, the study of structuredependent properties of surface-grafted polymers is interesting in its own right, from a fundamental as well as an applicative point of view. Second, PEG is well-known as a polymer with a number of unique properties, such as high protein resistance and biocompatibility, despite of its simple chemical nature.3 As one key point for this unique behavior, its strong interaction with water has been identified. For example, the protein resistance of PEG coatings is explained by steric repulsion forces exerted on adsorbing proteins by the hydrated polymer chains for which compression is entropically and dehydration energetically unfavorable.20 PEG-SH as investigated in this study has been found to show protein resistance at low as well as at high grafting densities.21 This raises the question of whether the mechanism of repulsion is the same in both density regimes. As a first step, in this investigation we study to which extent the amount of sorbed water depends on the film morphology. To get a detailed picture of the swelling behavior and to quantify the amount of sorbed water, we investigated PEG-SH layers of different density and structure formed on polycrystalline gold films during contact with deuterated water by neutron reflectometry (NR). To estimate the swelling, that is, the increase in the film thickness, as compared to the dry state, the film thickness under ambient conditions was determined by spectral ellipsometry. In the following, we present the results of the NR
Chemicals. DMF and acetone were purchased from Merck, and D2O was purchased from Aldrich. All chemicals were used as received. (1-mercaptoundec-11-yl) poly(ethylene glycol) monomethyl ether [CH3-(OCH2CH2)45-O-(CH2)11-SH; PEG-SH] was prepared as described elsewhere.19 Quartz substrates were purchased from CrysTec GmbH, Berlin, and prepared for film adsorption by evaporation of a nominally 1-nm-thick adhesion promoter layer of titanium and an additional 60-nm gold layer. Sample Preparation. For preparation of the PEG monolayers on the gold thin films, 22.2 mg of PEG-SH were dissolved in 200 mL of DMF to give a 50 µM solution. Quartz substrates were immersed into the solution for 2 and 10 min, respectively, to give films with a low thickness and 12 h to give films with a high thickness according to the adsorption kinetics as described elsewhere.19 Methods. The film thickness under ambient conditions was determined by means of a spectral ellipsometer (J. A. Woollam, Inc., model M-44) assuming a three-layer model for the gold film, the organic layer,22 and the ambient. To describe the refractive index of the organic layer, a Cauchy model was applied with parameters An ) 1.45 and Bn ) 0.01, which yields a refractive index of n ) 1.4760 at λ ) 620.0 nm. Results are shown in Table 1. NR measurements were performed at the Advanced Diffractometer for the Analysis of Materials (ADAM) at the Institut Laue Langevin in Grenoble.23 The setup utilizes a focusing graphite monochromator operated at a peak wavelength of 4.41 Å and gives a flux of 2 × 106 neutrons/(cm2 s) at the spot of the sample. The collimating slits were set to a 0.4-mm width yielding a resolution of 5 × 10-4 Å-1. Neutron measurements of samples in contact with the nitrogen atmosphere were carried out in a sealed aluminum box. Neutrons were reflected at the nitrogen/solid interface from the gas phase. In contrast, measurements in contact with water were performed with the samples mounted to a Teflon cell filled with D2O or a D2O/H2O mixture. In that case, neutrons were reflected at the solid/liquid interface from the solid phase. Typically, substrates
(20) Jeon, S. I.; Lee, J. H.; Andrade, J. D.; Gennes, P. G. d. J. Colloid Interface Sci. 1991, 142, 149. (21) Herrwerth, S.; Ph.D. Thesis, University of Heidelberg, Heidelberg, Germany, 2002.
(22) Tompkins, H. G.; McGahan, W. A. Spectroscopic Ellipsometry and Reflectometry; Wiley & Sons: New York, 1999. (23) Schreyer, A.; Siebrecht, R.; Englisch, U.; Pietsch, U.; Zabel, H. Physica B 1998, 248, 349.
Experimental Section
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Letters
Figure 1. (a) Scheme of the layer sequence as used in Parratt’s algorithm; (b) layer sequence as used for evaluation of the data. Assignments are made according to eq 1. were measured in contact with nitrogen before PEG film formation to obtain an independent set of data for the metal layers. After film preparation, samples were first measured in nitrogen, then in contact with D2O, and finally against the D2O/H2O mixture. A total of four samples, two each with crystalline and amorphous PEG layers, respectively, were examined this way.
Modeling Before analysis of the neutron reflectivity spectra, data were footprint- and background-corrected and normalized to the incident intensity I0. Then, the data were analyzed by a fitting routine24 based on Parratt’s algorithm.25,26 As shown in Figure 1a, the interface structure of our samples was described by a multilayer model consisting of a number of slabs, each of which was adjustable by two independent parameters, that is, the thickness di and the scattering length density (sld) Fi. To account for the surface roughness between two adjacent layers, the approach of Nevot and Croce27 is implemented in the fitting algorithm: the height z0i of the ideally flat interface i, that is, the interface between the ith and (i + 1)th layer, is assumed to vary according to a Gaussian distribution function: that is, the complex refractive index across the interface n˜ (z) is postulated to vary according to
n˜ (z) ) n˜ i + (n˜ i+1 - n˜ i)Fi(z)
∫
Fi(z) ) (1/σx2π)
(1) z
e-(1/2)[(ξ - z0i)/σi] dξ -∞ 2
Here, n˜ i and n˜ i+1 designate the refractive indices of the two adjacent layers i and i + 1 and Fi(z) is the so-called error function. σi is a measure for the roughness of the ith interface. For further details of the evaluation procedure, we refer to refs 25-27. It is important to note that the distribution function as given in eq 1 to describe surface roughness at the interface (24) Braun, C. Parratt32; HMI: Berlin, 1997-1999. (25) Parratt, L. G. Phys. Rev. 1954, 95, 359. (26) Parratt, L. G. J. Chim. Phys. Phys.-Chim. Biol. 1956, 43, 597. (27) Nevot, L.; Croce, P. Rev. Phys. Appl. 1980, 15, 761.
Figure 2. (a) Neutron reflectivity data (dots) and corresponding fits (lines) for an amorphous PEG-SH film in contact with nitrogen, deuterium oxide, and a mixture of water and deuterium oxide, which match the sld of the quartz substrate. For clarity, the two upper spectra are shifted vertically. (b) Real space profiles of the sld as resulting from the fits shown in the upper diagram.
particularly is a good model for the interface roughness between the PEG layer and the ambient, because the chain length variation of the PEG molecules and, thus, the local thickness of the PEG layer also are well-described by a Gaussian distribution function. The characteristic chain length variation σpol of the PEG-SH amounts to 5.4 ( 0.24 EG units according to the results of mass spectrometry.21 Results and Discussion In Figures 2 and 3, two sets of neutron reflectivity spectra, the corresponding fits according to the box model, and the real space profiles as derived from the fits are shown for an amorphous film and a densely packed monolayer of PEG-SH on polycrystalline gold, respectively. The spectra were obtained with the samples in contact with pure N2, against pure D2O, and in contact with a D2O/H2O mixture adjusted to match the sld of the quartz substrate. The latter was chosen to get an independent set of data for each sample and is called “index-matched quartz” (IMQ) in the following. During modeling, the sld’s of the quartz, the titanium, the gold, and the ambient (D2O, IMQ, or N2) were kept fixed to their bulk values as given in the literature. All other parameters, such as the layer thickness di, the interface roughness σi for the individual layers, and in addition the sld of the PEG layer, were free for optimization. The best fit in each case was
Letters
Figure 3. (a) Neutron reflectivity data (dots) and corresponding fits (lines) for a quasi-crystalline PEG-SH film in contact with nitrogen, deuterium oxide, and a mixture of water and deuterium oxide, which match the sld of the quartz substrate. For clarity, the two upper spectra are shifted vertically. (b) Real space profiles of the sld as resulting from the fits shown in the upper diagram.
found by minimization of the fit’s least-squares deviation χ2 and checked by plausibility considerations, such as the consistent layer thickness and interface roughness for the gold and titanium layers. In particular, the gold layer was allowed to deviate by only
