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Langmuir 2008, 24, 13828-13832
Adapting Low-Adhesive Thin Films from Mixed Polymer Brushes Roman Sheparovych, Mikhail Motornov, and Sergiy Minko* Department of Chemistry and Biomolecular Science, Clarkson UniVersity, Potsdam, New York 13699-5810 ReceiVed September 22, 2008. ReVised Manuscript ReceiVed NoVember 1, 2008 The concept of the responsive/adaptive mixed polymer brushes was applied to the development of the thin film coatings possessing low adhesive properties that were evaluated with AFM probes in different media. Mixed brushes composed of polydimethylsiloxane (PDMS) and polyethyleneoxide (PEO) revealed a selective layered segregation in air and water. Immersion of the sample into an aqueous environment drove PEO chains to the brush-water interface while upon drying the surface undergoing reconstruction and was occupied with PDMS. Low interfacial energies of PDMS in air and PEO in water provided low-adhesive properties of the PDMS-PEO brushes to the probes in both media due to the spontaneous and rapid reconstruction of the mixed brush.
Introduction Fabrication of the coatings responsive to changing environments and possessing low interfacial energy in the variable media is of great interest for many practical applications such as micro(nano)-electromechanical systems (MEMS/NEMS), microfluidic devices, biomedical equipment, antifouling coatings, etc.1-3 Operating in the changeable environment, where conditions repeatedly change between air and water, may worsen device performance or result in device failure. In air, adhesion to polar substrates occurs mostly as a result of the capillary forces because of the condensation of water vapors.4 Interactions in the aqueous environment are more complex, and multiple forces may contribute to the interaction between materials in water. The diverse nature of surface forces acting in air and water requires different approaches to low-adhesive (nonsticky) surfaces in different environments. Capillary condensation in air is minimized for hydrophobic coatings.5 The lowest adhesion between various materials was found for cases in which the surface of a material was modified by fluorinated and siloxane-based substances.6 Such coatings were synthesized using silane and thiol chemistry,7 covalent endgrafting of polymer chains,8 cross-linking of fluorinated polymers,9 polymerization, and deposition of fluorocarbons in plasma.10 However, in water, hydrophobic interactions enhance the attraction between nonpolar moieties of the solutes and substrates.11 Protein molecules, for example, show a high affinity for the hydrophobic materials.12 Application of PEO hydrophilic brushes is a common and efficient method of decreasing the * To whom correspondence should be addressed. Phone: (315)268-3807. Fax: (315)268-6610. E-mail:
[email protected]. (1) Delrio, F. W.; De Boer, M. P.; Knapp, J. A.; Reedy, E. D.; Clews, P. J.; Dunn, M. L. Nat. Mater. 2005, 4, 629–634. (2) Grayson, A. C. R.; Shawgo, R. S.; Johnson, A. M.; Flynn, N. T.; Li, Y. W.; Cima, M. J.; Langer, R. Proc. IEEE 2004, 92, 6–21. (3) Zhao, Y. P.; Wang, L. S.; Yu, T. X. J. Adhes. Sci. Technol. 2003, 17, 519–546. (4) Bhushan, B. J. Vac. Sci. Technol., B 2003, 21, 2262–2296. (5) Burton, Z.; Bhushan, B. Nano Lett. 2005, 5, 1607–1613. (6) Chaudhury, M. K. Mater. Sci. Eng. R 1996, 16, 97–159. (7) Ulman, A. Chem. ReV. 1996, 96, 1533–1554. (8) Burtovyy, O.; Klep, V.; Chen, H. C.; Hu, R. K.; Lin, C. C.; Luzinov, I. J. Macromol. Sci., Phys. 2007, 46, 137–154. (9) Taguet, A.; Ameduri, B.; Boutevin, B. Crosslinking of Vinylidene FluorideContaining Fluoropolymers. In Crosslinking in Materials Science; Ame´duri, B., Ed.; Springer: 2005; Vol. 184, pp 127-211. (10) Cunge, G.; Booth, J. P. J. Appl. Phys. 1999, 85, 3952–3959. (11) Chandler, D. Nature 2005, 437, 640–647.
adhesion of proteins and cells to the modified surfaces in an aqueous environment. Reduction of the substrate-water interfacial energy as well as steric repulsion exerted by hydrophilic polymer chains secured antifouling properties of the polymer brushes.13-15 Poly(ethylene oxide) (PEO) coatings are one of the most studied protein-resistant materials.16,17 PEO chains form a helix conformation that is incorporated into water structures.18 Steric repulsion exerted by the end-tethered PEO helixes increases at higher grafting densities and improves the antifouling properties of PEO brushes. Methods developed for the immobilization of PEO coatings involve physical and covalent networks from the PEO block copolymers,19-21 covalent grafting of the endfunctionalized homo- and block-copolymer PEO chains,22 and physical adsorption of copolymers incorporating PEO and hydrophobic arms.23 However, this method of surface modification fails to minimize the interaction of the modified surfaces with different hydrophilic materials in air because of the van der Waals interactions and the capillary condensation of moisture between the hydrophilic surfaces in contact with one another. Surface modification by responsive polymer coatings provides a simple route for adaptive surface behavior and regulating the interaction of the responsive coatings with different materials.24-26 Changing hydrophilic/hydrophobic properties of the medium results in a reconstruction of responsive polymer coatings (12) Neff, J. A.; Caldwell, K. D.; Tresco, P. A. J. Biomed. Mater. Res. 1998, 40, 511–519. (13) Jeon, S. I.; Lee, J. H.; Andrade, J. D.; De Gennes, P. G. J. Colloid Interface Sci. 1991, 142, 149–158. (14) Leckband, D.; Sheth, S.; Halperin, A. J. Biomater. Sci., Polym. Ed. 1999, 10, 1125–1147. (15) Szleifer, I. Biophys. J. 1997, 72, 595–612. (16) Morra, M. J. Biomater. Sci., Polym. Ed. 2000, 11, 547–569. (17) Li, L. Y.; Chen, S. F.; Zheng, J.; Ratner, B. D.; Jiang, S. Y. J. Phys. Chem. B 2005, 109, 2934–2941. (18) Kjellander, R.; Florin, E. J. Chem. Soc., Faraday Trans. 1 1981, 77, 2053–2077. (19) (a) Krishnan, S.; Ayothi, R.; Hexemer, A.; Finlay, J. A.; Sohn, K. E.; Perry, R.; Ober, C. K.; Kramer, E. J.; Callow, M. E.; Callow, J. A.; Fischer, D. A. Langmuir 2006, 22, 5075–5086. (b) Tokarev, I.; Krenek, R.; Burkov, Y.; Schmeisser, D.; Sidorenko, A.; Minko, S.; Stamm, M. Macromolecules 2005, 38, 507–516. (20) Park, J. H.; Bae, Y. H. Biomaterials 2002, 23, 1797–1808. (21) Vaidya, A.; Chaudhury, M. K. J. Colloid Interface Sci. 2002, 249, 235– 245. (22) Zdyrko, B.; Klep, V.; Luzinov, I. Langmuir 2003, 19, 10179–10187. (23) Currie, E. P. K.; Van der Gucht, J.; Borisov, O. V.; Stuart, M. A. C. Pure Appl. Chem. 1999, 71, 1227–1241. (24) Russell, T. P. Science 2002, 297, 964–967. (25) Lupitskyy, R.; Roiter, Y.; Tsitsilianis, C.; Minko, S. Langmuir 2005, 21, 8591–8593. (26) Crowe, J. A.; Genzer, J. J. Am. Chem. Soc. 2005, 127, 17610–17611.
10.1021/la803117y CCC: $40.75 2008 American Chemical Society Published on Web 11/20/2008
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Langmuir, Vol. 24, No. 24, 2008 13829
Figure 1. (a) Kinetics of grafting PDMS, obtained from the combinatorial experiment via gradual immersion of the sample in a PDMS melt at 70 °C, revealed three characteristic regimes: (1) fast grafting, (2) grafting controlled by the diffusion of PDMS through the primary layer of PDMS chains, and (3) “layer-assisted” acceleration of grafting. (b) Synthesis of mixed brushes of different compositions in the combinatorial experiment: grafting density of the (1) PEO brush, (2) PDMS brush, and (3) mixed PDMS-PEO brush vs the x coordinate on the long axis of the sample, which corresponds to different PDMS grafting times in the combinatorial experiment.
and switching of the interfacial energy.27,28 The assembly of block copolymer nanostructured layers,19 synthesis of polymer brushes with functional “buoy” groups,29 application of segmented polyurethanes (SPU),21 and synthesis of mixed polymer brushes30,31 present major routes for polymer-based responsive surface modification. In mixed polymer brushes, two different polymers are grafted to the same substrate. These two unlike polymers segregate microscopically. They form laterally segregated phases of characteristic sizes that scale with the polymer chain end-to-end distance.32 In a selective solvent, the mixed brush undergoes reconstruction, and the structure of the brush can be represented by a combination of the lateral segregation and a layered segregation (stratification) where the selective interaction drives segments of one of the polymers to the topmost layer.33 Thus, the mixed polymer brush architecture provides an efficient means to regulate surface composition of the topmost layer as a result of the stratification effect.34 Mixed polymer brushes can be grafted to the surface of various substrates using silane chemistry30,35 or polymeric anchoring layers.36 One of the practically important aspects is that the nanoparticles could be used as carriers for polymer brushes.37-39 Hence, the responsive coatings can be created by the simple deposition of nanoparticles decorated with polymer brushes on the surfaces of various substrates.40,41 The goal of this letter is to study the interaction of mixed binary PDMS-PEO brushes with AFM probes in both air and aqueous environments. We demonstrate that the combination of (27) Luzinov, I.; Minko, S.; Tsukruk, V. V. Prog. Polym. Sci. 2004, 29, 635– 698. (28) Minko, S. Polym. ReV. 2006, 46, 397–420. (29) Koberstein, J. T. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 2942– 2956. (30) Sidorenko, A.; Minko, S.; Schenk-Meuser, K.; Duschner, H.; Stamm, M. Langmuir 1999, 15, 8349–8355. (31) Motornov, M.; Sheparovych, R.; Tokarev, I.; Roiter, Y.; Minko, S. Langmuir 2007, 23, 13–19. (32) Muller, M. Phys. ReV. E 2002, 65, 30802. (33) Minko, S.; Muller, M.; Usov, D.; Scholl, A.; Froeck, C.; Stamm, M. Phys. ReV. Lett. 2002, 88, 035502. (34) Minko, S.; Luzinov, I.; Luchnikov, V.; Muller, M.; Patil, S.; Stamm, M. Macromolecules 2003, 36, 7268–7279. (35) Santer, S.; Ruhe, J. Polymer 2004, 45, 8279–8297. (36) Ionov, L.; Sidorenko, A.; Stamm, M.; Minko, S.; Zdyrko, B.; Klep, V.; Luzinov, I. Macromolecules 2004, 37, 7421–7423. (37) Luzinov, I.; Voronov, A.; Minko, S.; Kraus, R.; Wilke, W.; Zhuk, A. J. Appl. Polym. Sci. 1996, 61, 1101–1109. (38) Zhang, M. M.; Liu, L.; Wu, C. L.; Fu, G. Q.; Zhao, H. Y.; He, B. L. Polymer 2007, 48, 1989–1997. (39) Motornov, M.; Sheparovych, R.; Lupitskyy, R.; MacWilliams, E.; Hoy, O.; Luzinov, I.; Minko, S. AdV. Funct. Mater. 2007, 17, 2307–2314. (40) Motornov, M.; Sheparovych, R.; Lupitskyy, R.; MacWilliams, E.; Minko, S. J. Colloid Interface Sci. 2007, 310, 481–488. (41) Motornov, M.; Sheparovych, R.; Lupitskyy, R.; MacWilliams, E.; Minko, S. AdV. Mater. 2008, 20, 200–205.
these two polymers in the mixed PDMS-PEO brush provides fast reconstruction of the mixed brush if the environment repeatedly changes between air and water. This reconstruction is accompanied by switching the topmost layer composition so that the mixed brush adapts low-adhesive properties in the interactions with the AFM probes in changing environments.
Experimental Section Poly(dimethylsiloxane) with primary amine end-functional groups (PDMS, Mw ) 30 000 g/mol) was purchased from Gelest Inc., and poly(ethylene oxide) with primary amine end-functional groups (PEO, Mw ) 10 000 g/mol) was received from Polymer Source Inc. Poly(glycidyl methacrylate) (PGMA, Mw ) 70 000 g/mol) was obtained by the radical polymerization of glycidyl methacrylate.42 All solvents were used as received. Silicon wafers (purchased from Semiconductor Processing, Union Miniere USA Inc.) were first cleaned in an ultrasonic bath for 20 min with ethanol, were then placed in cleaning solution (prepared from NH4OH and H2O2) at 80 °C for 20 min, and finally were rinsed several times with Millipore water (18 MΩ cm). PDMS-PEO binary polymer brushes were synthesized using the “grafting to” approach. End-functionalized polymer chains were grafted to the Si wafers using PGMA as an anchoring layer with epoxy-functional groups. A thin PGMA layer was prepared by spin coating a 0.03% (mass) solution in MEK at 4000 rpm. The average thickness of the obtained PGMA layers was 2.1 ( 0.1 nm. Spin coating was performed in the low-humidity environment to avoid dewetting (relative humidity less than 10%). Afterwards, the PGMA layer was cross-linked at 110 °C for 10 min in a vacuum oven, and nonreacted polymer was washed out by sonication in MEK. The PGMA-modified samples were immersed in a bath with liquid PDMS. The grafting of PDMS was performed in a vacuum oven at 70 °C. The grafting time was varied in the range of 2-60 min to achieve different grafting densities of PDMS brushes (Supporting Information, Table 1). Nongrafted PDMS was washed out in MEK. We studied the kinetics of PDMS grafting to the epoxyfunctionalized substrate using a combinatorial approach. For this purpose, we synthesized gradient PDMS brushes on the silicon substrate. A PGMA-modified sample of the Si wafer was slowly immersed in the PDMS bath at 70 °C. Constant velocity of the sample set to 12 mm/h provided gradual immersion into the PDMS melt. After the moving stage reached its limiting point, the sample was removed from the polymer melt and rinsed with MEK. The grafting density of the obtained PDMS layer plotted versus grafting time (Figure 1a) revealed three characteristic regimes of the grafting kinetics: (1) fast grafting controlled by the segmental diffusion of PDMS molecules close to the substrate surface, (2) grafting controlled by the diffusion of PDMS through the primary-layer PDMS chains (42) Draper, J.; Luzinov, I.; Minko, S.; Tokarev, I.; Stamm, M. Langmuir 2004, 20, 4064–4075.
13830 Langmuir, Vol. 24, No. 24, 2008 in the mushroom regime, and (3) acceleration of the grafting rate assigned to “layer-assisted” polymer tethering.43 The PDMS brush approaches the saturation regime after 1 h of grafting, and the maximum brush grafting amount obtained at 70 °C is 5.5 mg/m2. The amino-terminated PEO was grafted to the samples with the PDMS brushes by placing the PEO polymer between the sample surface and a glass slide, which was used as a cover. PEO was grafted from the melt at 110 °C. Capillary forces held the polymer between closely located surfaces of the sample and the glass slide and provided a good contact between the PEO melt and the sample surface. Mapping of the brush thickness using ellipsometry revealed that bubbles stuck between the sample and the glass slide caused nonhomogeneous grafting of the PEO brush. To avoid this problem, the PEO powder deposited on the glass slide was evacuated at room temperature and than heated to 80 °C in a vacuum oven. When all of the bubbles escaped, we extracted the glass slide with the PEO melt from the vacuum oven and immediately attached the sample to the glass slide covered by the molten PEO. After evacuation, the temperature was raised to 110 °C. To achieve the maximum grafting density of the PEO brush, grafting was performed for 10 h. PDMS and PEO were covalently attached to the substrate by reaction between amine end groups of the polymers and epoxy functionalities of PGMA (Scheme 1 in Supporting Information). The combinatorial experiment helped us to identify conditions for grafting mixed brushes of different compositions as shown in Figure 1b. The amount of each deposited component was measured using null ellipsometry (Optrel Multiscope, Berlin, Germany). Measurements were performed at a 70° angle of incidence. Refractive indices used for the calculation of layer thickness were 1.406 for PDMS, 1.450 for PEO, 1.550 for PGMA, and 1.4598 for silica. The measurements were performed for each sample after each step of the modification in order to use the measurements of the previous step as a reference for the simulation of ellipsometric data. Initially, the thickness of the native SiO2 layer (usually 1.4 ( 0.2 nm) was calculated at refractive indexes of N ) 3.858-i0.018 for the Si substrate and n ) 1.4598 for the SiO2 layer. The thickness of the PGMA layer was evaluated using the two-layer model: SiO2/PGMA. The thickness of PDMS-NH2 as the first grafted layer (typically 1-4 nm) was evaluated with the three-layer model of SiO2/PGMA/PDMSNH2. Finally, the thickness of the whole polymer film (typically 4-7 nm) after the grafting of the second polymer was calculated using the two-layer model of the SiO2/polymer considering the thin polymer film to be an effective optical medium, with n ) 1.5. We estimated that this calculation results in an error of no larger than (5% for the 5-nm-thick films because the difference between refractive indexes of all organic ingredients is small. From the obtained values, we calculated the grafting amount of each polymer to be A ) HF, and the grafting density to be Σ ) ANA/Mw, where H is the ellipsometric thickness, F is the density, NA is Avogadro’s number, and Mw is the molecular weight of the polymer. An AFM Dimension 3100 microscope (Veeco, NY) was used for surface imaging and measurements of force-distance curves in air. Imaging and adhesion measurements in water were performed using a Multimode AFM microscope (Veeco, NY). For imaging in air, we used BS-TAP 300 silicon probes (Innovative Solutions Bulgaria Ltd., Sofia, Bulgaria), and in water we used NP-S Si3N4 probes (Veeco, NY). AFM images were processed using WSxM 4.0 Nanotec software.44 Adhesion experiments were performed with silicon tips in air and Si3N4 tips in the aqueous environment, respectively. In air, adhesion was measured under ambient conditions at 35% relative humidity. The measurements were performed in a single point by averaging five consecutive records of force-distance curves. This measurement cycle was not distractive for the sample. We did not observe a tendency toward changes in the force-distance curves with a number of measurements within the five record cycles. The measurements were performed at least in 10 different points on the sample with (43) Penn, L. S.; Huang, H.; Sindkhedkar, M. D.; Rankin, S. E.; Chittenden, K.; Quirk, R. P.; Mathers, R. T.; Lee, Y. Macromolecules 2002, 35, 7054–7066. (44) http://www.nanotec.es/wsxm_download.html.
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Figure 2. Force-distance measurements of the tip adhesion to PDMSPEO brushes of different compositions in air (1) and in water (2).
Figure 3. Advancing and receding contact angles for PDMS-PEO mixed brushes of different compositions. A high advancing angle (4) indicates the presence of PDMS in the topmost brush layer whereas low receding angles (O) indicate switching of the brush surface to the state with the enriched fraction of PEO in the topmost layer upon contact with water.
at least 50 µm spacing between them. The adhesion forces (in these experiments, pull-off forces) were evaluated from the force-distance curves. Examples of typical force-distance curves are shown in Figure S1-S3 in Supporting Information.
Results and Discussion We studied the adhesion of the AFM tip to the PDMS-PEO mixed brush surface in water and in the dry state using AFM force-distance measurements. The major result of this study is reported in Figure 2. In air only, mixed brushes with a high PEO fraction showed a high level of adhesion to the AFM tip. Symmetrical mixed brushes and those with high PDMS fractions showed adhesion to the AFM tip that was as low as for the homopolymer PDMS brush. In water, the mixed brushes with moderate and high PEO fractions show as low adhesion to the AFM probe as does the homopolymer PEO brush, whereas PDMS-enriched brushes reveal a strong interaction with the probe. Thus, we may speculate that the mixed brush approaches a low interfacial tension in both environments as a result of reconstruction caused by different environments. Contact angle measurements provide evidence for switching the PDMS-PEO brush structure upon contact with water and air
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Figure 4. (a, b) AFM topographical images, (c, d) cross-sectional profiles, and (e, f) schematics of PDMS-PEO brushes ((a, c, e) 33% PDMS, (b, d, f) 56% PDMS) in air. z scale ) 10 nm.
(Figure 3). For a rapidly adapting polymer system, the advancing contact angle reflects the presence of hydrophobic polymer on top of the brush in the dry state whereas the receding contact angle characterizes the switching of the surface layer due to contact with water.31 High values of advancing contact angles indicate the presence of PDMS chains in the top of the mixed brush. Switching of the brush surface in front of the contact line and spreading of the water droplet over the mixed brush surface do not occur because of an “activation barrier” created by PDMS chains located on top of the brush. The PDMS homopolymer brush demonstrates low wetting hysteresis (small difference between advancing and receding contact angles). Thus, we used receding contact angle measurements to characterize the switching of the mixed brush surface from the state where the topmost brush layer was enriched with PDMS chains to the state with a large fraction of PEO chains on the top upon contact with water. For the fraction of PEO in the mixed brush greater than 40%, the receding angle drops to 36° (Figure 3), which corresponds to the contact angle of the homopolymer PEO brushes (∼35°). The low receding angle of this sample can be explained by the layered phase separation in the mixed brush where swollen PEO chains cover segregated PDMS domains. Further increases in the PEO fraction have no effect on the receding contact angle, in good agreement with the results of force-distance measurements. AFM imaging of the mixed PDMS-PEO brushes of different compositions revealed the lateral phase segregation of PDMS and PEO chains (Figure 4). The PEO domains appear as lighter areas (higher regions) in the AFM images. Upon immersion of the PDMS-PEO brush sample in water, the size of the PEO clusters increased (Figure 5). The comparison of the samples with different compositions clearly demonstrates that for samples with a larger fraction of PEO (Figures 4a and 5a) the swollen PEO chains occupy the topmost layer in water and completely cover the PDMS domains located underneath. However, for the sample with the larger PDMS fraction (Figures 4b and 5b), the
Langmuir, Vol. 24, No. 24, 2008 13831
Figure 5. (a, b) AFM topographical images, (c, d) cross-sectional profiles, and (e, f) schematics of PDMS-PEO brushes ((a, c, e) PDMS 33%, (b, d, f) PDMS 56%) in water. z scale ) 25 nm.
Figure 6. AFM topography ((a, b) z scale ) 15 nm) and phase ((c, d) z scale ) 30°) images of the symmetrical PDMS-PEO brush in air at different tip-loading forces. At a low target amplitude voltage (1.5 V), (c) the phase image shows a uniform composition of the surface layer, which is PDMS, whereas at a high voltage (2.5 V) (d) the tip penetrates through the PDMS top layer and probes distinct domains of PEO and PDMS chains.
swollen PEO chains cover only some fraction of the sample surface. These results are in accord with the observed character of the force-distance curves in water. The samples with a larger fraction of PEO demonstrated repulsive interaction with the approaching AFM tip, whereas the repulsive forces were not well pronounced
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for the samples with the larger PDMS fraction (Figure S3, Supporting Information). Phase imaging of symmetrical PDMS-PEO brushes (Figure 6) performed in air at a low tip-loading force showed a uniform surface layer, but at the higher loading forces, the contrast between PEO and PDMS domains was much increased. The AFM tip in the latter case penetrated through the top PDMS layer and probed the brush structure close to the grafting surface. This result is in accord with the force-distance measurements, which revealed low adhesive properties of the mixed brush in air and hence an increased concentration of PDMS chains in the topmost layer. Thus, the PDMS-PEO brushes experience both lateral and layered phase segregation. The lateral segregation is clearly seen on the AFM images. The AFM phase imaging and contact angle measurements revealed the stratified structure of the PDMSPEO brushes due to the layered phase segregation. Probing of the mixed brush surface in the force-distance experiments provided data that are in accord with the imaging and contact angle experiments. Lowering of the interfacial energy is the driving force for the diffusion of PDMS chains to the brush surface in air and PEO chains in water. Low interfacial energies for PDMS in air and PEO in water result in lower adhesive properties of the adaptive mixed PDMS-PEO brushes in both air and aqueous environments. The flexibility of PDMS chains provides fast reconstruction of the brush in changing environments. In particular, we observed instant switching of the mixed brush surface from hydrophilic to hydrophobic if the sample was extracted from an aqueous medium. High advancing contact angles of the samples removed from water indicated fast diffusion of PDMS to the brush surface upon contact with air. The same
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fast switching to the hydrophilic property upon contact with a water droplet resulted in low receding contact angles. The same sample of the mixed brush was used in several consecutive force-distance experiments in water and in air and in multiple contact angle experiments. We noticed good reproducibility of the results (Figure S4 in Supporting Information). The multiple switching experiments of the mixed brushes in changing environments showed that all changes in the brushes were reversible.
Conclusions Mixed PDMS-PEO brushes demonstrate unique responsive properties. The stratified mixed brush in water is covered by PEO chains, whereas in air it is rapidly reorganized and formed the topmost layer constituted of PDMS chains. Because of the rapid switching of the mixed brush, it has low adhesive properties with respect to the AFM probes in both environments in water and in air. The low adhesive property is intact for the brushes with compositions close to 50:50. The developed approach to nonsticky surfaces in changing environments could find important applications in the surface modification of materials for medicine as well as for antifouling coatings and coatings for MEMS. Acknowledgment. We acknowledge financial support provided by the National Science Foundation (award DMR 0602528). Supporting Information Available: Grafting of mixed brushes, characteristics of the samples, and representative force-distance curves. This material is available free of charge via the Internet at http:// pubs.acs.org. LA803117Y
