Reduced Hydrophobic Interaction of Polystyrene Surfaces by

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Langmuir 2008, 24, 5527-5533

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Reduced Hydrophobic Interaction of Polystyrene Surfaces by Spontaneous Segregation of Block Copolymers with Oligo (Ethylene Glycol) Methyl Ether Methacrylate Blocks: Force Measurements in Water Using Atomic Force Microscope with Hydrophobic Probes Rui Zhang,† Akiko Seki,‡ Takashi Ishizone,‡ and Hideaki Yokoyama*,† Nanotechnology Research Institute, National Institute of AdVanced Industrial Science and Technology, Central 5, Higashi, 1-1-1, Tsukuba, Ibaraki 305-8565, Japan and Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1-H-119, Ohokayama, Meguro-ku, Tokyo 152-8552, Japan ReceiVed December 16, 2007. ReVised Manuscript ReceiVed February 21, 2008 Reduction of hydrophobic interaction in water is important in biological interfaces. In our previous work, we have found that poly(styrene-b-triethylene glycol methyl ether methacrylate) (PS-PME3MA) segregates the PME3MA block to the surface in hydrophobic environment, such as in air or in a vacuum, and shows remarkable resistance against adsorption or adhesion of proteins, platelets, and cells in water. In this paper, we report that atomic force microscopy (AFM) with hydrophobic probes can directly monitor the reduced hydrophobic interaction of the PS surfaces modified by poly(styrene-b-origoethylene glycol methyl ether methacrylate) (PS-PMENMA), where N is the number of ethylene glycol units. The pull-off forces between the hydrophobic probes that are coated with octyltrichlorosilane (OLTS) and the PS-PMENMA modified polystyrene (PS) surfaces in water were measured. The absolute spring constants and tip-curvatures of the AFM cantilevers were measured to compute the work of adhesion by the Johnson, Kendall, and Roberts (JKR) theory, which relates the pull-off force at which the separation occurs between a hemisphere and a plane to the work of adhesion. The hydrophobic interactions between the hydrophobic tip and polymer surfaces in water were greatly reduced with the segregated PMENMA blocks. The hydrophobic interactions decrease with increasing N of the series of PS-PMENMA and show a correlation with the amount of protein adsorbed.

Introduction Hydrophobic interactions play important roles in biological interfaces, an example of which is the interfaces between blood and polymeric medical devices. It has been extensively studied that grafting oligo- and polyethylene glycols (OEG and PEG) onto surfaces prevents adsorption or adhesion of proteins, platelets, and cells.1–5 However, attaching OEG or PEG onto surfaces requires functionality on the surfaces, which is not always available on most of the surface of polymeric materials. A similar effect can be obtained by the use of graft copolymers with OEG side chains as demonstrated by Mayes and co-workers.6–8 By using the graft copolymers to achieve good resistance against protein adsorption, at least 6–9 units of ethylene glycol (EG) are necessary.9 However, with increasing the number of EG units, the solubility of such random graft copolymers to water increases. * To whom correspondence should be addressed. E-mail:yokoyama@ molle.k.u-tokyo.ac.jp. Phone/Fax: +81-4-7136-3766. † National Institute of Advanced Industrial Science and Technology. ‡ Tokyo Institute of Technology. (1) Fan, X. W.; Lin, L. J.; Dalsin, J. L.; Messersmith, P. B. J. Am. Chem. Soc. 2005, 127, 15843–15847. (2) Bethencourt, M. I.; Barriet, D.; Frangi, N. M.; Lee, T. R. J. Adhes. 2005, 81, 1031–1048. (3) Zheng, J.; Li, L. Y.; Tsao, H. K.; Sheng, Y. J.; Chen, S. F.; Jiang, S. Y. Biophys. J. 2005, 89, 158–166. (4) Zheng, J.; Li, L. Y.; Chen, S. F.; Jiang, S. Y. Langmuir 2004, 20, 8931– 8938. (5) Herrmann, S.; Mohl, S.; Siepmann, F.; Siepmann, J.; Winter, G. Pharm. Res. 2007, 24, 1527–1537. (6) Banerjee, P.; Irvine, D. J.; Mayes, A. M.; Griffith, L. G. J. Biomed. Mater. Res. 2000, 50, 331–339. (7) Hester, J. F.; Mayes, A. M. J. Membr. Sci. 2002, 202, 119–135. (8) Hester, J. F.; Banerjee, P.; Mayes, A. M. Macromolecules 1999, 32, 1643– 1650. (9) Irvine, D J.; Mayes, A. M.; Griffith, L. G. Biomacromolecules 2001, 2, 85–94.

Therefore, there is a narrow window of the number of EG units for the random graft copolymer approach to satisfy two competing requirements. The minimum number of EG units required for the random graft copolymer approach is similar to that for the self-assembled monolayer of OEG investigated by Grunze’s group.10–12 In our previous study, we showed that poly(styreneb-triethylene glycol methyl ether methacrylates) (PS-PME3MA) preferentially segregates to the vacuum surface of polystyrene (PS), which is more hydrophobic and is likely to cover the surfaces.13 Interestingly, although PME3MA homopolymer is water-soluble at room temperature, the block copolymers of PS and PME3MA (PS-PME3MA) enrich the PME3MA block at the air surfaces even in the blends with PS. Furthermore, such spontaneously formed surfaces reconstruct upon contact with water and present remarkable antifouling properties against proteins, cells, and platelets in water although the number of EG units in PME3MA is only 3, which is much smaller than the number of units required by the graft random copolymer approach.9 In the case of our block copolymer approach for surface modification, water solubility of the entire hydrophilic blocks appears to be important. It is speculated that the entire PME3MA blocks covering the surface work as water-soluble polymer brushes on the PS surfaces and prevent hydrophobic interaction. Schematic illustration of surface segregation of PME3MA blocks in air and reconstructed structure in water are shown in Figure 1. (10) Seigel, R. R.; Harder, P.; Dahint, R.; Grunze, M.; Josse, F.; Mrksich, M.; Whitesides, G. M. Anal. Chem. 1997, 69, 3321–3328. (11) Zwahlen, M.; Herrwerth, S.; Eck, W.; Grunze, M.; Hahner, G. Langmuir 2003, 19, 9305–9310. (12) Herrwerth, S.; Eck, W.; Reinhardt, S.; Grunze, M. J. Am. Chem. Soc. 2003, 125, 9359–9366. (13) Yokoyama, H.; Miyamae, T.; Han, S.; Ishizone, T.; Tanaka, K.; Takahara, A.; Torikai, N. Macromolecules 2005, 38, 5180–5189.

10.1021/la703934u CCC: $40.75  2008 American Chemical Society Published on Web 04/16/2008

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Zhang et al. Scheme 1. Molecular Structure of PS-PMENMA Block Copolymers Where N ) 1, poly[styrene-b-2-methoxyethyl methacrylate] (PMEMA); N ) 2, poly[styrene-b-2(methoxyethoxy)ethyl methacrylate] (PME2MA); and N ) 3, poly[styrene-b-2-[2-(methoxyethoxy)ethoxy]ethyl methacrylate] (PME3MA)

Figure 1. Schematic illustration of surface segregation of PMENMA (left) and expected structure in water. Solid and dotted curves are PS and PMENMA chains, respectively. White circles represent water molecules.

Hydrophobic interactions between two surfaces in liquids have been studied using several different force-measuring apparatus. Atomic force microscopy (AFM) is one of the methods to measure adhesion forces between probes and surfaces and is frequently employed.14,15 A drawback of using AFM to measure interactions is that probe shapes and hence contact areas are not always welldefined. Probes with colloidal particles have been developed for force measurements; however, the actual contact areas are not always determined by the global curvatures of colloidal particles but by the roughnesses of the particles.16,17 In this paper, we extend the same methodology to poly(styreneb-origoethylene glycol methyl ether methacrylate) (PS-PME NMA), which is the generalization of PS-PME3MA with the different number of EG units, and show that AFM using probes with hydrophobic self-assembled monolayer (SAM) successfully measures the degree of hydrophobic interactions of the surfaces that are modified by spontaneous segregation of the series of PS-PMENMA provided that the radii of the probes and the spring constants of the cantilevers are calibrated for every tip.

Experimental Section Materials. Block copolymers of polystyrene (PS) and oligo (ethylene glycol) methyl ether methacrylates (PMENMA) were synthesized by sequential anionic polymerization. A block copolymer of polystyrene and poly(triethylene glycol mathacrylate) (PTEGMA), which has -OH terminal instead of -OCH3, was synthesized by the anionic polymerization of the triethylene glycol mathacrylate monomer protected with tert-butyldimethylsilane followed by deprotection.18 The details of the synthesis of monomers, polymerization, and characterization have been described elsewhere.18 PMENMA consists of N ethylene glycol (EG) units in the side chains of the methacrylates as shown in Scheme 1. N is the number of EG units in the side chains. The molecular characteristics of PS, PSPMENMAs, and PS-PTEGMA used in this study are summarized in Table 1. The PS-PMENMA block copolymers with approximately the same molecular weights and block fractions were used to contrast the difference in the monomeric structures of the hydrophilic PMENMA blocks. The PS-PMENMA and PS-PTEGMA copolymers were mixed with PS in a range of fractions (5, 10, 15, 20 vol %) and dissolved in toluene (Wako) with 3 wt % solid content. The solutions were (14) Nutt, H. J.; Cappella, B.; Kappl, M. Surf. Sci. Rep. 2005, 59, 1–152. (15) Leite, F. L.; Herrmann, P. S. P. J. Adhes. Sci. Technol. 2005, 19, 365–405. (16) Sindel, U.; Zimmermann, I. Powder Technol. 2001, 117, 247–254. (17) Jarvis, S. P.; Yamada, H.; Yamamoto, S.; Tokumoto, H. ReV. Sci. Instrum. 1996, 67, 2281–2285. (18) Han, S.; Hagiwara, M.; Ishizone, T. Macromolecules 2003, 36, 8312– 8319.

spin-coated onto silicon wafers (Shinetsu Co.) to form approximately 200 nm thick films. The films supported on the silicon wafers were heated up to 150 °C in a vacuum for 24 h for equilibration. X-ray Photoelectron Spectroscopy. XPS Spectra of the polymer blend films were acquired on a PHI Quantum 2000 spectrometer equipped with a hemispherical capacitor analyzer using monochromated X-ray from Al KR. A binding energy scale of the instrument was calibrated by setting Au 4f7/2 to 84.0 eV, Cu 2p3/2 to 932.6 eV, and Ag 3d5/2 to 368.3 eV. A chamber pressure during measurements was maintained at approximately 1 × 10-7 Pa. An X-ray beam operated at 20 W and focused to approximately 100 µm in diameter rastered over 500 µm by 500 µm square area. We obtained sharp C 1s and O 1s peaks without neutralization: a small shift in bonding energy was rescaled using the C 1s peak as a standard to 284.8 eV. High-resolution scans of C 1s and O 1s were acquired with a pass energy of 35.8 eV at a take-off angle of 45°. The atomic fractions were defined by neglecting the presence of hydrogen and were calculated using the atomic sensitivity factors supplied by PHI. Protein Adsorption Measurements. Protein adsorption measurement was carried out using albumin from human serum. Albumin is the most abundant plasma protein in human blood. The specimens of the mixtures of PS-PMENMA or PS-PTEGMA and PS with various fractions spin-coated on silicon wafers were immersed in a phosphatebuffered saline solution of albumin (460 mg/mL, Sigma-Aldrich, U.S.A.) at 37 °C for 2 h. The specimens were dipped into a cup of ultrapure water (Milli-Q) for 5 s and 3 times. XPS was used to estimate the amounts of proteins remaining on the surfaces of specimens. N 1s and C 1s intensities were measured to estimate the atomic fraction of nitrogen that is proportional to the amount of albumin on the surface. Adhesion Force Measurements. Force curves between octyltrichlorosilane (OLTS) coated cantilevers and polymer surfaces in water were obtained with an atomic force microscope (SPA300HV, Seiko Instrument). A custom-designed fluid cell was mounted on SPA300HV for the measurements in fluids. Milli-Q water with a resistance of 18.0 MΩ cm was used for the force measurement. AFM tips were immersed in a 5% toluene solution of OLTS which forms monolayers on the tips. Dehydrated toluene and OLTS were purchased from Wako and Aldrich, respectively, and were used as received. Silicon nitride rectangular cantilevers (OMCL-RC800PSA, Olympus Co.) were cleaned with a UVO cleaner 342–101 (Jelight, Inc.) for 15 min and were dipped into the OLTS solution for 30 min. Then, the OLTS coated cantilevers were rinsed with toluene and dried in air. The silicon wafers, which will be used for control measurements, were also treated by the same method. Calibration of Spring Constants. The silicon nitride tips supplied from Olympus have a typical spring constant of 0.76 Table 1. Molecular Characteristics of PS, PS-PMNMA, and PS-PTEGMA polymer

PDI

Mn (PS)

Mn (PMENMA)

fwt (PMENMA)

PS PS-PMEMA PS-PME2MA PS-PME3MA PS-PTEGMA

1.08 1.05 1.07 1.06 1.04

13 800 12 600 9300 9500 9700

0 11 400 9700 8200 7700

0 48 51 43 46

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Figure 2. Example of the shape of the tip used for a force measurement. The estimated profile (O) and the best fit (-) with a circle of radius 15 nm for the top 5 nm of the tip.

N/m. We, however, found significant variation of spring constants among cantilevers even in the same lot. Therefore, we calibrated the spring constant of every cantilever with the reference cantilever that is made of silicon and is precalibrated by the manufacturer (Model: CLFC-NOBO, Part: 00–103–0994, Veeco Inc., spring constant, Kref, of 0.349 N/m). The calibration method is the following: The deflection of a cantilever under test, δtest, was measured on a silicon wafer. The total deflection, δtot, was measured by pushing the cantilever under test onto the free end of the reference cantilever. The spring constant of the cantilever under test (Ktest) can be calculated as follows:

Ktest ) Kref (δtot - δtest)/(δtestcos θ)

(1)

θ is the angle between the cantilever under test and the silicon wafer (or the reference cantilever) and is 11° for our setup. We obtained the values of Ktest ranging from 0.493 to 1.497 N/m even in the same lot of cantilevers. The details of this calibration method has been described elsewhere.19 Estimation of Radii of Tips. TipCheck (Aurora Nano Devices Inc.) and the software to reverse-image tip shapes from the scanned images (scanning probe image processor 4.2, Image Metrology) were used to estimate the radii of tips. The greater the scanned TipCheck image looses the resolution, the blunter the tip is. An example of the tip profiles is shown in Figure 2. The top 5 nm part of the tip was fit with a radius R ) 15 nm. An example of TipCheck images and the detail of the method to estimate the tip radii are found in Supporting Information. The tip radii were measured before and after the force measurement, and the values of R and the force measurement results were accepted only when the radii agree each other. Radii before and after a force measurement often disagree apparently because of the hydrophobic tips attracting contaminants in water during the force measurement.

Results and Discussions PS-PMENMA block copolymers were mixed with homo-PS, dissolved in toluene and spin-coated onto silicon wafers. After thermal annealing at 150 °C for 24 h, the surface compositions were measured using XPS. The oxygen atomic fractions of the series of mixtures are plotted against the volume fractions of the block copolymers in the mixtures. We observed the surface excess of oxygen as shown in Figure 3. The source of O 1s photoelectron is the side chains of PMENMA. Similar to the segregation of PS-PME3MA to the surface of PS, which has been reported (19) Tortonese, M.; Kirk, M. Micromachining and Imaging 1997, 3009, 53– 60.

Figure 3. Oxygen atomic fractions measured by XPS plotted against volume fractions of PS-PMENMA block copolymers in homo-PS. The solid line represents the average oxygen atomic fraction in the hypothetical ideal homogeneous mixtures.

previously,20 preferential segregation of PMENMA is evidently observed irrespective of the number of EG units (N) in the side chains. Such segregation is caused by the apparent low surface energy of the hydrophobic methyl termini of side chains pointing out to and covering the surface as previously probed by sumfrequency generation spectroscopy.13 In order to clarify the effect of methyl termini on the surface segregation, PS-PTEGMA, which has 3 units of EG terminated by -OH instead of -OCH3, was mixed into PS. The surface oxygen atomic fraction was measured by XPS and overlaid in Figure 3. No preferential segregation of PTEGMA is observed; moreover, the atomic fractions of oxygen of the mixtures of PS-PTEGMA and PS are even lower than the average value of the ideal homogeneous mixtures. Therefore, PTEGMA is repelled from the surface and embedded in the bulk to avoid the excess surface energy. We clearly reconfirm that the lower surface energy of the methyl termini of the side chains in addition to the increasing entropy of the flexible side chains drive the hydrophilic blocks (polymers) to the surface of PS in a hydrophobic environment such as air or a vacuum. Surface segregation of a particular component in miscible and immiscible multicomponent systems is a general phenomenon, and the surface segregation of a chain end group has been studied widely.21–23 Using the Scheutjens-Fleer lattice self-consistent mean-field model and Monte Carlo simulation, Koberstein and co-workers found that end group segregation is primarily controlled by the surface energetic differences between the chain ends and the chain middle moieties.21 They found that a functional group that has a lower surface energy than its polymer backbone will segregate to the surface in order to decrease the overall surface energy and free energy of the system.22 The presence of even minute concentrations of certain additives in the bulk polymer can dominate the properties of the surface of the material as a result of surface-segregation effects.24 In our case, hydrophilic PME3MA block prefer to stay at the surface of air. The terminal methyl groups of the side chains instead of chain ends of the backbone are attracted to the air surface to minimize the free energy of the system. Such segregation driving force is even (20) Oyane, A.; Ishizone, T.; Uchida, M.; Furukawa, K.; Ushida, T.; Yokoyama, H. AdV. Mater. 2005, 17, 2329–2332. (21) Jalbert, C.; Koberstein, J. T.; Hariharan, A.; Kumar, S. K. Macromolecules 1997, 30, 4481–4490. (22) O’Rourke-Muisener, P.; Koberstein, J. T.; Kumar, S. Macromolecules 2003, 36, 771–781. (23) O’Rourke-Muisener, P.; Jalbert, C. A.; Yuan, C.; Baetzold, J.; Mason, R.; Wong, D.; Kim, Y. J.; Koberstein, J. T.; Gunesin, B. Macromolecules 2003, 36, 2956–2966. (24) Koberstein, J. T. Mater. Res. Soc. Bull. 1996, 21, 19–23.

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Figure 4. Nitrogen atomic fraction measured by XPS plotted against volume fraction of PS-PMENMA copolymers in the mixture with PS. Nitrogen atomic fraction (the amount of albumin adsorbed) decreases sharply with increasing volume fraction of PS-PMENMA with N g 2.

stronger in our case because of the greater number of side chain termini. It should be noted that polymethyl methacrylates (PMMA), which correspond to PMENMA with N ) 0, do not segregate to the surface in a PS matrix.25 Therefore, methyl termini of the side chains are not enough to drive the surface segregation. It is apparently important that the flexible PMENMA (N g 1) side chains have more conformational freedom near the surface and thus have greater conformation entropy. Mayes and co-workers theoretically26 and experimentally27 found the entropically driven segregation of a higher-energy polymer additive to the surface of a polymer blend due to the highly branched nature of the polymer additive. This entropic contribution also plays a part of the surface segregation of PMENMA onto PS surfaces. We have previously reported that the surfaces spontaneously modified by PME3MA blocks become hydrophilic upon contact with water and effectively hinder the adsorption and adhesion of proteins, cells, and platelets.20 We conducted the same protein adsorption measurements on the series of PSPMENMA-modified surfaces as described in Experimental Section. The amount of albumin adsorbed on the polymer surfaces was quantified by the nitrogen atomic fraction measured by XPS after the specimens were rinsed with pure water and dried. The nitrogen atomic fractions as a function of volume fraction of copolymers are plotted in Figure 4. The PS surface shows 6 atom % nitrogen indicating that significant adsorption of albumin occurred. The mixture of PS-PTEGMA and PS shows almost the same amount of albumin adsorption as that of PS, which is consistent with the fact that PS-PTEGMA does not segregate to the surface and thus the surface is nearly pure PS. With increasing volume fraction of PS-PMEMA albumin adsorption decreases down only to half-the amount of the adsorption of the PS surface. PS-PME2MA and PS-PME3MA show even more significant reductions of albumin adsorption only with 5 vol % additions of the copolymers. Our previous work on the solubility of PMEMA, PME2MA, and PME3MA homopolymers revealed that PME2MA and PME3MA are watersoluble polymers with lower critical solution temperature while PMEMA is not water-soluble.28 Apparently, solubility of (25) Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 1996, 29, 3232– 3239. (26) Walton, D. G.; Mayes, A. M. Phys. ReV. E 1996, 54, 2811–2815. (27) Walton, D. G.; Soo, P. P.; Mayes, A. M.; Sofia Allgor, S. J.; Fujii, J. T.; Griffith, L. G.; Ankner, J. F.; Kaiser, H.; Johansson, J.; Smith, G. D.; Barker, J. G.; Satija, S. K. Macromolecules 1997, 30, 6947–6956. (28) Ishizone, T.; Han, S.; Hagiwara, M.; Yokoyama, H. Macromolecules 2006, 39, 962–970.

Zhang et al.

Figure 5. Example of force curves measured between OLTS coated tips and polystyrene films in water. The pull-off force, P, is defined by the difference between the minimum force and the baseline after complete detachment.

PMENMA block to water plays an important role in preventing adsorption of proteins. Surface rearrangement is often observed in many functional polymer surfaces,24,29–31 including the surface of biomaterials.32 In our case, the pseudohydrophobic methyl surface barely covers the hydrophilic ethylene oxide side chains and quickly rearranges to become a hydrophilic surface in a water environment as observed in the large hysteresis of advancing and receding water contact angles.13 The large number of methyl chain ends enhances such properties similar to the surfaces of dendrimers33 or branched molecules.26,27 We proposed that the swollen PME3MA blocks in water work as a surface polymer brush similar to PEG chains attached to surfaces. The EG side chains themselves are not long enough to work as a brush layer that prevents the adsorption of proteins; however, the whole block can work as a brush layer if the whole block is water soluble.13,34 Such polymer brushes are expected to screen out the hydrophobic interactions that initiate the adsorption and adhesion in biological interfaces. The study of the interfacial structure between the PS-PMENMA modified surfaces and (deuterated) water by neutron reflectivity is undergoing to prove such hypothesis. In this report, we directly measured the strengths of hydrophobic interactions in water between the PS-PMENMA modified surfaces and the hydrophobic AFM probes that are modified by self-assembled monolayer of alkyl chains. The detail of the experimental setting and treatment of the cantilever has been given in the experimental section. We first show an example of force curves of octyltrichlorosilane(OLTS) coated tips in contact with polystyrene thin films in water. A force curve of an OLTS coated tip with a curvature of 30 nm in contact with PS in water is plotted in Figure 5. The cantilever was pushed onto the surface until the force reached some value in the range of 6.6–15.2 nN and then was retracted with the same speed (30 to 120 nm/s). Neither the variation of forces nor the approach and pull-off speeds affected the results. As seen in Figure 5, negative (pull-off) forces are necessary to (29) Pike, J. K.; Ho, T.; Wynne, K. J. Chem. Mater. 1996, 8, 856–860. (30) Chapman, T. M.; Benrashid, R.; Marra, K. G.; Keener, J. Macromolecules 1995, 28, 331–335. (31) Granville, A. M.; Boyes, S. G.; Akgun, B.; Foster, M. D.; Brittain, W. J. Macromolecules 2004, 37, 2790–2796. (32) Koberstein, J. T. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 2942– 2956. (33) Sheiko, S. S.; Buzin, A. I.; Muzafarov, A. M.; Rebrov, E. A.; Getmanova, E. V. Langmuir 1998, 14, 7468–7474. (34) Andruzzi, L.; Senaratne, W.; Hexemer, A.; Sheets, E. D.; Ilic, B.; Kramer, E. J.; Baird, B.; Ober, C. K. Langmuir 2005, 21, 2495–2504.

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separate the surface of polystyrene and the OLTS tip in contact. We define the pull-off force, P, from the difference between the negative maximum force and the baseline after complete detachment as shown in Figure 5. For contact of a sphere and a flat surface, the JKR theory predicts the work of adhesion in the case that all the bodies are purely elastic and relatively compliant.35–40 In our case, the use of the JKR theory seems to be the most reasonable model since OLTS coated tip, which can be approximated by a hemisphere, is in contact with a flat compliant polymer film. The surface is covered with the PMENMA layer possibly swollen with water, but such a potentially viscous layer is solidly attached to the underlying PS film. The PMENMA layer flows only the distance that is in the same order of the chain dimension but is smaller than the typical radius of a tip. Therefore, the JKR theory is still a valid model for the precision required in our experiments. The JKR theory predicts the pull-off force, P, at which the separation occurs between a sphere with radius R and a plane as in eq 2:

P ) -1.5πRW

(2)

where W is the work of adhesion between a hemisphere and a flat sheet. To check the accuracy of the force measurement and the applicability of the JKR theory, we conducted a control symmetric adhesion measurement between an OLTS-coated silicon wafer and an OLTS-coated AFM tip in water. The measured pull-off force was 9.0 ( 0.2 nN with a tip radius of 19.5 nm. For the OLTS-OLTS symmetric interface, W ) 2γOLTS-w, where γOLTS-w is the interfacial tension between OLTS and water. γOLTS-w is estimated to be 48.9 mJ/m2 using eq 2. Good et al. proposed to split the asymmetric acid–base parts of a bipolar system into separate surface tension components: Lewis acid (electron acceptor) γ+ and Lewis base (electron donor) γ-.41 With that method, the interfacial energy between the solid and a liquid was calculated using eq 3:

γSL ) ((γSLW)0.5 - (γLLW)0.5)2 + 2((γS+γS-)0.5 +

(γL+γL-)0.5 - (γS+γL-)0.5 - (γS-γL+)0.5)

(3)

where S and L denote solid and liquid, respectively. For the case of water (L) and n-octadecyltrichlorosilane (OTS) (S) interface, 42 γLLW ) 21.8 mJ/m2, γL+ ) 34.2 mJ/m2, γL- ) 19 mJ/m2; γSLW ) 23.7 mJ/m2, γS+ ) 0, and γS- ) 0. γSL is calculated to be 51.02 mJ/m2, which is in a good accord with the value of 55.2 mJ/m2 obtained by a previous adhesion force measurement using AFM tips and substrates modified by alkyl chains.43–45 Our estimated γOLTS-w is also close to the calculation and the value from the literature; therefore, it is reasonable to assume that the tip is fully covered by the OLTS layer and the measured force is accurate enough to discuss the work of adhesion between the OLTS coated tip and the PMENMA modified surface in water. (35) Shull, K. R. Mater. Sci. Eng. R. 2002, 36, 1–45. (36) Deruelle, M.; Leger, L.; Tirrell, M. Macromolecules 1995, 28, 7419– 7428. (37) Chaudhury, M. K.; Whitesides, G. M. Langmuir 1991, 7, 1013–1025. (38) Silberzan, P.; Perutz, S.; Kramer, E. J. Langmuir 1994, 10, 2466–2470. (39) Tirrell, M. Langmuir 1996, 12, 4548–4551. (40) Schneider, J.; Dori, Y.; Haverstick, K.; Tirrell, M. Langmuir. 2002, 18, 2702–2709. (41) Van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. ReV. 1988, 88, 927–941. (42) Peters, R. D.; Yang, X. M.; Kim, T. K.; Sohn, B. H.; Nealey, P. F. Langmuir 2000, 16, 4625–4631. (43) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985, 229, 1088–1089. (44) Xu, G. H.; Ko, H. Chin. J. Chem. Eng. 1999, 7, 345–350. (45) Xu, G. H.; Ko, H Acta Physico-Chimica Sin. 1999, 15, 458–461.

We performed the same force measurements on the PS surfaces spontaneously modified with PS-PMENMA. We observed significantly weaker forces between the hydrophobic probes and the PS-PMENMA modified surfaces. Since the radius of the probe used for the force measurement had a significant impact on the pull-off force, the radii, R, of all AFM tips were determined from reverse imaging of sharp features before and after the force experiments as described in the experimental section. The results of the force measurement were accepted only when the radii before and after the force measurement agreed each other. The pull-off forces, P, are plotted as functions of R in Figure 6 for various volume fractions of PS-PMENMA. Straight lines with zero intersection fit all of the data sets very well. Those linear relations are consistent with the Derjaguin approximation.46–48 Let us calculate the work of adhesion between the PSPMENMA modified PS surfaces and the hydrophobic tips in water. The values estimated from the pull-off forces in Figure 6 and eq 2 are plotted in Figure 7 as a function of PS-PMENMA volume fraction in PS. W is greatly reduced by the spontaneous segregation of PMENMA block to 14–28 mJ/m2 from the value (79 mJ/m2) of the polystyrene surface. At full coverage (20% copolymer content), the values of work of adhesion of PMEMA, PME2MA, and PME3MA decrease down to 20.3 ( 2.3, 16.8 ( 2.4, and 14.2 ( 3.7 mJ/m2, respectively. However, it should be noted that interactions between PMENMA modified surfaces and OLTS coated tips are not purely repulsive but are still weakly attractive. Let us discuss the origin of the adhesion forces acting between OLTS coated tips and polymer surfaces. In case of OLTS modified probes in contact with polymer surfaces in water, the equation is given by

WP-OLTS ) γP-w + γOLTS-w - γP-OLTS

(4)

where P and w denote the polymer and water, respectively. The interfacial energy between OLTS and water γOLTS-w has already been estimated to be 48.9 mJ/m2. The interfacial tensions for PS and polymethylmethacrylate (PMMA) with methyl terminated self-assembled monolayers in the literature42 are γPS-CH3 ) 1.2 mJ/m2 and γPMMA-CH3 ) 1.9 mJ/m2, respectively, which are negligibly small compared with γOLTS-w (48.9 mJ/m2). Let us assume that the interfacial energy between polymer and OLTS, γP-OLTS, is 1.5 mJ/m2 irrespective of the type of polymer instead of completely neglecting it. For the PS surface, γP-w is calculated to be 28.6 mJ/m2, which is a reasonable value considering that PS is less hydrophobic than hydrocarbons (48.9 mJ/m2). However, for the PMENMA covered surfaces, the measured work of adhesion is smaller than γOLTS-w, and γP-w has to be negative to satisfy the equality of eq 4. Therefore, the PMENMA brush not only reduced the interfacial energies between the polymer surface and water but also increased the interfacial energies between the polymer and OLTS in water. Schematic pictures of contacts of tips and PMENMA modified surfaces are shown in Figure 8. The presence of water between the OLTS and the polymer surfaces prevents the OLTS probe from being in full contact (Figure 8a) with the polymer surfaces. The partial contact (Figure 8b) induces a positively large apparent interfacial tension γP-OLTS due to the remaining contact of water with the OLTS coated tip. This is a strong indication of the presence of water-swollen layer on the polymer surface. The values of work of adhesion estimated by the AFM force measurement qualitatively predict the result of protein adsorption. (46) Derjaguin, B. V. Kolloid-Z. 1934, 69, 155–164. (47) Derjaguin, B. V. Trans. Faraday Soc. 1940, 36, 203–215. (48) Roth, R.; Evans, R.; Dietrich, S. Phys. ReV. E: Stat., Nonlinear, Soft Matter Phys. 2000, 62, 5360–5377.

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Figure 6. Dependences of the pull-off forces, P, on the radii, R, of tips for the mixtures of the series of PS-PMENMA copolymers of various fractions. For example, PME2MA20 indicates the mixture of 20 vol % of PS-PME2MA in PS. The standard deviations of the estimated radii and the measured forces are shown as the x and y error bars, respectively.

Figure 7. Work of adhesion, W, between OLTS coated tips and polystyrene surfaces modified with PS-PMENMA copolymers of various volume fractions.

However, protein adsorption measurement is more sensitive to the addition of PS-PMENMA and the number of EG units. During the work of adhesion measurement, a maximum force of ∼10 nN is applied onto an area of ∼1000 nm2; therefore, the pressure applied to the contact area reaches ∼10 MPa. While the sharp AFM tips ensure the single point contact, which is easier to handle theoretically, such a sharp tip increases the pressure significantly and makes the quantitative comparison with protein adsorption results difficult.

Conclusion Atomic force microscopy (AFM) with hydrophobic probes can measure the hydrophobic interactions of polymer surfaces. On the basis of the estimation of the absolute force constants

Figure 8. Schematic illustration of the moment of pull-off. (a) Full contact between the PMENMA blocks (black curves) and the OLTS coated tip (hemisphere). (b) Partial contact between the PMENMA blocks and the OLTS coated tip. When the PMENMA blocks are highly swollen, the water molecules (gray circles) impede the contact between the polymer and the OLTS coated tip. With increasing hydrophilicity of PMENMA, the OLTS is in contact with water even before detachment. The images a and b represent the PMEMA modified and the PMNMA (N g 2) modified surfaces, respectively.

Reduced Hydrophobic Interaction of PS Surfaces

and tip curvatures of AFM cantilevers, the pull-off forces of the hydrophobic probes from PS-PMENMA modified polystyrene (PS) surfaces in water were determined. The work of adhesion was obtained according to the JKR theory. The hydrophobic interactions are greatly reduced with the segregated PMENMA blocks. For the series of PS-PMENMA block copolymers (N ) 1, 2, and 3), the work of adhesion decreases with the increasing number of ethylene glycol units (N). With increasing water solubility of PMENMA by increasing the number of EG units in the side chains, the work of adhesion is reduced (PMEMA > PME2MA ) PME3MA), which qualitatively agrees with protein adsorption results.

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Acknowledgment. We thank Ayako Oyane for the protein adsorption measurements, and Kei-ichi Akabori for estimation of radii of tips. This study is supported in part by KAKENHI (Grant-in-Aid for Scientific Research) on Priority Area “Soft Matter Physics” from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Supporting Information Available: Estimation of tip radius by reverse imaging. This material is available free of charge via the Internet at http://pubs.acs.org. LA703934U