Reduction of Friction at Oxide Interfaces upon Polymer Adsorption

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Reduction of Friction at Oxide Interfaces upon Polymer Adsorption from Aqueous Solutions Xiaoping Yan,† Scott S. Perry,*,† Nicholas D. Spencer,‡ Ste´phanie Pasche,‡ Susan M. De Paul,‡ Marcus Textor,‡ and Min Soo Lim† Department of Chemistry, University of Houston, Houston, Texas 77204-5003, and Laboratory for Surface Science and Technology, Department of Materials, ETH-Zu¨ rich, Sonneggstrasse 5, Zu¨ rich, CH-8092, Switzerland Received September 24, 2003. In Final Form: November 5, 2003 Reduction of the interfacial friction for the contact of a silicon oxide surface with sodium borosilicate in aqueous solutions has been accomplished through the adsorption of poly(L-lysine)-graft-poly(ethylene glycol) on one or both surfaces. Spontaneous polymer adsorption has been achieved via the electrostatic attraction of the cationic polylysine polymer backbone and a net negative surface charge, present for a specific range of solution pH values. Interfacial friction has been measured in aqueous solution, in the absence of wear, and on a microscopic scale with atomic force microscopy. The successful investigation of the polymer-coated interfaces has been aided by the use of sodium borosilicate microspheres (5.1 µm diameter) as the contacting probe tip. Measurements of interfacial friction as a function of applied load reveal a significant reduction in friction upon the adsorption of the polymer, as well as sensitivity to the coated nature of the interface (single-sided versus two-sided) and the composition of the adsorbed polymer. These measurements demonstrate the fundamental opportunity for lubrication in aqueous environments through the selective adsorption of polymer coatings.

1. Introduction The reduction of interfacial friction and the control of interfacial wear are critical to the continued and successful operation of all moving mechanisms. Many common mechanisms are lubricated by hydrocarbon oils which serve to lower the shear strength of an interface and physically prevent the contact of surfaces in motion, thereby reducing wear. While this approach has been highly refined and has resulted in significant increases in performance specifications, alternative approaches are needed for many advanced technologies that inherently prevent the use of such lubricants. Examples of these technologies include microelectromechanical (MEM) devices, mechanisms developed for some aerospace applications, and biomedical implants used for joint repair. Research in the fields of hard coatings, vapor phase lubrication, self-assembled monolayers, and polymeric coatings has begun to address these needs. Among the alternative approaches, lubrication in aqueous solutions has received recent attention due to the need to solve tribological problems in biomedical applications, microfluidic MEMs, and advanced steam engines. Water-based lubrication approaches also offer the potential for reduced environmental impact. A specific approach to lubrication in aqueous solutions has involved the introduction of boundary layers of surfactant-like species to the interface.1 In particular, this has been demonstrated with a number of polymer “brush” structures which involve components that interact favorably with both the surface and aqueous solution, thereby altering the surface composition (through coating) and the interfacial interactions during contact and sliding.2-5 This report describes experiments that explore the frictional properties of such interfaces under * Corresponding author. E-mail: [email protected]. Fax: (713) 7432709. † University of Houston. ‡ ETH-Zu ¨ rich. (1) Fundamentals of Friction: Microscopic and Macroscopic Progress; Singer, I. L., Pollock, H. M., Eds.; Kluwer: Dordrecht, 1992.

aqueous solutions, specifically addressing the influence of polymer composition on adsorption and interfacial shear. Poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) is a member of the family of polycationic copolymers that have been shown to chemisorb on negatively charged surfaces, including various metal oxides surfaces, providing a high degree of resistance to protein adsorption.6-8 As a result, PLL-g-PEG modified surfaces have received attention with respect to a variety of applications involving sensors in bioaffinity assays and coatings for bloodcontacting biomedical devices.9,10 In addition, the adsorption and wetting properties of PLL-g-PEG offer the potential for significant reductions in interfacial friction at metal oxide interfaces in aqueous environments. The general structure of PLL-g-PEG is depicted in Figure 1a. It consists of a polylysine backbone with multiple poly(ethylene glycol) side chains grafted to the backbone via the amine groups of some of the lysine side chains. As schematically shown in Figure 1b, PLL-g-PEG adsorption occurs through the electrostatic attractions between a negatively charged surface and the multiple positive charges of the PLL backbone (-NH3+).6 Adsorbed polymer mass and architecture have been characterized quantitatively using optical waveguide lightmode spectroscopy (OWLS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS), respectively, for polymers such as PLL(2) Klein, J.; Kumacheva, E.; Mahalu, D. Perahia, D.; Fetters, L. J. Nature 1994, 370, 634. (3) Kreer, T.; Mu¨ser, M. H.; Binder, K.; Klein, J. Langmuir 2001, 17, 7804. (4) Raviv, U.; Tadmor, R.; Klein, J. J. Phys. Chem. B 2001, 105, 8125. (5) Grest, G. S. Phys. Rev. Lett. 1996, 76 (26), 4979. (6) Kenausis, G. L.; Vo¨ro¨s, J.; Elbert, D. L.; Huang, N.; Hofer, R.; Ruiz-Taylor, L.; Textor, M.; Hubbell, J. A.; Spencer, N. D. J. Phys. Chem. B 2000, 104, 3298. (7) Huang, N.; Michel, R.; Voros, J.; Textor, M.; Hofer, R.; Rossi, A.; Elbert, D. L.; Hubbell, J. A.; Spencer, N. D. Langmuir 2001, 17, 489. (8) Elbert, D. L.; Hubell, J. A. Chem. Biol. 1998, 5, 177. (9) Ruiz-Taylor, L. A.; Martin, T. L.; Wagner, P. Langmuir 2001, 17, 7313. (10) Huang, N.; Vo¨ro¨s, J.; De Paul, S. M.; Textor, M.; Spencer, N. D. Langmuir 2002, 18 (1), 220.

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Figure 1. (a) Schematic structure of a PLL-g-PEG copolymer consisting of a polylysine backbone and grafted poly(ethylene glycol) side chains. In this scheme, x represents the fraction of the lysine units bearing grafted PEG side chains. (b) PLL-g-PEG adsorbs onto a SiO2 surface at pH values above 2.0 through the electrostatic attraction between the negatively charged surface and the positively charged PLL backbone, leaving PEG chains to extend away from the surface. The schematic representation illustrates the brushlike character of the adsorbed species.

(20)-g-PEG(1), PLL(20)-g-PEG(2), and PLL(20)-g-PEG(5) possessing different grafting ratios (including the ones used in this work).11 In this study, interfacial friction measurements have been conducted in aqueous solutions for the contact of oxidized Si(100) substrates and silicate microspheres mechanically attached to atomic force microscopy (AFM) cantilevers. A series of PLL-g-PEG polymers have been investigated with respect to the specific influence of PEG chain length on adsorption and friction properties. These measurements have demonstrated that the adsorption of PLL-g-PEG polymers onto metal oxide surfaces strikingly reduces the interfacial friction measured in aqueous solutions and that interfacial forces are dependent on the specific composition of the polymer coating. 2. Experimental Section 2.1. Synthesis of PLL-g-PEG. In this study, polymer substrates are designated as PLL(x)-g[y]-PEG(z), where the (11) Pasche, S.; De Paul, S. M.; Vo¨ro¨s, H.; Spencer, N. D.; Textor, M. Langmuir, submitted.

copolymer consists of a PLL backbone of molecular weight (MW) x kDa and grafted PEG side chain of molecular weight z kDa with a grafting ratio of (lysine-mers)/(PEG side chains) of y. PLLg-PEG copolymers were synthesized according to a previously described method.6,10 Briefly, poly(L-lysine) hydro-bromide (MW ) 10-30 kDa (average MW ) 20 kDa); Sigma, St. Louis, MO) was dissolved in 50 mM sodium borate buffer solution, followed by filter sterilization of the solution (0.22 µm pore-size filter). Then the N-hydroxysuccinimidyl ester of methoxypoly(ethylene glycol) propionic acid (SPA-PEG, Shearwater Polymers, Inc., U.S.) was added to the dissolved PLL. The reaction was allowed to proceed for 6 h at room temperature, after which the reaction mixture was dialyzed (SpectraPor, MW cutoff size of 6-8 kDa; Spectrum, Houston, TX) against deionized water for 48 h. The product was freeze-dried and stored at -20 °C. By varying the molecular weight of the starting material (SPA-PEG) and effectively controlling the reaction process, a series of PLL-gPEG graft copolymers with varying PEG side-chain lengths were prepared via the method described above. Detailed preparation procedures and analytical information for the products obtained via this method have been reported elsewhere.6,7,10 2.2. Preparation of PLL-g-PEG Coated SiO2 Substrate. Silicon (100) wafers were employed as substrates. Prior to immobilization of PLL-g-PEG onto the surface, the Si(100) wafers

Reduction of Friction at Oxide Interfaces (0.5 cm × 0.5 cm) were treated by the following procedure: sonicated in toluene for 2 min and 2-propanol for 10 min, extensively rinsed with ultrapure water (EM Science, Gibbstown, NJ), dried in a nitrogen flow, and exposed for 2 min to an oxygenplasma PDC-32G (Harrick Scientific Corp., Ossining, NY). The oxidized substrate was immediately transferred to a 1 mg/mL solution of PLL-g-PEG in 10 mM HEPES buffer solution (4-[2hydroxyethyl]piperazine-1-[2-ethanesulfonic acid], pH 5.4) for at least 40 min. Polymer-coated substrates were stored in HEPES solution (in the absence of PLL-g-PEG) until the time of the AFM experiments. At the time of transfer, the immersed substrates were withdrawn from the polymer solution, rinsed with HEPES buffer solution and water to remove free PLL-gPEG, and then dried under a nitrogen flow. 2.3. PLL-g-PEG Coated AFM Microsphere Tips. In this study, AFM cantilevers, modified by the attachment of a sodium borosilicate microsphere of known diameter at the end of the cantilever (Novascan Technologies, Inc., Ames, IA), were employed as the sliding counterface. All the measurements were performed with microspheres 5.1 µm in diameter. For friction measurements involving bare oxide counterfaces, the microsphere tips were first exposed to an oxygen plasma for 15 s. An immersion procedure similar to the preparation of the PLL-g-PEG modified substrates was employed to generate polymer-coated tips. The plasma-cleaned tips were immediately immersed in 1.0 mg/mL solutions of PLL-g-PEG in 10 mM HEPES buffer solutions. Following 40 min of immersion, the modified tip was withdrawn, rinsed with HEPES solution and ultrapure water to remove physisorbed PLL-g-PEG, and then directly used in AFM measurements. 2.4. Friction Measurements with Atomic Force Microscopy. AFM was used to probe both frictional and adhesion forces at the interfaces of polymer-modified SiO2 substrates under liquid environments. The microscope was equipped with a liquid cell/ tip holder (Digital Instruments, Santa Barbara, CA) and controlled by SPM 1000 electronics and software (RHK Technology, Inc., Troy, MI). The microscope makes use of a single-tube scanner and a beam-deflection technique where light from a laser diode is reflected from the back of a microfabricated cantilever onto a four-quadrant photodetector. Greater details of this instrumental design have been reported previously.12,13 Deflection of the cantilever normal to the surface served to monitor surface topography and interfacial adhesion. Torsion or twisting of the cantilever was indicative of frictional forces at the tip-sample interface. Kinetic friction data were acquired by monitoring the lateral deflection of the cantilever as a function of position across the sample surface and the normal applied load. This was accomplished by rastering the sample in a line-scan mode while first increasing and then decreasing the applied load. During this procedure, frictional forces and normal forces were measured simultaneously with a scan speed of 1400 nm/s over a distance of 100 nm. The reported frictional data represent the average of at least six results obtained at different locations across the surface. Valid comparisons of frictional data have been enabled through the use of the same cantilever assembly throughout a set of measurements. Normal loads were limited in order to avoid wear of tip and surface, thus affecting the effective comparison of frictional data. Generally, the measurement of at least one tip-sample combination was repeated at the end of a series to ensure that no significant wear had occurred during the course of measurements. In all experiments, substrates were scanned with respect to a fixed tip position. AFM measurements were all carried out in aqueous HEPES solutions. The composition of the liquid environment encompassing the tip-sample interface was controlled by transferring aliquots of solution in and out of the liquid cell through the use of two 5 mL syringes. For the isoelectric point measurements, the pH and ionic strength of the solution were adjusted using NaCl, NaOH, and HCl to minimize the number of ion species in the solution and to maintain an approximately constant ionic strength. Solution pH values were measured with a VWR Scientific 8000 pH meter incorporating a Gel Triode pH electrode with a Ag/AgCl internal reference. (12) Lee, S.; Shon, Y.-S.; Colorado, R.; Guenard, R. L.; Lee, T. R.; Perry, S. S. Langmuir 2000, 16, 2220. (13) Kim, H. I.; Koini, T.; Lee, T. R.; Perry, S. S. Langmuir 1997, 13, 7192.

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3. Results The tribological properties of PLL-g-PEG-modified surfaces have been studied through a series of friction measurements performed with AFM at the interface of a flat surface and a 5.1 µm diameter microsphere. The sodium borosilicate microsphere, attached to the end of an AFM cantilever, was used in this study in order to avoid substantial deformation of the polymer layer that would occur under the high contact pressures common in the use of standard AFM tips. The composition of the microsphere also enables the facile adsorption of PLLg-PEG onto the probe surface. The following paragraphs describe the details of polymer adsorption, the influence of polymer adsorption on interfacial friction, and the influence of polymer architecture. 3.1. PLL-g-PEG Adsorption. The adsorption of PLLg-PEG onto oxide substrates is known to occur through the electrostatic interaction of the positively charged PLL backbone and a net negative surface charge.6,7 Negative surface charges will exist at oxide surfaces in aqueous solutions at pHs above the isoelectric point (IEP) of the surface. While the IEP of SiO2 is known to be ∼2.0,14,15 the IEP was independently characterized to ensure successful polymer adsorption under neutral pH conditions. Previous studies have reported that the interfacial forces between a tungsten oxide surface and bare or alumina-coated Si3N4 tips greatly depend on solution pH and the respective isoelectric points of the contacting surfaces.14,16-18 In the current study, pull-off forces between SiO2 surfaces and the sodium borosilicate microsphere tip were measured in order to estimate the isoelectric point of the microsphere tip.14 From these measurements, we determined that the microsphere tip has an isoelectric point of pH ∼ 5.0. As a result, the adsorption of PLL-g-PEG onto the surface of the sodium borosilicate microspheres is expected in solutions with a pH greater than 5.0. 3.2. Influence of Polymer Adsorption on Interfacial Friction. Interfacial friction was measured for the sliding contact of the sodium borosilicate microspheres and SiO2 substrates by the procedures previously described. Measurements were performed at bare oxide interfaces, the interface of a coated microsphere and a bare oxide substrate, the interface of a bare oxide microsphere and a coated substrate, and the interface of a coated microsphere and a coated tip. The same microsphere/cantilever assembly was used throughout the measurements; control experiments reproducing entire data sets verified that polymer transfer to the tip did not contribute to the observed results. In this series of experiments, microspheres and substrates were coated with PLL(20)-g[3.5]-PEG(2) and friction measurements were performed in polymer-free 10 mM HEPES buffer solutions. The data in Figure 2 portray distinct differences in the rate of increase in friction with increasing load19 for interfaces, depending on the presence and specific location of the polymer coating. Each of the interfaces in which the substrate has been coated with the polymer exhibited lower frictional forces than those of the bare microsphere/ bare oxide substrate contact measured under HEPES solution. For the interfaces represented in Figure 2, the rate of increase in friction with increasing load is lowest (14) Lin, X.; Creuzet, F.; Arribart, H. J. Phys. Chem. 1993, 97, 7272. (15) Parks, G. A. Chem. Rev. 1965, 65, 177. (16) Lim, M. S.; Perry, S. S.; Galloway, H. C.; Koeck, D. C. J. Vac. Sci. Technol., B 2002, 20 (2), 575. (17) Marti, A.; Ha¨hner, G.; Spencer, N. D. Langmuir 1995, 11, 4632. (18) Ha¨hner, G.; Marti, A.; Spencer, N. D. Tribol. Lett. 1997, 3, 359. (19) Carpick, R. W.; Agrait, N.; Ogletree, D. F.; Salmeron, M. Langmuir 1996, 12, 3334.

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Figure 2. Interfacial friction measured as a function of decreasing load in aqueous buffer solutions (HEPES) following adsorption of PLL(20)-g[3.5]-PEG(2). The measurements have all been performed using the same AFM cantilever assembly modified with a sodium borosilicate microsphere (5.1 µm diameter). Results are reported for the contact of a bare microsphere/bare oxide substrate, coated microsphere/bare oxide substrate, bare microsphere/coated substrate, and coated microsphere/coated substrate as indicated by the figure legend.

for the contact situation in which polymer has been adsorbed on both surfaces. Slight changes in interfacial adhesion, judged through the negative load at which tipsample separation occurs, are also observed. For surfaces coated on only a single side of the interface, these forces are higher for the polymer-coated microsphere. The shape of the friction-load plots (curvature at low loads) further indicates a different degree of deformation of the interfacial contact for the cases where the polymer has not been adsorbed on the substrate. 3.3. Influence of Polymer Architecture on Interfacial Friction. Interfacial friction was measured on three silicon oxide substrates coated with PLL-g-PEG polymers varying in PEG side-chain length. The use of a single sodium borosilicate microsphere throughout the measurements ensured valid comparison of the friction data. The three polymers, PLL(20)-g[4.1]-PEG(1), PLL(20)-g[3.5]-PEG(2), and PLL(20)-g[3.5]-PEG(5), primarily differed only in PEG side-chain length. To avoid artifacts resulting from contamination of the tip, the tip assembly was cleaned after each measurement by rinsing in ultrapure water and subsequent exposure to an oxygen plasma for 15 s. The influence of PEG chain length on the frictional properties of PLL-g-PEG coated silicon oxide and bare sodium borosilicate interfaces immersed in a 10 mM HEPES buffer solution is illustrated in Figure 3. The data exhibit a distinct dependence on polymer architecture. Both the interfacial adhesion and frictional forces are reduced with increasing PEG chain length. Data (open circles) collected for the PLL(20)-g[3.5]-PEG(2) coated surface following the series of measurements match well with the former data (filled circles) collected for this interface with the same cantilever assembly, indicating that neither wear nor contamination of the microsphere contributed to the observed effect. A similar influence of polymer architecture was observed in measurements performed at interfaces composed of two polymer-coated surfaces. As shown in Figure 4, interfacial friction was observed to decrease with increasing PEG chain length. Again, to allow a comparison of the friction data, the same tip was used throughout the measurements. Polymer coatings were removed from the microsphere tip by immersion in HCl aqueous solutions (pH ∼ 2.0) for 20 min, rinsing with ultrapure water, and exposure to oxygen plasma for 15 s. Microspheres cleaned by this procedure

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Figure 3. Interfacial friction measured as a function of decreasing load for the contact of a bare microsphere probe and SiO2 substrates coated with PLL-g-PEG polymers varying in PEG side-chain length (1, 2, and 5 kDa). A single bare microsphere probe was employed throughout the series of measurements; the second set of results, obtained for PEG(2) following the series of measurements, indicate the reproducibility of the approach and verify that changes in the nature of the probe have not influenced the reported trends. Lower interfacial friction is observed with increasing PEG chain lengths.

Figure 4. Interfacial friction measured as a function of decreasing load and polymer architecture for the contact of symmetrically coated SiO2 surfaces and microsphere probes. To employ the same AFM cantilever/microsphere assembly throughout, polymer coatings were removed and then redeposited between each trial. Again, the second set of results obtained for PEG(2) validate the integrity of this approach. The trend of reduced friction with increased PEG chain length is also evident for polymer-polymer contacts.

were recoated with PLL-g-PEG according to the procedures described previously. As before, measurements repeated for the PEG(2)/PEG(2) interface following the series of tip cleaning/coating procedures (Figure 4, open circles) agree well with the former measurement (filled circles) and indicate the validity of this approach. 4. Discussion 4.1. Adsorption Properties of PLL-g-PEG. Previous studies of the adsorption of the same type of PLL-g-PEG polymers onto a number of different oxide surfaces, by means of in situ OWLS, ex situ X-ray photoelectron spectroscopy (XPS), and ex situ ToF-SIMS, have demonstrated that these polymers spontaneously adsorb to negatively charged surfaces and form adlayers that are stable in buffers of the type and ionic strength used in this work.6,11 The electrostatic interaction between polycations of the PLL polymer backbone and a negative

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surface charge leads to strong attraction of the polymer backbone structure and the oxide surfaces under certain solution conditions. Negatively charged surfaces were encountered in solution pHs greater than the IEP of the oxide.16 The isoelectric point is the pH at which a material immersed in the solution has zero net surface charge. Below the isoelectric point, the surface is more fully protonated and positively charged; conversely, the surface is deprotonated at a pH above its isoelectric point and negatively charged as a result. In the present study, the adsorption of PLL-g-PEG onto SiO2 surfaces has been performed from pH 5.4, 10 mM HEPES solutions. As the IEP of SiO2 is ∼2.0,14,15 SiO2 surfaces are negatively charged under these conditions. As a result, strong adsorption of PLL-g-PEG occurred on the SiO2 surface, as indicated by the relative effort required to remove the polymer from the surface throughout these investigations. For the surface of the sodium borosilicate microsphere, the isoelectric point was estimated to be ∼5.0. Thus, the surface of the microsphere tip immersed in HEPES solution also was negatively charged, and PLL-g-PEG adsorption similarly occurred at this surface through electrostatic interactions, as indicated by the interfacial forces experienced upon adsorption. 4.2. Reduction in Friction upon Polymer Adsorption. The data presented in the previous sections portray the significant modification of the tribological properties of silicon oxide through the adsorption of PLL-g-PEG. This modification is evident in the reduction in critical load, that force that must be overcome to separate the surfaces, and in the rate of friction increase with increasing loads. The reduction in critical load, measured as a negative load in the friction-load data obtained as a function of decreasing load, points to a change in area of contact (Ac) due to a change in the energy of adhesion.19 The rate of friction increase will depend on both the area of contact and the interfacial shear strength (Ff ) τAc). Although both will influence the shape of the friction-load curves, it is clear that the drastic reduction in the rate of friction increase with polymer adsorption must involve a reduction in shear strength in addition to a reduction in adhesion energy.19 The reduction of interfacial forces exists for all cases where PLL-g-PEG has been introduced to the substrate as a result of the formation of a boundary layer film. However, the degree of reduction depends on the location of the adsorbed polymer. The data presented in Figure 2 demonstrate that while friction is reduced upon adsorption of PLL-g-PEG onto one surface, the greatest reduction in interfacial friction is realized when polymer is adsorbed on both surfaces, resulting in a polymer-polymer interface. For interfaces consisting of single-sided adsorption, friction is higher for the polymer-coated microsphere tip, as compared to the forces measured between a bare microsphere tip and a coated silicon oxide substrate. Subsequent measurements to be reported elsewhere have demonstrated that this effect arises from the difference in IEPs of the two surfaces and the resulting difference in the extent of adsorption and in the stability of the adsorbed layer. Additional contributions to the relative differences in measured friction may also result from the degree to which the polymer coatings are deformed during sliding; polymer adsorbed on the microsphere tip undergoes continuous deformation during sliding, while polymer adsorbed on the substrate only undergoes local deformation as the tip slides across a given region. Significantly deformed or partially desorbed polymer films would be expected to give rise to higher interfacial forces.

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Frictional forces measured at the polymer-polymer interfaces are substantially lower than those encountered for single-sided adsorption. This significant finding clearly portrays that appropriately solvated polymer-polymer interfaces are characterized by very low shear strengths. These low shear strengths can be understood by considering the architecture of the adsorbed polymer structures in solution. Under the conditions investigated here, the cationic PLL adsorbs to the oxide surfaces through electrostatic interactions, leaving the PEG chains to extend away from the surface into solution. Huang et al.7 have demonstrated that PEG side chains under these conditions extend away from the charged surface to form a flexible, comblike structure. Such structures are facilitated by the excluded-volume effect, that is, the tendency for the PEG chains to be wet by the solvent (water) as much as possible. For PEG/PEG interfaces in aqueous solutions, chainchain interactions across the interface (interpenetration) are mitigated by the presence of a water layer and a significant component of the shear likely occurs within this thin interfacial layer of water. Lower interfacial shear strengths appear to be a common property of well-solvated polymer-polymer interfaces. Klein et al. have reported low friction for the sliding contact of tethered polystyrene chains in toluene, as measured with the surface forces apparatus.3,4,20,21 4.3. Influence of Polymer Architecture. The frictional properties of the solvated, sliding polymer-polymer interface are also found to depend on the specific PLLg-PEG composition. Data presented in Figures 3 and 4 reveal a strong dependence of interfacial friction on the length of the PEG side chains (due to the use of a different cantilever, the data cannot be directly compared to Figure 2). A significant reduction in friction is observed with longer PEG chains for both single-sided and fully coated interfaces. In terms of the polymer structure, previous studies7 have indicated that the polymer film is composed of an interfacial layer of the cationic PLL backbone having a thickness of ca. 0.6 ( 0.2 nm, in intimate contact with the negatively charged oxide surface and poly(ethylene glycol)grafted side chains extending from the surface to form an outer layer of an essentially close-packed, compressed PEG layer possessing a thickness of 1.1 ( 0.3 nm in a “dried” state. Ruiz-Taylor et al.9 also reported a similar structure of biotin-derivatized PLL-g-PEG monolayers on metal oxides as measured by XPS, with the PLL backbone located with the first 1.0 nm on the substrate and with the PEG side chains located another 1.0-1.5 nm from the surface. In a recent paper, adsorbed layers of the same polymers have been investigated quantitatively using OWLS and ToF-SIMS.11 For all three polymers, highly packed PEG brush-type structures were identified with PEG chain densities per nm2 of 0.60 (1 kDa PEG), 0.53 (2 kDa PEG), and 0.27 (5 kDa PEG), corresponding to respective monomer ethylene glycol surface densities of 13.7, 24.4, and 30.9 nm2. Contact-mode AFM images obtained in the present study with both microspheres and conventional tips revealed no evidence of voids or exposure of the underlying substrate, leading us to conclude that island formation is not a predominant quality of the films. From these data, we suggest that the predominant mechanism of friction reduction with increasing chain length entails the arrangement of the PEG comblike structure. Specifically, we propose that longer PEG chains possess the ability to adopt structures that optimize (20) Klein, J. Annu. Rev. Mater. Sci. 1996, 26, 581. (21) Klein, J.; Peraha, D.; Warburg, S. Nature 1991, 352, 143.

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chain-chain and chain-solvent interactions and in turn lower the interfacial frictional forces. Correlations between interfacial friction and molecular organization have been confirmed in a number of self-assembled monolayer systems.22,23 Furthermore, similar arguments have been proposed for the frictional behavior of adsorbed poly(ethylene oxide) layers immersed in toluene.24 The work presented here demonstrates that the modification of oxide surfaces through the adsorption of PEG significantly reduces interfacial friction in an aqueous environment and that the length of the PEG chain influences friction. In related work, we have recently reported that the PLL-g-PEG copolymer functions as an effective boundary lubricant to reduce the interfacial friction in macrotribological measurements.25-27 Trends of decreasing friction with increasing PEG chain length were similarly observed. 5. Conclusion We have presented a systematic study of the frictional properties of oxide surfaces modified through the adsorp(22) Perry, S. S.; Lee, S.; Shon, Y.-S.; Colorado, R., Jr.; Lee, T. R. Tribol. Lett. 2001, 10, 81-87. (23) Lee, S.; Shon, Y.-S.; Guenard, R.; Colorado, R., Jr.; Lee, T. R.; Perry, S. S. Langmuir 2000, 16 (5), 2220-2224. (24) Raviv, U.; Tadmor, R.; Klein, J. J. Phys. Chem. B 2001, 105, 8125. (25) Spencer, N. D.; Perry, S. S.; Lee, S.; Mu¨ler, M.; Pasche, S.; De Paul, S. M.; Textor, M.; Yan, X.; Lim, M. S. Proceedings of the 29th Leeds-Lyon Symposium on Tribology, 2003; pp 411-416. (26) Lee, S.; Mu¨ller, M.; Ratoi-Salagean, M.; Vo¨ro¨s, J.; Pasche, S.; De Paul, S. M.; Spikes, H. A.; Textor, M.; Spencer, N. D. Tribol. Lett. 2003, 15(3), 231-239. (27) Muller, M.; Lee, S.; Spikes, H. A.; Spencer, N. D. Tribol. Lett. 2003, 15 (4), 395.

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tion of water-soluble grafted copolymers (PLL-g-PEG). Microscopic measurements of film formation and interfacial shear have been accomplished through the use of AFM. These measurements have been conducted under aqueous environments and have employed sodium borosilicate microsphere probes, thus allowing polymer adsorption on both counterfaces and the facile investigation of polymer-polymer interfaces. Our results reveal that the adsorption of these polymers significantly reduces friction, with the lowest friction being observed when polymer is adsorbed on both surfaces. In addition, these results indicate that interfacial adhesion and friction can be further modified through control of the polymer composition. An investigation of three PLL-g-PEG polymers, differing only in PEG side chain length, revealed that interfacial forces measured under aqueous conditions are reduced with increasing PEG chain length. Together, the results of this study highlight the opportunity for controlling the tribological properties of interfaces immersed in aqueous solutions through the design and introduction of adsorbed copolymer structures. The relevant structural features of such copolymers include functionalities required to promote both adsorption on the substrate and solvation at the polymer-liquid interface. Acknowledgment. This work has been supported by the U.S. Air Force Office of Scientific Research under Contract No. F49620-02-1-0346. The authors are also grateful for financial support received from the TopNano21 Program of the Council of the Swiss Federal Institutes of Technology (ETH-Rat). LA035785B