Effect of Brush Thickness and Solvent Composition on the Friction

Feb 14, 2011 - Biocompatibles UK Ltd., Chapman House, Farnham Business Park, Weydon ..... a silicon substrate using WVASE32 software (J.A. Woolam Co.)...
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Effect of Brush Thickness and Solvent Composition on the Friction Force Response of Poly(2-(methacryloyloxy)ethylphosphorylcholine) Brushes Zhenyu Zhang,† Andrew J. Morse,† Steven P. Armes,† Andrew L. Lewis,‡ Mark Geoghegan,§ and Graham J. Leggett*,† †

Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K. Biocompatibles UK Ltd., Chapman House, Farnham Business Park, Weydon Lane, Farnham, Surrey GU9 8QL, U.K. § Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, U.K. ‡

ABSTRACT: The frictional properties of poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC) brushes grown from planar silicon surfaces by atom transfer radical polymerization (ATRP) have been characterized using in situ friction force microscopy (FFM). The dry thicknesses of the PMPC brushes ranged from 20 to 421 nm. For brush layers with dry thicknesses greater than ca. 100 nm, the coefficient of friction decreased with increasing film thickness. For shorter brushes, the coefficient of friction varied little with brush thickness. We hypothesize that the amount of bound solvent increases as the brush length increases, causing the osmotic pressure to increase and yielding a reduced tendency for the brush layer to deform under applied load. A comparison of the force-displacement plots acquired for various PMPC brushes under water supports this hypothesis, since a greater repulsive force is measured for thicker brushes. FFM was also used to investigate the well-known co-nonsolvency behavior exhibited by PMPC chains. For a PMPC brush layer of 307 nm dry thickness, the friction force was determined as a function of the volume fraction of alcohol in alcohol/water mixtures. Unlike a previous macroscopic study, a significant increase in the coefficient of friction was observed for ethanol/water mixtures at a volume fraction of 90%. This is attributed to brush collapse due to cononsolvency, leading to loss of hydration of the brush chains and hence substantially reduced lubrication. Force measurements normal to the surface indicate much greater hysteresis between approaching and retraction curves under co-nonsolvency conditions. However, no such effect was observed for 2-propanol/water and methanol/water mixtures over a wide range of volume fractions, in agreement with recent ellipsometric studies of PMPC brushes.

’ INTRODUCTION Anchoring polymer chains to surfaces is an effective way to reduce interfacial friction.1,2 For example, Klein et al.3,4 found that covering surfaces with end-grafted polystyrene in a good solvent reduced friction by two orders of magnitude in comparison with two bare surfaces under a given normal load. Such phenomena have been attributed to the very limited interpenetration between opposing polymer brushes, even at quite high compressions, as suggested by some theoretical studies.4,5 Many experimental6-11 and computer simulation12-14 studies have been performed to investigate the mechanisms of friction reduction by tethering polymer chains onto surfaces. Drobek et al.10 examined the lubrication properties of adsorbed poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) in aqueous buffer solution. It was found that two opposing brushes of PLL-g-PEG were able to slide against each other without measurable friction forces, even at very high applied pressure (∼10 MPa). Owing to its excellent biocompatibility and resistance to protein adsorption and cell adhesion, poly[2-(methacryloyloxy)r 2011 American Chemical Society

ethylphosphorylcholine] (PMPC) has been used widely in biomedical applications.15-19 For example, grafting PMPC chains onto the wearing surfaces of artificial hip joints can significantly reduce osteolysis.20 A recent study of the behavior of PMPC brushes in sliding contacts has confirmed their remarkable lubricating properties: a friction coefficient, μ, as low as 0.0004 was determined at an applied pressure of up to 7.5 MPa using a surface force balance (SFB). This value is smaller than that recorded for any other boundary lubricant system.16 In the present study, friction force microscopy (FFM)21-23 has been used to investigate the frictional properties of PMPC brushes in more detail so as to extend earlier work.15,16,24 FFM is a variant of atomic force microscopy (AFM) in which lateral deflections of the cantilever are measured as it slides across a sample surface.21,22,25,26 Measurements by FFM can yield Received: November 3, 2010 Revised: January 9, 2011 Published: February 14, 2011 2514

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Langmuir quantitative insights into various phenomena, including the effect of varying the surface composition, brush molecular weight, and molecular organization on the mechanical properties of the brush chains.27-29 For example, McNamee et al. studied poly(ethylene glycol) (PEG) brushes.30 They measured greater friction forces for longer brushes, which was attributed to higher degrees of entanglement occurring in the longer brushes. Using a colloid probe, Nordgren and Rutland measured small coefficients of friction for films of poly[2-(dimethylamino)ethyl methacrylate] at low pH, which they attributed to the formation of a repulsive, highly charged, hydrated cushion.11 At high pH, these workers measured a significant increase in the coefficient of friction, which is consistent with brush collapse. Kitano et al.24 conducted a FFM study in which the lubricious properties of three PMPC brushes with mean degrees of polymerization of 50, 100, and 150 were compared. Although the frictional coefficients of the PMPC brush layers were found to be approximately 0.09, 0.06, and 0.03 for a normal load of over 20 nN, no detailed explanation for these observations was offered. The conformations of polymer brushes are influenced by their solvent environment, and this may yield substantial changes in nanoscale friction.31 However, one relatively unusual property of PMPC is its co-nonsolvency behavior: it is soluble in both water and ethanol but becomes insoluble in certain ethanol/water mixtures.32,33 Extensive quantitative data on the effect of solvent composition on brush thickness for PMPC are available from many studies by ellipsometry.34 A polyionic brush may undergo a change in its net charge following a change in pH, resulting in brush collapse.11 However, the origin of the co-nonsolvency effect in PMPC is quite different: while both ethanol and water are, separately, able to solvate the brush, there is a critical concentration at which the polymer is insoluble in a mixture of the two liquids. The charge on the polymer is unchanged (it remains zwitterionic throughout the concentration range). Studies of such effects may yield fundamental insights into mechanisms of macromolecular solvation. Co-nonsolvency effects have been reported for various other polymers in binary solvent mixtures, such as poly(N-isopropylacrylamide) in binary mixtures of water with either methanol, tetrahydrofuran (THF), or 1,4-dioxane;35,36 polystyrene in N,Ndimethylformamide (DMF) and cyclohexane;37 and poly(vinyl alcohol) in water and dimethyl sulfoxide.38 The co-nonsolvency behavior of a lightly cross-linked PMPC hydrogel has been reported by Kiritoshi and Ishihara.39,40 In a recent study,34 it was found that PMPC brushes exhibited very similar (de)swelling characteristics to PMPC hydrogels on systematic variation of the alcohol/water composition. Polymer brushes in good solvents adopt an extended conformation due to the osmotic pressure caused by the high local chain density. The effect of solvent on the chain conformation, and therefore on the tribological properties of surface-bound, brushlike polymer chains, has been studied by various groups.8,31,41 It has been suggested that a high coefficient of friction is associated with the collapsed conformation of polymer chains on the surface, while extended conformations confer a significantly reduced friction force.11 The influence of the solvent environment on the contact mechanics of tip-sample interactions in FFM has also been examined previously for model surfaces.42,43 However, FFM studies of co-nonsolvency effects for polymer brushes immersed in different liquids are lacking. In the present work, interfacial friction and normal force-displacement measurements were conducted using FFM in order to

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systematically examine the effect of brush thickness and solvent environment on the lubricious properties of PMPC brushes.

’ EXPERIMENTAL METHODS Materials. Silicon wafers (Æ100æ orientation, boron doped, 0-100 Ω cm (the dopant is present at very low concentration and does not influence the brush density)) were purchased from Compart Technology (Peterborough, UK). 2-(Methacryloyloxy)ethylphosphorylcholine monomer (MPC, >99% purity) was kindly donated by Biocompatibles UK Ltd. and used as received. Glycerol monomethacrylate (GMA) monomer was kindly donated by Cognis Ltd. (Hythe, UK) and used as received. 2-Bromoisobutyryl bromide (BIBB, 98%), triethylamine (99%), copper(I) bromide [Cu(I)Br, 99.999%], copper(II) bromide [Cu(II)Br2, 99.999%], copper(I) chloride [Cu(I)Cl, 99.999%], and 2,20 -bipyridine (bpy, 99%) were purchased from Sigma-Aldrich UK. All were used as received apart from the triethylamine, which was refluxed over potassium hydroxide, distilled, and stored over fresh potassium hydroxide prior to use. 3-Aminopropyltriethoxysilane (APTES, >98%) was purchased from Fluka and used as received. Methanol, ethanol, and 2-propanol (HPLC grade) were obtained from Fisher Scientific (Loughborough UK) and used as received. Deionized water was obtained using an Elga Elgastat Option 3 system. Preparation of Surface-Initiated Silicon Wafers. To remove any grease or other contaminants, silicon wafers were washed in turn with acetone, 2-propanol, and water. The cleaned substrate was then immersed in a cleaning bath containing a mixture of ammonia solution (28 mL, 35 wt %), hydrogen peroxide solution (28 mL, 30 wt %), and water (142 mL) at 70 C. After 15 min, any contaminants on the water surface were removed by overflowing the bath with deionized water three times. The cleaned substrate was then removed, washed with water, and dried under a stream of nitrogen gas. The silicon wafer substrate was then amine-functionalized by exposure to APTES vapor under reduced pressure (in a glass chamber containing liquid APTES, pumped down to 0.2 mbar and then sealed) for 30 min. The reagent was evacuated, and the sample was annealed for a further 30 min at 110 C. The substrate was then immersed in dry THF (100 mL) under a nitrogen atmosphere. Triethylamine (1.39 mL) was added, followed by BIBB (1.24 mL), and the reaction solution was allowed to stand for 3 h without stirring at 20 C. The substrate was removed, washed with THF, water, methanol, and acetone, and then dried under a nitrogen stream. The final initiator-coated wafer was hydrophobic. The total surface layer thickness (i.e., the native silicon dioxide layer plus the ATRP initiator) was ∼2.2 nm, as judged by ellipsometry. Preparation of PMPC-Coated Silicon Wafers. Methanol and water were deoxygenated using a nitrogen purge for 30 min. MPC (4.00 g, 0.0135 mol, 60 equiv) was added to a flask and placed under nitrogen using three pump-refill cycles. Solvents (2.0 mL of methanol, 2.0 mL of water) were added, and the mixture was stirred to aid dissolution. Meanwhile, the ATRP initiator-functionalized substrate was cut into suitable sizes (ca. 1 cm2) and placed under nitrogen using three pump-refill cycling of up to six tubes in a Radley’s Carousel 12 Reaction Station. Cu(I)Br (32.3 mg, 0.2251 mmol, 1 equiv), Cu(II)Br (15.1 mg, 0.0676 mmol, 0.3 equiv), and bpy (98.7 mg, 0.6319 mmol, 2.8 equiv) were added to the MPC monomer solution, with ultrasonication and stirring being employed to aid dissolution (relative molar ratios were MPC:Cu(I)Br:Cu(II)Br2:bpy = 60:1:0.3:2.8). The reaction solution was injected into the carousel tubes over the ATRP initiator-functionalized substrates and maintained at ambient temperature and pressure without stirring. These polymerizations were terminated at various desired time intervals to produce PMPC brushes of varying thicknesses. The substrates were removed and rinsed with excess methanol, water, and methanol again to remove unreacted MPC monomer and any ATRP catalyst. These substrates were then soaked 2515

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Langmuir in methanol overnight to remove any nongrafted PMPC chains and then dried under a dry nitrogen stream. Preparation of PGMA-Coated Silicon Wafers. Methanol and water were deoxygenated using a nitrogen purge for 30 min. GMA (6.00 g, 0.0374 mol, 60 equiv) was added to a flask and deoxygenated using a nitrogen purge for 30 min. Solvents (3.0 mL of methanol, 3.0 mL of water) were added, and the mixture was stirred to aid dissolution. Meanwhile, the ATRP initiator-functionalized substrate was cut into suitable sizes (ca. 1 cm2) and placed under nitrogen using three pump-refill cycling in a Radley’s Carousel 12 Reaction Station. Cu(I)Cl (61.8 mg, 0.624 mmol, 1 equiv), Cu(II)Br (41.8 mg, 0.187 mmol, 0.3 equiv), and bpy (273 mg, 1.745 mmol, 2.8 equiv) were added to the GMA monomer solution, with ultrasonication and stirring being employed to aid dissolution (relative molar ratios were GMA:Cu(I)Cl: Cu(II)Br2:bpy = 60:1:0.3:2.8). The reaction solution was injected into the carousel tubes over the ATRP initiator-functionalized substrates and maintained at ambient temperature and pressure without stirring. These polymerizations were terminated at different times to produce PGMA brushes of varying thickness. The substrates were removed and rinsed with excess methanol and water and with methanol again to remove unreacted GMA monomer and any ATRP catalyst. These substrates were then soaked in methanol overnight to remove any nongrafted PGMA chains and then dried under a dry nitrogen stream. Ellipsometry. A Jobin Yvon UVISEL spectroscopic ellipsometer was used to determine the mean thickness of both the ATRP initiator layer and also the various PMPC (or PGMA) brushes grown from the silicon wafers. Measurements were recorded at 10 nm wavelength intervals from 300 to 700 nm at a 70 incident angle under ambient conditions. These ellipsometric data were then fitted to a model comprising a PMPC (or PGMA) brush layer of variable thickness on a silicon substrate using WVASE32 software (J.A. Woolam Co.). The refractive index (RI) of the PMPC brush layer is described by the Cauchy approximation (RI = An þ Bn/λ2), where An = 1.4778 and Bn = 0.004 775 μm2 (these two values were calculated by fitting ellipsometric data obtained from thick PMPC brushes). This approach gives PMPC brushes a refractive index of 1.490 at λ = 632.8 nm. The refractive index of PGMA brushes was also calculated using the Cauchy approximation, where An = 1.5113 and Bn = 0.005 261 μm2. This approach gives a refractive index for PGMA brushes of 1.5113 at λ = 630 nm. Softwaresupplied tabulated refractive indices were used for silicon. The actual polymer brush thickness was calculated by subtracting the combined silicon dioxide and initiator layer thickness of 2.2 nm from the fitted layer thickness (n.b. the error incurred by not treating the silicon dioxide and initiator layers separately is negligible for these relatively thick polymer brushes). The mean brush thickness was determined at five random points along a polymer-coated silicon wafer. These data gave an indication of the brush uniformity (typically (8-10%) on such wafers, and these results were combined with the fitting error of ∼1% obtained using the WVASE32 software to give an overall uncertainty for each of the mean brush thicknesses. Friction Force Microscopy. Friction force measurements were performed using a Digital Instruments Nanoscope IV multimode atomic force microscope (Digital Instruments, Cambridge, UK) operating in contact mode using a gold-coated silicon nitride rectangular probe (Olympus, Germany), with a nominal spring constant of 0.05 N m-1. Gold-coated silicon nitride triangular probes (NP-10, Veeco, Cambridge, UK) were used for the measurements in 2-propanol/water mixtures. The spring constants of the cantilevers were calibrated by measuring their thermal spectra, following the method of Hutter and Bechhoeffer.44 Bare gold-coated probes were used throughout the work described here: brushes were not grown from the probes. To coat the cantilevers with gold, an Edwards Auto 306 bell jar vacuum coater system was first used to deposit a 5 nm thick layer of

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Figure 1. Representative friction force as a function of the applied load for 307 nm PMPC brushes immersed in pure water. Inset: schematic illustrating the interaction between a gold-coated probe and a polymer brush layer. chromium, with a deposition rate in the region of 0.05 nm s-1. Following deposition of this adhesion layer, the system was allowed to cool down for ∼20 min prior to deposition of a 20 nm thick gold coating. Measurements were acquired using a liquid cell fitted with a silicone O-ring. FFM measurements were taken from friction loops acquired by obtaining forward-reverse scan cycles along a single line with the microscope employed in “scope” mode. The range of the scan was maintained at 3.0 μm and the scan rate at 3.05 Hz. The friction signal was obtained by subtracting the mean signals in both directions, giving a resultant force that is twice the frictional force. The lateral signal was converted from V to nN by using the wedge method,45-47 in which the cantilever is scanned across a calibration grating (TGF11, MikroMasch, Tallinn, Estonia) and the friction signal is measured as a function of applied load. The refractive indices of the liquids used varied by a negligible amount over the range of compositions explored.48 Force curves were obtained at a minimum of 500 locations on the sample surface for each tip and solvent system. Pull-off forces were then extracted from these curves using Carpick’s Toolbox.49

’ RESULTS Effect of Brush Length on Friction Force of Polymer Brush. Figure 1 shows a typical plot of the variation in the

lateral force with applied load for a PMPC brush layer with a dry thickness of 307 nm following immersion in pure water. There is a linear relationship between the friction force and the load on the normal direction, suggesting conformity with Amontons’ law FF ¼ μFN where FF is the friction force, FN is the load applied perpendicular to the sample surface, and μ is the coefficient of friction. Although the line of best fit does not pass exactly through the origin in Figure 1, the friction force at zero normal load, 20 pN, corresponds to a very small lateral photodetector signal, which probably reflects the small experimental uncertainty in the signal corresponding to zero lateral deflection. Friction measurements were made on PMPC brushes of different thicknesses immersed in either water or ethanol. In this work, the samples are categorized by the thickness of the dry brush film. In liquid, the polymer molecules may remain in a collapsed “mushroom”-type morphology or, if they are well solvated, they may extend into the solvent as brushes. The extent of the swelling of the brush film was expected to vary with the solvent, so the dry thickness was used to identify specimens 2516

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Figure 2. Coefficients of friction for PMPC brushes immersed in water as a function of the mean brush thickness for dry PMPC. The error bars represent the standard (statistical) error of the mean of five repeated measurements. Some error bars were similar in magnitude to the dimensions of the symbols used in the graph.

rather than the thickness in the solvent medium. It is important to stress that, in contrast to brush films formed by grafting a presynthesized polymer to a substrate, the films studied here were grown by surface-initiated ATRP in the absence of any soluble initiator. A direct measurement of the polymer brush molecular weight is thus not possible, unlike the grafting-to approach. Consequently, the degree of polymerization can only be expressed indirectly in terms of the brush thickness, not the molecular weight, although the length of a fully solvated, extended brush molecule will increase with its molecular weight. In our ATRP protocol, the polymer chains grow from initiator molecules that are confined at the surface, and the polymer chain density is thus determined by the surface density of initiator sites. The length of the polymer chains depends on the polymerization reaction time, and the density of polymer chains is thought to be invariant with the polymerization time. Hence the brush density should not change with polymer chain length While this cannot be confirmed directly, close agreement was found between the ellipsometric thickness data reported by Edmondson et al.34 and brush thickness measurements made in the present work following AFM tip-induced scratching. This suggests that brush density does not change with brush thickness because such a change would likely cause a change in the apparent thickness measured by ellipsometry. Figure 2 displays the mean coefficients of friction obtained for PMPC brushes in water as a function of the dry brush film thickness. Each datum is the mean of five separate measurements. The highest coefficients of friction were measured for the thinnest brushes. As the brush thickness increases above 100 nm, the coefficient of friction decreases with increasing brush thickness. For the 421 nm PMPC brush, the coefficient of friction was approximately one-third of the mean value determined for brushes with thicknesses in the range 0-200 nm. For the shorter brushes, the coefficient of friction was independent of brush thickness within experimental error. This is in agreement with the FFM work of PMPC by Kitano et al.,24 in which the friction coefficients of PMPC brushes of different mean degrees of polymerization are very close when the normal load is less than 20 nN. Figure 3 depicts representative approach and retraction components of force-displacement curves acquired using a goldcoated tip for PMPC brushes immersed in water. The curves shown are individual measurements that are typical of behavior

Figure 3. Normal load detected as a function of the z-displacement of the piezoelectric scanner, which presents the interaction between PMPC brushes and the gold-coated AFM cantilever under water. The approach curve (a) represents increasing load, while the retracting curve (b) represents decreasing load.

observed in some 500 repeat measurements acquired at multiple different locations on the sample. Little point-to-point variation was observed in the behavior. The approach curves (Figure 3a) indicate a repulsive force between the brush and cantilever that becomes stronger for thicker brushes; this is attributed to the presence of additional water molecules at the brush surface. Unfortunately, the precise separation distance of the probe and sample surface is convolved with the elastic deformation of the PMPC brush, but the qualitative behavior is clear. In a good solvent, the osmotic pressure within a brush leads to highly extended chains. It has been reported that there are up to 25 water molecules associated with each MPC residue.50-52 Thus, when the highly hydrated PMPC brushes are compressed by an AFM probe, a repulsive force is expected. We suggest that these bound water molecules confer a resistance to deformation of the PMPC chains by the AFM probe during sliding and hence reduce the friction force. The data in Figure 3a also show that the depth of penetration by the probe is substantial. For a modest loading (up to 10 nN), the tip penetrates to a significant depth within the brush film. For the brushes studied here, the swollen thickness was estimated to be ca. 5 times the dry thickness. For the thicker brushes, the depth of penetration of the AFM probe is significantly less than the brush thickness. However, for the 20 nm brush, the depth of penetration corresponds to the full thickness of the brush layer. It is likely that the somewhat different dependence of the coefficient of friction on brush thickness observed for the thinner layers is due to the probe penetrating further into the brush layer, while for the longer brushes, the probe compresses a 2517

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Figure 4. Coefficients of friction, μ, obtained for PMPC brushes immersed in ethanol as a function of the brush thickness.

progressively smaller fraction of the film as the brush thickness is increased. Limpoco et al. reported that polystyrene brushes in toluene (a good solvent) were compressed over substantial distances during loading and also yielded a small coefficient of friction, while in alcohols (poor solvents, in which the brushes were not extended) the film compressed over a shorter distance during loading and yielded a larger coefficient of friction.31 The data shown in Figure 3 are in agreement with this work. Limpoco et al. suggested that the compression observed for the extended brushes arose largely from the reversible displacement of solvent molecules by the probe, an explanation that is also consistent with the data shown in Figure 3. For the thicker PMPC brushes (237, 307, and 402 nm), negligible hysteresis was observed between the approach and retraction curves (Figure 3b). A small amount of hysteresis was observed in the retraction component of the force curves for thinner PMPC brushes (20, 73, and 103 nm), although this was very small indeed even for the 103 nm brushes. For the 20 nm thick brush, a small negative load was measured during retraction of the probe from the surface, indicating that there was a weak adhesive interaction between one or more PMPC chains and the AFM tip. It is suggested that this adhesive force resulted from the more substantial chain deformation because penetration of the brush layer by the probe is essentially complete. Ethanol is also known to be a good solvent for PMPC. The coefficient of friction was thus measured as a function of the brush thickness in ethanol to examine whether a similar trend was observed. The resulting data are shown in Figure 4. The coefficients of friction obtained for the three thinnest brushes were somewhat higher than the corresponding values determined in water and were greater than twice the value measured for the 307 nm brushes. For thicker brushes, the coefficient of friction decreased with increasing brush thickness, in agreement with the data acquired in water. However, this reduction was observed at lower brush thicknesses in ethanol, with the coefficient of friction of the 192 nm brushes being significantly smaller than that determined for the 103 nm brushes. Comparative measurements were made for PGMA brushes immersed in ethanol. Figure 5 shows the coefficients of friction of a range of PGMA and PMPC brushes with mean thicknesses up to 100 nm. For the thinnest brushes, the coefficients of friction were similar for PMPC and PGMA. However, while for PMPC μ varied little in this thickness range, the coefficient of friction for PGMA increased significantly: 60 nm brushes had a coefficient of

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Figure 5. Comparison of the coefficients of friction obtained for PMPC and PGMA brushes immersed in ethanol as a function of brush thickness.

Figure 6. Coefficients of friction obtained for 307 nm PMPC brushes immersed in various ethanol/water mixtures as a function of the volume fraction of ethanol. Some error bars were similar in magnitude to the size of the symbols used in the graph.

friction approximately twice that of 10 nm brushes. Thus, the zwitterionic PMPC brushes exhibit a lubricious character that seems to be qualitatively different from the nonionic, hydrophilic PGMA brushes. Co-nonsolvency Behavior of PMPC Brushes. Coefficients of friction were determined in binary mixtures of water with various lower alcohols (methanol, ethanol, or 2-propanol). While pure ethanol and water are known to be good solvents for PMPC, mixtures of these two liquids are known to exhibit co-nonsolvency behavior. In particular, it is known that a 90/10 v/v binary mixture of ethanol and water is a nonsolvent for PMPC.34,39,40 Extensive ellipsometry studies, allied with other techniques, have demonstrated that a substantial change in the brush conformation occurs at this composition, yielding a film thickness that is much reduced compared to the fully solvated brush.34 This change is best explained by the collapse of the brush structure due to desolvation of the polymer chains. Figure 6 shows the mean coefficients of friction of 307 nm PMPC brushes in ethanol/water mixtures as a function of the volume fraction of ethanol. The coefficient of friction starts to increase at a 70/30 ethanol/water composition, and reaches a maximum at a 90/10 composition, as expected. The maximum value of μ is ∼7 times higher than that determined in pure water, reflecting a very substantial change in the tribological properties of this PMPC brush layer. 2518

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Figure 7. Coefficients of friction obtained for 20 nm PMPC brushes immersed in various ethanol/water mixtures as a function of the volume fraction of ethanol. Some error bars were similar in size to the symbols used in the graph.

For the 90/10 ethanol/water composition, the PMPC chains collapse and their lubricious character is lost. This leads to a very different interaction with the AFM probe, which is able to deform the nonsolvated PMPC chains much more readily, thus yielding a wide range of pathways for energy dissipation through polymer chain deformation and increased plowing. This leads to a concomitant increase in the coefficient of friction. The value of μ obtained for the dehydrated 307 nm PMPC brushes immersed in a 90/10 ethanol/water binary mixture is much larger than that determined for hydrated short PMPC (e.g., 103 nm) brushes, although they are comparable in terms of their physical thickness (the former is collapsed while the latter is swollen). Here the brush thickness is determined by ellipsometry and is equivalent to the distance between the substrate-brush interface and the brush-liquid interface. For collapsed thick brushes, the brush layer contains disordered, entangled chains in which molecular conformational changes may readily be induced during “plowing”; in contrast, the thin, highly swollen brushes retain substantial lubricity because the bound solvent molecules prevent significant deformation of the extended chains and reduce the extent of energy dissipation at a given load. Frictional measurements were repeated for 20 nm PMPC brushes (Figure 7) to examine the generality of the phenomenon. The coefficient of friction increased by a factor of 7 on switching from water to the poorest solvent conditions (90/10 ethanol/ water), in agreement with the behavior reported above for the 307 nm PMPC brushes. For brushes of different thickness, it is not surprising that the coefficient of friction in water is lower than that in ethanol because the PMPC chains are less well-solvated in the latter solvent.34 Typical examples of the retraction component of the forcedisplacement curve for 20 nm PMPC brushes under selected ethanol/water compositions are shown in Figure 8a. A pronounced force minimum was recorded for PMPC brushes immersed in 70/30, 80/20, and 90/10 ethanol/water mixtures. This indicates brush collapse: poorly solvated brushes yield a stronger adhesive interaction with the probe compared to solvated brushes. It is noteworthy that the largest force minimum was measured for the 90/10 ethanol/water mixture, which is consistent with this hypothesis. To understand better the adhesive interaction between a 20 nm PMPC brush and a goldcoated AFM cantilever, we show in Figure 8b a histogram of the force minima recorded for the 20 nm PMPC brush immersed in

Figure 8. (a) Normal load detected as a function of the z-displacement of the piezoelectric scanner under decreasing load, which reveals the nature of the interaction between a 20 nm PMPC brush and a goldcoated AFM probe when immersed in various solvent mixtures. (b) Histogram of the force minima obtained for the same solvent mixtures.

Figure 9. Friction coefficients obtained for 307 nm PMPC brushes immersed in various 2-propanol/water mixtures. Some error bars were similar in size to the symbols used in the graph.

binary solvent mixtures of various compositions. The dramatically greater force minimum obtained for certain binary mixtures (such as the 90/10 solvent composition) correlates well with the observed changes in the coefficient of friction between the AFM cantilever and the PMPC brush. Using the same methodology, the friction coefficient of the 307 nm PMPC brushes was determined in various 2-propanol/ water mixtures, as shown in Figure 9. In contrast to ethanol, 2519

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Figure 10. Friction coefficients obtained for 307 nm PMPC brushes in various methanol/water mixtures.

2-propanol is only a marginal solvent for PMPC.33,40 For water/ 2-propanol mixtures, the maximum μ value was measured in pure 2-propanol. It is likely that this is because the PMPC chains are only weakly solvated by this poorer solvent, leading to the formation of a rather collapsed conformation which results in a higher friction coefficient. It should also be noted that the friction coefficient of the 307 nm PMPC brushes is significantly higher when immersed in pure 2-propanol than that obtained for the same brush immersed in either pure water or pure ethanol. Essentially, the effect of 2-propanol on the frictional coefficient of this thick PMPC brush is broadly consistent with ellipsometry observations,34 whereby the PMPC chains begin to collapse at 50 vol % 2-propanol and are most deswollen at approximately 88-98 vol % 2-propanol. Finally, we examined the behavior for the 307 nm PMPC brushes immersed in various methanol/water mixtures (see Figure 10). Methanol is a more polar solvent than higher alcohols and may interact with PMPC in a similar manner to that of water, as suggested by previous studies.34,40 Therefore, the conformation of PMPC brushes should not change in methanol/water binary mixtures, and indeed co-nonsolvency does not occur over the entire composition range. In this case there is barely any change in the friction coefficient, which suggests that the PMPC chains remain well solvated across a wide range of methanol/ water compositions. Again, comparable results were reported by Edmondson et al.,34 who used ellipsometry to monitor the dimensions of PMPC brushes immersed in various methanol/ water mixtures. The co-nonsolvency behavior of PMPC chains in the form of hydrogels,39,40 colloidal particles,53 and surface-anchored brushes has been previously reported for certain alcohol/water mixtures. In the present work we have utilized FFM to confirm that such behavior directly influences the lubricious character of PMPC brushes, which supports the hypothesis that a high degree of solvation is essential for a low friction coefficient.

’ DISCUSSION The data presented here clearly demonstrate that in a good solvent, such as water or ethanol, PMPC brushes exhibit substantially reduced coefficients of friction. The large difference between the frictional behavior in good and poor solvents is most likely to be due to the high degree of solvation of PMPC in good solvents. Similar results were reported for other polymer brushtype systems in good solvents, such as adsorbed poly(L-lysine)graft-poly(ethylene glycol) copolymers immersed in aqueous

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buffer10 or polystyrene brushes in toluene.3 Klein and co-workers studied the frictional properties of PMPC brushes using surface forces apparatus16 and concluded that the extremely low coefficients of friction recorded for PMPC brushes at pressures of up to 7.5 MPa could be attributed to the exceptionally high level of hydration of the zwitterionic PC group in each MPC residue: in an aqueous environment, the highly hydrated PMPC chains barely perturb the hydrogen-bonding network formed by the water molecules.33,40 The water molecules associated with the PMPC chains are able to rapidly exchange with free water molecules; hence, PMPC brushes are highly lubricious when sheared at rates lower than this water exchange rate.16 We have found that PMPC brushes immersed in aqueous solutions are highly lubricious, with lower coefficients of friction being achieved for longer brushes. This is consistent with previous studies of the effect of chain length on the frictional properties of PMPC brushes. Force measurements normal to the surface support the hypothesis that the very high degree of hydration is the main contribution to the lubricious character of PMPC brushes. Because the amount of solvent molecules bound to polymer chains increases with longer brushes, it is more difficult for the tip to interact with the highly hydrated PMPC chains, leading to a lower frictional coefficient being observed. The frictional properties of PMPC brushes have previously been studied at different length scales. For example, Chen et al.16 used a surface force balance to measure the nanoscale friction between two mica surfaces coated with PMPC brushes, whereas Kobayashi et al.15 investigated the friction behavior of surface grown PMPC brushes on the macroscopic scale by sliding a 10 mm glass sphere on the samples. Kitano et al.24 used FFM to compare the lubricity of PMPC brushes of differing mean degrees of polymerization in both air and water, but no detailed studies have been made to date. Herein we have utilized FFM to obtain quantitative data on the frictional properties of PMPC brushes to investigate the effect of systematically varying the brush thickness and to make a direct comparison with PGMA brushes. The co-nonsolvency behavior of PMPC brushes has been studied by Edmondson et al.34 using ellipsometry. In the present study, varying the alcohol/water composition allows the solvency of brushes to be adjusted, which in turn affects their conformation: the interaction between the AFM probe and the PMPC brushes is much stronger for collapsed brushes compared to highly solvated brushes.

’ CONCLUSIONS For zwitterionic PMPC brushes prepared with a wide range of dry thicknesses, it was found that the friction coefficient is independent of the brush thickness up to a certain limiting thickness. Thereafter, the coefficient of friction decreases with increasing brush thickness. In contrast, the coefficient of friction increased with brush thickness for nonionic PGMA brushes. These results suggest that plowing is reduced for PMPC brushes by the osmotic pressure resulting from extensive brush hydration under good solvent conditions. This conclusion is supported by force measurements normal to the surface, which indicate that greater repulsion and weaker attraction occurs between the AFM probe and relatively thick brushes. FFM has also been used to investigate the co-nonsolvency behavior of PMPC brushes immersed in ethanol/water, 2-propanol/water, and methanol/water binary mixtures. Substantially 2520

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Langmuir increased friction coefficients were observed in 90/10 ethanol/ water mixtures, as confirmed by repeated measurements on PMPC brushes of different mean thickness. Force-displacement experiments suggest much greater adhesion when PMPC brushes experience a co-non-solvency conditions. Similar results were obtained for brushes immersed in a 2-propanol/water mixture. In contrast, no co-nonsolvency behavior was observed for water/ methanol mixtures, which agrees with previous studies in which different techniques were used.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: Graham.Leggett@sheffield.ac.uk.

’ ACKNOWLEDGMENT The authors thank EPSRC (grant EP/039999/1) for financial support. Dr. S. Sugihara is thanked for fruitful discussions. Dr. C. Vasilev is thanked for assistance in the analysis of the force curves. Biocompatibles is thanked for supplying the MPC monomer and for permission to publish this work. ’ REFERENCES (1) Klein, J.; Kumacheva, E.; Mahalu, D.; Perahia, D.; Fetters, L. J. Nature 1994, 370, 634. (2) Raviv, U.; Giasson, S.; Kampf, N.; Gohy, J.-F.; Jer^ome, R.; Klein, J. Nature 2003, 425, 163. (3) Klein, J.; Kamiyama, Y.; Yoshizawa, H.; Israelachvili, J. N.; Fredrickson, G. H.; Pincus, P.; Fetters, L. J. Macromolecules 1993, 26, 5552. (4) Klein, J. Annu. Rev. Mater. Sci. 1996, 26, 581. (5) Wijmans, C. M.; Zhulina, E. B.; Fleer, G. J. Macromolecules 1994, 27, 3238. (6) Currie, E. P. K.; Norde, W.; Cohen Stuart, M. A. Adv. Colloid Interface Sci. 2003, 100-102, 205. (7) Raviv, U.; Tadmor, R.; Klein, J. J. Phys. Chem. B 2001, 105, 8125. (8) Brady, M. A.; Limpoco, F. T.; Perry, S. S. Langmuir 2009, 25, 7443. (9) Tadmor, R.; Janik, J.; Klein, J.; Fetters, L. J. Phys. Rev. Lett. 2003, 91, 115503. (10) Drobek, T.; Spencer, N. D. Langmuir 2008, 24, 1484. (11) Nordgren, N.; Rutland, M. W. Nano Lett. 2009, 9, 2984. (12) Koike, A.; Yoneya, M. J. Chem. Phys. 1996, 105, 6060. (13) Grest, G. S. Phys. Rev. Lett. 1996, 76, 4979. (14) Yin, F.; Bedrov, D.; Smith, G. D. Eur. Polym. J. 2008, 44, 3670. (15) Kobayashi, M.; Terayama, Y.; Hosaka, N.; Kaido, M.; Suzuki, A.; Yamada, N.; Torikai, N.; Ishihara, K.; Takahara, A. Soft Matter 2007, 3, 740. (16) Chen, M.; Briscoe, W. H.; Armes, S. P.; Klein, J. Science 2009, 323, 1698. (17) Lobb, E. J.; Ma, I.; Billingham, N. C.; Armes, S. P. J. Am. Chem. Soc. 2001, 123, 7913. (18) Feng, W.; Zhu, S.; Ishihara, K.; Brash, J. L. Biointerphases 2006, 1, 50. (19) Feng, W.; Zhu, S.; Ishihara, K.; Brash, J. L. Langmuir 2005, 21, 5980. (20) Moro, T.; Takatori, Y.; Ishihara, K.; Konno, T.; Takigawa, Y.; Matsushita, T.; Chung, U.-I.; Nakamura, K.; Kawaguchi, H. Nature Mater. 2004, 3, 829. (21) Overney, R.; Meyer, E. MRS Bull. 1993, 18, 26. (22) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (23) Gnecco, E.; Bennewitz, R.; Gyalog, T.; Meyer, E. J. Phys.: Condens. Matter 2001, 13, R619.

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