Reinforcement of a Surfactant Boundary Lubricant Film by a

The hydrophobic block anchors in the surfactant films, modifying their ..... The σ−V sliding curve includes a stick−slip regime in the high-veloc...
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Langmuir 2008, 24, 1560-1565

Reinforcement of a Surfactant Boundary Lubricant Film by a Hydrophilic-Hydrophilic Diblock Copolymer† C. Drummond, G. Marinov, and P. Richetti* UniVersite´ de Bordeaux 1, Centre de Recherche Paul Pascal, CNRS, UPR 8641 AVenue Schweitzer, 33600 Pessac Cedex, France ReceiVed August 3, 2007. In Final Form: October 26, 2007 The coadsorption from aqueous solutions of an anionic-neutral hydrophilic-hydrophilic diblock copolymer onto a mica-suported surfactant bilayer of a cationic oligomeric surfactant has been investigated. By using an atomic force microscope and a surface forces apparatus nanotribometer, we studied the resulting film morphology, the interactions between two coated surfaces, and the frictional properties of the boundary film. When the coated surfaces were compressed while being fully immersed in an aqueous surfactant solution, the hemifusion of the adsorbed surfactant bilayers could be easily induced. Noticeable friction forces could then be measured between the monolayer-coated surfaces. Coadsorbing poly(acrylic acid)-poly(acrylamide) diblock copolymer with the cationic surfactant changes the cohesion of the adsorbed layers. When the copolymer concentration is sufficiently high, the hemifusion instability of the adsorbed layers can be inhibited, considerably improving its lubricant properties.

Introduction In both natural and engineering applications, the use of surfactants and polymers to reduce or control friction is widespread.1,2 There is a need for the development and understanding of these lubricants that are used under conditions where more common hydrocarbon lubricants are inadequate (e.g., in biomedical applications). Water-based lubricants facilitate the sliding of proximate surfaces through a variety of mechanisms including steric or electrostatic repulsion and surface hydration. Ultimately, a boundary lubricant film should exhibit relatively weaker friction forces in comparison to the nonlubricated system. In typical applications, the smooth sliding of the rubbed surfaces is also desirable in order to avoid high accelerations that might enlarge the rate of energy dissipation, increasing the probability of irreversible wear and surface damage. The investigation of the frictional properties of surfactant and polymeric films has progressed greatly over the past 20 years owing to the application of direct lateral force measurement. Surfactant films deposited or self-assembled from solution have been demonstrated to modify the frictional characteristics of solid substrates. The behavior of deposited surfactant layers under shear in air depends on many parameters such as temperature, pressure, humidity, sliding velocity, and density of the deposited layers3 as well as intrinsic characteristics of the surfactant investigated such as chain length, degree of unsaturation, and endgroup functionality.4 Of perhaps greater utility are the frictional characteristics of surfaces in aqueous environments. It has been shown that charged surfactants that self-assemble on surfaces may be very effective boundary lubricants.5,6 Because of the electrostatic repulsion between the adsorbed layers, the friction force between coated surfaces is extremely low. However, after some shear-induced †

Part of the Molecular and Surface Forces special issue. * Corresponding author. E-mail: [email protected].

(1) Persson, B. N. J. Sliding Friction: Physical Principles and Applications; Springer: Heidelberg, Germany, 1998. (2) Scherge, M.; Gurb, S. N. Biological Micro- and Nano-tribology: Nature’s Solutions; Springer: Heidelberg, Germany, 2001. (3) Yoshizawa, H.; Chen, Y. L.; Israelachvili, J. J. Phys. Chem. 1993, 97, 4128. (4) Lui, Y.; Evans, F.; Song, Q.; Grainger, D. Langmuir 1996, 12, 1235.

disruption of the adsorbed layer takes place, the behavior of the system under shear becomes more complicated. This is particularly so if the adsorbed bilayers hemifuse as a result of shear or compression. Different dynamical regimes may then be observed, depending on the relative velocity of the rubbed surfaces and the experimental conditions.5-7 The general behavior of the hemifused bilayers under shear can then be described in terms of an adhesive-viscous model based on the kinetics of formation and rupture of adhesive links with an added viscous term, as we have reported previously.5-7 The adsorption of polymer lubricants has also been demonstrated to modify the frictional characteristics of solid substrates. Klein et al.8 measured a substantial reduction in the effective friction coefficient when mica surfaces were coated with a polystyrene brush layer in toluene under good solvent conditions. The reduced friction was attributed to a long-ranged entropic repulsion acting to maintain a finite surface separation with a relatively fluid intervening film. Further studies have shown that the shear properties of adsorbed or anchored polymer molecules are strongly dependent on different parameters but first of all on solvent quality.9,10 End-tethered polymer chains are much less effective lubricants when exposed to near-θ solvents whereas charged polymers seem to display superior lubrication properties in water.11 The morphology of the polymer film also plays a fundamental role in the behavior of the adsorbed layers under shear and compression. Whereas nonadhesive smooth layers may oppose an extremely small resistance to sliding, the presence of asperities even on the nanometric scale brings about a frictional resistance to the motion.12 (5) Richetti, P.; Drummond, C.; Israelachvili, J.; In, M.; Zana, R. Europhys. Lett. 2001, 55, 653. (6) Blom, A.; Drummond, C.; Wanless, E.; Richetti, P.; Warr, G. Langmuir 2005, 21, 2779. (7) Drummond, C.; Israelachvili, J.; Richetti, P. Phys. ReV. E 2003, 67, 066110. (8) Klein, J.; Kumacheva, E.; Mahalu, D.; Perahla, D.; Fetters, L. J. Nature 1994, 370, 634. (9) Schorr, P. A.; Kwan, T. C. B.; Kilbey II, S. M.; Shaqfeh, E. S. G.; Tirrell, M. Macromolecules 2003, 36, 389. (10) Granick, S.; Demirel, A. L.; Cai, L. L.; Peanasky, J. Isr. J. Chem. 1995, 35, 75. (11) Raviv, U.; Giasson, S.; Kampf, N.; Gohy, J-F.; Jeroˆme, R.; Klein, J. Nature 2003, 425, 163. (12) Drummond, C.; Rodrı´guez-Herna´ndez, J.; Lecommandoux, S.; Richetti, P. J. Chem. Phys. 2007, 126, 184906.

10.1021/la702392v CCC: $40.75 © 2008 American Chemical Society Published on Web 01/08/2008

Surfactant Boundary Lubricant Film Reinforcement

Fewer studies have addressed the behavior of surfactant/ polymer mixtures as boundary lubricants. In previous studies6,13,14 we demonstrated the anchoring of low-molecular-weight hydrophobic-neutral diblock copolymers on an adsorbed surfactant layer on mica, although the copolymers alone did not adsorb on this substrate. The hydrophobic block anchors in the surfactant films, modifying their morphology and their cohesion under stress. The coadsorption of diblock copolymers on the preadsorbed surfactant bilayer has important implications for the behavior of the surfaces under shear6,14 but does not significantly improve their lubrication proprieties. At low grafted copolymer density, the cohesion of the boundary surfactant bilayers is changed. At high grafting densities, the mixed adsorbed layer is significantly modified compared to bare surfactant bilayers, with many surfactant molecules replaced by diblock copolymer molecules. A short-range attraction emerges between two mixed adsorbed layers that is responsible for a non-negligible shear stress measured between the sheared surfaces. The extra adhesion is ascribed to spatial fluctuations of density with nanodomains bridging the two films either by hydrophobic interaction or confinementinduced phase separation. In the present study, a new class of copolymers has been tested with the aim of improving the cohesion of the surfactant lubricant film and avoiding any systematic desorption. We have chosen an associative copolymer, poly(acrylic acid)-poly(acrylamide) (PAA-PAM) with a PAA polyelectrolyte block of opposite charge to that of the surfactant. To get a fully hydrosoluble system despite the electrostatic association between charged molecules, the second block, PAM, is also chosen to be a hydrophile although it is uncharged. We report here on the structural and functional modifications caused by the coadsorption of PAA-PAM with a cationic trimeric surfactant that self-assembles in the form of bilayers on mica surfaces. We investigate how the copolymer modifies the morphology of the adsorbed layer as well as its cohesion and lubrication properties. To assess the influence of copolymer incorporation, atomic force microscopy (AFM) and surface forces apparatus nanotribometry (SFA-N) have been used: the former was used to monitor the lateral homogeneity and the morphology of the adsorbed layer, and the latter was used to study the interaction force between the mixed layers and their behavior under shear. Experimental Section Materials. 12-3-12-3-12-trimeric (methyldodecylbis[3-(dimethyldodecylammonio) propyl]ammonium tribromide), a cationic oligomeric surfactant with dodecyl ammonium moieties connected at the ammonium groups by propyl chains, was a generous gift from Dr M. In, University of Montpellier, France. This cationic surfactant spontaneously self-assembles as spherical micelles in water solution (cmc ) 0.16 mM15) and adsorbs as flat bilayers on the negatively charged mica surfaces from aqueous solutions. PAA-PAM has been provided by Rhodia and has been used as received. The results presented here have been obtained with a PAAPAM of 3-30K molecular weight (corresponding to about 40 and 400 monomers, respectively). The addition of the hydrophilichydrophilic PAA-PAM copolymer to a cationic surfactant solution induces the formation of submicrometric supramicellar colloids. The core is believed to be a dense assembly of surfactant micelles connected by the negatively charged PAA blocks whereas the corona is a diffuse shell of neutral PAM chains.16 For adsorption on mica, the surfaces were fully immersed in solutions of the copolymer and (13) Robelin, C.; Duval, F. P.; Richetti, P.; Warr, G. G. Langmuir 2002, 18, 1634. (14) Drummond, C.; In, M.; Richetti, P. Eur. Phys. J. E 2004, 15, 159. (15) Zana, R.; Levy, H.; Papoutsi, D.; Beinert, G. Langmuir 1995, 11, 3694.

Langmuir, Vol. 24, No. 4, 2008 1561 the trimeric surfactant of controlled concentration. AFM and SFA-N measurements were always performed with the surfaces fully immersed in the solution after at least overnight adsorption. In the absence of the cationic surfactant, the low-molecular-weight copolymer does not adsorb on the bare mica surfaces and is easily displaced from the contact region under compression. The results that we report here are reproducible over a few consecutive days and in independent experiments. However, the investigated system exhibits some aging effects, as will be discussed below. Methods. Atomic Force Microscopy. The structure of the layers adsorbed at the solid/solution interface was examined using a Digital Instruments NanoScope IIIa Multimode. The solution was held in a standard fluid cell sealed with a silicone O-ring. Both were cleaned by rinsing with ethanol and distilled water and dried using filtered nitrogen. The solid substrate used in all experiments was muscovite mica, which was cleaved using adhesive tape immediately before use. Experiments were performed at room temperature (approximately 22 °C). Millipore water with a conductivity of 18 MΩ cm-1 was used for the solution preparation. The AFM micrographs reported were measured in soft contact mode.17 In this mode, molecular adsorption on both the tip and the substrate allow imaging using electrical double-layer forces. This method enables the generation of a double-layer force map of the adsorbed layer without the tip physically contacting the sample. Typical imaging scan rates varied between 1 and 3 Hz; proportional and integral gains between 1 and 5 were used. The variation of frequency, scan angle, and gains had no effect on the observed morphology. All images are unmodified except for flattening along scan lines. The cantilevers used were standard Si3N4 with sharpened tips (Digital Instruments), with a nominal spring constant of 0.12 N/m and a nominal tip radius of 40 nm, irradiated with ultraviolet light for 30 min prior to use. All of the AFM force curves reported were measured with the same tip to avoid discrepancies due to the variability in the spring constant of the cantilevers and the radius of the tip. They were measured at a frequency of 1.5 Hz and a ramp size of 1 µm, but variations in these parameters do not affect the observed results. Surface Forces Apparatus (SFA-N). An SFA-N18 was used to measure the interaction between surfactant/copolymer-coated mica surfaces. This technique has been extensively described in the past. In brief, two back-silvered molecularly smooth mica surfaces are glued to cylindrically curved silica lenses. With the SFA-N, it is possible to study a single asperity contact, owing to the molecular smoothness of cleaved mica surfaces and the crossed-cylinder configuration used. The separation between the surfaces, D, can be controlled to an accuracy of a fraction of a nanometer by using a piezoelectric nanopositioner. Multiple beam interferometry (MBI) is used to measure D with subnanometric resolution, the local radius of curvature R, and the refractive index of the confined film.19 The force-distance profile between the surfaces is measured by changing the position of the double cantilever spring attached to the lower surface and measuring by MBI the actual variation in the separation between the surfaces. The interaction force, F, is then determined from the deflection of the spring by using Hooke’s law. If a normal force, L, is applied to the surfaces in contact, then the cylinders are elastically flattened at the point of contact. The load-induced contact radius of a few tens of micrometers, Rc, and the area of contact, A, can be directly measured by MBI. We are able to induce a lateral relative motion between the surfaces by voltage-driven bimorph strips attached to the lower surface.20 Shearing cycles are carried out by moving this surface at constant velocity, V, over a certain distance, after which the driving direction is reversed. The upper surface is attached to a vertical double(16) Berret, J.-F.; Vigolo, B.; Eng, R.; Herve´, P.; Grillo, I.; Yang, L. Macromolecules 2004, 37, 4922. (17) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. E.; Hansma, P. K. Langmuir 1994, 10, 4409. (18) Homola, A.; Israelachvili, J. N.; Gee, M. L.; McGuiggan, P. J. Tribol. 1989, 111, 675. (19) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259. (20) Luengo, G.; Schmitt, F. J.; Hill, R.; Israelachvili, J. Macromolecules 1997, 30, 2482.

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Figure 1. AFM study of the adsorbed layers. (a) 1 µm × 1 µm deflection image of a trimer bilayer (b). 1 µm × 1 µm deflection image of a mixed adsorbed layer of trimer and PAA-PAM at Z ) 1/1. (c, d) Normal tip-surface interaction force measured by AFM in solutions at Z ) 0 and 1/1 (c) at different adsorption times and (d) during an approach-separation cycle between the tip and the substrate after 24 h of adsorption at Z ) 0 (×) and Z ) 1/1 (b, O). cantilever spring whose deflection is monitored using strain gauges (SurForce Inc). The friction force, Ff, induced by the relative displacement between the surfaces in contact can then be calculated from the experimentally measured spring force, Fs, obtained from the deflection of this double-cantilever spring, to an accuracy of (5 µN.

Results Solutions. Solutions of different surfactant-copolymer concentration ratios were studied at a fixed surfactant concentration of 8 cmc. The same surfactant concentration has been used throughout the study. Increasing the number of charged monomers per surfactant polar head, Z, does not seem to greatly modify the size of the supramicellar aggregates in solution when Z is varied from 1/10 to 1/1. Over this range, the mean hydrodynamic diameter of the aggregates is found to be 90 ( 10 nm as measured by dynamic light scattering. Atomic Force Microscope: Morphology of the Adsorbed Layer and Normal Interaction Forces. Figure 1 shows AFM deflection micrographs and force profiles measured for the mixed 12-3-12-3-12/PAA-PAM system for different copolymer concentrations. With no added PAA-PAM, the interaction force profile shows the hard wall repulsion at tip-substrate separations between 2 and 3 nm characteristic of an intercalated bilayer resistant to compression by the tip, preceded by long-range doublelayer repulsion at larger separations. The measured separation at breakthrough of 2.2 ( 0.3 nm is consistent with AFM results reported elsewhere.21 In the presence of PAA-PAM, for Z between 1/10 and 1/1, the main trends in the force profiles are preserved. However, the compression barrier for the breakthrough of the adsorbed films increased with the amount of copolymer coadsorbed and the adsorption time. It becomes increasingly difficult to pierce the self-assembled layer with the AFM tip, providing clear evidence of the copolymer-induced enhanced cohesion of the mixed layer. As illustrated in Figure 1d, the tip-substrate interaction force profile eventually becomes reversible with no adhesion observed: the AFM tip fails to penetrate the adsorbed layer at the maximal normal forces applied. (21) Manne, S.; Gaub, H. E. Science 1995, 270, 1480.

Figure 2. Normal force profile between two mica surfaces measured by SFA-N in solutions of (a) trimer at 8 cmc and (b) trimer and PAA-PAM at different Z ratios. (For clarity, only the results measured during the approach of the surfaces are represented in b.)

AFM images were obtained at weak normal forces corresponding to the compression barrier but well below the rupture by the tip of the adsorbed layer. The adsorbed layers are seemingly smooth on the micrometric scale with a rougher aspect in the presence of the copolymer in solution. A typical rms roughness of 0.1 nm was determined in a 1 µm × 1 µm height image of the adsorbed surfactant bilayer, which can be compared with the typical roughness of 0.05 nm measured on molecularly smooth mica surfaces and with the typical rms roughness value of 1 nm measured in the presence of PAA-PAM at Z ) 1/1. However, we do not observe any adsorption of supramicellar aggregates even for the highest copolymer concentration investigated, Z ) 1/1. Surface Forces Apparatus-N: Normal Interaction Forces. Interaction force curves between mica surfaces immersed in 123-12-3-12/PAA-PAM solutions were measured by SFA-N. Unlike the AFM, in this experiment the interaction force between identical mica surfaces is measured under symmetric conditions. Representative curves of the measured interaction force normalized by the local radii of curvature of the mica surfaces are presented in Figure 2. In general, the force profile between the surfaces immersed in a pure solution of 12-3-12-3-12 at Z ) 0 (Figure 2a) is similar to those reported previously for other bilayer-forming surfactants when the bulk concentration is above the cmc. At large separation, electrostatic repulsion between the charged bilayers is observed. When the adsorbed bilayers are approached at less than 8 nm, a hydration force barrier that deviates from the DLVO theory predictions can be observed. Under greater pressure, a cohesive instability is induced between the two compressed bilayers. They hemifuse by “jumping” from 6.5 to 3.5 nm into monolayermonolayer contact, expelling from the contact zone the outer monolayer of each adsorbed bilayer. Hemifusion brings the two adsorbed monolayers into adhesive (hydrophobic) contact. Indeed, a pull-off force is measured upon decompression. The break-

Surfactant Boundary Lubricant Film Reinforcement

through due to hemifusion is easily achieved, underlying the weak cohesion of the adsorbed bilayers at room temperature. Increasing the copolymer concentration does not significantly reduce the magnitude of the long-range electrostatic repulsion between the surfaces, suggesting a conservation of their charge density, as can be seen in Figure 2b. Nevertheless, at large copolymer concentrations the characteristic decay length of the electrostatic repulsion is reduced. The coadsorption of copolymer molecules also introduces an extra short-range repulsion above the level of the hydration force observed in the absence of the copolymer. At low copolymer concentration, Z ) 1/10, the repulsive hard wall appears at larger surface separations whereas the hemifusion critical pressure is increased by a factor of 3 compared with that of the surfactant-coated surfaces. The coadsorption of the PAAPAM copolymer reinforces the cohesion of the adsorbed bilayers; this effect is even more obvious at higher copolymer concentration. As shown in Figure 2b for Z ) 1/3 or 1/1, the position of the high repulsive wall is progressively displaced to larger separations, ranging from around 8 nm at Z ) 1/10 to around 11 nm at Z ) 1/1 for a normalized applied force of F/R ) 3 × 102 mN/m. At Z g 1/3, the cohesion becomes strong enough that neither the hemifusion nor the desorption of the adsorbed mixed film could be induced by increasing the pressure, even at the highest normal pressures accessible with our experimental setup (on the order of 10 MPa). At these copolymer concentrations, the force profiles between the nonsheared boundary layers are always repulsive and virtually reversible in a loading-unloading cycle. The displacement of the position of the repulsive wall and the cohesion reinforcement clearly indicate that diblock molecules coadsorb with the trimeric surfactant onto the surfaces and remain trapped in the contact area when the surfaces are approached. Surface Forces Apparatus-N: Behavior under Shear. After the overnight self-assembly of mixed surfactant-diblock copolymer layers, the coated SFA-N surfaces still immersed in the solutions were brought into contact under a normal load L, and the friction force was measured as a function of V. In the absence of the diblock copolymers (Z ) 0), a cohesive instability occurs at some weak critical compression, and the bilayers hemifuse into adhesive monolayer-monolayer contact, with the removal of the outer monolayer of each adsorbed bilayer. Outside of the contact area, the bilayers are preserved. The two domains are separated by a circular defect line. At low normal load (i.e., before hemifusion is achieved), we do not detect any frictional resistance provided that the bilayers are not altered either by normal compression or by shear. This condition is difficult to maintain because of the weak cohesion of 12-3-12-3-12 adsorbed bilayers: the hemifusion is always triggered by the combined effects of normal load and shear. In the hemifused state, the contact area appears to be flattened by the elasticity of the film of glue used to bond the mica surfaces. An interesting feature of such an adhesive contact is that the shear stress, defined as σ ) Ff/A, is independent of L at fixed temperature T and V. This implies that the friction force is proportional to A: the main contribution to the dissipation arises from the adhesive region. Figure 3 shows the variation of σ with V at fixed temperature and normal load when the surfactant concentration is 8 cmc. The surfactant-coated surfaces display shear-thinning behavior, as we have reported in the past.5,7 The σ-V sliding curve includes a stick-slip regime in the high-velocity range for which the extreme values of the observed oscillations are represented. A different scenario is observed with the surfactant-coated surfaces in the presence of the diblock copolymer. Increasing Z

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Figure 3. Sliding-velocity dependence of the shear stress between two adsorbed layers of the 12-3-12-3-12 surfactant measured after the hemifusion of the adsorbed layers (open symbols) and mixed trimer/PAA-PAM copolymer Z ) 1/10 (closed symbols). Stickslip is observed with the surfactant layers under shear at high velocity. The maxima and minima values of the stick-slip oscillations are also displayed (+). T ) 22 °C, L ) 1.6 mN.

improves the cohesion of the layer and makes it increasingly difficult to achieve hemifusion even under shear. At Z ) 1/10, it is possible to stabilize to a certain extent the contact along the steep repulsive barrier preceding the hemifusion critical pressure, as shown in Figure 2. However, the low-friction regime remains limited to a small velocity range, and at some critical velocity, which depends on the applied load, irreversible hemifusion occurs. Similar trends to those observed in the absence of copolymer are found. The hemifusion instability arises, and a decrease in film thickness equivalent to a bilayer thickness of around 3.5 nm is observed. The contact then becomes adhesive because a pull-off force is measured as the surfaces are moved apart. Before shear-induced hemifusion, the friction force is not discernible from the experimental noise. Once hemifusion has been achieved, the friction resistance suddenly increases, but the level of the shear stress is a factor of 2 lower than the one observed in the absence of PAA-PAM, as can be observed in Figure 3. This transition is irreversible: to reform complete bilayers with good lubrication properties, the two surfaces must be separated. Over the explored velocity range, the shear stress is found to be relatively constant. At Z ) 1/3, it was impossible to disrupt the adsorbed layers by compression only (maximum applied load 10 MPa) after overnight adsorption. However, a slow shear-induced jump-in could be observed during the first few hours following the immersion of the surfaces, from the larger thickness of the mixed film adsorbed, 8.5 nm, to an equivalent monolayer-monolayer surfactant film thickness, 3.5 nm. After the jump-in, the contact becomes adhesive, and a similar level of friction force to the one previously measured in the hemifused state is obtained. Nevertheless, aging of the mixed layer is observed. After 48 h of adsorption, it is no longer possible to shear induce the transition to the high-friction adhesive state regardless of the applied shear rate. In the unperturbed quiescent state, a weak but finite shear stress is then measured, as shown in Figure 4. It seems to increase as a power law of V over 3 orders of magnitude, with an exponent of about 1/3. No hysteresis was observed in the sliding curves when the driving velocity was first increased and then decreased. We observed that the film thickness is gradually reduced under shear even though L has been kept fixed, typically from about 9.5 to 8 nm. The film thickness reduction is accompanied by the appearance of an adhesive interaction between the surfaces. After the films have been conditioned by shearing, the normal force profile still presents an exponential long-range repulsion and a final stiffer barrier under compression, but the separation of surfaces from the contact proceeds by an abrupt jump-out, as can be observed in Figure 5.

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Drummond et al. Table 1. Shear Stress and Film Thickness Measure under Compression before and after Hemifusion and after Shear for the Different Z Ratios Investigateda

Z

D before D after hemifusion hemifusion (nm) (nm)

0 6.5 ( 0.2 1/10 8.0 ( 0.5 1/3 9.5 ( 0.5 1/1.8 10.0 ( 0.5 1/1 11.5 ( 0.5

3.5 ( 0.2 4.0 ( 0.5

D after shear (nm)

sliding curve σ(V)

shear stress, σ (MPa) (0.1 µm/s)

3.5 ( 0.2 4.0 ( 0.5 8.0 ( 0.5 4.0 ( 0.5 3.5 ( 0.5

bell-shaped constant σ ≈ VR; R ) 1/3 σ ≈ VR; R ) 2/3 σ ≈ VR; R ) 2/3

0.350 0.220 0.007 0 0

For Z g 1/3, the applied normal force is F/R ) 300 mN/m. The shear stress is measured at V ) 0.1 µm/s. a

Figure 4. Sliding-velocity dependence of the shear stress between two adsorbed mixed layers of 12-3-12-3-12 surfactant and PAAPAM copolymer. T ) 22 °C, Z ) 1/3. Adsorption time, 48 h.

forces are weaksthe corresponding friction coefficients, defined as the ratio between the measured friction force and the applied normal load, are lower than 10-2sa shear-induced adhesive interaction is also observed in this case. At higher copolymer concentrations (Z ) 1/1), the selfadaptation scenario described above is confirmed, and a velocity dependence of the shear stress that is still compatible with a power law with an exponent of 2/3 is observed. However, the cohesion of the mixed boundary film declines. A complete characterization of the behavior of the system at this Z will be reported in a forthcoming publication. A synopsis of the main experimental results, including the evolution of the film thickness and the shear response of the confined boundary layers at the different Z ratios investigated, is presented in Table 1.

Figure 5. Normal force profile between two mica surfaces measured by SFA-N in a solution of trimer and PAA-PAM before and after surface shear. Z ) 1/3.

Discussion

Figure 6. Sliding-velocity dependence of the shear stress between two adsorbed mixed layers of 12-3-12-3-12 surfactant and PAAPAM copolymer. T ) 22 °C, Z ) 1/1.8. The dashed line indicates the detection limit of our experimental setup. The data points below this value are reported as zero shear stress.

At even larger copolymer concentrations (Z ) 1/1.8), the transition to the adhesive high-friction state is never observed in the accessible loading and shearing ranges after overnight adsorption. As observed at Z ) 1/3, the friction force is very weak and seems to increase with the driving velocity as an algebraic law, with no hysteresis observed in the measured sliding curve. However, a larger exponent close to 2/3 is measured, as illustrated in Figure 6. The thickness of the film confined in the contact gradually decreases during shearing, reaching an ultimate value of 4 nm, which is smaller that the one observed at Z ) 1/3. Once the thickness has been stabilized, the mixed copolymersurfactant film exhibits superior lubrication, mechanical proprieties, and cohesive stability. Fretting tests under severe conditions over a few hours did not lead to any damage of the boundary lubricant films or the surfaces. Even though the friction

The results obtained with both techniques, AFM and SFA-N, clearly demonstrate that the diblock copolymer chains are coadsorbed with the surfactant, leading to significant changes in the composition of the films and their response to both applied normal load and lateral shear. The coadsorption of PAA-PAM copolymer with the adsorbed 12-3-12-3-12 film is expected to occur via electrostatic association of their moieties of opposite charge: the PAA-PAM chains by themselves do not adsorb on the mica surfaces. However, even for the largest copolymer concentration investigated, Z ) 1/1, no dramatic changes in the morphology of the adsorbed film were observed in the AFM images. The boundary films remains molecular in thickness, without the adsorption of large aggregates. The main question is, what is the structure of the boundary films in the presence of the PAA-PAM copolymer? The experimental results suggest that a surfactant bilayer is the base of the nonsheared films. The copolymers would then be mainly adsorbed onto the outer layer of the bilayers. Several experimental observations support this picture. The indentation of the films by the AFM tip gives a quite constant breakthrough of about 2.5 nm consistent with the thickness of a compressed surfactant bilayer, regardless of the copolymer concentration (Figure 1). Furthermore, for Z e 1/3 the cohesion instability triggered under loading or shearing stress leads to an adhesive contact with a final film thickness compatible with a surfactant monolayermonolayer contact as measured in a pure 12-3-12-3-12 solution (Figure 2, Table 1). Moreover, the double-layer repulsions of the SFA-N force profiles have equivalent ranges in the absence and in the presence of diblock copolymer, suggesting that the charge density of the films is fairly conserved (Figure 2). Because no evidence of charge inversion is observed by increasing the copolymer concentration, the density of coadsorbed PAA-PAM appears to be low. The mixed film would be reminiscent of a surfactant bilayer adsorbed on the mica surfaces overcoated by copolymer chains.

Surfactant Boundary Lubricant Film Reinforcement

Langmuir, Vol. 24, No. 4, 2008 1565

The PAA blocks are electrostatically adsorbed onto the bilayer in a rather flat conformation whereas the PAM blocks are left free in solution. Such a picture is compatible with the cohesion reinforcement of films under compression observed by AFM and SFA-N. The copolymer chains would form a 2D physical gel on the outer monolayer because a 12-3-12-3-12 trimeric surfactant molecule may cross link up to three copolymer chains. The lateral mobility of the surfactant molecules is then gradually restricted as the coadsorbed copolymer density increases and the hemifusion instability is consequently inhibited. However, as the copolymer concentration is increased, extra surfactant molecules may accompany the coadsorbed copolymer chains while the surfactant bilayer is gradually destructured by the internal presence of the copolymers. With such a complex film, irreversible molecular rearrangements are expected either by aging or under shear. Whereas the cohesion of the quiescent boundary films is reinforced under compression over the copolymer range up to Z ) 1/1, the behavior under shear is more subtle. At low (Z ) 1/10) or high (Z ) 1/1) copolymer concentration, the films undergo abrupt damage under shear, leading to poor lubricant behavior. For intermediate copolymer concentrations, the films undergo gradual and limited rearrangements under shear, which is progressively more important with an increasing Z ratio, and exhibit extremely good lubrication properties. The corresponding sliding curve σ(V) can be described by a sublinear scaling power law, σ ≈ VR, suggesting that the dominant contribution is viscous in nature with some shear-thinning effect: the effective viscosity decreases with the shear rate, ηeff ≈ VR - 1. The value of R varies with the Z ratio. In the framework of bilayers decorated by coadsorbed PAA-PAM copolymers, when the two surfaces are brought in contact the interfacial film is made of compressed PAM chains. Under shearing, the frictional resistance may emerge as a consequence of at least two different mechanisms: first, the friction between PAM monomers and an effective medium involving solvent with opposite PAM chains and second, viscous resistance to the displacement of PAA moieties of the copolymer associated with surfactant molecules in the underlying adsorbed surfactant layer. Given that the copolymer molecules are not chemically bonded to the mica surfaces, a pulled chain may be displaced relatively to the substrate. The dissipation would be viscous with shear-thinning arising from the combination of stretching and the mobility of the chains. Both the alignment22 and the mobility23 of weakly entangled macromolecules have been theoretically predicted to cause a viscous force opposing the movement of the copolymer molecules proportional to the 1/3 power of the shear rate, in agreement with our results at Z ) 1/3. A value of R ) 1/3 has been observed experimentally several times in shear studies of thin lubricant films made of linear molecules.24 Raviv and coworkers reported an exponent of R close to 1/3 in studies of dense PEO layers that are strongly attached to mica surfaces.25

We have found a similar exponent for PEO layers grafted onto fluid underlayers.14 In the present work, we found R to be close to 1/3 only for Z ) 1/3, when the shear-induced reduction of the film thickness is limited to less than 30%. At this composition, the lubricant film must be close to a PAM interfacial film confined between two surfactant bilayers. On the contrary, when the shear-induced film thickness reduction is more important, as is the case for Z g 1/1.8, a larger R exponent close to 2/3 is observed. At these copolymer concentrations, important rearrangements of the lubricant layer take place because the surfactant and copolymer layers are extremely confined and no longer well-segregated. This will limit the macromolecular stretching that is assumed to be at the origin of shear thinning. Sublinear R exponents higher than 1/3 have also been reported in several frictional studies of other complex boundary lubricants in the past. In a study of friction between hydrogels, Kagata and co-workers reported R exponents greater than 0.5.26 More interestingly, a similar change in regime, with R varying from 1/3 to 2/3, was observed by Brown in a study of friction between a PDMS elastomer network and a solid substrate covered by chains of the same material, by increasing the density of copolymer chains grafted to the solid.27 Rearrangements of the mixed films under shear induce some adhesion between the two surfaces (Figure 5). This attraction occurs only when the two coated surfaces are brought into close contact. The bridging of adsorbed surfactant molecules by PAA blocks is probably at the origin of the measured adhesion. Nevertheless, under these conditions the friction forces are very weak; this strongly suggests that the time needed to form the adhesive bridges is long compared with the shear rates investigated.

(22) Subbotin, A.; Semenov, A.; Manias, E.; Hadziioannou, G.; Brinke, G. Macromolecules 1995, 28, 3898. (23) Bright, J. N.; Williams, D. R. Langmuir 1999, 15, 3836. (24) Hu, W.-H.; Carson, G. A.; Granick, S. Phys. ReV. Lett. 1991, 66, 2758.

(25) Raviv, U.; Frey, J.; Sak, R.; Laurat, P.; Tadmor, R.; Klein, J. Langmuir 2002, 18, 7482. (26) Kagata, G.; Ping Gong, J.; Osada, Y. J. Phys. Chem. B 2002, 106, 4596. (27) Brown, H. R. Faraday Discuss. 1994, 98, 47.

Conclusions We have shown that an anionic-neutral hydrophilichydrophilic diblock copolymer coadsorbs with an oligomeric cationic surfactant in the form of a molecular film on mica substrates. At low copolymer concentrations, the boundary mixed film appears to be layered. A surfactant bilayer is adsorbed onto the mica whereas the copolymer is adsorbed onto this bilayer. As the copolymer concentration increases, the layered structure seems to become more and more diffuse. The coadsorption of the copolymer mechanically reinforces the adsorbed surfactant bilayer. The hemifusion, a cohesive instability of the surfactant bilayer under mechanical stress, is inhibited by this coadsorption; the boundary mixed film then exhibits excellent lubrication properties. There is an optimal copolymer concentration for lubrication purposes. Excess coadsorbed copolymer decreases the cohesion of the film under shear and impairs the lubrication properties of the mixed layer. LA702392V