Article pubs.acs.org/Langmuir
Boundary Lubricant Polymer Films: Effect of Cross-Linking Suzanne Giasson,† Jeanne-Marie Lagleize,‡,§,⊥ Juan Rodríguez-Hernández,∥ and Carlos Drummond*,‡,§ †
Department of Chemistry and Faculty of Pharmacy, Université de Montréal, C.P. 6128, succursale Centre-Ville, Montréal, QC, Canada H3C 3J7 ‡ CNRS, Centre de Recherche Paul Pascal (CRPP), UPR 8641, F-33600 Pessac, France § Université Bordeaux 1, CRPP, F-33600 Pessac, France ∥ Instituto de Ciencia y Tecnología de Polímeros, CSIC, Juan de la Cierva 3, 28006, Madrid, Spain S Supporting Information *
ABSTRACT: We have studied the adsorption and lubricant properties of a multifunctional triblock copolymer poly(L-lysine)-b-poly(acrylic acid)-bpoly(L-lysine). In particular, we investigated the nature of the layer adsorbed under different conditions of polymer and salt concentration and the lubricant properties of the polymer layer before and after its chemical crosslinking by bridging the poly(acrylic acid) blocks. We found that the amount of polymer adsorbed is controlled by the ionic strength and the polymer concentration in the solution. In all cases, the self-assembled polymer layer is a poor lubricant before cross-linking, but the cohesion and load-carrying ability of the layer are substantially improved by this reaction. However, the chemically cross-linked coating has a limited deformation capacity as a consequence of its permanent network nature, and irreversible damage is observed after excessive strain of the film.
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INTRODUCTION Water-based lubricants are necessary for biomedical and environmentally friendly applications, where commonly used oil-based lubricants are forbidden. In virtue of its low viscosity, water opposes a small resistance to shear; however, it is apparent that its load-bearing capabilities are generally poor. For this reason, different additives are generally dissolved in water to improve its lubricant properties; water-soluble salts, surfactant, and polymer are regularly used as friction modifiers, which commonly results in the adsorption of a coating layer on the surfaces. An ideal boundary lubricant layer must satisfy a harsh set of specifications: it must adhere well to the substrate to avoid being expelled under pressure, leaving the surfaces unlubricated; it must be cohesive enough to endure shear preventing surface−surface contact and wear; and most importantly, it must help reducing friction between the rubbing surfaces; this is achieved through a variety of mechanisms including steric or electrostatic repulsion and surface hydration. The use of polyelectrolytes PE as boundary lubricants has often been proposed. Remarkable lubrication properties have been reported for dense end-grafted nonadsorbing polyelectrolytes (polymer brushes).1 This performance has been attributed to the small interlayer interpenetration between the polymer brushes together with the hydration layers surrounding the charged monomers; when two opposing PE brushes are subjected to compression, their hydration layers overlap and water in the interlayer region effectively acts as a low-viscosity lubricating fluid. Indeed, molecular dynamics simulations have also shown that two opposing polyelectrolyte brushes in good © 2013 American Chemical Society
solvent avoid mutual interpenetration upon compression by folding in upon themselves while maintaining a polymer-free gap as the compression increases.2−4 However, if the interpenetration between the polymer layers is forced by either higher compressive loads or worsening solvent conditions, a sharp deterioration of the lubrication properties is observed.5,6 This is a result of the higher energy dissipation occurring between the sheared interpenetrating polymer chains. The force experienced by two opposing polymer brushes is believed to arise from the polymer segments being dragged through the interpenetration zone.7 It has been shown that the compression of charged brushes results in less interpenetration relative to neutral brushes when considering equivalent grafting density and molecular weight.3 The charges on the polymer chains provide additional osmotic repulsions upon compression enhancing the ability of the polymer layers to separate the surfaces. Therefore, for a given applied load, a smaller number of segment collisions occur per unit time in the case of charged polymers under shear, resulting in lower friction forces. The scenario is somehow different for adsorbing polyelectrolytes: intersurface bridging attraction or shear-induced expulsion of polymer chains can occur which can increase adhesion and friction between the surfaces. Some experimental studies have addressed a number of natural and biomimetic lubricant systems based on charged biopolymers. Hyaluronic Received: May 30, 2013 Revised: August 22, 2013 Published: September 20, 2013 12936
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acid, 8 lubricin 9 other mucins, 10 chitosan,11 and other polysaccharides12 are some of the natural polyelectyrolytes investigated in biotribology. Most of these studies involve physisorbed polymers with rather complicated multicomponent structure. Paradoxically, there are only few reported studies of the tribological behavior of adsorbed homopolyelectrolytes. While there seems to be a reasonable good understanding of polyelectrolyte adsorption and of the normal interaction forces between polyelectrolyte coated surfaces, less is known about the lubricant properties of adsorbing charged polymers. Friction experiments between poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC)-coated glass surfaces produced by grafting-from synthesis revealed very low friction forces under large pressures.13 On the contrary, studies with weakly charged adsorbing polyelectrolytes have shown only a limited improvement of lubrication properties at large adsorbing densities, although the resistance to wear is notoriously improved.6,14 Rutland and co-workers studied adsorbed layers of polycations of different charge density; poor lubrication properties were observed.15 Similar results were reported by Kampf and co-workers in a study of interaction between chitosan-coated mica surfaces.11 The influence of the preparation path on the behavior under shear of adsorbed PE layers of quaternized poly(4-vinylpyridine) was verified by Ruths and co-workers.16 However, a more exhaustive investigation of simple PE systems might improve our understanding of this type of boundary lubricant. The density of self-assembled layers is commonly limited by steric restrictions or electrostatic repulsion between the adsorbed species. This usually implies low cohesion and density of the protective layers, decreasing its effectiveness against wear. Several strategies have been explored to improve the density of self-assembled layers. These include end-grafted polymer brushes, as previously discussed, prepared via graftingfrom polymer synthesis or polymer layer transfer via the Langmuir−Blodgett (L−B) technique. However, grafting-from or L−B transfer strategies are rather expensive and difficult to implement in most practical cases. Another route proposed to improve the density of coating polymer layers is to use bottlebrush copolymers, with an adsorbing backbone and lubricant side chains coupled at a controlled density. Poly(L-lysine)-graf tpoly(ethylene glycol), PLys-g-PEG, is probably the most extensively studied example of this type of copolymer.17 The PLys backbone is positively charged at neutral or acidic pH. Electrostatic attraction drives the adsorption of this polymer onto negatively charged surfaces. In this way, the PEO chains form a dense brushlike structure on the surface. There is a natural limit to this approach: increasing the density of side chains reduces the interaction between the backbone and the surface because of steric hindrance. It has been shown that this type of molecule can achieve exceptional lubrication properties, which can be tuned by varying the molecular architecture.18 However, the wear resistance under moderate and high loads is somehow restricted. A different strategy that has been proposed to improve the lubricant properties of a coating is in-situ crosslinking. For instance, it has been shown that cross-linking of adsorbed hyaluronic acid molecules confers greater cohesion and load bearing ability to the resulting hylan layer although the friction remains largely unaffected.19,20 Enhanced layer cohesion was also reported in a study of a polylysine-based triblock copolymer cross-linked via ester bonds between the lysine moieties.21 Analogously, Kobayashi and co-workers found that cross-linking substantially improved wear resistance
of a layer of PMPC, increasing the friction to some extent.22 Kampf and co-workers reported a marked increase in friction forces for chitosan lubricant layers after cross-linking.11 Different studies have shown that the mechanical properties of the lubricant layers can be tuned by controlled crosslinking.23,24 The purpose of this study is twofold. First, we investigated the effect of a polylysine-based multiblock polyelectrolyte on adhesion and friction between coated mica surfaces. Second, we studied the effect of chemical cross-linking on these properties. The earlier studies described above have investigated homopolymers; with a single functionality, it is difficult to chemically modify the coating layer without altering its adsorbing properties or interfacial composition. For these reason we have designed and synthesized a multifunctional molecule: poly(L-lysine)-b-poly(acrylic acid)-b-poly(L-lysine), PLys23-b-PAA19-b-PLys23, a triblock copolymer in which the different blocks have specific functions. The driving force for the adsorption and the polyelectrolyte character for lubrication purposes are given by the positive charges on the polylysine (PLys) blocks. The carboxylic acid functions of the central block provide reactive points for the cross-linking of the adsorbed layer, with the aim of improving its cohesion. The different blocks of this copolymer are sensitive to pH. In particular, the PLys blocks become positively charged at neutral or acidic pH values, while the PAA block remains neutral, becoming negatively charged at high pH. Most of the tests reported in this work were performed at acidic conditions (ca. pH 3.5); in this condition the charged PLys blocks act as polyelectrolyte, while the polyacrylic block remains in the uncharged form.
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EXPERIMENTAL SECTION
Materials. tert-Butyl acrylate (tBA) (Sigma-Aldrich, 98%) was distilled under reduced pressure over calcium hydride (CaH2) prior to use. Tetrahydrofuran (THF) (J.T. Baker, >99%) was distilled under benzophenone and stored under vacuum before use. Nε-Trifluoroacetyl-L-lysine N-carboxyanhydride (TFA-Lys NCA) was supplied by Isochem-SNPE (France). Copper(I) bromide (CuBr) (Sigma-Aldrich, 98%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) (Sigma-Aldrich, 99%), sodium hydroxide (NaOH), sodium nitrate (NaNO3), nitric acid (HNO3), dimethylformamide (DMF), 1-ethyl-3(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS), and 2,2′-(ethylenedioxy)diethylamine (Aldrich, >98%), and other solvents were used as received. EDC was always stored under dark at a temperature below −20 °C. Copolymer Synthesis. The preparation of the triblock copolymer PLys23-b-PAA19-b-PLys23 was carried out as follows (detailed schematics of the process of copolymer synthesis and 1HNMR spectra of the reaction intermediate are presented as Supporting Information): (1) Preparation of Dibromo End-Terminated Poly(tert-butyl acrylate) (Br-PtBuA19-Br). The central block of the triblock copolymer was prepared by atom-transfer radical polymerization (ATRP). All polymerizations were performed in Schlenk flasks previously flamed and dried under vacuum. ATRP was carried out using the following stoichiometry [M]/[I]/[CuBr]/[L] = 60:1:2:2, where M = tert-butyl acrylate (monomer), I = diethyl meso-2,5-dibromoadipate (initiator), and L = PMDETA (ligand). The reactants were added under N2. The reaction mixture was then degassed by three freeze−pump−thaw cycles and placed in a thermostated oil bath at 70 °C. After polymerization, the mixture was cooled to room temperature, diluted with dichloromethane (CH2Cl2), and passed through a neutral alumina column to remove the copper salt. After evaporation, the polymer was precipitated in ethanol, filtered, washed, and dried under vacuum. Polymerizations carried out during 15 min produced polymers with an average degree of polymerization (DP) of 19 12937
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units, as calculated from 1H NMR (nuclear magnetic resonance) (size exclusion chromatography: Mn = 6360 g/mol, PD = 1.12). (2) Preparation of Diamino End-Terminated Poly(tert-butyl acrylate) NH2-PtBuA19-NH2. End-bromo functional groups were transformed into primary amine groups (which will be able to initiate the ringopening polymerization of TFA-Lys NCA) by nucleophilic substitution of the bromo-end group with ethylene diamine (EA) in the presence of triethylamine. The reaction was carried out as follows: 0.95 g of Br-PtBuA19-Br was dissolved in 8 mL of DMF. 10 equiv of triethylamine and 10 equiv of ethylenediamine were added to the polymer solution. The reaction was allowed to proceed at room temperature for 3 days. (3) Polymerization of TFA-Lys NCA f rom the Telechelic Diamino End-Functionalized Macroinitiator (PTFAlys-PtBuA19-PTFAlys). A thoroughly dried Schlenk flask was charged with the amount of Nεtrifluoroacetyl-L-lysine NCA and dissolved in THF. Then, α,ωdiamino end-functionalized PtBuA previously dissolved in the same solvent mixture was added to the solution. Several cycles vacuum/N2 were performed to purge the Schlenk and to avoid the presence of humidity in the reaction media. After 5 days at room temperature, the reaction product was precipitated in water and extensively washed with diethyl ether in order to eliminate rests of decomposed monomer and unreacted initiator. (4) Deprotection of the tert-Butyl Protective Groups of PAA and Trifluoroacetyl Groups of PLys (PLys23-PAA19-PLys23). The triblock copolymers were deprotected in two steps. In order to remove the trifluoroacetyl (TFA) protective groups, the polymer was first dissolved in the minimum amount of THF. Then, 1.5 equiv (related to the number of TFA groups to be deprotected) of KOH was added to the solution and allowed to react for 3 days. During the reaction the polymer precipitated and was purified by centrifugation. The second step consists on the hydrolysis of the PtBuA block in the PLys23PtBuA19-PLys23 block copolymers. The block copolymers were first dissolved in CH2Cl2. Trifluoroacetic acid (TFA) was then added (10 equiv to tert-butyl ester units), and the mixture was stirred at room temperature for 3 days. The deprotected polymers precipitated in the reaction media and were filtered, washed with CH2Cl2, and finally dried under vacuum. Copolymer Adsorption. We investigated the adsorption and interaction forces of self-assembled layers of the triblock copolymer (PLys23-PAA19-PLys23) on silica and mica before and after crosslinking of the PAA block. Spontaneous adsorption of the block copolymers onto negatively charged surfaces is driven by electrostatic interaction with the positively charged PLys blocks in acidic environment. We explored the influence of salt and polymer concentration, Cs and Cp, on this process. Salinity and pH of aqueous solutions were adjusted using NaNO3 and HNO3 (Aldrich). We avoided using buffer solutions through the whole study to keep the ionic environment as simple and controlled as possible. After adsorbing for a given time, we removed the nonadsorbed polymer by extensive rinsing with an aqueous solution of similar or different Cs. As discussed below, the rinsing procedureintrinsic to each experimental techniquehas implications on the resulting (preparation-path-dependent) self-assembled layer. Cross-Linking Reaction of the Adsorbed Copolymer Layer. The self-assembled polymer layer was cross-linked by the intermolecular formation of amide bonds between the carboxyl groups of the PAA blocks and the amino group of the cross-linking agent. A 1 mg/mL solution of EDC, NHS, and 2,2′-(ethylenedioxy)diethylamine (NH2(CH2)2-O-(CH2)2−O-(CH2)2-NH2; the cross-linking agent) was prepared right before its use, and its pH was adjusted to 7 ± 1 by adding a few drops of concentrated HNO3 (the outcome of the reaction was greatly affected by pH). Prior to use, EDC was kept under dark, at −20 °C, in an inert atmosphere (dry N2). Otherwise, it decomposed rapidly. The polymer-coated mica surfaces were then immersed in this solution. The reaction was performed under dark promptly after preparing the solution. It was allowed to proceed for 12 h. The unreacted cross-linking agent was removed by extensive successive rinsing with neutral water and water pH 4. The cross-linking reaction was performed in situ in the chamber of the surface forces
apparatus or in the liquid cell of the atomic force microscope; the polymer-coated surfaces were never dried during a particular experiment. Methods. Quartz Crystal Microbalance with Dissipation Monitoring. Polymer adsorption on silica was measured in a commercial quartz crystal microbalance with dissipation monitoring (QCMD E1, Q-Sense). The principles of the technique have been extensively described in the literature.25 Briefly, the resonance frequency f of an AT-cut quartz resonator is measured; a change in the effective mass of the resonator due to material adsorption translates into a variation in the resonance frequency, Δf. In addition, the damping of the oscillation of the crystal was measured and used to calculated the “dissipation factor”, D, defined as the inverse of the quality factor of the resonance peak.26 A large value of D indicates quickly decaying oscillations of the crystal, which is observed for thicker and nonrigidly attached layers. In a QCMD experiment, the measured Δf and ΔD can be related to the properties of the material adsorbed on the quartz crystal by using adequate models.25 If the adsorbed mass is evenly distributed, rigidly attached, and small compared to the mass of the crystal, Δf can be related to the adsorbed mass per unit area (Γ ≡ Δm/A) by the Sauerbrey equation27 Δf =
− 2f 2 Δm Aρq c
where ρq and c are the density and speed of sound in the quartz and A is the surface area of the resonator. Δf measures the resonance frequency change before and after adsorption, but always in the presence of the same bulk liquid phase. Otherwise, the influence of the variation in solvent viscosity must be considered. If the adsorbed layer (adlayer) is not thin and rigidly attached, more complicated models, which consider the influence of the viscoelastic properties of the adsorbed material on the propagation of the shear wave, must be used. One common practice is to describe the adsorbed film as a homogeneous viscoelastic Voigt-like film with a complex shear modulus G*= G′ + iG″ (G′ and G″ are the storage and loss modulus of the film) in contact with a semi-infinite viscous solvent. In this model G′ and the film viscosity (η ≡ G″/ω, with ω the angular frequency 2πf for a given harmonic) are considered frequencyindependent quantities. A complete description of this (and other) models have been reported in the literature.25,27 We used 5.0 MHz quartz resonators with silica-coated gold electrodes. The sensors were rinsed twice with Milli-Q water and ethanol, then were irradiated with ultraviolet light for 15 min, and rinsed again with ethanol and blow-dried with nitrogen gas prior use. At the beginning of each experiment the resonator was placed in the cell and immersed in Milli-Q water adjusted to the pH and salt concentration of the polymer solution to be studied for at least 30 min, until a stable baseline was established. This enabled the system to thermally equilibrate and the silica surface charge to equilibrate at the measurement conditions. Solutions of the polymer at different concentrations (0.01−100 μg/mL) in water were prepared by dilution of a 1 mg/mL stock solution (at pH 3.5 in all cases). The effects of polymer concentration Cp and background salt concentration Cs (NaNO3) on polymer adsorption were investigated. We fitted the odd harmonics (n = 3 to n = 13) QCMD data by using the Sauerbrey and Voigt models using the QTM software, written by Diethelm Johannsmann.28 Atomic Force Microscopy. The structure of the layers adsorbed at the solid/solution interface was examined using a Digital Instruments NanoScope IIIa Multimode atomic force microscope in contact mode in a standard fluid cell. Standard triangular cantilevers were used with sharpened Si3N4 tips (Digital Instruments, Santa Barbara, CA). These were irradiated with ultraviolet light for 15 min prior to use. The solution was held in a fluid cell and sealed by a silicone O-ring. Both were rinsed in ethanol and deionized water and then dried using filtered nitrogen gas. The solid substrate used in all experiments was muscovite mica (METAFIX, France) cleaved using adhesive tape immediately before use. Experiments were performed in Millipore 12938
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Figure 1. Triblock copolymer adsorption: QCMD. (a) Mass of polymer adsorbed at different Cp values. Cs = 10 mM. The mass was calculated by using Sauerbrey equation (open circles), Voigt viscoelastic model of the layer with variable bulk liquid viscosity (closed circles), and Voigt viscoelastic model of the layer with fix fluid viscosity (open squares). Partial desorption is observed upon rinsing; the dashed lines indicate the effective mass of the hydrated PE layer after rinsing at different pH values, calculated using Sauerbrey equation. (b) Effect of salt. Sauerbrey and Voigt mass measured after adsorption and rinsing, for different Cs (no difference in the values obtained by both methods was observed for Cs 3 or 10 mM). Cp = 0.5 mg/mL. between the surfaces in contact can then be calculated from the experimentally measured spring force, Fspring, obtained from the deflection of this vertical double-cantilever spring, with an accuracy of ±5 μN.34
water with a resistivity of 18 MΩ·cm, adjusting pH and salt concentration by adding NaNO3 and HNO3. Images show height and deflection data captured using the soft contact method,29 in which the repulsion between the polymer layer adsorbed on the substrate and the Si3N4 AFM tip allows imaging using steric or electrical double layer forces. This method enables generation of a repulsive force map of the adsorbed layer without the tip physically contacting the sample. Typical imaging scan rates varied between 0.5 and 2 Hz, and proportional and integral gains between 0.5 and 1 were used. Variation of scan frequency, scan angle, and gains had no effect on the observed morphology. All images are unmodified except for flattening along scan lines. Force versus tip−substrate separation curves were measured with a 200 μm long cantilever. We considered the tip to be in contact with the surface when the voltage vs displacement response varied steeply in an approach−retraction cycle: we have used this data to calibrate the response of the photodiode for each force curve, as has been described in the literature.30 We did not attempt to calculate the actual interaction force from the deflection data because the tip geometry and the precise spring constant of the cantilever were not known. Surface Forces Apparatus (SFA). A surface forces apparatus (SFA) modified for nanotribological studies was used to measure the interaction between polymer-coated mica surfaces. This technique has been extensively described in the past.31 In brief, two back-silvered molecularly smooth mica surfaces are glued to cylindrically curved silica lenses. With the SFA 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 can be controlled with an accuracy of a fraction of a nanometer by using a piezoelectric nanopositioner. Multiple beam interferometry (MBI) is used to measure the separation between the surfaces, D, with subnanometric resolution.32 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 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, the cylinders are elastically flattened at the point of contact. The load-induced contact radius, Rc, and the area of contact, A, can also 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. Shearing cycles are carried out by moving this surface at constant velocity, V, over a certain distance, after which the driving direction is reversed.33 The upper surface is attached to a vertical double cantilever spring whose deflection is monitored using strain gauges (SurForce Inc.). The friction force, Ff, induced by the relative displacement
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RESULTS AND DISCUSSION Adsorption. Quartz Crystal Microbalance. We have observed that surface adsorption of the copolymer is very sensitive to the environmental conditions and in particular to Cs. Two regimes could be identified. At low Cs ( 10 mN/m) was ca. 10 nm. As previously mentioned, poor reproducibility between different contact spots and different experiments was observed, indicating again that the adsorbed layer results of an out-of-equilibrium process determined by the deposition conditions. Although the hard wall thickness was relatively small, the extremely large extension of the repulsive force appears to be inconsistent with the thickness that could be estimated from QCMD results (assuming a reasonable density of the adsorbed layer). This seems to indicate the presence of a loosely adsorbed thick layer, which remains on the surfaces due to the limited rinsing performed in the SFA experiments, as mentioned previously. As will be discussed below, the effective thickness of the coating layer is substantially reduced by shear and compression− decompression cycles. ii. Low Cs during Adsorption (1−10 mM NaNO3). As evidenced by the QCMD and AFM results, at low Cs a thinner polymer layer was deposited; as a consequence, the range of the interaction force between polymer-coated surfaces was much smaller than at high Cs, as can be observed in Figure 3a. However, the polymer concentration C p modifies the morphology of the adsorbed layer. These changes have some consequences on the interaction force between the coated surfaces, as can be observed in Figure 3b. A nonmonotonic evolution of the interaction force with Cp is observed. At low and high Cp a long-range electrostatic repulsive force and a jump in contact at closer separations are observed. On the contrary, at intermediate Cp the interaction force measured upon surface approach remains extremely low until the jump to adhesive contact appears. This behavior suggests that at low Cp (10−4 mg/mL) the amount of polymer adsorbed is not enough 12942
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sliding, providing a satisfactory protection against wear for many thousand shear cycles. Nevertheless, continuous shearing progressively reduced the thickness of the adsorbed layer, as evidenced by the decreasing range of the repulsive force after shear (Figure 4b). The rate of material removal was determined by the operational conditions (L, V, and time of shear); we did not pursue a comprehensive characterization of this process. ii. Low Cs during Adsorption (1−10 mM NaNO3). As mentioned before, an adsorbed layer at equilibrium was obtained when Cs was low. A better description of the PE layer under shear can then be achieved. As for the case of normal forces, one must differentiate the scenarios of low and high Cp during adsorption, which produced two markedly distinct layers. In both cases, wear of the adsorbed layer often appeared upon shear under substantial load (larger than ca. 1 mN), and a weak dependence of Ff on V (after sliding was initiated) was observed. However, the behavior at low speeds and at the initiation of sliding was different on each case. Typical results for Ff vs L for low Cp are presented in Figure 5. Some variability in the measured forces was observed
quickly and the surfaces remain under high compression no more than few seconds. We calculated the adhesion free energy per unit area after the surfaces have been in contact longer than ca. 30 s, E0, using the expression E0 = (2F0/3πR′), where F0 is the pull-off force, measured from the jump-out distance of the surfaces upon separation.55 The magnitude of E0 varied nonmonotonically with Cp between 1.7 and 8 mJ/m2, as reported in the inset of Figure 3b. The maximum adhesion force corresponds to the case of minimum electrostatic repulsion (charge compensation). The large magnitude of the adhesive forcemuch stronger than can be expected from the dispersion attractive forces aloneand its variation with time in contact (cf. Figure 4a) indicate that the adhesive force is not only due to attractive dispersive interaction between the surfaces but rather due to some specific interactions. Two potential candidates are bridging forces and hydrogen bonding. Bridging is probably more important at low Cp: as can be guessed from the AFM force results (cf. Figure 2f), the possibility of a polymer chain traversing the midplane and/or reaching the opposite surface is greater at low coverage conditions. Hydrogen bonding will be more important at large Cp. The higher density of adsorbed polymer molecules augments the probability of hydrogen bonding between acrylic acid segments adsorbed on opposite surfaces. Surface Forces Apparatus: Friction Forces. We investigated the behavior under shear of polymer-coated mica surfaces in the different conditions described in previous sections. Obviously, the multiple scenarios observed for the adsorbed polymer layer translate in different responses under shear, making it difficult to completely characterize the system. However, distinctive trends are observed. In general, two regimes can be identified. At very low loads, (e.g., before the adhesive jump-in was observed at low Cs) friction forces were smaller than the detection limit of our instrument. The surfaces remained lubricated by the adsorbed polymer layer and/or a thin liquid film sustained by the electrostatic repulsion between them. On the contrary, at larger loads (after the jump-in in the case of low Cs or for loads larger than ca. 0.1 mN in the case of large Cs) the friction force increased substantially. A similar behavior has been reported before for different self-assembled layers of charged polymers or surfactants: low friction is observed as long as the layers remain intact, but higher friction appears after the surfactant or polymer layers are disrupted under compression and shear.1,56 The lubricant properties of the polymer coating in the high adhesion state were rather poor: friction coefficients, μ (defined as the ratio between the friction force Ff and the applied normal force L), of the order of 0.3 were typically observed. The appearance of stiction spikes upon commencement of sliding or direction reversal was often detected. Other groups have reported similar friction coefficient for surfaces lubricated by adsorbed polyelectrolytes and conclude that adsorbing PE layers are not good lubricants at intermediate or high loads.11,57,58 i. Large Cs during Adsorption (100 mM NaNO3). As previously mentioned under these adsorption conditions the thickness of the adsorbed layer did not reach an equilibrium value. Therefore, it is difficult to carry out a quantitative description of the lubricant properties for this out-ofequilibrium layer. Nevertheless, the possibility of assembling a thin multilayer coating whose thickness is controlled by the adsorption conditions might be exploited in practical situations. Although relatively large μ values were typically observed (μ ≥ 0.3), the thick adsorbed layers kept the surfaces apart while
Figure 5. Normal load dependence of the friction force. Cp = 10−4 mg/mL; Cs = 5 mM. The data sets correspond to maxima and minima values observed in independent experiments. Results for different contact positions in three independent experiments are within in the shadow area. The dotted line corresponds to a fit Ff ∼ CL2/3.
between different contact spots and pairs of mica surfaces: the two sets of data reported in Figure 5 represent the lowest and highest value of Ff measured among all experiments carried out under the same conditions. Smooth sliding was always observed. When present, the static friction force exceeded the kinetic friction force by less than 10%. The measured Ff increases as a 2/3 power law of the applied load L, suggesting the measured force is proportional to the area of contact between the sliding surfaces, as frequently observed for singleasperity contacts. For loads larger than ca. 1 mN we had to limit the shear to few cycles; otherwise, irreversible damage of the polymer layers and mica surfaces was promptly observed. The behavior is substantially different for polymer layers adsorbed at low Cs and large Cp. Representative friction traces are reproduced in Figure 6. Extremely large stiction spikes were always present; stick−slip friction was observed at low driving velocities. Substantial dilatancy of the confined film was commonly observed upon shear: the surface separation increased ca. 2 nm after initiating sliding. Analogously, an oscillation of amplitude 2 nm in film thickness was observed during stick−slip motion. The large amplitude of the static force is probably related to the observed adhesive interaction: for the conditions of largest adhesion (charge-compensated layer) we could not overcome the static force without damaging the lubricant film. The large stiction spikes observed impaired a complete characterization of the lubricant film: shearing for 12943
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Figure 6. Typical friction traces measured between coated surfaces. Cp = 0.1 mg/mL; Cs = 5 mM. L = 0.1 mN. (a) V = 120 nm/s. Driving direction was reversed at t ∼ 97 s. (b) V = 1200 nm/s. Driving direction was reversed before every stiction spike.
long time the surfaces in contact ultimately cause wear of the coating, due to the extremely large stiction forces. In summary, a rather complex behavior under shear is observed for the self-assembled layer of this simple triblock copolymer. The combination of electrostatic and specific interactions controls the morphology and lubricant properties of the self-assembled layer. In general, poor lubricant properties were observed, with relatively large μ and weak wear resistance when shear was applied at intermediate loads (ca. 1 mN). As described in next section, this scenario is substantially modified upon cross-linking of the adsorbed layer. Boundary Layer Lubricant: Cross-Linking. We crosslinked the self-assembled polymer layer with the purpose of increasing its cohesion following the procedure described in the Experimental Section. The morphology of the adsorbed layer after the cross-linking reaction was investigated by AFM. Typical results obtained at different values of Cp are presented in Figure 7. As can be observed in the micrographs, surface features look much better defined after cross-linking. However, it is difficult to assess if this is due to a less fluctuating adsorbed layer or to the higher tip−substrate repulsive force, which makes it easier to image the polymer adlayer. Indeed, substantial changes were observed in the tip−substrate interaction after the reaction; enhanced tip−substrate steric repulsiona clear signature of enhanced cohesionis observed after cross-linking. Some examples of these force curves are presented in the Figure S6 of the Supporting Information. We performed an extensive study of the modified layers using the SFA. We observed that the cross-linking reaction modified the normal and shear interaction forces between the coated surfaces, reducing adhesion and shear-induced wear. (As mentioned in the Experimental Section, the reaction was performed under dark, turning off the white light used in the SFA. Otherwise, a less marked modification of the properties of the adsorbed layer was achieved.) The most important effects were observed on the relatively thin layers adsorbed at low Cs, which are described in the following section. Cross-Linked Layer: Normal Interaction Forces. After crosslinking, we observed a marked reduction of the adhesive force between the surfaces and a modification of the long-range electrostatic repulsion. Jump-in was no longer observed upon surface approach, suggesting increased film cohesion. Typical force profiles after cross-linking are presented in Figure 8. Once again, they depend on Cp during adsorption. At low Cp (Figure 8a) the magnitude of the long-range electrostatic repulsion decreases after cross-linking. A clear jump-out from ca. 5 nm is observed upon surface separation, but the magnitude of the pull-off force is less significant than that measured before cross-
Figure 7. Soft-contact AFM deflection images and typical height profiles for polymer layers adsorbed on mica from a solution at pH 3.5, after rinsing with water at pH 3.5, (a, c) before and (b, d) after crosslinking reaction. Adsorption conditions: (a, b) Cp 10−4 mg/mL and Cs 5 mM NaNO3. (c, d) Cp 0.1 mg/mL and Cs 5 mM NaNO3.
linking. This reduction in adhesion probably signals that crosslinking makes it more difficult for intersurface bridging to occur. In addition, the decrease of the amount of free hydroxyl groups from the PAA block (due to amide bonds formation) limits the hydrogen bonding between the adsorbed layers. The decrease in repulsive force is most likely due to the diamino molecules that have not been bonded in both ends. As described before, after adsorbing at low Cp the coated mica surfaces remain negatively charged. The nonbonded amino groups, positively charged at acidic pHs, contribute to further neutralize the surfaces. The opposite effect is observed for the case of polymer layers adsorbed at large Cp (Figure 8b). In this case the electrostatic repulsive force increases after cross12944
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Figure 8. SFA normal force profile between coated mica surfaces before (open symbols) and after (closed symbols) cross-linking reaction. Cs = 5 mM. (a) Cp = 10−4 mg/mL; (b) Cp = 0.5 mg/mL. Circles: surfaces approach. Squares: surfaces separation.
Figure 9. (a) Temporal evolution of Ff when V is changed from 120 to 300 nm/s at t = 140 s; after V is changed the system evolves from a high friction state to a low friction state. (b) Driving-velocity dependence of Ff. L = 0.08 mN (full circles); 0.25 mN (open circles); 0.51 mN (crosses). Cp = 0.5 mg/mL; Cs = 5 mM.
cycles agrees with the fact that small friction is observed at large V and with the disappearance of the stick−slip response. A different response is observed for larger applied L (L > ca. 1 mN): in this case a large elastic response is observed. We defer its description to the next section. (ii) For low Cp during adsorption (10−4 mg/mL), the friction remains relatively large after cross-linking. Friction coefficients of the order of 0.3 are typically observed at low loads, as was observed before the cross-linking reaction (Figure 10). However, the wear resistance of the coating layer at larger loads is greatly improved, as long as the driving amplitude remains small. Under these conditions, the elastic character of the adsorbed layer dominated the dynamic response of the film. It is interesting to consider the behavior of the system when the movement of the lower surface is initiated. In a typical friction experiment, the surfaces move closely coupled together until the force due to the deflection of the shear-measuring spring, Fspring, is large enough to overcome the static friction force, Fs. If the maximal deformation of the spring determined by the amplitude of the triangular wave displacement imposed to the lower surfaceis not large enough to overcome Fs, the surfaces remain practically coupled through the cycle. On the contrary, when Fspring > Fs, sliding of the surfaces is initiated; friction is reduced from Fs to the kinetic friction Ff. Hence, the deformation of the spring is reduced until Fspring = Ff, and the displacement of the driven surface (the lower surface in our experimental setup) continues while the surface attached to the friction measuring spring (the upper surface in our setup) remains almost stationary as the surfaces slide past each other. Upon direction reversal the spring quickly starts deflecting in the opposite direction, and the same sequence of events is repeated. The scenario just described was always observed before the cross-linking reaction, as illustrated
linking, suggesting an enlarged net positive charge of the coated surfaces, due again to the unreacted amino groups. The pull-off force is also substantially reduced after cross-linking. As before cross-linking, adhesive interaction was only observed after the surfaces were kept in contact (D < 10 nm) for few seconds. However, the jump-out is reduced, and long-ranged attractive forces that extend to ca. 50 nm are observed upon retraction (Figure 8b). The cross-linking also modified the lubricant properties of the adsorbed layers, as described in the following section. Cross-Linking: Shear Forces. Adding intermolecular crosslinking bridges to the coating polymer layer led to a significant reduction of the adhesion between the surfaces and to changes in the tribological behavior of the polymer-coated surfaces. In general, the magnitude of the friction at low loads (L < 0.1 mN) is not greatly modified: the lubrication properties of the lubricant layers under low compression are similar before and after cross-linking. On the contrary, at higher loads the wearprotection ability of the cross-linked layer is greatly improved for limited sliding amplitudes; two different regimes are observed depending on Cp during the adsorption. (i) For large Cp (Cp > 0.1 mg/mL), the static force is greatly reduced after cross-linking, and the stick−slip regime is no longer observed. The shear response strongly depends on the applied load. At intermediate L (L < 1 mN) usual frictional sliding is observed. Nevertheless, there is a substantial variation of F with V. Typical friction traces are reproduced in Figure 9a. At sufficiently large V, the friction is relatively low and velocity independent. On the contrary, at lower V, a much higher friction is observed. As can be observed in Figure 9b, the transition velocity V between both regimes is load-dependent: the larger L, the larger the transition V. The fact that no adhesion was observed for fast compression−decompression 12945
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(otherwise, an excessive deformation of the confined thin film would have to be assumed). At low driving amplitudes, the displacement of the upper surface is proportional (albeit significantly smaller) to the driving amplitude; some deviation of linearity is observed at larger amplitudes. As can be observed in Figure 11b, which shows maximum relative displacement between the surfaces vs maximum Fspring in a cycle, the force opposing the lateral movement may be significant. However, as long as the driving amplitude of the reciprocate motion is not too large, the cross-linked layer can sustain high normal loads for long times without noticeable damage and with reduced energy dissipation. Imposing large driving amplitudes pushes the layer beyond its breaking limit and damage readily appears. When this happens, the amplitude of the movement of the upper surface quickly increases and the lubrication properties are rapidly worsened, as illustrated in Figure 11c. The rupture point varies with the applied normal load and the history of the sample. In summary, chemical cross-linking the adsorbed polymer layer enhances its load-bearing capacity, making it more resistant to shear-induced wear under large loads. However, the elastic component dominates the response of the system, making it more difficult to apply large deformations without damaging the polymer network. AFM micrographs measured ex situ after the friction experiments showing intact and sheardamaged zones on the surfaces are presented in Figure 12. A network-like region can be observed surrounding the damaged region. In addition, a step of height 2 nm is observed between coated and uncoated (damaged) zones (Figure 12a). On the contrary, a homogeneous coating remains on the undamaged region of the surface (Figure 12b). These results suggest that the rupture of chemically cross-linked PE layer propagates to large areas; as opposite to physical cross-linking, which can readily rebuild after shear-induced degradation,59 the chemical cross-linked layer gets irreversibly damaged. Reducing the cross-linking degree of the PE layer may reduce the propagation of wear while preserving the good layer cohesion obtained by covalent cross-linking.
Figure 10. Maximum spring force before (full circles) and after (open circles) cross-linking of the preadsorbed polymer layer. Cp = 10−4mg/ mL; Cs = 5 mM. V = 300 nm/s. Lower surface displacement 3.95 μm p-p (except for the point of largest friction, measured while displacing the lower surface 5.9 μm p-p). The slope of the dashed line is 0.3. Panels a−d display the position of the lower surface and the friction traces recorded under different conditions, as indicated in the main panel.
in Figure 10d. A similar behavior was observed after crosslinking at low L (L < 1 mN), when the compression of the shearing boundary layers was not substantial, as illustrated in Figure 10a. Simple frictional sliding occurs in these cases. On the contrary, the behavior at high L is markedly modified after the cross-linking of the boundary layer (Figure 10c). Even though no clear plateau was observed in the friction traces, the displacement of the upper surface is much smaller than the movement of the lower surface, indicating that significant sliding is taking place, as can be observed in Figure 11a
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CONCLUSIONS The adsorption of PLys23-PAA19-PLys23 copolymer is determined by polymer and salt concentration during adsorption; by tuning these parameters, different configurations of the adsorbed layers exhibiting different lubrication properties can be obtained. These results are relevant for the understanding of
Figure 11. (a) Amplitude of the displacement of the upper surface for different amplitude of motion of lower surface after cross-linking. No clear plateau is observed in the friction signal. L = 1.56 mN. V = 150 nm/s. Adsorption conditions: Cp = 10−4 mg/mL; Cs = 5 mM. (b) Difference in distance traveled by upper and lower surfaces vs spring force (both measured right before motion reversal). (c) Friction signal recorded during shearinduced damage of adsorbed layers. Damage of the boundary layer (indicated by the arrow) appeared immediately when the amplitude of movement of the lower surface was increased from 5.99 to 11.98 μm. L = 2.60 mN. 12946
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Figure 12. Height tapping mode AFM micrographs of a polymer-coated mica surface taken in air after an SFA experiment (polymer cross-linking performed inside the SFA chamber): Cp = 10−4 mg/mL; Cs = 5 mM. (a) Shear-induced damaged region. The coating is nonuniform; fibrils are observed in the upper right corner of the image. Lower panel: height profile measured in the zone indicated by the white line. (b) Undamaged area. A uniform coating is present.
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more complicated polyelectrolyte lubricants (e.g., biolubricants). The chemically cross-linked polymer layer presented here behaves as a strong gel, with a solidlike response at small deformations and irreversible rupture above a critical deformation. Although the chemically cross-linked layer shows improved cohesion, it lacks the self-healing abilities observed for physically (noncovalently) cross-linked boundary layers, where bonds can break and re-form without extensive layer damage. A lesser degree of chemical cross-linking, possibly below the percolation of the confined layer, might be the right compromise between enhanced cohesion and localized deformation of the boundary lubricant.
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ASSOCIATED CONTENT
S Supporting Information *
A detailed description of the method of synthesis of the triblock copolymer, raw QCMD data, and some examples of AFM tip− substrate force curves before and after cross-linking. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Ph +33-556845612; e-mail
[email protected] (C.D.). Present Address ⊥
J.-M.L.: Rescoll, 8 allée Geoffroy Saint Hilaire, CS 30021, 33615 Pessac, France.
Notes
The authors declare no competing financial interest.
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REFERENCES
ACKNOWLEDGMENTS
SG thanks the CNRS-CRPP, France, the Natural Sciences and Engineering Research Council (NSERC) Canada and the Centre for self-assembled chemical structures (CSACS) Montreal for the financial support. 12947
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