Modification of Surface Interactions and Friction by Adsorbed

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Modification of Surface Interactions and Friction by Adsorbed Dendrimers: 1. Low Surface-Energy Fifth-Generation Amino Acid-Modified Poly(propyleneimine) Dendrimers Xueyan Zhang,§ Manfred Wilhelm,‡ Jacob Klein,* Marcus Pfaadt,† and E. W. Meijer† Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel, and Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands Received March 31, 1999. In Final Form: November 10, 1999 The interactions between two mica surfaces bearing fifth-generation amino acid-modified poly(propyleneimine) dendrimers (the dendritic box that exposes methyl groups at their outer surface) were studied across a toluene medium, using a surface force balance capable of measuring shear as well as normal forces. Normal force measurements indicated that dendrimers adsorb from dilute toluene solution (ca. 5 × 10-5-3 × 10-4 w/w) as a monolayer on the surfaces. Two interacting dendrimer monolayer-bearing surfaces experience a van der Waals attraction followed by steric repulsion on compression. The dendrimer bilayer could be compressed reversibly, yielding a measure of the compressibility of the molecules. Frictional force versus normal load profiles were measured at different shear velocities, and reveal both solidlike and liquidlike behavior of the confined dendrimers, consistent with NMR measurements on the dendritic box. The results show that the yield stress increases with compression of the layers. Observation of the relaxation behavior of sheared dendrimer layerssfor adsorption from dilute solutionssuggest that, within the parameters of our experiments, the relaxation times are insensitive to the compression. For the case of surface interactions after incubation in more concentrated dendrimer/toluene solutions (ca. 10-3 w/w), the results of both normal and shear force measurements suggested aggregation of much thicker, loose dendrimer layers on the mica surfaces. These layers resulted in hysteretic and longer-ranged monotonic repulsion, and much weaker frictional forcessat comparable loads and shear velocitiessthan in the case of the monolayers adsorbed from dilute solutions.

Introduction Dendrimers are well-defined three-dimensional highly branched macromolecular structures whose physical and chemical properties can be controlled over a wide range.1,2 They may be synthesized either by a divergent route, growing in generations from a central core, or by a convergent route, through assembly of so-called dendrons.3,4 Structurally, they may be viewed as intermediate between colloidal nanoparticles and polymeric molecules, and indeed exhibit characteristics of both. Dendrimers have attracted considerable interest because of their large size (e.g., with the feasibility of encapsulating guest species),5 the controllable chemistry of their surfaces, and the possibility of more subtle variations through the variation of the dendrimer growth generation and endgroup modification. These properties hold potential both * Corresponding author. E-mail: [email protected]. § Present address: Department of Materials Science and Engineering, University of Illinois at Urbana-Champagne, UrbanaChampagne, IL. † Eindhoven University of Technology. ‡ Present address: Max Planck Institute for Polymer Research, Ackermannweg 10, D-55021 Mainz, Germany. (1) Tomalia, D. A.; Naylor, A.; Goddard, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (2) Tomalia, D. A. Sci. Am.. May 1995, 42. (3) Voit, B. I. Acta Polym. 1995, 46, 87. (4) Janssen, H. M.; Meijer, E. W. In Polymer Synthesis; Schluter, A.-D., Ed.; Wiley-VCH: Weinheim, 1999; p 403. (5) Jansen, J. F. G. A.; de Brabander-van den Berg, E. M. M.; Meijer, E. W. Science 1994, 266, 1226.

for chemical and biological applications.6-8 In recent years, interest has progressively turned not only to the synthetic aspects and chemical properties of dendrimers, but also to their physical properties.2,9-13 Thus, they can be modified to expose a variety of outer groups that determine, for example, optical activity14 or molecular surface energies.15 It is also possible to create dendrimers (by the convergent approach) that expose different groups on different regions of their surfaces, rendering them amphiphilic. This allows intriguing possibilities for the modification of macroscopic surfaces or interfaces at which they may adsorb.16 The adhesion forces per molecule may be large (as several surface groups can attach to the surface) while at the same time the molecules themselves (6) Astruc, D. C. R. Acad. Sci. Ser. II: Mec., Phys., Chim., Astron. 1996, 322, 757. (7) Wallimann, P.; Seiler, P.; Diederich, F. Helv. Chim. Acta 1996, 79, 779. (8) Kim, Y.; Zimmermann, S. C. Curr. Opin. Chem. Biol. 1998, 2, 733. (9) Wooley, K. L.; Hawker, C. J.; Pochan, J. M.; Frechet, J. M. J. Macromolecules 1993, 26, 1514. (10) Sheiko, S. S.; Eckert, G.; Ignateva, G.; Muzafarov, A. M.; Spickermann, J.; Rader, H. J.; Mo¨ller, M. Macromol. Rapid Commun. 1996, 17, 283. (11) Bar, G.; Rubin, S.; Cutts, R. W.; Taylor, T. N.; Zawodzinski, T. Langmuir 1996, 12, 1172. (12) Tsukruk, V. V.; Rinderspracher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (13) Kopelman, R.; Shortreed, M.; Zhong-You, S.; Weihong, T.; Zhifu, X.; Moore, J.; Bar-Haim, A.; Klafter, J. Phys. Rev. Lett. 1997, 78, 1239. (14) Peerlings, H. W. I.; Meijer, E. W. Chem. Eur. J. 1997, 3, 974. (15) Service, R. F. Science 1995, 267, 458. (16) Bosman, A. W.; Bruining, M.; Kooijmans, H.; Spek, A. L.; Janssen, R. A. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199.

10.1021/la990378e CCC: $19.00 © 2000 American Chemical Society Published on Web 02/04/2000

Low-Surface-Energy Poly(propyleneimine) Dendrimers

are compact and semirigid, leading to surface layers with properties that may differ qualitatively from adsorbed polymers. Several recent studies have investigated the structure of films or surface-adsorbed layers of dendrimers using Langmuir-Blodgett and scanning probe microscopy methods.10-12,16-18 The aim of this investigation is to determine how adsorbed dendrimers modify interactions between surfaces, using a surface force balance (SFB) capable of measuring both normal and lateral surface forces with high resolution. At the same time it is of interest to determine the mechanical properties of the dendrimer molecules by observing their deformation in response to applied stress. Previous studies19-21 on the use of surfaceattached polymers in a good solvent as lubricants reveal that they may lead to striking reduction in friction between sliding surfaces. The origin of this is the large contribution of configurational entropy of the flexible polymer chains to the osmotic pressure that keeps the two surfaces aparts and the interface between them fluidseven under large compressions. At the same time, simple (small molecule) liquids of quasi-spherical shape undergo confinementinduced solidification when compressed between solid surfaces, with a resulting high yield stress of the confined films and consequent large friction.22,23 Confined films of linear chains and of branched chains exhibit intermediate behavior.24,25 Surface layers of dendrimers have characteristics of both spherical and polymeric molecules, and a chemistry of exposed groups that may be well controlled, and are thus attractive candidates for investigation. In the present paper we describe interactions between mica surfaces bearing a fifth-generation amino acid-modified poly(propyleneimine) dendrimer, also known as the rigid dendritic box, exposing low-energy methyl groups at its outer surface. In the following paper, by way of contrast, we extend this to the case of carbosilane dendrimers terminated with -OH groups of much higher surface energy. Experimental Section Materials. The molecular structure of the dendrimer used in our experiments is shown in Figure 1. The precursor of this molecule, synthesized by the divergent approach,26 is a poly(propyleneimine) dendrimer with 64 amine end groups. Subsequently the end groups were modified with phenylalanine, where the amino group was protected as a tert-butyl carbamate forming the rigid shell,5 so that the outer surface consists of 192 densely packed methyl groups. The molecular weight of the dendrimer is ca. 22400 as measured by MALDI-TOF mass spectrometry (with some polydispersity), compared with 22560 from its formula. The diameter of the molecules in solution is d ) 4.5 nm as determined by small-angle X-ray scattering, dynamic light scattering and computer modeling.27 This yields a mean density F ) 0.79 g/mL within the volume pervaded by the polymer. The outer shell of this dendrimer is known to be closely packed and (17) Sheiko, S. S.; Buzin, A. I.; Muzafarov, A. M.; Rebrov, E. A.; Getmanova, E. G. Am. Chem. Soc. Polym. Abstr. 1998, 215, 195. (18) Sheiko, S. S.; Buzin, A. I.; Muzafarov, A. M.; Rebrov, E. A.; Getmanova, E. V. Langmuir 1998, 14, 7468. (19) Klein, J. Pure Appl. Chem. 1992, 64, 1577. (20) Klein, J.; Kumacheva, E.; Mahalu, D.; Perahia, D.; Fetters, L. J. Nature 1994, 370, 634. (21) Klein, J. Annu. Rev. Mater. Sci. 1996, 26, 581. (22) Klein, J.; Kumacheva, E. J. Chem. Phys. 1998, 108, 6996. (23) Kumacheva, E.; Klein, J. J. Chem. Phys. 1998, 108, 7010. (24) Gee, M. L.; McGuiggan, P. M.; Israelachvili, J. N.; Homola, A. M. J. Chem. Phys. 1990, 93, 1895. (25) Granick, S. Science 1991, 253, 1374. (26) de Brabander-van den Berg, E. M. M.; Meijer, E. W. Angew. Chem., Int. Ed. Engl. 1993, 32, 2, (27) Jansen, J. F. G. A.; de Brabander, E. M. M.; Meijer, E. W. In New Macromolecular Architecture and Function; Kamachi, M., Nakamura, A. Eds.; Springer-Verlag: Berlin, 1996; p 99.

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Figure 1. Two-dimensional representation of the modified fifth-generation poly(propyleneimine) dendrimer used in this study. The dendrimer is composed of a poly(propyleneimine) core and a phenylalanine outer shell, exposing 192 -CH3 groups. therefore rather stiff, and indeed it is not possible to synthesize the next generation of this molecule. NMR relaxation data confirmed the solidlike character of the shell.5 Intramolecular hydrogen bonding in the shell is proposed to be responsible for this solidlike character. The solvent used was hypergrade toluene [99.8%, from Merck (Germany)], a good solvent for the dendrimer, used as received from freshly opened bottles. The mica was grade 1 Ruby Clear, supplied by S. & J. Trading Inc., New York. Apparatus and Procedure. The SFB used is a modified version of one that has been described in detail earlier,22 and is capable of measuring normal forces and shear forces between atomically smooth mica surfaces as a function of the separation, load, and (in the case of the shear forces) the sliding velocity vs. Mica sheets (backsilvered and in the thickness range 1-2 mm) were glued using glucose on cylindrical quartz lenses (radius R ) 1 cm), and mounted in a cross-cylindrical configuration into the instrument. The top lens is mounted on a sectored piezoelectric tube (PZT) that can move it both normally and laterally (and parallel) relative to the lower lens, which is fixed on two orthogonal sets of leaf springs (a schematic is shown as an inset to Figure 3). These have constants K2 ) 68 N/m (normal force spring) and K1 ) 30 N/m (shear spring), respectively. The separation D of closest approach between the surfaces was measured to (1-2 Å in the usual way via white-light interferometry, and the normal forces determined as previously described22 by monitoring the bending of K2 as a function of D. Lateral shear motion at a controlled velocity vs is provided by the sectored PZT,22 which can be adjusted via a simple feedback control to eliminate unwanted nonparallel motion. The shear force Fs(D, vs) between the surfaces is measured via the bending of K1, which is monitored (to ca. 2-3 Å) as a change in the capacitance of an air gap by a bridge (Accumeasure 5000). The bridge response was recorded on a digital storing oscilloscope (Lecroy 9304A). The experimental procedure was as follows: after mounting and calibrating the apparatus and air-contact fringe position,

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Figure 2. SFM micrograph of the dendrimer from a droplet of 0.05 mg/mL solution in toluene on a freshly cleaved mica surface. The solvent was evaporated before taking the micrograph. normal forces F(D) and shear forces Fs(D) between mica surfaces immersed in pure toluene were first measured. Then the surfaces were separated to about 1-2 mm apart and dendrimer solution was transferred into the box. The box could be either completely filled with liquid (in the case of the lower dendrimer concentrations) or by introducing a droplet between the mica surfaces (used for the higher-concentration solutions). Force measurements were carried out at 24 ( 0.5 °C after incubation times of 2-3 h. Further details are given in the following section.

Results and Discussion We consider our results separately for the case of lowconcentration solutions (0.05-0.3 mg/mL) and for the highconcentration solutions (0.5-1 mg/mL). This is because in the former case there are clear indications that we are dealing both with molecularly dispersed dendrimer solutions and with well-defined molecular layers on the surfaces. In experiments using the high-concentration solutions, on the other hand, there are indications of dendrimer aggregate formation both in the bulksas observed also by others in different dendrimer systems28s and particularly on the surfaces, leading to very different behavior both of the normal and of the lateral surface inteactions. We first describe the low solution concentration regime. Scanning Force Microscopy Results. A droplet (ca. 0.02 mL) of dendrimer solution (0.05 mg/mL) was put on a freshly cleaved mica surface, and spread, spontaneously and by tilting, over an area of ca. 10 cm2, and allowed to evaporate. An image of the mica surface observed using scanning force microscopy (SFM) is shown in Figure 2, and is very typical of images taken elsewhere on the surface under the evaporated solution (a Topometrix TMX2000 Discoverer was used in the intermittent contact mode; the microfabricated Si probes had a resonance frequency around 300 kHz). Areas of quite smooth, uniform film of thickness 3.0 ( 0.2 nm covering part of the surface are clearly seen, with rounded edges suggestive of growth of drying holes during the evaporation process. A simple estimate shows that the amount of dendrimer in the spread droplet suffices to cover only some 40% of the surface with a 3-nm-thick film, which is consistent with the partial coverage of the surface by the coherent films. We conclude therefore that the films observed are dendrimer monolayers, which are deposited on the mica and then assemble into coherent films as the solvent (toluene) evaporates and recedes. We note that these films have a thickness only some 2/3 the diameter of the dendrimer in bulk solution, suggesting that the molecules underwent a (28) Jackson, C. L.; Chanzy, H. D.; Booy, F. P.; Drake, B. J.; Tomalia, D. A.; Bauer, B. J.; Amis, E. J. Macromolecules 1998, 31, 6259.

Figure 3. Normal force (F)-distance (D) profiles between curved mica surfaces (radius of curvature R) immersed in toluene. The force profile is normalized as F(D)/R in the Derjaguin approximation.38 The open symbols correspond to measurements between approaching surfaces, and solid symbols to receding surfaces. Arrows marked J show positions of jumps in or out, whereas the solid line has a slope equal to the spring constant K2. The broken curve is the expected van der Waals attraction between crossed cylinders of radius R, (F/R) ) -A/ 6D2, where A ) 1.3 × 10-20. J is the appropriate Hamaker constant estimated for interaction between mica sheets across toluene. The inset shows schematically the configuration of the measurements where the top mica surface is mounted on the sectored PZT and the normal forces F and shear forces Fs are given by the bending of the springs K2 and K1 respectively.

compaction of some 30-35% of their diameter normal to the surface as a result of the solvent evaporation.29 Earlier measurements by different groups11,12 of the thickness of polyamidoamine dendrimer monolayers adsorbed on a silicon surface using X-ray photoelectron spectroscopy, X-ray reflectivity, and atomic force microscopy (AFM), showed an even greater compaction normal to the adsorbing surface, of some 60% of the unperturbed dendrimer diameter. The smaller compaction in the present case is consistent with the greater rigidity expected for the modified poly(propyleneimine) dendrimer used in our study.5 Normal Interactions. Before studying the dendrimer solution, the forces between the surfaces were measured in pure toluene. Figure 3 shows typical profiles of force versus distance as measured in our experiments [normalized as F(D)/R versus D, see caption]. No force was detected from large separations (>1000 nm) down to about 20 nm, when attraction commenced, and a jump from D ) Dj j 12 nm into a new contact position at D ≈ 2.2 nm was observed, followed by a hard-wall repulsion on further compression and a jump-out on receding the surfaces. This is very similar to profiles measured in toluene in earlier studies,30 and does not exhibit structural forces, probably as the solvent (as in the earlier work) was not especially dried to remove trace water, which is known to suppress (29) AFM images of mica surfaces that had been incubated in a 0.05 mg/mL dendrimer/toluene solution and then removed and dried suggest the surfaces are coated with dendrimer films of similar thickness. The extent of coverage varied from a fully covered surface (continuous unbroken dendrimer layer) to one where irregular holes formed in an otherwise fully coated surface. These holes may have formed as a result of solvent evaporation. (30) Luckham, P. F.; Klein, J. Macromolecules 1985, 18, 721.


Figure 4. Normal force (F)-distance (D) profiles between curved mica surfaces (radius of curvature R) after incubation in 0.05 mg/mL dendrimer/toluene solution. Empty symbols correspond to an approaching motion, filled symbols to receding motion. For other lines and curves see caption to Figure 3.

any oscillating force behavior. Also shown (broken line) is the expected van der Waals attraction between crossed mica cylinders across toluene, and the line corresponding to the spring constant K2: a jump into contact is expected whenever ∂F/∂D g K2, or at ca. D ) 9 nm in this case. This is close to if somewhat smaller than the observed value Dj. The normal forces between the two surfaces after incubation to equilibrium in 0.05 mg/mL dendrimer solution are shown in Figure 4 (results in 0.2 and 0.3 mg/mL dendrimer solutions gave similar force profiles, save that on the very first approach there was a transient weak repulsion that disappeared after a first compression). The (F/R) versus D profile on initial approach again shows little interaction at large separations, with attraction commencing at D j 20 nm. On closer approach, an inward jump is observed from D j 12 nm into a new equilibrium position at D ) 6.5 ( 0.2 nm. The depth of the attractive well (ca. 100 µN/m) is much weaker than for the bare mica surfaces in toluene (ca. 500 µN/m, Figure 3). Indeed, the experimental profiles in both Figures 3 and 4 are, within the scatter, very close to each other and to the predicted expression for van der Waals attraction,31 save that the profile in the dendrimer solution is cut off by a repulsive “wall” at D ) 6.5 nm: the adsorbed dendrimers in the gap between the two surfaces prevent closer approach at under the attractive force at this separation. This suggests that in both cases it is the van der Waals interactions between the mica substrate sufaces that comprise the main contribution to the attractive surface forces. It is of interest to consider in more detail the changes in the adsorbed dendrimer layers as the surfaces approach each other. We assume that each mica surface has adsorbed on it a layer of dendrimers within the toluene whose effective thickness L is comparable with the (31) The refractive indices of the dendrimer and of toluenesand thus their Hamaker constantssare similar, so that the dendrimer layer, to a first approximation, does not significantly change the van der Waals forces between the mica sheets across the dendrimer/toluene sandwich in the gap relative to pure toluene.

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Figure 5. The variation of F/R versus D at small separations after incubation in 0.05 mg/mL dendrimer/toluene solution. The inset illustrates schematically the compressed dendrimers in the intersurface gap.

unperturbed molecules, L ≈ 4.5 nm. There are two reasons why L should be larger, when the layers are immersed in toluene, than the dry film thickness. One is that there may be some solvation of the dendrimers by the solvent, though this is likely to be limited by the closed-shell dendrimer structure and by the moderately high density of its core. The other is that the adhesive interaction of dendrimers with the mica surface when they are immersed in toluene is much weaker than it is in air (because the van der Waals interactions with both species are similar, so that the net adhesion of the surface with the dendrimers is smaller when immersed in the solvent), leading to reduced compressive stresses on the adsorbed molecules. We also assume that the dendrimers on the surface are quite closely packed, that is, more or less in contact on the surfaces. Because adsorption occurs and the bulk concentration of 5 × 10-4 w/w is likely to be within the plateau regime for the adsorbing polymers,32 such close packing is expected (in other words, the free energy gain per adsorbed dendrimer makes it worthwhilesin this bulk concentration regimesfor as many to adsorb as possible). As the surfaces approach each other, the attractive forces between the mica surfaces pull them together, compressing the adsorbed dendrimer double layer to below its unperturbed thickness 2L until the gap has decreased to some 6.5 nm (Figure 5). At this point the attractive forces are balanced by the repulsion due to the resistance of the dendrimers to further compression. Increasing the normal load results in further compression of the dendrimer layers. After the jump into the attractive well at D ) 6.5 nm, the double dendrimer layer may be further compressed from 6.5 ( 0.2 nm to 4.6 ( 0.2 nm on going to a normal force corresponding to ca. 2000 µN/m, beyond which additional normal loads result in little detectable further compaction even up to 4000 µN/m, as shown in Figure 5. That is, the effective thickness of each layer goes from (32) Fleer, G. J.; Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993.

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some (2/3) (just after the jump) to (1/2) of the unperturbed mean dendrimer diameter. The compaction is reversible on decreasing the pressure: the separation between the surfaces remains essentially unchanged as F/R goes from 5000 µN/m to ca. 2000 µN/m, and then increases from 4.5 ( 0.2 nm to 6.4 ( 0.1 nm as F/R decreases from about 2000 µN/m to ca. -100 mN/m, when the surfaces jump apart. This reversible compaction may be used to extract a rough measure of the effective compressibility of the surface-attached dendrimer molecules. To do this we evaluate the pressure P(D) at the point of closest approach D. This pressure is given by

P ) (1/2πR)(∂F/∂D)

(1)

and may be viewed as the effective normal stress σ on the dendrimers at that point. The corresponding strain on these dendrimers may be taken as their relative distortion from their unperturbed shape, ) [L - (D/2)]/L. Then an effective modulus may be defined as

Kd ) σ/ ) [(1/2πR)(∂F/∂D)]/[(L - (D/2)]/L]

(2)

This modulus clearly increases rapidly at higher compressions, as the slope (∂Fn/∂D) becomes larger. For example, at D ) 5.5 nm, we estimate Kd ≈ (3 ( 1) × 105 N/m2, whereas at D ) 5 nm, Kd ) (4.5 ( 1) × 105 N/m2, and for D j 4.5 nm the slope (∂F/∂D) is too high to be reasonably estimated from the force profile. These values are comparable with the bulk modulus of cross-linked rubber, for example, or with the osmotic pressure of a polymer/good-solvent solution at the segment concentrations within the core of the dendrimer molecules.33 However, it should be emphasized that several assumptions have gone into this estimate, which should be viewed as approximate only. In particular, we have taken the dendrimer layer to fully cover the surface; a less-than-full coverage would imply a higher value for the dendrimer modulus, as fewer dendrimers would then be bearing the load. It would be of interest, however, to calculate the elastic response of the close-packed outer shell of methyl groups on the dendrimer and the elastic/ osmotic response expected from compression of the core of the molecule to see which of these makes the dominant contribution to the dendrimer modulus. The overall picture that emerges is one where the dendrimers are “softer” to weak distortions, but become progressively more difficult to compress at higher distortions. This is qualitatively in line with the schematic diagram of the dendrimer, which suggests that the central core is less dense and thus initially easier to deform, whereas the shell exhibits solidlike character.5,16,27 Shear Forces. The shear response was first monitored in pure toluene, before dendrimer was added to the solution, and is shown in Figure 6. Figure 6a, top trace, shows the back-and-forth motion applied via the sectored PZT to the top mica surface, whereas the lower traces show the variation of the force transmitted to the lower surface. A clear stick-slip motion during sliding is observed, with a yield value Fy (the top of the stick part of the cycle), which increases with increasing normal load (traces b-e); this behavior persisted over a range of applied shear velocities (vs ) 17-186 nm/s). Similar stick-slip behavior has been observed previously in toluene and in other confined simple liquids.20,24,23 Shear forces were then measured between the surfaces after their incubation in 0.05 mg/mL dendrimer solution. (33) Noda, I.; Kato, N.; Kitano, T.; Nagasawa, M. Macromolecules 1981, 16, 668.

Figure 6. Traces b-e show the variation with time of the shear forces (RH axis) across mica surfaces immersed in toluene, at a separation D ) 2.2 ( 0.3 nm, in response to a back-andforth lateral motion (LH axis), trace a, applied to the top surface. The applied shear velocity of the top surface in each direction is 40 nm/s. The normal load (F/R) increases from b to e: (b) 30 µN/m, (c) 137.5 µN/m, (d) 233.2 µN/m, (e) 428 µN/m. Fy is the value of Fs at the yield point as shown.

Figure 7. Traces b-d show the variation with time of the shear forces (RH axis) across mica surfaces after incubation in 0.05 mg/mL dendrimer solution in toluene, in response to a back-and-forth lateral motion (LH axis), trace a, applied to the top surface. The applied shear velocity of the top surface in each direction is 40 nm/s. The normal load (F/R) increases from b to d: (b) 352 µN/m, (c) 1132 µN/m, (d) 1595 µN/m. Fsp is the shear force required for steady sliding (plateau region of traces) as indicated.

No shear forces are detectable as the surfaces approach until after their jump (see Figure 4) to a surface separation at D ) 6.4 ( 0.1 nm, whereon a shear response becomes measurable. Steady sliding takes place once the shear force attains its “plateau” value Fsp, as indicated, and increases progressively with load (and decreasing D) as shown in Figure 7. In Figure 7 a back-and-forth motion (at vs ) 40 nm/s) is applied to the top surface (top trace), and the force transmitted to the lower surface is recorded (lower traces). Similar responses were observed over a


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where Fp is the pull-off force (at the minimum of the adhesive well, Figure 3). Here K is related to the Youngs

modulus of the substrate (mica + glue layer) and has a value estimated22 as K ) (1 ( 0.3) × 109 N/m2. In general, the yield stress Sy may itself depend on the pressure Pm, as has been demonstrated for the shear of simple organic liquids.23 At the simplest level we may postulate a linear relation, Sy ) Sy0 + RPm, where Sy0 ) [Fy(F)0)/A0] is the yield stress at zero applied normal load (F ) 0), A0 being the corresponding contact area. This is the expression corresponding to the solid line through the toluene yield stress versus normal pressure data in Figure 8, with R ) 0.3 and Sy0 ) 1.8 × 104 N/m2. For the case of the compressed dendrimer layers, these are much softer than the substrate on initial compression (K ) ca. 105 N/m2), but we should not use eq 3, as the dendrimer layers are too thin. For the effective contact area we use rather the expression for the flat truncated area exposed by a sphere, radius R, when a spherical section of thickness δ , R is removed from its surface: πa2 ) 2πRδ, where for the compressed dendrimers δ ) L - (D/2). There is a clear difference between the shear behavior across the dendrimer-free toluene and across the surfaceattached dendrimer layers. In the former case the yield force rises sharply, and has a finite value even at zero (or slightly negative) applied normal stress, as expected between surfaces in adhesive contact. The shear of molecularly thin layers of simple organic liquids has been investigated in detail, and has been discussed in terms of the layered structures that such liquids form when confined between the sheared surfaces.23-25 The present case is somewhat different in that the toluene used has not been dried (water content typically of the order of some tens ppm) and the force profile shown in Figure 3 does not exhibit the oscillatory forces associated with layering, but suggests rather some effect of capillary water condensation.35 The increase of the yield stress with normal pressure is similar to that observed in dried (hence layered) simple organic solvents.23 Over the limited range of loads and yield strengths measured, the values R ) 0.3 and Sy0 ) 0.18 × 105 N/m2 in our experiments compare with values of order R ≈ 1-3 and Sy0 ≈ (0.2-0.5) × 105 N/m2 (depending on the number of confined monolayers) obtained for shear of dry cyclohexane, for example.23 The lower values in our study are probably due to the perturbation of the toluene layering by water molecules at the mica surface. The shear stresses for yield and sliding after the incubation in dendrimer solution are much weaker than between the bare mica surfaces; again the data suggest a rather small but finite yield stress even at zero load, consistent with the weak attraction between the layers (Figure 4). However, because the dendrimer layer is much more compressible than the toluene layers, and we did not use eq 3 for the interaction area, we do not discuss in detail the variation of Ssp with Pm. We note merely that the measured yield stress on sliding the dendrimer monolayers increases roughly linearly with normal stress, and that it is only weakly dependent on the shear velocity over a fivefold range of vs (Sy is slightly higher at the highest vs). We discuss possible physical origins of this later; here we note that the effective friction coefficient between the sliding dendrimer layers, that is, the slope of the Ssp vs Pm plot, varies between ca. 0.1-0.2 as the load increases. We next investigated some of the dynamic aspects of the lateral forces. It is in principle straightforward to apply

(34) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, Ser. A 1971, 324, 301.

(35) Christenson, H. K.; Israelachvili, J. N. J. Colloid Interface Sci. 1987, 119, 194.

Figure 8. The variation of shear stress S with normal stress Pm for sliding of mica surfaces. 9, yield stress across toluene;; S ) Sy ) Fy/A and Pm ) F/A, where Fy is the yield shear force (see Figure 6) and A is the contact area given via the JKR expression eq 3 at shear velocity vs ) 40 nm/s. The solid line through the toluene data is the fit to Sy ) Sy0 + RPm, with R ) 0.3 and Sy0 ) 1.8 × 104 N/m2. Other symbols (for the dendrimer-covered surfaces): sliding shear stress S ) Ssp ) Fsp/(area), where Fsp is the shear force required for sliding the surfaces steadily (see Figure 7), after incubation in 0.05 g/mL dendrimer solution, and the area is as in the text, at different applied shear velocities vs of the top surface. 0, 42 nm/s; O, 125 nm/s; 4, 210 nm/s; 3, 340 nm/s; ], 413 nm/s.

velocity range vs ) 17.4 nm/s to 1530 nm/s. We note that stick-slip motion on sliding the adsorbed dendrimers past each other does not occur (though at high applied lateral velocities a broad maximum in the shear force occurs before steady sliding, see traces in Figure 9 and Figure 11). It is interesting to contrast this with the clear stick-slip motion observed when sliding across confined toluene at similar velocities and loads (Figure 6a). This suggests that the greater compressibility of the dendrimer layers leads to a yield mechanism that is softer and has less of a solidlike character. We return to this issue later. Figure 8 summarizes the yield stress Sy (for toluene) and the steady-sliding stress Ssp (for dendrimer-covered surfaces) as a function of the mean normal pressure Pm at different applied lateral velocities of the top surface, where Sy ) (Fy/area of interaction), Ssp ) Fsp/(area of interaction), and Pm ) (F/area of interaction). The area of interaction A is evaluated as πa2 from the radius a of the flattened region between the crossed cylindrical surfaces. For the toluene data we use the Johnson-KendallRoberts (JKR) expression:34,22

A ) πa2 ) π

{2KR [F + 2F

p

}

+ 2(FFp + Fp2)1/2]

2/3

(3)

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Figure 9. Variation of shear force as function of time between compressed surfaces (F/R ) 420 µN/m) after incubation in 0.05 g/mL dendrimer solution. Lower trace: lateral motion applied to top mica surface (RH axis). Region I, top surface at rest; region II, steady sliding motion (lateral velocity of top surface in this region is vs ) 480 nm/s); region III, top surface at rest. Upper trace: the corresponding force transmitted to the lower surface. The straight broken line is the rigid-coupling (nosliding) response. The solid curve through the trace in region C-D is an exponential decay from C to the plateau at D with characteristic time 0.047 s.

a sinusoidal shear motion and to extract from the sinusoidal shear-force response the viscoelastic parameters (e.g. G′ and G′′) of the sheared interfacial layer as a function of shear rate and so on. In the present configuration, however, all the interfacial shear takes place at the surfaces of the mutually compressed dendrimer molecules, which drag past each other, and it is probably not appropriate to analyze the data as though one were dealing with a uniform viscoelastic fluid permeating the intermica gap. Rather, we applied a different kind of motion to the top surface, as shown in Figure 9: initially the surfaces are at rest, region I of bottom trace in the figure, followed by a steady motion (region II; in Figure 9, the lateral velocity in this region is vs ) 480 nm/s) that is stopped after some time (region III). In regime II, the shear force initially rises in a “stick” fashion: the frictional force between the two surfaces at first exceeds the shear force between them, and they move together in tandem as the top surface moves laterally. As the shear force increases it begins to exceed the static frictional force between the dendrimer layers, at around A, at which point the actual shear force falls below the stick response (broken line). At B there is a yield point, at which the shear force falls to a plateau value that is the force required to slide the surfaces steadily past each other. Finally, when the applied lateral motion (regime II) is stopped, the shear forces relax from point C to a long-lived plateau D in the shear force, representing a residual stress. The reduction of the shear force from C to its residual value is well described as an exponential decay with a characteristic decay time τ (solid curve through trace in region C-D). Shear responses to the same pattern of applied lateral motion, but under different normal loads, are shown in Figure 10. The applied motion is the lower trace in the figure; the lateral velocity in regime II is vs ) 732 nm/s (very much lower than for the data in Figure 9). The top traces (a-d) show the full shear force responses, which each exhibit similar features to the trace in Figure 9:

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Figure 10. Variation of shear force as function of time between compressed surfaces at different loads after incubation in 0.05 g/mL dendrimer solution, in response to the pattern of lateral motion applied to the top surface and shown as the bottom trace h (the lateral velocity applied to the top surface is vs ) 732 nm/s). Trace a, F/R ) 370 µN/m; trace b, F/R ) 500 µN/m; trace c, F/R ) 910 µN/m; trace d, F/R ) 1100 µN/m; trace e, F/R ) 2130 µN/m; trace f, F/R ) 2950 µN/m; trace g, F/R ) 4410 µN/m. Table 1. Normal Loads and Relaxation Times for Data of Figure 10 trace

normal load (µN/m)

relaxation time τ (s)

a b c d

371.20 503.80 908.10 1102.0

0.030 0.028 0.030 0.029

initially a stick response, followed by partial sliding as the shear force exceeds the frictional force, then a yield point and a sliding regime, and finally a relaxation (of characteristic time τ) to a residual stress, once the applied motion stops. At higher loads, traces e-g, the frictional force is sufficiently high that the yield point is not reached before the applied lateral motion is stopped. It is interesting, however, that some relaxation of the stress occurs even in these high-compression traces when the applied lateral motion ceases, showing that confined dendrimer layers do not behave in a solidlike manner even at the highest compressions investigated. In Table 1 the relaxation times τ for traces a-d are tabulated. We note that over the threefold range of normal load for these traces, the relaxation times (for a given pattern of the applied lateral motion) remain roughly constant. It is of interest to consider, qualitatively, the molecular origins of the shear forces and the relaxation of the dendrimer layerssadsorbed from dilute solutionsas they are compressed and made to slide past each other. The attraction between the mica surfaces adds to the applied normal loads, and presses the dendrimer shells from opposing layers against each other. On starting to slide one surface against the other the frictional force between them initially exceeds the shear force and they move in a stick mode. This gives rise to a yield stress, due to weak van der Waals adhesion between the opposing outer dendrimer shells that consist primarily of -CH3 groups, butssee Figure 1salso some phenyl and carbamate groups. We should note that the amide groups in the outer shells may induce local dipole moments and contribute to the interactions. It is worth recalling that both polymeric


molecules in good solvents, and also conventional boundary lubricants36 exposing -CH3 groups, tend to have quite low friction coefficients. The higher frictional forces in the present case are probably due to the weakness of conventional excluded volume effects associated with flexible chains, which in that case prevent too intimate a contact between chains because of osmotic pressure.21 At the same time the dendrimer layers are somewhat softer than conventional boundary lubricants, and this probably increases the frictional dissipation by introducing a ploughing effect during sliding. After a sufficiently large shear stress has accumulated because of progressive bending of the shear springs, some sliding begins (point A in Figure 9). As the shear force increases further a soft yield point is reached (B in Figure 9scontrast with the sharp slip at the yield of the confined solvent layers alone, Figure 6), after which the shear stress relaxes to the value required for steady sliding. On stopping the applied lateral motion, the shear stress relaxes further, with characteristic time τ, to a long-lived residual value (C to D in Figure 9). One may speculate that this relaxation process involves some short-range rearrangements of the interacting segments on the outer shells of the dendrimers. Because many outer groups need to rearrange cooperatively, and because they are under strong confinement and thus not very mobile, these relaxations take place over macroscopic times (typically tens of milliseconds, see Table 1). Overall, viewed as lubricants, the dendrimer monolayers have characteristics of both solids (friction only weakly dependent on sliding velocity, long-lived residual shear stresses) and of liquid lubricants [(partial) relaxation of shear stresses].24,37 Interactions after Incubation in High-Concentration Dendrimer Solution. Surface interactions after incubation in 0.5 and 1.0 mg/mL solutions of the dendrimer in toluene were very different from those described above, and clearly indicated aggregation of the molecules into loose multilayers at the surface of each mica substrate. Detailed analysis in terms of well-defined surface layers is thus more difficult, and we describe only the broad features of the normal and shear interactions. Figure 11 shows the normal force-distance profile after the mica surfaces have been incubated in 0.5 mg/mL dendrimer solution. In contrast to the attractive interactions in low-concentration profiles (Figure 4), monotonically increasing repulsion is measured on a first approach, curve a, which commences at ca. 50 nm. On a first separation a large hysteresis is observed, leading to a much shorter-ranged repulsion, curve b, whereas curve c, showing the interactions on a second approach, closely follows curve b again. The second separation, curve d, is again hysteretic and of shorter range, whereas the third approach and third separation (curves e, f) are similar to each other, and slightly closer in than d. By the time of the third approach the range of the repulsion has dropped to ca. 15 nm. These profiles provide a clear indication of an aggregation of material between the surfaces, which appears to be either compacted or squeezed out on progressive compressions. The amount of material remaining after the third approach still results in a repulsion whose range (ca. 15 nm) is sufficient to mask the van der Waals attraction between the mica surfaces. (36) Tabor, D. In Boundary Lubrication; Georges, J. M., Ed.; Elsevier: New York, 1982; p 651. (37) Tabor, D. Friction; Doubleday: New York, 1973. (38) The Derjaguin approximation enables the conversion of the force data F(D) between the curved mica surfaces (mean radius R) a closest distance D apart to the interaction energy per unit area E(D) ) F(D)/ 2πR between parallel plates a distance D apart.

Langmuir, Vol. 16, No. 8, 2000 3891

Figure 11. Force versus distance profile between mica surfaces after incubation in 0.5 mg/mL dendrimer/toluene solution. Curve a, first approach of surfaces; curve b, first separation of surfaces; curve c, second approach; curve d, second separation; curves e, f, third approach and separation.

Figure 12. Force versus distance profile between mica surfaces after incubation in 1.0 mg/mL dendrimer/toluene solution. Empty symbols are for approaching surfaces, full symbols for receding surfaces. 0 9, first approach and separation; O b, second approach and separation; 4 2, third approach and separation; ] [, fourth approach and separation. The final two approach cycles (4, ], solid curve to guide the eye) are identical within the scatter, and represent the steady-state approach F(D)/R profile for approaching surfaces in these conditions.

Incubation in a 1 mg/mL solution of the dendrimer results in similar but more marked behavior, as shown in the force-distance profile of Figure 12. Here again monotonic repulsion in the first approach profile commences from relatively large surface separations, D ≈ 100 nm, and as in Figure 11, hysteretic compaction takes place on separating the surfaces; subsequent profiles are similar but move in to lower D values. However, after three or four approach/separation cycles a limiting repulsive interaction profile on approach of the surfaces is attained,

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by the shear measurements themselves. The magnitude of the shear forces is clearly very much lower than the normal loads (on approach), and this is highlighted in the inset. Over nearly the entire surface separation regime, from 40 to 10 nm, over which the shear forces were measured, the effective friction coefficientsdefined as (force to slide surfaces/normal load)sis of the order of 0.05. This is significantly lower than for the case of sliding of the single monolayers, and indicates that the loosely attached dendrimer layers form a more efficient lubricant. Qualitatively, the origin of this is probably via an osmotic pressure within the solvated, aggregated dendrimers, which can support a large normal load while maintaining a fluid interface.

Figure 13. Shear force Fs(D)/R and normal force F(D)/R versus distance profiles between mica surfaces after incubation in 1.0 mg/mL dendrimer/toluene solution. Two in/out normal force profiles (4 2 and ] [ respectively) were followed by a shear forces profile as the surfaces approached (×). After this, two further in/out normal force profiles were measured (0 9 and O b respectively): these are closely similar to the preshear profiles, indicating that little dendrimer is removed from between the surfaces during shear. The solid curve is a guide to the eye to the limiting F/R profile for approach. The inset shows the Fs(D)/R profile on an expanded scale, where the solid curved line corresponds to the limiting F(D)/R variation (as solid curve in main figure). The ratio of the two, Fs/F, is roughly 0.05 and may be viewed as the effective coefficient of friction.

commencing at ca. 40 nm; here again the force-distance profile upon separation of the surfaces is of much shorter range (ca. 12 nm), and indicates that compaction of the surface-associated dendrimers has occurred. However, on re-approaching, the range of repulsion is shifted out again to ca. 40 nm. This strongly suggests that, after the incubation in the 1 mg/mL solution, there remains a loose aggregation of dendrimers at the surface that are compacted about three- to fourfold on compression but recover their loose structure within some minutes after the surfaces are separated. Figure 13 shows both normal and shear force profiles taken on such a limiting adsorbed layer. Normal profiles were taken both before and after the shear measurements, whereas the shear itself was carried out at different D values as the surfaces were made to approach from a large separation. The normal interactions before and after the shear are similar, both commencing at ca. 40 nm, though the postshear profiles have repulsive walls somewhat closer in; this suggests that little material was removed

Summary We have characterized for the first time the modification of surface interactions by adsorbed dendrimers, and of shear and frictional forces between sliding dendrimerbearing surfaces. Our results reveal that for the case of a fifth-generation poly(propyleneimine) dendrimer modified to expose low-energy methyl groups on its outer surface, the molecules adsorb in monolayers from dilute solution in toluene onto high-energy solid surfaces (mica). From the force-distance profile we obtain a measure of their compressibility, which decreases as the dendrimers are progressively compressed. Frictional forces between sliding surfaces bearing the dendrimer monolayers show a moderate effective friction coefficient (0.1-0.2) over the range of pressures and shear velocities in our experiments, suggestive of a behavior intermediate between a solid and a flexible polymer. The frictional profiles also reveal rich yield and relaxation behavior of the sheared dendrimer layers. Incubation at high dendrimer concentrations (> 0.5 mg/mL) indicates the formation of much thicker, loose aggregated mutlilayers on the surfaces, which leads to longer-ranged repulsion between the surfaces, and to rather low effective friction coefficients over the range studied (ca. 0.05). This is probably due to an osmotic repulsion between the aggregated layers, which can support a normal load and yet remain relatively fluid. In the following paper39 we consider surface modification by dendrimers exposing -OH groups; the very different behavior suggests that through tailoring of the exposed groups one can tune the surface modification by these molecules over a large spectrum. Acknowledgment. We thank the Deutsches-Israel Program (DIP), the US-Israel Binational Science Foundation, and the Israel Science Foundation for support of this work. Support from the Council for Chemical Sciences of The Netherlands Organization for Scientific Research (CW-NWO) is gratefully acknowledged. LA990378E (39) Zhang, X.; Klein, J.; Sheiko, S. S.; Muzafarov, A. M. Langmuir 2000, 16, 3893.

Modification of Surface Interactions and Friction by Adsorbed

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