NANO LETTERS
Tunable Nanolubrication between Dual-Responsive Polyionic Grafts
2009 Vol. 9, No. 8 2984-2990
Niklas Nordgren and Mark W. Rutland* Department of Chemistry, Surface and Corrosion Science, Royal Institute of Technology, 10044 Stockholm, Sweden Received May 3, 2009; Revised Manuscript Received June 18, 2009
ABSTRACT This study reports on a direct approach of quantitatively probing the nanotribological response of chemically end-grafted polyions. A combination of a quartz crystal microbalance with dissipation and atomic force microscopy, in the now well established colloidal probe mode, was utilized to investigate the stimuli-induced lubrication behavior between poly(2-(dimethylamino)ethyl methacrylate) grafts on gold. Force and friction measurements showed reversible transitions of up to an order of magnitude difference induced by varying the solvent conditions. The greatly enhanced lubrication observed at low pH was attributed to the formation of a repulsive, highly charged, hydrated cushion. At high pH the friction was significantly increased. The system turned attractive above the lower critical solution temperature with a small friction reduction interpreted as being due to nanoscopic flattening at the interfacial boundary.
The use of polymers that respond to external stimuli such as temperature, pH, magnetism, or changes in electrolyte concentration has been widely studied, mainly with the aim of producing nanomaterials with novel functionalities. When attached to a solid support, changes in the solvent conditions can result in swelling and collapse of the tethered macromolecules, a trait which is of high relevance in medicine and biotechnology. Potential applications range from controlled release and delivery1 to biomimetic materials2 and sensors.3 Moreover, the growing interest in miniaturizing different processes has led to implications concerning surface interactions on the nanoscopic level. These piconewton-scale forces may have both normal and lateral (friction) components of significance. Well-defined surfaces bearing endgrafted polyions can be used for fundamental studies of these interactions. In addition, such surfaces can act as simplistic models for obtaining insight into the behavior of more complex biological processes or to improve interfacial properties to facilitate development of medical implants and nanoelectromechanical systems (NEMS) suffering from interfacial friction and wear. A technique that lends itself well to studying nanotribological phenomena in polymer films is atomic force microscopy (AFM)4,5 particularly in colloidal probe mode,6 which is the focus of the current paper. For example, poly-N-isopropylacrylamide (PNIPAAM)7,8 has been extensively investigated due to its thermosensitive solution properties, exhibiting a lower critical solution temperature (LCST) of 32 °C below which the polymer is fully soluble in aqueous media and above which it exists in a collapsed, nonsoluble state. Recently, multiresponsive polymers sensitive to more than one stimulus have drawn considerable attention. These 10.1021/nl901411e CCC: $40.75 Published on Web 07/27/2009
2009 American Chemical Society
materials can be either copolymers8 or homopolymers9 built up from monomers exhibiting the desired properties. An example of the latter is the polycation based on the monomer 2-(dimethylamino)ethyl methacrylate (DMAEMA) which can undergo both temperature and pH induced conformational transitions in aqueous media. Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) in bulk exhibits an LCST of 40-50 °C10-12 which in turn is pH-tunable due to the ionizable tertiary amine moieties on the polymer backbone (apparent pKa of 7.0-7.5).13,14 Thus the LCST can be raised at acidic pH (LCST > 95 °C at pH 4)11 where the induced charge increases the hydrophilicity and lowered at basic pH. Comprehensive literature is available concerning the synthesis of PDMAEMA15,16 as well as formation and properties of hydrogels17-20 and micellar21-23 structures based on its homo- and copolymers.24-26 Moreover, PDMAEMA, which can also be used as a precursor for producing antibacterial surfaces,27 has been chemically grafted or physisorbed onto a variety of substrates for the manufacture of functional nanomaterials. These substrates include synthetic polymers,28,29 but biofibers and polysaccharides like cellulose,30 starch,28 and chitosan31 have also been modified. The present study focuses on the interactions between PDMAEMA-grafted gold substrates. The main goal was to elucidate how the interfacial nanotribology is quantitatively affected by conformational changes of the surface bound species induced by varying pH and temperature of the surrounding media. In addition, the described experimental configuration can be viewed as a model for better understanding interactions between biomacromolecular systems for which lateral forces have shown to be of significance.32-36
Figure 1. Synthetic pathway for the preparation of monothiol-functionalized PDMAEMA by conversion of the dithioester RAFT moiety (upper). The subsequent grafting of the polymer chains to a gold QCM crystal is schematically presented (lower).
In this study, PDMAEMA was synthesized by the versatile living free radical polymerization technique: reversible addition-fragmentation chain transfer (RAFT)37-39 and the macromolecules (weight-average molecular mass Mw of 2.8 × 104 with a polydispersity index (Mw/Mn) of 1.42) were subsequently chemically end-grafted to gold via thiol bonds (see the Supporting Information for details regarding polymer synthesis and characterization). The inherent living property of macromolecules produced via RAFT is that they contain chain-end functionality in the form of a dithioester (in this case, dithiobenzoate). This has been utilized herein for mediating chemical surface anchorage. Conversion of the dithioester into a monothiol group was performed as previously described for nonionic RAFT-polymers40 by the addition of NaBH4 (0.5 mL, 1.0 M) to an aqueous solution of PDMAEMA (5.0 mL, 0.1 mM). The conversion and endgrafting of the PDMAEMA chains to gold were performed according to the scheme in Figure 1. The grafting was monitored in situ by the use of a quartz crystal microbalance with dissipation41-44 (QCM-D Q-Sense E4 system). Adsorption of PDMAEMA (0.1 mM) to the gold surface was monitored at pH 5.7 at room temperature (24 °C). The change in adsorbed mass per unit surface area, ∆m(Γ), was calculated according to the Sauerbrey equation45 Nano Lett., Vol. 9, No. 8, 2009
∆m ) -
C∆fn n
(1)
where C is a calibration constant for the crystal, n is the overtone number, and ∆fn is the frequency change of that overtone. In this case C ) 0.177 mg m-2 Hz-1 and n ) 3. Once the ac voltage driving the crystal, oscillating at its resonance frequency, is turned off, the decay rate of the amplitude can be monitored and gives rise to a dissipation factor. Dissipation (D) is defined as the ratio between energy dissipated and the stored energy during a single crystal oscillation41 D)
EDissipated 2πEStored
(2)
In order to investigate the significance of the conversion on the grafting behavior, it was compared with the sample containing neat PDMAEMA (0.1 mM) with the dithioester still intact. This was necessary in order to justify the extra chemical reduction step since dithioesters have also previously been shown to have an affinity for gold.46 Figure 2 depicts the grafting behavior where the lower curve shows the neat sample and the upper curve shows the modified sample. The apparent adsorbed amount (Γ) was calculated 2985
Figure 2. Adsorption isotherms for the grafting of neat PDMAEMA (lower curve) and thiol-capped PDMAEMA (upper curve) to gold obtained by QCM-D. Measurements were conducted in water at pH 5.7 with a maximum flow rate of 150 µL/min.
using the Sauerbrey relation (eq 1) which is applicable for relatively packed layers where the dissipation-mass change ratio is small.47,48 Initially, the adsorbed amounts are similar in the two cases, reflecting similar conformation on adsorption. After only a few minutes the difference between the two cases is substantial with a much higher adsorbed amount observed for the sample containing the monothiolated species. Importantly, after the sample was carefully rinsed in order to remove any nonchemically attached material, the irreversibly adsorbed amount for the monothiolated PDMAEMA was reproducible at 11.7 ( 1.3 mg/m2 in comparison with the lower value of 4.5 ( 0.96 mg/m2 observed for the dithioester-capped (neat) chains. The conformation of the chemisorbed chains can be estimated by considering the geometry of the macromolecules. Assuming the adsorbed amount is solely due to the mass of the polymer, excluding any coupled water,49 it can be shown that the area occupied by a single chain is about 4 nm2 for the higher adsorbed amount (monothiolated polymer) and about 10 nm2 for the neat polymer. A single chain adsorbing in a flat conformation would occupy a space of about 38 nm2 and a fully extended freely rotating chain about 1.7 nm2. The monothiolated PDMAEMA is thus in an extended brushlike regime- and the dithioester-capped PDMAEMA (control) is in a compromise adsorption state between pancake and end-graft since there is a small affinity of the terminal group of the latter for gold.46 Of course these numbers provide only an estimate of the upper limit to the grafting density (since associated water is ignored); a lower limit will be obtained presently from consideration of the force curves. (Note that under certain conditions50 the Sauerbrey relation has been shown to somewhat underestimate the adsorbed amount. Such a correction would push the graft into a even more tighly packed palisade layer.) Analysis of the change in dissipation (∆D) as a function of the adsorbed amount (Γ) (Figure 3) supports the conclusions drawn from the adsorption isotherms. From the open symbols, representing the dithioester-capped chains, it can 2986
Figure 3. Dissipation (∆D) versus the adsorbed amount (Γ) for the grafting of neat PDMAEMA (open squares) and thiol-capped PDMAEMA (closed circles) to gold obtained by QCM-D. Measurements were conducted at pH 5.7 with a maximum flow rate of 150 µL/min.
be seen that the buildup is fairly linear with a very low dissipation suggesting a rather flat “pancake-like” conformation. Interestingly, in the case of the monothiolated sample, three distinctly different regimes can be distinguished. Initially, in regime I the dissipation is very low corresponding to a flat conformation in which the adsorption is probably due to center-of-mass diffusion from the solution.9 In regime II (onset at an adsorbed amount of about 2 mg/m2) a more extended “mushroom-like” conformation (rapid increase in dissipation) occurs which levels off in regime III where the ∆D/Γ ratio is lower. The occurrence of the plateau in regime III is a strong indication of a densification/compaction of the layer. We ascribe this behavior to be due to the buildup of a more closely packed layer in, or close to, a “brushlike” chain conformation. The conformational changes of the grafted film induced by varying pH and temperature were investigated and are summarized in Figure 4. Here, the mass and dissipation have been arbitrarily set to zero at the final levels of the graft. The pH was adjusted using either HCl or NaOH of analytical grade. For an adsorbed amount of 11.7 mg/m2 it can clearly be seen that swelling induces a 70% mass increase at pH 3 (24 °C). Moreover, the dissipation also increases significantly indicating a high water uptake of the brush. When the temperature is raised to 50 °C at pH 11 (above the LCST), a full collapse of the layer is observed with release of the water bound in the brush, resulting in a mass loss of about 50% (see Supporting Information for experimental details). This fact, combined with the large reduction in the dissipation (close to the final value obtained during the grafting), strongly indicates that the brush is in a fully collapsed conformation under these conditions. The collapsed mass thus provides a better estimate of the grafted mass of PDMAEMA and revises the area per chain to roughly 8 nm2. In order to study the interactions at the single contact level in a symmetric configuration, the colloidal probe technique6 was utilized. Surface force and friction measurements Nano Lett., Vol. 9, No. 8, 2009
Figure 4. Relative changes in mass (black squares) and dissipation (gray squares) due to changes in pH and temperature for a grafted layer of thiol-capped PDMAEMA on gold obtained by QCM-D. Measurements were conducted in water at pH 5.7 with a maximum flow rate of 150 µL/min.
between an identically treated gold sphere and the underlying grafted QCM crystal were conducted. Experiments (full details in Supporting Information) were performed in a fused silica liquid cell using a MultiMode Picoforce with Nanoscope III (AFM) controller (Veeco, USA) following the procedures extensively described previously.51,52 In order to perform measurements at elevated temperature, the instrument was equipped with a thermal application controller attached to a Bio-Heater element mounted on top of the Picoforce-scanner. A thermocouple attached to the liquid cell facilitated accurate monitoring of the temperature in close proximity to the sample surfaces. Friction was measured as in earlier work32,33 as a function of increasing and decreasing load with a scan size of 2 µm and a scan rate of 1 Hz. The normal forces on approach under different solution conditions are summarized in Figure 5. In all cases, a longrange electrostatic repulsion appears to be present indicating that the brush carries charge which should increase with decreasing pH. In the low temperature case, at pH 3, the force deviates from Poisson-Boltzmann theory, which predicts a decay length of about 10 nm for a 1:1 electrolyte at a concentration of 1 mM. The mean extended contour length of PDMAEMA (Mw ) 2.8 × 104) can be estimated (180 repeat units × 0.25 nm/repeat unit) to be 45 nm. The fact that the repulsive force starts to increase more steeply at a range about twice that number (noticeable as a kink in the force curve at a separation of about 80 nm) strongly suggests the presence of an electrosteric force.53 Hence, the repulsion in this case is thought to originate from the full extension of highly charged, strongly hydrated polymer species which act as both a physical and electrostatic barrier. It is noted that the repulsive force is even more long ranged than what would be expected from the calculated average contour length. This is an effect of the non-negligible polydispersity present which is also indicated by the lower magnitude of the force and tends to suggest that a few longer chains are in physical contact up to a separation of about Nano Lett., Vol. 9, No. 8, 2009
Figure 5. Normalized force profiles on approach between a gold sphere and a gold-coated QCM crystal both bearing a grafted layer of PDMAEMA. Measurements (ramp size 1 µm at a rate of 2 µm/ s) were conducted in water at pH 3 and pH 11 (24 or 50 °C). The ionic strength of the solutions corresponds to a Debye length of 10 nm in all cases. The arrow indicates the jump in resulting from an attractive force for the case of pH 11, 50 °C. (This attraction can be more clearly observed in Figure 8). The displayed results are taken from representative individual force curves. The solid gray line represents the theoretical steric repulsion between brushes calculated using eq 3 with L ) 40 nm and s ) 4.5 nm at 24 °C.
120 nm. Increasing pH leads to less repulsive forces, as the charging of the amine groups is gradually switched off. For the case of pH 11 (24 °C) where the charge is expected to be zero, it is not completely clear whether the force is electrostatic or steric in origin. If purely steric repulsion (no electrostatic contribution) is assumed between the brushes, the theory of Alexander54 and de Gennes55 can be applied to the system. The interaction between brushes under favorable solvent conditions can thus be described by the following relationship for a sphere on flat geometry56,57 8kBTL 2L F(d) ) 7 2πR d 35s3
5/4
[( )
+5
( 2Ld )
7/4
- 12
]
(3)
for d < 2L where kB is the Boltzmann constant, L is the brush length, s is the mean distance between the grafted polymer chains, T is the absolute temperature, and d is the surface separation. For example, if considering L ) 40 nm (onset of repulsion ∼ 80 nm) and s ) 4.5 nm, the fit in Figure 5 (solid gray curve) is obtained. This would correspond to an area per molecule of about 20 nm2, which is higher than the calculations based on the QCM mass. This value is no more than a rough estimate since the theory does not take into account polydispersity or any residual electrostatic contribution to the force profile.53,58 As with increase of the pH, raising the temperature also results in less repulsive forces (collapse of the polymer layer) as the hydration of the hydrocarbon regions becomes less entropically favorable. When the temperature is raised to 50 °C, which is above the LCST at pH 11, the surfaces become attractive at long range as shown by the “jump into physical 2987
Figure 6. Friction measurements between a gold sphere and a goldcoated QCM crystal both bearing a grafted layer of PDMAEMA. The data shown are from measurements conducted at 24 °C in aqueous solutions at pH 11 (open squares) and at pH 3 (open triangles). The slopes of the solid lines represent the friction coefficients, calculated as µ ) 1.06 (pH 11) and µ ) 0.07 (pH 3). In order to facilitate comparison with the force data, the normalized applied load (FLoad/2πR) is displayed, where R is the radius of the spherical probe.
contact” at a separation of about 40 nm. This is due to solvophobic attraction since the brushes are exposed to a poor solvent.59,60 The effects of the conformational changes observed from the normal forces can be further investigated by studying the lateral forces as a function of the normal applied load. In Figure 6, typical friction data are displayed for pH 11 and pH 3 at ambient temperature (24 °C). In both cases a linear relation between the observed friction force and the applied load is observed, indicative of Amontonian behavior. However, the presence of a nonzero friction force at zero load at pH 11 indicates that a small adhesion is present.61 Therefore the frictional behavior on both loading and unloading instead follows the slightly modified version of Amonton’s law FFriction(FLoad) ) FFriction(0) + µFLoad
(4)
where FFriction(FLoad) is the total friction force, FLoad is the applied load, µ is the friction coefficient, and FFriction(0) is the friction force at zero applied load. The friction behavior as a function of pH and temperature is displayed in Figure 7. The friction coefficients displayed are the averages taken from the slopes of several individual friction curves (such as those in Figure 6). The lowest friction coefficients (µ ≈ 0.1) and friction forces were observed at pH 3 where the PDMAEMA chains are highly charged. This finding is consistent with the normal forces where both the range and the magnitude of the repulsive forces were highest at low pH where the chains are the most hydrated and loadbearing. An increase in the pH significantly increased the 2988
Figure 7. Friction measurements between a gold sphere and a goldcoated QCM crystal both bearing a grafted layer of PDMAEMA. The friction coefficient (µ) averaged over several individual measurements is displayed as a function of pH at 24 °C (closed squares) and at 50 °C (open squares). The dashed lines have been added for visual guidance.
measured friction coefficient which reached a maximum value of µ ≈ 1. The repulsive forces associated with the “hydration cushion” of the brushes are of much lower significance when the brushes are less charged. A secondary effect which is noteworthy is that at elevated temperature (50 °C) the friction coefficient is reduced at all but the lowest pH. As mentioned earlier, the QCM-D results indicate that the layer collapse is more pronounced at higher temperature and the force measurements are consistent with this. Thus the slightly lower friction at elevated temperature is not due to improved load bearing properties of the film, and another explanation must be sought. We speculate that a combination of factors is responsible. First, and most importantly, the more collapsed film is likely to give rise to a smoother interface as the polymer strives to minimize its contact with the solvent, and the effects of polydispersity on the topography should thus be much less. The surface roughness of polymer interfaces has previously been shown to have a large influence on the friction coefficient.32 Second, the higher temperature may lead to increased polymer mobility and thus a more liquid-like polymer surface.62 A related effect has been observed when imaging surfaces lithographically patterned with PDMAEMA copolymer hydrogels. A sharp decrease in the lateral force signal related to the friction between the uncoated probe and the substrate was observed above the LCST.17 With the aim of connecting the observations from the nanotribological measurements with the earlier discussion, the normal forces have been displayed on a linear scale in Figure 8. The proposed conformations of the brush in the two extreme cases are also schematically presented. At pH 3 (24 °C) the brush is fully extended due to the highly hydrated polymer chains and this is reflected both in a longrange repulsive force and low friction. The reduced charge of the species at pH 11 (24 °C) yielded a 10-fold increase in the friction coefficient compared to that observed under acidic Nano Lett., Vol. 9, No. 8, 2009
Figure 8. Normalized force profiles on approach between a gold sphere and a gold-coated QCM crystal both bearing a grafted layer of PDMAEMA. The displayed results are from individual force curves (black squares) and averages taken from 10 force curves (red line). Measurements (ramp size 1 µm at a rate of 2 µm/s) were conducted in water at pH 3 (24 and 50 °C) and pH 11 (24 and 50 °C). The insets schematically show the proposed conformational changes of the polymer brush.
conditions. Raising the temperature to 50 °C causes the brush to fully collapse due to unfavorable solvent conditions. The smaller influence of the temperature on the lubrication (Figure 7), effectively reducing the friction coefficient by about 30%, is attributed to the flattening of the interface due to the collapse (discussed above). The effect of temperature on the friction coefficient is minimal at low pH because the LCST is higher than 50 °C under those conditions. In summary, the interactions between responsive polymer brushes are strongly dependent on the conformation of the brush, which can be finely controlled through both temperature and pH, without changing the density of attached molecules. The effects of these two parameters on the brush collapse are subtly different, since the pH affects primarily the charging of the amine moieties, whereas the temperature controls the strength of the hydrophobic interactions between the polymer chains and the solvent. The nanotribological properties of the films reflect the brush conformationsfor extended highly charged hydrated layers, the friction is minimized, due to their cushioning effect. Lower charged brushes exhibit higher friction since this cushioning is removed. Finally, stiff, hydrated biopolymers are often implicated in biolubrication mechanisms,63 and while PDMAEMA is not per se a biopolymer, the lowered friction in the extended hydrated case clearly demonstrates the importance of such factors. An interesting and somewhat unexpected additional conclusion is that the most collapsed brushes display lower friction coefficients than partially collapsed films, which we speculate is due to the improved smoothness of the slightly polydisperse brush. Acknowledgment. This work was supported by the Swedish Foundation for Strategic Research (SSF) through a Nano Lett., Vol. 9, No. 8, 2009
“Chemistry for Life Sciences” project grant and through Biomime, the Swedish Center for Biomimetic Fiber Engineering (http://www.biomime.org/). The AFM was financed by a generous grant from the Knut and Alice Wallenberg foundation. M.R. is a fellow of the Swedish Research Council. Professor R. G. Gilbert and the Key Centre for Polymers & Colloids (http://www.kcpc.usyd.edu.au/) are acknowledged for access to the lab for polymer synthesis. Dr. Camilla Nilsson and Dr. Nikolas Zwaneveld are acknowledged for providing the chain transfer agent. Dr. Harry Brumer is thanked for useful discussions and Jerker Nordgren (http://www.jerkernordgren.com/) is acknowledged for provision of certain graphics. Supporting Information Available: Detailed experimental procedures. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Hinrichs, W. L. J.; Schuurmans-Nieuwenbroek, N. M. E.; van de Wetering, P.; Hennink, W. E. J. Controlled Release 1999, 60, 249. (2) Kato, N.; Yamanobe, S.; Takahashi, F. Mater. Sci. Eng., C 1997, C5, 141. (3) Arndt, K.-F.; Kuckling, D.; Richter, A. Polym. AdV. Technol. 2000, 11, 496. (4) Gotsmann, B.; Duerig, U. T.; Sills, S.; Frommer, J.; Hawker, C. J. Nano Lett. 2006, 6, 296. (5) Maye, M. M.; Luo, J.; Han, L.; Zhong, C.-J. Nano Lett. 2001, 1, 575. (6) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature (London, U.K.) 1991, 353, 239. (7) Plunkett, M. A.; Wang, Z.; Rutland, M. W.; Johannsmann, D. Langmuir 2003, 19, 6837. ¨ stmark, E.; Antoni, P.; Carlmark, A.; (8) Lindqvist, J.; Nystro¨m, D.; O Johansson, M.; Hult, A.; Malmstro¨m, E. Biomacromolecules 2008, 9, 2139. (9) Liu, G.; Yan, L.; Chen, X.; Zhang, G. Polymer 2006, 47, 3157. 2989
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Nano Lett., Vol. 9, No. 8, 2009
