Macrotribological Studies of Poly(L-lysine)-graft-Poly(ethylene glycol) in Aqueous Glycerol Mixtures
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- Nalam, P.C., Clasohm, J.N., Mashaghi, A. et al. Tribol Lett (2010) 37: 541. doi:10.1007/s11249-009-9549-9
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We have investigated the tribological properties of surfaces with adsorbed poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) sliding in aqueous glycerol solutions under different lubrication regimes. Glycerol is a polar, biocompatible liquid with a significantly higher viscosity than that of water. Macrotribological performance was investigated by means of pin-on-disk and mini-traction-machine measurements in glycerol-PLL-g-PEG-aqueous buffer mixtures of varying compositions. Adsorption studies of PLL-g-PEG from these mixtures were conducted with the quartz-crystal-microbalance technique. The enhanced viscosity of the glycerol-containing lubricant reduces the coefficient of friction due to increased hydrodynamic forces, leading to a more effective separation of the sliding partners, while the presence of hydrated polymer brushes at the interface leads to an entropically driven repulsion, which also helps mitigate direct asperity–asperity contact between the solid surfaces under boundary-lubrication conditions. The combination of polymer layers on surfaces with aqueous phases of enhanced viscosity thus enables the friction to be reduced by several orders of magnitude, compared to the behavior of pure water, over a large range of sliding speeds. The individual contributions of the polymer and the aqueous glycerol solutions in reducing the friction have been studied across different lubrication regimes.
KeywordsBoundary lubricationAqueous lubricationGlycerolPolymer brushesViscosity
The study of macromolecules at the solid–liquid interface has led to improved understanding and new technologies in many fields including colloid science, biomedicine, and tribology . Klein et al. have studied the shear forces between polymer-bearing surfaces with the surface-forces apparatus, to understand the frictional forces at the interface. These studies show that when two surfaces covered with a high density of terminally attached polymers are immersed in a good solvent and brought into contact, the swollen polymer brushes reduce interfacial frictional forces . As they approach each other, opposing polymer brushes exhibit repulsive forces due to osmotic effects on the one hand and the free-energy penalty (due to reduced configurational entropy) resulting from the overlap of the brush layers on the other. There have been several studies, both theoretical  and experimental [4, 5] that have investigated the origin of frictional forces between contacting brushes at different shear rates. For water-soluble polymer brushes in aqueous environments, the presence of bound (or ‘hydration’) water surrounding the polymer chains can result in structural forces between the hydrated brushes . Strongly hydrated polymers, together with a continuous rapid exchange of bound water with other free water molecules, keep the surfaces separated while maintaining a high fluidity at the brush–brush interface at high compressions, thus leading to a very low coefficient of friction [7, 8].
Aqueous lubrication is of interest in a number of technological applications where lubrication with oil presents contamination problems. This is the case, for example, in the food, textile, and pharmaceutical industries. The adsorption of synthetic, hydrated polymer brushes at the interface overcomes the drawback of the low viscosity of water and, to some extent, mimics the situation encountered in nature . Adsorption of the poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) copolymer has been extensively used [10, 11] as a facile approach for the attachment of water-compatible polymer brushes to surfaces. Studies have been conducted to understand the lubrication properties of these polymer brushes both at macroscopic [11, 12] and at nanoscopic scales [13, 14] in aqueous environments. PLL-g-PEG contains a positively charged polypeptide backbone that adsorbs spontaneously via electrostatic interactions onto several metal oxide surfaces, such as TiO2, Nb2O5, and SiO2 at neutral pH. In aqueous media, PEG chains become hydrated to form “brush-like” structures at the interface, which reduce the frictional forces when surfaces are rubbed against each other. If sheared off under tribological stress, the electrostatically attached polymers can immediately be replaced by molecules readsorbing from solution and thus can act as better lubricants when compared to covalently attached polymers , since the latter generally require specific, non-aqueous reaction conditions for reattachment.
Along with polymer architecture , the quality of the solvent surrounding the polymer brush is an important parameter for determining both adsorption kinetics and lubrication properties [16–18]. For end-grafted polymers in poor solvents, the cohesive forces between polymer molecules (both inter- and intrachain) or polymers and surface dominate, resulting in a dense collapsed structure of the polymer when adsorbed on the surface (pancake structure). In contrast, good solvents can induce, at low surface coverages, a structure resembling that of the free polymer chains in solution (mushroom structure), or, at high coverages, a significant stretching of the polymers leading to a polymer brush. Several studies have been conducted to understand the effect of solvent quality on the structure and stability of brushes [18, 19]. The structural changes and preferential solvation of polymer brushes have been studied in detail [20–22] by varying the solvent quality using binary solutions, which contain varying volume fractions of good and bad solvents in the solution. Müller et al.  studied the frictional properties of adsorbed PLL-g-PEG polymers on silica surfaces using colloidal-probe AFM for binary mixtures of water and 2-propanol. They observed little or no variation in the frictional properties of the brushes until the critical volume fraction of ϕ = 0.85 (2-propanol) is reached, beyond which the friction increases remarkably with even a slight increase in the volume fraction of the solvent.
In this study, we have investigated the tribological properties of PLL-g-PEG copolymer brushes in binary mixtures of buffer solution and glycerol. Studies of the fluidity of water, when it is confined as a molecularly thin film between two solid surfaces, show that there is only a nominal increase in the viscosity of the confined water at the interface . In contrast to the behavior of oils, the low pressure-viscosity coefficient of water can impose a major constraint for aqueous tribology at high loads, since the boundary regime is extended to higher speeds. Increasing viscosity by the addition of water-compatible viscous fluids is an alternative approach to rectifying this situation. Glycerol is a polar, biocompatible, and highly viscous liquid, which readily dissolves in water. As PEG does not dissolve in glycerol, glycerol behaves as a poor solvent in the buffer-solution–glycerol binary mixture. We have conducted tribological tests at various speeds and loads with buffer-solution–glycerol solutions of different compositions and viscosities and explored the effect of polymer brushes across different lubrication regimes. It was found that a combination of polymer brushes and the enhanced viscosity obtained by glycerol addition provided effective lubrication over a wide range of speeds, and therefore lubrication regimes. While the enhanced viscosity fluids were highly effective in extending the hydrodynamic regime to lower speeds, it was clear that the polymer brushes enhanced lubrication within the boundary and mixed regimes. The adsorption kinetics of polymers from viscous binary solutions has also been investigated. Lastly, a calculation of the lubricating film thickness at the interface determines the importance of polymer brushes at the interface under different lubrication regimes.
2 Materials and Methods
All tribological experiments were conducted with a steel ball loaded against a glass disk. HEPES [10 mM of 4-(2-hydroxyethyl)-1-piperazine-1-ethanesulfonic acid (Sigma, St. Louis, MO, USA), with 6.0 M NaOH solution] was used as the aqueous buffer to maintain the pH at 7.4. Due to the low isoelectric point of silicon dioxide (~2), negative charges reside on the surface at neutral pH. These negatively charged surfaces adsorb the positively charged backbones of PLL-g-PEG copolymers to form brush-like structures spontaneously upon immersion in the aqueous-polymer-containing solution.
2.1 Materials Used
Concentration of glycerol/ethylene glycol in water (vol.%)
Dynamic viscosity of glycerol–water mixture at 25 °C (mPa s)
Dynamic viscosity of ethylene-glycol–water mixture at 25°C (mPa s)
2.2 Tribological Experiments
Disks and balls used for the tribological tests were sonicated in ethanol absolute (Scharlau, Analytical grade, ACS, Sentmenat, Spain) in Teflon boxes for 30 min. N2-dried samples were then plasma-treated in an oxygen environment (for pin-on-disk) and in air (for MTM) for 90 s to remove adventitious organic matter. Treated disks and balls were transferred to the polymer solution and the experiments conducted after soaking for a minimum of 30 min.
2.3 Pin-on-disk Measurements
Pin-on-disk tribometers (CSEM, Neuchâtel, Switzerland) were used to measure macroscopic frictional forces under pure sliding conditions. Two tribometers operating in different speed ranges were employed to enable the sliding speed to be varied over a wide range. The slower tribometer measures frictional forces in the speed range of 0.1–20 mm/s and the faster tribometer from 25 to 400 mm/s. A fixed pin that holds the steel ball (diameter = 6 mm, DIN 5401-20 G20, Hydrel AG, Romanshorn, Switzerland) was brought into contact with the flat, rotating glass slide (2.5 × 2.5 cm2, 1-mm thick; Super Frost microscope slides, Menzel Gläser, Braunschweig, Germany with a composition as specified by the manufacturer: 72.2% SiO2, 14.3% Na2O, 1.2% K2O, 6.4% CaO, 4.3% MgO, 1.2% Al2O3, 0.03% Fe2O3, and 0.3% SO3). RMS roughness values of the steel ball and the glass disk were measured by AFM as 32 and 5 nm, respectively. A stainless steel cup held the polymer solution (capacity ~20 ml) such that the pin and disk were completely immersed in the solution. The coefficient of friction (μ) is plotted as a function of number of laps under a normal load of 2 N for all experiments (Hertzian contact pressure = 0.34 GPa). Experiments were conducted at ambient temperature and a fresh track and new pin were used for every measurement. Data acquisition and operating speeds were controlled by means of Tribo X software (InstrumX version 2.5A, CSM Instruments, Switzerland). Friction coefficients were averaged over 200 laps for speeds above 20 mm/s and over 50 laps for speeds below 20 mm/s.
2.4 Mini-Traction-Machine Measurements
The mini-traction-machine (MTM, PCS instruments, London, UK) was used to measure frictional forces in rolling contact between PEG-coated surfaces immersed in the copolymer solutions. In the experimental setup, a 9.5-mm radius steel ball (AISI 52100, RMS roughness = 11 nm, PCS Instruments, London, UK) was brought into contact with a 46-mm diameter glass disk (RMS roughness = 2 nm, PCS Instruments, London, UK). Only one track with a radius of 20.7 mm per disk was used. The rotation of the ball and the disk can be independently controlled and thus a mixture of sliding and rolling can be achieved. The slide/roll ratio (SRR) is defined as the percentage ratio of the difference between the ball and the disk speed to the mean of ball (uball) and disk speed (udisk); SRR = (uball − udisk)/[(uball + udisk)/2]. The SRR varies from 0 to 200% with SRR = 0% (uball = udisk) representing pure rolling and SRR = 200% for complete sliding conditions. A SRR of 10% was used for all experiments to maintain the conditions of near-pure rolling. Using the manufacturer’s software (PCS Instruments, MTM version 1.0, London, UK) the speed can be varied from 0 to 2500 mm/s. A load of 10 N was applied (Hertzian contact pressure = 0.42 GPa) and the coefficient of friction measured as a function of the mean speed of the disk and the ball. A temperature of 25 °C was maintained by means of a water bath. New disks and balls were used for every measurement.
2.5 Quartz Crystal Microbalance
The quartz crystal microbalance (QCM) is a mass-sensing device. In contrast to many other mass-sensing techniques that work under liquids, QCM is sensitive to the mass of both the adsorbed polymer layer and the mass of the solvent associated with it. The measurements were performed with a commercially available QCM with dissipation monitoring (Q-sense, Gothenburg, Sweden).
Quartz crystals were cleaned in ethanol for 30 min and then ozone-treated for 30 min before placing them in the QCM chamber. Inlet and outlet tubing and the QCM chambers were rinsed with ultra pure water (GenPure UV, TKA GmbH, Niederelbert, Germany) before use. The fundamental frequencies were characterized in pure water. The chamber is designed to provide a non-perturbing exchange of liquids over the quartz crystal by means of a pump. A flow rate of 20 μl/min was used and the chamber temperature was maintained at 25 °C during all of the measurements.
3 Results and Discussion
3.1 Tribological Studies of PLL-g-PEG in Aqueous Glycerol Solutions
In the polymer-adsorption studies carried out with different HEPES–glycerol solutions, the baseline was first obtained in HEPES buffer and the solution subsequently exchanged with the HEPES–glycerol mixture (arrow marked ‘A’ in Fig. 2). A decrease in the resonant frequency of the crystal was observed due to the drag forces on the crystal originating from the viscosity of the solution. PLL-g-PEG dissolved in a HEPES–glycerol solution of the same mixing ratio, at a polymer concentration of 0.25 mg/ml, was injected into the cell (B). The kinetics of adsorption of the copolymers onto the bare SiO2 surface are observed. The adsorption was carried out in flow mode until no further decrease in the frequency was seen. The crystal was then rinsed with the HEPES–glycerol mixture to remove any physisorbed or loosely bound copolymers from the crystal (C). The solution was finally exchanged back to HEPES solution (D) and the change in the frequency with respect to the baseline noted.
The change in frequency and the corresponding mass of PLL-g-PEG adsorbed on the SiO2-coated QCM crystal from HEPES–glycerol mixtures
Concentration of glycerol in water (vol.%)
Change in frequency upon polymer adsorption relative to baseline (Hz)
Mass of the adsorbed polymer, including solvent (from Sauerbrey equation) (ng/cm2)
The polymer, when adsorbed from viscous solutions assists in the reduction in the coefficient of friction (Fig. 1) under the boundary- and mixed-lubrication conditions of pin-on-disk tribometry. The reduction in the friction was observed to be about 60% (from 0.25 to 0.1) at 0.1 mm/s when the polymer was adsorbed at the interface from 50% v/v HEPES–glycerol solution in comparison to a similar system with no polymer at the interface. The coefficient of friction also appears to converge at high speed (150 mm/s) in the presence of polymer for all HEPES–glycerol mixtures, indicating no effect of the polymer on friction as the surfaces become completely separated by a fluid film.
At higher operating speeds, the viscous solutions form a complete lubricating film. For 50% v/v glycerol in HEPES solution, the coefficient of friction increases with increasing speed above 1000 mm/s, indicating the onset of full-fluid-film lubrication. Upon complete film formation between the contacts, the presence of polymer at the interface no longer has any effect on the frictional properties and thus the frictional curves (Fig. 3) show similar behavior with and without polymer. At high lubricant viscosity (i.e., 75% v/v glycerol in HEPES) the increase in friction forces occurs at speeds as low as 100 mm/s due to the onset of the hydrodynamic regime at much lower contact speeds.
3.2 “Rehealing” Studies of PLL-g-PEG in Aqueous Glycerol Mixtures
There is a continuous shear of the polymer at the interface when the two contacting surfaces slide against each other. Under the conditions used in the pin-on-disk tribological experiments, the shear forces exerted on the polymer chains in the vicinity of underlying asperities were comparable to the binding strength of the polymer to the substrate and thus the polymer was partially removed from the interface after each cycle. In order to maintain a low coefficient of friction over a large number of rotations there is a need for either a strong polymer attachment with the surface or replacement of the sheared polymer by fresh polymer from the bulk solution. PLL-g-PEG interacts with the surface via a weak electrostatic attraction and thus the polymer is partially sheared from the interface under tribological stress. Lee et al.  have studied the rehealing of the tribo-stressed contact by diffusion of the polymer to the surface by means of tribometry and fluorescence microscopy. It was shown that a high concentration of polymer in the bulk lubricant (0.25 mg/ml) will provide sufficient polymer in the vicinity of the stressed area and thus rehealing of the sheared area can occur before the onset of the next rotation. The diffusivity of PLL-g-PEG and the concentration of the polymer in the vicinity of the contact are important parameters that influence the rehealing process. By maintaining the same concentration as used by Lee et al., we have explored the effect of solution viscosity (and thus the diffusion rate of the PLL-g-PEG) on the rehealing mechanism. Viscosity was varied by adding different volume fractions of either glycerol or ethylene glycol to HEPES solution.
According to Lee et al. , the rapid rehealing properties observed for PLL-g-PEG can be attributed to the fast adsorption kinetics of the polymer through a low-viscosity bulk solution to the surface. The present rehealing studies of PLL-g-PEG in viscous solutions also show a rapid replacement of the polymer when the surfaces are significantly covered with polymer, i.e., after a fraction has been removed by shear (Case III). However, similar kinetics were not observed for surfaces from which polymer had been completely tribologically removed (Case IV). This suggests that the time required to reestablish a monolayer of polymer on these essentially bare contact regions is noticeably increased with increasing viscosity. In the case of the largely covered surface (Case III), the need for polymer adsorption to reestablish the monolayer is much less pronounced, and therefore the effects of viscosity less noticeable on the timescales probed in these experiments. The experiments of Case IV show that adsorption is slowed down by diffusion of molecules from the bulk solution, which, in turn, is slowed down at higher viscosities.
The adsorption kinetics of the PLL-g-PEG from HEPES–glycerol mixtures, as monitored by QCM measurements, also show that the rate of adsorption of the polymer onto the surface has a clear dependence on the concentration of the glycerol in the solution (Fig. 2). The time required to form a fully covered polymer film on the surface increases with increasing glycerol content in the HEPES–glycerol solution.
3.3 Film-thickness Calculations for Lubricants Consisting of PLL-g-PEG in Aqueous Glycerol Mixtures
Pressure-viscosity coefficient values (α) for different concentrations of glycerol in water (vol.%)
Concentration of glycerol in water (vol.%)
Pressure-viscosity coefficient (α) (×10−9 m2 N−1)
The RMS roughness values for steel ball and silica disk used for MTM are 11 and 2 nm, respectively (obtained from PCS instruments) and those for the steel ball and glass wafer used for POD experiments are 32 and 5 nm, respectively (measured by AFM). Figure 9b plots the λ values against speed for all of the POD and MTM experiments. The curves show that all the contacts during POD measurements were in the boundary-lubrication regime. Though there was an increase in the λ value with addition of glycerol, the values still lie below 1, indicating a high probability for asperity contact. For MTM measurements, on the other hand, although contacts tested at high speeds and in viscous lubricant (0.5 or 0.75 volume fraction of glycerol in HEPES) showed λ values above 3, for other operating conditions the λ values indicated either the boundary- or mixed-lubrication regime. Thus, there is a need for adsorbed copolymers of PLL-g-PEG on the sliding surfaces to reduce the interfacial friction generated at the asperity contacts, even in the presence of aqueous viscous lubricants.
It is has been shown that when poly(L-lysine)-g-poly(ethylene glycol) is dissolved in aqueous glycerol solutions, the tribological properties can be improved both in the boundary- and the mixed-lubrication regimes. Different percentages of glycerol in HEPES have been used to vary the viscosity of the solvent, which enables hydrodynamic lubrication to take place over a wider speed range than for the pure HEPES case. The effect of the polymer (in glycerol-containing solution) in different lubrication regimes was demonstrated by means of a Stribeck plot, which shows that the presence of polymer at the interface can reduce the friction as long as there exists asperity–asperity contact between the tribo pair; the viscous solvents separate the contacting surfaces due to hydrodynamic forces and the presence of hydrated polymer brushes reduces the interfacial friction between contacting surface asperities. Also, Esfahanian–Hamrock–Dowson film-thickness calculations show that the lubricating HEPES–glycerol films formed in our pin-on-disk experiments are very thin, so that numerous asperity–asperity contacts are expected, and thus the presence of copolymer at the surface is necessary to further reduce the friction. In mini-traction-machine measurements in rolling contact, on the other hand, the full-fluid-film-lubricated region could be examined, in which the presence of the polymer was found to have negligible effect.
Quartz-crystal microbalance measurements showed that the total amount of adsorbed polymer appeared unaffected by the presence of glycerol. The kinetics of adsorption of PLL-g-PEG from the HEPES–glycerol solution to the interface was investigated to help understand the effect of the increased viscosity on the rehealing of the tribo-stressed contact. Although there is an inverse relation between the viscosity, the solvent, and the rate of diffusion of the polymer to the interface, the time required for the polymer to reheal wear-damaged polymer layers appears to be less than that between contacts in successive laps, and thus rehealing of the contact remains independent of the viscosity of the solvent under the conditions investigated. These results show that the use of HEPES–glycerol solutions as a viscosity-enhanced solvent for PLL-g-PEG can expand the applicability of aqueous lubrication to a significantly larger range of operating conditions.
The financial assistance of the European Science Foundation, through their Eurocores (FANAS) program is gratefully acknowledged. We would also like to thank Dr. Rowena Crockett of EMPA, Dübendorf, Switzerland for allowing us to use her mini-traction-machine and Prof. Hugh Spikes of Imperial College, London for his valuable suggestions.