Article pubs.acs.org/Langmuir
Low-Friction Adsorbed Layers of a Triblock Copolymer Additive in Oil-Based Lubrication Shinji Yamada,*,†,# Ami Fujihara,‡ Shin-ichi Yusa,‡ Tadao Tanabe,§ and Kazue Kurihara*,§,∥ †
New Industry Creation Hatchery Center, Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan § Institute of Multidisciplinary Research for Advanced Materials and ∥Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡
S Supporting Information *
ABSTRACT: The tribological properties of the dilute solution of an ABA triblock copolymer, poly(11-acrylamidoundecanoic acid)-block-poly(stearyl methacrylate)-blockpoly(11-acrylamidoundecanoic acid (A5S992A5), in poly(αolefin) (PAO) confined between mica surfaces were investigated using the surface forces apparatus (SFA). Friction force was measured as a function of applied load and sliding velocity, and the film thickness and contact geometry during sliding were analyzed using the fringes of equal chromatic order (FECO) in the SFA. The results were contrasted with those of confined PAO films; the effects of the addition of A5S992A5 on the tribological properties were discussed. The thickness of the A5S992A5/PAO system varied with time after surface preparation and with repetitive sliding motions. The thickness was within the range from 40 to 70 nm 1 day after preparation (the Day1 film), and was about 20 nm on the following day (the Day2 film). The thickness of the confined PAO film was thinner than 1.4 nm, indicating that the A5S992A5/PAO system formed thick adsorbed layers on mica surfaces. The friction coefficient was about 0.03 to 0.04 for the Day1 film and well below 0.01 for the Day2 film, which were 1 or 2 orders of magnitude lower than the values for the confined PAO films. The time dependent changes of the adsorbed layer thickness and friction properties should be caused by the relatively low solubility of A5S992A5 in PAO. The detailed analysis of the contact geometry and friction behaviors implies that the particularly low friction of the Day2 film originates from the following factors: (i) shrinkage of the A5S992A5 molecules (mainly the poly(stearyl methacrylate) blocks) that leads to a viscoelastic properties of the adsorbed layers; and (ii) the intervening PAO layer between the adsorbed polymer layers that constitutes a high-fluidity sliding interface. Our results suggest that the block copolymer having relatively low solubility in a lubricant base oil is effective at forming low-friction adsorbed layers in oil-based lubrication.
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INTRODUCTION Reducing friction is one of the most important requirements for improving the performance of moving components and saving energy in many industrial applications of mechanical systems. For this purpose, low-viscosity lubricant oils are widely used to obtain low friction at high sliding velocity/low applied load conditions (in the hydrodynamic lubrication regime). However, such low-viscosity lubricant oils are easily squeezed out from the contact interfaces at low sliding velocity/high applied load conditions (boundary lubrication regime). Therefore, realizing low friction in both the hydrodynamic and boundary lubrication regimes using low-viscosity lubricant oils is one of the most important problems in oil-based lubrication technology. Decades of fundamental research on the dynamics of confined simple liquids have greatly improved our understanding of the squeezing of lubricant liquids between solid surfaces.1−5 Both experimental and theoretical investigations revealed that at least a few layers of liquid molecules could © 2015 American Chemical Society
remain at the solid/solid contact interface even under extremely large load (pressure). However, the intervening molecules tend to pack into structures in confinement and often exhibit an extreme increase in viscosity and/or solid-like shear responses such as stiction and stick−slip sliding.1−4,6−8 The fundamental physics underlying the squeezing dynamics and friction properties of confined simple liquids has been extensively studied; now much of the effort is devoted to designing low friction interfaces by use of practical lubricant systems. One of the approaches to achieve low friction and wear in boundary lubrication regime using low-viscosity lubricant oils is to add friction modifiers; a variety of lubricant additives are dissolved in base lubricant oils. One of the representative examples is fatty acids (FAs).9−13 The FA molecules form selfassembled monolayers on surfaces and increase the boundary Received: September 28, 2015 Revised: October 19, 2015 Published: October 19, 2015 12140
DOI: 10.1021/acs.langmuir.5b03620 Langmuir 2015, 31, 12140−12147
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Langmuir
oven at 40 °C for 24 h (0.948 g, 56.8%). Synthetic route of PAaU is shown in Scheme S1 (Supporting Information). Gel permeation chromatography (GPC) indicated that number-average molecular weight (Mn (GPC)) and molecular weight distribution (Mw/Mn) of PAaU were 3.77 × 103 and 1.05, respectively (Figure S1, Supporting Information). The number-average molecular weight (Mn (NMR)) and number-average degree of polymerization (DP) estimated from 1 H NMR in d6-DMSO of PAaU were 2.84 × 103 and 10, respectively, calculated from an integral intensity ratio of the NMR bands at 1.0 ppm of the methyl protons of BMAT at both polymer chain ends and 2.2 ppm for the pendent methylene protons (Figure S2a, Supporting Information). Synthesis of ABA Triblock Copolymer (A5S992A5). PAaU (25.1 mg, 8.85 μmol, Mn (NMR) = 2.84 × 103, and Mw/Mn = 1.05), SMA (3.00 g, 8.86 mmol), and AIBN (0.58 mg, 3.53 μmol) were dissolved in THF (2.63 mL). The solution was degassed by purging with Ar gas for 30 min. Polymerization was performed at 60 °C for 24 h. 99.2% conversion was estimated from 1H NMR for the polymerization solution before purification. After polymerization, the polymer was purified by reprecipitation from THF into a large excess of methanol repeating three times. The triblock copolymer (A5S992A5; Figure 1)
film thickness and improve load-bearing capability (reducing wear). However, friction reduction is not sufficient because the thickness of the lubricant oils between the adsorbed FA monolayers is generally a few molecular layers or less; the lubricant oils do not have enough fluidity for low friction. Polymer additives are also dissolved in base lubricant oils and used as friction modifiers.14 In most cases, polymer additives are used to reduce the change in the bulk viscosity of the base oils against temperature change (this type of additive is referred to as “viscosity index improver”). Recently, block copolymers with functional groups have been receiving attention as friction modifiers; they are expected to adsorb on surfaces and restrain the decrease of lubricant film thickness. For water-based lubricant systems, water-soluble block copolymers (biopolymers or polyelectrolytes) having adsorption sites in the molecules are extensively studied because of the importance of biological lubrication.15−18 For oil-based lubricant systems, there are some reports in the literature on the macroscopic friction behavior and effectiveness of block copolymer additives as friction modifiers in the boundary friction regime.19−21 However, the interfacial film structures and molecular friction mechanisms of block copolymer additive/oil systems, which are essential to optimize the polymer/oil systems for low-friction interfaces, have not been investigated in detail. In this study, the tribological properties of the dilute solution of a triblock copolymer, poly(11-acrylamidoundecanoic acid)block-poly(stearyl methacrylate)-block-poly(11-acrylamidoundecanoic acid (henceforth: A5S992A5), in poly(α-olefin) (PAO) confined between mica surfaces were investigated using the surface forces apparatus (SFA). The results were contrasted with those of confined PAO films; the effects of the addition of A5S992A5 on the tribological properties were investigated. The A5S992A5/PAO system formed adsorbed layers on mica surfaces; the thickness of the adsorbed layers and resulting friction properties changed with time after surface preparation and repetitive sliding motions. The system showed low friction coefficients, well below 0.01, 2 days after surface preparation. The structures of the adsorbed polymer layers and the molecular mechanisms of friction were discussed, which gives new insights into designing low-friction surfaces by use of block copolymer additives in oil-based lubrication.
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Figure 1. Molecular structure of the triblock copolymer A5S992A5. was dried in a vacuum oven at 40 °C for 24 h (2.21 g, 73.8%). Synthetic route for A5S992A5 is shown in Scheme S2 (Supporting Information). GPC results indicated that the Mn (GPC) and Mw/Mn of A5S992A5 were 1.90 × 105 and 1.76, respectively (Figure S1, Supporting Information). DP for poly(stearyl methacrylate) (PSMA) block and Mn (theory) for A5S992A5 estimated from conversion were 992 and 3.39 × 105, respectively. NMR peaks for PAaU block were not observed, because the DP of PAaU was too small compared to that of PSMA (Figure S2b, Supporting Information). The major molecular parameters such as number-average degree of polymerization, numberaverage molecular weight, and Mw/Mn for PAaU10 and A5S992A5 are listed in Table 1.
Table 1. Number-Average Degree of Polymerization (DP), Number-Average Molecular Weight (Mn), and Molecular Weight Distribution (Mw/Mn) for PAaU10 and A5S992A5
EXPERIMENTAL SECTION
Materials. 11-Acrylamidoundecanoic acid (AaU)22 and S,S′bis(α,α-dimethylacetic acid) trithiocarbonate (BMAT)23 were synthesized according to previously reported methods. 4,4′-Azobis(4cyanovaleric acid) (V-501, 98.0%) from Wako Pure Chemical Industries was used as received. Stearyl methacrylate (SMA, >97.0%) from Wako Pure Chemical Industries was used after removing inhibitor by inhibitor remover column (Sigma-Aldrich). 2,2′-Azobis(isobutyronitrile) (AIBN, 98%) from Wako Pure Chemical Industries was used after recrystallization from methanol. Methanol and tetrahydrofuran (THF) were dried over 4 Å molecular sieves and distilled. Water was purified using a Millipore Milli-Q system. The base lubricant oil used in this study was PAO (hydrogenated oligomers of 1-decene, industrial grade). The bulk viscosity of PAO was 76 mPa s. Synthesis of Poly(11-acrylamidoundecanoic acid) (PAaU). AaU (1.50 g, 5.89 mmol), BMAT (165 mg, 0.584 mmol), and V-501 (65.7 mg, 0.234 mmol) were dissolved in methanol (5.90 mL). The solution was degassed by purging with Ar gas for 30 min. Polymerization was carried out at 70 °C for 4 h. 92.3% conversion was estimated from 1H NMR for the polymerization solution before purification. After polymerization, the polymer was purified by reprecipitation from methanol into a large excess of diethyl ether. Poly(11-acrylamidoundecanoic acid) (PAaU) was dried in a vacuum
PAaU10 A5S992A5
DP (theory)a
Mn × 10−3 (theory)a
DP (NMR)
Mn × 10−3 (NMR)
Mn × 10−3 (GPC)
Mw/ Mn
10 992
2.84 339
10 b
2.84 b
3.77 190
1.05 1.76
a
Theoretical values estimated from conversion and feed molar ratio (Supporting Information). bNMR signals for the PAaU block in A5S992A5 were not assigned, because the content of PAaU was too small. NMR and GPC Measurements. 1H NMR spectra were obtained with a Bruker DRX-500 spectrometer operating at 500 MHz. The sample solutions of PAaU and A5S992A5 for 1H NMR measurements were prepared in DMSO-d6 and CDCl3, respectively. GPC measurements were performed using a refractive index detector equipped with three Shodex KF-803L columns and a Shodex KF-805L column at 40 °C under a flow rate of 1.0 mL/min. THF was used as the eluent. The values of Mn (GPC) and Mw/Mn were calibrated with standard polystyrene samples. Preparation of A5S992A5/PAO Solution. A5S992A5 was dissolved in PAO at the concentration of 0.5 wt % by stirring with a magnetic 12141
DOI: 10.1021/acs.langmuir.5b03620 Langmuir 2015, 31, 12140−12147
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Langmuir stirrer at 45 °C for 3 to 5 h. The solution was clear after stirring, but gradually became cloudy after leaving for 5 to 8 h at room temperature. Friction Measurements Using the Surface Forces Apparatus (SFA). Friction behavior of the confined films of the A5S992A5/PAO solution was measured using the surface forces apparatus RSM-1 (Advance Riko, Inc., Japan)24 modified for sliding experiments. Briefly, two cylindrical mica surfaces were positioned in a crossed cylinder configuration and were used to confine the sample solution. When the mica substrates were installed into the apparatus, some P2O5 was placed inside the sealed chamber to keep the internal atmosphere completely dry during friction measurements. A droplet of the clear A5S992A5/PAO solution (vol. ∼0.1 mL) was injected between the mica surfaces. In this study, time-dependent change of the friction properties was investigated because the appearance of the bulk sample solution changed with time at room temperature. For this purpose, after the sample injection, the surfaces were left overnight (16 to 20 h) at the surface distance of more than several microns. Then the surfaces were brought together under load L and friction measurements were made (Day1). The following day (Day2, approximately 40 to 45 h after surface preparation), friction measurements were made at a new contact position that was far from the position measured at Day1 (the new contact position was compressed for the first time upon Day2 measurements). We could not perform the measurements for the Day3 film because we observed particles (possibly the precipitates of A5S992A5) at every contact positions. Friction measurements were made using a home-built bimorph slider.25,26 Lateral motion (reversible cyclings) at a constant sliding velocity V (ranged from 0.0056 to 5.6 μm/s) was applied to the lower surface. The resulting friction force F was measured using the resonance shear unit of the RSM-1;27,28 the deflection of the friction measuring springs which support the upper surface was measured by a capacitance probe. The applied load L (in the range ∼40 mN) was controlled by a normal force spring of the bimorph slider (spring constant K = 1810 N/m). Using multiple beam interferometry (MBI),29,30 a cross-sectional image of the contact area (contact geometry) can be continuously monitored during sliding. Fringes of equal chromatic order (FECO) are obtained by passing a beam of white light normally through the substrate surfaces, which allows the measurement of the film thickness D (accuracy of 0.2 nm) and the size of the contact area A in real time. Friction measurements were also made for confined PAO films. Schematic drawing of the contact interface in the SFA friction measurements is shown in Figure 2. The experimental room was kept at a fixed temperature of 23 °C ± 1 °C.
A5S992A5/PAO system (the Day1 and Day2 films, see Experimental Section) were measured as a function of applied load; the results are shown in Figure 3. The confined film
Figure 3. Film thickness (both static hard-wall thickness and dynamic thickness) of the A5S992A5/PAO system at two different days. Results of the PAO films are also shown for comparison. Dynamic thickness of the Day1 film varied with load and repetitive sliding motion; static thickness at each applied load is shown as open circles. Static and dynamic thickness are the same for the Day2 film and PAO film. Note that the thickness values plotted here were measured at the center of the contact area (see Figure 4).
thickness of PAO is also plotted for comparison. For the A5S992A5/PAO Day1 film, the static hard-wall thickness at L = 5 mN was 43.5 nm (shown as an open circle). The dynamic thickness of the Day1 film (shown as filled symbols) varied within the range from about 40 to 70 nm depending on the applied load and repetitive sliding motions. The variation in the dynamic thickness at a given load was particularly large for low load sliding conditions (L ≤ 20 mN), indicating that the structure of the Day1 film was not stable. For the Day2 film, the static hard-wall thickness was 20.0 nm at L = 5 mN, and no change in the thickness was observed when sliding motions were applied. Increasing load only slightly decreased the dynamic thickness for the Day2 film. The variation of the static hard-wall thickness of the A5S992A5/PAO system at different contact positions was approximately within the range of ±10% for the Day1 film. For the Day2 film, the hard-wall thicknesses at different contact positions varied from 12.5 to 20 nm. Note that the thicknesses plotted in Figure 3 were measured at the center of the contact area (see Figure 4). For the PAO films, two distinct static hard-wall thickness values (1.1 and 1.4 nm) were obtained at different contact positions. This is probably due to the different confinement rates (surface approach rates) at different contact positions.31 The dynamic thickness values for the PAO films were the same as the static hard-wall thickness. The thickness values for the A5S992A5/PAO system (both Day1 and Day2 films) were larger than those of the PAO system, indicative of the adsorption of the polymer additive on mica surfaces. The change in the thickness between the Day1 and Day2 films implies that the structure of the adsorbed layers was gradually altered with time after surface preparation and repetitive sliding motions. Observation of FECO fringes in the SFA enabled us to determine the thickness and contact geometry (shape) of the interface; the typical examples of the fringes obtained from the A5S992A5/PAO system (the Day1 and Day2 films) are shown in Figure 4. For the Day1 film, the fringe shape (contact geometry) was altered along with the increase of applied load
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RESULTS The confined film thicknesses, both static hard-wall thickness and dynamic thickness (thickness during sliding), of the
Figure 2. Schematic drawing of the SFA friction experiments. A droplet of A5S992A5/PAO solution (concentration of 0.5 wt %) is confined between molecularly smooth mica surfaces under applied load, L. Lateral motion at a constant sliding velocity V is applied to the lower surface and resulting friction force F is measured by friction measuring springs that support the upper surface. Thickness of the film D, real contact area A, and contact geometry are measured from an optical technique using FECO fringes. 12142
DOI: 10.1021/acs.langmuir.5b03620 Langmuir 2015, 31, 12140−12147
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Figure 4. Typical FECO fringes of the A5S992A5/PAO system observed during sliding. For the Day1 film (a), contact geometry changes with load and repetitive sliding motions. For the Day2 film (b), flat contact was obtained for all the sliding conditions. The thickness values plotted in Figure 3 were measured at the center of the contact area.
and repetitive sliding motions. At low load (L = 10 mN), fringe shape (contact shape) was rounded and the film thickness was a minimum at the center of the contact area. This fringe shape suggests the Hertz contact for soft (deformable) interface; the contact pressure is a maximum at the center of the contact area. Increasing load and repetitive sliding motions gradually flattened the contact interface (Day1, L = 20 mN), which was mainly due to the decrease in the thickness near the edge of the contact area. Further increase of L and repetitive sliding motions deformed the contact area into a recessed shape (Day1, L = 40 mN). In this case, the thickness at the edge of the contact area was 16.7 nm and that at the center was 44.7 nm. The thickness data plotted in Figure 3 were the values obtained at the center of the contact area. The recessed fringe shape should come from the inhomogeneity of the contact pressure within the contact area. The pressure is highest at the center of the contact area, so that molecules become kinetically trapped at the center and flow out of the contact at the edge. For the Day2 film, flat contact area was observed for all the sliding conditions. The relationship between friction force and sliding velocity at different applied load conditions for the A5S992A5/PAO system were measured at different days. Friction measurements were started from V = 0.11 μm/s and then measurements were made with decreasing V. After the measurement at V = 0.0056 μm/s (lowest V condition in this study), friction force at V = 0.11 μm/s was again measured, followed by the measurements with increasing V (see the arrows in Figure 5a). The friction force for the Day1 film (Figure 5a) changed with repetitive sliding motions. For L = 10 mN, the first friction force data at V = 0.11 μm/s was 0.34 mN, and the second datapoint at the same V was 0.24 mN. Likewise, for L = 40 mN, the first friction force datapoint at V = 0.11 μm/s was 1.33 mN, and the second datapoint at the same V was 0.97 mN. The second datapoint was always smaller than the first datapoint, indicative of a sliding-induced change of the film structures at a given load. The Day1 film exhibited stick−slip friction below some critical sliding velocity Vc; the Vc for L = 10 mN was near 0.05 μm/s and that for L = 40 mN was near 0.1 μm/s. The variation of the friction force at different contact positions for the Day1 film was within the range of about ±15%. For the Day2 film (Figure 5b), friction force did not vary with repetitive sliding motion; the first and second friction force datapoints at V = 0.11 μm/s were almost the same for all the
Figure 5. Friction force F as a function of sliding velocity V at different applied load L conditions measured at different days. The Day1 film (a) exhibited stick−slip friction at low V; both kinetic friction force Fk (filled symbols) and static friction force Fs (open symbols) are plotted. Friction behavior of the Day1 film was unstable and friction force changed with repetitive sliding motions (see text in detail). The Day2 film (b) exhibited smooth sliding for all the sliding conditions; the plotted data correspond to Fk. Note that the scales of the friction force axes in (a) and (b) are different by approximately an order of magnitude.
applied load conditions. Friction force was almost constant at low V (
