Friction behavior (MTM)
The efficacy of the developed PFMs was examined in MTM experiments by measuring their friction behavior, plotting the coefficient of traction (COT) against entrainment speed. A comparison of all synthesized PFMs as well as results for pure hexadecane and the GMO solution is shown in Fig. 3a. When the entrainment speed is higher than 1000 mm.s− 1, all the measurements overlap at very low friction values, corresponding to the onset of hydrodynamic lubrication. The results for the decreasing speed ramps are omitted for clarity, since no indication of hysteresis was observed.
In the mixed/boundary regime the results fall into three categories: a) Pure hexadecane shows extremely high friction, reaching 0.3 with a large scatter, b) most of the polymers appear to be as effective as GMO, with a friction coefficient of about 0.17, and c) PFM-B, which exhibits an exceptionally low friction coefficient (∼0.06) throughout the measured speed range of 1 to 100 mm s− 1, resembling the previously observed friction behavior for “grafted-from” poly(lauryl methacrylate) brushes in oil [30].
To have a better understanding of the efficacy of PFM B, similar MTM experiments were performed in different base oils such as hexadecane, PAO2 and PAO6. The results are plotted in Fig. 3 (b), where the viscosity of the base oil was factored on the X-axis. As expected, PFM-B consistently shows greater friction reduction, especially in the boundary regime, compared to GMO. Hardly any influence of the viscosity was detected in the boundary regime, confirming that a surface effect was being measured. Finally, the low-friction behavior of PFM B can be further examined by examining Stribeck curves that were obtained upon increasing normal load, as shown in Fig. 3 (c). No significant differences are seen here, even though the maximum contact pressure reaches 960 MPa.
Existence of an adsorbed film under unperturbed conditions (QCM-D)
Film formation by PFM B was examined using the quartz crystal microbalance with dissipation (QCM-D). The results are summarized in Fig. 4. As expected from the tribological results, the lubricant containing PFM B exhibits significantly different behavior compared to the others.
In Fig. 4a, the adsorbed masses are shown on the left axis, with film-thickness on the right axis, a conversion having been performed into an adsorbed film thickness of the polymer and incorporated solvent, assuming a density of 0.93 g cm-3 (LMA in bulk). For PFM B in hexadecane, the resulting thickness is 16.4 ± 0.7 nm, the highest of all samples measured. Assuming an all-trans zigzag conformation of the backbone chain, one would expect between 10.5 nm (for the oleophilic block only) and 19.5 nm for the entire polymer. GMO in hexadecane shows an equilibrium thickness of 1.3 nm, which corresponds to what has been reported using other techniques [42], which supports the validity of our experimental conditions and modelling used for the determination of film thickness.
These estimated thicknesses here are mainly for comparison, and are to be considered as approximations, especially since the Sauerbrey equation assumes only a solid adsorbed slab of material. The layer whose thickness we are measuring also incorporates an unknown amount of hexadecane as solvent, some of which may be partially protruding from the polymer layer. Finally, the polydispersity of the polymer leads to a fraction of longer chains protruding from the surface, leading to an overestimation of the adsorbed mass. In view of these caveats, it is likely that we are overestimating the thickness of the polymer layer for PFM B. We are within a thickness range consistent with the LMA chains being upright in a brush-like configuration, the space between them being filled with solvent (hexadecane) molecules. Potentially, some of the adjacent anchoring groups are also constituting part of the brush-forming chains.
The deduced thicknesses for all other PFMs are much lower, indicating unoriented, “mushroom” monolayer adsorption and neither brush nor multilayer formation. Additional ex-situ ellipsometry measurements of the additives adsorbed on Cr surfaces from hexadecane yielded dry thickness values corresponding to a packing density of ≈ 0.2 chains.nm-2 for PFMs A and B, which would be consistent with the formation of a polymer brush (see Supporting Information).
The raw data from QCM-D experiments are shown in the Supporting Information. Briefly, the adsorption of GMO (Figure S15 (f)) is distinguishable from that of the polymers in that the adsorbed film seems to be in dynamic equilibrium and is removed from the surface by rinsing with hexadecane. For PFM A (Figure S15 (a)) significant mass is adsorbed irreversibly, i.e. remains upon rinsing. The decrease in dissipation upon rinsing indicates, however, that a rearrangement of the film occurred. PFM B (Figure S15 (b)) has a visibly faster adsorption rate than the other polymers, and the thick film resulting after a few minutes shows no signs of rearrangement upon rinsing. Figure S15 (c) highlights the extremely slow adsorption kinetics of PFM C, which can be attributed to its high molecular weight.
Correlation between the presence of an adsorbed film and low friction (IRIS)
In order to confirm the role of the adsorbed film on the low friction behavior in the mixed/boundary lubrication regimes, solutions of PFMs A, B and C in hexadecane were investigated here. The results of the IRIS measurements, in which film thickness and friction coefficient are measured simultaneously, are summarized in Fig. 5.
For all polymers, the film thickness was large compared to that calculated for hexadecane, especially at low entrainment velocity, indicating that hydrodynamic effects alone could not explain the film formation mechanisms. Contact visualization, during and after friction experiments showed that a boundary film was adsorbed on the surfaces. This was also confirmed by the shape of the meniscus in the outlet zone at low entrainment velocity, a signature of a modification of surface properties. The thickness and heterogeneity of the adsorbed film and its capacity to withstand shear were dependent on the polymer architecture (see Figures S18, S19 and S20).
The friction measurement for PFM A shows a significant hysteresis for entrainment velocities larger than 20 mm/s. This hysteresis was concomitant with that seen in the film evolution. The point of highest measured friction coincides with visible onset of wear of the chromium coating of the disk (see also Figure S18). The measured film thickness is only approaching the regime expected for hexadecane at high entrainment speeds. In the lower speed range, however, a speed-independent film in the range of 7 nm was visible, which again was consistent with both the post-friction adsorbed film measurement and the QCM results.
PFM B shows the lowest friction coefficient, consistent with the MTM measurements, despite the different surface roughness and substrate chemistry in the IRIS measurements. Negligible hysteresis was observed in friction or film thickness, highlighting the robustness of the adsorbed polymer film. The low friction coincided with the presence of a measured adsorbed film in the range of 20 nm. This corresponds reasonably well with what was measured in QCM, keeping in mind that we expect an adsorbed film on each surface that is under significant compression (Pav=212 MPa). The film distribution was homogeneous, uniformly covering the surface, regardless of the entrainment velocity, confirming the strong anchoring of the polymers on metallic surfaces and the absence of contact bridging.
PFM C exhibited a continuous increase in measured film thickness while performing the IRIS experiment. At the end of the measurements, the film was patchy and thickness increased to 100 nm (Figure S20) compared to the thickness of 6 nm measured at the beginning of the experiment. The initially measured 6 nm thickness was consistent with the thickness derived from QCM-D experiment while the 100 nm thicknesses measured at the end of the IRIS measurement is comparable with the large PFM C molecule in a brush configuration. This increase in film thickness was not observed during the initial pure rolling step at constant velocity, indicating that shear in the contact might be favoring the formation of the adsorbed film. A possible interpretation is that the shearing at the inlet zone of the contact and/or in the high-pressure contact zone allows for re-arrangements of the polymer molecules and a densification of the formed film. The formation of multilayers of polymer aggregates could also be a reason of the thick adsorbed film. The friction trends show a hysteresis, where the largest difference between decreasing and increasing speed is at 100 mm/s—the same speed at which the film-thickness measurements start to show large deviations. The absolute friction values in the final measurement stages are in a similar range to those measured with MTM. This seemed to confirm the existence of a run-in phase in the film formation/friction evolution.