Low-Friction Adsorbed Layers of a Triblock Copolymer Additive in Oil-Based Lubrication

Abstract

The tribological properties of the dilute solution of an ABA triblock copolymer, poly(11-acrylamidoundecanoic acid)-block-poly(stearyl methacrylate)-block-poly(11-acrylamidoundecanoic acid (A5S992A5), in poly(α-olefin) (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.

Aqueous Lubrication with the Molecularly Confined Films of Silicone-Based Amphiphilic Block Copolymer Aggregates

“…If we consider the presence of water hydration shells (layers) at the aggregate/mica and opposing aggregate layer interfaces, then a dynamic thickness of about 3 nm obtained for the two adsorbed layers is almost the minimum thickness attainable for the adsorbed BIPA copolymer. The minimum thickness layer should have a rather rigid property (no more deformation is available for energy dissipation), which is also suitable for extremely low friction. , …”

“…The minimum thickness layer should have a rather rigid property (no more deformation is available for energy dissipation), which is also suitable for extremely low friction. 46,47 We should add one more point for the extremely low friction of the BIPA adsorbed layers. It is well known that charged polymer brushes in water are the representative example of low-friction systems; the μ values for the BIPA adsorbed layers are equivalent or smaller.…”

Effect of a Fatty Acid Additive on the Kinetic Friction and Stiction of Confined Liquid Lubricants

“…When a droplet of PAO was confined between two mica surfaces by normal compression, two distinct static hardwall thicknesses (compressed film thickness at which the repulsive surface force (steric force of trapped liquid molecules) diverges), D = 1.1 ± 0.1 nm and 1.4 ± 0.1 nm, were obtained at different contact positions (different measurements) [20]. This is probably due to the different confinement rates (surface approach rates) between different measurements [21].…”

Anchor Group Bottlebrush Polymers as Oil Additive Friction Modifiers

“…It has been shown that two electrostatic anchor blocks attached to a solvated bottlebrush segment provide effective surface activity; this ABA triblock bottlebrush mimicking lubricin displayed excellent lubrication performance. , Although research in this area has mostly focused on aqueous systems, recent studies have looked at oil-soluble block copolymers with long dodecyl side groups in the oleophilic segment (akin to a bottlebrush) and a polar segment for surface adsorption. , These additives are promising competitors for current additives in that they offer both friction and viscosity improvements from one material . ABA triblock copolymers with end groups introducing surface activity or an upper critical solution temperature have also been investigated for oil-based lubrication. , …”