Towards a Polymer-Brush-Based Friction Modifier for Oil

Abstract

To meet the need for oil-compatible friction modifier additives that can significantly reduce energy consumption in the boundary-lubrication regime, a macromolecular design approach has been taken. The aim was to produce a lubricious polymer film on the sliding surfaces. A series of readily functionalizable block copolymers carrying an oleophilic poly(dodecyl methacrylate) block and a functionalizable poly(pentafluorophenyl methacrylate) block of various lengths was synthesized by means of reversible addition-fragmentation chain-transfer (RAFT) polymerization. The poly(pentafluorophenyl methacrylate) block was used to attach surface-active nitrocatechol anchoring groups to the polymer. The friction-reduction properties of these polymers were assessed with 0.5 wt% solutions in hexadecane by means of rolling-sliding macroscopic tribological tests. Block copolymers with roughly equal block lengths and moderate molecular weights were significantly more effective at friction reduction than all other architectures investigated. They also displayed lower friction coefficients than glycerol monooleate—a commercially used additive. The film-formation ability of these polymers was examined using a quartz-crystal microbalance with dissipation (QCM-D), by monitoring their adsorption onto an iron oxide-coated QCM crystal. The polymer with highest lubrication efficiency formed a thin film of ~ 17 nm thickness on the crystal, indicating the formation of a polymer brush. Interferometric rolling-sliding experiments with the same polymer showed a separating film thickness of ~ 20 nm, which is consistent with the QCM-D value, bearing in mind the compression of the adsorbed layers on the two sliding surfaces during tribological testing. Graphical Abstract

Enhanced Adsorption of Epoxy‐Functional Nanoparticles onto Stainless Steel Significantly Reduces Friction in Tribological Studies

“…Such copolymers were prepared by RAFT solution polymerization using pentafluorophenyl methacrylate as reactive repeat units to enable introduction of nitrodopamine functional groups. The resulting anchor block led to a relatively high adsorbed mass in the form of a brush‐like surface layer and excellent friction reduction in tribological experiments [3] . Similar observations were reported by the same team for poly(lauryl methacrylate)‐based diblock copolymers bearing a carboxylic acid‐based anchoring block [4] .…”

“…It is well‐documented that the addition of oil‐soluble polymers to automotive engine oils confers various benefits. For example, polyolefins or poly( n ‐alkyl methacrylates) confer superior lubrication performance, [1–4] polystyrene‐based diblock copolymers can act as effective diesel soot dispersants [5] and star diblock copolymers can be used to control the viscosity‐temperature profile of engine oils [6]…”

Polymeric Friction Modifiers: Influence of Anchoring Chemistry on Their Adsorption and Effectiveness

“…28 The L H for hexadecane was determined to be 1.35 ± 0.2 nm, as shown in Figure 2b, corresponding to a few layers of hexadecane molecules lying down on the surfaces. 19 Reversible behavior was measured upon loading and unloading (see Figure 2a).…”

“…8,10,13−19 To better control the adsorption, block copolymers have been preferred to random ones. 13,18,19 The architecture and the chemical nature of the polymers also play a role in boundary-film formation and friction reduction capability, as shown in refs 18 and 19.…”

“…These surfaces were cleaned in an ultrasonic bath for 20 min in ethanol, before being rinsed with ultrapure water and blown dry with filtered nitrogen. 19 The rest of the measurement setup was cleaned by sonication in isopropanol and hexane. Each of the four cells was connected in parallel to a peristaltic pump and rinsed with hexadecane for 4 h for equilibration at a constant flow rate of 2.5 μL/s per sensor.…”