Towards a Polymer-Brush-Based Friction Modifier for Oil

In the search for new, oil-compatible friction-modifier additives that can significantly reduce energy consumption by reducing the friction in the boundary-lubrication regime, a macromolecular-design approach has been taken. This involved the synthesis of a series of readily functionalizable block copolymers carrying an oleophilic poly(lauryl methacrylate) block and a functionalizable poly(pentafluorophenyl methacrylate) block of various lengths by means of reversible addition-fragmentation chain-transfer (RAFT) polymerization. The poly(pentafluorophenyl methacrylate) block was used to attach surface-active nitro-catechol anchoring groups to the polymer. The friction-reduction properties of these polymers were assessed using 0.5 wt.% solutions in hexadecane using 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 iron-oxide coated QCM crystal, consistent with 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.