The Influence of Anchoring-Group Structure on the Lubricating Properties of Brush-Forming Graft Copolymers in an Aqueous Medium

Abstract

We have compared the lubricating properties of two different PEG-grafted, polycationic, brush-forming copolymers to gain a deeper understanding of the role of the polyionic backbone in the lubricating behavior of such materials, when used as additives in aqueous lubricant systems. Previously, poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) has been shown to adsorb onto oxide surfaces from aqueous solution and substantially lower frictional forces. Poly(allylamine)-graft-poly(ethylene glycol) (PAAmg-PEG), which also has a polycationic backbone, has been synthesized in several different architectures, and its performance investigated via adsorption tests, rolling-and sliding-contact tribometry, and the surface forces apparatus. These tests show a clear reduction of friction forces with PAAm-g-PEG compared to water alone. However, when compared with PLL-g-PEG, while PAAm-g-PEG copolymers did not adsorb to the same extent or exhibit as high a lubricity in sliding geometry, they showed a similar lubricating effect under rolling conditions. The difference in the chemical structure of the backbones, especially the flexibility of the anchoring groups, appears to significantly influence both the extent and kinetics of polymer adsorption, which in turn influences lubrication behavior.

Influence of the molecular architecture on the adsorption onto solid surfaces: comb-like polymers

“…As a consequence the steric repulsion between PEG chains with a weaker interaction with the surface makes the adsorption easier for the copolymer with PLL. A similar explanation has been also proposed by Hartung et al 16 as a characteristic of the interaction between amine based polymers and negatively charged surfaces.…”

“…The smaller differences observed between G2 and G3 may be due to the effect of the structure of the main chain where the higher flexibility of the amine groups of PLL increases the entropic contribution. 16,33 The increase on the number of segments protruding into the solution with polymer concentration also increases the water trapped by the adsorbed layer, thus reducing the entropic contribution (increase in the absolute value of the entropy) as shown in Fig. 10b for G1 (similar results were found for G2 and G3).…”

“…The decrease in the packing density with an increase in the concentration may be due to the higher mass adsorbed that can increase the steric effects. 16 The packing of the PEG chains is strongly correlated with the water content inside the adsorbed layer. It is important to remind that the charged polymer chains adsorb in a hydrated form which explains why in all the cases G ac > G op .…”

Surface Modification of Diamond‐Like Carbon Film with Polymer Brushes Using a Bio‐Inspired Catechol Anchor for Excellent Biological Lubrication

“…The water lubrication of brushes‐like poly (L‐lysine)‐graft‐poly (ethylene glycol) (PLL‐g‐PEG) was also reported by Spencer. The PLL backbone with multiple positive charges spontaneously adsorbed on negative charged surfaces, and PEG side chains extended towards solid‐liquid interface to lubricate the solid surface . The frictional properties of polymer brushes formed by in situ polymerization on solid surfaces have been widely reported by theoretical and experimental studies .…”

Aqueous Lubrication of SiC and Si3N4 Ceramics Aided by a Brush-like Copolymer Additive, Poly(l-lysine)-graft-poly(ethylene glycol)

“…Past approaches have included modifying the surface charge by adjusting the pH of the lubricant [16] or using additives such as ionic liquids [17]. An alternative, promising approach for aqueous lubrication of ceramics is to use water-soluble brush-like copolymers, which have recently been applied to several oxide-based tribosystems [18][19][20][21][22][23][24][25][26][27][28][29][30]. Poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) has a polycationic backbone that adsorbs onto negatively charged surfaces, such as SiO 2 under neutral pH conditions.…”