The Formation of Viscous Surface Films by Polymer Solutions: Boundary or Elastohydrodynamic Lubrication?

A range of functionalised polymethacrylate copolymers have been synthesised with different functionalities, polymer architecture and molecular weight. It is shown that appropriately functionalised block copolymers give enhanced film thickness and greatly reduced friction under low entrainment speed conditions, even with polymer concentration as low as 1% wt. This behaviour almost certainly results from the formation of an adsorbed brushlike film of thickness ca 20 nm on each polar surface. These films provide a highly viscous inlet that promotes fluid entrainment and thus maintains a separating film down to very low entrainment speed. The adsorbed polymer films are also able to maintain separation in stationary contact conditions. Randomly distributed copolymers do not show this type of behaviour. The friction reduction observed is more effective in unidirectional, mixed slidingrolling than in reciprocating, sliding conditions. However, it is found that functionalised polymers and conventional organic and molybdenum-based friction modifiers can be combined to provide effective friction reduction over the whole range of rubbing conditions.

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“…The results support the overall mechanism of boundary film formation previously suggested [13] and illustrated in Fig. 5.…”

Section: Film Thickness Resultssupporting

confidence: 88%

Reduction of Friction by Functionalised Viscosity Index Improvers

Fan

Müller²,

Stöhr³

et al. 2007

Tribol Lett

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“…Although dex(5.2) chains do not display noticeable surface-adsorption properties (Fig. 3), and thus are poor boundary-lubricating additives, they appear to facilitate the entrainment of aqueous lubricant due to their hydrophilic characteristics [52].…”

Section: Lubrication At Mixed Sliding/rolling Contacts: Mtmmentioning

confidence: 99%

End-grafted Sugar Chains as Aqueous Lubricant Additives: Synthesis and Macrotribological Tests of Poly(l-lysine)-graft-Dextran (PLL-g-dex) Copolymers

2008

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Comb-like graft copolymers with carbohydrate side chains have been developed as aqueous lubricant additives for oxide-based tribosystems, in an attempt to mimic biological lubrication systems, whose surfaces are known to be covered with sugar-rich layers. As adopted in the previous studies of the graft copolymer poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG), which showed both excellent lubricating and antifouling properties, a similar approach was chosen to graft dextran chains onto the same backbone, thus generating PLL-g-dex. PLL-g-dex copolymers readily adsorb from aqueous solution onto negatively charged oxide surfaces. Tribological characterization at the macroscopic scale, either under pure sliding conditions or a mixed sliding/rolling contact regime, shows that PLL-g-dex is very effective for the lubrication of oxide-based tribosystems. The relative lubricating capabilities of PLL-g-dex copolymers compared with PLL-g-PEG copolymers were observed to be highly dependent on the molecular structure of the copolymers (in particular, sidechain density along the backbone) and the measurement conditions (in particular, time between tribocontacts); the PLL-g-dex copolymers with a low degree of grafted side chains (B20% grafting of available protonated primary amine groups along the backbone) showed better lubricating performance than their PLL-g-PEG counterparts at high tribocontact frequency (Cca. 0.32 Hz).

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“…Recently there have been some attempts in the literature [31], however these rely on conventional EHL film thickness equations which are only applicable to systems where there is a significant degree of increase in the inlet viscosity [32]. BCS is essentially an aqueous suspension and the viscosity of water is low, but more importantly the pressure viscosity coefficient is tiny [32].…”

Section: Protein Gel Formation and Contact Entrymentioning

confidence: 99%