The viscosity of lubricating oils is generally dependent upon temperature. To reduce this dependence, additives commonly known as viscosity index improvers have been developed. Their efficiency is strongly controlled by the nature of their affinity with the base oil and their interactions with surfaces in case of confinement.
A molecular tribometre derived from a Surface Force Apparatus has been used to characterize the mechanical and tribological behaviour of two semi-dilute DVM solutions in a confined interface. At large distances, two regimes can occur:
(a) one polymer is repelled from the wall and a depletion layer is built-up ,
(b) the other one sticks to the wall and an adsorption layer is depicted .
The structure of these layers results in a gradient of viscosity from the wall to the bulk that can be mechanically [1-2] and physically  modelled.
The confinement at short distances governs the tribological behaviour of the polymer layer formed close to the surface. The frictional response of the interface has been observed for various normal loads up to 1 mN and sliding velocities ranging from 0.1 nm/s to 0.5 µm/s. This clearly reveals that the Amontons: proportionality between frictional and normal stresses does not always hold: in the case of the adsorbed polymer layer, the higher the contact pressure, the lower the friction. Besides, the sliding stress is strongly dependent on the velocity: it increases with the sliding velocity. The behaviour of this system is theoretically accounted for using a model based on the kinetics of formation and rupture of adhesive bonds between the two shearing surfaces [4-5]. This approach allows us to correlate the frictional properties to the molecular organization on the surfaces.
1. J. Cayer-Barrioz, D. Mazuyer, A. Tonck and E. Yamaguchi, Tribology Letters (2008) 32: 81-90.
2. J. Cayer-Barrioz, D. Mazuyer, A. Tonck and E. Yamaguchi, Langmuir (2009) 25: 10802-10810.
3. PG. de Gennes, Macromolecules (1981) 14: 1637-1644.
4. C. Drummond, J. Israelachvili and Ph. Richetti, Phys Rev E (2003) 67: 066110.
5. D. Mazuyer, J. Cayer-Barrioz, A. Tonck and F. Jarnias, Langmuir (2008) 24: 3857-3866.