LUBRICATION BY MOLTEN POLYMER BRUSHES

We study the viscous friction between two polymer molten layers grafted on parallel solid surfaces and rubbed against each other. The dissipation is dominated by the small zone at the contact between the two brushes where they interdigitate. A sliding viscosity is calculated via an analogy with the problem of end adsorption of a polymer brush. As the relative velocity of the two brushes increases, the polymer chains are stretched and the sliding viscosity decreases. For a finite critical value of the velocity, the chains can no longer sustain the viscous stress and disinterpenetrate. The sliding viscosity and the critical velocity are calculated within the framework of both the Rouse model and the reptation model to describe the polymer dynamics; the critical velocity decreases exponentially with molecular weight in the reptation model. For long chains, even at very low shear gradients, this instability could considerably reduce the role of entanglements in the rheology of polymer brushes. Possible applications to the rheology of block copolymer lamellar phases are also briefly discussed.