Shear rate threshold for the boundary slip in dense polymer films

TL;DR

This study uses molecular dynamics simulations to explore how shear rate influences slip behavior in dense polymer films, revealing a transition from no-slip to slip flow and detailing the interfacial friction mechanisms.

Contribution

It provides new insights into the shear rate-dependent slip length and interfacial friction in dense polymer films, linking molecular relaxation to slip behavior.

Findings

Slip length is negative at low shear rates, about the viscous layer thickness.

Velocity profiles become linear at high shear rates, with slip length increasing rapidly.

Friction coefficient decays as a power law with slip velocity, influenced by surface structure and temperature.

Abstract

The shear rate dependence of the slip length in thin polymer films confined between atomically flat surfaces is investigated by molecular dynamics simulations. The polymer melt is described by the bead-spring model of linear flexible chains. We found that at low shear rates the velocity profiles acquire a pronounced curvature near the wall and the absolute value of the negative slip length is approximately equal to thickness of the viscous interfacial layer. At higher shear rates, the velocity profiles become linear and the slip length increases rapidly as a function of shear rate. The gradual transition from no-slip to steady-state slip flow is associated with faster relaxation of the polymer chains near the wall evaluated from decay of the time autocorrelation function of the first normal mode. We also show that at high melt densities the friction coefficient at the interface between the polymer melt and the solid wall follows power law decay as a function of the slip velocity. At large slip velocities the friction coefficient is determined by the product of the surface induced peak in the structure factor, temperature and the contact density of the first fluid layer near the solid wall.