Friction between Ring Polymer Brushes

TL;DR

This study investigates how the topology of polymer brushes, specifically ring versus linear, affects frictional forces during sliding, revealing that ring brushes exhibit approximately half the friction of linear brushes across various velocities due to differences in inter-digitation and monomer collisions.

Contribution

The paper provides the first comprehensive comparison of frictional behavior between ring and linear polymer brushes using molecular dynamics simulations and scaling analysis.

Findings

Ring-polymer brushes exhibit half the friction of linear brushes over three decades of velocity.

Weak inter-digitation in ring brushes results in fewer monomer collisions at low velocities.

At high velocities, ring polymers adopt double-stranded conformations reducing monomer collisions.

Abstract

Friction between ring-polymer brushes at melt densities sliding past each other are studied using extensive course-grained molecular dynamics simulations and scaling arguments, and the results are compared to the friction between linear-polymer brushes. We show that for a velocity range spanning over three decades, the frictional forces measured for ring-polymer brushes are half the corresponding friction in case of linear brushes. In the linear-force regime, the weak inter-digitation of two ring brushes compared to linear brushes also leads to a lower number of binary collisions between the monomers of opposing brushes. At high velocities, where the thickness of the inter-digitation layer between two opposing brushes is on the order monomer size regardless of brush topology, stretched segments of ring polymers take a double-stranded conformation. As a result, monomers of the double-stranded segments collide less with the monomers of the opposing ring brush even though a similar number of monomers occupies the inter-digitation layer for ring and linear-brush bilayers. The numerical data obtained from our simulations is consistent with the proposed scaling analysis. Conformation-dependent frictional reduction observed in ring brushes can have important consequences in non-equilibrium bulk systems.