Frictional response of wall-brush systems composed of breakable polymer chains.

Abstract

Molecular dynamics simulations were performed to investigate the structural evolution and frictional behaviors of breaking polymer brushes subjected to shear imposed by a moving wall with systematic comparisons to non-breaking systems. Our results show that the structural and dynamical properties of breakable brushes are strongly coupled with the shear velocity. At low wall velocities, bond cleavage is rare, and the two systems exhibit nearly identical structural and frictional responses. With increasing wall velocity, brush chains become progressively stretched and oriented along the shear direction, leading to bond rupture. Bonds with the highest degree of shear-induced orientation are more susceptible to breaking, while the location of maximum orientation shifts with wall velocity, <i>i.e.</i>, the polydispersity of free and brush chains are altered by the velocity. When the wall velocity exceeds a critical value, a large number of free chains accumulate near the moving wall and form a mobile interfacial lubricating layer, resulting in a pronounced self-lubrication effect. In this high-velocity regime, the friction coefficient decreases by approximately one order of magnitude relative to the corresponding non-breaking system. Moreover, this critical velocity decreases with increasing chain length. Further analysis reveals that, compared with the contact number, which is conventionally used to estimate the strength of friction, chain orientation maintains a robust linear relation with the friction coefficient over a much broader shear range, therefore serves as a more reliable descriptor for the interfacial frictional response. This study elucidates the molecular mechanism of bond-breaking-induced self-lubrication in polymer brushes and provides physical insights for the design of polymer-brush interfaces with tunable frictional properties.