posted on 2015-12-17, 03:13authored bySissi de Beer
Stimulus-responsive (SR), solvated
polymers can switch between
an expanded state and a collapsed state via external stimuli. Using
molecular dynamics simulations, I show that such SR polymers can be
employed to control the frictional response between two opposing polymer
brushes in relative sliding motion. By using a brush composed of SR
polymers in contact with a nonresponding solvated polymer brush, the
presence of capillaries and the overlap between molecules of the opposing
brushes can be switched. When both brushes are solvated, a capillary
is formed and polymers of the opposing brushes interdigitate. Interdigitation
dominates friction upon shearing flat brush-bearing surfaces, while
the breaking and formation of capillaries dominate friction in the
low-velocity limit between rough brush-bearing surfaces. Thus, when
either rough or flat polymer-bearing surfaces are sheared, friction
between two swollen brushes can be high. In contrast, when the SR
brush is collapsed, the solvent absorbs only in the brush that does
not respond to the external stimulus. The latter circumvents the presence
of capillaries and interdigitation of the brushes, which results in
a low friction force upon shearing.