Effect of Crosslinking on the Microtribological Behavior of Model Polymer Brushes

Polymer brushes in good solvents are known to exhibit excellent tribological properties. We have modeled polymer brushes and gels using a multibead-spring model and studied their tribological behavior via nonequilibrium molecular dynamics. Simulations of brush-against-wall systems were performed using an implicit solvent-based approach. Polymer chains were modeled as linear chains, randomly grafted on a planar surface. Quantities extracted from the simulations are the normal stress, shear stress and concentration profiles. We find that while an increase in the degree of crosslinking leads to an increase in the coefficient of friction, an increase in the length of crosslinker chains does the opposite. The effect of crosslinking can be understood in two ways: (1) There is a lower polymer concentration in the outer layer to take part in brush-assisted lubrication as the degree of crosslinking increases and (2) crosslinked polymer chains are more resistant to shear than noncrosslinked ones.