Impact of chain morphology on the lubricity of surface-grafted polysaccharides

Surface-density gradients of dextran have been fabricated on silicon wafers by means of an intermediate azide-terminated monolayer, which binds to random segments of the dextran chains to form a complex loop–train–tail structure. Friction-force microscopy was employed to understand the relative contributions of chain density and other parameters to the tribological behaviour of the immobilized chains. The results are contrasted with similar investigations performed with density gradients of poly(L-lysine)-graft-dextran, a bottlebrush copolymer that adsorbs onto silica to form a well-characterized dextran brush. Both systems exhibit friction coefficients that vary over more than an order of magnitude with applied load, with a sharp transition from low- to high-friction regimes occurring upon increasing load. The brush architecture exhibited more extreme friction coefficients than the loop–train–tail architecture, lubricating better at low loads while exhibiting higher friction at high loads, despite involving less than a third of the amount of dextran (on a monomer basis) in comparison to the loop–train–tail system. The coefficient of friction at high loads decreased with increasing dextran surface density in the loop–train–tail system, while the opposite was true for the polymer brush. The surface density required to forestall the pressure-induced transition to high friction was also significantly higher for the loop–train–tail system than for the brush system. These results illustrate the influence of brush regularity on resistance to collapse under applied load, but also its role in exacerbating friction forces.