Substrate effect on topographical, elastic, and frictional properties of hydrogels

Cross-linked poly(N,N'-dimethyl acrylamide) hydrogels were prepared on hydrophilic silicon substrates or hydrophobic polystyrene substrates and their topographical elastic properties were investigated by atomic force microscopy (AFM). The gels prepared on the hydrophobic substrate have a much rougher surface morphology with a lower surface elastic modulus than those prepared on the hydrophilic substrate. The topographical and mechanical properties of the gel have little influence on the surface friction, and the lower surface friction observed for the gel prepared on the hydrophobic substrate was attributed to the presence of dangling free polymer chains formed in the course of the polymerization.