Mechanical and frictional properties of nanoparticle monolayers grafted on functionalized mica substrates

Normal and lateral forces between two opposing monolayers of grafted polymer nanoparticles (NPs) were measured using the Surface Forces Apparatus in a humid atmosphere. The NPs made of N, N-diethylacrylamide and 2-hydroxyethyl methacrylate have a hydrodynamic diameter of ca. 660 nm at 25 degrees C. The effect of surface roughness was studied by creating surface asperities using different NP grafting densities ranging from 0.41 to 2.63 NPs/mum (2). An increase in the NPs grafting density gave rise to an increase in surface roughness and to a deformation of the nanoparticles caused by the lateral pressure between neighboring particles. An elastoplastic behavior of the nanoparticles was observed for large grafting densities, while a purely elastic behavior was observed for small grafting densities. The lateral forces measured between two opposing NP monolayers sliding past each other followed Amontons' law for all grafting densities. The friction coefficient between the surfaces appeared to increase significantly with an increase in surface roughness, which was inherent to an increase in the elastoplastic behavior of the NP monolayers.