Layering transitions and tribology of molecularly thin films of poly(dimethylsiloxane)

Friction measurements were carried out for molecularly thin films of a poly(dimethylsiloxane) (PDMS) melt (M_w ≈ 80000) as a function of the applied load (pressure) and sliding velocity using the surface forces apparatus. The PDMS films exhibit apparent layering transitions when the thicknesses of the films are decreased to the order of molecular dimensions. For four-layer and three-layer films, "solidlike" sliding is observed and the shear stresses are on the order of 10⁵ Pa. Further compression and simultaneous lateral motion squeeze out the PDMS molecules to a final residual film two molecular layers in thickness, whose shear properties include "viscous" characters, and the shear stress increases abruptly by a factor of 6-8. This shear property change may arise from the different sliding mechanisms of "adsorbed" and "mobile" molecular layers. When thicknesses of the films are three layers and above, the first layers adjacent to mica substrates are strongly adsorbed onto substrate surfaces and immobile during sliding; shear is accomplished by the slipping of "mobile" middle layers (results in low friction). For the two-layer film (adsorbed layers in direct contact), sliding involves the deformation of adsorbed PDMS segments and wall slip, resulting in high friction and surface damage (wear). For PDMS films, a "fluidlike" response appears when molecules are squeezed out to a final residual thickness (two layers), which is very different from the typical behavior of most of the confined fluid systems (solidlike shift is commonly observed due to confinement). Effects of the substrate-molecule interaction strength on the layering structures and shear properties are also discussed.