Nanotribology of Poly(dimethylsiloxane) Melt Confined between Hydrophobic Surfaces

Abstract

Friction measurements were carried out for a poly(dimethylsiloxane) (PDMS) melt (Mw ≈ 80000) confined between hydrophobic surfaces using the surface forces apparatus. The PDMS films were prepared by two different procedures: i) compression of a droplet into a hard-wall state (compressed system); ii) adhesive contact of two thin films cast on each substrate from solution (cast system). The dynamic thicknesses were 1.4 nm for the compressed system and 2.0 nm for the cast system. Despite the large thickness, the friction of the cast system was larger than that of the compressed system. Large thicknesses generally give low friction; the unusual result suggests that the confined structures may be different between the two systems. The PDMS molecules in both systems lay parallel to surfaces, but the extent of ordering could be much higher for the compressed system. The compressed film has a layer structure and slipping mainly occurs between the layers, resulting in the low friction. On the contrary, the cast system should have a disordered structure; molecules may interdigitate to each other and possibly form bridges across the sliding surfaces, which could induce large friction. The effect of film the preparation procedures on molecular ordering is discussed.

Keywords

• surface forces apparatus
• nanotribology
• confinement
• poly(dimethylsiloxane)
• molecular layering
• squeeze flow