Investigation Of The Interfacial Friction And Adhesion Of Thin Pdms Network Lubricant Films
The lubrication properties of several variations on a polydimethylsiloxane (PDMS) network - self-assembled monolayer (SAM) hybrid lubricant and the effect that structural and chemical modifications can have on the friction, adhesion, and wear are investigated. The primary lubricant structure studied in this thesis consisted of a model cross-linked PDMS network tethered to an anchoring SAM layer. Modifications of this structure including ultrathin hyperbranched films, micron-thick films with pendent chains, networks swollen with free chains, and a deconstructed polymer brush are also studied. These hybrids created high degrees of internal tethering with a flexible surface layer exhibiting low interfacial shear and low surface energy. The friction and adhesion properties of these lubricants were determined from a series of experiments conducted with atomic force microscopy, nanoindentation, and bulk rheology and tribology. The PDMS-SAM lubricants demonstrated dramatically low friction coefficients, as low as [mu] = 0.0024, which is the lowest friction coefficient recorded for a dry polymer film. These low friction properties are a result of a negligible contribution of adhesion to the friction force. Structural modifications to the network allowed for the effect of the physical properties on the friction and lubrication to be determined. Incorporating pendent chains and free chains into the network layer increased the surface viscous dissipation. While the elastic moduli and the network stiffness decreased, the structural modifications also served to increase the surface shear and viscosity, resulting in larger friction coefficients. Wear measurements conducted on the networks swollen with free chains indicated that increasing the lubricant viscous dissipation also dramatically reduced the wear resistance of the film. Polymer brushes were developed at several surface coverages and with different polymer chemistries to determine the physical and chemical effects on friction and lubrication. High surface coverages creating stretched elastic chains yielded a significant reduction in the friction coefficient, relative to more deformed, mushroom conformations. PDMS brushes exhibited significantly lower friction than brushes made from polymers with higher surface energies, despite the chemical effects being minimized as a result of a minimal adhesion contribution to the friction force at the ultrathin lubricant lengthscale.
dissertation or thesis