Surface Force and Friction : effects of adsorbed layers and surface topography

Abstract: Interfacial features of polymers are a complex, fascinating topic, and industrially very important. There is clearly a need to understand interactions between polymer layers as they can be used for controlling surface properties, colloidal stability and lubrication. The aim of my Ph.D study was to investigate fundamental phenomena of polymers at interfaces, covering adsorption, interactions between polymer layers and surfactants, surface forces and friction between adsorbed layers.A branched brush layer with high water content was formed on silica surfaces by a diblock copolymer, (METAC)m-b-(PEO45MEMA)n, via physisorption. The adsorption properties were determined using several complementary methods. Interactions between pre-adsorbed branched brush layers and the anionic surfactant SDS were investigated as well. Surface forces and friction between polymer layers in aqueous media were investigated by employing the Atomic Force Microscopy (AFM) colloidal probe technique. Friction forces between the surfaces coated by (METAC)m-b-(PEO45MEMA)n in water are characterized by a low friction coefficient. Further, the layers remain intact under high load and shear, and no destruction of the layer was noted even under the highest pressure employed, about 50 MPa.Interactions between polymer layers formed by a temperature responsive diblock copolymer, PIPOZ60-b-PAMPTMA17 (phase transition temperature of 46.1 °C), was investigated in the temperature interval 25-50 °C by using the AFM colloidal probe technique. Friction between the layers increases with increasing temperature (25-45 °C), while at 50 °C friction was found to be slightly lower than that at 45 °C. We suggest that this is due to decreased energy dissipation caused by PIPOZ chains crystallizing in water above the phase transition temperature.The structure of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers was determined by X-ray reflectometry. Surface forces and friction between DPPC bilayer-coated silica surfaces were measured utilizing the AFM colloidal probe technique. Our study showed that DPPC bilayers are able to provide low friction forces both in the gel (below ≈ 41°C) and in the liquid crystalline state (above ≈ 41°C). However, the load bearing capacity is lower in the gel state. This is attributed to a higher rigidity and lower self-healing capacity of the DPPC bilayer in the gel state.Friction forces in single asperity contact acting between a micro-patterned silicon surface and an AFM tip was measured in air. We found that both nanoscale surface heterogeneities and the µm-sized depressions affect friction forces, and considerable reproducible variations were found along a particular scan line. Nevertheless, Amontons’ first rule described average friction forces reasonably well. Amontons’ third rule and Euler’s rule were found to be less applicable to our system.