An atomic force microscope colloidal probe technique has been employed to probe normal and friction forces between silica surfaces coated with adsorbed layers of a diblock copolymer of the composition poly(N-isopropylacrylamide) 48 -block-poly(3-acrylamidopropyl)trimethylammonium chloride) 20 , abbreviated PNIPAAM 48 -b-PAMPTMA(+) 20 . The interactions between the PNIPAAM 48 -b-PAMPTMA(+) 20 -coated surfaces across a 0.1 mM NaCl (pH 6) solution at 25 C are purely repulsive, due to a combination of steric and electrostatic double-layer forces. However, when the temperature is increased to 35 C, and subsequently to 45 C, an attractive force develops at short separations due to the unfavourable PNIPAAM-water interaction at these temperatures. The temperature-dependent polymer-water interaction has implications for the friction force between the layers. At 25 C a frictional force that increases linearly with increasing load is observed once the surfaces are brought into close contact. At higher temperatures significantly higher friction forces appear as a consequence of attractive segment-segment interactions. Further, a clearly expressed hysteresis between friction forces encountered on loading and unloading is detected. Our results demonstrate that both normal and friction forces between surfaces can be controlled by temperature changes when temperatureresponsive polymers are employed, and friction forces can be adjusted as required from low to high.

The adsorption behavior of bottle-brush polymers with different charge/PEO ratio on silica was studied using optical reflectometry and QCM-D. The results obtained under different solution conditions clearly demonstrate the existence of two distinct adsorption mechanisms depending on the ratio of charge/PEO. In the case of low-charge density brush polymers (0-10 mol %), the adsorption occurs predominantly through the PEO side chains. However, the presence of a small amount of charge along the backbone (as low as 2 mol %) increases the adsorption significantly above that of the uncharged bottle-brush polymer in pure water. As the charge density of the brush polymers is increased to 25 mol % or larger the adsorption occurs predominantly through electrostatic interactions. The adsorbed layer structure was studied by measuring the layer dissipation using QCM-D. The adsorbed layer formed by the uncharged brush polymer dissipates only a small amount of energy that indicates that the brush lie along the surface, the scenario in which the maximum number of PEO side chains interact with the surface. The adsorbed layers formed by the low-charge density brush polymers (2-10 mol %) in water are more extended, which results in large energy dissipation, whereas those formed by the high-charge density brush polymers (50-100 mol %) have their backbone relatively flat on the surface and the energy dissipation is again low.

Adsorbed layers of a series of bottle-brush polyelectrolytes with 45 units long poly(ethylene oxide) [PEO], side chains have been investigated by the quartz crystal microbalance technique with dissipation monitoring. The data have been evaluated with three different models, the Sauerbrey model, the Johannsmann model, and the Voigt model. It is found that all three models predict the same trend in the variations of sensed mass and hydrodynamic layer thickness with polymer architecture, that is, with the backbone charge to side chain density ratio. However, the simple Sauerbrey model underestimates the sensed mass by a factor of 1.15−1.45 compared to the more accurate Voigt model. By following the evolution of the layer dissipation, elasticity, and viscosity with increasing surface coverage it was concluded that the layers formed by brush polymers with intermediate charge densities undergo a structural change as the coverage is increased. Initially, the polymers are anchored to the surface via the PEO side chains. However, as the adsorption proceeds a structural change that brings the backbone to the surface and forces the side chains to extend from it is observed. The layer elasticity and viscosity as a function of surface coverage go through a maximum in this transition region.

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