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Thermo-Responsive Block Copolymers: Friction Control by Temperature Switch

By Andra Dedinaite, Esben Thormann, geoffrey Olanya, Per M. Claesson, Bo Nystrom and Kaizheng Zhu


An atomic force microscope colloidal probe technique was employed to probe normal and frictional forces between silica surfaces coated with layers of diblock copolymer of the composition (N-isopropylacrylamide)48-((3-acrylamido¬propyl)tri-methyl ammonium chloride)20, abbreviated PNIPAAm48-PN(+)20.The architecture of these polymers allowed exploring how normal and frictional forces between polymer-coated surfaces can be tuned by temperature changes. The interaction in 0.1 mM NaCl (pH 6) solution between the PNIPAAm48-PN(+)20-coated surfaces at 25°C is purely repulsive. The total surface forces can be interpreted as being due to additive contributions of two components – steric and electrostatic. However, when the temperature is increased to 36°C, and subsequently to 45°C, an attractive force develops at short separations. As the temperature is increased, the PNIPAAm-water interaction becomes unfavorable and the polymer change conformation due to dehydration. Thus, at higher temperatures the surface interactions become a composition of long-range double layer repulsion and short-range attraction between the PNIPAAm blocks on the opposite surfaces. The temperature-dependent behavior of the block co-polymer also has implication on friction forces: 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, as a consequence of adhesive interactions a sudden jump to higher friction forces occurs. 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 temperature responsive polymers are employed for surface modification, and friction forces can be switched from low to high values

Topics: surface forces, friction, aqueous lubrication, AFM colloidal probe, thermo-responsive polymer, adsorption
Publisher: RTU
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