Friction in aqueous media tuned by temperature-responsive polymer layers

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 degrees C are purely repulsive, due to a combination of steric and electrostatic double-layer forces. However, when the temperature is increased to 35 degrees C, and subsequently to 45 degrees 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 degrees 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 temperature-responsive polymers are employed, and friction forces can be adjusted as required from low to high.