Interaction forces between a poly (N-isopropylacryamide) layer grafted onto a solid surface and a hydrophobic particle

The interaction forces between poly (N-isopropylacrylamide) (PNIPAAm)-grafted surfaces and colloidal particles in an aqueous solution were investigated using atomic force microscopy. The measurements were conducted between a smooth silicon wafer on which PNIPAAm was terminally grafted and silica particles hydrophobized with a silanating reagent in an aqueous electrolyte solution under controlled temperature. Large repulsive forces were observed between the surfaces below the lower critical solution temperature (LCST) of PNIPAAm, while attractive forces were observed above the LCST. The magnitude of the attractive force tended to increase with the surface hydrophobicity of the particles. The temperature-induced changes in the hydration state of the grafted PNIPAAm chains are considered to greatly alter the interaction force properties. The role of the intermolecular interaction between the PNIPAAm chains and hydrophobic particles in the interaction forces is discussed.