Temperature-induced adsorption and optical properties of an amphiphilic diblock copolymer adsorbed onto flat and curved silver surfaces

Temperature-induced adsorption of a thermoresponsive amphiphilic copolymer, containing poly(N-isopropylacrylamide), on citrate-capped silver nanoparticles and planar silver surfaces has been studied with the aid of quartz crystal microbalance with dissipation monitoring (QCM-D) and dynamic light scattering (DLS). The results clearly show that both the amount of adsorbed copolymer and thickness of the adsorbed layer increase strongly at temperatures above the lower critical solution temperature (LCST). These findings are ascribed to enhanced hydrophobicity of the polymer and higher affinity for polymer adsorption at elevated temperatures and formation of intermicellar structures of the copolymer. The values of the layer thickness calculated from QCM-D data are practically identical to the values for the hydrodynamic thickness from DLS. This result suggests that the adsorbed layer is compact with few tails protruding out into the bulk. The surface plasmon peak for silver is observed at all temperatures, and the maximum is red-shifted with increasing temperature, which is attributed to an increase of the localized refractive index as more polymer chains are adsorbed onto the silver particles.