Molecular diffusion on surface tethered polymer layers: coupling of molecular thermal fluctuation and polymer chain dynamics

The dynamics of single probe molecules on surface-tethered poly(N-isopropylacrylamide) (PNIPAM) layers is studied by fluorescence correlation spectroscopy. It is surprisingly observed that molecular surface dynamics on polymer layers is much slower than that on surfactant monolayers, despite stronger interfacial interactions for the latter case, suggesting a strong coupling of molecular thermal fluctuation and polymer chain dynamics. The effect of collective dynamics of polymer chains on the surface diffusion of adsorbed molecules is further examined with varied polymer grafting density and thickness. At low grafting density or large brush thickness, the resulting surface inhomogeneity of PNIPAM layers from the respective polymer mushroom conformation and low polymer end-segment density leads to the significant slowing down in the surface diffusion of adsorbed probe molecules. An optimal range in brush thickness to facilitate molecular surface dynamics is observed, where PNIPAM layers adopt the brush conformation with high polymer end-segment density.