Small molecule probe diffusion in thin and ultrathin supported polymer films

The translational diffusion of two small molecule probes, decacyclene and lophine, has been studied in supported polymer films as a function of film thickness using fluorescence nonradiative energy transfer. These probes are sufficiently large that their translational motion is coupled substantially to the segmental mobility of the polymer matrix. Probe diffusion in polystyrene films on fused quartz substrates was found to slow as film thickness is decreased below approximately 150 nm. In contrast, no change in probe diffusion as compared to bulk values was found in poly(isobutyl methacrylate) films as thin as 50 nm and poly(2-vinylpyridine) films as thin as 88 nm. These differences cannot be explained in terms of differences in polymer affinity for the fused quartz substrate, indicating that polymer-substrate affinity does not influence probe diffusion in this Nm thickness range. Probe diffusion toward polymer-substrate and polymer-free surface interfaces has also been studied with little change in diffusive behavior as compared to bulk in regions as close as 40 nm from the interface. This indicates that the range of influence of a particular polymer-free surface or polymer-substrate interaction is of limited extent ((40 nm distance) in modifying thin and ultrathin behavior.