Tracer diffusion in thin polystyrene films

In order to determine how confinement of a polymer melt alters segmental scale mobility, we used fluorescence recovery after photobleaching (FRAP) to measure tracer diffusion coefficients, D, of a low molecular weight fluorescent probe in supported polystyrene (PS) thin films. The effect of film thickness was investigated for a polydisperse polymer over the range ≃102–104 Å at conditions above the bulk glass transition temperature, Tg. For relatively thick films (thicknesses ≃1 μm), the D agree with bulk values reported in the literature. As the film thickness decreases below ≃103 Å, the D increase significantly, by as much as two orders of magnitude. At the same time the temperature dependence of D becomes much weaker. Analysis by free volume theory shows the data are consistent with both the effective Tg and liquid state thermal expansion coefficient, α, decreasing with film thickness. This agrees qualitatively with a number of recent thermal property studies on thin PS films which suggest that changes in local packing near the free surface significantly enhance the average segmental scale mobility in sufficiently thin films.