Tracer diffusion in thin polystyrene films

In order to determine how confinement of a polymer melt alters segmental sc ale mobility, we used fluorescence recovery after photobleaching (FRAP) to measure tracer diffusion coefficients, D, of a low molecular weight fluores cent probe in supported polystyrene (PS) thin films. The effect of film thi ckness was investigated for a polydisperse polymer over the range similar o r equal to 10(2) - 10(4) Angstrom at conditions above the bulk glass transi tion temperature, T-g. For relatively thick films (thicknesses similar or e qual to 1 mu m), the D agree with bulk values reported in the literature. A s the film thickness decreases below similar or equal to 10(3) Angstrom, th e D increase significantly, by as much as two orders of magnitude. At the s ame time the temperature dependence of D becomes much weaker. Analysis by f ree volume theory shows the data are consistent with both the effective T-g and liquid state thermal expansion coefficient, alpha, decreasing with fil m thickness. This agrees qualitatively with a number of recent thermal prop erty studies on thin PS films which suggest that changes in local packing n ear the free surface significantly enhance the average segmental scale mobi lity in sufficiently thin films. (C) 2000 Elsevier Science Ltd. All rights reserved.