Molecular mobility in polymer thin films

Fluorescence recovery after photobleaching was used to measure in-plane dye-probe diffusion coefficients, D, in thin films of monodisperse polystyrene supported on fused quartz substrates. The substrates were prepared with a high density of surface hydroxyl groups which interact favorably with repeat units of the polymer. The effects of temperature and film thickness were investigated, at temperatures above the bulk glass transition of the polymer, $T_g,$ and in the range of film thicknesses from $1–{10}^2$ times the radius of gyration ${(R}_g)$ of individual polymer molecules. As the film thickness decreases towards $R_g$ the value of D increases above the bulk values, with significant effects first appearing in films $\simeq {20R}_g.$ In the thinnest films studied, about ${4R}_g,$ the values of D lie as much as two orders over bulk values. At the same time, the temperature dependence of D becomes much weaker than in bulk. Analysis by free volume theory indicates that apparent values of both $T_g$ and the thermal expansion coefficient for liquid state, ${\alpha }_L,$ decrease as the film thickness decreases. The possible effects of surface segregation of the dye probe are discussed.