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 repea t units of the polymer. The effects of temperature and film thickness were investigated, at temperatures above the bulk glass transition of the polyme r, 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 de creases towards Rg the value of D increases above the bulk values, with sig nificant effects first appearing in films similar or equal to 20R(g). In th e thinnest films studied, about 4R(g), the values of D lie as much as two o rders over bulk values. At the same time, the temperature dependence of D b ecomes much weaker than in hulk. Analysis by free volume theory indicates t hat apparent values of both T-g and the thermal expansion coefficient for l iquid state, alpha(L), decrease as the film thickness decreases. The possib le effects of surface segregation of the dye probe are discussed.