Diffusion of polystyrene chains and fluorescent dye molecules in semidilute and concentrated polymer solutions

Fluorescence correlation spectroscopy (FCS) experiments have been performed to measure the center-of-mass diffusion coefficient of fluorescently labeled polystyrene (M-W = 8100 g/mol) dissolved in polymer/toluene solutions above the overlap concentration. The diffusion coefficient was found to decrease by 2 orders of magnitude for a concentration change of 0.17-0.6 g/cm(3) at 34 degrees C. Less dramatic changes were observed at higher temperatures. The results are compared with the diffusion of free dyes in similar concentrated solutions. Vrentas-Duda free volume theory can explain the data reasonably well, from which the unit size of transport for the labeled macromolecule and the dye relative to the solvent is determined. The activation energy of diffusion was found to increase significantly as a function of concentration.