Collective and thermal diffusion in dilute, semidilute, and concentrated solutions of polystyrene in toluene

Abstract

Collective mass diffusion (D), thermal diffusion (DT), and Soret coefficients (ST) have been determined for solutions of polystyrene in toluene ranging from dilute (10−4 g/cm3) to concentrated (0.9 g/cm3) by a transient holographic grating technique and photon correlation spectroscopy (collective diffusion only). The molar mass range of the polymer was between 4.75 and 4060 kg/mol. With a slight exception at intermediate concentrations, DT is molar mass independent over the entire concentration range. Above a polymer concentration of C≈0.2 g/cm3 the rising glass transition temperature of the solution leads to a rapid slowing down of both mass and thermal diffusion (Ludwig Soret effect) and the structure relaxation enters the experimental time window. Both D and DT are governed by the same microscopic local viscosity ηeff, which increases dramatically at Tg. It cancels out in ST=DT/D. As a consequence, ST is completely insensitive to the glass transition and shows concentration-dependent scaling as expected for a fictive polymer solution with concentration-independent local friction. Where available, results are compared with literature data.