Coupling of density to concentration fluctuations in concentrated solutions of polystyrene in toluene

Abstract

In concentrated polystyrene (PS) solutions in toluene, close to the glass transition temperature (Tg), both concentration (q2 dependent) and density fluctuations (q independent) are simultaneously observable in the time window of the polarized (VV) dynamic light scattering experiment. At Φp=0.89, both modes (Γc and Γd) are well separated on the time scale. Close to Tg, the diffusion relaxation is ‘‘frozen-in,’’ a phenomenon anticipated from free-volume theory but not previously observed in polymer solutions. At Φp=0.78, a q2-dependent mode is found above about 40 °C, whereas it is q independent at 17 °C. At intermediate temperatures, the two modes are coupled when Γc≊Γd. The coupling of density to concentration fluctuations is analyzed here in terms of a recent theory of Jäckle and Frisch [J. Polym. Sci. Polym. Phys. Ed. 23, 675 (1985)]. The latter is found to provide a good semiquantitative description of the complex dynamic behavior in this system close to the glass transition.