COUPLING OF DENSITY TO CONCENTRATION FLUCTUATIONS IN CONCENTRATED-SOLUTIONS OF POLYSTYRENE IN TOLUENE

In concentrated polystyrene (PS) solutions in toluene, close to the gl ass transition temperature (T(g)), both concentration (q2 dependent) a nd density fluctuations (q independent) are simultaneously observable in the time window of the polarized (VV) dynamic light scattering expe riment. At PHI(p)=0.89, both modes (GAMMA(c) and GAMMA(d)) are well se parated on the time scale. Close to T(g), the diffusion relaxation is ''frozen-in,'' a phenomenon anticipated from free-volume theory but no t previously observed in polymer solutions. At PHI(p)=0.78, a q2-depen dent mode is found above about 40-degrees-C, whereas it is q independe nt at 17-degrees-C. At intermediate temperatures, the two modes are co upled when GAMMA(c) almost-equal-to GAMMA(d) . The coupling of density to concentration fluctuations is analyzed here in terms of a recent t heory of Jackle and Frisch [J. Polym. Sci. Polym. Phys. Ed. 23, 675 (1 985)]. The latter is found to provide a good semiquantitative descript ion of the complex dynamic behavior in this system close to the glass transition.