Solvent mobility in polystyrene/Aroclor solutions by depolarized Rayleigh scattering

We report evidence of modification of solvent reorientation induced by the presence of polymer in polymer/solvent systems. Depolarized Rayleigh spectroscopy (DRS) has been used to probe the rotational motion of neat Aroclor and polystyrene (PS) solutions with polymer concentration 0.05, 0.086, and 0.146 g/cm³. The depolarized (I_VH(ω)) spectra were recorded in the temperature range 40 to 140 °C using Fabry-Perot interferometry. Two Lorentzian lines were found to fit well the experimental I_VH(ω) of PS/Aroclor above 60 °C revealing two populations of solvent. The fast relaxation time τ_f is virtually insensitive to variations of PS content in contrast to a considerable slowing down effect on the slow reorientation time τ_s obtained more precisely from photon correlation and oscillatory electric birefrigence measurements at low temperatures (<0 °C). The intensity I_s of the narrow Lorentzian component, which was found to increase rapidly below a characteristic temperature at given PS concentration, is associated with the slow orientation of the solvent in the PS environment. Surprisingly, the onset of PS induced modification occurs at rather high temperatures (∼T_g + 110 K). The values of the intensity ratio I_s/I_f are rationalized in the framework of the restricted rotational diffusion model. Alternatively, the study of the single Lorentzian I_VH(ω) spectra of the neat solvent suggests a significant amount of orientational pair correlations and is consistent with clustering in liquid Aroclor.