Solvent mobility in poly(methyl methacrylate)/toluene solutions by depolarized and polarized light scattering

Abstract

Depolarized Rayleigh and polarized Rayleigh–Brillouin scattering are employed to examine the mobility of toluene in solutions of poly(methyl methacrylate) (PMMA) and for PMMA concentrations: 0≤cPMMA≤1. The depolarized Rayleigh measurements were performed in the temperature range 20–120 °C with several interferometer spacings, whereas the polarized Rayleigh–Brillouin measurements were made in the range from −20 to 140 °C. The reorientation times of toluene, obtained from a single Lorentzian fit to the experimental depolarized spectra, are in good agreement with earlier NMR and dielectric relaxation data. Two Lorentzians were necessary to fit the depolarized spectra of the PMMA/toluene solutions at all temperatures. The broader Lorentzian is due to fast toluene reorientation and the narrower Lorentzian contains contributions from both slowly relaxing polymer chains and slow solvent reorientation in the macromolecular environment. The reorientation time and the fraction of the ‘‘mobile’’ toluene molecules are obtained, respectively, from the width and the integrated intensity of the broader Lorentzian. We discuss the reorientational dynamics of the broader Lorentzian with respect to the normalized solvent relaxation time τ/τ0, where τ0 refers to the neat solvent. The principal conclusions of this work with respect to solvent dynamics are (i) the exponential concentration dependence of τ/τ0 at high temperatures and for polymer concentration up to 70% which is similar for the three polymer/solvent systems employed so far, (ii) the presence of two time scales for the solvent reorientation in these homogeneous polymer solutions, and (iii) these ‘‘fast’’ and ‘‘slow’’ relaxation processes resemble the toluene and PMMA bare dynamics, respectively, and their relative amplitudes depend on temperature. The experimental results are discussed in terms of recent models of orientational relaxation in dense systems. Furthermore, the polarized Rayleigh–Brillouin measurements on the PMMA/toluene system revealed the presence of significant rotational mobility of toluene acting as an initiator for the broad hypersonic attenuation observed at GHz frequencies.