Brillouin scattering and segmental motion of a polymeric liquid, II

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Abstract
We have studied the Brillouin scattering spectra of oligomers of polyethylene glycol, along with ethylene glycol. These systems vary greatly in viscosity as well as molecular weight. We have found that except for ethylene glycol (the monomer), the Brillouin frequencies and the linewidths of the rest of oligomers show more or less the same temperature dependence, independent of viscosity and molecular weight. These results indicate that the motion responsible for the hypersonic dispersion and attenuation is localized. We have also developed a theory based upon the Zwanzig-Mori formalism to account for the experimental results. We have then shown that in order to obtain good agreement between theory and the experiment, the usual Markov approximation describing the dynamic behaviour of the memory function cannot be used. Thus, the usual hydrodynamic equations with frequency-independent transport coefficients are inadequate in describing the Brillouin scattering spectra of viscous liquids.

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