Brillouin scattering and segmental motion of a polymeric liquid. I

Abstract

The effect of segmental motion on the density–density correlation function of a viscous polymer liquid has been analyzed using a generalized relaxation equation developed by Zwanzig and Mori. It is shown that for polymer liquids of high viscosity, Brillouin scattering is closely associated with the structural relaxation associated with the motion chain segments. A single relaxation time theory is shown to yield good agreement with the experimental results on polypropylene glycol. The torsional motion involving a small number of monomer units is shown to be responsible for the dispersion and attenuation of the hypersonic wave. The fact that the Brillouin scattering spectrum of a polymer liquid is insensitive to the change of molecular weight is discussed. We have shown that temporal modulation of the spatial second moment of the intermolecular or intersegmental interaction energy is responsible for the relaxation process involved in Brillouin scattering.