| Abstract
| - We present results of a Brillouin scattering experiment on solutions of poly(ethylene glycol) of mean molecularmass 600 g/mol (PEG600) in CCl4. The relaxation process detected has been assigned to conformationalrearrangements of the polymeric chains, triggered by reorientation of the side groups. The concentrationdependencies of the hypersound velocity and normalized absorption are compared against the indicationsfrom several models proposed in the literature. The concentration evolution of the system is described interms of two distinct regimes. At high polymer content, the system is dominated by the structure of the densepolymer, where polymer−polymer interactions, together with excluded volume effects, induce the existenceof a preferred local arrangement resulting in a narrow distribution of the relaxation times, with the averagevalue of the relaxation time following a simple Arrhenius temperature dependence. As the concentrationdecreases, the original structure of the hydrogen bonded polymer network is destroyed, and a number ofdifferent local configuration coexist, giving rise to a wider distribution of relaxation times or to a multiplerelaxation. At low concentrations, the experimental data are well fitted assuming a Vogel−Fulker−Tammonbehavior for the average relaxation time. In addition, the observed deviation from the ideal behavior for therefractive index and the density suggests that CCl4 does not behave as an inert solvent, and due to polarizationeffects, it can develop local hetero-associated structures via electrostatic interaction with the O−H end groupsof the polymeric chains. The hypothesis has been successfully tested by fitting the concentration behavior ofthe hypersonic velocity to a recent three-component model, suitable to describe the concentration dependenceof sound velocity in moderately interacting fluids. The indication of the model furnishes a very high valuefor the association constant of the PEG600, confirming the literature indication that, in polymeric systemscapable of developing long liner aggregates via hydrogen bonding interaction, the Brillouin probe is insensitiveto the true length of the polymeric chains. The Brillouin scattering experiment just sees an effective hydrogenbonded aggregate that is huge relative to the length of the single polymeric chain and becomes sensitive onlyto the density fluctuations of the local segmental motions.
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