Chain entanglement and normal mode motions in poly(dimethylsiloxane)+toluene mixtures

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Abstract
Acoustic absorption and velocity measurements are reported on mixtures of polydimethylsiloxane (PDMS) with toluene, covering a frequency range of 1–100 MHz and a concentration range from 0 to 1 volume fraction of polymer. Nine polymers were studied covering a molecular weight range ( text-decoration:overline M n) from the dimer to 8 × 104. Viscosity and density measurements were made at 303 K and excess properties for the mixtures calculated. These data were used in conjunction with the acoustic measurements to characterize the influence of chain entanglement on both the static and dynamic properties of the polymer solutions. The highest molecular weight polymers exhibit properties which can be associated with a significant degree of an entanglement interaction betwee chains as well as dense packing of solvent into or around the chains. In contrast, the lower molecular weight polymers behave as though forming simple binary mixtures. Comparison of the observed ultrasonic attenuatoin spectrum with that calculated from the viscoelastic relaxation spectrum shows that the low frequency acoustic process can be ascribed to normal mode motions of the chains, but that the time constants for these depend on the nature of the perturbing stress experienced by the molecules as well as on entanglement effects, the viscoelastic relaxation times being greater than the corresponding acoustic times. These results are the first which clearly illutrate the differences in normal mode motion stimulated under isovolumetric and isoentropic conditions.

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