Acoustic Studies of Polymers

Abstract

Acoustic propagation studies of dilute, semi-dilute and concentrated solutions of polystyrene in toluene are used to illustrate how this technique can be used to characterize high frequency normal mode and segmental relaxation of the polymer backbone. The amplitude of the acoustic relaxation is observed to vary in a non linear manner with increasing concentration of polymer and is indicative of the onset of polymer-polymer interactions. Investigations of the effect of variation of the molecular weight of the acoustic relaxation in polydimethylsiloxane are interpreted in terms of the effects of polymer entanglement. In the case of solid polymers, relaxation features observed in the megahertz frequency range correlate closely with similar phenomena detected using dielectric and dynamic mechanical relaxation methods and are characteristic of local and side chain motions of the polymer. Marked increases in the attenuation are observed when the wavelength of the sound wave becomes comparable with the domain size and scattering occurs. Scattering in the case of isotropic polyethylene can be attributed to the effects of domains, whereas in the case of drawn polyethylene it can be associated with the presence of air voids. Investigations of polar diagrams allows identification of crystal twinning in low draw ratio polyethylene. The use of high frequency sound waves in the detection of phase separation in solutions of polystyrenebutadiene-styrene triblock copolymer is also described.