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Absorption of 10-MHz to 110-MHz Ultrasonic Waves by Solutions of Polystyrene

The absorption of ultrasonic waves over the frequency range of 10 kHz to 110 MHz was measured for solutions of polystyrene in xylene. The samples were anion-polymerized, atactic polystyrenes with three different molecular weights. Both the strength and frequency of the low-frequency relaxation occuring between 10 and 100 kHz were found to depend on the molecular weight and were ascribed to heat-transfer between the polymer sphere (a polymer chain with included solvent) and the surrounding solvent. Spectra of the high-frequency relaxations observed at frequencies from 1 to 100 MHz were found to be independent of molecular weight and decomposed into two single relaxations. The high-frequency relaxation is concluded to arise from the volume change accompanying an internal rotation of a backbone segment consisting of a few monomeric units. The splitting into two mechanisms is interpreted on the basis of the tacticity of atactic polystyrene. The ultrahigh-frequency relaxation around 500 MHz which is discerned by replotting the data by Mikhailov and Safina is attributed to the relaxation of vibrational specific heat of the phenyl groups.