Viscoelastic Relaxation of the Bulk Modulus in Two Supercooled Benzene Derivatives

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Abstract
The velocity and absorption coefficient of ultrasonic longitudinal waves have been measured in sec-butyl benzene and 3-phenylpropyl chloride at temperatures between the glass transition temperature and room temperature and at frequencies between 5 and 55 MHz. A relaxation process caused by rotational isomerisation has been observed in sec-butylbenzene with ΔHb‡ = (3.6 ± 0.7) kcal mol−1. The total “structural” relaxation region of the liquids bears no resemblance to the shear relaxation region because the former contains contributions from two different processes. The first of these is the true structural relaxation process which is similar to the shear relaxation process. The other, lower temperature process gives rise to considerable excess absorption and velocity dispersion at temperatures below the structural relaxation region. It is caused not by a hysteresis loss mechanism but by a combination of non-Hookean behavior and internal molecular motion.

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