Pressure Dependence of the Viscoelastic Behavior of Polyisobutylene

The absorption of 4 mc/sec ultrasonic waves in polyisobutylene is measured as a function of temperature, for various pressures. The extreme limits of these variables are 1 to 1400 atmos and −20°C to 80°C, respectively. Approximate measurements of propagation velocity are also obtained. Pulse reflection apparatus is used. A magnetically actuated holder places the sample in the signal path or removes it. Pressure-volume data are obtained for polyisobutylene at 30°C by means of a capillary dilatometer fitted with electrical contacts, in which the sample is placed with mercury. The expected behavior of viscoelastic relaxation times with temperature and pressure is computed by adapting a theory due to Bueche. With additional assumptions concerning the compressibility, the theory gives a shift of the constant-frequency absorption maximum of 0.025 C°/atmos, in approximate agreement with experiment.