Pressure and Temperature Dependence of the Acoustic Velocities in Polymethylmethacrylate

The acoustic velocities in polymethylmethacrylate have been measured with an ultrasonic pulse-echo technique as functions of frequency, temperature, and pressure. At atmospheric pressure, data on the velocities and attenuation coefficients were obtained for the temperature range of 22°–75°C in the frequency range of 6–30 MHz. For the measurements of velocity and attenuation as a function of frequency, the complex adiabatic bulk modulus was calculated at room temperature and atmospheric pressure for the above frequencies. At temperatures of 25°, 40°, 55°, and 75°C, the pressure dependence of the longitudinal and shear velocity was determined to 150 kpsi at a frequency of 6 MHz. It was found that the measured velocities under increasing pressure conditions were generally lower than those of decreasing pressure by about 0.5% for the longitudinal measurements and about 1% for the shear measurements. However, measurements of the velocities at atmospheric pressure after the specimens had been exposed to 150 kpsi were usually within 0.1% of the initial values. A discussion is presented which compares the continuity of the present data with equation of state determinations in PMMA at elevated pressures.