Cavitation and Polymer Degradation

Abstract

Various dilute polymer solutions have been subjected to ultrasonic waves from a spherically concave transducer at 600 kc/sec. The decrease in molecular weight of the polymers has been examined as a function of the gas saturation of the polymer solution and the static pressure. If cavitation is suppressed either by degassing or by the use of high static pressures with only partially gas-saturated solutions, degradation does not appear to occur even at intensities approaching 103 watts/cm2 or acceleration amplitudes of 109 cm/sec2. With gas-saturated solutions, the degradation of the polymers decreases with increasing static pressure. The dependence of the rate and extent of degradation on ultrasonic intensity has also been studied with other equipment providing relatively uniform ultrasonic fields. In polymer solutions of relatively high viscosity, the development of the cavitation field is a slow process. Visual observations at 600 kc/sec indicate that only a few bubbles form initially and each of these bubbles grows until of sufficient size to support a low order surface wave. Very small bubbles are then seen to stream from each parent bubble with the most common case involving two streams in diagonally opposite directions. (This phenomenon is similar to that reported by G. Willard, paper C1, 47th meeting, Acoustical Society of America, June, 1954.) After a few seconds, the whole field is filled with such bubbles. This research was partially supported by the Office of Naval Research.