EXPERIMENTAL APPROACH TO ELUCIDATE THE MECHANISM OF ULTRASOUND-ENHANCED POLYMER EROSION AND RELEASE OF INCORPORATED SUBSTANCES

Studies were conducted on the mechanism of ultrasound-enhanced solid polymer degradation in the presence of a liquid. Polyanhydride degradation and release experiments were performed at different temperatures. Considering that the change of temperature affects degradation product solubility, solution gas content, and vapor pressure, the dependence of ultrasound-enhanced degradation on the above three factors was investigated. The role of free radicals produced by ultrasound and the effect of ultrasound on hydrolysis versus mechanical erosion were also investigated using a nonaqueous solvent as the liquid phase. The results suggest that ultrasound accelerates both polymer hydrolysis and mechanical surface erosion. These effects might be explained by enhanced transport (enhanced permeation of water into the polymer exposing more hydrolytically labile linkages for hydrolysis) and mechanical shear stress mainly caused by the liquid jets at high velocity produced when the gas bubbles collapse (cavitation). The rate of ultrasound-enhanced polymer degradation increases with increasing liquid vapor pressure. These two parameters control the extent of cavitation.