The Rate of Degradation by Ultrasonation of Polystyrene in Solution

Abstract

Abstract The rate of degradation by ultrasonation of polystyrene in tetrahydrofuran solutions has been measured at various concentrations, temperatures, and irradiation intensities. As reported earlier, the course of the changes in molecular weight distributions is independent of the molecular weight (MW) of the initial polymer and of the degradation conditions. Therefore, the time dependence of the extent of the degradation can be meaningfully compared for various reaction conditions and for polymers with different original MWs. The extent of the scission process was expressed in terms of the degradation index (DI), the ratio of the number of chain bonds broken to those initially present. The problem of the existence of a minimum MW required for sonic scission is considered and arguments are advanced in favor of the assumption of a minimum degradable chain length for the purpose of evaluation of degradation data. This minimum MW was found to be about 30,000. DI values were corrected for the "unbreakable" chain ends. It was observed that curves representing the time dependence of DI were linear on double logarithmic plots for all experiments, DI being proportional to t0.85. By shifting these lines along the time axis a master curve was obtained representing all data for all experimental conditions over almost three decades of DI. Thus the time-dependent rate of scission of any run relative to that under reference conditions can be expressed by a single value of the time transposition factor, a measure of the shift. This transposition factor provides a simple method for assessing the effects of experimental conditions. Some data are reported expressing the effect of MW of the initial polymer, concentration, temperature, and irradiation intensity on degradation rate.