Ultrasonic degradation of cellulose nitrate. II. Effects of temperature, solvent, and other process variables

Abstract

Abstract Cellulose nitrate was degraded ultrasonically at 0 to 85°C. in the alkyl acetates from methyl to heptyl. In each solvent there is a high and low limit temperature where cavitation and degradation both cease. The upper limits are at the temperatures where vapor pressure of the solvent is about one‐half atmosphere, and cavitation evidently ceases because the solvent vapor pressure becomes too great for rapid bubble collapse. The lower limits of pressure and temperature vary in a linear manner, but have not been explained. The results confirm the need for cavitation as an initiator of the degradation mechanism, and explain some effects previously ascribed to solvent viscosity and temperature. When radio frequency power was varied, the rate of degradation was found to be independent of power so long as cavitation was present. In other experiments, degradation was found to be approximately constant to concentrations much lower than previously tested, whereas some proposed mechanisms would require that it should be reduced by dilution. Addition of a nonsolvent reduced the intrinsic viscosity of the polymer solution but did not reduce the extent of degradation; thus degradation is a function of the chain's length but not of its solvation. Finally, comparison of a series of celluloses from diverse sources showed that all gave about the same minimum degradable chain length, of 800–1000 DP. It was concluded that there was no evidence for the existence of weak links in the celluloses at shorter intervals.