The
observed degradation mode depends on the nature of the molecular
weight distribution of the polymer: monodisperse polymers
break centrosymetrically with the PDI converging on a value of 2
and the higher molecular weight fractions of a polydisperse material
degrade preferentially leading to a random degradation
mechanism for the overall sample. A rapid decrease in molecular
weight was observed for the polydisperse PMMA 468 KDa in gbutyrolactone
with increasing sonication time, Fig. 3a. Degradation
rate is highest at low concentration as cavitation is supressed in higher viscosity fluids.