It is well documented that exposure of polymeric material to
ultrasound can result in irreversible molecular weight degradation
as a consequence of the high shear gradients generated [34]. 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.
In the case of MB and HB sonicated in xylene for a period of up to
10 min, Fig. 3b and c, it can be seen that molecular weight > 105 Da
are degraded preferentially leaving a shear resistant bimodal distribution
comprising the most highly branched, Fig. 1a. This suggests
that the branched chain ends are being preferentially shaved
leaving a branched central core, and hence the molecular weight
distribution retains a bimodal distribution rather than a Gaussian
distribution. Similar behaviour has been previously observed in the
sonication of star polymers with the arms being removed from a
central core.