Comparing molecular weight models for polymer degradation with ball-mill grinding

Kinetics studies provide important insight into the parameters that influence the degradation of polymers during ball-mill grinding (BMG). One of the simplest ways to calculate degradation rate constants is by fitting molecular weight data (the molecular weight typically decreases with increasing grinding time) with a molecular weight model. Most molecular weight models were developed for ultrasonic or other degradation processes, and a robust evaluation of their suitability for BMG had previously not been conducted. In this report, we evaluate the performance of six different molecular weight models in fitting the degradation data from polystyrene (PS) samples with varying initial molecular weight. We use fit statistics and rate constant trends to identify three molecular weight models that appear to be suitable phenomenological models for the degradation of PS with BMG. We also use these models to probe the generality of the glass transition temperature as a kinetic parameter in amorphous polymer degradation. This work highlights the importance of using a suitable molecular weight model, as they can influence the reactivity trends that are observed.