Comparing kinetic methods for ball-mill grinding: Molecular weight versus refractive index-based methods

Abstract

Polymers are commonly subjected to ball-mill grinding (BMG) to induce chain scission. Milling and polymer parameters can influence the kinetics of degradation, and various kinetic methods have been employed to study this process. The most common methods include molecular weight models (fitting the decrease in molecular weight with milling time) and refractive index (RI)-based methods (such as the Florea method, which fits the decrease in RI signal with milling time). In this report, we compare the rate constant trends obtained from three different molecular weight models and the Florea method to provide a deeper understanding of how the kinetic method employed in kinetic studies influences the observed reactivity trends. Specifically, each kinetic method was applied to BMG-induced degradation data from polystyrene (PS) and poly(lactide) (PLA) of varying initial molecular weight. Each kinetic method produced different rate constant trends (e.g., initial molecular weight vs. rate constant), which could also be influenced by changing the milling duration. Molecular weight models or the Florea method were better suited with longer or shorter milling durations, respectively. Furthermore, the rate constant trends for PS and PLA were relatively consistent if compared with the same kinetic method. We expect that this work will help guide future BMG-based degradation kinetics studies.