Diffusion-Ordered Spectroscopy for Rapid and Facile Determination of Consumer Plastic Molecular Weight

Abstract

Molecular weight (MW) is a key control of plastic polymer properties and their fate in the environment. However, the primary tool used to determine plastic MW, gel permeation chromatography (GPC), has major limitations, such as low precision and accuracy, requirements for dedicated instrumentation, production of high volumes of hazardous waste, and large sample sizes. In this study, we describe, validate, and apply a diffusion-ordered spectroscopy (DOSY) method for polymer MW determinations, with a focus on applications for consumer plastics. Several experimental conditions were systematically optimized and tested to validate the DOSY method, including the selection of pulse sequences, the effect of sample concentration, cross-validation with multiple sets of external standards, and long-term instrumental stability. Validation was performed for a wide range of polymers, solvents, and temperatures, highlighting its potential for broad applicability. A preliminary screening of polystyrene and polyethylene terephthalate consumer products revealed widely varying MWs (up to two-fold) for products made of the same polymer type. A preliminary experiment was also conducted to track the decrease in polystyrene MW via photochemical chain scission reactions, finding a 20% reduction in MW after less than 1 week of irradiation. Collectively, our results demonstrate the potential for DOSY to provide high-throughput, accurate, and precise measures of polymer MW, as well as the evolution of polymer MW during environmental weathering processes, such as photochemical degradation. We conclude with a discussion of (i) the many advantages of DOSY compared to GPC, (ii) future developments to enhance the depth of information obtained from DOSY, and (iii) approaches to broaden the accessibility of this promising analytical method to the research community.