Quantification of photooxidative defects in weathered microplastics using 13C multiCP NMR spectroscopy

• Published in: RSC advances Vol. 12; no. 18; pp. 10875 - 10885
• Main Authors: Mauel, Anika, Pötzschner, Björn, Meides, Nora, Siegel, Renée, Strohriegl, Peter, Senker, Jürgen
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 07.04.2022; The Royal Society of Chemistry
• Subjects: Carboxylic acids; Chemistry; Cross polarization; Defects; Esters; Excitation; Functional groups; Ketones; NMR spectroscopy; Photooxidation; Plastic pollution; Polyethylenes; Polystyrene resins; Spectrum analysis; Time measurement; Weathering
• ISSN: 2046-2069
• DOI: 10.1039/d2ra00470d

Weathering of microplastics made of commodity plastics like polystyrene, polypropylene and polyethylene introduces polar polymer defects as a result of photooxidation and mechanical stress. Thus, hydrophobic microplastic particles gradually become hydrophilic, consisting of polar oligomers with a significant amount of oxygen-bearing functional groups. This turnover continuously changes interactions between microplastics and natural colloidal matter. To be able to develop a better understanding of this complex weathering process, quantification of the corresponding defect proportions is a first and essential step. Using polystyrene, 13C enriched at the α position to 23%, we demonstrate that 13C cross polarisation (CP) NMR spectroscopy allows for probing the typical alcohol, peroxo, keto and carboxyl defects. Even the discrimination between in- and end-chain ketones, carboxylic acids and esters as well as ketal functions was possible. Combined with multiCP excitation, defect proportions could be determined with excellent accuracy down to 0.1%. For materials with 13C in natural abundance, this accounts for a detection limit of roughly 1%. The best trade-off between measurement time and accuracy for the quantification of the defect intensities for multiCP excitation was obtained for CP block lengths shorter than 250 μs and total build-up times longer than 2 ms. Further measurement time reduction is possible by using multiCP excitation to calibrate intensities obtained from series of 13C CP MAS NMR spectra. As photooxidation is an important degradation mechanism for microplastics in the environment, we expect these parameters to be transferable for probing defect proportions of weathered microplastics in general.