The effect of weathering environments on microplastic chemical identification with Raman and IR spectroscopy: Part II. polystyrene, polyethylene terephthalate, and nylon 6.

The use of spectroscopic techniques to identify microplastics found in the environment is challenging because weathered microplastics undergo chemical changes that make their spectra drastically different from their pristine counterparts. In previous work, we reported the findings from systematic artificial weathering of polyethylene and polypropylene for 0-26 weeks in four different weathering environments (air, DI water, artificial seawater, and Puget Sound seawater), characterized by Raman and IR spectroscopy (https://doi.org/10.1016/j.polymertesting.2022.107752). This manuscript provides the final part of the study's findings, which includes the Raman and IR spectroscopy information on the weathering effects on polystyrene (PS), polyethylene terephthalate (PET), and nylon 6 (PA6) with the goal to evaluate how these weathering effects affect the reliability of polymer identification. Our results show that spectral changes are often non-linear, lacking a clear exposure-time trend. While PS exhibited significant oxidation by IR in DI water, these changes were not detectable by Raman spectroscopy, highlighting a risk for researchers who rely on a single technique. Both PS and PA6 showed more degradation peaks in DI water than in seawater, which suggests that chlorine radicals from salt may inhibit the formation of some degradation products. This work underscores the need to use complementary IR and Raman analysis to avoid misinterpretation of environmental microplastics and their aging state.