Insights into the photodegradation mechanisms of PET and PA-6 via ATR-FTIR and two-dimensional correlation spectroscopy

Microplastic (MP) characterization in environmental samples relies predominantly on Fourier transform infrared (FTIR) spectroscopy; however, environmental aging induces subtle yet progressive spectral modifications that complicate accurate polymer identification and interpretation. Moreover, photo-oxidative degradation alters the surface chemistry of microplastics, generating oxygen-containing functional groups that may modify their reactivity and interactions with coexisting contaminants. In this study, the accelerated photodegradation of polyethylene terephthalate (PET) and polyamide-6 (PA-6) in aqueous media was investigated under UV irradiation to induce controlled and fast oxidative transformations. Surface chemical changes were analyzed using Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) combined with two-dimensional correlation spectroscopy (2D-COS), enabling enhanced spectral resolution and discrimination of overlapping vibrational bands. The 2D-COS analysis resolved the sequential evolution of degradation processes that were not readily distinguishable by conventional one-dimensional spectra. PET exhibited an initial crystalline disorder phase followed by ester backbone scission consistent with Norrish type II reactions and progressive accumulation of oxidation products. In contrast, PA-6 degradation involved crystalline perturbation, hydrogen-bond weakening, CN bond scission, and the accumulation of oxidation products and aliphatic reorganization. By revealing the temporal order and correlation of spectral variations across the full mid-infrared region, the combined ATR-FTIR/2D-COS methodology provides a robust analytical framework for elucidating polymer-specific aging pathways. This approach enhances the interpretative capability of FTIR-based microplastic analysis and supports the development of reliable spectral aging indicators, contributing to a more accurate assessment of degraded microplastics and their evolving environmental behavior.