A contribution to safe-and-sustainable-by-design polymers through integrated µFTIR, pyrolysis-GC/MS, and SEM analysis of accelerated UV photo-oxidation

Despite the widespread use of agricultural plastics, polymer-specific degradation pathways under realistic solar irradiation remain poorly resolved, particularly because chemical oxidation, chain integrity, and particle morphology can evolve in different ways. Here we show that a single analytical technique is insufficient to capture this evolution. We combine µFTIR, pyrolysis-GC/MS, and SEM, which probe functional-group chemistry, chain architecture/oligomer distributions, and surface damage/fragmentation features, respectively, and assess how consistently these readouts converge across polymers. Polypropylene (PP), polystyrene (PS), poly(lactic acid) (PLA), and poly(ethylene terephthalate) (PET) microplastics (200–400 µm) were subjected to 2275 MJ m⁻² of UV exposure over 48 days, approximating one year of Almería (Spain) greenhouse solar radiation. The combined evidence revealed four mechanistically distinct regimes. Polyolefin photooxidation (PP) yielded a 190-fold carbonyl-index increase, an 87% loss of backbone regularity, and surface crazing. Aromatic vinyl photooxidation (PS) was characterized by a 32.8-fold hydroxyl accumulation and progressive coalescence of C=O bands into a broad oxidation envelope. Biodegradable ester photoscission (PLA) showed decreases in three µFTIR degradation indices (aECI PLA −51%, HI PLA −68%, weakHI PLA −91%) consistent with ester bond consumption and brittle fragmentation. PET showed only modest diagnostic change by µFTIR (carbonyl index +55%), the smallest response of the four and consistent with its established UV stability, yet Py-GC/MS still resolved a clear shift in its pyrolysis products, showing that a single spectroscopic index can underrepresent the change even in a comparatively resistant polymer. These findings indicate that Safe and Sustainable by Design benchmarks should be polymer-specific and based on at least two orthogonal metrics capturing oxidation chemistry (µFTIR), chain integrity (Py-GC/MS), and/or morphological damage (SEM), to avoid false equivalence across materials.