Additive topology governs structural degradation of weathered plastic

Abstract Plastic degradation is commonly described as a surface-driven process regulating additive mobilization. However, the internal spatial organization of additives within polymer matrices remains poorly resolved. Here, three environmentally weathered polyethylene fragments were selected as chemically contrasted endmembers from a broader multivariate dataset to investigate how additive topology relates to structural degradation patterns. Using synchrotron-based micro-X-ray fluorescence (µXRF) and optical photothermal infrared (O-PTIR) spectroscopy, we spatially resolve metallic additive distributions and their association with oxidation gradients. We identify three distinct degradation archetypes: (i) rupture of additive-enriched domains associated with localized fragmentation, (ii) oxidation-mediated internal redistribution along crack networks, and (iii) homogeneous additive dispersion associated with limited structural alteration. Reactive metals such as titanium and chromium accumulate near oxidation fronts, whereas calcium-rich inclusions remain structurally isolated. These results indicate that additive topology influences structural accessibility pathways independently of total concentration. Although based on three archetypal fragments, this study provides spatially resolved evidence that internal chemical heterogeneity structures polyethylene aging and refines mechanistic interpretations of additive mobilization in environmental plastics.