Rheological Characterization of <scp>UV</scp> and Shear‐Induced Degradation of Poly(Ethylene Terephthalate): Linking Environmental and Processing Histories to Recyclability

ABSTRACT Throughout its lifecycle, poly(ethylene terephthalate) (PET) undergoes significant degradation from long‐term ultraviolet (UV) exposure, moisture, and shear forces, with these effects intensified during mechanical recycling. To characterize molecular degradation, PET samples subjected to UV aging in dry, marine, and freshwater environments, as well as diverse shear rates (90–450 s −1 ) are investigated using rheological analysis. Rheological analysis effectively distinguishes competing PET degradation effects under UV exposure from shear effects, highlighting crosslinking in dry conditions and hydrolytic chain scission under aqueous environments. Under shear, virgin PET shows an upshift in molecular weight ( Mw ) due to crosslinking reactions, while recycled PET experiences a drastic reduction in Mw owing to chain scission. When virgin PET goes through even a single shear‐processing cycle, its crystallite size decreases by 70%, increasing crystallinity and releasing volatile by‐products including aldehydes, acids, and alkanes that further compromise its recyclability. This study reveals that both UV and shear degradation significantly impair the mechanical properties of PET by reducing tensile strength and elongation at break by 55% and 99%, respectively. Linking degradation history to crystallization and mechanical behavior provides molecular‐level insights.