Mechanistic insights into photooxidation-enhanced biodegradation of polyamide 6 in marine environment

Polyamide 6 (PA6) is widely utilized because of its excellent mechanical properties. Despite its superior performance, there is an urgent need to evaluate marine biodegradation to mitigate the potential environmental risks associated with plastic waste pollution. While the biodegradation of PA6 has been previously reported, the specific influence of surface structural features and physicochemical properties on its biodegradation behavior in the marine environment remains poorly understood. The effect of ultraviolet (UV) light-induced photooxidation on the biodegradability of PA6 was systematically investigated. As-received, UV-exposed, and annealed PA6 films were characterized by wide- and small-angle X-ray scattering (WAXS and SAXS), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and nanoindentation. UV exposure induced photooxidation and chain scission, resulting in the introduction of oxygen-containing functional groups that increased surface hydrophilicity and decreased molecular mass. In contrast, annealing enhanced the crystallinity and surface hardness in the absence of oxidation. Biodegradation tests in extracted seawater revealed that the degradation of UV-exposed PA6 was accelerated, reaching a biodegradation degree of 18.4% after 60 days, whereas the degradation of the annealed film was negligible. Extensive biofilm formation and surface erosion occurred on the UV-exposed films, whereas minimal microbial adhesion occurred on the annealed films. These results demonstrate that photooxidation enhances PA6 biodegradability by promoting surface oxidation and microbial colonization, whereas increased crystallinity and mechanical stiffness suppress enzymatic degradation. This study provides mechanistic insights into the interplay between photooxidation, surface chemistry, and crystalline structure in controlling the environmental degradability of PA6, offering a strategy for designing more sustainable and degradable PAs as well as for developing effective waste PA treatment approaches.