Photoaging of stabilized and unstabilized low-density polyethylene films using Raman and DSI techniques

This study presents novel insights into the photodegradation behavior of both stabilized and unstabilized monolayer low-density polyethylene (LDPE) films using a combination of Raman spectroscopy and depth-sensing indentation (DSI) techniques. This integrated approach enables a comprehensive evaluation of chemical and mechanical property changes with depth. The primary objective was to elucidate heterogeneous degradation mechanisms and their influence on material performance under prolonged UV exposure. Films were exposed for 8 months (unstabilized) and 21 months (stabilized). Raman spectroscopy revealed the formation of carbonyl groups within a 60 µm depth from the exposed surface, exhibiting a non-uniform distribution due to oxygen diffusion and UV attenuation. Notably, the carbonyl index difference between the exposed and unexposed surfaces decreased over time, signifying degradation progression. Mechanical characterization via DSI demonstrated significant increases in surface hardness and modulus, attributed to chemo-crystallization and secondary crystallization induced by chain scission. The modulus of the unstabilized film surfaces nearly doubled after 8 months, whereas stabilized films exhibited a 50% increase after 21 months. These results highlight the surface-driven nature of degradation, with distinct depth-dependent patterns observed near the exposed and unexposed surfaces. The findings provide evidence of the complex interplay between chemical and mechanical transformations during photodegradation. This study underscores the importance of depth-resolved analysis in understanding photodegradation, offering critical insights for designing more durable polymeric materials. The results have broad applications in evaluating the long-term performance of LDPE in agricultural films and other outdoor uses, emphasizing the need for stabilization strategies to mitigate surface-driven degradation.