Could Superficial Chiral Nanostructures Be the Reason Polyethylene Yellows as It Ages?

Understanding the degradation of plastics, some of the most widely used materials on Earth, is crucial in a broad range of fields from materials design to environmental monitoring. Many polymers yellow as they age, but there is no chemical explanation that can describe the origin of this yellowing for polyolefins specifically. Here, we show that irradiated blown polyethylene sheets preferentially scatter circularly polarized light. Because scattering of circularly polarized light only occurs in the presence of chiral structures, our findings provide evidence of formation of chiral supramolecular structures responsible for preferential light scattering that may be the underpinnings for the perceived yellow/ brown tint as polyethylene ages. Further, we demonstrate incident polarization-dependent detection of colored light scattering from irradiated polyethylene films and that the scattered light is distinctly different in color. Overall, these results provide evidence that the yellowing of polyethylene, previously assumed to be caused by polymer backbone rearrangements, is actually the product of chiral, optically active structures that form on the plastic's surface due to UV irradiation. To the best of our knowledge, this is the first explanation for polymer discoloring that provides evidence for the development of supramolecular structures of polymers during aging. Because of this, our findings provide an alternative direction in plastic degradation research for understanding the chemical and structural changes. Findings presented here shift our understanding about materials degradation and can inform our future materials designs and recovery.