Long‐lasting hydrophilicity induced by ultraviolet light on surface modified hydrophobic polylactic acid

Surface treatments are used to tailor the wettability of compostable polymers for outdoor applications. However, imparting hydrophobicity can have direct and indirect consequences on the polymer's eventual degradation during use. In this research, a solvent-treated hydrophobic (water contact angle of 147.2° ± 0.6°) polylactic acid (PLA) substrate could be altered significantly to a wicking sample (22.5° ± 3.1°) with a single 2-h ultraviolet-C (UVC) exposure. Wicking behavior remains consistent even 1 year post-exposure, implying a long-lasting hydrophilic change to the polymer surface. UVC irradiation induced chain scission near the surface, reducing considerably the molecular weight. Reduction in molecular weight impacts properties, including lower glass transition, melting, and degradation temperatures. However, no significant chemical composition changes could be detected with X-ray Photoelectron Spectroscopy (XPS). Infrared spectroscopy has shown a very minor oxidation with an increasing signal of the peak related to carbonyl groups at 1724 cm⁻¹ from α-cleavage. Force-distance spectroscopy confirmed the increase in polarity of the UVC-exposed solvent-treated surfaces. The increase in wettability and more precisely surface polarity relates to an orientation of polar oxygen bonds towards the surface made possible by the chain scissions.