Quantifying Polymer Surface Degradation Using Fluorescence Spectroscopy

Abstract

One solution to minimizing plastic pollution is to improve reuse and recycling strategies. Recycling, however, is limited by the overall degradation of plastics being used. Photochemical or thermal driving forces facilitate the incorporation of oxygen into the backbone and chain cleavage; yet, current techniques for monitoring this plastic degradation fail to observe early stages of degradation, which is key for optimizing reusability. This research seeks to develop a cheap, reproducible, and nondestructive technique for monitoring degradation of polyethylene and polypropylene materials using Nile red as a fluorescent probe. Changes in Nile red’s fluorescence spectra were observed upon exposure to stained, aged polyethylene and polypropylene samples. As the surface hydrophobicity of the plastic decreases, Nile red’s fluorescence signal undergoes corresponding signal shift to longer wavelengths (lower energy). The trends seen in the fluorescent profile were related to more commonly used measurements of plastic degradation, namely carbonyl index from infrared spectroscopy and bulk crystallinity from calorimetry. Results demonstrate clear trends in fluorescence spectra shifts as related to the chemical and physical changes to the plastics, with trends dependent on polymer type but independent of polymer film thickness. The strength of this technique is divided into two defined fits of the fluorescence signal; one fit characterizes the degradation throughout the whole range of degradative oxidation and the other is tailored to provide insight into the early stages of degradation. Overall, this work establishes a characterization tool that assesses the extent of plastics’ degradation, which may ultimately impact our ability to recover plastics and minimize plastic waste.