Determination of complex refractive index of plastics from ultraviolet to mid-infrared by ellipsometry

Abstract

Plastic litter is a pervasive issue in the ocean, posing threats to both marine ecosystems and human health. In the field of marine remote sensing detection, one common method for detecting and identifying marine plastics is by analyzing their optical characteristics, specifically their spectral remote sensing reflectance. This reflectance is dependent on the complex refractive index of plastics. Therefore, understanding the complex refractive index, which is an intrinsic optical property of plastics, holds significant importance in marine plastics research. In this study, we utilized spectroscopic ellipsometry to measure the complex refractive index of 9 common ploymers including polymethyl methacrylate (PMMA), methyl methacrylate–styrene copolymer (MS), polyethylene (PE), poly (ethylene glycol-co-cyclohexane-1,4-dimethanol terephthalate) (PETG), polybutylene terephthalate (PBT), polyoxymethylene (POM), perfluoroalkoxy (PFA), acrylonitrile butadiene styrene (ABS), and styrene-acrylonitrile copolymer (SAN). In addition, we have expanded the complex range of refractive index measurements to include UV and MIR bands with wavelengths up to 32 μm. To ensure the reliability and accuracy of the measurements, experimental results for polycarbonate (PC) were compared with data from the existing literature. The variation of refractive index with wavelength is studied and the causes of absorption peaks in the infrared range are discussed. Furthermore, Considering the dispersion relationship and monomer composition of PMMA, MS, ABS and SAN plastics, a comparative analysis of plastics with commonly used monomers is conducted, and the influence of monomers on refractive index is discussed. Our study indicates that the presence of styrene monomers leads to an increase in the refractive index of plastics in the visible to near-infrared band, while the presence of butadiene monomers results in a decrease.The findings from this study provide fundamental data for future research on marine plastic remote sensing detection applications.