Characterization of Microplastics by Advanced Analytical Techniques

Plastic wastes in the oceans, rivers, and soil have become a serious issue in recent years, especially plastic fragments with the size of 5 mm or less, which are called microplastics (MPs). Characterization of MPs is complex because the weight of a single MP particle is less than 1 mg. In this study, microplastics collected from the surface layer of the sea near Japan were characterized. The target sample size in this study is on the order of mm, which can be collected by Neuston net and observable with the naked eye. Since MPs were floated on the surface of the sea, the collected MPs were mainly composed of polyolefins and expanded polystyrene (EPS), which have densities lower than that of seawater. Polymer components of MPs were characterized by polarizing optical microscopy (POM), transmission electron microscopy (TEM), Raman scattering spectroscopy, infrared (IR) spectroscopy, and wide-angle X-ray diffraction (WAXD). MPs that showed birefringence by POM were crystalline polyolefins, and those without birefringence were mainly EPS. Cross-sectional TEM observation of the MP identified as PP revealed the degraded amorphous phase on the surface with the internal crystalline phase. WAXD results indicated that the polyolefin film or plate was fragmented while maintaining the orientation state of the crystallite in the film to some extent. IR microscopic measurement of a cross-section of the polyolefin MP clarified that oxidation proceeded from the surface. In addition, the results of nanoindentation of the PP MP revealed that its surface became brittle due to photo-oxidative degradation.