Structural Mechanisms Governing Polyolefin Microplastic Formation in Marine Environments

This study investigates the higher-order structure and physical properties of polyolefin-based microplastics (MPs) collected from the ocean and elucidates their formation mechanisms. A combination of polarized optical microscopy (POM), scanning and transmission electron microscopy (SEM and TEM), small-angle X-ray scattering (SAXS), infrared (IR) spectroscopy, Raman spectroscopy, and nanoindentation was employed to characterize surface and internal structures, mechanical properties, crystalline organization, and molecular orientation of MPs. The results demonstrate that MPs are generated through brittle fracture initiated at cracks formed by photo-oxidative degradation and are subsequently released into the environment while largely retaining their original crystalline structure and molecular orientation. Furthermore, laboratory weathering tests of polypropylene that combine simulated sunlight exposure with external mechanical stimuli successfully reproduce this fragmentation behavior, indicating that such protocols can serve as an effective screening method to evaluate the susceptibility of polyolefin materials to MP formation. This study provides a comprehensive structural framework that correlates chemical degradation with mechanical embrittlement, supporting existing hypotheses on the fragmentation of semicrystalline polymers in marine environments.