Structural changes of polystyrene particles in subcritical and supercritical water revealed by in situ small-angle neutron scattering

Marine ecosystem degradation due to microplastic pollution is a significant environmental problem, as acknowledged by Sustainable Development Goal 14. Decomposition of plastics using near critical or supercritical water is a promising method to remove microplastics. To optimize this method for realizing environmental benefits, it is necessary to clarify the structural change of materials during the process. Thus, we investigated the decomposition processes of polystyrene particles dispersed in deuterated water (D 2 O) during heating under near critical or supercritical conditions by using in situ small-angle neutron scattering. Under subcritical conditions, the PS particles were swollen by D 2 O due to increased compatibility with temperature. In subcritical conditions near the critical point, cleavage of PS chains in the particles occurred, so that the swollen ratio was enhanced despite the PS particles keeping their shapes. Under supercritical conditions, the PS particles were degraded into oil, including oligomers or monomers and phase-separated structures with styrene-rich and D 2 O-rich regions.