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

Shibata, Motoki*; Nakanishi, Yohei*; Abe, Jun*; Arima, Hiroshi*; Iwase, Hiroki*; Shibayama, Mitsuhiro*; Motokawa, Ryuhei   ; Kumada, Takayuki   ; Takata, Shinichi  ; Yamamoto, Katsuhiro*; Takenaka, Mikihito*; Miyazaki, Tsukasa*

Marine ecosystem degradation due to micro plastics is a significant environmental problem, as acknowledged by Sustainable Development Goal 14. Decomposition of plastics using near critical or supercritical water is one of the promising methods to reduce micro plastics. To attain the optimization of the method for improving environmental friendliness, it is necessary to clarify the structural change of materials during the process. We, thus, investigated the decomposition processes of polystyrene particles dispersed in deuterated water (DO) 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 DO due to increased compatibility with temperature. Near the critical point in subcritical conditions, the cleavage of PS chains in the particles occurred, so that the swollen ratio was much enhanced though the PS particles kept their shapes. In a supercritical condition, the PS particles were degraded into oil including oligomers or monomers and the phase-separated structures with styrene-rich and DO-rich regions.