Photocatalytic aging process of Nano-TiO2 coated polypropylene microplastics: Combining atomic force microscopy and infrared spectroscopy (AFM-IR) for nanoscale chemical characterization

Microplastics (MPs) are considered to have greater environmental hazards than large plastics. Most MPs undergo different degrees of aging and aged MPs exhibit different physicochemical properties from pristine ones. This study successfully prepared a nano-TiO2 coated polypropylene MPs, and explored the nanoscale infrared, thermal, and mechanical properties of MPs before and after photo-aging using a combined AFM-IR technique. Surface height range of MPs was +/- 25 nm. The signal intensity of the absorption peak at 1654 cm(-1) in terms of vinylidene end groups gradually increased as the irradiation time prolonged. The softening temperature of MPs decreased from 126.7 degrees C to 108.5 degrees C as the irradiation time increased from 0 h to 4 h. The MPs after photo-aging became stiffer, especially for the components surrounding the nano-TiO2 particle, indicating that photocatalytic reaction accelerated the aging process of MPs. The resonance frequency of MPs surrounding the nano-TiO2 particle was stronger after photo-aging and the stiffer components were uniformly distributed, confirming that the thermal and mechanical properties of MPs changed after photo-aging. These novel findings are essential to better understand the changes in the surface microstructures, physical properties, and chemical compositions of MPs during aging process.