Nanoscale infrared, thermal and mechanical properties of aged microplastics revealed by an atomic force microscopy coupled with infrared spectroscopy (AFM-IR) technique

Microplastics (MPs) often undergo different degrees of aging, and the aged MPs exhibit different surface properties from pristine MPs. This study explored the nanoscale infrared, thermal and mechanical properties of TiO2-pigmented MPs before and after aging by using an AFM-IR technique. Results showed that the surface of MPs was relatively smooth before aging, and was rough with more granular domains after aging. The stronger band at 1706 cm(-1) (assigned to C=O) and the weaker band at 1470 cm(-1) (assigned to -CH2) were observed in aged MPs due to oxidation of C=H bond in low-density polyethylene (LDPE). The softening temperature of MPs was about 209.50 +/- 11.48 degrees C before aging, but after aging it dropped to 94.91 +/- 4.40 degrees C. Aging process mainly reduced the glass transition temperature of the continuous phase (LDPE) rather than the discrete phase (TiO2) in MPs. Resonance deviations of the two characteristic peaks (i.e., 299/645 kHz and 311/670 kHz) between unaged and aged MPs were observed, and these characteristic peaks obviously appeared at higher frequencies in aged MPs, suggesting that the MPs after aging became stiffer. A stronger signal at a high frequency and the uniform signal distribution at this frequency confirmed that the mechanical properties of MPs changed after aging. These findings help to better understand the effects of aging process on the physicochemical properties of MPs. (C) 2020 Published by Elsevier B.V.