Incognito forms of polyethylene small micro and nanoplastics in solvents: Changes in molecular vibrations

Plastic particles in the range of 1 nm to 1 μm in diameter are nano pollutants in all environments, ubiquitous throughout the air, all waters and living organisms. The formation of nanoplastics (NP) occurs in different ways that include material abrasion, light exposure and even from normal use of plastic materials. In solvents, NP and small microplastics (< 10 μm diameter) differ from larger plastic particles in that they can mix and suspend, similar to other colloidal sized particles. In this study, we used normal mixing conditions and our novel solubilization method to generate polyethylene (PE) small microplastic and nanoplastic solutions (sM&NP), both in water and in common organic solvents. The sM&NP were examined using Raman spectroscopy and microscopy, transmission electron microscopy (TEM) and particle size analysis using dynamic light scattering (DLS) methods. The Raman data showed notable spectral changes compared to solid PE, which indicates significant molecular and morphological changes of the PE polymer when it is part of these sM&NP solutions or suspensions. In organic solvents, the spectral changes for sM&NP signified a loss of polymer crystallinity, while the changes in aqueous solutions suggest greater molecular reorganization of the polymer structure. These structural changes were supported by TEM images of the particles that appeared mostly amorphous with varying thickness. Importantly, the changed spectra of these small particles of PE likely render them more difficult to detect and study, particularly in real-world aqueous systems. These findings indicate that solubilized sM&NP are routinely modified by solvent exposure, and thereby interact differently from larger plastic materials, particularly in aqueous environments.