The size distribution, obtained by OM, shows that the most numerous particle size is the 1.5 µm -2µm range, and there is a significant shoulder located in the 10 µm region. This bimodality appears in almost all of the MP samples, with a sharp peak in the low, 0.5 µm -2 µm range followed by either a shoulder or plateau in the larger size ranges. This could arise due to distinct underlying breakage mechanisms. For example, a single particle could be split into two similar magnitude, coarse pieces, with many fine particles being generated along the crack. Such a mechanism is likely the source of the bimodality in all samples. The shape characterization of the PET sample show narrow distributions of both high solidity and low AR across all length-scales, supporting the SEM characterization of the particles as blocky and boulder-like. This indicates that PET is highly brittle during the cryo-milling, as such particles are generally formed during rapid fracturing events, with minimal-to-no plastic deformation. Of note, the bimodal breakage mechanism results in particles of similar form across the two length-scales.