By comparing LDPE with the other polyolefins, HDPE and PP (Supplementary Fig. 2), it can be seen that the size distributions obtained by both LD and OM are similarly shaped, but with a slightly lower frequency of the larger size ranges. The HDPE sample has a narrower distribution of particles with low AR, suggesting 'blockier', less-stretched particles. Contrarily, the PP sample shows a distinct bimodality in the shapes of the MPs. The smallest particles are both high AR and low solidity indicating a highly tortuous, ribbon-like shape, whereas the largest particles are significantly more symmetrical. This may be due to only microscopic regions of the PP plastics becoming heated above their T g and producing small, deformed MPs. The SEM images of the cryo-milled PET shows that the particles are blocky but with a rough surface. Unlike the polyolefin samples they show no signs of having been stretched or deformed, indicating that all the fracturing events occurred above the T g (∼ 73 °C).