Polymer type alone is insufficient to predict freezing results. For example, PP-fibers b had higher 𝑛 𝑠 (𝑇)-values than PP-needles c . On the other hand, crystallinity may be important, as previous studies have shown that porous organic polymers of higher crystallinity exhibit greater ice nucleation efficiency. PP can exist in different structural forms (atactic, syndiotactic, isotactic), which affect crystallinity. Thus, differences in crystallinity could explain variations in ice nucleation behavior among PP samples. The morphology of microplastics also appears to be important. Both fiber samples (PP-fibers b and PET-fibers b ) showed the highest 𝑛 𝑠 (𝑇)-values, suggesting morphology may have a role in ice nucleation. However, PP-needles c also nucleated ice, indicating morphology alone is not the sole predictor.