(283e) Mechanisms of Nanoplastic Formation

Polymers are critical elements of our society but they have unintended negative consequences, such as the creation of microplastics (sizes between 1 micron and 3 mm) and nanoplastics (sizes between 10 nm and 1 micron) [MNPL] when they are exposed to the environment. These MNPL could potentially have deleterious consequences on health. It is well-known that MNPL formation is triggered by polymer chain scission caused by environmental cues. However, a thorough understanding of how such angstrom-scale bond-breaking events lead to the creation of these much larger-sized pollutants remains elusive. We hypothesize and show that two variables are critical in MNPL creation: (a) Polymer Morphology: Semicrystalline polymers form MNPL by tie-chain scission (and also scission of bridging entanglements) which frees up a single lamella or collections of lamellae into the environment. Similarly, entanglements provide strength to amorphous polymers, while crosslinks and trapped entanglements are critical for the mechanical properties of rubbers. Systematically reducing these key elements by bond cleavage ultimately leads to material failure and MNPL creation. (b) Methods of Bond Scission: In stress-driven MNPL formation experiments show that the degrading polymer has minimal changes in molecular weight. Thus, we hypothesize that the imposed stresses primarily break the (few) key connectors that lend mechanical integrity to the material. MNPLs are then created when the degraded material’s failure stress becomes smaller than the imposed stress. In contrast, in homogeneous, quiescent degradation (either by UV or by a solvent) experiments show a continuous decrease in polymer molecular weight with degradation time. We hypothesize that random chain cleavage results in a uniform reduction in molecular weight, and causes the material to embrittle, yielding MNPL.