(641b) Modeling Polymer Mechanical Degradation in Flowing Melts & Nanocomposites

Molecular dynamics simulations are used to study the flow-induced alignment and mechanical degradation of well-entangled polymer melts and nanocomposites for varying flow-rates and polymer backbone strengths. We find that, unlike dilute solutions, chain scission events in melts and composites are highly correlated in space and time due to the entanglement network that mediates chain elongation and subsequent scission. Scission events degrade the entanglement network and cause the rate of long-chain scission to decrease with increasing strain. Tracking chain conformations during flow reveals that the fracture of some chains prior to full alignment can decouple large regions of polymer from the macroscopic entanglement network, kinetically trapping a substantial number of unbroken chains in folded conformations that are less susceptible to elongating and breaking during subsequent flow. This suppression of long-chain degradation is shown to be highly sensitive to the specific flow geometry - with the counterintuitive result that uniaxial flows are the most suppressed. The effect of noncatalytic nanoparticles on melt processing and degradation is also considered. Nanoparticles are found to substantially enhance the rate of chain degradation in extensional flows. This enhancement is related to a nonequilibrium feedback between chain elongation and nanoparticle aggregation during flow that drives tension into chain backbones much more efficiently than in neat polymer melts. Understanding and controlling these physical behaviors are central to designing robust processing methods for efficient and scalable polymer reprocessing and recycling.