(527c) Effect of Chain Relaxation on Flow Enhanced Nucleation in Polymers

The fabrication of many polymer products involves melt phase processes like injection molding, blown-film extrusion and heat sealing. For semi-crystalline polymers, the flow can have a huge impact on crystallization kinetics and crystallinity. This phenomenon is known as Flow Induced Crystallization (FIC). The earliest stage of FIC, known as Flow Enhanced Nucleation (FEN), is a subject of on-going research and is especially challenging because it occurs at a very small spatio-temporal scale. Many industrial processes and experiments involving FIC have a flow protocol that has cessation of flow before the polymer melt is quenched to the crystallization temperature. This cessation of flow introduces the additional complexity of chain relaxation during nucleation.

Earlier work has shown that nucleation rates correlate with segmental orientation of the melt under steady state flow. In this work, we use molecular dynamics (MD) to simulate a flow protocol in which flow is stopped when the polymer is quenched to the crystallization temperature. A melt of linear polyethylene chains is simulated. We observe that stopping the flow leads to a reduction in nucleation rates. However, the flow history is not entirely lost, as higher strain rates in the melt lead to higher nucleation rates even after cessation of flow. We find that even as chains tend to relax in the absence of flow, the thermodynamically favored crystalline phase freezes in the residual orientation of the melt. In our simulations we observe this competition between relaxation and crystallization and rationalize the observed nucleation rates for some practically relevant flows.