We use both all-atom and united-atom molecular dynamics (MD) simulations to quantify the role of nematic order<br/>in both linear and short-chain-branched polyethylene (PE) nucleation when chains are at equilibrium and when stretched. We combine atomistic MD simulation and mean-field theory to determine the nematic coupling parameter by imposing a tension on a fraction of the chains, and measuring induced nematic order on the other. This effect can lead to phase separation of stretched chains. We also mix stretched periodic PE chains with free PE chains of length 500 and 1000 backbone carbons (C500 and C1000). By simulating isothermal nucleation and performing mean-first- passage time analysis for the growth of the largest nucleus in our simulations, we show that the nucleation rate of PE increases exponentially with increasing average nematic order of monomers and is strongly slowed by short-chain side branches. We also validate the predicted quiescent nucleation rates by computing the crystallization half-time at various temperatures using the crystal growth velocity, sampled from simulations of isothermal crystallization of free chains near crystalline slabs, and the Avrami equation. The predicted crystallization half-times are in agreement with experiments, suggesting our predicted nucleation rates are reasonable. The results provide information that can be used towards developing a quantitative theory of flow-enhanced crystallization of polyethylene.

Role of nematic order and chain tension in nucleation and growth of polyethylene crystals

ACS Spring Meeting 2021

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