Direct observation of long chain enrichment in flow-induced nuclei from molecular dynamics simulations of bimodal blends

Anwar, Muhammad; Graham, Richard S

Abstract

Modelling of flow-induced nucleation in polymers suggest that long chains are enriched in nuclei, relative to their melt concentration. This enrichment has important consequences for the nucle-ation rate and mechanism, but cannot be directly observed with current experimental techniques. Instead, we ran united atom molecular dynamics simulations of bimodal polyethylene blends, comprising linear chains at a 50:50 mix of long (1000 carbon) and short (500-125 carbon) chains, under shear flow. We developed a method to extract the nucleus composition during a transient start-up flow. Our simulations show significant and systematic enrichment of long-chains for all nucleus sizes up to and beyond the critical nucleus. This enrichment is quantitatively predicted by the recent polySTRAND model [Read et al. Phys. Rev. Lett. 2020, 124,147802]. The same model parameters also correctly capture the nucleus induction time in our simulations. All parameters of the model were fitted to a small subset of our data in which long chain enhancement was absent. We conclude that long-chain enrichment is central to the mechanism of flow-induced nucleation and that this enrichment must be captured to correctly predict the nucleation rate.

Citation

Anwar, M., & Graham, R. S. (2021). Direct observation of long chain enrichment in flow-induced nuclei from molecular dynamics simulations of bimodal blends. Soft Matter, 2021(10), 2872-2882. https://doi.org/10.1039/d0sm01361g

• Journal Article Type: Article
• Acceptance Date: Jan 31, 2021
• Online Publication Date: Feb 2, 2021
• Publication Date: Mar 14, 2021
• Deposit Date: Feb 3, 2021
• Publicly Available Date: Feb 3, 2022
• Journal: Soft Matter
• Print ISSN: 1744-683X
• Electronic ISSN: 1744-6848
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 2021
• Issue: 10
• Pages: 2872-2882
• DOI: https://doi.org/10.1039/d0sm01361g
• Public URL: https://nottingham-repository.worktribe.com/output/5289988
• Publisher URL: https://pubs.rsc.org/en/Content/ArticleLanding/2021/SM/D0SM01361G