Nonlinear shear of entangled polymers from nonequilibrium molecular dynamics

Anwar, Muhammad; Graham, Richard S.

Abstract

This work aims to improve the use of Molecular Dynamics simulations of Kremer-Grest chains to inform future developments of models of entangled polymer dynamics. We perform non-equilibrium molecular dynamics simulations, under shear flow, for well entangled Kremer-Grest chains. We study chains of 512 and 1000 Kremer-Grest beads, corresponding to 8 and 15 entanglements, respectively. We computed the linear rheological properties from equilibrium simulations of the stress auto-correlation function and obtained from these data the tube model parameters. Under non-linear shear flow, we compute the shear viscosity, the first and second normal stress differences and chain contour length. For chains of 512 monomers we obtain agreement with the shear stress results of Cao and Likhtman [ACS Macro Letters (2015) vol. 4 (12) pp. 1376-1381]. We also compare our non-linear results for both chain lengths with the GLaMM model, a widely used non-linear tube model. We identify some systematic disagreement that becomes larger for the longer chains. We made a systematic comparison of the transient shear stress maximum from our simulations, two non-linear models and experiments on a wide range of both melts and solutions, including polystyrene, polybu-tadiene and styrene-butadiene rubber. This comparison establishes that the polystyrene melt data show markedly different behaviour to all other melts and solutions and that Kremer-Grest simulations reproduce the polystyrene data more closely than either the GLaMM or Xie and Schweizer models. We also discuss the performance of these two models against the data and simulations. Finally, by imposing a rapid reversing flow, we produce a method to extract the recoverable strain from MD simulations, valid for sufficiently entangled monodisperse polymers. We then explore how the resulting data can be used to probe the melt state just before the reversing flow.

Citation

Anwar, M., & Graham, R. S. (2019). Nonlinear shear of entangled polymers from nonequilibrium molecular dynamics. Journal of Polymer Science Part B: Polymer Physics, 57(24), 1692-1704. https://doi.org/10.1002/polb.24904

• Journal Article Type: Article
• Acceptance Date: Oct 30, 2019
• Online Publication Date: Nov 25, 2019
• Publication Date: Dec 15, 2019
• Deposit Date: Nov 5, 2019
• Publicly Available Date: Dec 17, 2019
• Journal: Journal of Polymer Science Part B: Polymer Physics
• Print ISSN: 0887-6266
• Electronic ISSN: 1099-0488
• Publisher: Wiley
• Peer Reviewed: Peer Reviewed
• Volume: 57
• Issue: 24
• Pages: 1692-1704
• DOI: https://doi.org/10.1002/polb.24904
• Keywords: Molecular dynamics simulations; shear flow; non-linear rheology 1
• Public URL: https://nottingham-repository.worktribe.com/output/3059516
• Publisher URL: https://onlinelibrary.wiley.com/doi/10.1002/polb.24904
• Additional Information: Received: 2019-07-22; Accepted: 2019-10-30; Published: 2019-11-25