Molecular dynamics simulations of crystal nucleation in entangled polymer melts under start-up shear conditions

Abstract

Understanding the flow induced crystallisation (FIC) process is necessary due to its technological relevance to polymer processing. Polymer crystallisation controls the morphology of semi-crystalline polymers and hence the properties of the end product. We perform molecular dynamics simulations of polymer melts consisting of sufficiently entangled linear chains under shear flow. We determine the Rouse relaxation time (τ R ) for linear polymer chains using an established rheological model at different temperatures and fit the simulation data with the Arrhenius and Williams-Landel-Ferry (WLF) equations. We simulate the crystallisation induction times for different values of the Rouse Weissenberg number (W iR =γτ R ) at different temperatures. We observe that the level of strain and stretch required to induce crystallisation increases with temperature. We find that the induction times follow a power law in shear rate and observe a more pronounced effect of flow rate for higher temperatures than at lower temperatures. Moreover, we determine that nucleation events occur relatively early in the shear transient and at a stretch value that is smaller than its steady state value. We also report the values of strain at which the occurrence of a nucleation event is most likely to happen.

Divining the shape of nascent polymer crystal nuclei

“…As such, polymer crystallization is being increasingly probed with in silico tools, particularly Molecular Dynamics (MD) simulations, which provide fine-grain access in both time and space to the molecular-level details of polymer crystallization (e.g., see ref. 5,6,9,10,12,13,42,43).…”

Nonlinear shear of entangled polymers from nonequilibrium molecular dynamics

“…Recently, MD has been used to investigate the viscoelastic behavior of coarse‐grained polyethylene chains, in which C X denotes a melt of linear chains containing X carbon atoms. This system has about 75 carbon atoms per entanglement . Unentangled C50 and slightly entangled C178 chains have been simulated in bulk and in confined systems.…”

“…Unentangled C50 and slightly entangled C178 chains have been simulated in bulk and in confined systems. MD has been used to explore the structural, conformational, rheo‐optical, and topological properties of an entangled C400 chains in the linear and highly nonlinear regimes; the dynamics and rheology of supercooled melts of C10 C20 and C150, and C1000 chains; the steady and transient behavior of a C100 melt; the steady and transient behavior of a C400 melt; and steady and startup shear of a C700 melt …”

PolySTRAND Model of Flow-Induced Nucleation in Polymers

“…Molecular dynamics (MD) simulations can complement experiments. Recent simulations have resolved individual nucleation events from monodisperse chains, to quantify FIN [14][15][16][17][18][19]. Simulations of 150-carbon polyethylene [15], showed that the Kuhn segment nematic order, P 2,K , is the key parameter for FIN.…”