Molecular Dynamics Simulations of Short-Chain Branched Bimodal Polyethylene: Topological Characteristics and Mechanical Behavior

Abstract

It has previously been shown that polyethylene (PE) with a bimodal molar mass distribution has a high fracture toughness. Our approach has been to use coarse-grained (CG) molecular dynamics (MD) simulations to investigate the effects of including short-chain branches in the high molar mass fraction of bimodal PE on topological features and mechanical behavior of the material. The CG potentials were derived, validated, and utilized to simulate melt equilibration, cooling, crystallization, and mechanical deformation. Crystallinity, tie chain, and entanglement concentrations were continuously monitored. During crystallization, the branched bimodal systems disentangled to a lesser degree and ended up with a higher entanglement density than the linear bimodal systems simulated in our previous study. The increase in entanglement concentration was proportional to the content of the branched high molar mass fraction. A significantly higher tie chain concentration was obtained in the short-chain branched bimodal systems than in the linear systems. The increase in the number of ties was more pronounced than the increase in the number of entanglements. The tie chain concentration was not proportional to the content of the high molar mass fraction. Despite a lower crystal thickness and content, the elastic modulus and yield stress values were higher in the branched bimodal systems. A more pronounced strain hardening region was observed in the branched systems. It was suggested that the higher tie chain and entanglement concentration prior to the deformation, the more extensive disentanglement during the deformation, and the disappearance of formed voids prior to failure point were the reasons for the observed higher toughness of the short-chain branched bimodal PE compared with that of the linear bimodal systems. The toughest system, which contained respectively 25 and 75 wt % low molar mass and branched high molar mass fractions, had the highest tie chain concentration and the second highest entanglement concentration of the simulated systems.

Monodisperse Polymer Melts Crystallize via Structurally Polydisperse Nanoscale Clusters: Insights from Polyethylene

“…For reference, the simulation-based literature on polymer crystallization (e.g., [16,[18][19][20]24,25,28,30,34,38,41,42,48]) does not provide a clear precedent as to whether or not such short stems should be included when analyzing polymer crystallization simulations. In particular, some studies include such short stems [18,24,25,28,38,41,42,48], other studies exclude them by invoking a minimum length criterion [16,21,22,34], and still other studies allow for stems of variable length and use a length criterion to assign some stems as crystalline [19,20,30]. Therefore, we assessed the role of single-bead stems in the formation of large, post-critical crystal clusters.…”

“…With molecular simulations, one can directly study the molecular-level nanoscopic details of polymer crystal nuclei and stems during polymer crystallization. Previous in silico studies have probed stem-related properties and phenomena during crystallization, including the lamellar thickness of nuclei [14,15], intra-lamellar stem reordering mechanisms [16], stem growth rates as a function of stem length [16,17], the structural details of crystal growth fronts [16,18,19], and connections between average stem lengths and crystallization conditions (e.g., temperature and system composition) [20][21][22][23]. In particular, Doye and Frenkel [24] studied the distribution of stem lengths during the formation of crystalline layers from solution on polymer crystal growth faces of varying thickness, and thereby revealed that stem lengths can vary substantially.…”

Lamellae-controlled electrical properties of polyethylene – morphology, oxidation and effects of antioxidant on the DC conductivity

“…[38][39][40] Therefore, it was assumed that charge transport in the LDPE occurs dominantly in amorphous regions with higher concentrations of impurities, i.e. along the crystalline lamellar layers in the spherulites, 41 and along spherulite boundaries. [42][43][44][45] In the FEM model, the conductive paths were considered to exist dominantly in the thin amorphous regions along the spherulite boundaries.…”

Atomistic Investigation on the Wetting Behavior and Interfacial Joining of Polymer-Metal Interface

“…Therefore, in this atomic study, isotactic PP model was constructed in a cube with the dimensions of 8.0 nm × 8.0 nm × 8.0 nm. It is reported that PP has a semicrystalline structure with different crystallite sizes (>10 nm) and the amorphous phase [ 20 , 21 ]. In this present work, the PP structure was simplified as amorphous phases in small calculated volumes.…”

“…It has become a promising tool for observing the atomic movements that are allowed to interact and giving a view of the dynamic evolution at atomic level. Moyassari et al [ 20 , 21 ] used coarse-grained MD simulations method for the short-chain branched bimodal polyethylene and the linear polyethylene blend systems to investigate the topological features and mechanical behaviors of the semi-crystalline polymer. By using the fully atomistic MD simulations with the reactive force field (ReaxFF), Rout et al [ 22 ] studied the interface joining of metal and polymer during the mechanical friction process in order to check whether chemical bond between polymer and metal is formed.…”