Molecular Simulations of Controlled Polymer Crystallization in Polyethylene

Abstract

Multilamella polymer crystals are grown from the melt for the first time, in molecular dynamics simulations of a united-monomer model, with in excess of 1500000 unitedmonomers. Two-component systems comprised of equal weight fractions of 2000 united-monomer long chains and 200 unitedmonomer short chains are considered, with varying numbers of short butyl branches placed along the long chains. Utilizing two different cooling protocols, continuous-cooling and self-seeding, drastically different multilamella structures are revealed, which depend heavily on the branch content and crystallization protocol used. By self-seeding, well-aligned multilamella crystals are grown, which more clearly reveal the subtle alterations an increasing number of branches create on the size and shape of the crystallites in the early stages of spherulite formation. Under continuous cooling, this observation is almost completely obscured. At maximum thickness, chain portions as long as 100 united-monomers (200 carbons) are extended inside the crystalline lamella.

Coarse-Grained Simulations on Polyethylene Crystal Network Formation and Microstructure Analysis

“…To cool down the isotropic melt structure, temperature quenching from 450 K to 300 K and 1 atm condition using NPT ensemble is performed in two stages: a high quenching rate of 10 K/1 ns from 450 K to 400 K, and a slightly slower quenching rate of 5 K/1 ns from 400 K to 300 K. The variation in the quenching rates is because fast cooling could lead to glassy-state formation in plastics; this was observed as well in previous reports, where a much slower temperature-quenching protocol was adopted for crystallization [55,56]. Slower quenching rates are adopted in the simulations to avoid such phase change from the molten state and prepare the desired plastic PE model.…”

Preparation and Characterization of High-Density Polyethylene with Alternating Lamellar Stems Using Molecular Dynamics Simulations

“…Molecular simulation requires an appropriate semi-crystalline model that mimics bulk behavior through which numerical analysis and analytically solving the equations of motion can be carried out. In addition to this, advancements in MD simulation, which can document the molecular chain behavior over a millisecond time scale provided for polymers represented either as a united atom (UA) or as coarse-grained (CG) models, are indispensable for preparing a simple, more affordable HDPE semi-crystalline model structure comprising random or zigzag amorphous regions and ordered crystalline lamellar segments [ 6 , 7 , 8 , 9 , 10 , 11 ]. By taking advancements of UA or CG modeling in MD simulations, the macroscopic mechanical recycling methods would be simulated to predict the microstructure–mechanical property behavior [ 6 ].…”

Chain-Level Analysis of Reinforced Polyethylene through Stretch-Induced Crystallization

“…The process of crystallization has a significant effect on nanomechanics, which necessitates extensive investigations on the toughening of soft materials via stretch-induced crystallization (SIC) and flow-induced crystallization (FIC). − Molecular mechanisms of these phenomena should be understood by using molecular dynamics (MD) simulations for future molecular design and synthesis. Recent advances in computational power have facilitated large-scale MD simulations of crystal formation in semicrystalline polymers. − The insights − gained from small-scale MD simulations are incorporated to reproduce plausible systems in large-scale MD simulations. Initially, the molecular modeling of polymer crystallization was considered a potential model for understanding protein folding.…”