Molecular dynamics simulation of shish-kebab crystallization of polyethylene: Unraveling the effects of molecular weight distribution

Revealing the Effect of the Molecular Weight Distribution on the Chain Diffusion and Crystallization Process under a Branched Trimodal Polyethylene System

With the increasing demand for high-end materials, trimodal polyethylene (PE) has become a research hotspot in recent years due to its superior performance compared with bimodal PE. By means of molecular dynamics (MD) simulations, we aim to expound the effect of the molecular weight distribution (MWD) on the mechanism of nucleation and crystallization of trimodal PE. The crystallization rate is faster when short-chain branching is distributed on a single backbone compared to that on two backbones. In addition, as the content of high molecular weight backbone decreases, the time required for nucleation decreases, but the crystallization rate slows down. This is because low molecular weight backbones undergo intra-chain nucleation and crystallize earlier due to the high diffusion capacity, which leads to entanglement that prevents the movement of medium or high molecular weight backbones. Furthermore, crystallized short backbones hinder the movement and crystallization of other backbones. What is more, a small increase in the high molecular weight branched backbone of trimodal PE can make the crystallinity greater than that of bimodal PE, but when the content of high molecular weight backbone is too high, the crystallinity decreases instead, because the contribution of short and medium backbones to high crystallinity is greater than that of long backbones.

Unraveling the influential mechanism of short-chain branching on the crystallization of trimodal polyethylene by molecular dynamics simulation

Trimodal polyethylene (PE) has become the focus of research in recent years due to its excellent performance. By means of molecular dynamics simulations, we aim to expound the molecular mechanism of short-chain branching (SCB) in the nucleation process, crystallization process and chain entanglement of trimodal PE. In this study, a series of polyethylene models including different short-chain branching concentrations (SCBCs), short-chain branching lengths (SCBLs), and short-chain branching distributions (SCBDs) were considered. The increase of SCBCs greatly reduces the ability of flipping and movement of PE chains, resulting in more time for nucleation and crystallization and a significant reduction of crystallinity. In contrast, an increase in the SCBL only slightly slows down the diffusion rate of the chain, which leads to a little increase in crystallization time. Most important of all, in the study of SCBD, we find that the distribution of SCBs on a high molecular weight chain, which is the characteristic of trimodal PE, is conducive to the chain entanglement and prevents the occurrence of micro phase separation compared with the case where the SCBs are distributed on a medium molecular weight chain. The mechanism of chain entanglement is proposed to explain the effect of SCBs on tie chain entanglement.

Molecular dynamics simulation of extension-induced crystallization of branched bimodal HDPE: Unraveling the effects of short-chain branches

Bimodal HDPE models were designed for extension-induced crystallization imitating the architecture of industrial bimodal HDPE copolymerized with ethylene and 1-butene, 1-hexene, or 1-octene. Crystallites of bimodal HDPE experienced the emergence of precursors, shish nuclei, and lamellae. The compact conformation of branched polymers impeded the rolling-over, deposition, and folding of chains on the substrate, and thus the formation of nuclei and lamella. Moreover, this retardation was intensified with the rising branch density and length, causing a depression of crystallinity and an increment of tie-chains concentration. Besides, when branches were all located on long chains, the compact conformation enlarged the resistance to the disentanglement of main chains, thus relatively fewer branched long chains were involved in the precursors or nuclei, resulting in the attenuation of lamella formation. Furthermore, for ethyl branched polymers, the coexistent orthorhombic and monoclinic crystallites were built up, and a few expanded monoclinic cells occurred for butyl branches because of the larger butyl reeling into lamella, while hexagonal crystals were created for ethyl/1-hexyl copolymers because of cocrystallization. Additionally, relative to ethyl, larger butyl and hexyl were preferential to be repelled outside crystals to form tie-chains, and hexyl branched polymers acquired relatively fewer tie-chains because of hexagonal eutectoid.

Property Decoupling across the Embryonic Nucleus-Melt Interface during Polymer Crystal Nucleation

Spatial distributions are presented that quantitatively capture how polymer properties (e.g., segment alignment, density, and potential energy) vary with distance from nascent polymer crystals (nuclei) in prototypical polyethylene melts. It is revealed that the spatial extent of nuclei and their interfaces is metric-dependent as is the extent to which nucleus interiors are solid-like. As distance from a nucleus increases, some properties, such as density, decay to melt-like behavior more rapidly than polymer segment alignment, indicating that a polymer nucleus resides in a nematic-like droplet. This nematic-like droplet region coincides with enhanced formation of ordered polymer segments that are not part of the nucleus. It is more favorable to find nonconstituent ordered polymer segments near a nucleus than in the surrounding metastable melt, pointing to the possibility of one nucleus inducing the formation of other nuclei. In this vein, there is also a second region of enhanced ordering that lies along the nematic director of a nucleus, but beyond its nematic droplet and fold regions. These results indicate that crystal stacking, a key characteristic of lamellae in semicrystalline polymeric materials, begins to emerge during the earliest stages of polymer crystallization (i.e., crystal nucleation). More generally, the findings of this study provide a conceptual bridge between polymer crystal nucleation under nonflow and flow conditions and are used to rationalize previous results.