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

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.

Polymer crystallization under external flow

The general aspects of polymer crystallization under external flow, i.e., flow-induced crystallization (FIC) from fundamental theoretical background to multi-scale characterization and modeling results are presented. FIC is crucial for modern polymer processing, such as blowing, casting, and injection modeling, as two-third of daily-used polymers is crystalline, and nearly all of them need to be processed before final applications. For academics, the FIC is intrinsically far from equilibrium, where the polymer crystallization behavior is different from that in quiescent conditions. The continuous investigation of crystallization contributes to a better understanding on the general non-equilibrium ordering in condensed physics. In the current review, the general theories related to polymer nucleation under flow (FIN) were summarized first as a preliminary knowledge. Various theories and models, i.e., coil-stretch transition and entropy reduction model, are briefly presented together with the modified versions. Subsequently, the multi-step ordering process of FIC is discussed in detail, including chain extension, conformational ordering, density fluctuation, and final perfection of the polymer crystalline. These achievements for a thorough understanding of the fundamental basis of FIC benefit from the development of various hyphenated rheometer, i.e., rheo-optical spectroscopy, rheo-IR, and rheo-x-ray scattering. The selected experimental results are introduced to present efforts on elucidating the multi-step and hierarchical structure transition during FIC. Then, the multi-scale modeling methods are summarized, including micro/meso scale simulation and macroscopic continuum modeling. At last, we briefly describe our personal opinions related to the future directions of this field, aiming to ultimately establish the unified theory of FIC and promote building of the more applicable models in the polymer processing.