Dominant Effects of Short-Chain Branching on the Initial Stage of Nucleation and Formation of Tie Chains for Bimodal Polyethylene as Revealed by Molecular Dynamics Simulation

Abstract

The molecular mechanism of short-chain branching (SCB), especially the effects of methylene sequence length (MSL) and short-chain branching distribution (SCBD) on the initial stage of nucleation, the crystallization process, and particularly the tie chain formation process of bimodal polyethylene (BPE), were explored using molecular dynamics simulation. This work constructed two kinds of BPE models in accordance with commercial BPE pipe resins: SCB incorporated in the long chain or in the short chains. The initial stage of nucleation was determined by the MSL of the system, as the critical MSL for a branched chain to nucleate is about 60 CH2. SCB incorporated in the long chain led to a delay of the initial stage of nucleation relative to the case of SCB incorporated in the short chains. The increase of branch length could accelerate the delay to nucleation. The location of short chain relative to the long chain depended on the MSL of the short chain. As the MSL of the system decreased, the crystallinity decreased, while the tie chains concentration increased. The tie chains concentration of the BPE model with branches incorporated in the long chain was higher than that with branches incorporated in the short chain.

Chain-End Modification: A Starting Point for Controlling Polymer Crystal Nucleation

“…Other groups have leveraged molecular simulations to characterize how polymer crystal nucleation is affected at the molecular level by molecular weight distributions, − additives, , interfaces, − and changes in the polymer architecture ( i.e. , chain branching, − cross-linking, and ring vs linear chain topologies). The present study moves in a new direction by using molecular simulations to explore how possibly nucleation can be controlled through modifications to the end groups of polymer chains.…”

“…The P 2 order parameter measures the alignment of polymer chain segments. The P 2 order parameter and variations thereof (e.g., S and SOP) have been utilized in many previous in silico studies 13,31,[37][38][39][44][45][46]48,49,52,55,56,58,64,65,69,100,101 to monitor crystallization in polymeric systems. Therefore, we leveraged the P 2 order parameter to monitor crystallization in our crystal nucleation simulations.…”

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

“…The P 2 order parameter and variations thereof have been used to study polymer crystallization by many other researcher groups (e.g., see ref. 29,[33][34][35]43,47,56,57,69,73). For the purposes of this study, the backbone direction at a given coarse-grain segment (bead i) was estimated using the vector connecting its intramolecular nearest neighbours (i.e., beads i − 1 and i + 1).…”

“…Previous in silico work has probed polymer crystal nucleation in the context of isolated chains, 41,65,69 solutions, 49,62,63,69 and melts. 33,[35][36][37][38][39][40]52,53,55,57,60,64 Simulations have also been used to elucidate the effects of molecular weight distribution, 28,[31][32][33]44 chain topology (e.g., linear vs. ring chains), 30 chain branching, 33,34,40 and cross-linking 70 on polymer crystal nucleation. There has been much interest in the evolution of chain conformations, 32,33,39,53,60,61,69 topological details related to chain folding, 28,29,32,33,[37][38][39]49,57,64,71 and connect...…”

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

“…As such, understanding and controlling polymer crystallization is of great scientific and technological importance, especially in light of the ubiquity of polymeric materials. However, scientific understanding of polymer crystallization remains incomplete and an active area of research as evidenced by a number of recent reviews. − In the intervening decades since Keller’s seminal work , revealing that polyethylene chains adopt folded conformations upon crystallization, there has been a large number of experimental, − theoretical, − and computational − studies probing polymer crystallization and related phenomenology.…”