Molecular Dynamics Simulation of the Nascent Polyethylene Crystallization in Confined Space: Nucleation and Lamella Orientation

Abstract

Different nascent structures during polyethylene growth can impact the final polymer properties. So far, the evolution of the aggregation structure of nascent polyethylene is still unknown. We have developed a nascent polyethylene model and investigated the effect of confined space on the nucleation and crystallization process. The in situ polymerization characteristics were modeled through the chain-end fixation, temperature gradient, and sidewall setting. We observed a compromise in competition between high undercooling and heterogeneous nucleation in reducing the nucleation free energy, reflecting two nucleation mechanisms for different sidewall settings. Moreover, the contact of sidewall and lamella growth front with different directions had a diverse effect on the development of lamella. The contact of the side face can redress the lamella tilt angle and make its orientation along the Z-axis, while contact of the thickness face is proved to inhibit the increase of the stem length and reduce the thickness of the lamella. The results shed light on the crystallization process of nascent polyethylene and help to explain the role of the polyhedral oligomeric silsesquioxane modification at a microscopic level.

Molecular Dynamics Simulation of In Situ Polymerization: Chain Conformation Transition

“…In Situ Polymerization Model. Similar to our previous research, 53 we employed CGMD simulations to investigate the in situ polymerization of nascent polyethylene. Each hydrogen atom was explicitly paired with its corresponding carbon atom.…”

“…As shown in Figure 3c, by introducing two lateral walls positioned parallel to the XZ and YZ planes in system r = 0.004, a system (r = 0.004_side) considering spatial barriers was constructed to simulate a nanoparticle modified catalyst. 53 The sidewalls were composed of frozen particles arranged in a simple cubic stack with a lattice constant of a = 2.835 Å, and the dimensions were W = 74 Å, L = 170 Å. All the interaction potential patterns between the sidewalls and nascent chains followed the standard 12/6 Lennard-Jones potential, with a cutoff radius of 12 Å.…”

“…In the corresponding section, system r = 0.004 was referred to as S100 to facilitate the distribution of active sites. As shown in Figure c, by introducing two lateral walls positioned parallel to the XZ and YZ planes in system r = 0.004, a system ( r = 0.004_side) considering spatial barriers was constructed to simulate a nanoparticle modified catalyst . The sidewalls were composed of frozen particles arranged in a simple cubic stack with a lattice constant of a = 2.835 Å, and the dimensions were W = 74 Å, L = 170 Å.…”

“…Therefore, this study established an in situ polymerization model for the simulation of the nascent chain growth on supported Ziegler–Natta catalysts, explored the dominant mechanisms of the in situ growth rate, and elucidated the interplay between the chain growth and the chain conformational transition. Furthermore, based on our previous simulation and experimental , studies, we introduced sidewalls to simulate the spatial barrier on supported catalysts and observed the compromise in competition between the attractive energy arising from the sidewalls and the cohesive energy within the nascent chains, and its impacting mechanism on the orientation and length distribution of nascent chains.…”

Molecular dynamics investigation of benzoic acid in confined spaces

“…A similar strategy was employed by Chen and coworkers to simulate the nucleation of polyethylene, where van der Waals walls were composed of frozen particles arranged in a cubic stack with a lattice constant a = 2.835 Å. 42 A full description of the confinement algorithm and the required parameter files is available in the ESI † (Section S1).…”