Local structure order assisted two-step crystal nucleation in polyethylene

Abstract

Homogeneous nucleation process of polyethylene (PE) is studied with full-atom molecular dynamic simulation. To account the complex shape with low symmetry and the peculiar intra-chain conformational order of polymer, we introduce a shape descriptor OCB coupling conformational order and inter-chain rotational symmetry, which is able to differentiate hexagonal and orthorhombic clusters from melt. With the shape descriptor OCB, we find that coupling between conformational and inter-chain rotational orderings results in the formation of hexagonal clusters first, which is dynamic in nature. Whilst nucleation of orthorhombic structure occurs inside of hexagonal clusters later, which proceeds via the coalescence of neighboring hexagonal clusters rather than standard stepwise growth process. This demonstrates that nucleation of PE crystal is a two-step process with the assistance of OCB order, which is different from early models for polymer crystallization but similar with that proposed for spherical 'atoms' like colloid and metal. 3 Significance StatementBy introducing a shape descriptor OCB that couples intra-chain conformational order and inter-chain rotational order, we successfully differentiate local structures with hexagonal and orthorhombic symmetries and observe OCB order assisted two-step nucleation process in polyethylene crystallization. OCB order is demonstrated to promote the transformation from flexible chains to conformational ordered segments, which is the most peculiar and critical step in polymer crystallization. The shape descriptor OCB may be universal on differentiating local orders in polymer or systems with connectivity.

Monodisperse Polymer Melts Crystallize via Structurally Polydisperse Nanoscale Clusters: Insights from Polyethylene

“…The crystal cluster shown in Figure 1 corresponds to 600 carbon atoms, the approximate critical nucleus size for the conditions and molecular model used in this study (see [38]). The varying stem lengths in Figure 1 along with similar simulation snapshots from previous studies [17,19,20,[27][28][29][30][31][32][33][34] highlight that structural non-uniformity (heterogeneity) is present at the early stages of crystallization in a monodisperse polymer melt. This study contributes a detailed quantitative characterization and discussion of this structural heterogeneity.…”

“…Based on the visual inspection of snapshots from nucleation simulations, Meyer and Müller-Plathe [26] noted in passing that nuclei could be composed of stems of varying length and contain a variable number of stems. Similarly, images of polymer crystal nuclei from previous work [15,17,19,20,23,[27][28][29][30][31][32][33][34][35][36] exhibit stem length variability, and sometimes quite pronounced variability, though these studies did not directly explore nor quantify the stem length distributions of nuclei. More recently, Hagita et al [37] demonstrated that stem length probability distributions broaden as linear and ring polyethylene melts crystallize, though multiple nuclei of differing size formed in their systems, obfuscating the structural details of individual nuclei.…”

Polymer Nucleation under High-Driving Force, Long-Chain Conditions: Heat Release and the Separation of Timescales

“…With respect to alkane and polymer crystallization, there has been some discussion as to whether densification precedes local orientational ordering (i.e., what Anwar et al 10,11 call crystallinity), or local orientational ordering precedes densification. 13,14 Russo and Tanaka's recent review 43 on crystal nucleation highlights how the latter is generally operative during crystallization in colloidal and other spherical particle systems. The fact that this study's isochoric nucleation simulations (i.e., NVT and NVE simulations)…”

“…Therefore, it is reasonable to conjecture that the exothermic nature of crystallization and heat transfer effects might impact polymer nucleation. While there has been some work [7][8][9][10][11][12][13][14][15] The present study addresses this challenge through NPT, NVT, and NVE molecular dynamics simulations of crystal nucleation in entangled polyethylene (PE) melts, specifically n-C 720 H 1442 melts. During an isochoric, isothermal NVT simulation, the heat released during crystallization is removed nearly instantaneously from the crystallizing system (e.g., on picosecond timescales).…”

Polymer Nucleation under High-Driving Force, Long-Chain Conditions: Heat Release and the Separation of Timescales

“…Therefore, it is reasonable to conjecture that the exothermic nature of crystallization and heat transfer effects might impact polymer nucleation. While there has been much work [7][8][9][10][11][12][13][14][15] probing the molecular-level details of polymer crystallization using simulations, this previous work relied on isothermal simulations, and neglected the role of heat transfer during polymer crystallization.…”