Divining the shape of nascent polymer crystal nuclei

Abstract

We demonstrate that nascent polymer crystals (i.e., nuclei) are anisotropic entities, with neither spherical nor cylindrical geometry, in contrast to previous assumptions. In fact, cylindrical, spherical, and other high symmetry geometries are thermodynamically unfavorable. Moreover, post-critical transitions are necessary to achieve the lamellae that ultimately arise during the crystallization of semicrystalline polymers. We also highlight how inaccurate treatments of polymer nucleation can lead to substantial errors (e.g., orders of magnitude discrepancies in predicted nucleation rates). These insights are based on quantitative analysis of over four million crystal clusters from the crystallization of prototypical entangled polyethylene melts. New comprehensive bottom-up models are needed to capture polymer nucleation.

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

“…In order to quantitatively probe the characteristics of stem length distributions during polymer crystal nucleation, we performed microsecond molecular dynamics simulations of crystal nucleation in monodisperse polyethylene melts composed of n-C 720 H 1442 chains. The methodological details of the simulations are the same as our previous work [38], so only a summary of the calculations are provided herein (see [38] for more details). All simulations were conducted using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [39].…”

“…Barostat Details Anisotropic chain barostats [43] (4-member chains, coupling constants: 10 ps unscaled time) Consistent with previous work [14,18,26,34,38,41,42,[47][48][49][50][51], polymer crystallization was monitored using the P 2 order parameter. Note that the P 2 order parameter is sometimes alternatively referred to as S or P l depending on how it is calculated and on author preferences.…”

“…In any case, the order parameter provides a quantitative assessment of the alignment between polymer chain segments within a system. The P 2 analysis was performed according to the protocol detailed in [38,42]. In brief, coarse-grain beads with P 2 ≥ 0.85 were labelled crystalline, and cluster analysis was performed on crystalline coarse-grain beads to extract nuclei.…”

“…We also performed a substantial detailed analysis of our crystallization simulations to validate the inclusion of the single-bead stems in the current study. For reference, the simulation-based literature on polymer crystallization (e.g., [16,[18][19][20]24,25,28,30,34,38,41,42,48]) does not provide a clear precedent as to whether or not such short stems should be included when analyzing polymer crystallization simulations. In particular, some studies include such short stems [18,24,25,28,38,41,42,48], other studies exclude them by invoking a minimum length criterion [16,21,22,34], and still other studies allow for stems of variable length and use a length criterion to assign some stems as crystalline [19,20,30].…”

“…Therefore, we assessed the role of single-bead stems in the formation of large, post-critical crystal clusters. Based on earlier work [38], the approximate critical nucleus size (i.e., location of the free-energy barrier to crystallization) for the conditions and molecular model used in this study corresponds to a cluster size of ∼200 coarse-grain beads (i.e., ∼600 carbon atoms); see ref. [38] for more details concerning this estimate.…”