Polymer nucleation under high-driving force, long-chain conditions: Heat release and the separation of time scales

Abstract

This study reveals important features of polymer crystal formation at high-driving forces in entangled polymer melts based on simulations of polyethylene. First and in contrast to small-molecule crystallization, the heat released during polymer crystallization does not appreciably influence structural details of early-stage, crystalline clusters (crystal nuclei). Second, early-stage polymer crystallization (crystal nucleation) can occur without substantial chain-level relaxation and conformational changes. This study’s results indicate that local structures and environments guide crystal nucleation in entangled polymer melts under high-driving force conditions. Given that such conditions are often used to process polyethylene, local structures and the separation of time scales associated with crystallization and chain-level processes are anticipated to be of substantial importance to processing strategies. This study highlights new research directions for understanding polymer crystallization.

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

“…As such, small nuclei are not simply smaller versions of larger-scale crystals, which aligns with insights from previous work monitoring the shape of nascent polymer crystals [38]. Previous work [42] has demonstrated that nucleation in entangled polymer melts at high-driving forces (i.e., for the types of systems considered in this study) is a local event guided by local environments. Nucleation rates for long-and short-chain systems similarly indicate that nucleation is a local event [14].…”

“…By comparing SDK-based results for molten polyethylene and polyethylene crystallization to results from experiments and atomistic simulations, we have previously demonstrated that the SDK model is an appropriate model for studying polyethylene systems [41]. Due to their coarse-grain nature, molten SDK polyethylene chains diffuse ∼4.07 times faster than those in experimental samples, as discussed in our previous work [41,42]. As such, all simulation times reported in this study correspond to scaled times (i.e., internal LAMMPS times × 4.07), unless otherwise stated, in order to facilitate comparisons with experimental timescales.…”

“…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].…”