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

Abstract

Spatial distributions are presented that quantitatively capture how polymer properties (e.g., segment alignment, density, and potential energy) vary with distance from nascent polymer crystals (nuclei) in prototypical polyethylene melts. It is revealed that the spatial extent of nuclei and their interfaces is metric-dependent as is the extent to which nucleus interiors are solid-like. As distance from a nucleus increases, some properties, such as density, decay to melt-like behavior more rapidly than polymer segment alignment, indicating that a polymer nucleus resides in a nematic-like droplet. This nematic-like droplet region coincides with enhanced formation of ordered polymer segments that are not part of the nucleus. It is more favorable to find nonconstituent ordered polymer segments near a nucleus than in the surrounding metastable melt, pointing to the possibility of one nucleus inducing the formation of other nuclei. In this vein, there is also a second region of enhanced ordering that lies along the nematic director of a nucleus, but beyond its nematic droplet and fold regions. These results indicate that crystal stacking, a key characteristic of lamellae in semicrystalline polymeric materials, begins to emerge during the earliest stages of polymer crystallization (i.e., crystal nucleation). More generally, the findings of this study provide a conceptual bridge between polymer crystal nucleation under nonflow and flow conditions and are used to rationalize previous results.

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

“…In turn, we were able to represent an individual n -C 720 H 1442 chain using 240 SDK beads, substantially reducing the number of interaction sites required to simulate polyethylene crystallization in each of the 400-chain systems (96,000 coarse-grain sites vs over 850,000 atomic sites) and enabling us to undertake the current study. We have previously established that the SDK model is suitable for investigating polyethylene systems and processes in silico (see ref ) and have leveraged the SDK model to study various facets of crystal nucleation in entangled polyethylene melts. − , …”

“…, a CT coarse-grain bead). In accordance with previous work − , leveraging the SDK model to study polymer crystallization, segments with P 2 ≥ 0.85 were labeled crystalline. In turn, the percent crystallinity of each system/melt was estimated as the fraction of its constituent polyethylene chain segments with P 2 ≥ 0.85.…”

“…We have previously established that the SDK model is suitable for investigating polyethylene systems and processes in silico (see ref 84) and have leveraged the SDK model to study various facets of crystal nucleation in entangled polyethylene melts. [44][45][46]48 2.2. Chain-End Modifications.…”

“…While there has been, and continues to be, much experimental work − on polymer crystal nucleation as well as theoretical work − on nucleation processes, simulations are being increasingly leveraged to provide direct, unambiguous access to sub-nano spatiotemporal resolutions, namely, the time and length scales relevant to polymer crystal nucleation. For more than two decades, simulations have been used to study the molecular-level details of polymer crystal nucleation under nonflow conditions in both solutions − and melts. − In recent years, several groups have also started using molecular simulations to study polymer crystal nucleation in the presence of shear − and extension ,,,− conditions, emulating flow-induced 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.…”

Molecular dynamics simulations of stretch‐induced crystallization in layered polyethylene

“…Crystal nucleation under quiescent, nonflow conditions was also studied by Hall and co‐workers using a coarse‐grained model and exploring how polymer properties vary from the center of a baby nucleus into the surrounding metastable melt, thus revealing that there is a partial spatial decoupling of properties at the nucleus‐melt interface. The crystal nucleation took place on the microsecond time scale with two out of 10 configurations not crystallizing 37 …”