Flow-induced density fluctuation assisted nucleation in polyethylene

Abstract

The nucleation process of polyethylene under quiescent and shear flow conditions are comparatively studied with all-atom molecular dynamical simulations. At both conditions, nucleation are demonstrated to be two-step processes, which, however, proceed via different intermediate orders. Quiescent nucleation is assisted by local structure order coupling conformational and local rotational symmetric orderings, while flow-induced nucleation is promoted by density fluctuation, which is a coupling effect of conformational and orientation orderings. Flow drives the transformation from flexible chains to rigid conformational ordered segments and circumvents the entropic penalty, which is the most peculiar and rate-limited step in polymer crystallization. Current work suggests that flow accelerates nucleation in orders of magnitude is not simply due to flow-induced entropic reduction of melt as early models proposed, which is mainly attributed to the different kinetic pathway via conformational/orientational orderingdensity fluctuationnucleation.

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.…”

Property Decoupling across the Nucleus-Melt Interface during Polymer Crystal Nucleation

“…Our insights align well with results from previous computational studies 35,50,51,55,58,59 on polymer and alkane crystallization. Zhang and Larson 35 recently demonstrated that quenching polyethylene melts below their isotropic-nematic transition temperature (T IN ) results in rapid crystal nucleation on timescales <100 ns, which is much shorter than the microsecond induction times associated with this study's simulations (see Fig.…”

“…They also observed nascent hexagonal structuring temporally preceding changes in the Voronio volume (local density) of carbon atoms. Moreover, subsequent work by Tang et al 50 on shear-induced crystallization suggests that chain alignment and densification precede crystallization under shear conditions. For reference, Tang et al assigned particles as crystalline using a metric based on spherical harmonics and similar in spirit to q 6 q * 6 , so their observations could correspond to a scenario where the q 6 q * 6 and ρ curves in Fig.…”

Roles of solution concentration and shear rate in the shear-induced crystallization of P3HT

“…Some research groups have reported that shear temperatures above the equilibrium melting temperature change the features of FIPs, further affecting the crystallization morphology and crystallization rate of polymers in the melt. [20][21][22][23] In this work, we studied the effects of FIPs on the crystallization of P3HT in solution, and the equilibrium dissolution temperature T 0 d needs to be evaluated to determine a suitable shear temperature. The Hoffman-Weeks method was used to measure the T 0 d of P3HT toluene solution.…”