Flow-induced Density Fluctuation assisted Nucleation in Polyethylene

TL;DR

This study uses all-atom molecular dynamics simulations to compare polyethylene nucleation under quiescent and shear flow conditions, revealing distinct two-step pathways involving local order and density fluctuations.

Contribution

It uncovers the different mechanisms of nucleation under flow and quiescent conditions, emphasizing the role of density fluctuations and conformational ordering in flow-assisted nucleation.

Findings

Flow accelerates nucleation by coupling conformational and orientation orderings.

Flow-induced nucleation is driven by density fluctuations, not just entropic effects.

Nucleation pathways differ significantly between quiescent and flow conditions.

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 ordering_density fluctuation_nucleation.