Molecular dynamics (MD) simulations are employed to study biaxial stretch-induced crystallization of polymers, during which the individual roles of chain conformation and orientation on crystal nucleation and growth are clarified. Systems with different stiffness and orientations are constructed by changing the stretch ratios of the x-and y-axis, which allow us to figure out the individual contributions of chain conformation and orientation to flowenhanced nucleation. The results show that nucleation occurs in areas with high segment orientation, and the higher orientation corresponds to the shorter nucleation induction period. The relationship between the nucleation induction period and orientation is quantitatively expressed, which indicates that orientation plays a dominant role in flow-enhanced nucleation. On the other hand, the results show that chain stiffness exhibits a negative correlation with nucleation in biaxial stretch, supporting that conformational entropy reduction is not the main driving force in flow-induced crystallization of polymers. With the secondary nucleation model, the crystal growth rates in different directions correlate well with the orientation at the growth front of the clusters, further confirming the decisive role of orientation in crystal nucleation and growth. Finally, crystal cluster merging is proposed to be a way to form shish structures in highly oriented melts.

Homogeneous nucleation process of polyethylene (PE) is studied with full-atom molecular dynamic simulation. To account the complex shape with low symmetry and the peculiar intra-chain conformational order of polymer, we introduce a shape descriptor OCB coupling conformational order and inter-chain rotational symmetry, which is able to differentiate hexagonal and orthorhombic clusters from melt. With the shape descriptor OCB, we find that coupling between conformational and inter-chain rotational orderings results in the formation of hexagonal clusters first, which is dynamic in nature. Whilst nucleation of orthorhombic structure occurs inside of hexagonal clusters later, which proceeds via the coalescence of neighboring hexagonal clusters rather than standard stepwise growth process. This demonstrates that nucleation of PE crystal is a two-step process with the assistance of OCB order, which is different from early models for polymer crystallization but similar with that proposed for spherical 'atoms' like colloid and metal. 3 Significance StatementBy introducing a shape descriptor OCB that couples intra-chain conformational order and inter-chain rotational order, we successfully differentiate local structures with hexagonal and orthorhombic symmetries and observe OCB order assisted two-step nucleation process in polyethylene crystallization. OCB order is demonstrated to promote the transformation from flexible chains to conformational ordered segments, which is the most peculiar and critical step in polymer crystallization. The shape descriptor OCB may be universal on differentiating local orders in polymer or systems with connectivity.

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.