Primary nucleation of polyethylene: Embryogenesis from a semidilute solution

Abstract

Using “realistic” molecular dynamics simulation extended up to 100 ns, we have investigated the evolution of cluster size, intrachain vs. interchain potential energies and pair correlations of polyethylene (PE) in a semidilute (ca. 28 wt%) 1,2,4-trichlorobenzene solution at 300 K. Results indicate that the embryonic development begins with the aggregation of trans-rich sequences of characteristic length lo ≈ 2 nm, forming clusters of short stems. This is immediately followed by reorganization/thickening via intracluster axial translation and reeling-in of segments from the surrounding matrix in dynamic competition with neighboring embryos. Up to this stage, the embryonic clusters are loosely packed, retaining largely the conformer populations in the solution state but with gauche conformers enriched in the loose fold loops. After reaching a critical size with l∗ ≈ 4 nm, the intracluster order starts to significantly improve via a “solidification” process with sigmoidal decreases of valence and nonbonding energies, while axial diffusion dramatically slows down and intracluster torsions become fully adjusted to trans conformation by annihilation of gauche conformers. In these “solidified” embryos, although molecular packing remains deviated from the orthorhombic structure (as reflected in significant differences in pair correlations) while reminiscent of the mesomorphic “rotator” or hexagonal phase, the decrease in potential energy is already significant (corresponding to about half of the heat of crystallization) as the intrachain valence contribution is fully realized.