PE Crystallization and Rotator Phases.

Recent work on crystallization of polyethylene (PE) implicates a metastable rotator phase as the nucleating phase. This claim invites the questions: what is the structure of this phase, what is the free energy difference $\Delta F$ driving nucleation, and what is the surface free energy $\Sigma$ of the phase in contact with the melt? Related proposals for critical nuclei in polymer crystallization (``fringed micelles'') were dispensed with long ago, with estimates of $\Delta F$ and $\Sigma$ that were unduly pessimistic. With more recent theoretical tools, we can revisit PE nucleation, comparing crystalline and rotator phase nuclei. To do so requires a model that can describe the bulk and surface free energy of both phases. To compare bulk free energies, we use a 6-state generalized Potts model, in which the disordered phase represents the rotator phase. Using a multiscale approach, coupling constants are obtained from solid-state simulations of domain walls between six degenerate crystalline orderings. The surface free energies, dominated by entropic penalties of melt segments near the nucleus surface (the ``fringe''), are calculated using methods developed in the context of polymer brushes. Combining these ingredients, we can make a more enlightened comparison of the nucleation barrier for crystalline and rotator phase nuclei.