  We demonstrate that nascent polymer crystals (i.e., nuclei) are anisotropic
entities, with neither spherical nor cylindrical geometry, in contrast to
previous assumptions. In fact, cylindrical, spherical, and other high symmetry
geometries are thermodynamically unfavorable. Moreover, post-critical
transitions are necessary to achieve the lamellae that ultimately arise during
the crystallization of semicrystalline polymers. We also highlight how
inaccurate treatments of polymer nucleation can lead to substantial errors
(e.g., orders of magnitude discrepancies in predicted nucleation rates). These
insights are based on quantitative analysis of over four million crystal
clusters from the crystallization of prototypical entangled polyethylene melts.
New comprehensive bottom-up models are needed to capture polymer nucleation.
