How Flexible Chain Transforms into Stem in Crystal Growth: Simulation Insights from Polyethylene

Using self-seeding molecular dynamics simulations, we investigated how flexible chain transforms into stem in polyethylene crystal growth. Our simulations revealed a wedge-shaped growth front with a distinct interfacial layer of constant width. Outside this interfacial layer, chain segments retained flexible conformations indistinguishable from the amorphous. Upon entering the interfacial layer, van der Waals interactions from the crystal induced a conformational transition, leading to the formation of short initial stems. These initial stems then underwent linear followed by logarithmic thickening to form the long stems that consist of chain-folded lamellae. We confirmed that both thickening stages proceeded via a slip mechanism. The linear stage represented diffusion-controlled free slip, while the logarithmic regime corresponded to confined slip controlled by a barrier. The latter slip barrier was regulated by interfacial packing density, which is governed by amorphous conformation.