Abstract
We carried out large-scale atomistic molecular dynamics simulations to study the growth of twin lamellar crystals of polyethylene initiated by small crystal seeds. By examining the size distribution of the stems—straight crystalline polymer segments—we show that the crystal edge has a parabolic profile. At the growth front, there is a layer of stems too short to be stable, and new stable stems are formed within this layer, leading to crystal growth. Away from the edge, the lengthening of the stems is limited by a lack of available slack length in the chains. This frustration can be relieved by mobile crystal defects that allow topological relaxation by traversing through the crystal. The results shed light on the process of polymer crystal growth and help explain initial thickness selection and lamellar thickening.
Highlights
Most commodity and engineering plastics are semicrystalline, which means that their macroscopic properties strongly depend on the shape, size, and connectivity of the constituent microscopic crystallites.[1−3] understanding the process of polymer crystallization is key to understanding and controlling the properties of this technologically important group of materials
We showed that the profile has a constant parabolic shape that does not seem to be affected by the interplay of stem attachment and lengthening
We studied crystal growth in polyethylene with molecular dynamics in a large-scale system of 3 million united atoms with simulation times over 1 μs
Summary
Most commodity and engineering plastics are semicrystalline, which means that their macroscopic properties strongly depend on the shape, size, and connectivity of the constituent microscopic crystallites.[1−3] understanding the process of polymer crystallization is key to understanding and controlling the properties of this technologically important group of materials. Because of the limitations in available experimental methods, the molecular level kinetic mechanism of the growth of polymer crystals is still a subject of speculation.[7,8] Even though the large interfacial energy associated with the fold surface (where chains exit and enter the crystal) would thermodynamically favor thick extended-chain crystals, polymers are found to form thin folded lamellae only 10 or more nanometers thick. In these lamellae, the chains make a large number of folds that, increase the free energy, make crystallization more kinetically accessible. Computer simulations have started to shed light on the molecular level dynamics
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