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We study, via united atom molecular dynamics (MD) simulations, the crystallization of six polyethylene (PE) systems, each consisting of alternating layers of molten linear polymer of two different degrees of polymerization, under stretch, while at the same time comparing against the corresponding results from two homogeneous monodisperse systems. The “slab,” out of which the model bidisperse layered systems are formed by applying periodic boundary conditions, is comprised of two different molten film subsystems, both of initial thickness 3.66 nm in the z‐direction, adhering to each other; in the first the chain length is 200 carbon atoms and in the second 400 carbon atoms. Following a short pre‐equilibration at 350 K and 1 bar, a stretching simulation is conducted in the NėxxLyPzzT ensemble, with the engineering strain rate ėxx fixed in the machine (pulling) direction x, parallel to the interfaces; the width of the films Ly fixed in the transverse direction; and the pressure Pzz normal to the interfaces fixed at 1 bar. The temperature during stretching is held constant at T = 300 K, corresponding to more than 100 K subcooling for both molar masses. Deformation to a stretch ratio of Lxx/Lxx,0 = 2.6 within 120 ns is followed by annealing according to two different protocols. In the first protocol, stretching is stopped and the systems allowed to relax in the NLxLyPzzT ensemble under Pzz = 1 bar for 60 ns. In the second protocol, stretching is continued at a rate 10 times lower than the initial one for 60 ns and then stopped, letting the systems relax for 100 ns. Crystallization is observed following both protocols. A nucleus of ordered material emerges early on during the first stage of stretching at a deformation rate of 0.05 nm/ns. The crystalline structure is sharper and more extended following the second, slower protocol. Crystallized macromolecules lie in planes almost parallel to the interface, forming a tilt angle of 20° to 24° with the pulling direction. Long and short chains co‐crystallize into lamellae, which are slightly more extended in the long‐chain regions. This study sheds light on the morphology expected to be developed in monomaterial packaging consisting of oriented bidisperse PE layers, designed to combine low permeability with recyclability and low‐environmental impact.
In this article, the crystallization of six polyethylene (PE) systems is studied via united atom molecular dynamics (MD) simulations. It is found that long and short chains co‐crystallize into lamellae, which are slightly more extended in the long‐chain regions. This study sheds light on the morphology of oriented bidisperse PE layers in mono‐material packaging with a low‐environmental impact.