Molecular dynamics simulations of stretch‐induced crystallization in layered polyethylene

Abstract

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.

Simulating Stretch-Induced Crystallization of Polyethylene Films: Strain Rate and Temperature Effect on the Kinetics and Morphology

“…Throughout our study we performed a series of drawing Molecular Dynamics simulations at various strain rates and temperatures. The protocol we apply during these simulations mimics the drawing stage of a real Machine Direction Orientation (MDO) process, as described in a preliminary study by Romanos et al In particular, beginning from a purely amorphous PE melt configuration at temperature T , we stretch the sample in the drawing (machine) direction z applying a constant drawing rate ε̇ = L z ( t ) − L z ( 0 ) t L z ( 0 ) = λ̇ where t is the time needed to deform the sample from L z (0) to L z ( t ) and L z is the length of the simulation box in direction z . The draw ratio at any time is defined as λ ( t ) = L z ( t ) L z ( 0 ) .…”

“…50,51 It must be mentioned that UA force fields exhibit limitations in accurately capturing the orthorhombic crystal structure of PE, although such models have been previously used for PE crystallization with very reasonable results. 33−35 Initial configurations for all systems were generated at T = 450 K using the amorphous builder plugin of the MAPS platform developed by Scienomics 52 in cubic simulation boxes where three-dimensional periodic boundary conditions were applied. Then an energy minimization step followed to remove any possible atom overlaps created during the previous step.…”

Effect of Temperature on Flow-Induced Crystallization of Isotactic Polypropylene: A Molecular-Dynamics Study

“…It should be noted that charges are not taken into account. TraPPE-UA has been extensively used in crystallization studies of polymers with a focus on the interphase properties of semicrystalline iPP 33 and on the FIC of both polyethylene 34 and iPP. 30 Ranganathan et al 35 have claimed that TraPPE-UA is promising for the investigation of semicrystalline polymers with diverse branching architecture as well.…”

Molecular Dynamics of Topological Barriers on the Crystallization Behavior of Ring Polyethylene Melts with Trefoil Knots

“…Recent large-scale MD simulations have also rendered it possible to investigate lamellar thicknesses of ∼10 nm, wherein the order parameters (i.e., the crystallization-degree index) for the crystallinities of the PE melts are essential in determining the crystallization behavior of the subchain. The development of detection methods for local crystallization behaviors has also received recent attention in the context of strain-, stretch-, flow-, and shear-induced/enhanced crystallization. ,,− …”