Simulation of melting in crystalline polyethylene

For the first time, we carry out molecular dynamics (MD) simulation of shear-induced martensitic phase transitions between the orthorhombic and non-orthorhombic (triclinic and monoclinic) phases of crystalline polyethylene (PE) in the framework of a realistic all atom model of the polymer. We show that the variation of the shear rate allows observing on a nano-sample both a strongly nonequilibrium phase transition occurring by random nucleation and irregular growth of a new phase ('civilian' way, for rapid deformations) and the coherent, or 'military', kinetics (generally considered as usual for martensitic transformations). We induce transitions from the orthorhombic to the triclinic phase according to two transformation modes observed in experiment on PE single crystals. Rapid deformation favors the transition directly to the triclinic phase, slow deformation -first to the intermediate monoclinic, and only then -to the triclinic phase. The second way corresponds to the experiment on extended chain PE. We explain this result and analyze the competition between different transformation and plastic deformation modes. Rotations of PE chains around their axes necessary for the transition between the orthorhombic and non-orthorhombic phases are executed by short twist defects diffusing along the chains. The transition between the monoclinic and triclinic phases occurs through half-chain-period translations of the chains along their axes, mostly collectively, as crystallographic slips.

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“…6, on the left). Similar twist defects were observed in MD simulation of premelting in a PE crystal 34 .…”

Section: Boundaries Between the Phases: Twist Defects And Tension supporting

confidence: 79%

Shear-induced martensitic transformations in crystalline polyethylene: Direct molecular-dynamics simulations

Strelnikov

Zubova

2019

Phys. Rev. B

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“…More detailed applications of FK theory to all-atom PE crystals consider short chains (N < 1000) in unstressed crystals, and do not address yield. 30,31 We find that an FK model, parametrized from all-atom simulations of the orthorhombic PE crystal, describes the tensile yield in our detailed finite-chain simulations as chain slip by disloca-tion nucleation at chain ends. The properties of the dislocations allow us to understand trends in yield with N and place bounds on σ y that agree well with our simulations and experiments.…”

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confidence: 80%

“…A large group of researchers has extended their ideas to describe a multitude of condensed matter systems . This includes many who have invoked the ideas of Frank and van der Merwe to describe the dynamics of PE and other polymer crystals. − None have considered the actual geometry of chain ends in the orthorhombic PE crystal under tension, instead they typically apply FK theory to simple bead-spring or united-atom potentials. − , We find that explicit hydrogens are essential to the shear modulus and interchain forces within the PE crystal. Shear forces are underestimated by a factor of 2–3 in united-atom models.…”

mentioning

confidence: 99%

“…23 This includes many who have invoked the ideas of Frank and van der Merwe to describe the dynamics of PE and other polymer rystals. [25][26][27][28][29][30][31][32] None have considered the actual geometry of chain ends in the orthorhombic PE crystal under tension, instead they typically apply FK theory to simple bead-spring or united-atom potentials. [25][26][27][28][29]32 We find that explicit hydrogens are essential to the shear modulus and interchain forces within the PE crystal.…”

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confidence: 99%

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Chain Ends and the Ultimate Strength of Polyethylene Fibers

O'Connor

Robbins

2016

ACS Macro Lett.

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We use large scale molecular dynamics (MD) simulations to determine the tensile yield mechanism of orthorhombic polyethylene (PE) crystals with finite chains spanning $10^2-10^4$ carbons in length. We find the yield stress $\sigma_y$ saturates for long chains at 6.3 GPa, agreeing well with experiments. We show chains do not break but always yield by slip, after nucleation of 1D dislocations at chain ends. Dislocations are accurately described by a Frenkel-Kontorova model parametrized by the mechanical properties of an ideal crystal. We compute a dislocation core size $\xi\approx25$\AA\ and determine the high and low strain rate limits of $\sigma_y$. Our results suggest characterizing the 1D dislocations of polymer crystals as an efficient method for numerically predicting the ultimate tensile strength of aligned fibers

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“…Polymer crystallites can undergo a variety of structural phase transitions that is a subject of extensive studies. PE is widely used as a 'model system' for high-crystallinity polymer [1]. At normal conditions (room temperature and atmospheric pressure), the PE crystal is in orthorhombic [2] or monoclinic phase [3].…”

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confidence: 99%

Molecular dynamics simulation of chains mobility in polyethylene crystal

Sultanov¹,

Atrazhev

Дмитриев

et al. 2013

Jetp Lett.

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The mobility of polymer chains in perfect polyethylene (PE) crystal was calculated as a function of temperature and chain length through Molecular dynamics (MD) in united atom approximation. The results demonstrate that the chain mobility drastically increases in the vicinity of the phase transition from the orthorhombic to quasi-hexagonal phase. In the quasi-hexagonal phase, the chain mobility is almost independent on temperature and inversely proportional to the chain length.Mechanical properties of semi-crystalline polymers below the melting temperature are strongly governed by morphology of crystallites that depends on how the melt was prepared and treated. Polymer crystallites can undergo a variety of structural phase transitions that is a subject of extensive studies. PE is widely used as a 'model system' for high-crystallinity polymer [1]. At normal conditions (room temperature and atmospheric pressure), the PE crystal is in orthorhombic [2] or monoclinic phase [3]. The PE crystals undergo phase transition to quasi-hexagonal phase at elevated pressure (more than 400 MPa) and temperature (> 520K) [4][5][6].Quasi-hexagonal phase has been found in numerous diverse polymeric systems [7]. One common property of the quasi-hexagonal polymeric phase is some degree of conformational disorder, either in the main chain or in the side groups or in both. Compared with the ordered crystalline state, there is a high degree of molecular mobility, with the chain performing both rotational and translational motion. Translational chain mobility enables easy formation of extended chain crystals in polymers that exhibit the quasi-hexagonal phase; isothermal extension of the initially folded chains has been shown to occur in the quasi-hexagonal phase [7]. The PE chains in both monoclinic and quasihexagonal crystals are parallel to each other. However, the monomers of the chain in monoclinic phase predominantly belong to the same plane, while the monomers of the chain in quasi-hexagonal phase are randomly oriented. Therefore, the chains in quasi-hexagonal phase form close packing of rods [7].Diffusion rate of PE chains in quasi-hexagonal phase was measured experimentally in Ref.[6] using proton spin-lattice relaxation experiments and significant increase in the rate of chain diffusion from about 10 −12 cm 2 /s (orthorhombic phase) to 10 −9 cm 2 /s (quasi-hexagonal phase) was observed. The molecular dynamics modeling and understanding of the atomistic mechanisms of high chains mobility in the quasi-hexagonal phase is the purpose of the current work.The LAMMPS software package [8] was utilized for molecular dynamics simulations. Polyethylene chains were modeled in united atom version of Dreiding forcefield [9]. The Nose-Hoover style thermostat and barostat were used in these calculations [10]. The modeled samples were comprised of 64 polyethylene chains with 36, 100 and 200 carbon atoms in each chain. The simulations were performed in periodic boundary conditions in all directions. The chains were made 'infinite' via binding the ...

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Simulation of melting in crystalline polyethylene

Cited by 18 publications

References 45 publications

Shear-induced martensitic transformations in crystalline polyethylene: Direct molecular-dynamics simulations

Shear-induced martensitic transformations in crystalline polyethylene: Direct molecular-dynamics simulations

Chain Ends and the Ultimate Strength of Polyethylene Fibers

Molecular dynamics simulation of chains mobility in polyethylene crystal

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