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First-principle simulations of electronic structure in semicrystalline polyethylene
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. ABB Corporate Research, Sweden.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
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2017 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 146, no 20, article id 204901Article in journal (Refereed) Published
Abstract [en]

In order to increase our fundamental knowledge about high-voltage cable insulation materials, realistic polyethylene (PE) structures, generated with a novel molecular modeling strategy, have been analyzed using first principle electronic structure simulations. The PE structures were constructed by first generating atomistic PE configurations with an off-lattice Monte Carlo method and then equilibrating the structures at the desired temperature and pressure using molecular dynamics simulations. Semicrystalline, fully crystalline and fully amorphous PE, in some cases including crosslinks and short-chain branches, were analyzed. The modeled PE had a structure in agreement with established experimental data. Linear-scaling density functional theory (LS-DFT) was used to examine the electronic structure (e.g., spatial distribution of molecular orbitals, bandgaps and mobility edges) on all the materials, whereas conventional DFT was used to validate the LS-DFT results on small systems. When hybrid functionals were used, the simulated bandgaps were close to the experimental values. The localization of valence and conduction band states was demonstrated. The localized states in the conduction band were primarily found in the free volume (result of gauche conformations) present in the amorphous regions. For branched and crosslinked structures, the localized electronic states closest to the valence band edge were positioned at branches and crosslinks, respectively. At 0 K, the activation energy for transport was lower for holes than for electrons. However, at room temperature, the effective activation energy was very low (similar to 0.1 eV) for both holes and electrons, which indicates that the mobility will be relatively high even belowthe mobility edges and suggests that charge carriers can be hot carriers above the mobility edges in the presence of a high electrical field.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2017. Vol. 146, no 20, article id 204901
National Category
Chemical Sciences Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-208802DOI: 10.1063/1.4983650ISI: 000401778900042Scopus ID: 2-s2.0-85019720001OAI: oai:DiVA.org:kth-208802DiVA, id: diva2:1108961
Funder
Swedish Research Council, 621-2012-2673 621-2014-5398VINNOVA, 2015-06557
Note

QC 20170613

Available from: 2017-06-13 Created: 2017-06-13 Last updated: 2018-09-11Bibliographically approved
In thesis
1. Molecular Structure, Interfacial Chain Topology, Electronic Structure and Fracture Toughness of Polyethylene: A Multiscale Computational Study
Open this publication in new window or tab >>Molecular Structure, Interfacial Chain Topology, Electronic Structure and Fracture Toughness of Polyethylene: A Multiscale Computational Study
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The structure of semicrystalline polyethylene (PE) strongly affects its properties. Two important structural features, namely the concentrations of tie chains and entanglements cannot be directly assessed using experimental techniques. These parameters have a major impact on mechanical properties of the material, especially on its fracture toughness. The present study has therefore focused on developing methods based on computer simulation in order to determine the concentrations of tie chains and entanglements as a function of molecular structure in unimodal and bimodal PE systems.

An off-lattice Monte Carlo (MC) method was developed to simulate the semicrystalline PE. The code was able to input molar mass distribution, short-chain branch distribution, and crystallinity data and model the crystalline-amorphous lamellar structure with the focus on determining the concentrations of tie chains and entanglements. Introduction of the short-chain branches significantly increased the tie chain and entanglement concentrations. The method was then used to simulate a typical semicrystalline structure, and this structure as well as other simulated variations of the PE structure were equilibrated using molecular dynamics (MD) simulations. A linear-scaling DFT (density functional theory) method was then used in order to determine the electronic structure of the materials. Bandgap of the semicrystalline model was found to be smaller than both pure crystalline or amorphous systems. This could indicate the preference for electrons to reside in the interfacial regions rather than in crystalline or bulk amorphous regions. Low effective activation energies obtained indicated a high mobility of holes, excess electrons, and charge carriers at room temperature.

Coarse-grained (CG) potentials were derived using the iterative Boltzmann inversion (IBI) method to describe linear and branched PE. The potentials were then used in CG-MD simulations to crystallize and draw blends of low and high molar mass PE. The purpose was to determine the concentrations of tie chains and entanglements as well as their effect on the fracture toughness. Addition of a linear high molar mass component (only 25 % by weight) significantly increased the concentration of entanglements and thus the fracture toughness of the material. The introduction of a butyl-branched high molar mass fraction had an even stronger effect on the concentration of entanglements and, in particular, on the tie chain concentration. These latter systems exhibited the highest fracture toughness values of all systems studied.

Abstract [sv]

Strukturen hos delkristallin polyeten (PE) har stor inverkan på materialets egenskaper. Två viktiga strukturella parametrar, koncentrationerna av sammanbindningsmolekyler och låsta kedjeihoptrasslingar, kan inte direkt bestämmas med tillgängliga experimentella metoder. Dessa parametrar har stor inverkan på materialets mekaniska egenskaper, i synnerhet på dess brottseghet. Avhandlingen har därför haft ett fokus på att utveckla metoder baserade på datasimulering för att kunna bestämma koncentrationerna av sammanbindningsmolekyler och låsta kedjeihoptrasslingar som funktion av molekylär struktur i enmodala och bimodala PE system.

En icke-gitterbaserad Monte Carlo (MC) metod utvecklades för att simulera strukturen hos delkristallin PE. Den numeriska koden utgick från inmatade parametervärden för fördelningar av molmassa och förgreningsgrad, samt kristallinitetsgrad för att modellera den kristallina-amorfa lamellstrukturen med ett fokus på att bestämma koncentrationerna av sammanbindningsmolekyler och låsta kedjeihoptrasslingar. Införandet av kortkedjeförgrenat material gav en signifikant ökning av koncentrationerna av sammanbindningsmolekyler och låsta kedjeihoptrasslingar. Metoden nyttjades också för att bygga delkristallina multilamellstrukturer, som innan ytterligare simulering genomfördas bringades i jämvikt med hjälp av en molekyldynamik-(MD)-metod. En linjär DFT - (density functional theory) - metod användas för att kartlägga materialets elektriska egenskaper. Bandgapen hos den delkristallina modellen befanns vara mindre än för rena kristallina och rena amorfa modellsystem. Denna observation indikerar att elektroner har en tendens att befinna sig i gränsskikten mellan amorf och kristallin fas. De låga erhållna aktiveringsenergierna för mobilitet hos hål, elektroner och andra laddningsbärare vid rumstemperatur var anmärkningsvärd.

Coarse-grained (ungefär grovskaliga (CG)) potentialfunktioner togs fram med hjälp av en iterativ-Boltzmann-inversions-(IBI)-metod för att beskriva linjär och grenad PE. Metoden användas för CG-MD-simuleringar för att kristallisera blandningar av nämnda bimodala (lågmolekylär komponent samt högmolekylär komponent) polymersystem.  Studiens syfte var att bestämma koncentrationerna av sammanbindningsmolekyler och låsta kedjeihoptrasslingar samt deras effekt på krypkomplians (vid flytning) och brottseghet. Addition av en linjär högmolekylär komponent (även vid lägre andelar, mindre än 25 vikt%) ökade avsevärt koncentrationen av låsta kedjeihoptrasslingar och därmed materialets brottseghet. Introduktion av butylgrenat, högmolekylärt material hade en ännu starkare effekt på koncentrationen av låsta kedjeihoptrasslingar och i synnerhet på sammanbindningsmolekylkoncentrationen. Dessa senare system uppvisade de allra högsta brottseghetsvärdena av alla studerade system.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018
Series
TRITA-CBH-FOU ; 2018:40
National Category
Textile, Rubber and Polymeric Materials Polymer Technologies Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-234818 (URN)978-91-7729-925-7 (ISBN)
Public defence
2018-10-05, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20180911

Available from: 2018-09-11 Created: 2018-09-11 Last updated: 2018-09-11Bibliographically approved

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