| 1. |
- Moyassari, Ali, et al.
(författare)
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First-principle simulations of electronic structure in semicrystalline polyethylene
- 2017
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 146:20
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Moyassari, Ali, et al.
(författare)
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Molecular dynamics simulation of linear polyethylene blends : Effect of molar mass bimodality on topological characteristics and mechanical behavior
- 2019
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Ingår i: Polymer. - : ELSEVIER SCI LTD. - 0032-3861 .- 1873-2291. ; 161, s. 139-150
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Tidskriftsartikel (refereegranskat)abstract
- Blending different molar mass fractions of polyethylene (PE) in order to obtain materials with higher fracture toughness has previously proven beneficial. Our approach has been to use coarse-grained (CG) molecular dynamics (MD) simulations to obtain semicrystalline polyethylene systems on a nanoscale, and then draw them in order to mimic tensile testing. The CG potentials were derived, validated and utilized to simulate melt equilibration, cooling, crystallization and mechanical deformation. Crystallinity, tie chain and entanglement concentrations were continuously monitored. During crystallization, the low molar mass fraction disentangled to a greater degree and ended up with a lower entanglement density than the high molar mass fraction, although the tie chain concentration was higher for the low molar mass fraction. The deformation behavior of semicrystalline PE above its glass transition temperature was then assessed in a uniaxial tensile deformation simulation. The low-strain mechanical properties (i.e. elastic modulus, yield stress and strain) were in accordance with the literature. The high-strain mechanical features and toughness were improved in bimodal systems. The presence of a high molar mass fraction in bimodal systems was shown to affect the crystallinity and tie chain concentration during the strain hardening, leading to tougher model systems. Finally, the bimodal system with equal shares of the molar mass fractions showed the highest toughness and the best ultimate mechanical properties while having a concentration of tie chains and entanglements intermediate between the values for the other systems. This was a clear sign of the non-exclusive role of tie chains and entanglements in the mechanical behavior of bimodal PE and more generally of semicrystalline polymers at high strains.
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| 3. |
- Moyassari, Ali, et al.
(författare)
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Molecular Dynamics Simulations of Short-Chain Branched Bimodal Polyethylene : Topological Characteristics and Mechanical Behavior
- 2019
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Ingår i: Macromolecules. - : American Chemical Society (ACS). - 0024-9297 .- 1520-5835. ; 52:3, s. 807-818
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Tidskriftsartikel (refereegranskat)abstract
- It has previously been shown that polyethylene (PE) with a bimodal molar mass distribution has a high fracture toughness. Our approach has been to use coarse-grained (CG) molecular dynamics (MD) simulations to investigate the effects of including short-chain branches in the high molar mass fraction of bimodal PE on topological features and mechanical behavior of the material. The CG potentials were derived, validated, and utilized to simulate melt equilibration, cooling, crystallization, and mechanical deformation. Crystallinity, tie chain, and entanglement concentrations were continuously monitored. During crystallization, the branched bimodal systems disentangled to a lesser degree and ended up with a higher entanglement density than the linear bimodal systems simulated in our previous study. The increase in entanglement concentration was proportional to the content of the branched high molar mass fraction. A significantly higher tie chain concentration was obtained in the short-chain branched bimodal systems than in the linear systems. The increase in the number of ties was more pronounced than the increase in the number of entanglements. The tie chain concentration was not proportional to the content of the high molar mass fraction. Despite a lower crystal thickness and content, the elastic modulus and yield stress values were higher in the branched bimodal systems. A more pronounced strain hardening region was observed in the branched systems. It was suggested that the higher tie chain and entanglement concentration prior to the deformation, the more extensive disentanglement during the deformation, and the disappearance of formed voids prior to failure point were the reasons for the observed higher toughness of the short-chain branched bimodal PE compared with that of the linear bimodal systems. The toughest system, which contained respectively 25 and 75 wt % low molar mass and branched high molar mass fractions, had the highest tie chain concentration and the second highest entanglement concentration of the simulated systems.
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| 4. |
- Moyassari, Ali, et al.
(författare)
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Simulation of semi-crystalline polyethylene : Effect of short-chain branching on tie chains and trapped entanglements
- 2015
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Ingår i: Polymer. - : Elsevier BV. - 0032-3861 .- 1873-2291. ; 72, s. 177-184
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Tidskriftsartikel (refereegranskat)abstract
- A Monte-Carlo simulation method for assessing the tie chain and trapped entanglement concentration in linear polyethylene was extended to enable the simulation of explicitly branched polyethylene. A subroutine was added to the model making possible the incorporation of different branch lengths and distributions. In addition, the microstructure of branched polyethylene was considered to be made of lamellar stacks of different thicknesses, acknowledging the segregation phenomenon during crystallization. Also, based on complete exclusion of bulky branches from the crystal lattice, a 'pull-out' mechanism was developed for the relaxation of branched parts of polyethylene chains in the vicinity of the crystal layer. Simulations of two series of real polyethylene samples showed the effect of short-chain branching on the concentrations of tie chains and trapped entanglements. Introducing a few branches to an unbranched polyethylene increased the concentration of inter-lamellar connections significantly. This effect decayed if the number of branches was further increased. The tracking of the position of all the carbon atoms during the crystallization process was implemented in the model, making the average square end-to-end distance < r(2) > of polyethylene chains calculable. Simulation of chains with the same molar mass but with different branch contents showed a reduction in the average end-to-end distance with increased branching. The use of real molar mass distribution data was also added to the model features.
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| 5. |
- Moyassari Sardehaei, Ali
(författare)
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Molecular Structure, Interfacial Chain Topology, Electronic Structure and Fracture Toughness of Polyethylene: A Multiscale Computational Study
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 6. |
- Nilsson, Fritjof, Docent, 1978-, et al.
(författare)
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Modelling anti-icing of railway overhead catenary wires by resistive heating
- 2019
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Ingår i: International Journal of Heat and Mass Transfer. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0017-9310 .- 1879-2189. ; 143
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Tidskriftsartikel (refereegranskat)abstract
- Aggregation of ice on electrical cables and apparatus can cause severe equipment malfunction and is thus considered as a serious problem, especially in arctic climate zones. In particular, cable damage caused by ice accumulation on railway catenary wires is in wintertime a common origin for delayed trains in the northern parts of Europe. This study examines how resistive heating can be used for preventing formation of ice on metallic, non-insulated electrical cables. The heat equation and the Navier Stokes equations were solved simultaneously with FEM in 3D in order to predict the cable temperature as function of external temperature, applied voltage, wind speed, wind direction, and heating time. An analytical expression for the heat transfer coefficient was derived from the FEM simulations and it was concluded that the influence of wind direction can typically be neglected. Experimental validation measurements were performed on Kanthal cables in a climate chamber, giving temperature increase results in good agreement with the simulation predictions. The resistive heating efficiency, i.e. the ratio between applied electrical energy and resulting thermal energy, was found to be approximately 68% in this particular study.
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