| 1. |
- 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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| 2. |
- Nilsson, Fritjof, et al.
(författare)
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Modelling tie-chains and trapped entanglements in polyethylene
- 2012
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Ingår i: Polymer. - : Elsevier. - 0032-3861 .- 1873-2291. ; 53:16, s. 3594-3601
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Tidskriftsartikel (refereegranskat)abstract
- A Monte Carlo random walk model was developed to simulate the chain structure of amorphous layers in polyethylene. The chains emerging from the orthorhombic crystal lamellae were either folding back tightly (adjacent re-entry) or performing a random walk (obeying phantom chain statistics) forming statistical loops or tie chains. A correct amorphous density (ca. 85% of the crystalline density) was obtained by controlling the probability of tight folding. Important properties like fracture toughness depend on the number of chains covalently linking together the crystalline regions. The model structure was analysed with a novel numerical topology algorithm for calculating the concentration of tie chains and trapped entanglements. The numerical efficiency of the algorithm allowed molecular cubic systems with a side length of 100 nm to be readily analysed on a modern personal computer. Simulations showed that the concentration of trapped entanglements was larger than the concentration of tie chains and that the thickness of the amorphous layer (L a) had a greater impact than the crystal thickness (L c) on the tie-chain concentration. In several other commonly used models, such as the Huang-Brown model, the influence of trapped entanglements and the effect of the L a/L c ratio are neglected. Simulations using as input the morphology data from Patel generated results in agreement with experimental rubber modulus data.
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| 3. |
- Soroudi, Azadeh, 1979, et al.
(författare)
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Highly insulating thermoplastic nanocomposites based on a polyolefin ternary blend for high-voltage direct current power cables
- 2022
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3364 .- 2040-3372. ; 14:21, s. 7927-7933
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Tidskriftsartikel (refereegranskat)abstract
- Octyl-silane-coated Al2O3 nanoparticles are found to be a promising conductivity-reducing additive for thermoplastic ternary blends comprising low-density polyethylene (LDPE), isotactic polypropylene and a styrenic copolymer. The ternary blend nanocomposites were prepared by compounding the blend components together with an LDPE-based masterbatch that contained the nanoparticles. The nanoparticles did not affect the superior stiffness of the ternary blends, compared to neat LDPE, between the melting temperatures of the two polyolefins. As a result, ternary blend nanocomposites comprising 38 wt% polypropylene displayed a storage modulus of more than 10 MPa up to at least 150 °C, independent of the chosen processing conditions. Moreover, the ternary blend nanocomposites featured a low direct-current electrical conductivity of about 3 × 10−15 S m−1 at 70 °C and an electric field of 30 kV mm−1, which could only be achieved through the presence of both polypropylene and Al2O3 nanoparticles. This synergistic conductivity-reducing effect may facilitate the design of more resistive thermoplastic insulation materials for high-voltage direct current (HVDC) power cables.
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