Simulation of semi-crystalline polyethylene: Effect of short-chain branching on tie chains and trapped entanglements

Moyassari, Ali; Mostafavi, Hakhamanesh; Gkourmpis, Thomas; Hedenqvist, Mikael S.; Gedde, Ulf W.; Nilsson, Fritjof

doi:10.1016/j.polymer.2015.07.008

Cited by 42 publications

(44 citation statements)

References 33 publications

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“…This result suggests the exclusion of branches to the amorphous phase, but a small concentration of them may still be present at the crystal interface, as it has been reported both experimentally and by computer simulations . Recently, Moyassari et al have reported Monte Carlo simulations concerning the effect of branching on morphological aspects of semicrystalline branched PE . In their work, it is shown that the only way to achieve a correct value for the density of the amorphous layers is to allow butyl branches to be rejected to the crystal–amorphous boundary.…”

Section: Resultsmentioning

confidence: 67%

The influence of short-chain branching on the morphology and structure of polyethylene single crystals

Vega

Jargour

Núñez-Ramírez

et al. 2015

J. Polym. Sci. Part B: Polym. Phys.

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The influence of short‐chain branching on the formation of single crystals at constant supercooling is systematically studied in a series of metallocene catalyzed high‐molecular‐weight polyethylene samples. A strong effect of short‐chain branching on the morphology and structure of single crystals is reported. An increase of the axial ratio with short‐chain branching content, together with a characteristic curvature of the (110) crystal faces are observed. To the best of our knowledge, this is the first time that this observation is reported in high‐molecular‐weight polyethylene. The curvature can be explained by a continuous increase in the step initiation—step propagation rates ratio with short‐chain branching, that is, nucleation events are favored against stem propagation by the presence of chain defects. Micro‐diffraction and WAXS results clearly indicate that all samples crystallize in the orthorhombic form. An increase of the unit cell parameter a0 is detected, an effect that is more pronounced than in the case of single crystals with ethyl and propyl branches. The changes observed are compatible with an expanded lattice due to the presence of branches at the surface folding. A decrease in crystal thickness with branching content is observed as determined from shadow measurements by TEM. The results are in agreement with additional SAXS results performed in single crystal mats and with indirect calorimetry measurements. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1751–1762

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Section: Resultsmentioning

confidence: 67%

The influence of short-chain branching on the morphology and structure of polyethylene single crystals

Vega

Jargour

Núñez-Ramírez

et al. 2015

J. Polym. Sci. Part B: Polym. Phys.

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“…The strong influence of the molecular weight on the ability of the HDPE additive to prevent creep above the melting temperature of the LDPE matrix suggests the presence of a load‐bearing network. This network likely consists of the remaining HDPE‐rich crystallites that are connected through tie chains and trapped entanglements . Huang and Brown have proposed a simple model to describe the probability with which a polymer chain of a certain length forms a tie chain

P_{normalM} = \frac{1}{3} (1 - \frac{4 b^{3}}{\sqrt{π}} true \int_{0}^{L} r^{2} normal e^{- b^{2} r^{2}} normald r)

where the parameter

b^{2} = 3 / 2 {\overset{r}{true}}^{2}

is related to the end‐to‐end distance of a random coil

{\overset{r}{true}}^{2} = D n r_{normali}^{2}

with a chain extension factor in the melt of D = 6.8, the number of links given by n = M w /14 and a link length of r i = 0.153 nm for linear polyethylene.…”

Section: Resultsmentioning

confidence: 99%

Influence of Molecular Weight on the Creep Resistance of Almost Molten Polyethylene Blends

Andersson

Städler

Hagstrand

et al. 2017

Macro Chemistry & Physics

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The most common route to improve the creep resistance of low density polyethylene (LDPE) is crosslinking, which however results in volatile decomposition products that must be removed. Blends of LDPE and an additive‐like amount of a linear polyethylene are found to offer improved creep resistance. Above the melting temperature of LDPE, Tm ≈ 111 °C, a load‐bearing network of higher‐melting crystallites—connected through tie chains and trapped entanglements—provides additional form stability. The molecular weight of the linear polyethylene is found to be critical for the ability to arrest creep, which is correlated with the probability of tie chain formation as well as cocrystallization of the two polyethylenes. A number of high‐density polyethylenes (HDPE) and one ultrahigh molecular weight polyethylene (UHMW‐PE) are explored. For blends of LDPE and 2 wt% of the linear polyethylene, an HDPE with a weight‐average molecular weight Mw of 16 kg mol−1 is found to be sufficient to arrest creep at 115 °C. Further improvement in terms of creep resistance is obtained in case of UHMW‐PE with creep fracture occurring only at a stress of 12 kPa at 115 °C.

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“…2(a), was generated using the MC method developed in previous studies 11,13 . Due to the computational cost of the succeeding electronic structure calculations with LS-DFT, relatively small PE systems consisting of about 2000 CH x units were prepared.…”

Section: Methodology and Computational Detailsmentioning

confidence: 99%

First-principle simulations of electronic structure in semicrystalline polyethylene

Moyassari

Unge

Hedenqvist

et al. 2017

The Journal of Chemical Physics

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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 (∼0.1 eV) for both holes and electrons, which indicates that the mobility will be relatively high even below the 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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Simulation of semi-crystalline polyethylene: Effect of short-chain branching on tie chains and trapped entanglements

Cited by 42 publications

References 33 publications

The influence of short-chain branching on the morphology and structure of polyethylene single crystals

The influence of short-chain branching on the morphology and structure of polyethylene single crystals

Influence of Molecular Weight on the Creep Resistance of Almost Molten Polyethylene Blends

First-principle simulations of electronic structure in semicrystalline polyethylene

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