Features of the amorphous-crystalline structure of UHMWPE

Egorov, V. M.; Иванькова, Е. М.; Кулик, В. Б.; Lebedev, D. V.; Myasnikova, L. P.; Marikhin, V. A.; Radovanova, E. I.; Yagovkina, M. A.; Seydewitz, V.; Goerlitz, S.; Michler, Goerg H.; Nöchel, Ulrich; Baltà‐Calleja, F. J.

doi:10.1134/s1811238211050018

Cited by 13 publications

(17 citation statements)

References 41 publications

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“…are very thick due to days of annealing at low undercooling; for thick crystallites and resulting large long periods, the density reduction by chain ends is proportionally increased, as shown above. At ultrahigh molecular weights, well-stacked flat lamellae do not form easily, 59,60,63 and when lamellae are present, they probably consist of strongly tilted chains (ϕ ≥ 40°), as documented above (under "Chain Tilt in Aligned PE Lamellae") for sedimented UHMWPE single crystals from solution. The need for chain ends is even greater in melt-crystallized PE since, unlike folds or short loops, tie molecules and long loops enhance the density throughout the noncrystalline layer.…”

Section: ■ Results and Discussionmentioning

confidence: 73%

“…8 However, these problems have been ignored in most of the literature, where it is usually assumed implicitly that the presence of crystalline material implies the presence of wide, stacked crystalline lamellae with chains perpendicular to the lamellar surface, even for ultrahigh molecular weights. 55,59,97 The effects of chain ends are not necessarily negligible for some commercial UHMWPEs, since their polydispersity index (PDI = M w /M n ) is usually quite large; 59 M w = 3 000 000 g/mol and PDI = 10 results in M n = M w /PDI = 300 000 g/mol. Furthermore, these materials may have crystal thicknesses of 20−40 nm and long periods of 45−100 nm, 58,61 which are ∼3 times larger than in HDPE.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…On the other hand, TEM micrographs of some UHMWPEs do not show lamellae at all. 59,60,63 For instance, only short, poorly defined crystallites are seen in TEM of nascent UHMWPE reactor powders. 59,60 Several studies 59,97 assumed that UHMWPEs form a lamellar structure without chain folding, i.e., with many tie molecules, which would result in an amorphous layer with a density of >1.3 g/cm 3 , 6 incompatible with experiment.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

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Avoidance of Density Anomalies as a Structural Principle for Semicrystalline Polymers: The Importance of Chain Ends and Chain Tilt

2017

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On the basis of sufficiently realistic chain models and simulations, it is concluded that the commonly used models of the lamellar structure of melt-crystallized semicrystalline polymers unintentionally but inevitably contain layers with a higher density than in the crystallites (density anomalies). The density excess would be particularly pronounced in polymers with planar zigzag conformations in the crystallites, such as polyethylene (PE). To avoid density anomalies, the structural models must be modified, with chain ends at the crystal surface and/or chain tilt in the crystallites. NMR and X-ray evidence for these structural features in PE is presented. Termination of chains at the crystal surface keeps dangling chain ends out of the crowded interfacial layer, reducing the density at the interface by about 17% for M n = 15 kg/mol, a common value in commercial high-density polyethylenes. NMR of PEs shows that most CH3 end groups are indeed in all-trans chains in a nearly solid-like environment. When the ends of polydisperse polymers are trapped at the crystal surface, many chain folds cannot be tight, in agreement with NMR showing fast trans–gauche isomerization even for solution crystals of PE. We propose that for polydisperse PE, interfacial chain ends are required for the formation of regular stacks of flat melt-crystallized lamellae without extreme chain tilt or density anomalies. Chain tilt in the crystallites, which decreases the area density of chains emerging from the crystal surface, is another indispensable structural adjustment in PE that reduces excess noncrystalline density; it has occasionally been reported but was usually considered as incidental. For instance, in X-ray analyses of PEs, chain tilt was ignored, and differential broadening of the (hk0) Bragg peaks was mistakenly attributed to mosaicity, in extreme cases resulting in the assumption of overly thick, rod-shaped rather than lamellar crystallites. For various oriented PE samples, including blown films and annealed fibers, published scattering patterns exhibit clear evidence of macroscopically aligned lamellar stacks with pronounced chain tilt. Literature data for PE also show examples of the increasing importance of chain tilt at high molecular weights, where the density reduction by chain ends is minor and evidence of extreme chain tilts of up to 60° has been shown. Alternatively, when chain-end concentrations are very low, the assumption of wide regularly stacked lamellae may have to be given up in favor of ribbon-shaped crystallites. The interplay of the density effects of chain ends and chain tilt can explain many molecular-weight-dependent structural features in polyethylene. On the basis of the combined effects of chain ends at the crystal surface, chain tilt in the crystallites, and adjacent reentry of ∼1/3 of chains, we can construct a lamellar model of PE without density anomalies. Chain tilt and chain ends at the crystal surface are required to “make space” for short loops, in conjunction with noncrystalline chain segments eme...

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Section: ■ Results and Discussionmentioning

confidence: 73%

Section: ■ Results and Discussionmentioning

confidence: 99%

Section: ■ Results and Discussionmentioning

confidence: 99%

See 1 more Smart Citation

Avoidance of Density Anomalies as a Structural Principle for Semicrystalline Polymers: The Importance of Chain Ends and Chain Tilt

2017

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“…24 However, it does suffer from poor thermostability, fatigue resistance, and processability. In order to overcome these weaknesses, blending is widely used to improve the material's performance.…”

Section: ■ Introductionmentioning

confidence: 99%

Preparation of a Thermally Insulating Nanocomposite by Blending Ultra-High-Molecular-Weight Polyethylene with Gas-Phase Silica

Zhang

Chen

et al. 2015

Ind. Eng. Chem. Res.

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In this study, ultra-high-molecular-weight polyethylene (UHMWPE)/gas-phase silica (GS) nanocomposites were prepared by the melt extrusion method. Scanning electron microscopy images of the cross-sectional morphology of the nanocomposites showed that the GS was dispersed uniformly in the UHMWPE matrix. Thermogravimetric analysis confirmed that the addition of GS resulted in an increase in the decomposition temperature of the UHMWPE matrix. X-ray diffraction showed that the GS had an obstructive effect on the crystallization of the UHMWPE. However, the thermal diffusivity of nanocomposites decreased with increasing the GS content, which confirmed that the thermal insulation properties of UHMWPE/GS nanocomposites were improved. Mechanical tests in tensile strength and elongation demonstrated that the strength of nanocomposites increased by the addition of an increased amount of GS.

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“…10 The morphology of UHMWPE particles could be controlled by the synthesis temperature and the type of catalyst, albeit particles with similar morphologies may have differences in their internal structure, such as lamellas dimension and arrangement. 11 It is reported that only nodular structure among the different shapes and morphologies has good compatibility and fewer defects at grain boundaries. 12 Wu et al distinguished two different types of defect in the compression-molded samples.…”

Section: Introductionmentioning

confidence: 99%

The unique effect of chain entanglements and particle morphology on the sintering of ultrahigh molecular weight polyethylene

Hosseinnezhad

Talebi

Rezaei

2016

Journal of Elastomers & Plastics

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Nascent ultrahigh molecular weight polyethylene (UHMWPE) is a thermoplastic material, consisting of nodular-fibrillar structure making it too much porous. At the present work, the effect of both particle morphology and chain entanglements on the sintering of UHMWPE was investigated. Neck growth for particles of three different commercial grades was determined as a function of time at isothermal sintering temperatures. A comparison between the experimental data and predictions from a theoretical model based on Maxwell constitutive equations revealed a complicated effect of particle size and morphology. It was observed that at early stages of sintering, Bellehumeur’s model was in an excellent agreement with the experimental data, while it was deviated at late stages when the neck growth ratio ( y/ a) exceeds 0.8. Unlike commercially entangled samples, melted disentangled powder particles did not flow toward each other even after a long time had elapsed. Disentangled UHMWPE pellets, sintered for 10 min near the melting temperature, showed great transparency indicating low air voids and developing the sintering process, while grain boundaries remained undeveloped for entangled pellets.

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Features of the amorphous-crystalline structure of UHMWPE

Cited by 13 publications

References 41 publications

Avoidance of Density Anomalies as a Structural Principle for Semicrystalline Polymers: The Importance of Chain Ends and Chain Tilt

Avoidance of Density Anomalies as a Structural Principle for Semicrystalline Polymers: The Importance of Chain Ends and Chain Tilt

Preparation of a Thermally Insulating Nanocomposite by Blending Ultra-High-Molecular-Weight Polyethylene with Gas-Phase Silica

The unique effect of chain entanglements and particle morphology on the sintering of ultrahigh molecular weight polyethylene

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