Molecular fractionation in melt-crystallized polyethylene: 4. Fracture

Gedde, Ulf W.; Jansson, J.‐F.

doi:10.1016/0032-3861(85)90079-5

Cited by 33 publications

(19 citation statements)

References 19 publications

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“…The length of the co-monomer also appears to exert an influence on SCG, with HDPEs copolymerized with 1-hexene displaying superior performance compared to those co-polymerized with 1-butene [25,30,31]. Some authors also suggest that crack growth can be accelerated along regions of low molar mass molecules segregated along the boundaries of crystalline superstructures [32][33][34].…”

Section: Introductionmentioning

confidence: 97%

Analysis of ductile and brittle failures from creep rupture testing of high-density polyethylene (HDPE) pipes

Krishnaswamy

2005

Polymer

121

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Section: Introductionmentioning

confidence: 97%

Analysis of ductile and brittle failures from creep rupture testing of high-density polyethylene (HDPE) pipes

Krishnaswamy

2005

Polymer

121

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“…The quantity Pdc, the probability that a n arbitrary ethyl group is confined in the crystals, as calculated according to E q 6: Pdc = tc(%)Wc/%CH3 (6) Crystallization kinetic data of fractions from extr-HDPE and linear PE (sharp fractions) are presented in Figs. 1 1 and 12.…”

Section: Resultsmentioning

confidence: 99%

“…Molecular weight segregation occurs in linear PE on solidification (1)(2)(3)(4)(5)(6). The low molecular weight species are accumulated in the peripheral parts of the spherulites/axialites and between main lamellae within the spherulites/ axialites (5).…”

Section: Introductionmentioning

confidence: 99%

“…The low molecular weight species are accumulated in the peripheral parts of the spherulites/axialites and between main lamellae within the spherulites/ axialites (5). The low molecular weight segregated material is brittle due to the low concentration of interlamellar tie chains (6). and fracture thus propagates preferentially through these regions (4,6).…”

Section: Introductionmentioning

confidence: 99%

“…The low molecular weight segregated material is brittle due to the low concentration of interlamellar tie chains (6). and fracture thus propagates preferentially through these regions (4,6). Knowledge concerning the relationship between molecular structure, crystallization kinetics, morphology and material properties, and the ability to control the detailed molecular structure are essential if "weak-link properties, e.g.…”

Section: Introductionmentioning

confidence: 99%

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Molecular structure, crystallization behavior, and morphology of fractions obtained from an extrusion grade high‐density polyethylene

Gedde

Jansson

Liljenström

et al. 1988

Polymer Engineering & Sci

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An extrusion‐grade of high density polyethylene (HOPE) (3 ethyl groups per 1000 carbons) has been divided into 16 fractions by preparative GPC and selective p‐xylene extraction. The fractions, with molecular weights ranging from 900 to 1,000,000, have been studied by IR spectros‐copy, DSC, WAXS, polarized microscopy, and small‐angle light scattering (SALS), The average degree of chain branching (percent C2H5) is 0.5 percent for the part of the sample having a molecular weight lower than 10,000 and it decreases monotonically with increasing molecular weight, finally approaching 0.1 percent C2H5. A crystallinity depression with respect to linear PE equivalent to 20 percent/(percent C2H5) is recorded for all samples except for the very low molecular weight samples for which the crystallinity depression is much larger (30 to 35 percent/ (percent C2H5)). The unit cell volume increases with increasing percent C2H5, presumably due to the inclusion of ethyl groups in the crystals as interstitlals at 2gl kinks. The concentration of ethyl groups in the crystals (ϵc) unanimously follows the relationship: ϵc(percent) = 0.32 + 0.25 log(percent C2H5) except for the low molecular weight fractions which have significantly lower values for ϵc. Our admittedly speculative explanation for this major discrepancy between high and low molecular weight samples is based on the idea that segments with ethyl groups close to chain ends have a greater difficulty in crystallizing than segments containing ethyl groups located at positions far from the chain ends. The fractions obtained from the extrusion‐grade HDPE show a solidification temperature depression with respect to linear PE which can only be explained by the presence of chain branches in these samples. The depression is particularly pronounced for the low molecular weight samples as is expected from the data on molecular structure. Well‐developed non‐banded spherulites are observed in rapidly cooled (crystallized at about 35 K supercooling), low molecular weight samples (6,000 < Mw < 8,000)from the extrusion‐grade HDPE in contrast to the axialites observed in linear PE of the same molecular weight and thermal treatment. This discrepancy in morphology has been related to the presence of ethyl groups in the extrusion grade HDPE fractions. Higher molecular weight samples (20,000 < Mw < 1,000,000)from the extrusion‐grade HDPE and linear PE both display well‐developed banded spherulites of similar nature as is expected due to the similarity in molecular structure of the two sets of sample.

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The influence of process history on the ductile failure of polyethylene pipes subject to continuous hydrostatic pressure

Krishnaswamy

Lamborn

2005

Adv Polym Technol

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Failure times of plastic and metal pipes subjected to hydrostatic pressure at various levels assist pipe manufacturers to not only design pipes for certain applications, but also to give them an indication of the useful service lifetimes of these pipes. In this work, a medium density polyethylene was converted into various pipes by varying extrusion parameters. These pipes displayed low levels of lamellar orientation and a skin-core gradient in morphology. The hydrostatic pressure performance (ductile failure regime at 23˚C) of the pipes was observed to depend on morphology, which was inferred by the systematic dependence of the failure times on the tensile behavior of the pipes. It was also found that pipes annealed at 80˚C prior to testing displayed 400% longer failure times; annealing was found to increase the crystallinity of the pipe only marginally. Consequently, this performance improvement was attributed to relaxation of some of the frozen-in residual stresses in the pipe. C

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Molecular fractionation in melt-crystallized polyethylene: 4. Fracture

Cited by 33 publications

References 19 publications

Analysis of ductile and brittle failures from creep rupture testing of high-density polyethylene (HDPE) pipes

Analysis of ductile and brittle failures from creep rupture testing of high-density polyethylene (HDPE) pipes

Molecular structure, crystallization behavior, and morphology of fractions obtained from an extrusion grade high‐density polyethylene

The influence of process history on the ductile failure of polyethylene pipes subject to continuous hydrostatic pressure

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