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Molecular structure, crystallization behavior, and morphology of fractions obtained from an extrusion grade high‐density polyethylene
Author(s) -
Gedde U. W.,
Jansson JF.,
Liljenström G.,
Eklund S.,
Holding S. R.,
Wang PL.,
Werner PE.
Publication year - 1988
Publication title -
polymer engineering and science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.503
H-Index - 111
eISSN - 1548-2634
pISSN - 0032-3888
DOI - 10.1002/pen.760282003
Subject(s) - crystallinity , crystallization , branching (polymer chemistry) , materials science , polyethylene , extrusion , mass fraction , xylene , molecular mass , polymer chemistry , molar mass distribution , analytical chemistry (journal) , crystallography , polymer , chromatography , chemistry , organic chemistry , composite material , toluene , enzyme
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 C 2 H 5 ) 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 C 2 H 5 ) 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 C 2 H 5 )). The unit cell volume increases with increasing percent C 2 H 5 , presumably due to the inclusion of ethyl groups in the crystals as interstitlals at 2gl kinks. The concentration of ethyl groups in the crystals (ϵ c ) unanimously follows the relationship: ϵ c (percent) = 0.32 + 0.25 log(percent C 2 H 5 ) except for the low molecular weight fractions which have significantly lower values for ϵ c . 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 < M w < 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 < M w < 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.