Morphology and properties of low‐density (branched) polyethylene

Abstract

The different types of morphology that can be developed in a large number of low‐density (branched) polyethylene whole polymers, as well as in a series of fractions, have been studied for two different extreme crystallization modes. Concomitantly, thermodynamic properties of the same samples have also been determined. After isothermal crystallization at elevated temperatures, spherulitic structures are found in all the whole polymer samples. On the other hand, after rapid crystallization a variety of different types of supermolecular structures are observed which are shown to depend systematically on the concentration of side‐chain branches and the relative proportion of high molecular weight species in the sample. This temperature dependence of the morphological forms is opposite to that previously reported for linear polyethylene. The studies with the fractions show that the individual species are not the cause of this behavior; rather, the total composition is the important factor. The thermodynamic properties are also quite different from those of linear polyethylene in showing virtually no molecular weight dependence and being governed primarily by the concentration of short‐chain branches. The degrees of crystallinity as determined from density and enthalpy of fusion measurements do not vary much with the two extreme crystallization conditions employed, are not sensitive to the morphology, and differ from one another, even when well‐developed spherulites are formed. A major influence of the branching concentration on these properties is clearly indicated.

A Raman spectroscopic study of the morphological structure of the polyethylenes

“…For samples with a high content of chain irregularities the interfacial content can become comparable to that of the orthorhombic phase. As is reflected in other properties [17,18] the effect of thermal treatment is relatively minor. The level of crystallinity, and other properties, are primarily determined by the branching or co-unit content.…”

“…Their main molecular characteristics are listed in table 2. Included in this series are three conventional, commercial type branched (lowdensity) polyethylenes -samples B-l, B-2 and B-3 -whose characteristics have been previously described [17,18]. Sample B-4 is a hydrogenated polybutadiene obtained from Phillips Petroleum Co.…”

“…The branching characteristics given in table 2 were determined by carbon-13 nmr spectroscopy using methods previously developed [25]. Molecular weights were determined by gel permeation chromotography [17]. The ethylene-vinyl acetate copolymers studied were of the usual commercial variety.…”

Abstracts

“…The complex spherulitic morphology of semicrystalline polymers depends on the interplay between the chemical microstructure of the polymer and crystallization conditions 40–42. At any given crystallization temperature, chains/chain segments that are unable to crystallize are excluded from the growing crystal.…”

Einfluß der Molekülmasse auf morphologie und mechanische Eigenschaften von Polyethylen

“…Uber den EinfluB der Molekulmasse auf die uhermolekulare Struktur existieren verschiedene Untersuchungen mit den Methoden der Kleinwinkellichtstreuung und der Polarisationslichtmikroskopie an Polyethylen hoher Dichte (PE-HD) [l, 21 und in geringerem Umfang a n Polyethylen niederer Dichle (PE-ND) [ 3 ] . Ursache ist die erst in den letzten Jahren erfolgte genauere Aufklarung dcr ubermolekularen Struktur und die Kompliziertheit der Zusammenhange.…”