ISSN:
0022-3832
Schlagwort(e):
Chemistry
;
Polymer and Materials Science
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
,
Physik
Notizen:
The relationship at 190°C. between Newtonian melt viscosity η0 and the weight-average molecular weight, M̄w for polyethylenes has been developed by use of melt viscosities obtained from capillary viscometry and molecular weights from intrinsic viscosity and bulk flow measurements in addition to the usual light-scattering determinations. The same η0 vs. M̄w relationship is obeyed by whole and fractionated linear polyethylenes and also by ethylene-butene and ethylene-propylene copolymers. The Newtonian viscosity therefore seems virtually independent of molecular weight distribution and the influence of short (ethyl, methyl) chain branches. Low density polyethylenes have consistently lower Newtonian melt viscosities at given M̄w than their linear counterparts. At constant M̄w and temperature, therefore, η0 depends primarily on long-chain branching. A viscosity index of long-chain branching (v.b.i.) is defined by the ratio of Newtonian viscosities of a linear and a branched polyethylene of the same M̄w. The v.b.i. correlates well with an intrinsic viscosity branching index provided long branch incidence is high. The correlation breaks down otherwise, because the intrinsic viscosity is also sensitive to molecular weight distribution and the presence of short-chain branches. The v.b.i. is therefore more useful as a long branching index. It is a comparative index, however, and not an absolute evaluation of the number of long branches per molecule. The η0 vs. M̄w relationship of linear polyethylenes does not obey the often reported 3.5 power law for linear polymers, but is well fitted by a 4.2 power factor. The present work indicates the necessity of using polyethylenes which have been stabilized against the effects of thermally induced changes if meaningful values of Newtonian viscosity are to be obtained.
Zusätzliches Material:
6 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/pol.1962.1205816603
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