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Linear viscoelastic and extrusion flow measurements of an alleged long chain branched metallocene catalyzed polyethylenes
Autori: M. Fernāndez, J.J. Pena, A. Santamariā, J.F. Vega, A. Munoz-Escalona
Presentato al: Southern Europe Conference on Rheology
Anno: 1999
Volume: 2
Issue: Unico
Casa Editrice: SIR-Cuen Napoli
Lingua: English
Abstract
Metallocene catalysts can produce vinyl terminated chains, that is to say macromonomers with the ability of incorporate into the growing chain of polythylene (PE), giving rise to long chain side structures 1-4. The level of long chain branching, LCB, depends on the catalyst and polymerization conditions in a way which has not been yet clearly established. In some cases very small amounts of LCB are developed (less than one long branch per PE chain), which makes the system very difficult to characterize from a molecular point of view. Actually traditional methods like Size Exclusion Chromatography (SEC) combined with intrinsec viscosity and NMR are unable to measure such a low LCB levels. Rheological techniques based in shear and elongational flow measurements, have been proposed as reliable methods to detect very small amounts of LCB 5,6. Needless to say that the effect of low levels of LCB in flow properties of metallocene catalyzed polyethylenes is nowadays a subject of great interest for both industrial and scientific rheologists.
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