Creep recovery behavior of metallocene linear low-density polyethylenes |
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Authors: | Claus Gabriel Helmut Münstedt |
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Institution: | (1) Institute of Polymer Materials Department of Materials Science University Erlangen-Nürnberg Martensstrasse 7 D-91058 Erlangen, Germany, DE |
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Abstract: | The rheological behavior of two metallocene linear low-density polyethylenes (mLLDPE) is investigated in shear creep recovery
measurements using a magnetic bearing torsional creep apparatus of high accuracy. The two mLLDPE used are homogeneous with
respect to the comonomer distribution. The most interesting feature of the two mLLDPE is that their molecular mass distributions
are alike. Therefore, as one of the mLLDPE contains long-chain branches, the influence of long-chain branching on the elastic
properties of polyethylene melts could be investigated. It was found that long-chain branches increase the elasticity of the
melt characterized by the steady-state recoverable compliance. The long-chain branched mLLDPE has a flow activation energy
of 45 kJ/mol which is distinctly higher than that of the other mLLDPE. The shear thinning behavior is much more pronounced
for the long-chain branched mLLDPE. A discrepancy between the weight average molecular mass M
w calculated from size exclusion chromatography measurements by the universal calibration method and the zero shear viscosities
of the two mLLDPE was observed. These observations are discussed with reference to the molecular architecture of the long-chain
branched mLLDPE. The rheological properties of the long-chain branched mLLDPE are compared with those of a classical long-chain
branched LDPE. It is surprisingly found that the rheological behavior is very much the same for these two products although
their molecular mass distributions and presumedly the branching structures differ largely.
Received: 15 February 1999 Accepted: 10 June 1999 |
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Keywords: | Metallocene linear low-density polyethylene Creep recovery Short- and long-chain branching Chemical composition distribution Flow activation energy |
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