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Rheologica Acta

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01-09-2012 | Original Contribution

Detecting very low levels of long-chain branching in metallocene-catalyzed polyethylenes

Author: Florian J. Stadler

Published in: Rheologica Acta | Issue 9/2012

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Abstract

The detection of long-chain branches (LCBs) is an issue of significant importance in both basic research and industrial applications, as LCBs provide excellent means to improve the processing behavior, especially in elongation-dominated processing operations. In this article, different methods for the detection of very low amounts of LCBs in metallocene-catalyzed polyethylene are presented and compared with respect to their sensitivity. Depending on the molar mass, the zero shear rate viscosity increase factor η ₀/\(\eta_{0}^{\rm lin}\), the steady-state elastic recovery compliance \(J_{e}^{0}\), the complex modulus functions G′(ω) and G″(ω), and the thermorheological complexity were found to be sensitive. In general, the higher the molar mass, the more important the transient quantities become and the easier finding the long-chain branches gets. Although rheology is very sensitive, rheological methods in combination with size exclusion chromatography proved to be the most sensitive combination to detect even very low amounts of LCBs. Especially methods involving the elastic properties (G′(ω), \(J_{\rm e}^{0}\), and J _r(t)) react very sensitively, but these are also very distinctly influenced by the molar mass distribution.

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The two main relaxation times in lightly branched LCB-mPE are determined by the molar mass M _w. The shorter one (λ ₂), scaling with \(M_{\rm w}^{3.6}\), corresponds to the characteristic relaxation time λ of linear PE, while the longer one (λ ₁), scaling with \(M_{\rm w}^{5.15}\), is also influenced by the degree of branching and longer than the shorter one by a factor of about 1,000 at M _w ≈ 50 kg/mol (Stadler and Münstedt 2009).

To reach the terminal regime fully, a frequency of at least two more likely four decades would have to be measured, which would mean unreasonably long measurement times in the order of months. Besides the limitation of the lowest frequency the rheometer can obtain, also it is very doubtful that the thermal stability would be sufficiently long to allow such a measurement.

Many more reports of the activation energy E _a exist, but the determination of E _a is also dependent on the laboratory procedure (e.g., Keßner et al. 2009). E _a of mHDPE was reported to be in the interval between 22 and 31 kJ/mol by different authors (see references in Stadler et al. (2007b) for an overview). For this reason, only activation energies reported by the same group, using the same methods should be compared.

Alternative models, e.g., by Vega et al. (1998) require individual parameters for each comonomer length.

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Title: Detecting very low levels of long-chain branching in metallocene-catalyzed polyethylenes
Author: Florian J. Stadler
Publication date: 01-09-2012
Publisher: Springer-Verlag
Published in: Rheologica Acta / Issue 9/2012
Print ISSN: 0035-4511
Electronic ISSN: 1435-1528
DOI: https://doi.org/10.1007/s00397-012-0642-x

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