We have the question of how the molar mass dependence of flow activation energy for very low molar masses of polyolefins has been found to be experimentally. I measured the values for low to high molar masses of PE to be constant at ca. 27 kJ/mol but for molar masses below Mw=40 kg/mol, especially below 5 kg/mol, I am not aware of reliable data.
Doese anybody know of such investigations for PE or PP?
Determination of method-invariant activation energiesof long-chain branched low-density polyethylenes
Ute Keßner, Joachim Kaschta, and Helmut Münstedt
a)
Institute of Polymer Materials, Friedrich-Alexander-University Erlangen-Nürnberg, Martensstrasse 7, D-91058 Erlangen, Germany
(Received 29 January 2009; final revision received 6 April 2009
Synopsis
The idea to use the temperature dependence of rheological properties, especially the flowactivation energy, as a tool to investigate branching structures is well-known from literature.However, there is no common method to obtain activation energies, which are independent of themeasuring quantity chosen, particularly, in the case of slightly thermorheologically complexpolymers like low-density polyethylene
LDPE
. Hence, differing activation energies result, whichcannot unequivocally be correlated with the branching structure. This paper describes a method forthe determination of method-independent activation energies for thermorheologically complexpolymers like LDPE. From a generalized approach to the time-temperature superposition principle,a vertical shift factor is introduced, which is related to the temperature dependence of the linearsteady-state compliance. In the case of the complex LDPE, a decrease in the linear steady-statecompliance with temperature is found. Taking this experimentally determined shift factor intoaccount leads to activation energies independent of the rheological quantity chosen. These valuescan be taken to analyze differences of the branching architecture.
©
2009 The Society of Rheology.
DOI: 10.1122/1.3124682
this paper can help you.
Thank you very much for the answer.
Sorry, but this paper is not relevant. While I am not coauthor, I know the authors very well. The question is about activation energies of very low molecular materials not about LCB-materials.
The flow activation energy is likely to be related to the fractional free volume, which is constant until end-group contributions increase at low molar mass. I am not sure if anyone has measured the Fox-Flory parameter for polyethylene, or if that even makes sense in a semi-crystalline material.
Thanks, Peter.
I concur with your arguments, but I don't know any literature having actually done this.
I did DMA on various polyolefins and found that it is extremely difficult to find a Tg as the crystallinity is too high. So for PP it should be possible, but for PE, I guess this method would not work.
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