Thermorheological behavior analysis of mLLDPE and mVLDPE: Correlation with branching structure

In this article, the correlation between the thermorheological behavior and the molecular structure of two grades of metallocene polyethylene, namely linear low density and very low density polyethylene, is studied. The investigated polymers possess the same molecular weight and polydispersity index, but different levels of short branches. Increasing the number of short branches results in enhanced activation energy and delayed relaxation times of the polymers. Four methods including the timetemperature superposition (TTS), van Gurp-Palmen and activation energy (Ea) as a function of the phase angle, Ea(), and the storage modulus, Ea(G) are employed to study the thermorheological behavior of the samples. The results indicated that the thermorheologically simple behavior is dominant in the specimens. Both the Ea() and Ea(G) showed independency toward phase angle and the storage modulus. Moreover, the activation energy values obtained from the TTS principle and the Ea() and Ea(G) diagrams were in good agreement. The zero-shear rate viscosity of the samples also followed the equation of the linear polyethylene. Regarding the simple thermorheological behavior and the agreement of the zero shear rate viscosity with the relation of the linear polyethylene, one can conclude that long branches do not exist in the investigated metallocene polyethylenes of this article. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013