Thermorheological Behavior Analysis of mLLDPE and mVLDPE: Correlation with Branching Structure A. K. Dordinejad, 1 S. H. Jafari, 1 H. A. Khonakdar, 2 U. Wagenknecht, 3 G. Heinrich 3 1 School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran 2 Iran Polymer and Petrochemical Institute, Tehran, Iran 3 Leibniz-Institute of Polymer Research Dresden, Dresden, Germany Correspondence to: S. H. Jafari (E-mail: shjafari@ut.ac.ir) ABSTRACT: In this article, the correlation between the thermorheological behavior and the molecular structure of two grades of met- allocene 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 time–temperature superposition (TTS), van Gurp-Palmen and activation energy (E a ) as a function of the phase angle, E a (d), and the storage modulus, E a (G 0 ) are employed to study the thermorheological behavior of the samples. The results indicated that the thermorheologically sim- ple behavior is dominant in the specimens. Both the E a (d) and E a (G 0 ) showed independency toward phase angle and the storage modulus. Moreover, the activation energy values obtained from the TTS principle and the E a (d) and E a (G 0 ) 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. V C 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 129: 458–463, 2013 KEYWORDS: polyolefins; rheology; morphology Received 29 April 2012; accepted 19 October 2012; published online 19 November 2012 DOI: 10.1002/app.38745 INTRODUCTION Studying the thermorheological behavior of polymers as a favor- ite rheological tool gives us an understanding about molecular structure of polymers such as polyethylenes. Linear viscoelastic properties in different temperatures can be shifted using the time–temperature superposition (TTS) and making the master curve. Time-scale shift factor, a T , can be obtained using the method proposed by Mavridis and Shroff. 1 a T is expressed by an Arrhenius equation as follows: a T ¼ exp E a R 1 T  1 T 0     (1) where E a is the activation energy, T the measurement tempera- ture, T 0 the reference temperature and R is the universal gas constant. The influence of branching (short and long branches) on the rheological properties of polyethylenes has gained much atten- tion during the recent years. The effect of short branches on rheological properties is mostly considered to be negligible com- pared to long branches. However, it has been confirmed that short-chain branches affect the temperature dependence of rheological properties. Vega et al. 2 reported that the activation energy increases with the increase of short-chain branch content. Activation energy and thermorheological behavior could be different regarding the length of branches in polyethylenes and their molecular structure as well as their branch content. The activation energy of linear polyethylene is reported to be between 26–28 kJ/mol, while slightly higher values (30–34 kJ/mol) is obtained according to the content and kind of comonomer of linear low-density metallocene polyethylenes (mLLDPE). 3–13 In literature with incorporating long branches to metallocene polyethylene an increase in the activation energy compared to LLDPE is reported. 12 Also for LDPE which contains a great amount of long branches, activation energy of about 65 kJ/mol is found. 12 V C 2012 Wiley Periodicals, Inc. 458 J. APPL. POLYM. SCI. 2013, DOI: 10.1002/APP.38745 WILEYONLINELIBRARY.COM/APP