Various blend systems having controlled level of long-chain branching were prepared by melt mixing of different amounts (5,10,25, 50, 75, and 90 wt %) of low-density polyethylene (LDPE) with a linear low density polyethylene (LLDPE). Analysis of the branching structure in the blends as well as in the neat components was carried out via thermorheological method. For this purpose six methods including time-temperature superposition (TTS), Cole-Cole plot, van Gurp-Palmen curve, phase angle (d) versus reduced frequency curve, and activation energy as a function of the d were employed. The results of all these methods (except TTS) indicated a complex thermorheological behavior for the neat LLDPE and LLDPE/LDPE blends. The extent of complexity was intensified by increasing LDPE content of the blends. However, using TTS method, E a (d) and d(x) curves resulted in simple thermorheological behavior for neat LDPE. The simple thermorheological behavior of LDPE having high content of long-chain branches (LCB) was attributed to small differences in its branch structures. The zero-shear rate viscosities of all samples deviated from the power-law equation of linear PEs which confirmed the presence of LCB in all the systems. This study shows that thermorheological assessment can be used as an alternative powerful rheological tool for analysis of the branching structures in PE blends.
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 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 simple 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.
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