Shear and extensional rheology of sparsely branched metallocene-catalyzed polyethylenes

The purpose of this study was to identify any rheological effects that are consistent with the presence of sparse levels of long chain branching (LCB) in three commercial metallocene-catalyzed polyethylenes (MCPE) all of the same melt flow index of 1.0. Two Dow INSITE MCPEs with apparently varying levels of LCB of approximately 0.17 and 0.57/10 000 carbon atoms and one Exxon EXXPOL MCPE with no LCB were studied. The breadth of distribution as determined by Mw/Mn of the three samples was 2.11 for the Exxon and one of the Dow samples, and 2.42 for the other Dow sample that had the highest degree of LCB. The MCPE with the highest branching seemed to have a slightly higher molecular weight tail in the distribution. Both the Dow samples had significantly higher flow activation energies than the Exxon sample, consistent with the presence of LCB, but this method could not distinguish between the two branched polymers. The differences in Mw could also not account for the appreciably higher zero-shear viscosities ...

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