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 M (w) / M (n) 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 how 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 M (w) could also not account for the appreciably higher zero-shear viscosities of the branched samples relative to the linear sample. Despite the differences in M (w) and LCB content in the two Dow samples, they exhibited almost identical shear how curves at temperatures between 120 and 170 degrees C. They also exhibited very similar shear stress growth behavior. Under constant extension rate deformation, the two samples with LCB showed a significant degree of strain hardening relative to the linear sample. Comparison between the two Dow samples revealed that the sample with the higher degree of LCB showed a greater degree of strain-hardening behavior. The extensional behavior is consistent with the LCB content determined by means of solution light scattering. (C) 2000 The Society of Rheology. [S0148-6055(00)00905-6].